Xử lý nước thải bằng phương pháp sinh học

là một chương trong cuốn sách " quá trình xử lý nước thải bằng phương pháp sinh học". nó mô tả quá trình xử lý nước, định nghĩa mà giải thích rõ các thông số cơ bản trong ngành môi trường: xử lý hiếu khí, kị khí, COD, BOD, HDT. Và các nhân tố ảnh hưởng đến quá trình xử lý.

Wastewater Treatment

According to the Code of Federal Regulations (CFR) 40 CFR Part 403, regulations were established in the late 1970s and early 1980s to help publicly owned treatment works (POTW) control industrial discharges to sewers.

These regulations were designed to prevent pass-through and interference at the treatment plants and interference

in the collection and transmission systems.

Pass-through occurs when pollutants literally pass through

a POTW without being properly treated, and cause the POTW to have an effluent violation or increase the mag- nitude or duration of a violation.

Interference occurs when a pollutant discharge causes a POTW to violate its permit by inhibiting or disrupting treatment processes, treatment operations, or processes related to sludge use or disposal.

18.1 WASTEWATER OPERATORS

Like waterworks operators, wastewater operators are

highly trained and artful practitioners and technicians of

their trade Both operators are also required by the states

to be licensed or certified to operate a wastewater

treat-ment plant

When learning wastewater operator skills, there are anumber of excellent texts available to aid in the training

process Many of these texts are listed in Table 18.1

18.1.1 T HE W ASTEWATER T REATMENT P ROCESS :

T HE M ODEL

Figure 18.1 shows a basic schematic of an example

waste-water treatment process providing primary and secondary

treatment using the activated sludge process This is the

model, prototype, and paradigm used in this book Though

it is true that in secondary treatment (which provides

bio-chemical oxygen demand [BOD] removal beyond what is

achievable by simple sedimentation), there are actually

three commonly used approaches (trickling filter,

acti-vated sludge, and oxidation ponds) For instructive and

illustrative purposes, we focus on the activated sludge

process throughout this handbook The purpose of

Figure 18.1 is to allow the reader to follow the treatment

process step-by-step as it is presented (and as it is actually

configured in the real world) and to assist understanding

of how all the various unit processes sequentially follow

and tie into each other

We begin certain sections (which discuss unit processes)with frequent reference to Figure 18.1 It is important tobegin these sections in this manner because wastewatertreatment is a series of individual steps (unit processes)that treat the wastestream as it makes its way through theentire process It logically follows that a pictorial presen-tation along with pertinent written information enhancesthe learning process It should also be pointed out thateven though the model shown in Figure 18.1 does notinclude all unit processes currently used in wastewatertreatment, we do not ignore the other major processes:trickling filters, rotating biological contactors (RBCs), andoxidation ponds

18.2 WASTEWATER TERMINOLOGY AND DEFINITIONS

Wastewater treatment technology, like many other cal fields, has its own unique terms with their own meaning.Though some of the terms are unique, many are common

techni-to other professions Remember that the science of water treatment is a combination of engineering, biology,mathematics, hydrology, chemistry, physics, and other dis-ciplines Many of the terms used in engineering, biology,mathematics, hydrology, chemistry, physics, and othersare also used in wastewater treatment Those terms notlisted or defined in the following section will be defined

waste-as they appear in the text

18.2.1 T ERMINOLOGY AND D EFINITIONS

Activated sludge the solids formed when organisms are used to treat wastewater usingthe activated sludge treatment process Itincludes organisms, accumulated food materi-als, and waste products from the aerobicdecomposition process

micro-Advanced waste treatment treatment technology used

to produce an extremely high quality discharge

Aerobic conditions in which free, elemental oxygen

is present Also used to describe organisms,biological activity, or treatment processes thatrequire free oxygen

Anaerobic conditions in which no oxygen (free orcombined) is available Also used to describeorganisms, biological activity or treatment pro-cesses that function in the absence of oxygen.18

528 Handbook of Water and Wastewater Treatment Plant Operations

Anoxic conditions in which no free, elemental oxygen

is present The only source of oxygen is

com-bined oxygen, such as that found in nitrate

compounds Also used to describe biological

activity of treatment processes that function

only in the presence of combined oxygen

Average monthly discharge limitation the highest

allowable discharge over a calendar month

Average weekly discharge limitation t h e h i g h e s t

allowable discharge over a calendar week

Biochemical oxygen demand (BOD) the amount of

organic matter that can be biologically oxidized

under controlled conditions (5 days @ 20∞C inthe dark)

Biosolids (from 1977) solid organic matter recoveredfrom a sewage treatment process and used espe-cially as fertilizer (or soil amendment); usuallyused in plural (from Merriam-Webster’s Colle- giate Dictionary, 10th ed., 1998)

Note: In this text, biosolids is used in many places(activated sludge being the exception) toreplace the standard term sludge The authorviews the term sludge as an ugly, inappropriatefour-letter word to describe biosolids Biosolids

TABLE 18.1

Recommended Reference and Study Material

1 Kerri, K.D et al., Advanced Waste Treatment, A Field Study Program, 2nd ed., California State University, Sacramento, 1995.

2 U.S Environmental Protection Agency, Aerobic Biological Wastewater Treatment Facilities, EPA 430/9–77–006, Washington, D.C., 1977.

3 U.S Environmental Protection Agency, Anaerobic Sludge Digestion, EPA-430/9–76–001, Washington, D.C., 1977.

4 American Society for Testing Materials, Section 11: Water and environmental technology, in Annual Book of ASTM Standards, Philadelphia, PA.

5. Guidelines Establishing Test Procedures for the Analysis of Pollutants, Federal Register (40 CFR 136), April 4, 1995, Vol 60, No 64, p 17160.

6 HACH Chemical Company, Handbook of Water Analysis, 2nd ed., Loveland, CO, 1992.

7 Kerri, K.D et al., Industrial Waste Treatment: A Field Study Program, Vols 1 and 2, California State University, Sacramento, CA, 1996.

8 U.S Environmental Protection Agency, Environmental Monitoring Systems Laboratory-Cincinnati, Methods for Chemical Analysis of Water and Wastes, EPA-6000/4–79–020, revised March 1983 and 1979 (where applicable).

9 Water Pollution Control Federation (now called Water Environment Federation), O & M of Trickling Filters, RBC and Related Processes, Manual

of Practice OM-10, Alexandria, VA, 1988.

10 Kerri, K.D et al., Operation of Wastewater Treatment Plants: A Field Study Program, Vols 1 and 2, 4th ed., California State University, Sacramento, 1993.

11 American Public Health Association, American Water Works Association-Water Environment Federation, Standard Methods for the Examination

of Water and Wastewater, 18th ed., Washington, D.C., 1992.

12 Kerri, K.D et al., Treatment of Metal Wastestreams, 2nd ed., California State University, Sacramento, 1993.

13 Price, J.K., Basic Math Concepts: For Water and Wastewater Plant Operators, Technomic Publ., Lancaster, PA, 1991.

14 Haller, E., Simplified Wastewater Treatment Plant Operations, Technomic Publ., Lancaster, PA, 1999.

15 Qaism, S.R., Wastewater Treatment Plants: Planning, Design, and Operation, Technomic Publ., Lancaster, PA, 1994.

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol 1, Technomic Publ., Lancaster, PA, 1999.

FIGURE 18.1 Schematic of an example wastewater treatment process providing primary and secondary treatment using activated sludge process (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol 1, Technomic Publ., Lancaster, PA, 1999.)

Sludge disposal

Screenings

Influent

Grit

Sludge dewatering

Anaerobic digester

Collection system

Thickener

Screening and comminution

contact tank Activated sludge

Grit chamber

Primary settling

Secondary settling

Primary treatment Secondary treatment

Chlorine Effluent Air

Wastewater Treatment 529

is a product that can be reused; it has some

value Because biosolids has value, it certainly

should not be classified as a waste product, and

when biosolids for beneficial reuse is

addressed, it is made clear that it is not

Buffer a substance or solution which resists changes

in pH

Carbonaceous biochemical oxygen demand (CBOD 5 )

the amount of biochemical oxygen demand that

can be attributed to carbonaceous material

Chemical oxygen demand (COD) the amount of

chemically oxidizable materials present in the

wastewater

Clarifier a device designed to permit solids to settle

or rise and be separated from the flow Also

known as a settling tank or sedimentation basin

Coliform a type of bacteria used to indicate possible

human or animal contamination of water

Combined sewer a collection system that carries both

wastewater and storm water flows

Comminution a process that shreds solids into

smaller, less harmful particles

Composite sample a combination of individual

sam-ples taken in proportion to flow

Daily discharge the discharge of a pollutant measured

during a calendar day or any 24-h period that

reasonably represents a calendar day for the

purposes of sampling Limitations expressed as

weight is total mass (weight) discharged over

the day Limitations expressed in other units are

average measurements of the day

Daily maximum discharge the highest allowable

val-ues for a daily discharge

Detention time the theoretical time water remains in

a tank at a given flow rate

Dewatering the removal or separation of a portion of

water present in a sludge or slurry

Discharge monitoring report (DMR) the monthly

report required by the treatment plant’s

National Pollutant Discharge Elimination

Sys-tem (NPDES) discharge permit

Dissolved oxygen (DO) free or elemental oxygen that

is dissolved in water

Effluent the flow leaving a tank, channel, or treatment

process

Effluent limitation any restriction imposed by the

regulatory agency on quantities, discharge

rates, or concentrations of pollutants that are

discharged from point sources into state waters

Facultative organisms that can survive and function

in the presence or absence of free, elemental

Flume a flow rate measurement device

Food-to-microorganism ratio (F:M) a n a c t iva t e dsludge process control calculation based uponthe amount of food (BOD or COD) availableper pound of mixed liquor volatile suspendedsolids

Grab sample an individual sample collected at a domly selected time

ran-Grit heavy inorganic solids such as sand, gravel, eggshells, or metal filings

Industrial wastewater wastes associated with trial manufacturing processes

indus-Infiltration/inflow extraneous flows in sewers; ply, inflow is water discharged into sewer pipes

sim-or service connections from such sources asfoundation drains, roof leaders, cellar and yardarea drains, cooling water from air conditioners,and other clean-water discharges from commer-cial and industrial establishments Defined byMetcalf & Eddy as follows:1

• Infiltration water entering the collectionsystem through cracks, joints, or breaks

• Steady inflow water discharged from cellarand foundation drains, cooling water dis-charges, and drains from springs andswampy areas This type of inflow is steadyand is identified and measured along withinfiltration

• Direct flow those types of inflow that have

a direct stormwater runoff connection to thesanitary sewer and cause an almost immedi-ate increase in wastewater flows Possiblesources are roof leaders, yard and areawaydrains, manhole covers, cross connectionsfrom storm drains and catch basins, andcombined sewers

• Total inflow the sum of the direct inflow atany point in the system plus any flow dis-charged from the system upstream throughoverflows, pumping station bypasses, andthe like

• Delayed inflow stormwater that may requireseveral days or more to drain through thesewer system This category can include thedischarge of sump pumps from cellar drain-age as well as the slowed entry of surfacewater through manholes in ponded areas

Influent the wastewater entering a tank, channel, ortreatment process

530 Handbook of Water and Wastewater Treatment Plant Operations

Inorganic mineral materials such as salt, ferric

chlo-ride, iron, sand, gravel, etc

License a certificate issued by the state board of

water-works or wastewater water-works operators authorizing

the holder to perform the duties of a wastewater

treatment plant operator

Mean cell residence time (MCRT) the average length

of time a mixed liquor suspended solids particle

remains in the activated sludge process May

also be known as sludge retention time

Mixed liquor the combination of return activated

sludge and wastewater in the aeration tank

Mixed liquor suspended solids (MLSS) the

suspend-ed solids concentration of the mixsuspend-ed liquor

Mixed liquor volatile suspended solids (MLVSS) the

concentration of organic matter in the mixed

liquor suspended solids

Milligrams/Liter (mg/L) a measure of concentration

It is equivalent to parts per million

National Pollutant Discharge Elimination System

permit permit that authorizes the discharge of

treated wastes and specifies the condition,

which must be met for discharge

Nitrogenous oxygen demand (NOD) a measure of

the amount of oxygen required to biologically

oxidize nitrogen compounds under specified

conditions of time and temperature

Nutrients substances required to support living

organ-isms Usually refers to nitrogen, phosphorus,

iron, and other trace metals

Organic materials that consist of carbon, hydrogen,

oxygen, sulfur, and nitrogen Many organics are

biologically degradable All organic

com-pounds can be converted to carbon dioxide and

water when subjected to high temperatures

Pathogenic disease causing A pathogenic organism is

capable of causing illness

Point source any discernible, defined, and discrete

conveyance from which pollutants are or may

be discharged

Part per million (ppm) an alternative (but numerically

equivalent) unit used in chemistry is milligrams

per liter As an analogy, think of this unit as

being equivalent to a full shot glass in a

swim-ming pool

Return activated sludge solids (RASS) the

concen-tration of suspended solids in the sludge flow

being returned from the settling tank to the head

of the aeration tank

Sanitary wastewater wastes discharged from

resi-dences and from commercial, institutional, and

similar facilities that include both sewage and

Settleability a process control test used to evaluate thesettling characteristics of the activated sludge

Readings taken at 30 to 60 min are used tocalculate the settled sludge volume and thesludge volume index

Settled sludge volume (SSV) the volume in percentoccupied by an activated sludge sample after

30 to 60 minutes of settling Normally written

as SSV with a subscript to indicate the time ofthe reading used for calculation (SSV60) or(SSV30)

Sewage wastewater containing human wastes

Sludge the mixture of settleable solids and water that

is removed from the bottom of the settling tank

Sludge retention time (SRT) see mean cell residencetime

Sludge volume index (SVI) a process control lation that is used to evaluate the settling quality

calcu-of the activated sludge Requires the SSV30 andmixed liquor suspended solids test results tocalculate

Storm sewer a collection system designed to carryonly storm water runoff

Storm water runoff resulting from rainfall and melt

snow-Supernatant the amber-colored liquid above thesludge that is in a digester

Wastewater the water supply of the community after

it has been soiled by use

Waste activated sludge solids (WASS) the tion of suspended solids in the sludge, which isbeing removed from the activated sludge process

concentra-Weir a device used to measure wastewater flow

Zoogleal slime the biological slime which forms onfixed film treatment devices It contains a widevariety of organisms essential to the treatmentprocess

18.3 MEASURING PLANT PERFORMANCE

To evaluate how well a plant or treatment unit process isoperating, performance efficiency or percent removal isused The results can be compared with those listed in theplant’s operation and maintenance manual (O & M) todetermine if the facility is performing as expected In thischapter sample calculations often used to measure plantperformance and efficiency are presented

Wastewater Treatment 531

18.3.1 P LANT P ERFORMANCE AND E FFICIENCY

Note: The calculation used for determining the

per-formance (percent removal) for a digester is

different from that used for performance

(per-cent removal) for other processes Care must be

taken to select the right formula

The following equation is used to determine plant

perfor-mance and efficiency:

E XAMPLE 18.1

Problem:

The influent BOD is 247 mg/L and the plant effluent BOD

is 17 mg/L What is the percent removal?

Solution:

18.3.2 U NIT P ROCESS P ERFORMANCE

AND E FFICIENCY

Equation 18.1 is used again to determine unit process

effi-ciency The concentration entering the unit and the

con-centration leaving the unit (i.e., primary, secondary, etc.)

are used to determine the unit performance

E XAMPLE 18.2

Problem:

The primary influent BOD is 235 mg/L and the primary

effluent BOD is 169 mg/L What is the percent removal?

18.3.3 P ERCENT V OLATILE M ATTER R EDUCTION

IN S LUDGE

The calculation used to determine percent volatile matter

(%VM) reduction is more complicated because of the

changes occurring during sludge digestion:

Raw sludge volatile matter = 74%

Digested sludge volatile matter = 54%

18.4 HYDRAULIC DETENTION TIME

The term detention time (DT) or hydraulic detention time(HDT) refers to the average length of time (theoreticaltime) a drop of water, wastewater, or suspended particlesremains in a tank or channel It is calculated by dividingthe water or wastewater in the tank by the flow rate throughthe tank The units of flow rate used in the calculation aredependent on whether the detention time is to be calcu-lated in seconds, minutes, hours or days Detention time

is used in conjunction with various treatment processes,including sedimentation and coagulation and flocculation

Generally, in practice, detention time is associatedwith the amount of time required for a tank to empty Therange of detention time varies with the process For exam-ple, in a tank used for sedimentation, detention time iscommonly measured in minutes

The calculation methods used to determine detentiontime are illustrated in the following sections

18.4.1 D ETENTION T IME IN D AYS

Use Equation 18.3 to calculate the detention time in days:

E XAMPLE 18.4

Problem:

An anaerobic digester has a volume of 2,400,000 gal.

What is the detention time in days when the influent flow

A settling tank has a volume of 44,000 ft 3 What is the

detention time in hours when the flow is 4.15 MGD?

18.4.3 D ETENTION T IME IN M INUTES

E XAMPLE 18.6

Problem:

A grit channel has a volume of 1340 ft 3 What is the

detention time in minutes when the flow rate is 4.3 MGD?

Solution:

Note: The tank volume and the flow rate must be in

the same dimensions before calculating thehydraulic detention time

18.5 WASTEWATER SOURCES AND CHARACTERISTICS

Wastewater treatment is designed to use the natural fication processes (self-purification processes of streamsand rivers) to the maximum level possible It is alsodesigned to complete these processes in a controlled envi-ronment rather than over many miles of a stream or river.Moreover, the treatment plant is also designed to removeother contaminants that are not normally subjected tonatural processes, as well as treating the solids that aregenerated through the treatment unit steps The typicalwastewater treatment plant is designed to achieve manydifferent purposes:

puri-1 Protect public health

2 Protect public water supplies

3 Protect aquatic life

4 Preserve the best uses of the waters

5 Protect adjacent lands

Wastewater treatment is a series of steps Each of thesteps can be accomplished using one or more treatmentprocesses or types of equipment The major categories oftreatment steps are:

1 Preliminary treatment — Removes materials thatcould damage plant equipment or would occupytreatment capacity without being treated

2 Primary treatment — Removes settleable andfloatable solids (may not be present in all treat-ment plants)

3 Secondary treatment — Removes BOD and solved and colloidal suspended organic matter bybiological action Organics are converted to sta-ble solids, carbon dioxide and more organisms

dis-4 Advanced waste treatment — Uses physical,chemical, and biological processes to removeadditional BOD, solids and nutrients (notpresent in all treatment plants)

5 Disinfection — Removes microorganisms toeliminate or reduce the possibility of diseasewhen the flow is discharged

6 Sludge treatment — Stabilizes the solidsremoved from wastewater during treatment,inactivates pathogenic organisms, and reducesthe volume of the sludge by removing water

The various treatment processes described above arediscussed in detail later

d 0.07 MGD 1, 000, 000 gal MG

Wastewater Treatment 533

18.5.1 W ASTEWATER S OURCES

The principal sources of domestic wastewater in a

com-munity are the residential areas and commercial districts

Other important sources include institutional and

recre-ational facilities and storm water (runoff) and groundwater

(infiltration) Each source produces wastewater with specific

characteristics In this section wastewater sources and the

specific characteristics of wastewater are described

18.5.1.1 Generation of Wastewater

Wastewater is generated by five major sources: human and

animal wastes, household wastes, industrial wastes, storm

water runoff, and groundwater infiltration

1 Human and animal wastes — Contains the solid

and liquid discharges of humans and animals and

is considered by many to be the most dangerous

from a human health viewpoint The primary

health hazard is presented by the millions of

bacteria, viruses, and other microorganisms

(some of which may be pathogenic) present in

the wastestream

2 Household wastes — Consists of wastes, other

than human and animal wastes, discharged from

the home Household wastes usually contain

paper, household cleaners, detergents, trash,

garbage, and other substances the homeowner

discharges into the sewer system

3 Industrial wastes — Includes industry specific

materials that can be discharged from industrial

processes into the collection system Typically

contains chemicals, dyes, acids, alkalis, grit,

detergents, and highly toxic materials

4 Storm water runoff — Many collection systems

are designed to carry both the wastes of the

community and storm water runoff In this type

of system when a storm event occurs, the

waste-stream can contain large amounts of sand,

gravel, and other grit as well as excessive

amounts of water

5 Groundwater infiltration — Groundwater will

enter older improperly sealed collection

sys-tems through cracks or unsealed pipe joints Not

only can this add large amounts of water to

wastewater flows, but also additional grit

18.5.2 C LASSIFICATION OF W ASTEWATER

Wastewater can be classified according to the sources of

flows: domestic, sanitary, industrial, combined, and storm

water

1 Domestic (sewage) wastewater — Contains

mainly human and animal wastes, household

wastes, small amounts of groundwater tion and small amounts of industrial wastes

infiltra-2 Sanitary wastewater — Consists of domesticwastes and significant amounts of industrialwastes In many cases, the industrial wastes can

be treated without special precautions ever, in some cases, the industrial wastes willrequire special precautions or a pretreatmentprogram to ensure the wastes do not cause com-pliance problems for the wastewater treatmentplant

How-3 Industrial wastewater — Consists of industrialwastes only Often the industry will determinethat it is safer and more economical to treat itswaste independent of domestic waste

4 Combined wastewater — Consists of a nation of sanitary wastewater and storm waterrunoff All the wastewater and storm water ofthe community is transported through one sys-tem to the treatment plant

combi-5 Storm water — Contains a separate collectionsystem (no sanitary waste) that carries stormwater runoff including street debris, road salt,and grit

18.5.3 W ASTEWATER C HARACTERISTICS

Wastewater contains many different substances that can

be used to characterize it The specific substances andamounts or concentrations of each will vary, depending

on the source It is difficult to precisely characterize water Instead, wastewater characterization is usuallybased on and applied to an average domestic wastewater

waste-Note: Keep in mind that other sources and types

of wastewater can dramatically change thecharacteristics

Wastewater is characterized in terms of its physical,chemical, and biological characteristics

2 Odor — Odors in domestic wastewater usuallyare caused by gases produced by the decompo-sition of organic matter or by other substances

added to the wastewater Fresh domestic

waste-water has a musty odor If the wastewaste-water is

allowed to go septic, this odor will significantly

change to a rotten egg odor associated with the

production of hydrogen sulfide (H2S)

3 Temperature — the temperature of wastewater

is commonly higher than that of the water

sup-ply because of the addition of warm water from

households and industrial plants However,

sig-nificant amounts of infiltration or storm water

flow can cause major temperature fluctuations

4 Flow — the actual volume of wastewater is

commonly used as a physical characterization

of wastewater and is normally expressed in

terms of gallons per person per day Most

treat-ment plants are designed using an expected flow

of 100 to 200 gallons per person per day This

figure may have to be revised to reflect the

degree of infiltration or storm flow the plant

receives Flow rates will vary throughout the

day This variation, which can be as much as

50 to 200% of the average daily flow is known

as the diurnal flow variation

Note: Diurnal means occurring in a day or daily.

18.5.3.2 Chemical Characteristics

In describing the chemical characteristics of wastewater,

the discussion generally includes topics such as organic

matter, the measurement of organic matter, inorganic

mat-ter, and gases For the sake of simplicity, in this handbook

we specifically describe chemical characteristics in terms

of alkalinity, BOD, chemical oxygen demand (COD),

dis-solved gases, nitrogen compounds, pH, phosphorus, solids

(organic, inorganic, suspended, and dissolved solids), and

water

1 Alkalinity — This is a measure of the

waste-water’s capability to neutralize acids It is

mea-sured in terms of bicarbonate, carbonate, and

hydroxide alkalinity Alkalinity is essential to

buffer (hold the neutral pH) of the wastewater

during the biological treatment processes

2 Biochemical oxygen demand — This is a

mea-sure of the amount of biodegradable matter in

the wastewater Normally measured by a 5-d test

conducted at 20∞C The BOD5 domestic waste

is normally in the range of 100 to 300 mg/L

3 Chemical oxygen demand — This is a measure

of the amount of oxidizable matter present in

the sample The COD is normally in the range

of 200 to 500 mg/L The presence of industrial

wastes can increase this significantly

4 Dissolved gases — These are gases that aredissolved in wastewater The specific gases andnormal concentrations are based upon the com-position of the wastewater Typical domesticwastewater contains oxygen in relatively lowconcentrations, carbon dioxide, and hydrogensulfide (if septic conditions exist)

5 Nitrogen compounds — The type and amount

of nitrogen present will vary from the rawwastewater to the treated effluent Nitrogen fol-lows a cycle of oxidation and reduction Most

of the nitrogen in untreated wastewater will be

in the forms of organic nitrogen and ammonianitrogen Laboratory tests exist for determination

of both of these forms The sum of these twoforms of nitrogen is also measured and is known

as total kjeldahl nitrogen (TKN) Wastewaterwill normally contain between 20 to 85 mg/L ofnitrogen Organic nitrogen will normally be inthe range of 8 to 35 mg/L, and ammonia nitro-gen will be in the range of 12 to 50 mg/L

6 pH — This is a method of expressing the acidcondition of the wastewater pH is expressed on

a scale of 1 to 14 For proper treatment, water pH should normally be in the range of6.5 to 9.0 (ideally 6.5 to 8.0)

waste-7 Phosphorus — This element is essential to logical activity and must be present in at leastminimum quantities or secondary treatmentprocesses will not perform Excessive amountscan cause stream damage and excessive algalgrowth Phosphorus will normally be in therange of 6 to 20 mg/L The removal of phos-phate compounds from detergents has had asignificant impact on the amounts of phospho-rus in wastewater

bio-8 Solids — Most pollutants found in wastewatercan be classified as solids Wastewater treatment

is generally designed to remove solids or to vert solids to a form that is more stable or can

con-be removed Solids can con-be classified by theirchemical composition (organic or inorganic) or

by their physical characteristics (settleable,floatable, and colloidal) Concentration of totalsolids in wastewater is normally in the range of

350 to 1200 mg/L

A Organic solids — Consists of carbon, gen, oxygen, nitrogen and can be converted

hydro-to carbon dioxide and water by ignition at

550∞C Also known as fixed solids or loss

on ignition

B Inorganic solids — Mineral solids that areunaffected by ignition Also known as fixedsolids or ash

Wastewater Treatment 535

C Suspended solids — These solids will not

pass through a glass fiber filter pad Can be

further classified as Total suspended solids

(TSS), volatile suspended solids, and fixed

suspended solids Can also be separated into

three components based on settling

charac-teristics: settleable solids, floatable solids,

and colloidal solids Total suspended solids

in wastewater are normally in the range of

100 to 350 mg/L

D Dissolved solids — These solids will pass

through a glass fiber filter pad Can also be

classified as total dissolved solids (TDS),

volatile dissolved solids, and fixed dissolved

solids TDS are normally in the range of

250 to 850 mg/L

9 Water — This is always the major constituent

of wastewater In most cases water makes up

99.5 to 99.9% of the wastewater Even in the

strongest wastewater, the total amount of

con-tamination present is less than 0.5% of the total

and in average strength wastes it is usually less

than 0.1%

18.5.3.3 Biological Characteristics and Processes

(Note: The biological characteristics of water were

dis-cussed in detail earlier in this text.)

After undergoing physical aspects of treatment (i.e.,

screening, grit removal, and sedimentation) in preliminary

and primary treatment, wastewater still contains some

sus-pended solids and other solids that are dissolved in the

water In a natural stream, such substances are a source

of food for protozoa, fungi, algae, and several varieties of

bacteria In secondary wastewater treatment, these same

microscopic organisms (which are one of the main reasons

for treating wastewater) are allowed to work as fast as

they can to biologically convert the dissolved solids to

suspended solids that will physically settle out at the end

of secondary treatment

Raw wastewater influent typically contains millions

of organisms The majority of these organisms are

non-pathogenic, but several pathogenic organisms may also be

present (These may include the organisms responsible for

diseases such as typhoid, tetanus, hepatitis, dysentery,

gas-troenteritis, and others.)

Many of the organisms found in wastewater are

micro-scopic (microorganisms); they include algae, bacteria,

protozoa (e.g., amoeba, flagellates, free-swimming

cili-ates, and stalked ciliates), rotifers, and viruses

Table 18.2 is a summary of typical domestic

waste-water characteristics

18.6 WASTEWATER COLLECTION SYSTEMS

Wastewater collection systems collect and convey water to the treatment plant The complexity of the systemdepends on the size of the community and the type of systemselected Methods of collection and conveyance of waste-water include gravity systems, force main systems, vacuumsystems, and combinations of all three types of systems

waste-18.6.1 G RAVITY C OLLECTION S YSTEM

In a gravity collection system, the collection lines aresloped to permit the flow to move through the system with

as little pumping as possible The slope of the lines mustkeep the wastewater moving at a velocity (speed) of 2 to

4 ft/sec Otherwise, at lower velocities, solids will settleout and cause clogged lines, overflows, and offensiveodors To keep collection systems lines at a reasonabledepth, wastewater must be lifted (pumped) periodically sothat it can continue flowing downhill to the treatmentplant Pump stations are installed at selected points withinthe system for this purpose

18.6.2 F ORCE M AIN C OLLECTION S YSTEM

In a typical force main collection system, wastewater iscollected to central points and pumped under pressure tothe treatment plant The system is normally used for con-veying wastewater long distances The use of the forcemain system allows the wastewater to flow to the treatmentplant at the desired velocity without using sloped lines Itshould be noted that the pump station discharge lines in

a gravity system are considered to be force mains sincethe content of the lines is under pressure

TABLE 18.2 Typical Domestic Wastewater Characteristics

Characteristic Typical Characteristic

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol 1, Technomic Publ., Lan-

caster, PA, 1999.

Note: Extra care must be taken when performing

maintenance on force main systems since the

content of the collection system is under pressure

18.6.3 V ACUUM S YSTEM

In a vacuum collection system, wastewaters are collected

to central points and then drawn toward the treatment plant

under vacuum The system consists of a large amount of

mechanical equipment and requires a large amount of

maintenance to perform properly Generally, the

vacuum-type collection systems are not economically feasible

18.6.4 P UMPING S TATIONS

Pumping stations provide the motive force (energy) to

keep the wastewater moving at the desired velocity They

are used in both the force main and gravity systems They

are designed in several different configurations and may

use different sources of energy to move the wastewater

(i.e., pumps, air pressure or vacuum) One of the more

commonly used types of pumping station designs is the

wet well/dry well design

18.6.4.1 Wet Well–Dry Well Pumping Stations

The wet well–dry well pumping station consists of two

separate spaces or sections separated by a common wall

Wastewater is collected in one section (known as the wet

well section); the pumping equipment (and in many cases,

the motors and controllers) is located in a second section

known as the dry well There are many different designs for

this type of system, but in most cases the pumps selected

for this system are of a centrifugal design There are a couple

of major considerations in selecting centrifugal design:

1 This design allows for the separation of

mechanical equipment (pumps, motors,

con-trollers, wiring, etc.) from the potentially

cor-rosive atmosphere (sulfides) of the wastewater

2 This type of design is usually safer for workers

because they can monitor, maintain, operate,

and repair equipment without entering the

pumping station wet well

Note: Most pumping station wet wells are confined

spaces To ensure safe entry into such spaces,

compliance with Occupational Safety and

Health Administration’s 29 CFR 1910.146

(Confined Space Entry Standard) is required

18.6.4.2 Wet Well Pumping Stations

Another type of pumping station design is the wet well

type This type consists of a single compartment that

col-lects the wastewater flow The pump is submerged in the

wastewater with motor controls located in the space or

has a weatherproof motor housing located above the wetwell In this type of station, a submersible centrifugalpump is normally used

18.6.4.3 Pneumatic Pumping Stations

The pneumatic pumping station consists of a wet well and

a control system that controls the inlet and outlet valueoperations and provides pressurized air to force or pushthe wastewater through the system The exact method ofoperation depends on the system design When operating,wastewater in the wet well reaches a predetermined leveland activates an automatic valve that closes the influentline The tank (wet well) is then pressurized to a predeter-mined level When the pressure reaches the predeterminedlevel, the effluent line valve is opened and the pressurepushes the wastestream out the discharge line

18.6.4.4 Pumping Station Wet Well Calculations

Calculations normally associated with pumping stationwet well design (determining design lift or pumpingcapacity, etc.) are usually left up to design and mechanicalengineers However, on occasion, wastewater operators orinterceptor’s technicians may be called upon to make cer-tain basic calculations Usually these calculations dealwith determining either pump capacity without influent(e.g., to check the pumping rate of the station’s constantspeed pump) or pump capacity with influent (e.g., to checkhow many gallons per minute the pump is discharging)

In this section we use examples to describe instances onhow and where these two calculations are made

E XAMPLE 18.7: D ETERMINING P UMP C APACITY WITHOUT I NFLUENT

Problem:

A pumping station wet well is 10 ¥ 9 ft The operator needs to check the pumping rate of the station’s constant speed pump To do this, the influent valve to the wet well

is closed for a 5-min test, and the level in the well dropped 2.2 ft What is the pumping rate in gallons per minute?

Solution:

Using the length and width of the well, we can find the area of the water surface:

10 ft ¥ 9 ft = 90 ft 2 The water level dropped 2.2 ft From this we can find the volume of water removed by the pump during the test:

A ¥ = D v

90 ft2 2 2 ft 198 ft

Wastewater Treatment 537

One cubic foot of water holds 7.48 gal We can convert

this volume in cubic feet to gallons:

The test was done for 5 min From this information,

a pumping rate can be calculated:

E XAMPLE 18.8: D ETERMINING P UMP C APACITY

WITH I NFLUENT

Problem:

A wet well is 8.2 ¥ 9.6 ft The influent flow to the well,

measured upstream, is 365 gal/min If the wet well rises

2.2 in in 5 min, how many gallons per minute is the pump

discharging?

Solution:

Influent = Discharge + Accumulation

We want to calculate the discharge Influent is known and

we have enough information to calculate the accumulation.

Using Equation 18.7:

Subtracting from both sides:

The wet well pump is discharging 343.4 gal each minute.

18.7 PRELIMINARY TREATMENT

The initial stage in the wastewater treatment process lowing collection and influent pumping) is preliminarytreatment Raw influent entering the treatment plant maycontain many kinds of materials (trash) The purpose ofpreliminary treatment is to protect plant equipment byremoving these materials that could cause clogs, jams, orexcessive wear to plant machinery In addition, theremoval of various materials at the beginning of the treat-ment process saves valuable space within the treatmentplant

(fol-Preliminary treatment may include many differentprocesses Each is designed to remove a specific type ofmaterial — a potential problem for the treatment process.Processes include: wastewater collections (influent pump-ing, screening, shredding, grit removal, flow measure-ment, preaeration, chemical addition, and flow equaliza-tion) The major processes are shown in Figure 18.1 Inthis section, we describe and discuss each of these pro-cesses and their importance in the treatment process

Note: As mentioned, not all treatment plants will

include all of the processes shown in Figure 18.1.Specific processes have been included to facil-itate discussion of major potential problemswith each process and its operation; this isinformation that may be important to the waste-water operator

18.7.1 S CREENING

The purpose of screening is to remove large solids, such

as rags, cans, rocks, branches, leaves, roots, etc., from theflow before the flow moves on to downstream processes

Note: Typically, a treatment plant will remove

any-where from 0.5 to 12 ft3 of screenings for eachmillion gallons of influent received

A bar screen traps debris as wastewater influent passesthrough Typically, a bar screen consists of a series ofparallel, evenly spaced bars or a perforated screen placed

in a channel (see Figure 18.2) The wastestream passesthrough the screen and the large solids (screenings) aretrapped on the bars for removal

Note: The screenings must be removed frequently

enough to prevent accumulation that will blockthe screen and cause the water level in front ofthe screen to build up

The bar screen may be coarse (2 to 4-in openings) orfine (0.75 to 2.0-in openings) The bar screen may bemanually cleaned (bars or screens are placed at an angle

of 30∞ for easier solids removal; see Figure 18.2) ormechanically cleaned (bars are placed at 45∞ to 60∞ angle

to improve mechanical cleaner operation)

37.48 gal

The screening method employed depends on the

design of the plant, the amount of solids expected, and

whether the screen is for constant or emergency use only

18.7.1.1 Manually Cleaned Screens

Manually cleaned screens are cleaned at least once per

shift (or often enough to prevent buildup that may cause

reduced flow into the plant) using a long tooth rake Solids

are manually pulled to the drain platform and allowed to

drain before storage in a covered container

The area around the screen should be cleaned

fre-quently to prevent a buildup of grease or other materials

that can cause odors, slippery conditions, and insect and

rodent problems Because screenings may contain organic

matter as well as large amounts of grease they should be

stored in a covered container Screenings can be disposed

of by burial in approved landfills or by incineration Some

treatment facilities grind the screenings into small

parti-cles; these particles are then returned to the wastewater

flow for further processing and removal later in the process

18.7.1.1.1 Operational Problems

Manually cleaned screens require a certain amount of

operator attention to maintain optimum operation Failure

to clean the screen frequently can lead to septic wastes

entering the primary, surge flows after cleaning, and low

flows before cleaning On occasion, when such

opera-tional problems occur, it becomes necessary to increase

the frequency of the cleaning cycle Another operational

problem is excessive grit in the bar screen channel

Improper design or construction or insufficient cleaning

may cause this problem The corrective action required is

either to correct the design problem or increase cleaning

frequency and flush the channel regularly Another

com-mon problem with manually cleaned bar screens is theirtendency to clog frequently This may be caused by exces-sive debris in the wastewater or the screen being too finefor its current application The operator should locate thesource of the excessive debris and eliminate it If thescreen is the problem, a coarser screen may need to beinstalled If the bar screen area is filled with obnoxiousodors, flies, and other insects, it may be necessary todispose of screenings more frequently

18.7.1.2 Mechanically Cleaned Screens

Mechanically cleaned screens use a mechanized rakeassembly to collect the solids and move them (carry them)out of the wastewater flow for discharge to a storage hop-per The screen may be continuously cleaned or cleaned

on a time or flow controlled cycle As with the manuallycleaned screen, the area surrounding the mechanicallyoperated screen must be cleaned frequently to preventbuildup of materials, which can cause unsafe conditions

As with all mechanical equipment, operator vigilance

is required to ensure proper operation and proper nance Maintenance includes lubricating equipment andmaintaining it in accordance with manufacturer’s recom-mendations or the plant’s O & M manual

mainte-Screenings from mechanically operated barscreens aredisposed of in the same manner as screenings from man-ually operated screens These include landfill disposal,incineration, or the process of grinding into smaller par-ticles for return to the wastewater flow

18.7.1.2.1 Operational Problems

Many of the operational problems associated with ically cleaned bar screens are the same as those for manualscreens These include septic wastes entering the primary,surge flows after cleaning, excessive grit in the bar screenchannel, and a screen that clogs frequently Basically thesame corrective actions employed for manually operatedscreens would be applied for these problems in mechanicallyoperated screens In addition to these problems, mechani-cally operated screens also have other problems Theseinclude the cleaner failing to operate; and a nonoperatingrake, but operating motor Obviously, these are mechanicalproblems that could be caused by jammed cleaning mech-anism, broken chain, broken cable, or a broken shear pin.Authorized and fully trained maintenance operators should

mechan-be called in to handle these types of problems

18.7.1.3 Safety

The screening area is the first location where the operator

is exposed to the wastewater flow Any toxic, flammable

or explosive gases present in the wastewater can bereleased at this point Operators who frequent enclosedbar screen areas should be equipped with personal airmonitors Adequate ventilation must be provided It is also

FIGURE 18.2 Bar screen (From Spellman, F.R., Spellman’s

Standard Handbook for Wastewater Operators, Vol 1,

Tech-nomic Publ., Lancaster, PA, 1999.)

Drain

Flow in

Wastewater Treatment 539

important to remember that, due to the grease attached to

the screenings this area of the plant can be extremely

slippery Routine cleaning is required to minimize this

problem

Note: Never override safety devices on mechanical

equipment Overrides can result in dangerous

conditions, injuries, and major mechanical

failure

18.7.1.4 Screenings Removal Computations

Operators responsible for screenings disposal are typically

required to keep a record of the amount of screenings

removed from the wastewater flow To keep and maintain

accurate screenings’ records, the volume of screenings

withdrawn must be determined Two methods are commonly

used to calculate the volume of screenings withdrawn:

(18.6)

(18.7)

E XAMPLE 18.9

Problem:

A total of 65 gal of screenings are removed from the

wastewater flow during a 24-h period What is the

screen-ings removal reported as cubic feet per day?

Solution:

First, convert gallons screenings to cubic feet:

Next, calculate screenings removed as cubic feet per day:

E XAMPLE 18.10

Problem:

During 1 week, a total of 310 gal of screenings were

removed from the wastewater screens What is the average

screening removal in cubic feet per day?

18.7.2.1 Comminution

The comminutor is the most common shredding deviceused in wastewater treatment In this device all the waste-water flow passes through the grinder assembly Thegrinder consists of a screen or slotted basket, a rotating

or oscillating cutter, and a stationary cutter Solids passthrough the screen and are chopped or shredded betweenthe two cutters The comminutor will not remove solids,which are too large to fit through the slots, and it will notremove floating objects These materials must be removedmanually

Maintenance requirements for comminutors includealigning, sharpening and replacing cutters and correctiveand preventive maintenance performed in accordance withplant O & M manual

18.7.2.1.1 Operational Problems

Common operational problems associated with tors include output containing coarse solids When thisoccurs it is usually a sign that the cutters are dull ormisaligned If the system does not operate at all, the unit

comminu-is either clogged, jammed, a shear pin or coupling comminu-isbroken or electrical power is shut off If the unit stalls orjams frequently, this usually indicates cutter misalign-ment, excessive debris in influent, or dull cutters

Note: Only qualified maintenance operators should

perform maintenance of shredding equipment

3d

Screenings Removed ft Screenings ft

Q MG3

3MG

( )

65 gal 7.48 gal ft 3 ft

3

= 8 7 screenings

Screenings Removed ft 8.7 ft

1 d 8.7 ft

3

= 41 4 screenings

Screenings Removed ft 41.4 ft

7 d 5.9 ft

for removal at a later process In operation each device’s

cutter alignment and sharpness are critical factors in

effec-tive operation Cutters must be sharpened or replaced and

alignment must be checked in accordance with

manufac-turer’s recommendations Solids, which are not shredded,

must be removed daily, stored in closed containers, and

disposed of by burial or incineration

Barminutor operational problems are similar to those

listed above for comminutors Preventive and corrective

maintenance as well as lubrication must be performed by

qualified personnel and in accordance with the plant’s

O & M manual Because of higher maintenance

require-ments the barminutor is less frequently used

18.7.3 G RIT R EMOVAL

The purpose of grit removal is to remove the heavy

inor-ganic solids that could cause excessive mechanical wear

Grit is heavier than inorganic solids and includes, sand,

gravel, clay, egg shells, coffee grounds, metal filings,

seeds, and other similar materials

There are several processes or devices used for grit

removal All of the processes are based on the fact that

grit is heavier than the organic solids, which should be

kept in suspension for treatment in following processes

Grit removal may be accomplished in grit chambers or by

the centrifugal separation of sludge Processes use gravity

and velocity, aeration, or centrifugal force to separate the

solids from the wastewater

18.7.3.1 Gravity and Velocity Controlled

Grit Removal

Gravity and velocity controlled grit removal is normally

accomplished in a channel or tank where the speed or the

velocity of the wastewater is controlled to about 1 foot

per second (ideal), so that grit will settle while organic

matter remains suspended As long as the velocity is

con-trolled in the range of 0.7 to 1.4 ft/sec the grit removal

will remain effective Velocity is controlled by the amount

of water flowing through the channel, the depth of the

water in the channel, the width of the channel, or the

cumulative width of channels in service

18.7.3.1.1 Process Control Calculations

Velocity of the flow in a channel can be determined either

by the float and stopwatch method or by channel dimensions

E XAMPLE 18.11: V ELOCITY BY F LOAT AND S

TOP-WATCH

Problem:

A float takes 25 sec to travel 34 ft in a grit channel What

is the velocity of the flow in the channel?

Solution:

E XAMPLE 18.12: V ELOCITY BY F LOW AND C HANNEL D IMENSIONS

Note: This calculation can be used for a single

chan-nel or tank or multiple chanchan-nels or tanks withthe same dimensions and equal flow If the flowthrough each unit of the unit dimensions isunequal, the velocity for each channel or tankmust be computed individually

Problem:

The plant is currently using two grit channels Each nel is 3 ft wide and has a water depth of 1.2 ft What is the velocity when the influent flow rate is 3.0 MGD?

chan-Solution:

Note: The channel dimensions must always be in feet.

Convert inches to feet by dividing by 12 in./ft

E XAMPLE 18.13: R EQUIRED S ETTLING T IME

Note: This calculation can be used to determine the

time required for a particle to travel from thesurface of the liquid to the bottom at a givensettling velocity In order to compute the settlingtime, the settling velocity in feet per secondmust be provided or determined experimentally

in a laboratory

Velocity, feet second Distance Traveled, feet

Time Required, Seconds

.

7 2

0 65

2

Wastewater Treatment 541

Problem:

The plant’s grit channel is designed to remove sand and

has a settling velocity of 0.085 ft/sec The channel is

currently operating at a depth of 2.2 ft How many seconds

will it take for a sand particle to reach the channel bottom?

Solution:

E XAMPLE 18.14: R EQUIRED C HANNEL L ENGTH

Note: This calculation can be used to determine the

length of channel required to remove an object

with a specified settling velocity

Problem:

The plant’s grit channel is designed to remove sand and

has a settling velocity of 0.070 ft/sec The channel is

currently operating at a depth of 3 ft The calculated

velocity of flow through the channel is 0.80 ft/sec The

channel is 35 ft long Is the channel long enough to remove

the desired sand particle size?

Solution:

Yes, the channel is long enough to ensure all of the sand

will be removed.

18.7.3.1.2 Cleaning

Gravity type systems may be manually or mechanically

cleaned Manual cleaning normally requires that the

chan-nel be taken out of service, drained, and manually cleaned

Mechanical cleaning systems are operated continuously

or on a time cycle Removal should be frequent enough

to prevent grit carryover into the rest of the plant

Note: Always ventilate the area thoroughly before and

during cleaning activities

Aerated grit removal systems use aeration to keep thelighter organic solids in suspension while allowing theheavier grit articles to settle out Aerated grit removal may

be manually or mechanically cleaned; the majority of thesystems are mechanically cleaned

During normal operation, adjusting the aeration rateproduces the desired separation This requires observation

of mixing and aeration and sampling of fixed suspendedsolids Actual grit removal is controlled by the rate ofaeration If the rate is too high, all of the solids remain insuspension If the rate is too low, both grit and organicswill settle out

The operator observes the same kinds of conditions

as those listed for the gravity and velocity-controlled tem, but must also pay close attention to the air distributionsystem to ensure proper operation

sys-The cyclone degritter uses a rapid spinning motion

(centrifugal force) to separate the heavy inorganic solids

or grit from the light organic solids This unit process isnormally used on primary sludge rather than the entirewastewater flow This critical control factor for the process

is the inlet pressure If the pressure exceeds the mendations of the manufacturer, the unit will flood andgrit will carry through with the flow

recom-Grit is separated from flow, washed, and dischargeddirectly to a strange container Grit removal performance

is determined by calculating the percent removal for ganic (fixed) suspended solids

inor-The operator observes the same kinds of conditionslisted for the gravity and velocity-controlled and aeratedgrit removal systems, with the exception of the air distri-bution system

Typical problems associated with grit removal includemechanical malfunctions and rotten egg odor in the gritchamber (hydrogen sulfide formation), which can lead tometal and concrete corrosion problems Low recovery rate

of grit is another typical problem Bottom scour, aeration, or a lack of detention time normally causes this

Settling, Velocity, fps

=

Settling Time sec .2 ft

.085 ft sec sec

( )=

=

2 0

25 9

Required Channel Length

Channel Depth, ft Flow Velocity, fps

Settling Velocity, fps

=

¥

Required Channel Length ft ft sec

.070 ft sec ft

( )= ¥

=

3 0 80 0

34 3

ft

.

When these problems occur, the operator must make the

required adjustments or repairs to correct the problems

18.7.3.2 Grit Removal Calculations

Wastewater systems typically average 1 to 15 ft3 of

grit/MG of flow (sanitary systems average 1 to 4 ft3/MG;

combined wastewater systems average from 4 to 15 ft3/MG

of flow), with higher ranges during storm events

Generally, grit is disposed of in sanitary landfills

Because of this practice, for planning purposes, operators

must keep accurate records of grit removal Most often,

the data is reported as cubic feet of grit removed per

million gallons of flow:

(18.8)

Over a given period, the average grit removal rate at

a plant (at least a seasonal average) can be determined and

used for planning purposes Typically, grit removal is

cal-culated as cubic yards because excavation is normally

expressed in terms of cubic yards:

(18.9)

E XAMPLE 18.15

Problem:

A treatment plant removes 10 ft 3 of grit in 1 d How many

cubic feet of grit are removed per million gallons if the

plant flow was 9 MGD?

Solution:

E XAMPLE 18.16

Problem:

The total daily grit removed for a plant is 250 gal If the

plant flow is 12.2 MGD, how many cubic feet of grit are

removed per million gallons of flow?

Solution:

First, convert gallon grit removed to cubic feet:

Next, complete the calculation of cubic feet per million gallons:

E XAMPLE 18.17

Problem:

The monthly average grit removal is 2.5 ft 3 /MGD If the monthly average flow is 2,500,000 gal/d, how many cubic yards must be available for grit disposal pit to have a 90-

d capacity?

Solution:

First, calculate the grit generated each day:

The cubic feet grit generated for 90 d would be:

Convert cubic feet grit to cubic yard grit:

18.7.4 P REAERATION

In the preaeration process (diffused or mechanical), weaerate wastewater to achieve and maintain an aerobic state(to freshen septic wastes), strip off hydrogen sulfide (toreduce odors and corrosion), agitate solids (to releasetrapped gases and improve solids separation and settling),and to reduce BOD All of this can be accomplished byaerating the wastewater for 10 to 30 min To reduce BOD,preaeration must be conducted from 45 to 60 min

Grit Removed ft Grit Volume ft

MG3

3MG

3

21

=

Wastewater Treatment 543

18.7.4.1 Operational Observations, Problems,

and Troubleshooting

In preaeration grit removal systems, the operator is

con-cerned with maintaining proper operation and must be

alert to any possible mechanical problems In addition, the

operator monitors DO levels and the impact of preaeration

on influent

18.7.5 C HEMICAL A DDITION

Chemical addition is made (either via dry chemical

meter-ing or solution feed metermeter-ing) to the wastestream to

improve settling, reduce odors, neutralize acids or bases,

reduce corrosion, reduce BOD, improve solids and grease

removal, reduce loading on the plant, add or remove

nutri-ents, add organisms, and aid subsequent downstream

processes The particular chemical and amount used

depends on the desired result Chemicals must be added

at a point where sufficient mixing will occur to obtain

maximum benefit Chemicals typically used in wastewater

treatment include chlorine, peroxide, acids and bases,

miner salts (ferric chloride, alum, etc.), and bioadditives

and enzymes

18.7.5.1 Operational Observations, Problems,

and Troubleshooting

In adding chemicals to the wastestream to remove grit,

the operator monitors the process for evidence of

mechan-ical problems and takes proper corrective actions when

necessary The operator also monitors the current chemical

feed rate and dosage The operator ensures that mixing at

the point of addition is accomplished in accordance with

standard operating procedures and monitors the impact of

chemical addition on influent

18.7.6 E QUALIZATION

The purpose of flow equalization (whether by surge,

diur-nal, or complete methods) is to reduce or remove the wide

swings in flow rates normally associated with wastewater

treatment plant loading; it minimizes the impact of storm

flows The process can be designed to prevent flows above

maximum plant design hydraulic capacity, reduce the

magnitude of diurnal flow variations, and eliminate flow

variations Flow equalization is accomplished using

mix-ing or aeration equipment, pumps, and flow measurement

Normal operation depends on the purpose and

require-ments of the flow equalization system Equalized flows

allow the plant to perform at optimum levels by providing

stable hydraulic and organic loading The downside to flow

equalization is the additional costs associated with

con-struction and operation of the flow equalization facilities

18.7.6.1 Operational Observations, Problems,

and Troubleshooting

During normal operations, the operator must monitor allmechanical systems involved with flow equalization andmust watch for mechanical problems and take the appro-priate corrective action The operator also monitors DOlevels, the impact of equalization on influent, and waterlevels in equalization basins; any necessary adjustmentsare also made

18.7.7 A ERATED S YSTEMS

Aerated grit removal systems use aeration to keep thelighter organic solids in suspension while allowing theheavier grit particles to settle out Aerated grit removalmay be manually or mechanically cleaned; the majority

of the systems are mechanically cleaned

In normal operation, the aeration rate is adjusted toproduce the desired separation, which requires observation

of mixing and aeration and sampling of fixed suspendedsolids Actual grit removal is controlled by the rate ofaeration If the rate is too high, all of the solids remain insuspension If the rate is too low, both the grit and theorganics will settle out

18.7.8 C YCLONE D EGRITTER

The cyclone degritter uses a rapid spinning motion trifugal force) to separate the heavy inorganic solids orgrit from the light organic solids This unit process isnormally used on primary sludge rather than the entirewastewater flow The critical control factor for the process

(cen-is the inlet pressure If the pressure exceeds the mendations of the manufacturer, the unit will flood andgrit will carry through with the flow Grit is separated fromthe flow and discharged directly to a storage container.Grit removal performance is determined by calculating thepercent removal for inorganic (fixed) suspended solids

recom-18.7.9 P RELIMINARY T REATMENT S AMPLING

AND T ESTING

During normal operation of grit removal systems (withthe exception of the screening and shredding processes),the plant operator is responsible for sampling and testing

as shown in Table 18.3

18.7.10 O THER P RELIMINARY T REATMENT P ROCESS

C ONTROL C ALCULATIONS

The desired velocity in sewers in approximately 2 ft/sec

at peak flow; this velocity normally prevents solids fromsettling from the lines When the flow reaches the gritchannel, the velocity should decrease to about 1 ft/sec topermit the heavy inorganic solids to settle In the example

calculations that follow, we describe how the velocity of

the flow in a channel can be determined by the float and

stopwatch method and by channel dimensions

E XAMPLE 18.18: V ELOCITY BY F LOAT

AND S TOPWATCH

Problem:

A float takes 30 sec to travel 37 ft in a grit channel What

is the velocity of the flow in the channel?

Solution:

E XAMPLE 18.19: V ELOCITY BY F LOW

AND C HANNEL D IMENSIONS

Note: This calculation can be used for a single

chan-nel or tank or for multiple chanchan-nels or tanks

with the same dimensions and equal flow If the

flow through each of the unit dimensions is

unequal, the velocity for each channel or tank

must be computed individually

Problem:

The plant is currently using two grit channels Each nel is 3 ft wide and has a water depth of 1.3 ft What is the velocity when the influent flow rate is 4.0 MGD?

chan-Solution:

Note: Because 0.79 is within the 0.7 to 1.4 level, the

operator of this unit would not make any ments

adjust-Note: The channel dimensions must always be in feet.

Convert inches to feet by dividing by 12 in./ft

E XAMPLE 18.20: R EQUIRED S ETTLING T IME

Note: This calculation can be used to determine the time

required for a particle to travel from the surface

of the liquid to the bottom at a given settlingvelocity To compute the settling time, settlingvelocity in feet per second must be provided ordetermined by experiment in a laboratory

TABLE 18.3 Sampling and Testing Grit Removal Systems

Grit removal (velocity) Influent Suspended solids (fixed) Variable

Channel Depth of grit Variable Grit Total solids (fixed) Variable Effluent Suspended solids (fixed) Variable Grit removal (aerated) Influent Suspended solids (fixed) Variable

Grit Total solids (fixed) Variable Effluent Suspended solids (fixed) Variable Chemical addition Influent Jar test Variable

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators,

Vol 1, Technomic Publ., Lancaster, PA, 1999.

Velocity, feet second Distance Traveled, ft

Time required, seconds

.

Wastewater Treatment 545

Problem:

The plant’s grit channel is designed to remove sand and

has a settling velocity of 0.080 ft/sec The channel is

currently operating at a depth of 2.3 ft How many seconds

will it take for a sand particle to reach the channel bottom?

Solution:

E XAMPLE 18.21: R EQUIRED C HANNEL L ENGTH

Note: This calculation can be used to determine the

length of channel required to remove an object

with a specified settling velocity

Problem:

The plant’s grit channel is designed to remove sand and

has a settling velocity of 0.080 ft/sec The channel is

currently operating at a depth of 3 ft The calculated

velocity of flow through the channel is 0.85 ft/sec The

channel is 36 ft long Is the channel long enough to remove

the desired sand particle size?

The purpose of primary treatment (primary sedimentation

or primary clarification) is to remove settleable organic

and flotable solids Normally, each primary clarification

unit can be expected to remove 90 to 95% settleable solids,

40 to 60% TSS, and 25 to 35% BOD

Note: Performance expectations for settling devices

used in other areas of plant operation is

nor-mally expressed as overall unit performance

rather than settling unit performance

Sedimentation may be used throughout the plant toremove settleable and floatable solids It is used in primarytreatment, secondary treatment, and advanced wastewatertreatment processes In this section, we focus on primarytreatment or primary clarification, which uses large basins

in which primary settling is achieved under relatively escent conditions (see Figure 18.1) Within these basins,mechanical scrapers collect the primary settled solids into

qui-a hopper where they qui-are pumped to qui-a sludge-processingarea Oil, grease, and other floating materials (scum) areskimmed from the surface The effluent is discharged overweirs into a collection trough

18.8.1 P ROCESS D ESCRIPTION

In primary sedimentation, wastewater enters a settling tank

or basin Velocity is reduced to approximately 1 ft/min

Note: Notice that the velocity is based on minutes

instead of seconds, as was the case in the gritchannels A grit channel velocity of 1 ft/secwould be 60 ft/min

Solids that are heavier than water settle to the bottom,while solids that are lighter than water float to the top.Settled solids are removed as sludge and floating solidsare removed as scum Wastewater leaves the sedimentationtank over an effluent weir and on to the next step intreatment Detention time, temperature, tank design, andcondition of the equipment control the efficiency of theprocess

18.8.1.1 Overview of Primary Treatment

1 Primary treatment reduces the organic loading

on downstream treatment processes by ing a large amount of settleable, suspended, andfloatable materials

remov-2 Primary treatment reduces the velocity of thewastewater through a clarifier to approximately

1 to 2 ft/min, so that settling and floatation cantake place Slowing the flow enhances removal

of suspended solids in wastewater

3 Primary settling tanks remove floated greaseand scum, remove the settled sludge solids, andcollect them for pumped transfer to disposal orfurther treatment

4 Clarifiers used may be rectangular or circular

In rectangular clarifiers, wastewater flows fromone end to the other, and the settled sludge ismoved to a hopper at the one end, either byflights set on parallel chains or by a single bot-tom scraper set on a traveling bridge Floatingmaterial (mostly grease and oil) is collected by

a surface skimmer

Settling Time sec .3 ft

.080 ft sec sec

( )=

=

2 0

28 7

Required Channel Length

Channel Depth, ft Flow Velocity, fps

fps

=

¥

0 080

R equired Channel ength ft L ft ft sec

.080 ft sec ft

( )= ¥

=

3 0 85 0

31 9

.

5 In circular tanks, the wastewater usually enters

at the middle and flows outward Settled sludge

is pushed to a hopper in the middle of the tank

bottom, and a surface skimmer removes floating

material

6 Factors affecting primary clarifier performance

include:

A Rate of flow through the clarifier

B Wastewater characteristics (strength;

tem-perature; amount and type of industrial

waste; and the density, size, and shapes of

particles)

C Performance of pretreatment processes

D Nature and amount of any wastes recycled

to the primary clarifier

7 Key factors in primary clarifier operation

include the following concepts:

18.8.2 T YPES OF S EDIMENTATION T ANKS

Sedimentation equipment includes septic tanks, two story

tanks, and plain settling tanks or clarifiers All three

devices may be used for primary treatment; plain settling

tanks are normally used for secondary or advanced

waste-water treatment processes

18.8.2.1 Septic Tanks

Septic tanks are prefabricated tanks that serve as a combined

settling and skimming tank and as an unheated–unmixed

anaerobic digester Septic tanks provide long settling times

(6 to 8 h or more), but do not separate decomposing solids

from the wastewater flow When the tank becomes full,

solids will be discharged with the flow The process is

suitable for small facilities (i.e., schools, motels, homes,etc.), but due to the long detention times and lack ofcontrol, it is not suitable for larger applications

18.8.2.2 Two-Story (Imhoff) Tank

The two-story or Imhoff tank is similar to a septic tank inthe removal of settleable solids and the anaerobic diges-tion of solids The difference is that the two story tankconsists of a settling compartment where sedimentation isaccomplished, a lower compartment where settled solidsand digestion takes place, and gas vents Solids removedfrom the wastewater by settling pass from the settlingcompartment into the digestion compartment through aslot in the bottom of the settling compartment The design

of the slot prevents solids from returning to the settlingcompartment Solids decompose anaerobically in thedigestion section Gases produced as a result of the solidsdecomposition are released through the gas vents runningalong each side of the settling compartment

18.8.2.3 Plain Settling Tanks (Clarifiers)

The plain settling tank or clarifier optimizes the settlingprocess Sludge is removed from the tank for processing

in other downstream treatment units Flow enters the tank,

is slowed and distributed evenly across the width anddepth of the unit, passes through the unit, and leaves overthe effluent weir Detention time within the primary set-tling tank is from 1 to 3 h (2-h average)

Sludge removal is accomplished frequently on either

a continuous or intermittent basis Continuous removalrequires additional sludge treatment processes to removethe excess water resulting from the removal of sludge,which contains less than 2 to 3% solids Intermittentsludge removal requires the sludge be pumped from thetank on a schedule frequent enough to prevent large clumps

of solids rising to the surface but infrequent enough toobtain 4 to 8% solids in the sludge withdrawn

Scum must be removed from the surface of the settlingtank frequently This is normally a mechanical process,but may require manual start-up The system should beoperated frequently enough to prevent excessive buildupand scum carryover but not so frequent as to cause hydrau-lic overloading of the scum removal system

Settling tanks require housekeeping and maintenance.Baffles (devices that prevent floatable solids and scum fromleaving the tank), scum troughs, scum collectors, effluenttroughs, and effluent weirs require frequent cleaning to pre-vent heavy biological growths and solids accumulations.Mechanical equipment must be lubricated and maintained

as specified in the manufacturer’s recommendations or inaccordance with procedures listed in the plant O & Mmanual

Retention Time h v gal 2 h d

urface Loading Rate gal d ft

gal d ft

olids into Clarifier lb d

Wastewater Treatment 547

Process control sampling and testing is used to

eval-uate the performance of the settling process Settleable

solids, DO, pH, temperature, TSS and BOD5, as well as

sludge solids and volatile matter testing are routinely

accomplished

18.8.3 O PERATOR O BSERVATIONS , P ROCESS

P ROBLEMS , AND T ROUBLESHOOTING

Before identifying a primary treatment problem and

pro-ceeding with appropriate troubleshooting effort, the operator

must be cognizant of what constitutes normal operation

(i.e., Is there a problem or is the system operating as per

design?)

Several important items of normal operation can have

a strong impact on performance In the following section,

we discuss the important operational parameters and

nor-mal observations

18.8.3.1 Primary Clarification: Normal

Operation

In primary clarification, wastewater enters a settling tank

or basin Velocity reduces to approximately 1 ft/min

Note: Notice that the velocity is based on minutes

instead of seconds, as was the case in the grit

channels A grit channel velocity of 1 ft/sec

would be 60 ft/min

Solids that are heavier than water settle to the bottom,

while solids that are lighter than water float to the top

Settled solids are removed as sludge and floating solids

are removed as scum Wastewater leaves the sedimentation

tank over an effluent weir and on to the next step in

treatment Detention time, temperature, tank design, and

condition of the equipment control the efficiency of the

process

18.8.3.2 Primary Clarification: Operational

Parameters (Normal Observations)

1 Flow distribution — Normal flow distribution

is indicated by flow to each in-service unit

being equal and uniform There is no indication

of short-circuiting The surface-loading rate is

within design specifications

2 Weir condition — Under this condition, weirs are

level, flow over the weir is uniform, and the weir

overflow rate is within design specifications

3 Scum removal — The surface is free of scum

accumulations, and the scum removal does not

operate continuously

4 Sludge removal — No large clumps of sludge

appear on the surface The system operates as

designed The pumping rate is controlled to

pre-vent coning or buildup, and the sludge blanketdepth is within desired levels

5 Performance — The unit is removing expectedlevels of BOD5, TSS, and settleable solids

6 Unit maintenance — Mechanical equipment ismaintained in accordance with planned sched-ules; equipment is available for service asrequired

To assist the operator in judging primary treatmentoperation, several process control tests can be used forprocess evaluation and control These tests include thefollowing:

1 pH (normal range: 6.5 to 9.0)

2 DO (normal range is <1.0 mg/L)

3 Temperature (varies with climate and season)

4 Settleable solids (influent is 5 to 15 mL/L; ent is 0.3 to 5 mL/L)

efflu-5 BOD (influent is 150 to 400 mg/L; effluent is

50 to 150 mg/L)

6 Percent solids (4 to 8%)

7 Percent volatile matter (40% to 70%)

8 Heavy metals (as required)

9 Jar tests (as required)

Note: Testing frequency should be determined on the

basis of the process influent and effluent ability and the available resources All should

vari-be performed periodically to provide referenceinformation for evaluation of performance

18.8.4 P ROCESS C ONTROL C ALCULATIONS

As with many other wastewater treatment plant unitprocesses, process control calculations aid in determiningthe performance of the sedimentation process Processcontrol calculations are used in the sedimentation process

to determine:

1 Percent removal

2 Hydraulic detention time

3 Surface loading rate (surface settling rate)

4 Weir overflow rate (weir loading rate)

5 Sludge pumping

6 Percent total solids (% TS)

In the following sections, we take a closer look at afew of these process control calculations and exampleproblems

Note: The calculations presented in the following

sec-tions allow you to determine values for eachfunction performed Keep in mind that an opti-mally operated primary clarifier should havevalues in an expected range

18.8.4.1 Percent Removal

The expected range of percent removal for a primary

clar-ifier is:

18.8.4.2 Detention Time

The primary purpose of primary settling is to remove

settleable solids This accomplished by slowing the flow

down to approximately 1 ft/min The flow at this velocity

will stay in the primary tank from 1.5 to 2.5 h The length

of time the water stays in the tank is called the hydraulic

detention time

18.8.4.3 Surface Loading Rate (Surface Settling

Rate and Surface Overflow Rate)

Surface loading rate is the number of gallons of

wastewa-ter passing over 1 ft2 of tank/d This can be used to

com-pare actual conditions with design Plant designs generally

use a surface loading rate of 300 to 1200 gal/d/ ft2

Other terms used synonymously with surface loading

rate are surface overflow rate and surface settling rate The

equation for calculating the surface loading rate is as

follows:

(18.10)

E XAMPLE 18.22

Problem:

The settling tank is 120 ft in diameter and the flow to the

unit is 4.5 MGD What is the surface loading rate in

gallons per day per square foot?

Solution:

18.8.4.4 Weir Overflow Rate (Weir Loading Rate)

Weir overflow rate (weir loading rate) is the amount ofwater leaving the settling tank per linear foot of weir Theresult of this calculation can be compared with design.Normally weir overflow rates of 10,000 to 20,000 gal/d/ftare used in the design of a settling tank:

Solution:

18.8.4.5 Sludge Pumping

Determination of sludge pumping (the quantity of solidsand volatile solids removed from the sedimentation tank)provides accurate information needed for process control

of the sedimentation process:

Surface Loading Rate gal d ft

Q gal d Settling Tank Area ft

gal MGD 0.785 120 ft 120 ft gal d ft

Weir Overflow Rate gal d ft

Q gal d Weir Length ft

The sludge pump operates 20 min/h The pump delivers

20 gal/min of sludge Laboratory tests indicate that the

sludge is 5.2% solids and 66% volatile matter How many

pounds of volatile matter are transferred from the settling

tank to the digester?

Solution:

Pump Time = 20 min/h

Pump Rate = 20 gal/min

A settling tank sludge sample is tested for solids The

sample and dish weigh 74.69 g The dish weighs 21.2 g.

After drying, the dish with dry solids now weighs 22.3 g.

What is the percent total solids (% TS) of the sample?

18.8.4.6 BOD and Suspended Solids Removal

To calculate the pounds of BOD or suspended solids (SS)

removed each day, you need to know the milligrams per

liter of BOD or suspended solids removed and the plant

flow Then you can use the milligrams per liter to poundsper day equation:

Solution:

Calculate the milligrams per liter of BOD removed:

Next calculate the pounds per day of BOD removed:

solu-in problem analysis is to solve the immediate problem.The long-term goal is to ensure that the problem does notpop up again, causing poor performance in the future

Solids Pumped lb d Pump Rate

Pump Time 8.34 lb gal Solids

Volume of Solids lb d Pump Rate

Pump Time 8.34 % Solids % VM

In this section, we cover a few indicators and

obser-vations of operational problems with the primary treatment

process The observations presented are not all-inclusive,

but highlight the most frequently confronted problems

1 Poor suspended solids removal (primary clarifier)

Causal factors:

A Hydraulic overload

B Sludge buildup in tanks and decreased

vol-ume and allows solids to scour out tanks

C Strong recycle flows

D Industrial waste concentrations

E Wind currents

F Temperature currents

2 Floating sludge

Causal factors:

A Sludge becoming septic in tank

B Damaged or worn collection equipment

C Recycled waste sludge

D Primary sludge pumps malfunctions

E Sludge withdrawal line plugged

F Return of well-nitrified waste-activated sludge

G Too few tanks in service

H Damaged or missing baffles

3 Primary sludge solids concentration too low

Causal factors:

A Hydraulic overload

B Overpumping of sludge

C Collection system problems

D Decreased influent solids loading

4 Septic wastewater or sludge

Causal factors:

A Damaged or worn collection equipment

B Infrequent sludge removal

C Insufficient industrial pretreatment

D Septic sewage from collection system

E Strong recycle flows

F Primary sludge pump malfunction

G Sludge withdrawal line plugged

H Sludge collectors not run often enough

I Septage dumpers

5 Primary sludge solids concentrations too high

Causal factors:

A Excessive grit and compacted material

B Primary sludge pump malfunction

C Sludge withdrawal line plugged

D SRT is too long

E Increased influent loadings

18.8.6 E FFLUENT FROM S ETTLING T ANKS

Upon completion of screening, degritting, and settling in

sedimentation basins, large debris, grit, and many

settle-able materials have been removed from the wastestream

What is left is referred to as primary effluent Usuallycloudy and frequently gray in color, primary effluent stillcontains large amounts of dissolved food and other chem-icals (nutrients) These nutrients are treated in the nextstep in the treatment process, secondary treatment, which

is discussed in the next section

Note: Two of the most important nutrients left to

remove are phosphorus and ammonia While

we want to remove these two nutrients from thewastestream, we do not want to remove toomuch Carbonaceous microorganisms in sec-ondary treatment (biological treatment) needboth phosphorus and ammonia

18.9 SECONDARY TREATMENT

The main purpose of secondary treatment (sometimesreferred to as biological treatment) is to provide BODremoval beyond what is achievable by primary treatment.There are three commonly used approaches, and all takeadvantage of the ability of microorganisms to convertorganic wastes (via biological treatment) into stabilized,low-energy compounds Two of these approaches, thetrickling filter (and its variation, the RBC) and the activatedsludge process, sequentially follow normal primary treat-ment The third, ponds (oxidation ponds or lagoons), canprovide equivalent results without preliminary treatment

In this section, we present a brief overview of thesecondary treatment process followed by a detailed dis-cussion of wastewater treatment ponds (used primarily insmaller treatment plants), trickling filters, and RBCs Wethen shift focus to the activated sludge process, the sec-ondary treatment process, which is used primarily in largeinstallations and is the main focus of the handbook.Secondary treatment refers to those treatment pro-cesses that use biological processes to convert dissolved,suspended, and colloidal organic wastes to more stablesolids that can either be removed by settling or discharged

to the environment without causing harm

Exactly what is secondary treatment? As defined bythe Clean Water Act (CWA), secondary treatment pro-duces an effluent with nor more than 30 mg/L BOD and

30 mg/L TSS

Note: The CWA also states that ponds and trickling

filters will be included in the definition of ondary treatment even if they do not meet theeffluent quality requirements continuously.Most secondary treatment processes decompose solidsaerobically, producing carbon dioxide, stable solids, andmore organisms Since solids are produced, all of thebiological processes must include some form of solidsremoval (settling tank, filter, etc.)

sec-Wastewater Treatment 551

Secondary treatment processes can be separated into

two large categories: fixed film systems and suspended

growth systems

Fixed film systems are processes that use a biological

growth (biomass or slime) that is attached to some form

of media Wastewater passes over or around the media and

the slime When the wastewater and slime are in contact,

the organisms remove and oxidize the organic solids The

media may be stone, redwood, synthetic materials, or any

other substance that is durable (capable of withstanding

weather conditions for many years), provides a large area

for slime growth and an open space for ventilation, and

is not toxic to the organisms in the biomass Fixed film

devices include trickling filters and RBCs

Suspended growth systems are processes that use a

biological growth that is mixed with the wastewater

Typ-ical suspended growth systems consist of various

modifi-cations of the activated sludge process

18.9.1 T REATMENT P ONDS

Wastewater treatment can be accomplished using ponds

Ponds are relatively easy to build and manage, can

accom-modate large fluctuations in flow, and can also provide

treatment that approaches conventional systems

(produc-ing a highly purified effluent) at much lower cost It is the

cost (the economics) that drives many managers to decide

on the pond option The actual degree of treatment

pro-vided depends on the type and number of ponds used

Ponds can be used as the sole type of treatment or they

can be used in conjunction with other forms of wastewater

treatment (i.e., other treatment processes followed by a

pond or a pond followed by other treatment processes)

18.9.1.1 Types of Ponds

Ponds can be classified (named) based upon their location

in the system, the type wastes they receive, and the mainbiological process occurring in the pond First we look atthe types of ponds according to their location and the typewastes they receive: raw sewage stabilization ponds (seeFigure 18.3), oxidation ponds, and polishing ponds In thefollowing section, we look at ponds classified by the type

of processes occurring within the pond: Aerobic Ponds,anaerobic ponds, facultative ponds, and aerated ponds

18.9.1.1.1 Ponds Based on Location and Types

of Wastes They Receive

The types of ponds based on location and types of wastesthey receive include raw sewage stabilization ponds, oxi-dation ponds, and polishing ponds

18.9.1.1.1.1 Raw Sewage Stabilization Ponds

The raw sewage stabilization pond is the most commontype of pond (see Figure 18.3) With the exception ofscreening and shredding, this type of pond receives noprior treatment Generally, raw sewage stabilization pondsare designed to provide a minimum of 45 d detention timeand to receive no more than 30 lb of BOD /d/acre Thequality of the discharge is dependent on the time of theyear Summer months produce high BOD removal, butexcellent suspended solids removals

The pond consists of an influent structure, pond berm,

or walls and an effluent structure designed to permit tion of the best quality effluent Normal operating depth

selec-of the pond is 3 to 5 ft

The process occurring in the pond involves bacteriadecomposing the organics in the wastewater (aerobicallyand anaerobically) and algae using the products of the

FIGURE 18.3 Stabilization pond processes (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol 1,

Technomic Publ., Lancaster, PA, 1999.)

Anaerobic digestion (settled solids)

Solids IN

Photosynthesis (Algae-producing oxygen)

Aerobic decomposition (bacteria producing CO2)

552 Handbook of Water and Wastewater Treatment Plant Operations

bacterial action to produce oxygen (photosynthesis)

Because this type of pond is the most commonly used in

wastewater treatment, the process that occurs within the

pond is described in greater detail below

When wastewater enters the stabilization pond severalprocesses begin to occur These include settling, aerobic

decomposition, anaerobic decomposition, and

photosyn-thesis (see Figure 18.3) Solids in the wastewater will settle

to the bottom of the pond In addition to the solids in the

wastewater entering the pond, solids, which are produced

by the biological activity, will also settle to the bottom

Eventually this will reduce the detention time and the

performance of the pond When this occurs (usually 20 to

30 years) the pond will have to be replaced or cleaned

Bacteria and other microorganisms use the organicmatter as a food source They use oxygen (aerobic decom-

position), organic matter, and nutrients to produce carbon

dioxide, water, stable solids (which may settle out), and

more organisms The carbon dioxide is an essential

com-ponent of the photosynthesis process occurring near the

surface of the pond

Organisms also use the solids that settled out as foodmaterial Because the oxygen levels at the bottom of the

pond are extremely low the process used is anaerobic

decomposition The organisms use the organic matter to

produce gases (hydrogen sulfide, methane, etc.), which are

dissolved in the water; stable solids; and more organisms

Near the surface of the pond a population of greenalgae will develop that can use the carbon dioxide pro-

duced by the bacterial population, nutrients, and sunlight

to produce more algae and oxygen, which is dissolved

into the water The DO is then used by organisms in the

aerobic decomposition process

When compared with other wastewater treatment tems involving biological treatment, a stabilization pond

sys-treatment system is the simplest to operate and maintain

Operation and maintenance activities include collecting

and testing samples for DO and pH, removing weeds and

other debris (scum) from the pond, mowing the berms,

repairing erosion, and removing burrowing animals

Note: DO and pH levels in the pond will vary

through-out the day Normal operation will result in veryhigh DO and pH levels because of the naturalprocesses occurring

Note: When operating properly the stabilization pond

will exhibit a wide variation in both DO and

pH This is due to the photosynthesis occurring

in the system

18.9.1.1.1.2 Oxidation Ponds

An oxidation pond, which is normally designed using the

same criteria as the stabilization pond, receives flows that

have passed through a stabilization pond or primary

set-tling tank This type of pond provides biological treatment,

additional settling, and some reduction in the number offecal coliform present

10 ft Excessive detention time or too shallow a depth willresult in algae growth, which increases influent, suspendedsolids concentrations

18.9.1.1.2 Ponds Based on the Type of Processes

Occurring within the Ponds

The type of processes occurring within the pond may alsoclassify ponds These include the aerobic, anaerobic, fac-ultative, and aerated processes

18.9.1.1.2.1 Aerobic Ponds

In aerobic ponds, which are not widely used, oxygen ispresent throughout the pond All biological activity isaerobic decomposition

18.9.1.1.2.2 Anaerobic Ponds

Anaerobic ponds are normally used to treat high strengthindustrial wastes No oxygen is present in the pond andall biological activity is anaerobic decomposition

18.9.1.1.2.3 Facultative Ponds

The facultative pond is the most common type pond (based

on processes occurring) Oxygen is present in the upperportions of the pond and aerobic processes are occurring

No oxygen is present in the lower levels of the pond whereanoxic and anaerobic processes are occurring

18.9.1.1.2.4 Aerated Ponds

In the aerated pond, oxygen is provided through the use

of mechanical or diffused air systems When aeration isused, the depth of the pond and the acceptable loadinglevels may increase Mechanical or diffused aeration isoften used to supplement natural oxygen production or toreplace it

18.9.1.2 Process Control Calculations

(Stabilization Ponds)

Process control calculations are an important part of water treatment operations, including pond operations Moresignificantly, process control calculations are an importantpart of state wastewater licensing examinations — you sim-ply cannot master the licensing examinations withoutbeing able to perform the required calculations Wheneverpossible, example process control problems are provided

waste-to enhance your knowledge and skills

Note: Acre-feet (ac-ft) is a unit that can cause

confu-sion, especially for those not familiar with pond

or lagoon operations The measurement of

1 ac-ft is the volume of a box with a 1-acre top

and 1 ft of depth — but the top does not have

to be an even number of acres in size to use

Note: Hydraulic detention time normally ranges from

30 to 120 d for stabilization ponds

E XAMPLE 18.29

Problem:

A stabilization pond has a volume of 53.5 ac-ft What is

the detention time in days when the flow is 0.30 MGD?

Solution:

Determine the flow rate in acre-feet per day:

Determine the detention time:

18.9.1.2.6 Hydraulic Loading in Inches per Day

(Overflow Rate)

(18.20)

(18.21)

Note: Population loading normally ranges from 50 to

500 people per acre

18.9.1.2.7 Organic Loading

Organic loading can be expressed as pounds of BOD peracre per day (most common), pounds BOD5 per acre-footper day, or people per acre per day

A wastewater treatment pond has an average width of

380 ft and an average length of 725 ft The influent flow rate to the pond is 0.12 MGD with a BOD concentration

of 160 mg/L What is the organic loading rate to the pond

in pounds per day per acre?

DT d Pond Volume

Influent Flow

DT

ft d d

d 53.5 acre0.92 ac-

= 58 2

Hydraulic Loading in dInfluent Flow acre- Pond Area acres

acre

=

.

18.9.2 T RICKLING F ILTERS

Trickling filters have been used to treat wastewater since

the 1890s It was found that if settled wastewater was

passed over rock surfaces, slime grew on the rocks and the

water became cleaner Today we still use this principle, but

in many installations we use plastic media instead of rocks

In most wastewater treatment systems, the trickling filter

follows primary treatment and includes a secondary settling

tank or clarifier as shown in Figure 18.4 Trickling filters are

widely used for the treatment of domestic and industrial

wastes The process is a fixed film biological treatment

method designed to remove BOD and suspended solids

A trickling filter consists of a rotating distribution arm

that sprays and evenly distributes liquid wastewater over

a circular bed of fist-sized rocks, other coarse materials,

or synthetic media (see Figure 18.5) The spaces between

the media allow air to circulate easily so that aerobic

conditions can be maintained The spaces also allow

wastewater to trickle down through, around, and over the

media A layer of biological slime that absorbs and

con-sumes the wastes trickling through the bed covers themedia material The organisms aerobically decompose thesolids and produce more organisms and stable wastes thateither become part of the slime or are discharged backinto the wastewater flowing over the media This slimeconsists mainly of bacteria, but it may also include algae,protozoa, worms, snails, fungi, and insect larvae Theaccumulating slime occasionally sloughs off (sloughings)individual media materials (see Figure 18.6) and is col-lected at the bottom of the filter, along with the treatedwastewater, and passed on to the secondary settling tankwhere it is removed

The overall performance of the trickling filter isdependent on hydraulic and organic loading, temperature,and recirculation

18.9.2.1 Trickling Filter Definitions

To clearly understand the correct operation of the tricklingfilter, the operator must be familiar with certain terms Thefollowing list of terms applies to the trickling filter process

FIGURE 18.4 Simplified flow diagram of trickling filter used for wastewater treatment (From Spellman, F.R., Spellman’s Standard

Handbook for Wastewater Operators, Vol 1, Technomic Publ., Lancaster, PA, 1999.)

FIGURE 18.5 Schematic of cross-section of a trickling filter (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater

Operators, Vol 1, Technomic Publ., Lancaster, PA, 1999.)

Influent Bar racks Grit

chamber

Primary sedimentaion Tricklingfilter Settlingtank

Chlorine contact tank

Rock Bed

Underdrain system

Rotating arm

Influent spray

Influent

Effluent Rock bed

Wastewater Treatment 555

We assume that other terms related to other units within the

treatment system (plant) are already familiar to operators:

Biological towers a type of trickling filter that is very

deep (10 to 20 ft) Filled with a lightweight

synthetic media, these towers are also know as

oxidation or roughing towers or (because of

their extremely high hydraulic loading)

super-rate trickling filters

Biomass the total mass of organisms attached to the

media Similar to solids inventory in the

acti-vated sludge process, it is sometimes referred

to as the zoogleal slime

Distribution arm the device most widely used to

apply wastewater evenly over the entire surface

of the media In most cases, the force of thewastewater being sprayed through the orificesmoves the arm

Filter underdrain the open space provided under the

media to collect the liquid (wastewater andsloughings) and to allow air to enter the filter

It has a sloped floor to collect the flow to acentral channel for removal

Hydraulic loading the amount of wastewater flow

applied to the surface of the trickling filter media

It can be expressed in several ways: flow persquare foot of surface per day, flow per acre perday, or flow per acre-foot per day The hydraulicloading includes all flow entering the filter

High-rate trickling filters a classification (see Table

18.4) in which the organic loading is in therange of 25 to 100 lb BOD /1000 ft3 of media/d.The standard rate filter may also produce ahighly nitrified effluent

Media an inert substance placed in the filter to provide

a surface for the microorganism to grow on.The media can be field stone, crushed stone,slag, plastic, or redwood slats

Organic loading the amount of BOD or COD applied

to a given volume of filter media It does notinclude the BOD or COD contributed to anyrecirculated flow and is commonly expressed aspounds of BOD or COD per 1000 ft3 of media

Recirculation the return of filter effluent back to the

head of the trickling filter It can level flow

FIGURE 18.6 Filter media showing biological activities that

take place on surface area (From Spellman, F.R., Spellman’s

Standard Handbook for Wastewater Operators, Vol 1,

Tech-nomic Publ., Lancaster, PA, 1999.)

Trickling Filter Classification

Hydraulic Loading (gal/d/ft 2 ) 25–90 90–230 230–900 350–2100 >900

Organic Loading BOD/1000 ft 3 5–25 15–30 25–300 Up to 300 >300

Sloughing frequency Seasonal Varies Continuous Continuous Continuous

Distribution Rotary Rotary fixed Rotary fixed Rotary Rotary Fixed

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol 1, Technomic Publ., Lancaster,

PA, 1999.

variations and assist in solving operational

prob-lems such as ponding, filter flies, and odors

Roughing filters a classification of trickling filters

(see Table 18.4) in which the organic is in

excess of 200 lb BOD /1000 ft3 of media/d A

roughing filter is used to reduce the loading on

other biological treatment processes to produce

an industrial discharge that can be safely treated

in a municipal treatment facility

Sloughing the process in which the excess growths

break away from the media and wash through

the filter to the underdrains with the wastewater

These sloughings must be removed from the

flow by settling

Staging the practice of operating two or more trickling

filters in series The effluent of one filter is used

as the influent of the next This practice can

produce a higher quality effluent by removing

additional BOD or COD

18.9.2.2 Trickling Filter Equipment

The trickling filter distribution system is designed to

spread wastewater evenly over the surface of the entire

media The most common system is the rotary distributor,

which moves above the surface of the media and sprays

the wastewater on the surface The force of the water

leaving the orifices drives the rotary system The

distrib-utor arms usually have small plates below each orifice to

spread the wastewater into a fan-shaped distribution

sys-tem The second type of distributor is the fixed nozzle

system In this system, the nozzles are fixed in place above

the media and are designed to spray the wastewater over

a fixed portion of the media This system is used frequently

with deep bed synthetic media filters

Note: Trickling filters that use ordinary rock are

normally only about 3 m in depth because of

structural problems caused by the weight of

rocks, which also requires the construction of

beds that are quite wide (in many applications,

up to 60 ft in diameter) When synthetic media

is used, the bed can be much deeper

No matter which type of media is selected, the primary

consideration is that it must be capable of providing the

desired film location for the development of the biomass

Depending on the type of media used and the filter

clas-sification, the media may be 3 to 20 or more ft in depth

The underdrains are designed to support the media,

collect the wastewater and sloughings and carry them out

of the filter, and provide ventilation to the filter

Note: In order to ensure sufficient airflow to the filter,

the underdrains should never be allowed to flow

more than 50% full of wastewater

The effluent channel is designed to carry the flow fromthe trickling filter to the secondary settling tank

The secondary settling tank provides 2 to 4 h of tion time to separate the sloughing materials from thetreated wastewater Design, construction, and operationare similar to the primary settling tank’s Longer detentiontimes are provided because the sloughing materials arelighter and settle more slowly

deten-Recirculation pumps and piping are designed to culate (and thus improve the performance of the tricklingfilter or settling tank) a portion of the effluent back to bemixed with the filter influent When recirculation is used,pumps and metering devices must be provided

90 gal/d/ft3 and a seasonal sloughing frequency It doesnot employ recirculation and typically has a 80–85% BODremoval rate and 80 to 85% TSS removal rate

The high rate filter has a hydraulic loading of 230 to

900 gal/d/ft3 and a continuous sloughing frequency Italways employs recirculation and typically has a 65 to80% BOD removal rate and 65 to 80% TSS removal rate.The roughing filter has a hydraulic loading of

>900 gal/d/ft3 and a continuous sloughing frequency Itdoes not normally include recirculation and typically has

a 40 to 65% BODremoval rate and 40 to 65% TSS removalrate

18.9.2.4 Standard Operating Procedures

Standard operating procedures for trickling filters includesampling and testing, observation, recirculation, mainte-nance, and expectations of performance

Collection of influent and process effluent samples todetermine performance and monitor process condition oftrickling filters is required DO, pH, and settleable solidstesting should be collected daily BOD and suspendedsolids testing should be done as often as practical to deter-mine the per cent removal

The operation and condition of the filter should beobserved daily Items to observe include the distributormovement, uniformity of distribution, evidence of operation

or mechanical problems, and the presence of objectionableodors In addition to the items above the normal observa-tion for a settling tank should also be performed

Wastewater Treatment 557

Recirculation is used to reduce organic loading,

improve sloughing, reduce odors, and reduce or eliminate

filter fly or ponding problems The amount of recirculation

is dependent on the design of the treatment plant and the

operational requirements of the process Recirculation flow

may be expressed as a specific flow rate (i.e., 2.0 MGD)

In most cases, it is expressed as a ratio (e.g., 3:1, 0.5:1.0,

etc) The recirculation is always listed as the first number

and the influent flow listed as the second number

Note: Since the second number in the ratio is always

1.0, the ratio is sometimes written as a single

number (dropping the 1.0)

Flows can be recirculated from various points

follow-ing the filter to various points before the filter The most

common form of recirculation removes flow from the filter

effluent or settling tank and returns it to the influent of the

trickling filter as shown in Figure 18.7

Maintenance requirements include lubrication of

mechanical equipment, removal of debris from the surface

and orifices, as well as adjustment of flow patterns and

maintenance associated with the settling tank

Expected performance ranges for each classification

of trickling filter The levels of BOD and suspended solids

removal are also dependent on the type of filter

18.9.2.5 General Process Description

The trickling filter process involves spraying wastewater

over a solid media such as rock, plastic, or redwood slats

(or laths) As the wastewater trickles over the surface of

the media, a growth of microorganisms (bacteria, protozoa,

fungi, algae, helminthes or worms, and larvae) develops

This growth is visible as a shiny slime very similar to the

slime found on rocks in a stream As the wastewater passes

over this slime, the slime adsorbs the organic (food) matter

This organic matter is used for food by the

microorgan-isms At the same time, air moving through the open

spaces in the filter transfers oxygen to the wastewater This

oxygen is then transferred to the slime to keep the outer

layer aerobic As the microorganisms use the food and

oxygen, they produce more organisms, carbon dioxide,

sul-fates, nitrates, and other stable by-products; these materials

are then discarded from the slime back into the wastewater

flow and are carried out of the filter The process is shown

in the following equation:

(18.23)

The growth of the microorganisms and the buildup ofsolid wastes in the slime make it thicker and heavier Whenthis slime becomes too thick, the wastewater flow breaksoff parts of the slime These must be removed in the finalsettling tank

In some trickling filters, a portion of the filter effluent

is returned to the head of the trickling filter to level outvariations in flow and improves operations (recirculation)

18.9.2.5.1 Overview and Brief Summary

of Trickling Filter Process

The following list provides an overview of the tricklingfilter process:

1 A trickling filter consists of a bed of coarsemedia, usually rocks or plastic, covered withmicroorganisms

2 The wastewater is applied to the media at acontrolled rate, using a rotating distributor arm

or fixed nozzles Organic material is removed

by contact with the microorganisms as thewastewater trickles down through the mediaopenings The treated wastewater is collected

by an underdrain system

3 The trickling filter is usually built into a tankthat contains the media The filter may besquare, rectangular, or circular

4 The trickling filter does not provide any actualfiltration The filter media provides a largeamount of surface area that the microorganismscan cling to and grow in a slime that forms onthe media as they feed on the organic material

in the wastewater

5 The slime growth on the trickling filter mediaperiodically sloughs off and is settled andremoved in a secondary clarifier that followsthe filter

6 Key factors in trickling filter operation includethe following concepts:

FIGURE 18.7 Common form of recirculation (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators,

Vol 1, Technomic Publ., Lancaster, PA, 1999.)

Primary settling

Trickling filter

Secondary settling

Recirculating

Organics OrganismsMore Organisms CO Solid Wastes

O22

A Hydraulic loading rate

B Organic loading rate

C Recirculation

18.9.2.6 Operator Observations, Process

Problems, and Troubleshooting

Trickling filter operation requires routine observation,

meter readings, process control sampling and testing, and

process control calculations Comparison of daily results

with expected normal ranges is the key to identifying

problems and appropriate corrective actions

18.9.2.6.1 Operator Observations

1 Slime — The operator checks the thickness of

slime to ensure that it is thin and uniform

(normal) or thick and heavy (indicates organic

overload) The operator is concerned with

ensuring that excessive recirculation is not

tak-ing place and checks slime toxicity (if any) The

operator is also concerned about the color of

the slime Green slime is normal, dark green or

black slime indicates organic overload Other

colors may indicate industrial waste or

chemi-cal additive contamination The operator should

check the subsurface growth of the slime to

ensure that it is normal (thin and translucent)

If growth is thick and dark, organic overload

conditions are indicated Distribution arm

oper-ation is a system function important to slime

formation It must be checked regularly for

proper operation For example, the distribution

of slime should be even and uniform Striped

conditions indicate clogged orifices or nozzles

2 Flow — Flow distribution must be checked toensure uniformity If nonuniform, the arms arenot level or the orifices are plugged Flow drain-age is also important Drainage should beuniform and rapid If not, ponding may occurfrom media breakdown or debris on surface

3 Distributor — Movement of the distributor iscritical to proper operation of the trickling filter.Movement should be uniform and smooth.Chattering, noisy operation may indicate bear-ing failure The distributor seal must be checked

to ensure there is no leakage

4 Recirculation — The operator must check therate of recirculation to ensure that it is withindesign specifications Rates above design spec-ifications indicate hydraulic overloading, whilerates under design specifications indicatehydraulic underloading

5 Media — The operator should check to ensurethat media are uniform

18.9.2.6.2 Process Control Sampling and Testing

To ensure proper operation of the trickling filter, samplingand scheduling are important For samples and the testsderived from the samples to be beneficial, operators mustperform a variety of daily or variable tests Individual testsand sampling may be needed daily, weekly, or monthly,depending on seasonal change Frequency may be lowerduring normal operations and higher during abnormalconditions

The information gathered through collection and ysis of samples from various points in the trickling filterprocess is helpful in determining the current status of theprocess as well as identifying and correcting operationalproblems

anal-The following routine sampling points and types oftests will permit the operator to identify normal and abnor-mal operating conditions

1 Filter influent — Tests include DO, pH, perature, settleable solids, BOD, suspended sol-ids, and metals

tem-2 Recirculated flow — Tests include DO, pH,flow rate, and temperature

3 Filter effluent — Tests include DO, pH, and jartests

4 Process effluent — Tests include DO, pH,settleable solids, BOD, and suspended solids

18.9.2.6.3 Troubleshooting Operational Problems

(Note: Much of the information in this section is based

on the Environmental Protection Agency’s (EPA)

Perfor-mance Evaluation and Troubleshooting at Municipal

Hydraulic Loading Rate gal d ft

Q gal d including recirculation

Media Top Surface ft

( )=

( ) ( )

the trickling filter process They do provide information

on the most common operational problems

E Medium is uniform, but is too small

F Debris (moss, leaves, sticks) or living isms (snails) clog the void spaces

organ-3 Corrective actionsCorrective actions are listed in increasingimpact on the quality of the plant effluent:

A Remove all leaves, sticks, and other debrisfrom the media

B Increase recirculation of dilute, strength wastes to improve sloughing tokeep voids open

high-C Use high-pressure stream of water to agitateand flush the ponded area

D Rake or fork the ponded area

E Dose the filter with chlorine solution for 2 to

4 h The specific dose of chlorine requiredwill depend on the severity of the pondingproblem When using elemental chlorine,the dose must be sufficient to provide aresidual at the orifices of 1–50 mg/L If thefilter is severely clogged, the higher residu-als may be needed to unload the majority ofthe biomass If the filter cannot be dosed byelemental chlorine, chlorinated lime or hightest hypochlorite powder may be used Dos-ing should be in the range of 8 to 10 lb ofchlorine/1000 ft2 of media

F If the filter design permits, the filter mediacan be flooded for a period of 4 h Remem-ber, if the filter is flooded, care must be taken

to prevent hydraulic overloads of the finalsettling tank The trickling filter should bedrained slowly at low flow periods

G Dry the media By stopping the flow to thefilter, the slime will dry and loosen Whenthe flow is restarted, the loosened slime willflow out of the filter The amount of dryingtime will be dependent on the thickness ofthe slime and the amount of removaldesired Time may range from a few hours

to several days

Note: Portions of the media can be dried without ing the filter out of service by plugging theorifices that normally service the area

tak-Note: If these corrective actions do not provide thedesired improvement, the media must be care-fully inspected Remove a sample of the mediafrom the affected area Carefully clean it,inspect for its solidity, and determine its sizeuniformity (3 to 5 in.) If it is acceptable, themedia must be carefully replaced If the mediaappear to be decomposing or are not uniform,then they should be replaced

18.9.2.6.3.2 Odors

Frequent offensive odors usually indicate an operationalproblem These foul odors occur within the filter periodi-cally and are normally associated with anaerobic condi-tions Under normal circumstances, a slight anaerobicslime layer forms due to the inability of oxygen to penetrateall the way to the media Under normal operation, the outerslime layers will remain aerobic, and no offensive odorsare produced

1 Causal factors

A Excessive organic loading due to poor filtereffluent quality (recirculation), poor primarytreatment operation, and poor control ofsludge treatment process that results in highBOD recycle flows

B Poor ventilation because of submerged orobstructed underdrains, clogged vent pipes,

or clogged void spaces

C Filter is overloaded hydraulically or ically

organ-D Poor housekeeping

2 Corrective actions

A Evaluate the operation of the primary ment process Eliminate any short-circuiting.Determine any other actions that can betaken to improve the performance of theprimary process

B Evaluate and adjust control of sludge

treat-ment processes to reduce the BOD or recycle

flows

C Increase recirculation rate to add additional

DO to filter influent Do not increase

recir-culation rate if the flow rate through the

underdrains would cause less than 50%

open space

D Maintain aerobic conditions in filter influent

E Remove debris from media surface

F Flush underdrains and vent pipes

G Add one of the commercially available

masking agents to reduce odors and prevent

complaints

H Add chlorine at a 1 to 2 mg/L residual for

several hours at low flow This will reduce

activity and cut down on the oxygen

demand Chlorination only treats symptoms;

a permanent solution must be determined

and instituted

18.9.2.6.3.3 High Clarifier Effluent Suspended Solids

and BOD

1 Symptom

A The effluent from the trickling filter

process-settling unit contains a high concentration

of suspended solids

2 Causal factors

A Recirculated flows are too high, causing

hydraulic overloading of the settling tank

In multiple unit operations, the flow is not

D Effluent weirs are not level

E Short-circuiting occurs because of

tempera-ture variations

F Improper sludge withdrawal rate or frequency

G Excessive solids loading from excessive

sloughing

3 Corrective actions

A Check hydraulic loading and adjust

recircu-lated flow if hydraulic loading is too high

B Adjust flow to ensure equal distribution

C Inspect sludge removal equipment Repair

broken equipment

D Monitor sludge blanket depth and sludge

solids concentration; adjust withdrawal rate

and/or frequency to maintain aerobic

condi-tions in settling tank

E Adjust effluent weir to obtain equal flow

over all parts of the weir length

F Determine temperature in the clarifier atvarious points and depths throughout theclarifier If depth temperatures are consis-tently 1 to 2°F lower than surface readings,

a temperature problem exists Baffles may beinstalled to help to break up these currents

G High sloughing rates because of the ical activity or temperature changes maycreate excessive solids loading An addition

biolog-of 1 to 2 mg/L biolog-of cationic polymer may behelpful in improving solids capture Remem-ber, if polymer addition is used, solids with-drawal must be increased

H High sloughings because of organic loading, toxic wastes, or wide variations ininfluent flow are best controlled at theirsource

over-18.9.2.6.3.4 Filter Flies

1 Symptoms

A The trickling filter and surrounding areabecome populated with large numbers ofvery small flying insects (psychoda moths)

A Increase recirculation rate to obtain ahydraulic loading of at least 200 gal/d/ft2

At this rate, filter fly larvae are normallyflushed out of the filter

B Clean filter walls and remove weeds, brush,and shrubbery around the filter Thisremoves some of the area for fly breeding

C Dose the filter periodically with low rine concentrations (less than 1 mg/L) Thisnormally destroys larvae

chlo-D Dry the filter media for several hours

F Flood the filter for 24 h

G Spray area around the filter with insecticide

Do not use insecticide directly on the media,because of the chance of carryover andunknown effects on the slime populations

Wastewater Treatment 561

2 Causal factors

A Recirculation causes increased temperaturedrops and losses

B Strong prevailing winds cause heat losses

C Intermittent dosing allows water to stand toolong, causing freezing

3 Corrective actionsAll corrective actions are based upon a need toreduce heat loss as the wastes move through thefilter

A Reduce recirculation as much as possible tominimize cooling effects

B Operate two stage filters in parallel to reduceheat loss

C Adjust splash plates and orifices to obtain acoarse spray

D Construct a windbreak or plant evergreens

or shrubs in the direction of the prevailingwind

E If intermittent dosing is used, leave dumpgates open

F Cover pump wet wells and dose tanks toreduce heat losses

G Cover filter media to reduce heat loss

H Remove ice before it becomes large enough

to cause stoppage of arms

Note: During periods of cold weather, the filter will

show decreased performance However, the ter should not be shut off for extended periods

fil-Freezing of the moisture trapped within themedia causes expansion and may cause struc-tural damage

18.9.2.7 Process Calculations

Several calculations are useful in the operation of a

trick-ling filter, these include: total flow, hydraulic loading, and

organic loading

18.9.2.7.1 Total Flow

If the recirculated flow rate is given, total flow is:

Note: The total flow to the tricking filter includes the

influent flow and the recirculated flow This can

be determined using the recirculation ratio:

E XAMPLE 18.31

Problem:

The trickling filter is currently operating with a lation rate of 1.5 What is the total flow applied to the filter when the influent flow rate is 3.65 MGD?

recircu-Solution:

18.9.2.7.2 Hydraulic Loading

Calculating the hydraulic loading rate is important inaccounting for both the primary effluent as well as therecirculated trickling filter effluent Both of these are com-bined before being applied to the surface of the filter Thehydraulic loading rate is calculated based on the surfacearea of the filter

E XAMPLE 18.32

Problem:

A trickling filter 90-ft in diameter is operated with a primary effluent of 0.488 MGD and a recirculated effluent flow rate of 0.566 MGD Calculate the hydraulic loading rate on the filter in units gallons per day per square foot.

Solution:

The primary effluent and recirculated trickling filter ent are applied together across the surface of the filter, therefore:

efflu-18.9.2.7.3 Organic Loading Rate

As mentioned earlier, trickling filters are sometimes sified by the organic loading rate applied The organic

clas-Total Flow MGD Influent Flow MGD

Recirculation Flow MGDTotal Flow gal d Total Flow MGD

ft ft gal d

562 Handbook of Water and Wastewater Treatment Plant Operations

loading rate is expressed as a certain amount of BOD

applied to a certain volume of media

E XAMPLE 18.33

Problem:

A trickling filter, 50 ft in diameter, receives a primary

effluent flow rate of 0.445 MGD Calculate the organic

loading rate in units of pounds of BOD applied per day

per 900 ft 3 of media volume The primary effluent BOD

concentration is 85 mg/L The media depth is 9 ft.

Solution:

To determine the pounds of BOD/1000 ft 3 in a volume of

thousands of cubic feet, we must set up the equation as

shown below:

Regrouping the numbers and the units together:

18.9.2.7.4 Settling Tanks

In the operation of settling tanks that follow trickling

filters, various calculations are routinely made to determine

detention time, surface settling rate, hydraulic loading and

sludge pumping

18.9.3 R OTATING B IOLOGICAL C ONTACTORS

The RBC is a biological treatment system (see Figure 18.8)

and is a variation of the attached growth idea provided by

the trickling filter Still relying on microorganisms that

grow on the surface of a medium, the RBC is a fixed film

biological treatment device; the basic biological process

is similar to that occurring in the trickling filter An RBCconsists of a series of closely spaced (mounted side byside), circular, plastic (synthetic) disks that are typicallyabout 3.5 m in diameter and attached to a rotating hori-zontal shaft (see Figure 18.8) Approximately 40% of eachdisk is submersed in a tank containing the wastewater to

be treated As the RBC rotates, the attached biomass film(zoogleal slime) that grows on the surface of the diskmoves into and out of the wastewater While submerged

in the wastewater, the microorganisms absorb organics;while they are rotated out of the wastewater, they aresupplied with needed oxygen for aerobic decomposition

As the zoogleal slime reenters the wastewater, excess ids and waste products are stripped off the media assloughings These sloughings are transported with thewastewater flow to a settling tank for removal

sol-Modular RBC units are placed in series (seeFigure 18.9) simply because a single contactor is not suffi-cient to achieve the desired level of treatment; the resultingtreatment achieved exceeds conventional secondary treat-ment Each individual contactor is called a stage and thegroup is known as a train Most RBC systems consist oftwo or more trains with three or more stages in each Thekey advantage in using RBCs instead of trickling filters

is that RBCs are easier to operate under varying loadconditions, since it is easier to keep the solid medium wet

at all times The level of nitrification, which can beachieved by a RBC system, is also significant This isespecially the case when multiple stages are employed

mg L

8 lb gal 3 BOD applied d

Surface Area 0.785 Diameter

ft ft

315.5 BOD d

17 662 5

1000 1000

lb

FIGURE 18.8 Cross-section of a rotating biological contactor (RBC) treatment system (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol 1, Tech- nomic Publ., Lancaster, PA, 1999.)

Organic matter Sloughings

Wastewater holding tank

Oxygen

Media

Zoogleal slime

Wastewater Treatment 563

density), a center shaft, drive system, tank, baffles,

hous-ing or cover, and a settlhous-ing tank

The rotating biological contactor consists of circularsheets of synthetic material (usually plastic) that are

mounted side by side on a shaft The sheets (media) contain

large amounts of surface area for growth of the biomass

The center shaft provides the support for the disks ofmedia and must be strong enough to support the weight

of the media and the biomass Experience has indicated a

major problem has been the collapse of the support shaft

The drive system provides the motive force to rotatethe disks and shaft The drive system may be mechanical,

air driven, or a combination of each When the drive

system does not provide uniform movement of the RBC,

major operational problems can arise

The tank holds the wastewater where the RBC rotates

It should be large enough to permit variation of the liquid

depth and detention time

Baffles are required to permit proper adjustment ofthe loading applied to each stage of the RBC process

Adjustment can be made to increase or decrease the

sub-mergence of the RBC

RBC stages are normally enclosed in some type ofprotective structure (cover) to prevent loss of biomass due

to severe weather changes (snow, rain, temperature, wind,

sunlight, etc.) In many instances this housing greatly

restricts access to the RBC

The settling tank is provided to remove the sloughingmaterial created by the biological activity and is similar

in design to the primary settling tank The settling tank

provides 2- to 4-h detention times to permit settling of

lighter biological solids

18.9.3.2 RBC Operation

During normal operation, operator vigilance is required

to observe the RBC movement, slime color, and

appear-ance If the unit is covered, observations may be limited

to that portion of the media, which can be viewed through

the access door

Sampling and testing should be conducted daily for DOcontent and pH BOD and suspended solids testing should

also be accomplished to aid in assessing performance

18.9.3.3 RBC: Expected Performance

The RBC normally produces a high quality effluent withBOD at 85 to 95% and suspended solids removal at 85 to95% The RBC treatment process may also significantlyreduce (if designed for this purpose) the levels of organicnitrogen and ammonia nitrogen

18.9.3.4 Operator Observations, Process

Problems, and Troubleshooting

Rotating biological filter operation requires routine tion, process control sampling and testing, troubleshooting,and process control calculations Comparison of dailyresults with expected normal ranges is the key to identi-fying problems and appropriate corrective actions

observa-18.9.3.4.1 Operator Observations

Note: If the RBC is covered, observations may belimited to the portion of the media that can beviewed through the access door

1 Rotation — The operator routinely checks theoperation of the RBC to ensure that smooth,uniform rotation is occurring (normal opera-tion) Erratic, nonuniform rotation indicates amechanical problem or uneven slime growth If

no movement is observed, mechanical lems or extreme excess of slime growth areindicated

prob-2 Slime color and appearance — Slime color andappearance can indicate process condition.Gray, shaggy slime growth on the RBC indi-cates normal operation Reddish brown orgolden brown shaggy growth indicates normalduring nitrification A very dark brown, shaggygrowth (with worms present) indicates a veryold slime White chalky growth indicates highinfluent sulfur or sulfide levels No visible slimegrowth to the RBC indicates a severe pH ortemperature change

FIGURE 18.9 Rotating biological contactor (RBC) treatment system in series (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol 1, Technomic Publ., Lancaster, PA, 1999.)

Rotating biological contactors Cl 2

Effluent

Solids disposal

Secondary settling tanks

Primary settling tank Influent

564 Handbook of Water and Wastewater Treatment Plant Operations

18.9.3.4.2 Process Control Sampling and Testing

For process control, the RBC process does not require

large amounts of sampling and testing to provide the

infor-mation required The frequency for performing suggested

testing depends on available resources and variability of

process Frequency may be lower during normal operation

and higher during abnormal conditions

The following routine sampling points and types of

tests will permit the operator to identify normal and

abnor-mal operating conditions:

1 RBC train influent — Tests include pH,

tem-perature, settleable solids, BOD, suspended

sol-ids, and metals

2 RBC — Test includes speed of rotation

3 RBC train effluent — Tests include DO, pH, jar

tests

4 Process effluent — Tests include DO, pH,

settle-able solids, BOD, and suspended solids

18.9.3.4.3 Troubleshooting Operational Problems

(Note: Much of the information in this section is based

on material provided by EPA in Performance Evaluation

and Troubleshooting at Municipal Wastewater Treatment

Facilities,Washington, D.C., current edition.)

The following sections are not all-inclusive; they do

not cover all of the operational problems associated with

the rotating biological contactor process They do provide

information on the most common operational problems

A Aerate RBC or plant influent

B Add sodium nitrate or hydrogen peroxide to

influent

C Adjust baffles between stages 1 and 2 to

increase fraction of total surface area in first

A Implement and enforce pretreatment program

B Install pH control equipment

C Equalize flow to acclimate organisms

A Repair mechanical problem

B Increase rotational speed

C Adjust baffles to decrease loading

A Identify and correct grit removal problem

B Identify and correct primary settling problem

18.9.3.5 RBC: Process Control Calculations

Several process control calculations may be useful in theoperation of a RBC These include soluble BOD, totalmedia area, organic loading rate, and hydraulic loadingrate Settling tank calculations and sludge pumping cal-culations may be helpful for evaluation and control of thesettling tank following the RBC

18.9.3.5.1 RBC: Soluble BOD

The soluble BOD concentration of the RBC influent can

be determined experimentally in the laboratory or it can

be estimated using the suspended solids concentration andthe K factor The K factor is used to approximate the BOD(particulate BOD) contributed by the suspended matter.The K factor must be provided or determined experimentally

Wastewater Treatment 565

in the laboratory The K factor for domestic wastes

nor-mally ranges from 0.5 to 0.7

E XAMPLE 18.34

Problem:

The suspended solids concentration of a wastewater is

250 mg/L If the normal K value at the plant is 0.6, what

is the estimated particulate BOD concentration of the

wastewater?

Solution:

The K value of 0.6 indicates that about 60% of the

sus-pended solids are organic sussus-pended solids (particulate

BOD):

E XAMPLE 18.35

Problem:

An RBC receives a flow of 2.2 MGD with a BOD content

of 170 mg/L and suspended solids concentration of 140

mg/L If the K value is 0.7, how many pounds of soluble

BOD enter the RBC daily?

Solution:

Now the pounds per day of soluble BOD may be

deter-mined:

18.9.3.5.2 RBC: Total Media Area

Several process control calculations for the RBC use the

total surface area of all the stages within the train As was

the case with the soluble BOD calculation, plant design

information or information supplied by the unit

manufac-turer must provide the individual stage areas (or the totaltrain area) because physical determination of this would

be extremely difficult

(18.26)

18.9.3.5.3 RBC: Organic Loading Rate

If the soluble BOD concentration is known, the organicloading on a RBC can be determined Organic loading on

a RBC based on soluble BOD concentration can rangefrom 3 to 4 lb/d/1000 ft2

E XAMPLE 18.36

Problem:

An RBC has a total media surface area of 102,500 ft 2 and receives a primary effluent flow rate of 0.269 MGD If the soluble BOD concentration of the RBC influent is

159 mg/L, what is the organic loading rate in pounds per

1000 ft 2 ?

Solution:

18.9.3.5.4 RBC: Hydraulic Loading Rate

The manufacturer normally specifies the RBC media face area and the hydraulic loading rate is based on themedia surface area (usually in square feet) Hydraulicloading on a RBC can range from 1 to 3 gal/d/ft2

sur-E XAMPLE 18.37

Problem:

An RBC treats a primary effluent flow rate of 0.233 MGD What is the hydraulic loading rate in gal/d/ft 2 if the media surface area is 96,600 ft 2 ?

Solution:

18.10 ACTIVATED SLUDGE

The biological treatment systems discussed to this point(ponds, trickling filters, and RBCs) have been around foryears The trickling filter, for example, has been aroundand successfully used since the late 1800s The problem

Soluble BOD Total BOD

K Factor Total Suspended Solids

with ponds, trickling filters, and RBCs is that they are

temperature sensitive and remove less BOD In addition,

trickling filters cost more to build than the activated sludge

systems that were later developed

Note: Although trickling filters and other systems cost

more to build than activated sludge systems, it

is important to point out that activated sludge

systems cost more to operate because of the

need for energy to run pumps and blowers

As shown in Figure 18.10, the activated sludge

pro-cess follows primary settling The basic components of an

activated sludge sewage treatment system include an

aer-ation tank and a secondary basin, settling basin, or clarifier

(see Figure 18.10) Primary effluent is mixed with settled

solids recycled from the secondary clarifier and is then

introduced into the aeration tank Compressed air is

injected continuously into the mixture through porous

dif-fusers located at the bottom of the tank, usually along one

side

Wastewater is fed continuously into an aerated tank,

where the microorganisms metabolize and biologically

flocculate the organics Microorganisms (activated sludge)

are settled from the aerated mixed liquor under quiescent

conditions in the final clarifier and are returned to the

aeration tank Left uncontrolled, the number of organisms

would eventually become too great; therefore, some must

periodically be removed (wasted) A portion of the

con-centrated solids from the bottom of the settling tank must

be removed from the process (waste activated sludge)

Clear supernatant from the final settling tank is the plant

effluent

18.10.1 A CTIVATED S LUDGE T ERMINOLOGY

To better understand the discussion of the activated sludge

process presented in the following sections, you must

understand the terms associated with the process Some

of these terms have been used and defined earlier in the

text, but we list them here again to refresh your memory

Review these terms and remember them They are usedthroughout the discussion

Absorption taking in or reception of one substance

into the body of another by molecular or ical actions and distribution throughout theabsorber

chem-Activated to speed up reaction When applied to

sludge, it means that many aerobic bacteria andother microorganisms are in the sludge particles

Activated sludge a floc or solid formed by the

micro-organisms It includes organisms, accumulatedfood materials, and waste products from theaerobic decomposition process

Activated sludge process a biological wastewater

treatment process in which a mixture or influentand activated sludge is agitated and aerated Theactivated sludge is subsequently separated fromthe treated mixed liquor by sedimentation and

is returned to the process as needed The treatedwastewater overflows the weir of the settlingtank in which separation from the sludge takesplace

Adsorption the adherence of dissolved, colloidal, or

finely divided solids to the surface of solid ies when they are brought into contact

bod-Aeration mixing air and a liquid by one of the

follow-ing methods: sprayfollow-ing the liquid in the air, fusing air into the liquid, or agitating the liquid

dif-to promote surface adsorption of air

Aerobic a condition in which free or dissolved oxygen

is present in the aquatic environment Aerobicorganisms must be in the presence of DO to beactive

Bacteria single-cell plants that play a vital role in

stabilization of organic waste

Biochemical oxygen demand (BOD) a measure of

the amount of food available to the isms in a particular waste It is measured by theamount of dissolved oxygen used up during a

microorgan-FIGURE 18.10 The activated sludge process (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol.

1, Technomic Publ., Lancaster, PA, 1999.)

Air

Activated sludge