ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - BIOLOGICAL TREATMENT OF WASTEWATER doc

In the operation of biological treatment facili-ties, the characteristics of wastewater are measured in terms of its chemical oxygen demand, COD, biochemical oxygen demand, BOD, total or

B BIOLOGICAL TREATMENT OF WASTEWATER

1 INTRODUCTION

Biological treatment is the most widely used method for

removal, as well as partial or complete stabilization of

bio-logically degradable substances present in waste-waters

Most often, the degradable substances are organic in nature

and may be present as suspended, colloidal or dissolved

matter The fraction of each form depends on the nature of

wastewater In the operation of biological treatment

facili-ties, the characteristics of wastewater are measured in terms

of its chemical oxygen demand, COD, biochemical oxygen

demand, BOD, total organic carbon, TOC, and volatile

sus-pended solids, VSS; concepts of which have been discussed

elsewhere. 1

Most of the conventional biological wastewater

treat-ment processes are based on naturally occurring

biologi-cal phenomena, but are carried out at accelerated rates

These processes employ bacteria as the primary

organ-isms; however, certain other microorganisms may also

play an important role Gates and Ghosh 2 have presented

the biological component system existing in the BOD

pro-cess and it is shown in Figure 1 The degradation and

sta-bilization of organic matter is accomplished by their use

as food by bacteria and other microorganisms to produce

protoplasm for new cells during the growth process When

a small number of microorganisms are inoculated into a

bacteriological culture medium, growth of bacteria with

time follows a definite pattern as depicted in Figure 2 by

plotting viable count and mass of bacteria against time. 3

The population dynamics of bacteria in biological

treat-ment processes depends upon various environtreat-mental

fac-tors including pH, temperature, type and concentration

of substrate, hydrogen acceptor, availability and

concen-tration of essential nutrients like nitrogen, phosphorous,

sulfur, etc., and essential minerals, osmotic pressure,

tox-icity of media or by-products, and degree of mixing. 4 In

recent years, cultures have been developed for biological

treatment of many hard-to-degrade organic wastes

2 METABOLIC REACTIONS The metabolic reactions occurring within a biological treat-ment reactor can be divided into three phases: oxidation, syn-thesis and endogenous respiration Oxidation–reduction may proceed either in the presence of free oxygen, aerobically,

or in its absence, anaerobically While the overall reactions

SUBSTRATE

ORGANICS OXYGEN GROWTH FACTORS LYSISED

PRODUCTS OXYGEN

BACTERIA (PRIMARY FEEDERS) DEAD

BIOMASS

DESTRUCTION

AUTO-OXYGEN

GROWTH FACTORS

ENERGY OTHER PRODUCTS

P R O T O Z O A

B A C T E R I A

PROTOZOA (SECONDARY FEEDERS)

FIGURE 1 Biological component system existing in BOD process.

carried out may be quite different under aerobic and

anaero-bic conditions, the processes of microbial growth and energy

utilization are similar Typical reactions in these three phases

are formulated below:

• Organic Matter Oxidation (Respiration)

Therefore, bacterial respiration in living protoplasm is a

biochemical process whereby energy is made available for

endothermic life processes Being dissimilative in nature,

respiration is an important process in wastewater

treat-ment practices On the other hand, endogenous

respira-tion is the internal process in microorganisms that results

in auto-digestion or self-destruction of cellular material. 3

Actually, bacteria require a small amount of energy to

main-tain normal functions such as motion and enzyme activation

and this basal-energy requirement of the bacteria has been

designated as endogenous respiration Even when

nutri-ents are available, endogenous metabolism proceeds with

the breakdown of protoplasm. 5 According to Bertalanffy’s

hypothesis, 6 the microbial growth is the result of competition

between two opposing processes: Aufban—assimilation, and

Abban—endogenous metabolism The rate of assimilation is proportional to the mass of protoplasm in the cell and the surface area of the cell, whereas the endogenous metabolism

is dependent primarily on environmental conditions

In the presence of enzymes produced by the living organisms, about 1/3 of the organic matter removed is oxi-dized into carbon dioxide and water in order to provide energy for synthesis of the remaining 2/3 of the organic matter into the cell material Metabolism and process reactions occur-ring in typical biological wastewater treatment processes are explained schematically by Stewart 7 as shown in Figure 3.Thus, the basic equations for biological metabolisms are: Organic matter metabolized

micro-= Protoplasm synthesized  Energy for synthesis and

Net protoplasm accumulation

= Protoplasm synthesized  Endogenous respiration

ASSBIOMASS EFFLUENT

ASSIMILATED

SYNTHESIZED

UNUSED BOD (SOLUBLE AND VSS) INFLUENT NON-BIODEGRADABLE FSS AND VSS

RESPIRA TION

FIGURE 3 Metabolism and process reactions.

FIGURE 2 Growth pattern of microorganisms.

k = Logarithmic growth rate constant, time 1

In wastewater treatment practices, the growth pattern

based on mass of microorganisms has received more

atten-tion than the number of viable microorganisms If each

microorganism is assumed to have an average constant mass,

then N in Eq 1 can be replaced with X, the mass of active

microorganisms present per unit volume to obtain:

dd

X

The growth of bacterial population may become limited

either due to exhaustion of available nutrients or by the

accu-mulation of toxic substances The growth rate of bacteria

starts slowing down, and Eq 1 changes to the form:

dd

N

where growth rate factor k t , varies with time and becomes a

function of temperature, T, pH, substrate concentration, S,

and concentration of various nutrients, C n 1 , C n 2 , etc., i.e.:

k t = V 1 ( T, pH, C s , C n 1 , C n 2 , … )

Figure 4 shows variation in growth rate k t with change in

nutrient concentrations, assuming that T and pH are held

con-stant and substrate concentration, S, is greater than the critical

substrate concentration, S * , above which k t, is independent

of S Several interesting observations are made from these

curves. 8 First, the maximum value of k t is essentially constant

Second, the shape of the curve and the limiting concentration

is different for each nutrient Third, k t is shown to be zero

when any of the nutrients is missing Fourth, as the biological

reaction proceeds, all nutrients are consumed Thus, even if

all nutrients are initially in excess, the growth may eventually

become limited Finally, as the concentration drops to zero, a

stationary phase is reached, i.e., d N /d t becomes zero

In case of a substrate limited system, rate of growth is

given by:

dd

N

or

dd

dd

where K is a constant called half velocity coefficient and µ max

is maximum specific growth rate

It is postulated that the same amount of substrate is porated in each cell formed Therefore, the rate of increase

incor-in number or mass of microorganisms incor-in logarithmic growth

phase, d N /d t, or d X /d t, is proportional to the rate of substrate consumption, d S /d t, or d L /d t, if the substrate concentration

is measured in terms of its BOD, L, and the following

rela-tionship can be stated:

dd

dd

X

S t

or

∆ X = Y ∆ S where Y is called the growth yield coefficient, ∆ X is the

cell mass synthesized in a given time, and ∆ S is substrate removed in the same time The substrate utilization rate, q,

per unit biomass has been defined as:

Combining Eqs 4, 6 and 7 yields:

q Y

Under conditions of rate limited growth, i.e., nutrient

exhaustion or auto-oxidation, Eq 6 becomes:

dd

dd

where b is the auto-oxidation rate or the microbial decay rate

In absence of substrate, this equation is reduced to:

dd

X

Several kinetic equations have been suggested for

analy-sis and design of biological wastewater treatment systems

and the following have been applied frequently: 10 – 13

dd

S t

d

S

dd

S

t qX

S S

0

(14)

where S 0 is the initial substrate concentration Combining

Eqs 10 and 12 gives the net specific growth rate:

dd

A similar kinetic relationship can be obtained by combining

Eq 10 with Eqs 13 and 14

Effect of Temperature

One of the significant parameters influencing biological

reaction rates is the temperature In most of the

biologi-cal treatment processes, temperature affects more than one

reaction rate and the overall influence of temperature on the

process becomes important The applicable equation for the

effect of temperature on rate construct is given by:

k T = k 20 uT– 20 (16)

where u is the temperature coefficient This equation shows

that reaction rates increase with increase in temperature

Methods of BOD Removal

In wastewater treatment processes, the microorganisms are not present as isolated cells, but are a collection of microorganismssuch as bacteria, yeast, molds, protozoa, rotifers, worms and insect larvae in a gelatinous mass. 13 These microorganisms tend to collect in a biological floc, called biomass, which is expected to possess good settling characteristics The bio-logical oxidation or stabilization of organic matter by the microorganisms present in the floc is assumed to proceed in the following sequence: 13,14

(a) An initial high rate of BOD removal from water on coming in contact with active biomass

waste-by adsorption and absorption The extent of this removal depends upon the loading rate, the type of waste, and the ecological condition of the biomass (b) Utilization of decomposable organic matter in direct proportion to biological cell growth Substances concentrating on the surface of biomass are decomposed by the enzymes of living cells, new cells are synthesized and end products of decom-position are washed into the water or escape to the atmosphere

(c) Oxidation of biological cell material through endogenous respiration whenever the food supply becomes limited

(d) Conversion of the biomass into settleable or erwise removable solids

The rates of reactions in the above mechanisms depend upon the transport rates of substrate, nutrients, and oxygen in case

of aerobic treatment, first into the liquid and then into the biological cells, as shown in Figure 5. 15 Any one or more of these rates of transport can become the controlling factors in obtaining the maximum efficiency for the process However, most often the interfacial transfer or adsorption is the rate determining step. 14

In wastewater treatment, the biochemical oxygen demand

is exerted in two phases: carbonaceous oxygen demand to oxidize organic matter and nitrogenous oxygen demand

to oxidize ammonia and nitrites into nitrates The nous oxygen demand starts when most of the carbonaceousoxygen demand has been satisfied. 15 The typical progression

nitroge-of carbonaceous BOD removal by biomass with time, during biological purification in a batch operation, was first shown

by Ruchhoft 16 as reproduced in Figure 6 The corresponding metabolic reactions in terms of microorganisms to food ratio, M/F, are shown in Figure 7 This figure shows that the food

to microorganisms ratio maintained in a biological reactor is

of considerable importance in the operation of the process

At a low M/F ratio, microorganisms are in the log-growth phase, characterized by excess food and maximum rate of metabolism However, under these conditions, the settling characteristic of biomass is poor because of their dispersed

continued aertion under these conditions results in oxidation of biomass Although the rate of metabolism is relatively low at high M/F ratio, settling characteristics of biomass are good and BOD removal efficiency is high Goodman and Englande 17 have suggested that the total

auto-mass concentration of solids, X T , in a biological reactor is

composed of an inert fraction, X i , and a volatile fraction, X v ,

which can be further broken down into an active fraction, X, and non-biodegradable residue fraction, X n , resulting from

endogenous respiration, i.e.:

X T = X i + X v = X i + X + X n (17) The total mass concentration of solids in wastewater treat-ment is called suspended solids, whereas its volatile fraction

is called volatile suspended solids, X In a biological tor, volatile suspended solids, X, is assumed to represent the

reac-mass of active microorganisms present per unit volume

3 TOXICITY Toxicity has been defined as the property of reaction of a substance, or a combination of substances reacting with each other, to deter or inhibit the metabolic process of cells without completely altering or destroying a particular species, under a given set of physical and biological environmental conditions for a specified concentration and time of exposure. 18 Thus, the toxicity is a function of the nature of the substance, its concen-tration, time of exposure and environmental conditions Many substances exert a toxic effect on biological oxida-tion processes and partial or complete inhibition may occur depending on their nature and concentration Inhibition may result from interference with the osmotic balance or with the enzyme system In some cases, the microorganisms become more tolerant and are considered to have acclima-tized or adapted to an inhibitory concentration level of a toxic substance This adaptive response or acclimation may result from a neutralization of the toxic material produced by the biological activity of the microorganisms or a selective

CELL

CELL MEMBRANE LIQUID FILM

LIQUID FILM

PRODUCT

BY-OXYGEN

SUBSTRA TE

BIOCHEM REACTION RD

CELL

BIOCHEMICALREACTION

WASTE PRODUCTS

1.0 UNUSED

RESPIRATION

INITIAL SYN THESIS

ENDOGENOUS RESPIRATION

NET BIOMASS INCREASE

SHORT-TERM AERATION

TIONAL

CONVEN-EXTENDED AERATION RELATIVE ORGANISM WEIGHT (M/F)

FIGURE 7 Metabolic reactions for the complete spectrum.

growth; also, the BOD removal efficiency is poor as the

excess unused organic matter in solution escapes with the

effluent On the other hand, high M/F ratio means the

opera-tion is in the endogenous phase Competiopera-tion for a small

amount of food available to a large mass of micro organisms

results in starvation conditions within a short duration and

growth of the culture unaffected by the toxic substance In

some cases, such as cyanide and phenol, the toxic substances

may be used as substrate Rates of acclimation to lethal

fac-tors vary greatly Thus, the toxicity to microorganisms may

result due to excess concentrations of substrate itself, the

presence of inhibiting substances or factors in the

environ-ment and/or the production of toxic by-products. 19 – 23

The influence of a toxicant on microorganisms depends

not only on its concentration in water, but also on its rate

of absorption, its distribution, binding or localization in the

cell, inactivation through biotransformation and ultimate

excretion The biotransformations may be synthetic or

non-synthetic The nonsynthetic transformations involve

oxida-tion, reduction or hydrolysis The synthetic transformation

involve the coupling of a toxicant or its metabolite with a

carbohydrate, an amino acid, or a derivative of one of these

According to Warren 19 , the additive interaction of two toxic

substances of equal toxicity, mixed in different proportions, may show combined toxicity as shown in Figure 8 The com-bined effects may be supra-additive, infra-additive, no inter-action or antagonism The relative toxicity of the mixture is measured as the reciprocal of median tolerance limit Many wastewater constituents are toxic to microorgan-isms A fundamental axiom of toxicity states that all com-pounds are toxic if given to a text organism at a sufficiently high dose By definition, the compounds that exert a delete-rious influence on the living microorganisms in a biological treatment unit are said to be toxic to those microorganisms

At high concentrations, these substances kill the microbes whereas at sublethal concentrations, the activity of microbes

is reduced The toxic substances may be present in the influent stream or may be produced due to antagonistic interactions Biological treatment is fast becoming a preferred option for treating toxic organic and inorganic wastes in any form;

FIGURE 8 Possible kinds of interactions between two hypothetical toxicants, A and B.

solid, liquid or gaseous The application of biological

pro-cesses in degradation of toxic organic substances is

becom-ing popular because (i) these have an economical advantage

over other treatment methods; (ii) toxic substances have

started appearing even in municipal wastewater treatment

plants normally designed for treating nontoxic substrates;

and (iii) biological treatment systems have shown a

resil-iency and diversity which makes them capable of

degrad-ing many of the toxic organic compounds produced by the

industries. 24 Grady believes that most biological treatment

systems are remarkably robust and have a large capacity for

degrading toxic and hazardous materials. 25 The bacteria and

fungi have been used primarily in treating petroleum-derived

wastes, solvents, wood preserving chemicals and coal tar

wastes The capability of any biological treatment system is

strongly influenced by its physical configuration

As mentioned previously, the Michelis–Menten or

Monond equation, Eq 5, has been used successfully to

model the substrate degradation and microbial growth in

biological wastewater treatment process However, in the

presence of a toxic substance, which may act as an

inhibi-tor to the normal biological activity, this equation has to be

modified The Haldane equation is generally accepted to be

quite valid to describe inhibitory substrate reactions during

the nitrification processes, anaerobic digestion, and

treat-ment of phenolic wastewaters. 24,26,27

where K i is the inhibition constant

In the above equation, a smaller value for K i indicates

a greater inhibition The difference between the two kinetic

equations, Monod and Haldane, is shown in Figure 9, in which the specific growth rate,  , is plotted for various sub-

strate concentrations, S The values for  max , K s and K i are assumed to be 0.5 h– 1 , 50 mg/L and 100 mg/L, respectively

Behavior of Biological Processes

The behavior of a biological treatment process, when jected to a toxic substance, can be evaluated in three parts:

1 Is the pollutant concentration inhibitory or toxic

to the process? How does it affect the tion rate of other pollutants?

2 Is the pollutant concentration in process effluent reduced to acceptable level? Is there a production

Generally, biological processes are most cost-effective methods to treat wastes containing organic contaminants However, if toxic substances are present in influents, certain pretreatment may be used to lower the levels of these con-taminants to threshold concentrations tolerated by acclimated microorganisms present in these processes Equalization of toxic load is an important way to maintain a uniform influ-ent and reduce the shock load to the process Also, various physical/chemical methods are available to dilute, neutralize and detoxicate these chemicals

FIGURE 9 Change of specific growth rate with substrate concentration (inhibited and uninhibited).

SUBSTRATE CONCENTRATION, S,mg/L

HALDANE EQUATION MONOD EQUATION

0.1 0.2 0.3 0.4 0.5

Genetically Engineered Microorganisms

One of the promising approaches in biodegradation of

tox-ic organtox-ics is the development of genettox-ically engineered

microorganisms Knowledge of the physiology and

biochem-istry of microorganisms and development of appropriate

process engineering are required for a successful system to

become a reality The areas of future research that can benefit

from this system include stabilization of plasmids, enhanced

activities, increased spectrum of activities and development

of environmentally safe microbial systems. 30

4 TYPES OF REACTORS

Three types of reactors have been idealized for use in

bio-logical wastewater treatment processes:

(a) Batch Reactors in which all reactants are added at

one time and composition changes with time;

(b) Plug Flow or Non-Mix Flow Reactors in which

no element of flowing fluid overtakes another

ele-ment; and

(c) Completely Mixed or Back-Mix Reactors in

which the contents are well stirred and are

uni-form in composition throughout

Most of the flow reactors in the biological treatment are

not ideal, but with negligible error, some of these can be

sidered ideal plug flow or back-mix flow Others have

con-siderable deviations due to channeling of fluid through the

vessel, by the recycling of fluid through the vessel or by the

existence of stagnant regions of pockets of fluid. 31 The

non-ideal flow conditions can be studied by tagging and

follow-ing each and every molecule as it passes through the vessel,

but it is almost impossible Instead, it is possible to measure

the distribution of ages of molecules in the exit stream

The mean retention time, t - for a reactor of volume V

and having a volumetric feed rate of Q is given by t -V Q In

non-ideal reactors, every molecule entering the tank has a

different retention time scattered around t - Since all

biologi-cal reactions are time dependent, knowledge on age

distribu-tion of all the molecules becomes important The distribudistribu-tion

of ages of molecules in the exit streams of both ideal and

non-ideal reactors in which a tracer is added instantaneously

in the inlet stream is shown in Figure 10 The spread of

con-centration curve around the plug flow conditions depends

upon the vessel or reactor dispersion number, Deul, where D

is longitudinal or axial dispersion coefficient, u is the mean

displacement velocity along the tank length and l is the

length dimension. 32 In the case of plug flow, the dispersion

number is zero, whereas it becomes infinity for completely

mixed tanks

Treatment Models

Lawrence and McCarty 11 have proposed and analyzed

the following three models for existing continuous flow

aerobic or anaerobic biological wastewater treatment configurations:

(a) a completely mixed reactor without biological solids recycle,

(b) a completely mixed reactor with biological solids recycle, and

(c) a plug flow reactor with biological solids recycle These configurations are shown schematically in Figure 11

In all these treatment models, the following equations can

be applied in order to evaluate kinetic constants, 33 where ∆ indicates the mass or quantity of material:

• Solid Balance Equation



CellsReactor

CellsGrowth

CellsDecay

⎡

⎣

⎢ ⎤⎦⎥ ⎡⎣⎢ ⎤⎦⎥ ⎡⎣⎢ ⎤⎦⎥ ⎡⎣⎢CEffluent LossCells ⎤⎦⎥ (19)

• Substrate Balance Equation

Reactor

SubstrateInfluent

Parameters for Design and Operation

Various parameters have been developed and used in the design and operation of biological wastewater treatment pro-cesses and the most significant parameters are:

u x– Biological Solids Retention Time, or Sludge Age, or Mean Cell Retention Time, is defined

Plug Flow Condition (Dispersion Number = 0)

Non-ideal Flow Condition (Large Dispersion Number) Uniformly Mixed Condition (Dispersion Number = 0)

Time of Flow to Exit / Mean Retention Time

as the ratio between total active microbial mass

in treatment system, X T , and total quantity of

active microbial mass withdrawn daily,

includ-ing solids wasted purposely as well as those

lost in the effluent, ∆ X T/ ∆ t Regardless of the

fraction of active mass, in a well-mixed system

the proportion of active mass wasted is equal

to the proportion of total sludge wasted,

mak-ing sludge age equal for both total mass and

active mass

U – Process Loading Factor, or Substrate Removal

Velocity, or Food to Microorganisms Ratio,

or Specific Utilization, is defined as the ratio

between the mass of substrate utilized over a

period of one day, ∆ S / ∆ t, and the mass of active

microorganisms in the reactor, X T

t¯– Hydraulic Retention Time or Detention Time,

or Mean Holding Time, is defined as the ratio

between the volume of Reactor, V, and the

volu-metric feed rate, Q

B V– Volumetric Loading Rate or Hydraulic Loading

Rate is defined as the ratio between the mass of

substrate applied over a period of one day, S T/ ∆ t

and the volume of the reactor, V

E – Process Treatment Efficiency or Process

Perform-ance is defined as percentage ratio between the

substrate removed, ( S 0– S e ), and influent

sub-strate concentration, S 0

A desired treatment efficiency can be obtained

by control of one or more of these parameters separately or in combination

5 BIOLOGICAL TREATMENT SYSTEMS The existing biological treatment systems can be divided into the following three groups:

(a) Aerobic Stationary-Contact or Fixed-Film tems: Irrigation beds, irrigation sand filters, rotating biological contactors, fluidized bed reactors, and trickling filters fall in this group In these treatment processes, the biomass remains stationary in contact with the solid supporting-media like sand, rocks or plastic and the waste-water flows around it

(b) Aerobic Suspended-Contact Systems: Activated sludge process and its various modifications, aerobic lagoons and aerobic digestion of sludges are included in this group In these treatment pro-cesses, both the biomass and the substrate are in suspension or in motion

(c) Anaerobic Stationary-Contact and Suspended Contact Systems: Anaerobic digestion of sludges and anaerobic decomposition of wastewater in anaerobic lagoons fall in this category

A typical layout of a wastewater treatment plant incorporating biological treatment is shown in Figure 12.Primary sedimentation separates settleable solids and the aerobic biological treatment is designed to remove the sol-uble BOD The solids collected in primary sedimentation tanks and the excess sludge produced in secondary treat-ment are mixed together and may be digested anaerobically

in digesters Trickling filter and activated sludge processes are most common secondary treatment processes for aer-obic treatment and are discussed in detail Discussion of sludge digestion by anaerobic process and use of biologi-cal nutrient removal as a tertiary treatment have also been included

In addition to conventional pollutants present in pal and industrial wastewaters, significant concentrations of toxic substances such as synthetic organics, metals, acids, bases, etc., may be present due to direct discharges into the sewers, accidental spills, infiltration and formation during chlorination of wastewaters It is import to have a knowl-edge of both the scope of applying biological treatment and the relevant engineering systems required to achieve this capability Thus, the kinetic description of the process and the deriving reactor engineering equations and strategies for treatment of conventional and toxic pollutants are essential for proper design and operation of biological waste treat-ment systems. 24

munici-I- Completely Mixed-No biological solids recycle

II- Completely Mixed-Biological solids recycle

III- Plug Flow-Biological solids recycle

Settling Tank

Sludge

Sludge

FIGURE 11 Treatment models.

The available information strongly indicates that

immo-bilized biological systems are less sensitive to toxicity and

have a higher efficiency in degrading toxic and hazardous

materials. 34 Fixed-film wastewater treatment processes are

regarded to be more stable than suspended growth processes

because of the higher biomass concentration and greater

mass transfer resistance from bulk solution into the biofilm

in fixed-films. 35 The mass transfer limitation effectively

shields the microorganisms from higher concentrations of

toxins or inhibitors during short-term shock loads because

the concentrations in biofilms change more slowly than in

the bulk solution Also, since the microorganisms are

physi-cally retained in the reactor, washout is prevented if the

growth rate of microorganisms is reduced. 34,35 The biofilm

systems are especially well suited for the treatment of slowly

biodegradable compounds due to their high biomass

concen-tration and their ability to immobilize compounds by

adsorp-tion for subsequent biodegradaadsorp-tion and detoxificaadsorp-tion. 34

Trickling Filters

Wastewater is applied intermittently or continuously to a

fixed bed of stones or other natural synthetic media resulting

in a growth of microbial slime or biomass on the surface of

this media Wastewater is sprayed or otherwise distributed so

that it slowly trickles through while in contact with the air

For maximum efficiency, food should be supplied

continu-ously by recirculating, if necessary, the treated wastewater or

settled sludge or both Oxygen is provided by the dissolved

oxygen in influent wastewater, recirculated water from the

air circulating through the interstices between the media to

maintain aerobic conditions

Active microbial film, biomass, consisting primarily

of bacteria, protozoa, and fungi, coats the surface of filter

media The activity in biological film is aerobic, with

move-ment of oxygen, food and end-products in and out of it as

shown in Figure 13 However, as the thickness of the film

increases, the zone next to the filter medium becomes bic Increased anaerobic activity near the surface may liquify the film in contact with the medium, resulting in sloughing

anaero-or falling down of the old film and growth of a new film The sloughed solids are separated in a secondary settling tank and a part of these may be recirculated in the system Two types of trickling filters are recognized, primarily on the basis of their loading rates and method of operation, as shown in Table 1 In low-rate trickling filter, the wastewater passes through only once and the effluent is then settled prior

to disposal In high-rate trickling filter, wastewater applied

FIGURE 12 Typical wastewater treatment sequence.

ANAER

OBIC

AEROBIC

FIGURE 13 Process of BOD removal in trickling filters.

to filters is diluted with recirculated flow of treated effluent,

settled effluent, settled sludge, or their mixture, so that it is

passed through the filter more than once Several

recircula-tion patterns used in high-rate filter systems are shown in

ASCE Manual. 36 Sometimes two filter beds are placed in

series and these are called Two-Stage Filters

The advantages and disadvantages of recirculation are

listed below:

Advantages of Recirculation

(a) Part of organic matter in influent wastewater is

brought into contact with growth on filter media

more than once

(b) Recirculated liquid contains active

microor-ganisms not found in sufficient quantity in raw

wastewater, thus providing seed continually This

continuous seeding with active microorganisms

and enzymes stimulates the hydrolysis and

oxi-dation and increases the rate of biochemical

stabilization

(c) Diurnal organic load is distributed more

uni-formly Thus, when plant flow is low, operation is

not shut off Also, stale wastewater is freshened

(d) Increased flow improves uniformity of

distribu-tion, increases sloughing and reduces clogging

tendencies

(e) Higher velocities and continual scouring make

con-ditions less favourable for growth of filter flies

(f) Provides for more flexibility of operation

Disadvantages

(a) There is increased operating cost because of

pumping Larger settling tanks in some designs

may increase capital cost

(b) Temperature is reduced as a result of number of

passes of liquid In cold weather, this results in

decreased biochemical activity

(c) Amount of sludge solids to digesters may be increased

The ACE Manual 36 lists the following factors affecting the design and operation of filters:

(a) composition and characteristics of the wastewater after pretreatment,

(b) hydraulic loading applied to the filter, (c) organic loading applied to the filter, (d) recirculation, system, ratio and arrangement, (e) filter beds, their volume, depth and air ventilation, (f) size and characteristics of media, and

S

t qSX=k S f

or

S S

11



where trickling filter rate coefficient, k f , is a function of

active film mass per unit volume and remains constant for a

given specific area and uniform slime layer Contact time, t,

TABLE 1 Comparison of low-rate and high-rate filters

Hydraulic Loading

Organic Loading (BOD)