Development of a simplified concept for process benchmarking of urban wastewater management

LIST OF FIGURES AND TABLESFigures Figure 1.1 Representation of a Centralized Wastewater Collection and Treatment System Figure 1.2 Representation of a Decentralized Wastewater Collection

DRESDEN UNIVERSITY OF TECHNOLOGY VNU UNIVERSITY OF SCIENCE

DRESDEN UNIVERSITY OF TECHNOLOGY VNU UNIVERSITY OF SCIENCE

Le Quynh Dung

DEVELOPMENT OF A SIMPLIFIED CONCEPT

FOR PROCESS BENCHMARKING OF URBAN

WASTEWATER MANAGEMENT

Major: Waste Management and Contaminated Site Treatment

MASTER THESIS

SUPERVISOR: PROF DR RER NAT DR H C PETER WERNER

MSC-ENG PHAN HOANG MAI

Hanoi - 2011

TABLE OF CONTENTS

ABBREVIATIONS

LIST OF FIGURES AND TABLES

ACKNOWLEDGEMENTS

INTRODUCTION

CHAPTER I

Theoretical Foundations of Urban Wastewater Management System

1.1 Characteristics of Urban Wastewater

1.1.1What is Urban Wastewater?

1.1.2Constituents of Wastewater

1.2 Overview of the Urban Wastewater Management System

1.2.1Components of Urban Wastewater Management System

1.2.2Types of Wastewater Management System

1.3 Sub-processes of Wastewater Management System

1.3.1Collection Systems

1.3.2Wastewater Treatment

1.3.3Sludge Treatment and Disposal

1.3.4Effluent Disposal and Reuse

1.4 Current situation of Urban Wastewater Management in Vietnam

1.4.1The Development of the Urban Drainage System

1.4.2Current Structure and Operation of Urban Drainage Systems

1.4.3The Organizations of Urban Drainage Services in Vietnam

1.4.4Financial Aspects of Urban Drainage Companies

1.4.5Legal and Institutional Frameworks

1.4.6Investment and Management of Urban Drainage System

CHAPTER II

Benchmarking in the Urban Wastewater Management Sector

2.1 Fundamentals of Benchmarking

2.1.1Definition of benchmarking

2.1.2Types and elements of benchmarking

2.2 International Benchmarking System in Water Industry

2.2.1Benchmarking of large Municipal Wastewater Treatment Plants in Austria

2.2.2Benchmarking in Canada

2.2.3North European Benchmarking Co-operation

2.3 Process Benchmarking in Wastewater Sector

2.3.1What is Process Benchmarking?

2.3.2The Objectives of Process Benchmarking

2.3.3Methodology in Process benchmarking

2.3.4Different Process Benchmarking Concepts

CHAPTER III 55

Performance Indicators of Benchmarking in Wastewater Service 55

3.1 Basis of Performance Indicators 55

3.1.1 Systems of Performance Indicators 55

3.1.2 The Usage of Performance Indicators (PIs) 57

3.1.3 Performance Indicators – A component of Benchmarking 59

3.2 The System of IWA-PIs for Wastewater Services 60

3.2.1 Context Information 60

3.2.2 Performance Indicators 62

CHAPTER IV 65

Performance Assessment and Data Collection for Benchmarking in Wastewater Services of Vietnam

4.1Approach of the Performance Assessment

4.1.1Classification of various Undertakings

4.1.2Performance Indicators

4.1.3Confidence Grades

4.1.4Structure of Questionnaire

4.2Questionnaire of Wastewater Management System

4.3Performance Indicators for Wastewater Management System 4.3.1Environmental Impacts

4.3.2Operation and Maintenance

4.3.3Quality of services

4.3.4Employees

4.3.5Economic and financial aspects

4.4Data Collection

CONCLUSIONS

REFERENCES

APPENDIX

Performance Indicators Population Equivalent Wastewater Treatment Plants

LIST OF FIGURES AND TABLES

Figures

Figure 1.1 Representation of a Centralized Wastewater Collection and Treatment System

Figure 1.2 Representation of a Decentralized Wastewater Collection and Treatment System

Figure 1.3 Schematic of unit operations and processes in a wastewater treatment plant Figure 1.4 Schematic of plug flow and complete mix activated sludge process

Figure 1.5 Schematic of trickling filter with rock packing and plastic packing

Figure 2.1 Main steps of a benchmarking process

Figure 2.2 Extended process model for wastewater treatment plants above 100,000 PE Figure 2.3 Methodology for the development of process performance indicators

Figure 2.4 NEBC’s benchmarking model

Figure 2.5 Procedure of process benchmarking

Figure 3.1 Structure of Wastewater Context Information & Performance Indicator Figure 3.2 Wastewater undertaking context

Tables

Table 1.1 Principal constituents of concern in wastewater treatment

Table 1.2 Important metals in Wastewater Management

Table 1.3 Comparison of ratios of various parameters used to characterize wastewater Table 1.4 Typical wastewater flowrates from urban residential sources in the USA Table 1.5 Typical wastewater flowrates from commercial sources in the USA

Table 1.6 Typical composition of untreated domestic wastewater

Table 1.7 Typical wastewater constituent data for various countries

Table 1.8 Major biological treatment processes used for wastewater treatment

Table 2.1 Holistic approach versus Selective approach in Process benchmarking

Table 3.1 Reliability bands of collected data

Table 3.2 The IWA Performance Indicators

Table 4.1 Summary of Performance Indicators for Urban Wastewater Management

I would like to send my special thanks to Msc Phan Hoang Mai, (IAA-TUD), who gave

me this topic, supervised and encouraged me to write my thesis I have learnt some things for

my studying from her

Also I would like to thank Dr Catalin Stefan (IAA-TUD) because of his kind helpduring the time I was in Dresden Thanks are also expressed to Msc Le Thi Hoang Oanh (IAA-TUD) who was always willing to help me as I need

Especially, I would like to thank my family and friends, who always support andencourage me to finish my thesis

Benchmarking was first time introduced by Xerox Company in the late 1970s whentheir peer company Fuji produced the photocopiers with better quality and lower prices Xeroxwas forced to critically review their products and production costs by adopting the Japanese

philosophy: gaining the best of the bests by learning, adapting and improving (Parena et al., 2001) That was how benchmarking appeared.

In many countries experiences (Xerox model inspired) have been developed to adapt

benchmarking procedures in the water context (Parena et al., 2001) Benchmarking has been

conducted in many developed countries such as Australia, Canada, England, Germany etc toassess the performance of water and wastewater service providers, to estimate the quality ofservices as well as the satisfaction of customers These benchmarking projects have achievedinitial success and are supposed to sustain Some systems of performance indicators have beendeveloped with the purpose of large scale application such as the system International WaterAssociation or Qualserve system, etc In some developing countries such as India, Vietnam, etc.certain benchmarking projects regarding the issue of water and sanitation have been carried outunder the support of the World Bank

Aiming at developing a simplified concept for process benchmarking of urbanwastewater management which can be applied in developing countries, performance indicatorsand questionnaire prepared for benchmarking in Vietnam, a representative of developing

countries are adapted in this thesis based on the International Water Association (IWA) system ofperformance indicators for wastewater services There are four chapters in the thesis Chapter Iconsiders the foundation of urban wastewater management in general and the current situation ofwastewater management in urban areas of Vietnam In chapter II, fundamentals of benchmarkingand process benchmarking for the water industry are discussed To make clear the tool ofperformance assessment presented in the thesis the performance indicators for wastewaterservices of IWA as well as the basis of performance indicators are given in chapter III Chapter

IV explains the performance indicators selected for process benchmarking in wastewaterservices of Vietnam; also, the questionnaire as well as the excel file to collect data fromwastewater undertakings are presented

Benchmarking of wastewater utilities is emerging as an important tool of performanceimprovement by regular monitoring and analyses can be the solution to this reality It can play asignificant role in the sector as a tool for institutional strengthening Sustained benchmarkingcan help utilities in identifying performance gaps and gaining improvements by the sharing ofinformation and best practices, ultimately resulting in better services to people It is expectedthat benchmarking in wastewater services in developing countries will soon be supported toimplement

CHAPTER I Theoretical Foundations of Urban Wastewater Management System

In this chapter the theoretical foundations of urban wastewater management will beconsidered, including: (1) characteristics of urban wastewater, (2) overview of the urbanwastewater management system, (3) sub-processes of wastewater management system and (4)current situation of urban wastewater management in Vietnam

1.1 Characteristics of Urban Wastewater

1.1.1 What is Urban Wastewater?

According to Tchobanoglous et al 2003, urban wastewater components may varydepending on type of collection system and may include:

1.Domestic (sanitary) wastewater Wastewater discharged from residential areas, and

from commercial, institutional and similar facilities

2.Industrial wastewater Wastewater in which industrial wastes predominate.

3.Stormwater Runoff resulting from rainfall

4.Infiltration/Inflow Water that enters the collection system through indirect and

direct means Infiltration is extraneous water that enters the collection system throughleaking joints, cracks and breaks, or porous walls Inflow is stormwater that enters thecollection system from storm drain connections, roof leaders, foundation and basementdrains, or through access port (manhole) covers

1.1.2 Constituents of Wastewater

The constituents of wastewater can be classified as physical, chemical and biological

Of the constituents listed in table 1.1, suspended solids, biodegradable organics and pathogenorganisms the most concerning ones are referred All wastewater treatment facilities aredesigned to remove these constituents completely

1.1.2.1 Physical Characteristics

Solids

There are many kind of solids present in wastewater, varying from coarse to colloidalones Before any analysis of solids in wastewater the coarse material should be removed Inwastewater treatment, the solids can be classified by their size and state (suspended solids &

dissolved solids), by their chemical characteristics (volatile & fixed solids) and by settleability

(settable suspended solids & non-settable suspended solids) (Sperling, 2007)

Table 1.1 Principal constituents of concern in wastewater treatment a

Priority organic pollutants

a: From Crites & Tchobanoglous, 1998

Particle Size Distribution

The determination of particle size is to understand more about nature of particlescomposing TSS in wastewater In addition, this analysis is also used to assess the effectiveness

of treatment process (biological treatment, disinfection process, sedimentation, etc)

Color in wastewater is caused by suspended solids, colloidal matters and dissolved

substances With suspended solids, it is called apparent color whereas true color is caused by

colloidal matters and dissolved substances

The sources of color in wastewater include infiltration/inflow (humic substances),industrial discharges (e.g dyes or metallic compounds, etc) and the decomposition of organiccompounds in wastewater.

Transmittance/ Absorption

Transmittance is the ability of a liquid to transmit light of a specified wavelengththrough a known depth of solution Absorbance is the loss of radiant energy as light pass

through a fluid (Tchobanoglous et al., 2003).

The components that affect transmittance include selected inorganic compounds (e.g.iron and copper), organic compounds (e.g organic dyes, humic substances, and conjugated ring

compounds such as benzene), and TSS (Tchobanoglous et al., 2003).

Odor

Variety of malodorous compounds released under anaerobic conditions in biologicalprocess of wastewater treatment

The typical compound that cause bad odor is hydrogen sulfide Other compounds such

as indole, skatole and mercaptanes, in anaerobic conditions may cause odors that are muchmore offensive than that of hydrogen sulfide

Temperature

The measurement of temperature is very important because most wastewater treatmentfacilities include the biological step, a temperature-dependent process The temperature ofwastewater depends on season and location In cold regions, the temperature will vary from 7 to

180C, in warmer regions it will vary from 13 to 300C

Temperature is a fatal parameter in water because it affects the chemical reactions,reaction rates and aquatic life The change in temperature can be a significant factor for thesurvival of a means of fish species In addition, higher temperature means lower dissolvedoxygen in water The increase in biochemical reaction rate due to higher temperature probably

leads to the depletion of oxygen in water (Tchobanoglous et al., 1998).

Density, Specific Gravity and Specific Weight

The density of wastewater, ρw is defined as its mass per unit volume expressed as g/l orkg/m3 (SI) Density is an important parameter because it is needed for the design of sometreatment units such as sedimentation tanks, constructed wetland, etc

The electrical conductivity (EC) of a liquid is the ability of that liquid to conduct anelectrical current Because the electricity is transported by ions in solution, the measured value

of EC is used to determine the concentration of total dissolved solids

The electrical conductivity is expressed in SI units as millisiemens per meter (mS/m) 1.1.2.2 Inorganic Chemical Characteristics

The chemical constituents of wastewater can be classified as inorganic and organic Inthis section, the inorganic constituents are considered

(Tchobanoglous et al., 1998).

Nitrogen

Nitrogen and phosphorus are essential nutrients for biological growth or biostimulants.Because nitrogen is a building block in the synthesis of protein, sufficient nitrogen is required tomake wastewater treatable All kinds of nitrogen present in wastewater are ammonia, nitrite,

nitrate, and organic nitrogen Organic nitrogen corresponds to amina groups (Sperling, 2007).

Ammonia exists in aqueous solution in two forms the ammonium ion or ammonia gas,depending on the pH of solution as the following equilibrium reaction:

fish and other aquatic species In wastewater, the concentration of nitrite seldom exceeds 1mg/l

(Tchobanoglous et al., 1998).

Nitrate nitrogen, the highest oxidized form of nitrogen found in wastewater becomestoxic only under conditions in which it is reduced to nitrite Therefore it is important wheneffluent from wastewater treatment is used as recharge for ground water Nitrate can be a veryserious problem because it causes blue baby syndrome or methemoglobinemia in young infants

at a certain concentration Nitrate varies from 2 to 30 mg/l as N in wastewater effluents

(Tchobanoglous et al., 1998).

Phosphorus

Phosphorus is also an essential nutrient to the growth of biological organisms but theassimilation of phosphorus in water bodies causes the problem of eutrophication In effort toprevent the eutrophication of water bodies, phosphorus in domestic and industrial wastewaterand natural runoff is concerned

The usual forms of phosphorus found in aqueous solution are orthophosphate,polyphosphate and organic phosphate The orthophosphates (e.g PO43-, HPO42-, H2PO4-,

H3PO4 and HPO42- complexes) can be absorbed by organisms without any breakdown.Polyphosphates convert to orthophosphate by hydrolysis process but this process is normallyquite slow Organic phosphate can be an important constituent in industrial wastewater and

wastewater sludges (Tchobanoglous et al., 1998).

Alkalinity

Alkalinity of a solution is the ability of acid-neutralizing of that solution

Alkalinity in wastewater results from the presence of the hydroxides [OH-], carbonates[CO32-], and bicarbonates [HCO3-] of elements such as calcium, magnesium, sodium,potassium Of these elements, calcium and magnesium are the most common Borates, silicates,phosphates and some similar compounds can cause alkalinity but insignificantly; perhaps in

industrial or agricultural wastewater (Tchobanoglous et al., 1998).

Wastewater is normally alkaline; this alkalinity comes from water supply, thegroundwater and domestic use

For most practical purposes alkalinity can be defined in terms of molar quantities as:

[Alk], mole/l = [HCO3-] + [CO32-] + [OH-] - [H+]

In terms of equivalents:

[Alk], eq/m3 = meq/l = [HCO3-] + 2[CO32-] + [OH-] - [H+]

Sulfur

The sulfate ion occurs naturally in most water supplies and as a result is present inwastewater Sulfur is required in the synthesis of proteins and released during the digestion ofproteins Sulfate is reduced biologically to sulfide under anaerobic condition, and then sulfidecombines with hydrogen to form hydrogen sulfide

Hydrogen sulfide is concerned because of the oxidation to sulfuric acid which iscorrosive to concrete sewer pipes Hydrogen sulfide gas accumulated at the crown of the pipedue to the not flowing full of the sewer pipe can be oxidized biologically to sulfuric acid, which

is corrosive to concrete sewer pipes Also in digester H2S is corrosive to gas piping

(Tchobanoglous et al., 2003) Sulfates are reduced to sulfides in sludge digester can upset the process if the concentration exceeds 200 mg/l (Tchobanoglous et al., 1998).

Metals

All living organisms require an amount of metallic compounds (from micro to macro)such as iron, copper, zinc…for proper growth Though certain amounts of metals are necessary,the elevated concentration of them can be toxic to all kinds of creatures Therefore, metals are

always the concern in wastewater treatment (Tchobanoglous et al., 1998).

The sources of metals in wastewater include residential areas, groundwater infiltration,commercial and industrial discharges

The importance of metals are given in table 1.2

Table 1.2 Important metals in wastewater management a

a: From Tchobanoglous et al 1998

b: often identified as trace elements needed for biological growth

Henry’s law is the basis to consider the activity of dissolved gas in water Henry’s law

(More et al., 2008) expresses the relationship between gas pressure and solubility as follow:

Sg = kH * PgWhere: Sg: the solubility of the gas in the liquid

Pg: the pressure of the gas above the solution (or the partial pressure of the gas if thesolution is in contact with a mixture of gases)

kH: Henry constant (depends on temperature as well as characteristics of the gas and the liquid) [Pa m3/mol]

1.1.2.3 Organic Compounds Characteristics

Purposes of the analyses of aggregate organic constituents are to characterize untreatedand treated wastewater, to assess the performance of treatment process and to study receivingwaters In this section biochemical oxygen demand (BOD), chemical oxygen demand (COD),total organic carbon (TOC), oil and grease and surfactants are discussed

Biochemical Oxygen Demand (BOD)

The most used parameter of organic pollution applied to both wastewater and surfacewater is the 5-day BOD (BOD5) This determination involves the measurement of the dissolvedoxygen used by microorganisms in the biochemical oxidation of organic matter

The normal incubation time for BOD test is normally 5 or 7 days at 200C, but otherlength of time or temperature can be used

Chemical Oxygen Demand (COD)

“The COD test is used to measure the oxygen equivalent of the organic material in

wastewater that can be oxidized chemically by using dichromate in an acid solution”

(Tchobanoglous et al., 1998)

An advantage of COD test is that it can complete in 2.5 hours (compare to 5-day test ofBOD)

Total Organic Carbon (TOC)

The TOC test is used to determine the total organic carbon in a sample The TOC value

of a wastewater sample can be used to assess its pollutional characteristics or sometimes torelate TOC to BOD and COD TOC test is also in favor because it only takes 5-10 minutes toget the result If a reasonable relationship between TOC and BOD can be established in awastewater sample, use of TOC test is recommended

The relationship of BOD, COD and TOC

There are interrelationships between BOD/COD and BOD/TOC and they are shown intable 1.3 This relationship can be used to determine whether a wastewater sample can be

or greater, it can be treated biologically; if this ratio is below about 0.3, the sample needed to betreated before any biological process.

Table 1.3 Comparison of ratios of various parameters used to characterize wastewater a Type of wastewater

Untreated

After primary settling

Final effluent

a: From Tchobanoglous et al., 2003

b:CBOD / COD (CBOD: Carbonaceous Biological Oxygen Demand – the oxygen demand exerted by oxidizable carbon in the sample when the nitrification occurs.) c:CBOD / TOC

Oil and Grease

“The term oil and grease as commonly used, includes fats, oils, waxes and other related constituents found in wastewater.” (Tchobanoglous et al., 1998)

The sources of fats and oil contributed to domestic wastewater includes: butter,margarine, vegetable fats and oils Fats can be found in meats, in cereals, in seeds, in nuts and

in certain fruits (Tchobanoglous et al., 2003)

Oil and grease in liquid form may not appear to be harmful but as being cooled itbecomes solids and causes many problems It sticks to inner lining of drainage pipes andrestrains the flow leading to blockages in sewers These blockages can cause the flooding in

sewer and odor problems (Best Management Practice for Catering Outlets, Welsh Water – Water UK).

Surfactants

Surfactants are organic molecules that are composed of strongly hydrophobic (insoluble

in water) and hydrophilic group (soluble in water)

The presence of surfactants in wastewater results from household detergents, laundryindustries, and other cleaning operations Surfactants tend to collect at the air-water interfaceand can cause foaming in wastewater treatment facilities or at the surface of discharge receivingwater

1.1.2.4 Biological Characteristics

Biological characteristics of wastewater are in major importance not only because of thehygienic issues but also the significance of microorganisms in water and wastewater

treatment In this section, these following subjects will be discussed: (1) microorganisms found

in wastewater, (2) pathogenic organisms related to human diseases, and (3) the use of indicatororganisms

Microorganisms found in wastewater

The microorganisms found in wastewater can be classified as eukaryotes, eubacteria andarchaea Their cell structure, typical size, characterization and representative member areillustrated in table 1 (appendix)

Pathogenic Microorganisms

Pathogens found in wastewater may be discharged by human who are suffering withdiseases or who are carriers of a particular disease The pathogenic microorganisms found inwastewater can be classified into three broad categories: bacteria, parasites (protozoa andhelminths) and viruses

Bacteria

There are many types of harmless bacteria in human intestinal track and human feces.Pathogens are only found in the infected humans feces therefore wastewater contain bothpathogenic and nonpathogenic bacteria

One of the most common pathogenic organisms found in domestic wastewater is the

genus Salmonella The Salmonella group contains variety species causing diseases to human and animals Other bacteria isolated from raw wastewater which causes cholera is Vibrio

cholerae.

Protozoa

Because of their significant impact on individuals with compromised immune systemsincluding very young children, persons with cancer and individuals with AIDS,

Cryptosporidium parvum, Cyclospora and Giardia lamblia are the most concerning protozoan.

It is important to note that these protozoans are found in wastewater because conventionaldisinfection techniques (UV radiation or chlorine) have not proven their inactivation ordestruction

Helminths

The most important helminthic parasites probably found in wastewater are intestinalworms The infective stage of some helminths is adult organisms or larvae The eggs and larvaewith the size from 10 µm to more than 100 µm are resistant to environmental stresses

and survive normal disinfection procedures though eggs can be removed by common treatment

processes such as sedimentation, filtration (Tchobanoglous et al., 1998) Viruses

More than hundred types of enteric viruses capable of producing infection or disease arereleased in the fecal matter of infected humans Of the most important human enteric viruses,only Norwalk virus and rotavirus which cause diarrheal disease have been shown to be major

waterborne pathogens (Tchobanoglous et al., 1998).

Use of Indicator Organisms

The Coliform organism which is numerous and easily tested for is commonly used as anindicator organism Each person discharges from 100 to 400 billion Coliform bacteria withother kinds of bacteria per day Therefore, the presence of Coliform bacteria can be anindication that other pathogens may be present

Easy and common but the limitation of Coliform test is that it only indicates for thepresence of pathogenic bacteria and viruses, not for waterborne protozoa or pathogenicorganisms that may arise from nonhuman sources Therefore the use of new indicator

bacteriophages is much more concerned (Tchobanoglous et al., 1998).

1.1.3 Flowrates and Composition of Wastewater

The analysis of wastewater data involves the determination of the flowrate and massloading variations From the standpoint of treatment processes, average flowrates and average

BOD and TSS loadings are two of the most concerning parameter in design (Tchobanoglous et al., 2003) In this section, flowrates and composition of wastewater will be considered.

1.1.3.1 Wastewater Flowrates

The hydraulic design of both collection and treatment facilities is influenced byvariations in wastewater flowrates; therefore the flowrate characteristics have to be analyzedcarefully

Wastewater flowrates vary during the day, day of the week, season of the year or depend

on the sources of discharge to the collection system Short-term variations have diurnal patternwhich minimum flows occur during the early morning, the first and the second peak flowsoccur in late morning and early evening respectively Seasonal variations are normally observed

in small communities with college campuses and in communities which have seasonalcommercial and industrial activities Industrial variations are difficult to predict and

the most troublesome in smaller wastewater treatment plants where the loading capacity is

limited (Tchobanoglous et al., 2003).

The principal sources of domestic wastewater in a community are the residential areasand commercial districts Data on ranges and typical flowrate values from urban residential andcommercial sources in the United States are illustrated in table 1.4 and 1.5 as followings

Table 1.4 Typical wastewater flowrates from urban residential sources in the USA a

a: From Tchobanoglous et al., 2003.

Table 1.5 Typical wastewater flowrates from commercial sources in the USA a

Mobile home park

Motel (with kitchen)

Motel (without kitchen)

The principal factors responsible for variations of loading are (1) the established habits

of community residents which cause short-term variations, (2) seasonal changes which oftencause long-term variations and (3) industrial activities which cause both long and short-termvariations

Typical data on the composition of raw domestic wastewater found in wastewatercollection systems in the USA is presented in table 1.6 It should be noted that there is no

“typical” wastewater therefore the data in this table is only used as a guide The amounts ofwastewater discharged by individuals in countries can vary significantly because of

differences in culture and socioeconomic conditions The comparison between wastewater discharged by individuals in the USA and other countries is illustrated in table 1.7.

Table 1.6 Typical composition of untreated domestic wastewater a

Total organic carbon (TOC)

Chemical oxygen demand

Oil & grease

Volatile organic compounds

(VOCs)

Total Coliform

Fecal Coliform

Cryptosporidum oocysts

Giardia lamblia cysts

a: From Tchobanoglous et al., 2003

b: Low strength is based on an approximate wastewater flowrate of 750 L/capita.dMedium strength is based on an approximate wastewater flowrate of 460 L/capita.d High strength is based on an approximate wastewater flowrate of 240 L/capita.d

c: Values should be increased by amount of constituent present in domestic waster

21

Table 1.7 Typical wastewater constituent data for various countries a

a: From Tchobanoglous et al., 2003

1.2 Overview of the Urban Wastewater Management System

1.2.1 Components of Urban Wastewater Management System

Wastewater Management System includes three main components: (1) collection, (2)treatment and (3) disposal or reuse

The first step in any wastewater management system of a community is the collectionand conveyance of wastewater from various sources The pipes that collect and transport away

wastewater from its sources are called sewers and the network of sewers is a collection system

(George Tchobanoglous, 1981) Most sewers are placed underground to prevent interference due to repair of this system (Punmia and Jain, 1998) The types of collection systems will be

discussed later

Treatment is an essential step in a wastewater management system This step not onlyreduces the amount of pollutants coming into the environment but also protects humans from

pathogens (Punmia and Jain, 1998) The treatment of wastewater is carried out by combination

of many unit processes The methods of treatment are various, including mechanical, physical,chemical, biological methods or the combination of these ones such as physiochemical methodetc The variety of these methods will be considered in following sections

After treatment, water will be disposed or reused The disposal of treated effluent isrelated very closely to self-purification of water bodies Based on the selected receiving water

or effluent standard engineers will decide the degree of treatment and type of plant required.Treated wastewater can be discharged into lakes, rivers or the ocean The reuse of treatedeffluent can be applied for groundwater recharge, irrigation, etc These issues will be referredlater

1.2.2 Types of Wastewater Management System

There are two typical types of wastewater management system, including centralizedand decentralized model The former one is the traditional system and applied successfully inmany industrialized countries over decades However, the cost of investment andimplementation of this system is a big problem for any community Decentralized wastewatersystems in which wastewater are treated near the source of generation are getting more concern

as a potential alternative of traditional centralized wastewater management system These twowastewater management systems will be discussed as followings

Centralized Wastewater Management

Figure1.1 Representation of a Centralized Wastewater Collection and Treatment System

(Source: Wilderer and Schreff, 2000)

Centralized wastewater management is used to describe the system consisting of acollection system that collect all wastewater from households, industrial zones, smallenterprises, storm water runoff and convey to the treatment plant located very far or outside thecity or village boundary (fig 1.1) The treated wastewater which meets the standard will bedischarged to the closest receiving water The remaining part after eliminating pollutants from

wastewater (waste sludge in summary) will also be treated before any further use (Wilderer and Schreff, 2000).

Decentralized Wastewater Management

In contrast with the centralized model, the treatment plants in decentralized system areclose to the original sources of waste (fig 1.2) Also, the wastewater is transported by means ofpipes but the length of these sewers is shorter There will be some on-site treatment plants inwhich the wastewater and sludge treatment processes are executed The treated wastewater and

sludge are discharged to water bodies or reused for irrigation, toilet flushing, etc (Wilderer and Schreff, 2000).

Figure1.2 Representation of a Decentralized Wastewater Collection and Treatment System

(Source: Wilderer and Schreff, 2000) Advantages and disadvantages

The centralized wastewater management systems have achieved certain success indeveloped countries for a long time Sewage and stormwater are collected and transported out

of the urban area by system of sewers; receive advanced treatment and are controlled beforedischarge into receiving bodies Waste sludge is treated, utilized or disposed in proper way One

of achievements of this system is the reliable and efficient management and control of treatmentplants Besides, it is assumed that one large treatment plant is less expensive than many small

plants serving the same urban area, regarding both capital and operation costs (Lettinga et al., 2001).

Despite of undeniable advantages, centralized systems have many limitations First ofall, this system requires very long sewer pipes and as a result, the cost for constructing andmaintaining the sewers is very high According to the 2005 survey of DWA in 2003, Germanyspent about 1.6 billion euro on rehabilitation of sewer system which have the total length of

515,000 km and collect wastewater of nearly 82.5 million (Profile of the German water industry 2008) More over, the collection and treatment system is typically designed with the

capacity which is proposed to satisfy the upcoming population Before getting that point, thecapacity is far higher than actually required Consequently, the operation costs are high and theplants work under non-optimal conditions The high cost of construction leads to large amount

of investment to be spent within a short period, thus the pressure on local economy is very high

(Wilderer and Schreff, 2000) As respective to environmental aspects, water balance can be

affected in a negative way because water is taken from discrete location but treated effluent isdischarged into an area distant from origin It is also obvious that the combination ofwastewater and stormwater from various sources leads to a highly complex of pollutants thatfluctuates heavily in composition and concentrations; thus the efficient removal becomes more

difficult to be achieved (Lettinga et al., 2001).

Decentralized wastewater management systems exist in many parts around the world,mainly in rural areas As compare to centralized management system, the decentralized modelhas obvious advantages, including: (1) lifting stations and storage tanks to handle combinedsewage flow is not needed leading to the reduction in construction as well as operation andmaintenance cost, (2) more possibility of water reuse and groundwater recharge because itseems to be unfeasible to transport treated wastewater from the treatment plant to the place ofutilization in case of centralized system, (3) failures of single units will not cause the break

down of the whole system (Wilderer and Schreff, 2000), (4) more flexible in case of fast

population growth because the capacity is easily adjusted

The major concerns in the decentralized wastewater management system include: (1)effluent quality is low and rarely complied with the water reuse standard, (2) treatment plants

are not operated properly, (3) plants are difficult to control and supervise by water authorities

The current methods are primitive (e.g pit latrines) or low technology (e.g one, two or three

chamber septic tanks) (Lettinga et al., 2001) Normally the owners of the on-site treatment

facilities are responsible for the proper operation and maintenance of their own treatment

stations However, the fact is that almost no treatment facility owner has any in-depthknowledge of processes on which the systems work correctly and efficiently as well as has any

motivation to do that (Wilderer and Schreff, 2000).

Decentralized systems can be the solution for wastewater management in part ofindustrialized countries and particularly for developing countries, because of flexibility andsimple However, various problems as referred above needed to be solved before implementingthis system

1.3 Sub-processes of Wastewater Management System

1.3.1 Collection Systems

The function of a collection system in the urban wastewater management system is tocollect all wastewater from domestic and non-domestic sources as well as stormwater andconvey to treatment plants In this section, typical components of a collection system will beconsidered Also, types of sewers are referred

1.3.1.1 Typical Components of Collection System

Typical components of an urban drainage system include: building drainage, roofdrainage and main sewer networks Building drainage carries all kinds of wastewater to themain sewer and roof drainage conveys stormwater to the main sewer In this section, the main

components or “hardware” of any drainage system will be discussed (Butler et al., 2004).

Sewers Sewers are components of sewerage that carry flow from groups of properties

or larger areas to wastewater treatment plants or receiving bodies Sewers can be made ofvitrified clay, concrete, cement… depending on types of sewers There are three types of

sewers, including: sanitary sewer, stormwater sewer and combined sewer (Butler et al., 2004).

Manholes In sewer system, manholes are access points for testing, inspection and

blockage clearance Manholes are usually deep and can be entered if necessary Manholes areprovided at: (1) changes in direction, (2) heads of runs, (3) changes in gradient, (4) changes insize, (5) major conjunction with other sewers The diameter of manhole depends on the size of

sewer and the orientation and number of inlets (Butler et al., 2004).

Gully inlets The surface runoff enters the sewer via inlets called gullies Gully consists

of a grating and normally an underlying sump to collect heavy material in the flow Gully isconnected to sewer by a lateral pipe and water seal is incorporated in case of connecting with

the extent of surface ponding of runoff during storm events Gullies are always placed at lowpoints and typically along the road The simplest approach for the distance of gullies is 50 mspacing or per 200 m2 of imperious area (Butler et al., 2004).

Ventilation Ventilation is required in all urban drainage system but particularly in

sanitary and combined sewers It is to ensure the aerobic condition within the pipe and to avoidthe possibility of toxic and explosive gas build-up

(gravity sanitary sewer) or pressure/vacuum (pressure sanitary sewer) (Tchobanoglous, 1981).

Some basic considerations in design of sanitary sewers include: (1) design flows, (2)hydraulic design equation, (3) sewer pipes and materials, (4) minimum and maximum

velocities, (5) minimum slopes, (6) alternative design, (7) sewer appurtenances and (8) sewer

ventilation (Tchobanoglous, 1981).

b/ Storm-water sewer

The water sewer, as the name is intended to collect storm water Design of water sewer is similar to the one of sanitary sewer except some difference The easiest realizeddifference is that the storm-water sewer is designed to overflow periodically For instance, astorm-water sewer designed based on a 10 year rainfall frequency that means one storm every

storm-10 year will exceed the capacity of the sewer In contrast, the sanitary sewer system is designed

to prevent the surcharge because of high contents of pollutants If surcharge occurs it will bedue to an unexpected break down Another easily seen difference is the diameter of the pipe inthese two systems The pipe of a sanitary sewer system is many times smaller than of a storm-

water sewer, therefore only a small amount of excess infiltration can lead to overload (Hammer

The main role of a CSO is to take an inflow and divides it into two outflows in case ofhigh flow rates, one to the treatment plant and one to the receiving bodies The normal means ofachieving this is a weir If the surface of the flow passing through CSO is below the weir, flowcontinues to the treatment plants only When the surface is above the weir, some of flow passesthe weir and the rest flows to treatment plants Thus, hydraulic design of a CSO requires care toavoid premature overflow leading to an unnecessary volume of polluted flow discharged towater bodies or too much flow leading surcharging in the sewer system The other main role ofCSO is related to pollution A CSO will be designed to keep as much as possible the content of

fine suspended and dissolved materials in continuation flow to treatment plants (Butler et al., 2004).

In combined sewer, the design of CSO is the most concerning Basic considerationswhen considering this component include: diameter of inflow pipe, control of outflow, weirs,

chamber invert, design return period, top water level and human access (Butler et al., 2004).

1.3.2 Wastewater Treatment

Typical wastewater treatment consists of preliminary processes, primary settling toremove heavy solids and floatable materials, and secondary treatment, normally biologicalprocesses to metabolize and flocculate colloidal and dissolved organics Waste sludge drawnfrom these units is thickened and processed for ultimate disposal, usually either land application

or landfilling Tertiary treatment is an additional step that follows primary and secondarytreatment when these steps can not comply with the requirement The schematic of unitoperations and processes in a wastewater treatment plant is illustrated in figure 1.3

This section will discuss briefly about the following treatment steps: (1) preliminary andprimary treatment, (2) secondary treatment and (3) tertiary treatment Because of the

purpose to give an overview of wastewater treatment, these topics are introduced and discussedvery briefly in following sections.

Figure 1.3 Schematic of unit operations and processes in a wastewater treatment plant

(Source: Tchobanoglous et al., 2003)

1.3.2.1 Preliminary and Primary treatment

The purpose of preliminary steps is to remove wastewater constituents such as rags,sticks, floatables, grit, and grease that may cause maintenance or operation problems to the

treatment operations, processes and ancillary systems (Tchobanoglous et al., 2003).

Primary steps can remove a portion of suspended solids and organic matters from

wastewater (Tchobanoglous et al., 2003).

Most units in preliminary and primary treatment are physical ones Some of most commonlyused unit operations in preliminary and primary treatment of wastewater include: (1)screenings, (2) grit removal, (3) flow equalization, (4) sedimentation, (5) flotation

Screenings Screening is typically the first unit operation encountered in a wastewater

treatment plant Screenings are classified into three types depending on the size of removalsolids: (1) bar racks for coarse screens such as debris, leaves, paper, tree roots, plastic and rags,

(2) fine screens for removal of small particles such as undecomposed food waste, feces…and

(3)micro screens for removal of fine solids, floatable matter and algae (Tchobanoglous et al., 1998; Tchobanoglous et al., 2003).

Grit removal Normally, the location of grit chamber is after the bar racks and before

the primary sedimentation tanks (Ramalho, 1983).Grit is composed of sand, gravel, cinders, egg

shells, bone chips, or other heavy solid materials that have specific gravities greater than those

of organic putrescible solids in wastewater Quantities of grit varied in range of 0.0037-0.22

m3/103 m3 Particles identified as the cause of most problems in downstream treatment units aretypically 0.2 mm and larger There are three types of grit chamber including: (1) horizontal

flow, (2) aerated and (3) vortex type (Tchobanoglous et al., 1998).

Flow equalization Flow equalization is used (1) to smooth out individual wastewater

stream flows so that a mixing stream of relatively constant flow rate is fed to treatment plant,

(2) to equal variations of BOD concentrations in influents, (3) probably to neutralize thewastewater Flow equalization can be applied in different purposes, especially in small plantswhere experience high peak - to - average flow and organic loading ratios If the peak - to -average flow rate is 2 or less, the use of flow equalization is not economically feasible

(Tchobanoglous et al., 1998).

Sedimentation Primary sedimentation is responsible to remove readily settleable solids

and floating materials There are two types of sedimentation tanks including: rectangular tanksand circular tanks An efficiently designed and operated tank should remove

about 50-70% SS and 25-40% of BOD5. In small treatment plants, primary sedimentation is

normally omitted (Tchobanoglous, 1979).

Flotation In wastewater treatment, flotation is used principally to remove suspended

solids and to concentrate biological sludges The most advantage of flotation compare tosedimentation is that very small and light particles that settle very slowly can be removedcompletely and in shorter time The surface floated particles can be collected by a skimming

operation (Tchobanoglous, 1979).

1.3.2.2 Secondary treatment

Biological treatment, the typical secondary level in a municipal wastewater treatmentplant is discussed in this section The following subjects will be considered: (1) introduction ofmicroorganisms in biological treatment, (2) objectives of biological treatment and (3) types ofbiological processes in wastewater treatment

a/ Introduction of microorganisms

Microorganisms are the core components of a biological treatment system Their roles

as well as what they need to fulfill the treatment are following discussions

Role of microorganisms In wastewater, microorganisms convert the colloidal and

dissolved carbonaceous organic matter into various gases and cell tissue Because cell tissue has

a slightly greater specific density than water it can be removed from treated liquid by gravity

settling (Tchobanoglous, 1979).

Microorganism’s requirement To continue to produce and function properly

microorganisms need to have sources of energy (light, oxidation) and carbon (carbon dioxide,organic matter) to synthesize new cell tissue Certain inorganic elements such as nitrogen andphosphorus and trace elements such as sulfur, potassium, calcium, magnesium are also vital to

cell synthesis (Tchobanoglous, 1979).

Treatment processes Based on metabolic function, biological processes used in

wastewater treatment can be classified into following groups: (1) aerobic processes, (2) anoxicprocesses, (3) anaerobic processes, (4) a combination of the aerobic/anoxic or anaerobic

processes, (5) facultative processes (Tchobanoglous et al., 2003).

b/ Objectives of biological treatment

In wastewater treatment, the overall objectives of the biological processes include: (1) totransform dissolved and particulate biodegradable constituents into acceptable end products,

(2)to capture and incorporate suspended and non settable colloidal solid into a biological floc

or biofilm, (3) to transform or remove nutrients, such as nitrogen and phosphorus, and (4) in

some cases to remove specific trace organic constituents and compounds (Tchobanoglous et al., 2003).

c/ Types of Biological processes in wastewater treatment

The principal biological processes used in wastewater treatment can be classified intotwo main categories: (1) suspended growth processes and (2) attached growth processes

(Tchobanoglous et al., 2003) Typical process applications for suspended and attached growth

biological treatment processes are present in table 1.8

Table 1.8 Major biological treatment processes used for wastewater treatment a

33

Suspended growth processes

In suspended growth processes, the microorganisms are maintained in liquid suspension

by appropriate mixing methods Many suspended growth processes used for treatment ofmunicipal and industrial wastewater are operated in aerobic conditions However, suspendedgrowth anaerobic reactors are used for high concentration industrial wastewaters and organicsludges

The most common suspended growth process used for municipal wastewater treatment

is the activated sludge process (see figure 1.4) In the aeration tank of activated sludge process,influent wastewater is aerated and mixed with the microbial suspension by mechanicalequipment The mixed liquor then flows to a clarifier where the microbial suspension is settledand thickened The settled biomass, called activated sludge because active microorganisms arereturned to the aeration tank to continue the biodegradation of the influent organic matters Aportion of thickened solids is removed periodically to avoid the entering of excess biomass intothe system effluent An important characteristic of the activated sludge process is the formation

of floc particles (sizes of 50-200 µm), which can be removed greater than 99 percent byclarifier, leaving a relatively clear liquid as the treated effluent

Figure 1.4 Schematic of (a) plug flow and (b) complete mix activated sludge process

(Source: Tchobanoglous et al., 2003) Attached growth processes

In attached growth processes, the microorganisms responsible for biodegradation oforganic materials or nutrients are attached to an inert parking material The organic material andnutrients are removed from the wastewater flowing past the attached growth known as biofilm.There is a wide range of packing materials used in attached growth processes,

including: rock, gravel, slag, redwood, plastics and other synthetic materials Attached growthprocesses can be operated in aerobic or anaerobic conditions The packing materials can besubmerged completely in liquid or non-submerged with air or gas space above the biofilmliquid layer.

The most common aerobic attached growth process used is trickling filter (see figure 1.5

in which water is distributed over the top area of a column containing packing materials Rockwas the most commonly used in trickling filters, with typical depths ranging from 1.25 to 2 m.Most modern trickling filters vary in height from 5 to 10 m and are filled with plastic packingmaterials for biofilm attachment Air circulation in the void space between packing materialsprovides aerobic condition for microorganisms growing as an attached biofilm Influentwastewater is distributed over the packing materials and flows as a non-uniform liquid filmover the attached biofilm Excess biomass sloughs from the attached growth periodically andclarification is required for liquid/solids separation to provide an effluent with an acceptablesuspended solids concentration The solids are collected at the bottom of the clarifier andremoved for sludge processing

Figure 1.5 Schematic of trickling filter with (a) rock packing and (b) plastic packing

(Source: Tchobanoglous et al., 2003)

1.3.2.3 Tertiary Treatment

Advanced wastewater treatment is defined as the additional treatment needed to removesuspended, colloidal, and dissolved constituents remaining after conventional secondarytreatment Dissolved constituents may be relatively simple inorganic ions such as

calcium, potassium, sulfate, nitrate, and phosphate or highly complex synthetic organic

compounds (Tchobanoglous et al., 2003).

The need for advanced wastewater treatment is based on the consideration of one ormore of following factors: (1) the removal of organic matter and total suspended solids thatcan’t be accomplished by conventional secondary treatment processes to meet the stringentdischarge and reuse standards, (2) the removal of residual total suspended solids to conditionthe treated wastewater for more effective disinfection, (3) the removal of nutrients of treatedeffluent from convention secondary treatment to limit eutrophication of sensitive water bodies,

(4) the need to remove specific inorganic (e.g heavy metals) and organic constituents (e.g.MTBE) to meet the stringent discharge and reuse requirements, (5) the need to remove specificinorganic and organic constituents for industrial reuse (e.g cooling water, process water, etc.)

Filtration, adsorption, gas stripping, ion exchange, advanced oxidation processes,

distillation are some of common processes used in wastewater treatment

1.3.3 Sludge Treatment and Disposal

In wastewater treatment sludge comes from preliminary, primary and secondary steps.These sludges contain 95-99% water and probably pathogens from raw sewage The disposal ofsludge into sea has been forbidden and required treatment before any disposal in case of plants

which have size more than 2000 people equivalents (Forster, 2003).

As referred above, sludge comes from both preliminary and primary treatment stepssuch as screenings, grit removal, primary sedimentation as well as secondary treatment such asactivated sludge process, secondary sedimentation The types of solids are different fromvarious sources It should be noted that sludge also comes from processes used for thickening,

digesting, conditioning and filtering (Tchobanoglous, 1979).

To treat and dispose of sludges produced from wastewater treatment plants, it is verynecessary to know the physical, chemical as well as thermal characteristics of them Thecharacteristics vary depending on the origin of sludge, the amount of aging that has carried out,

and the processing type to which they have been subjected (Tchobanoglous, 1979).

Certain options which are now available for sludge treatment include: thickening,stabilization (chemical processes, heat treatment, aerobic digestion, anaerobic digestion),