Development of a Simplified Concept for Process Benchmarking of Urban Wastewater Management

LIST OF FIGURES AND TABLES 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 mo

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 3

LIST OF FIGURES AND TABLES 4

ACKNOWLEDGEMENTS 5

INTRODUCTION 6

CHAPTER I 8

Theoretical Foundations of Urban Wastewater Management System 8

1.1 Characteristics of Urban Wastewater 8

1.1.1 What is Urban Wastewater? 8

1.1.2 Constituents of Wastewater 8

1.2 Overview of the Urban Wastewater Management System 22

1.2.1 Components of Urban Wastewater Management System 22

1.2.2 Types of Wastewater Management System 23

1.3 Sub-processes of Wastewater Management System 26

1.3.1 Collection Systems 26

1.3.2 Wastewater Treatment 28

1.3.3 Sludge Treatment and Disposal 36

1.3.4 Effluent Disposal and Reuse 37

1.4 Current situation of Urban Wastewater Management in Vietnam 37

1.4.1 The Development of the Urban Drainage System 37

1.4.2 Current Structure and Operation of Urban Drainage Systems 38

1.4.3 The Organizations of Urban Drainage Services in Vietnam 39

1.4.4 Financial Aspects of Urban Drainage Companies 40

1.4.5 Legal and Institutional Frameworks 40

1.4.6 Investment and Management of Urban Drainage System 41

CHAPTER II 42

Benchmarking in the Urban Wastewater Management Sector 42

2.1 Fundamentals of Benchmarking 42

2.1.1 Definition of benchmarking 42

2.1.2 Types and elements of benchmarking 43

2.2 International Benchmarking System in Water Industry 46

2.2.1 Benchmarking of large Municipal Wastewater Treatment Plants in Austria 46

2.2.2 Benchmarking in Canada 48

2.2.3 North European Benchmarking Co-operation 49

2.3 Process Benchmarking in Wastewater Sector 52

2.3.1 What is Process Benchmarking? 52

2.3.2 The Objectives of Process Benchmarking 52

2.3.3 Methodology in Process benchmarking 53

2.3.4 Different Process Benchmarking Concepts 53

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 65

4.1 Approach of the Performance Assessment 65

4.1.1 Classification of various Undertakings 65

4.1.2 Performance Indicators 66

4.1.3 Confidence Grades 67

4.1.4 Structure of Questionnaire 67

4.2 Questionnaire of Wastewater Management System 68

4.3 Performance Indicators for Wastewater Management System 79

4.3.1 Environmental Impacts 79

4.3.2 Operation and Maintenance 80

4.3.3 Quality of services 86

4.3.4 Employees 89

4.3.5 Economic and financial aspects 90

4.4 Data Collection 92

CONCLUSIONS 93

REFERENCES 95

APPENDIX 99

Performance Indicators Population Equivalent Wastewater Treatment Plants

LIST OF FIGURES AND TABLES

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

This thesis has been developed in Dresden, Germany with the support of some people

to whom I would like to express my special thanks

I would like to thank Prof Nguyen Thi Diem Trang - Hanoi University of Science and Prof Bernd Bilitewski - Institute of Waste Management and Contaminated Site Treatment (IAA), Dresden University of Technology (TUD) as well as DAAD because of giving me the chance to do my thesis in Germany

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 help during 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 and encourage me to finish my thesis

INTRODUCTION

Wastewater Management is one of the most concerns in any urban area An efficient management contributes to the wealth of a community, never the less a poor management leads to unpredictable hazards related to health, environmental pollution, etc In developed countries, the issues of water and sanitation are solved, floodings are well controlled However, the issues of water supply and sanitation are not solved in developing countries, poor management of floodings as well as improper operation and maintenance of sewer systems are very popular Therefore, it is an urgent requirement to improve the system of wastewater management in developing countries Benchmarking is promised to be a solution to this problem as it is always the useful tool for improvement in management

Benchmarking was first time introduced by Xerox Company in the late 1970s when their peer company Fuji produced the photocopiers with better quality and lower prices Xerox was 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 to assess the performance of water and wastewater service providers, to estimate the quality of services as well as the satisfaction of customers These benchmarking projects have achieved initial success and are supposed to sustain Some systems of performance indicators have been developed with the purpose of large scale application such as the system International Water Association 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 out under the support of the World Bank

Aiming at developing a simplified concept for process benchmarking of urban wastewater management which can be applied in developing countries, performance indicators and questionnaire prepared for benchmarking in Vietnam, a representative of developing

countries are adapted in this thesis based on the International Water Association (IWA) system

of performance indicators for wastewater services There are four chapters in the thesis Chapter I considers the foundation of urban wastewater management in general and the current situation of wastewater management in urban areas of Vietnam In chapter II, fundamentals of benchmarking and process benchmarking for the water industry are discussed

To make clear the tool of performance assessment presented in the thesis the performance indicators for wastewater services 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 wastewater services of Vietnam; also, the questionnaire as well as the excel file to collect data from wastewater undertakings are presented

Benchmarking of wastewater utilities is emerging as an important tool of performance improvement by regular monitoring and analyses can be the solution to this reality It can play

a significant role in the sector as a tool for institutional strengthening Sustained benchmarking can help utilities in identifying performance gaps and gaining improvements by the sharing of information and best practices, ultimately resulting in better services to people It is expected that benchmarking in wastewater services in developing countries will soon be supported to implement

CHAPTER I Theoretical Foundations of Urban Wastewater Management System

In this chapter the theoretical foundations of urban wastewater management will be considered, including: (1) characteristics of urban wastewater, (2) overview of the urban wastewater 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 vary depending 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 through leaking joints, cracks and breaks, or porous walls Inflow is stormwater that enters the collection system from storm drain connections, roof leaders, foundation and basement drains, 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 pathogen organisms the most concerning ones are referred All wastewater treatment facilities are designed 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 colloidal ones Before any analysis of solids in wastewater the coarse material should be removed In wastewater 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

Total suspended solids Sludge deposits and anaerobic conditions

Biodegradable organics Depletion of natural oxygen resources and the

development of septic conditionsDissolved inorganics (e.g total

dissolved solids)

Inorganic constituents added by usage Recycling and reuse applications

Heavy metals Metallic constituents added by usage Many metals are

also classified as priority pollutants Nutrients Excessive growth of undesirable aquatic life,

eutrophication, nitrate contamination of drinking water

Priority organic pollutants Suspected carcinogenicity, mutagenicity, teratogenicity, or

high acute toxicity Many priority pollutants resist conventional treatment methods (known as refractory organics)

a: From Crites & Tchobanoglous, 1998

Particle Size Distribution

The determination of particle size is to understand more about nature of particles composing 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 organic compounds in wastewater

Transmittance/ Absorption

Transmittance is the ability of a liquid to transmit light of a specified wavelength through 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 biological process 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 much more offensive than that of hydrogen sulfide

Temperature

The measurement of temperature is very important because most wastewater treatment facilities include the biological step, a temperature-dependent process The temperature of wastewater 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 the survival of a means of fish species In addition, higher temperature means lower dissolved oxygen 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 or kg/m3 (SI) Density is an important parameter because it is needed for the design of some treatment units such as sedimentation tanks, constructed wetland, etc

Conductivity

The electrical conductivity (EC) of a liquid is the ability of that liquid to conduct an

electrical 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 In this 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

to make 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:

NH4+ + OH- ↔ NH3 + H2O

At pH levels above 9.3, ammonia gas is predominant; at level below 9.3 the ammonium ion is

major

Nitrite nitrogen is an unstable form, easily oxidized to nitrate form Although present

in wastewater at low concentration nitrite can be very important because it is toxic with almost

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 becomes toxic only under conditions in which it is reduced to nitrite Therefore it is important when effluent from wastewater treatment is used as recharge for ground water Nitrate can be a very serious 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 the assimilation of phosphorus in water bodies causes the problem of eutrophication In effort to prevent the eutrophication of water bodies, phosphorus in domestic and industrial wastewater and 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 normally quite 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, the groundwater 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 in wastewater Sulfur is required in the synthesis of proteins and released during the digestion of proteins Sulfate is reduced biologically to sulfide under anaerobic condition, and then sulfide combines with hydrogen to form hydrogen sulfide

Hydrogen sulfide is concerned because of the oxidation to sulfuric acid which is corrosive to concrete sewer pipes Hydrogen sulfide gas accumulated at the crown of the pipe due 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

Nutrients necessary for biological growth

Concentration thresholds of inhibitory effect

on heterotrophic organisms (mg/l)

Used to determine whether biosolids are suitable for land application

10c, 1d1.0 0.1

0.1 1.0

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 * Pg Where: Sg: the solubility of the gas in the liquid [mol/m3]

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

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 untreated

and treated wastewater, to assess the performance of treatment process and to study receiving waters 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 surface water is the 5-day BOD (BOD5) This determination involves the measurement of the dissolved oxygen 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 other

length 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 of BOD)

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 to relate TOC to BOD and COD TOC test is also in favor because it only takes 5-10 minutes to get the result If a reasonable relationship between TOC and BOD can be established in a wastewater 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 in table 1.3 This relationship can be used to determine whether a wastewater sample can be treated by biological process or not For example, if the BOD/COD ratio of wastewater is 0.5

or greater, it can be treated biologically; if this ratio is below about 0.3, the sample needed to

be treated before any biological process

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

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 it

becomes solids and causes many problems It sticks to inner lining of drainage pipes and

restrains 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, laundry

industries, and other cleaning operations Surfactants tend to collect at the air-water interface

and can cause foaming in wastewater treatment facilities or at the surface of discharge

receiving water

1.1.2.4 Biological Characteristics

Biological characteristics of wastewater are in major importance not only because of

the hygienic 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 indicator organisms

Microorganisms found in wastewater

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

Pathogenic Microorganisms

Pathogens found in wastewater may be discharged by human who are suffering with diseases or who are carriers of a particular disease The pathogenic microorganisms found in wastewater can be classified into three broad categories: bacteria, parasites (protozoa and helminths) 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 both pathogenic 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 systems including 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 conventional disinfection techniques (UV radiation or chlorine) have not proven their inactivation or destruction

Helminths

The most important helminthic parasites probably found in wastewater are intestinal worms The infective stage of some helminths is adult organisms or larvae The eggs and larvae with 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 are released 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

an indicator organism Each person discharges from 100 to 400 billion Coliform bacteria with other kinds of bacteria per day Therefore, the presence of Coliform bacteria can be an indication that other pathogens may be present

Easy and common but the limitation of Coliform test is that it only indicates for the presence of pathogenic bacteria and viruses, not for waterborne protozoa or pathogenic organisms 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 mass loading 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 by variations in wastewater flowrates; therefore the flowrate characteristics have to be analyzed carefully

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 pattern which minimum flows occur during the early morning, the first and the second peak flows occur in late morning and early evening respectively Seasonal variations are normally observed in small communities with college campuses and in communities which have seasonal commercial 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 areas

and commercial districts Data on ranges and typical flowrate values from urban residential

and commercial 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

Household size

no of persons

Flowrate, L/capita.d Range Typical

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

Industrial building

Restaurant

a: From Tchobanoglous et al., 2003

1.1.3.2 Composition of Wastewater

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 often

cause long-term variations and (3) industrial activities which cause both long and short-term

variations

Typical data on the composition of raw domestic wastewater found in wastewater

collection 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 of

wastewater 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

Contaminants Unit

Concentration b Low

strength

Medium strength

High strength

Chemical oxygen demand

Volatile organic compounds

Total Coliform No./100ml 106-108 107-109 107-1010

Fecal Coliform No./100ml 103-105 104-106 105-108

Cryptosporidum oocysts No./100ml 10-1-100 10-1-101 10-1-102

Giardia lamblia cysts No./100ml 10-1-101 10-1-102 10-1-103

a: From Tchobanoglous et al., 2003

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

Medium 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 supply

Table 1.7 Typical wastewater constituent data for various countries a

Country/

Constituent

BOD g/capita.d

TSS g/capita.d

TKN g/capita.d

NH 3 -N g/capita.d

Total P g/capita.d

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 collection

and 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 only

reduces 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 method etc The variety of these methods will be considered in following

sections

After treatment, water will be disposed or reused The disposal of treated effluent is related 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 treated effluent can be applied for groundwater recharge, irrigation, etc These issues will be referred later

1.2.2 Types of Wastewater Management System

There are two typical types of wastewater management system, including centralized and decentralized model The former one is the traditional system and applied successfully in many industrialized countries over decades However, the cost of investment and implementation of this system is a big problem for any community Decentralized wastewater systems 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 two wastewater 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 a collection system that collect all wastewater from households, industrial zones, small enterprises, storm water runoff and convey to the treatment plant located very far or outside the city or village boundary (fig 1.1) The treated wastewater which meets the standard will be discharged 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 are close to the original sources of waste (fig 1.2) Also, the wastewater is transported by means

of pipes but the length of these sewers is shorter There will be some on-site treatment plants

in which 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)

Advantages and disadvantages

The centralized wastewater management systems have achieved certain success in developed 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 before discharge 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 treatment plants 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)

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

(Source: Wilderer and Schreff, 2000)

Despite of undeniable advantages, centralized systems have many limitations First of all, this system requires very long sewer pipes and as a result, the cost for constructing and maintaining the sewers is very high According to the 2005 survey of DWA in 2003, Germany spent 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, the capacity is far higher than actually required Consequently, the operation costs are high and the plants 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 is discharged into an area distant from origin It is also obvious that the combination

of wastewater and stormwater from various sources leads to a highly complex of pollutants that fluctuates 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 model has obvious advantages, including: (1) lifting stations and storage tanks to handle combined sewage flow is not needed leading to the reduction in construction as well as operation and maintenance cost, (2) more possibility of water reuse and groundwater recharge because it seems to be unfeasible to transport treated wastewater from the treatment plant to the place of utilization 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-depth knowledge 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 of industrialized countries and particularly for developing countries, because of flexibility and simple However, various problems as referred above needed to be solved before implementing this 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 to collect all wastewater from domestic and non-domestic sources as well as stormwater and convey to treatment plants In this section, typical components of a collection system will be considered 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, roof drainage and main sewer networks Building drainage carries all kinds of wastewater to the main 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 of vitrified 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 are provided at: (1) changes in direction, (2) heads of runs, (3) changes in gradient, (4) changes in size, (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 is connected to sewer by a lateral pipe and water seal is incorporated in case of connecting with the combined sewer The size, number and spacing of gullies will determine

the extent of surface ponding of runoff during storm events Gullies are always placed at low points and typically along the road The simplest approach for the distance of gullies is 50 m spacing 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 avoid the 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 storm-water sewer, as the name is intended to collect storm water Design of storm-water sewer is similar to the one of sanitary sewer except some difference The easiest realized difference is that the storm-water sewer is designed to overflow periodically For instance, a storm-water sewer designed based on a 10 year rainfall frequency that means one storm every 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 be due to an unexpected break down Another easily seen difference is the diameter of the pipe in these 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 et al., 2008)

The procedures used to design the storm-water sewer are similar to that needed for the sanitary sewer system, excluding some differences in design flow, minimum velocities and

pipe materials and sizes (Tchobanoglous, 1981)

The main role of a CSO is to take an inflow and divides it into two outflows in case of high flow rates, one to the treatment plant and one to the receiving bodies The normal means

of achieving this is a weir If the surface of the flow passing through CSO is below the weir, flow continues to the treatment plants only When the surface is above the weir, some of flow passes the weir and the rest flows to treatment plants Thus, hydraulic design of a CSO requires care to avoid premature overflow leading to an unnecessary volume of polluted flow discharged to water bodies or too much flow leading surcharging in the sewer system The other main role of CSO 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 considerations when 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 to remove heavy solids and floatable materials, and secondary treatment, normally biological processes to metabolize and flocculate colloidal and dissolved organics Waste sludge drawn from 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 secondary treatment when these steps can not comply with the requirement The schematic of unit operations 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 and primary treatment, (2) secondary treatment and (3) tertiary treatment Because of the

purpose to give an overview of wastewater treatment, these topics are introduced and discussed very 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 commonly used 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 removal solids: (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 are typically 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 the wastewater Flow equalization can be applied in different purposes, especially in small plants where 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 tanks and 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 to sedimentation is that very small and light particles that settle very slowly can be removed completely 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 treatment plant is discussed in this section The following subjects will be considered: (1) introduction of microorganisms in biological treatment, (2) objectives of biological treatment and (3) types of biological 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)

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 and phosphorus 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) anoxic

processes, (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)

to transform 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 into two 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

Trickling filters/ activated sludge

Carbonaceous BOD removal, nitrification Carbonaceous BOD removal, nitrification Stabilization, carbonaceous BOD removal Carbonaceous BOD removal, nitrification Carbonaceous BOD removal, nitrification Carbonaceous BOD removal, nitrification Carbonaceous BOD removal, nitrification

Carbonaceous BOD removal

Stabilization, solid destruction, pathogen kill

Carbonaceous BOD removal, waste stabilization, denitrification

Carbonaceous BOD removal, especially high strength wastes

Carbonaceous BOD removal

Combined aerobic, anoxic, and anaerobic processes

Suspended growth

Hybrid

Single or multi stage processes, various proprietary processes Single or multi stage processes with packing for attached growth

Carbonaceous BOD removal, nitrification, denitrification, and phosphorus removal Carbonaceous BOD removal, nitrification,denitrification, and phosphorus removal

Anaerobic lagoons

Carbonaceous BOD removal Carbonaceous BOD removal, nitrification Carbonaceous BOD removal

Carbonaceous BOD removal, waste stabilization

a : From Tchobanoglous et al., 2003

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 of municipal and industrial wastewater are operated in aerobic conditions However, suspended growth anaerobic reactors are used for high concentration industrial wastewaters and organic sludges

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 mechanical equipment The mixed liquor then flows to a clarifier where the microbial suspension is settled and thickened The settled biomass, called activated sludge because active microorganisms are returned to the aeration tank to continue the biodegradation of the influent organic matters A portion of thickened solids is removed periodically to avoid the entering of excess biomass into the 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 by clarifier, leaving a relatively clear liquid as the treated effluent

(a) (b) 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 of organic materials or nutrients are attached to an inert parking material The organic material and nutrients 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 growth processes can be operated in aerobic or anaerobic conditions The packing materials can be submerged completely in liquid or non-submerged with air or gas space above the biofilm liquid 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 Rock was 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 packing materials for biofilm attachment Air circulation in the void space between packing materials provides aerobic condition for microorganisms growing as an attached biofilm Influent wastewater is distributed over the packing materials and flows as a non-uniform liquid film over the attached biofilm Excess biomass sloughs from the attached growth periodically and clarification is required for liquid/solids separation to provide an effluent with

an acceptable suspended solids concentration The solids are collected at the bottom of the clarifier and removed for sludge processing

(a) (b) 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 remove suspended, colloidal, and dissolved constituents remaining after conventional secondary treatment 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 or more of following factors: (1) the removal of organic matter and total suspended solids that can’t be accomplished by conventional secondary treatment processes to meet the stringent discharge and reuse standards, (2) the removal of residual total suspended solids to condition the treated wastewater for more effective disinfection, (3) the removal of nutrients of treated effluent 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 specific inorganic 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

of sludge 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 steps such as screenings, grit removal, primary sedimentation as well as secondary treatment such as activated sludge process, secondary sedimentation The types of solids are different from various 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 very necessary to know the physical, chemical as well as thermal characteristics of them The characteristics 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),

dewatering, composting and for the disposal can be: landfill or utilization for agricultural land

(Forster, 2003; Tchobanoglous, 1979)

1.3.4 Effluent Disposal and Reuse

A treatment plant is designed to accomplish as much removal of pollutants as probably required Treated effluents which meet quality criteria will be disposed of or reused

The basic principle of the effluent disposal and the regulation of pollution is to make the treatment plants do part of the work and to let nature complete it If this balance is used improperly the receiving water will be polluted Therefore the standard is set to ensure the safe disposal of treated effluents into the receiving water Depending on the local regulations there are different fundamental considerations applicable for setting standard but the basic ones are: (1) degradable organic matter, (2) bacterial content, (3) conservative pollutants, (4) nutrients and (5) temperature Appropriate treated effluents can be disposed into lakes, rivers, estuaries

and the ocean (Tchobanoglous, 1979)

A wide range of options for water reuse exists, including: (1) agricultural irrigation, (2) landscape irrigation, (3) industrial reuse, (4) recreational impoundments, (5) groundwater recharge, (6) habitat wetlands, (7) miscellaneous uses, (8) augmentation of potable supplies

Also the reuse of effluents needs standard to ensure the safety (Tchobanoglous et al., 1998)

1.4 Current situation of Urban Wastewater Management in Vietnam

1.4.1 The Development of the Urban Drainage System

In the period of 1858-1945, Vietnam started to build urban drainage systems Drainage systems were built by bricks and collected both wastewater and stormwater Collected

wastewater was discharged into lakes, canals or rivers (Trinh, 2007)

During the period of 1945-1975, sewerage systems in urban areas were expanded but without planning Sewers are mainly made of precast concrete and bricks and the covers are

broken because of no maintenance or destruction by bombs in the war (Trinh, 2007)

In the next fifteen years from 1975 to 1990, all concern was about the unification of the South and the North Thus, the government did not pay much attention on sewerage system After 1990, as the renovation started, the authorities were more interested in urban drainage system though as compare to water supply it received less priority From the early of

the 21st century the authorities have become more aware of significance of sewerages as a part

of urban infrastructure systems

1.4.2 Current Structure and Operation of Urban Drainage Systems

In this section, the current situation of urban drainage systems in Vietnam including collection systems and treatment systems will be discussed

Collection of wastewater

Existing sewer networks in towns of class IV and higher are combined systems, including precast concrete pipes, brick canals with concrete panel covers, open channels, ponds and stabilization pond systems Sewerage system was constructed in the past without a master plan of urban development; thus many sewers have smaller capacity than as required and without proper maintenance Most of sewers and canals were not designed to have self-

cleaning properties or not included ventilation to prevent odour in dry season (Trinh, 2007)

At present, there is no complete drainage system for collection of stormwater and

wastewater nor wastewater treatment plants in towns of class V (Trinh, 2007) The rate of

household with hygiene latrines is very low and the use of bucket toilets and open defecation

is very popular Double vault composting latrines are also operated but generally maintained improperly Some households have septic tanks but do not connect to public sewerage system; thus wastewater is discharged to small ditches or seeps into soil nearby People often throw rubbish directly to sewers, canals and ditches leading to blockages in sewers and flooding in rain season

The coverage of drainage services in urban arrears has not been investigated However, according to estimation of experts from the Department of Urban Infrastructure – The Ministry of Construction and Vietnam Drainage and Water Supply Association, the coverage

of drainage service is lower than that of water supply service The average coverage is

approximately 40-50% (from 1-2% in towns of class V to 70% in large towns) (Trinh, 2007)

The ratio of length in big towns is 0.2-0.5 m/person, but only 0.05-0.08 m/person in small

towns (Status Report, Water Sector Review Project, 2008) Many households served by septic

tanks but not connected to public sewerage systems; thus wastewater flows over to open ditches or seeps into the ground Some households which have water pour toilets discharge

wastewater directly into the public sewerage systems without any preliminary treatment (Trinh,

2007)