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Effectiveness Of Biologigal Wastewater Treatment Environmental Sciences Essay

Paper Type: Free Essay Subject: Environmental Sciences
Wordcount: 5349 words Published: 1st Jan 2015

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Wastewater treatment is a serious environmental concern due to the hazards of discharging poorly treated effluent to the environment. Poor wastewater treatment poses a pollution threat to receiving water bodies, groundwater pollution, soil contamination and resulting loss of biodiversity (Mantila, 2002).

Dandora Estate Sewerage Treatment Works treats on average 62,000m3 per day annually of wastewater from Nairobi city and its environs through biological treatment and will form the study area. The population targeted in this study is wastewater received and treated at DESTW.

The purpose of this study is to find out the effectiveness of biological wastewater treatment and the pollution potential of DESTW activities to the environment.

An experimental research design will be used to determine the wastewater characteristics and contaminant removal while a descriptive design will be used to determine the environmental implications of wastewater treatment.

The instruments used in the study are observation, laboratory experiments, leopold matrix, network analysis, and impact characteristic analysis.

Data analysis will be done using both inferential and descriptive statistics.

Wastewater treatment has been defined as the process of removing contaminants from wastewater produced by both domestic and industrial sources (Tchobanglous, 1993). Its objective is to produce treated effluent and sludge suitable for discharge or reuse back into the environment which is achieved through physical, chemical and biological processes.

The issue of wastewater treatment and disposal assumed increasing importance in the early 1970s as a result of the general concern expressed in the United States and worldwide about the wider problem of pollution of the human environment, the contamination of the atmosphere, rivers, lakes, oceans, and groundwater by domestic, municipal, agricultural, and industrial waste (Oswald, 1996)

A great deal of wastewater treatment plants are scattered all over the world and until recently not much scientific attention was given to these plants. They were considered to solve local problems so specific that one did not want to think it worthwhile to discuss design and operation of them in international fora.

However, the interest shown for the 1st International Specialized Conference on Design and Operation of Wastewater Treatment Plants (Trondheim, 1989), and the IAWQ Specialist Group on the same subject (formed in 1991), demonstrated that there is a need to discussion on international scale the strategies for planning and the technical development of such plants.

The reason for this interest must be found in the abundance of cases around the world where small wastewater treatment plants have to be put in operation to prevent environmental pollution and hazards.

There is a global shift from the traditional centralized wastewater treatment system to locally based wastewater solutions (Hallvard, 1993) following the UN Decade for Water and Sanitation recommendations. The need for good solutions for wastewater treatment plants is therefore crucial in many developing countries.

Developed countries mainly use mechanical and chemical treatment processes which though requiring less land are very expensive to establish and maintain.

Alabaster (1994) cites that many developing countries favour the use of biological treatment which uses wastewater stabilization ponds since climate favours its operation and it is a low-cost, low-maintenance, highly efficient and natural method of wastewater treatment.

The Dandora Sewerage Treatment Works (DESTW) which treats wastewater from Nairobi city and its environs uses biological treatment. However, due to stricter discharge standards set by National Environmental Management Authority (NEMA), DESTW is increasingly falling short of those standards.

Parr and Horan (1994) highlight three principal reasons for wastewater treatment plants failure: a lack of technical knowledge, failure to consider all relevant local factors at pre-design stage and inappropriate discharge standards.

Mara (1992) cites the following broad impacts to the environment due to poorly treated effluent: pollution of receiving aquatic water body, groundwater pollution from seepage of effluent, soil pollution from dumping sludge and health impacts from drinking contaminated water or food grown by the same water.

1.2 Problem Statement

The problem under investigation in this study is the effectiveness of biological treatment in removing contaminants from wastewater and pollution potential of DESTW activities.

Factors making the problem a critical issue to warrant research are: the physical treatment unit at DESTW has not been operational for the past four years; all pond series apart from series 3 and 5 lack anaerobic ponds; closure of series 8 due to water hyacinth infestation may overload series 7; lack of pretreatment facilities in many industries that discharge into the Nairobi city sewer network may reduce treatment effectiveness; and the environmental implications of groundwater pollution by effluent seepage and soil pollution by dumping of toxic sludge.

Purpose of the Study

Based on the problem stated the purpose of this study is to investigate the effectiveness of biological treatment at removing contaminants from wastewater through empirical method of inquiry and propose sustainable methods of improving treatment effectiveness at DESTW.

This study also aims at identifying the potential impacts to the environment resulting from DESTW activities and proposes methods of mitigating negative impacts based on findings.

1.4 Objectives of the Study

The objectives of this study are as follows:

To analyze the composition of wastewater received at DESTW

To analyze the effectiveness of contaminants mass removal at DESTW

To determine the pollution potential in relation to activities of DESTW

To identify alternative uses of treated effluent

1.5 Hypothesis

There is a positive relationship between the functioning of biological treatment and the quality of effluent at DESTW.

1.6 Significance and Justification of the Study

This study addresses gaps in knowledge that exist in biological treatment effectiveness in treating wastewater from Nairobi, sewage effluent has long been cited as the cause of Nairobi River pollution, this study will quantify the extent to which effluent from DESTW pollutes the river.

By addressing the above gaps in knowledge, the study will add to the body of knowledge in the field of wastewater treatment in Kenya.

This study is important since the results will influence future environmental policies on wastewater management, recommendations will propose sustainable methods suitable for Kenya of further treating the effluent to ensure compliance with discharge standards, and they will also propose methods on improving existing methods of treating wastewater e.g. by harvesting methane gas from anaerobic ponds to provide electricity for running the physical treatment works.

The findings and recommendations will mitigate negative impacts to the environment as a result of DESTW activities.

Beneficiaries from findings of this study are the community surrounding DESTW who will enjoy cleaner groundwater resources and decrease health risks from eating vegetables grown by effluent or eating fish caught from oxidation ponds.

Downstream users of R. Nairobi will enjoy cleaner river water which will decrease prevalence of waterborne diseases.

DESTW will benefit from this study’s recommendations by increased environmental compliance and they will also cut down on operational costs through generating electricity from anaerobic ponds methane gas.

Researchers will benefit from this study’s findings which will form background information and methodology reference for future related studies.

Policy makers will use the findings and recommendations of this study in formulating policies for wastewater management in Kenya.

1.7 Limitations and Assumptions


Length of the study was limited to 3 months from January to March 2008 where data was to be collected. To overcome this limitation, data for previous years was obtained from the DESTW database.

Breakdown of some laboratory machines hindered analysis of samples e.g. water distiller breakdown prevented analysis on some days due to lack of distilled water.

Lack of a permanent vehicle at DESTW prevented final effluent sampling on some days.


It is assumed that the reagents were not contaminated.

It is assumed that the measuring equipments were calibrated properly.

It is assumed that sampling and storage cans were kept clean to prevent sample contamination.

1.8 Study Area

This study will be carried out at the Dandora Estate Sewerage Treatment Works (DESTW) which treats wastewater from Nairobi city and its environs using biological treatment process. The study area was chosen since it forms a representative sample of Nairobi city wastewaters.


The first phase was completed in 1977 and commissioned on 1978. The second phase was completed in 1990 and commissioned on 1992.


DESTW is located at Ruai in Embakasi division approximately 30km from the city center and about 3km off Kangundo road. Access to the plant is on a permanent earth road. The site is approximately 1000ha and the oxidation ponds are on 200ha.


The climate is a typical Nairobi climate with temperature ranging between 15-30 degrees centigrade. The average rainfall is approximately 760mm with the most of the rains falling in two seasons, March to May (long rains) and October to December (short rains).

Geology, soils and topography

The geology of the area mainly comprise of Nairobi volcanics covered by black cotton clay soils. The area is generally flat with Nairobi River forming the north Eastern boundary of the land.

Flora and fauna

The area is generally arid with scanty vegetation cover, mainly sisal and shrubs. The ponds have attracted crocodiles and hippos from the nearby Nairobi River since they provide habitat and cheap source of food to for fauna and flora. Large colonies of different species of birds such as birds of prey (e.g., buzzard, golden eagle, and barn-owl), garden and woodland birds (e.g., pigeon, crow, and sparrow) water-birds and sea-birds (e.g., heron, swans, kingfisher, and curlew), and game birds such as quail hovered around the stabilization ponds during the day. Mudfish and tilapia fish have also been introduced in the maturation ponds to assist in quality monitoring.

Number of ponds and arrangement

There are a total of 38 waste stabilization ponds at DESTW which occur in 8 series. Facultative and maturation (aerobic) ponds run in parallel. Only series 3 and 5 have anaerobic ponds.

Types of ponds

There are three types of ponds at DESTW and these are:

Anaerobic ponds- they are 4.0m deep and measure 100m by 100m. They are deigned for organic matter removal e.g. helminth eggs.

Facultative ponds – they are 2.5 m deep and measure 700m by 300m. They are designed for BOD5 removal.

Maturation ponds- they are 1.5m deep and measure m by m. They are designed for nitrogen and phosphorus removal.

Pretreatment and flow measurement facilities

DESTW has a conventional inlet works where large suspended solids are screened by coarse bar screens before being automatically raked by cup screens. Grit is removed by use of constant velocity grit traps.

A venturi flume is provided for flow measurement.


2.1 Nature of Wastewater

2.1.1 Origin and Quantity

Wastewater originates mainly from domestic, industrial, groundwater, and meteorological sources and these forms of wastewater are commonly referred to as domestic sewage, industrial waste, infiltration, and storm-water drainage, respectively(Mara, 1997).

Domestic sewage results from people’s day-to-day activities, such as bathing, body elimination, food preparation, and recreation, averaging about 90 liters per person daily in Kenya (Asano, 1998). The quantity and character of industrial wastewater is highly varied, depending on the type of industry, the management of its water usage, and the degree of treatment the wastewater receives before it is discharged.

A typical metropolitan area discharges a volume of wastewater equal to about 60 to 80 percent of its total daily water requirements, the rest being used for washing cars and watering lawns, and for manufacturing processes such as food canning and bottling (WHO, 1992).

2.1.2 Composition

The composition of wastewater is analyzed using several physical, chemical, and biological measurements. The most common analyses include the measurements of solids, biochemical oxygen demand (BOD5), chemical oxygen demand (COD), and pH (Pena, 2002). The solid wastes include dissolved and suspended solids. Dissolved solids are the materials that will pass through a filter paper, and suspended solids are those that do not.

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The concentration of organic matter is measured by the BOD5 and COD analyses. The BOD5 is the amount of oxygen used over a five-day period by microorganisms as they decompose the organic matter in sewage at a temperature of 20° C. Similarly, the COD is the amount of oxygen required to oxidize the organic matter by use of dichromate in an acid solution and to convert it to carbon dioxide and water. The value of COD is always higher than that of BOD 5 because many organic substances can be oxidized chemically but cannot oxidize biologically (Curtis, 1992) .

Commonly, BOD5 is used to test the strength of untreated and treated municipal and biodegradable industrial wastewaters. COD is used to test the strength of wastewater that is either not biodegradable or contains compounds that inhibit activities of microorganisms.

The pH analysis is a measure of the acidity of a wastewater sample.

2.2 Biological Wastewater Treatment

2.2.1 Waste Stabilization Ponds Technology Overview

Waste stabilization ponds (WSPs) are usually the most appropriate method of domestic and municipal wastewater treatment in developing countries, where the climate is most favourable for their operation WSPs are low-cost (usually least-cost), low-maintenance, highly efficient, entirely natural and highly sustainable (Alabaster, 1994). The only energy they use is direct solar energy, so they do not need any electromechanical equipment, saving expenditure on electricity and more skilled operation. They do require much more land than conventional electromechanical treatment processes such as activated sludge – but land is an asset which increases in value with time, whereas money spent on electricity for the operation of electromechanical systems is gone forever).

WSP systems comprise one or more series of different types of ponds. Usually the first pond in the series is an anaerobic pond, and the second is a facultative pond. These may need to be followed by maturation ponds, but this depends on the required final effluent quality – which in turn depends on what is to be done with the effluent: used for restricted or unrestricted irrigation; used for fish or aquatic vegetable culture; or discharged into surface water or groundwater (Horan, 1994).

Prior to treatment in the WSPs, the wastewater is first subjected to preliminary treatment −screening and grit removal − to remove large and heavy solids.

Basically, primary treatment is carried out in anaerobic ponds, secondary treatment in facultative ponds, and tertiary treatment in maturation ponds. Anaerobic and facultative ponds are for the removal of organic matter (normally expressed as “biochemical oxygen demand” or BOD), Vibrio cholerae and helminth eggs; and maturation ponds for the removal of faecal viruses (especially rotavirus, astrovirus and norovirus), faecal bacteria (for example, Salmonella spp., Shigella spp., Campylobacter spp. and pathogenic strains of Escherichia coli), and nutrients (nitrogen and phosphorus). Due to their high removal of excreted pathogens, WSPs produce effluents that are very suitable for reuse in agriculture and aquaculture.

2.2.2 Related Research on Biological Wastewater Treatment

Mandi (1993) in his comparative study of “Wastewater treatment by stabilization ponds with and without macrophytes under arid climate” found that ponds using water hyacinth proved most efficient than those using microphytic plants (algae). Howver, the process based on water hyacinth for wastewater purification is faced with two major problems: first the water loss by evapotranspiration reaches 60% during summer time and secondly the development of mosquito during summer time.

He however does not address the huge quantities of biomass produced from water hyacinth treatment systems and the resulting increase in sludge deposition rate.

Ghrabi (1989) in his experimental study “Treatment of wastewater by stabilization ponds application to Tunisian conditions” concluded that sediment accumulation occurs mainly in the first pond: the deposition rate is high (5 cm/year). In the maturation ponds, it ranges from 1.3 cm/year to 1.6 cm/year. The first pond can be desludged yearly or once each two years.

He however in his study doesn’t mention the environmental impacts of sludge to the soil and he also doesn’t suggest methods of decreasing the amounts reaching the wastewater stabilization ponds.

Jensen (1992) in his study on the “Potential use of constructed wetlands for wastewater treatment in Northern environments” concluded that wetlands achieve 98% phosphorus removal, 88% BOD removal and 55% nitrogen removal respectively. COD removal was only 64% due to discharge of organic matter that is slowly biodegradable e.g. humic acids.

This study however didn’t estimate the productive lifespan of the constructed wetlands.

2.3 Problems in Wastewater Treatment and Disposal

2.3.1 Wastewater Treatment Plant Problems

Many wastewater treatment plants (WwTP) of all kinds in developing countries do not function properly. Parr and Horan (1994) found that there are three principal reasons for WwTP failure: a lack of technical knowledge; failure to consider all relevant local factors at the pre-design stage; and inappropriate discharge standards. As a result, wrong decisions are often made and inappropriate unsustainable treatment processes are selected and implemented. This is then exacerbated by the absence of any real incentive to operate the WwTP correctly once it has been commissioned. It is therefore essential for the long-term sustainability of WwTP that simple efficient technologies such as WSPs are always considered at the pre-design (or feasibility) stage. An honest comparison of the cost-effectiveness of wastewater treatment technologies will almost always favour the selection of WSPs in warm-climate countries.

2.3.2 Environmental Problems of Wastewater Treatment and Disposal

If wastewater is discharged before it is properly treated, it can adversely affect the environment, public health and destination’s economic well-being. The cost of these negative impacts can be expressed in monetary, health and ecological terms (Mara, 1997).

Mantila (2002) identifies a number of consequences of poorly treated wastewater:

Health Impacts from pathogenic bacteria, viruses and toxic algae cause diarrhoea, shellfish poisoning and other diseases; bathing in polluted water causes gastroenteritis and upper respiratory diseases; eating polluted shellfish results in hepatitis, liver damage and in some cases death.

Impact on Marine Environment in the form of suspended solids may cause excessive turbidity and shading of sea grasses, produce sedimentation, damaging benthic (bottom layer) habitats and affect anaerobic conditions at the sea bottom; high BOD levels may cause severe oxygen depletion especially in shallow and enclosed aquatic systems such as estuaries that are ideal breeding grounds for various marine species resulting in fish deaths and anaerobic conditions which release bad odors(hydrogen sulfide); adverse nutrient levels cause algal blooms, resulting in the death of coral and sea grasses and eutrophication leading to severe oxygen depletion which kills living resources; many toxic materials and suspected carcinogens and mutagens can concentrate in fish tissue, putting humans at risk when they eat them; metals in specific forms can be toxic to humans and various marine organisms especially shellfish which is vulnerable, in areas with highly contaminated sediment layers; fats, oil and grease that float on the water surface interfere with natural aeration, are possibly toxic to aquatic life, destroy coastal vegetation and reduce recreational use of waters and beaches.

Impact on Groundwater and Water Resources in the form of improper disposal of wastewater can directly impact the quality of an area’s groundwater and water resources and since their movements are dynamic, contaminants can spread far beyond the immediate pollution area.

2.4 Wastewater Management Options

Oswald (1995) states that the following issues should be addressed before designing an effective wastewater management plan: assess current wastewater management practice before water is discharged to the municipal treatment facility, identification of sources of wastewater, determine whether discharged wastewater quality meets effluent standards, identify whether industries carry out pre-tretment of their wastewater and finally assessing complaints from users of reclaimed wastewater effluent. Once the situation has been assessed, a range of approaches and techniques to deal with wastewater can be considered.

Bartone (1996) argues that to ensure effective treatment o wastewater, the volume has to be reduced to prevent overloading of wastewater treatment plants and this can only be achieved at the source through installation of water efficiency equipment e.g. ultra-low flush toilets, spray nozzles, low-flow showerheads, water spigots, all which reduce overall water consumption.

Collection of domestic wastewater and transportation to a distant treatment plant is a difficult and highly expensive task, if the catchment area to be served is low in population density (Tchobanoglous, 1993). Onsite treatment of sewage is the alternative and has been applied al around the world for many centuries.

However, purification achieved by traditional onsite treatment systems such as septic tanks (DIN, 1993) is rather poor especially with respect to nutrient removal and as a result impacts on the quality of groundwater are inevitable.

The basic idea of the biofilter septic tank was introduced by Toshio Yahata (1981) and further developed by Stubner and Sekoulov (1987). The biofilm reactor septic tank has been found to be more efficient (Robert, 1996) and effluent can be reused for irrigating or flushing toilets.

2.5 Conceptual Framework

This study is based on the conceptual framework below that aims at optimal use of resources in an environmentally sustainable manner.



The main sources of generation are households, commercial and industrial sources.

This is done through the sewer network in Nairobi and conveyed to DESTW. An annual average of 62000 m3 wastewater reaches DESTW daily

It aims at screening solids and grit removal from wastewater stream.

Coarse bar screens- remove large suspended solids

Medium bar screens – remove smaller suspended solids

Cup screens- remove finer suspended solids

Grit traps- remove grit and sand particles from wastewater

Involves use of wastewater stabilization ponds

Anaerobic ponds – are designed for organic matter removal

Facultative ponds- are designed for BOD removal

Maturation ponds- designed for nitrogen and phosphorus removal

Treated effluent disposed of in Nairobi River

Effluent reused for agricultural irrigation and livestock watering.

Fig 1: Conceptual framework for wastewater treatment and disposal in Nairobi.(Adapted from WHO,1992)


3.1 Research Design

The design used in this research is experimental since analysis of wastewater quality is done in the laboratory.

It is also descriptive since the state of the environment and biological treatment process are described.

The approach used in this study is deductive since it begins with the perceptual experience and observation of an environmental problem, leads to hypothesis formulation, experimental design, data collection, statistical analysis, theory construction, and finally to explanation.

3.2 Population and Sample


The population targeted in this study is the wastewater received and treated at DESTW which averages 62,000m3 per day annually.

Sample types

Grab samples were necessary for parameters such as pH, ammonia, and faecal indicator bacteria.

Flow-weighted composite samples were necessary for raw sewage parameters such as electrical conductivity, dissolved oxygen,

Frequency of sampling

Raw sewage was sampled hourly because its composition varies considerably throughout the day.

Flow was sampled hourly throughout the day.

Final effluents were sampled once daily before noon.

Pond series were sampled once every week.

Nairobi River upstream and downstream was sampled once a week.

Data Collection Instruments

3.3.1 Field Observation

Environmental impacts will be identified using field observation which will be aided by the following instruments

a) Leopold matrix

It is a grid-like table that is used to identify the interaction between project activities, which are displayed along one axis, and environmental characteristics, which are displayed along the other axis. Using the table, environment-activity interactions can be noted in the appropriate cells or intersecting points in the grid. ‘Entries’ are made in the cells to highlight impact severity or other features related to the nature of the impact, e.g. numbers in this study are used to indicate scale in this study.

This instrument was chosen for environmental impact identification because it links the action to the impact, shows impact magnitude and significance, and is a good way of displaying environmental impact results.

b) Network analysis

Networks illustrate the cause-effect relationship of project activities and environmental characteristics. They are, therefore, particularly useful in identifying and depicting secondary impacts (indirect, cumulative, etc). They are drawn by identifying first order impacts first then linking them to second order impacts and third order impacts by use of an arrow.

This instrument was chosen for environmental impact identification since it links the actions to the impacts, is useful I simplified form for checking for second order impacts and can handle direct and indirect impacts.

c) Impact characteristics analysis

It is normally in the form of a summary table and this instrument was chosen for environmental impact identification because it shows impact nature, magnitude, extent/location, timing, duration, reversibility, likelihood (risk), and significance.

3.3.2 Laboratory experiments

Experiments were performed to determine the composition of wastewater at DESTW and the mass removal of contaminants from the wastewater. The apparatus below will be used during the laboratory experiments:

Plastic sampling cans were used to collect and store samples.

A wooden pole with 1cm graduations was used to measure depth at the venturi flume.

A refrigerator was used to store samples at below 4 degrees Celsius.

Burettes, conical flasks, pipettes, beakers, and digestion tubes were used to hold samples and reagents when analyzing for various parameters in the laboratory.

Ovens, digestion blocks, water baths, and fume chambers were used in creating conducive conditions for chemical reactions to take place in the laboratory.

Pan balances, beam balances, UV spectrophotometers, atomic absorption spectrophotometers, water quality meters and flame photometers were used to measure values of various parameters in the samples.

3.4 Data Collection Procedures

3.4.1 Laboratory Analysis Procedures

Parameters will be analyzed according to Alabaster’s 1989 “Practical Guide to the Monitoring of Waste Stabilization Ponds” standard operations manual that was adopted by the DESTW laboratory.

a) Flow

This will be measured on the raw sewage and final effluents using the venturi flume which is a restriction in the channel carrying wastewater. The formula below was used to calculate flow.

Q =2∕3 √2/3 g CV.CD . b. h3/2

Where Q= flowrate m3/s CV = coefficient of velocity

CD = coefficient of discharge b = width of throat (m)

h = upstream depth (m)

b) COD total and filtered

The micro-digestion sealed tube method will be used with potassium dichromate as digestion solution and ferrous ammonium sulphate as titration solution.


1.5 ml of digestion solution is dispensed into a digestion tube, 2.5 ml of sample is added using a pipette and mixed well, 3.5 ml of catalyst solution (silver sulphate in 2.5 liters of sulphuric acid ) is added, the tube is capped tightly using a PTFE sealing gasket, the tubes contents are then mixed by gentle swirling, the tubes are then placed in a digestion block at 1500 C for 120 minutes, contents of the tube are transferred quantitavely to 100ml conical flask and sufficient water added to a final volume around 25 ml , 1 drop of ferroin indicator is added and the solution mixed well, it is titrated with FAS (N/40) until the faint blue colour changes to red and the value of the titre T ml recorded, a blank titration is carried out following the same procedure but using distilled water instead and the value of blank titre B ml recorded.

COD calculated as follows: COD = (B-T) / S Ã- 1000 mg/l

c) BOD total

The standard 5 days, 20 0C, BOD bottle test will be used.


Dilution water, ferric chloride solution, manganous sulphate solution, sodium azide solution, alkali- iodide solution, 90 % orthophosphoric acid, N/40 sodium thiosulphate, starch solution.


Dilution water is prepared, sample added and incubated at 200C for 5 days to determine dissolved oxygen, remove stopper from the BOD bottle and 2ml each of manganous sulphate solution, sodium azide solution, alkali- iodide solution, immediately after the addition of alkali-iodide reagent a brown flocculent precipitate forms therefore the bottle is shaken to ensure that all the dissolved oxygen reacts with the reagents, when the floc settles add 2ml orthophosphoric acid and shaken until the bottle contents turn yellow, 205 ml of the bottle contents is titrated with N/40 sodium thiosulp


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