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Department of Environmental Engineering

Sitemap Index. General Imprint:. Wali, U. The Open Environmental Engineering Journal , , 4, p, refereed. And Kimwaga, R.


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Nhapi I. Research Journal of Soil and Water Management 2 2 : p refereed. Nakibuuka, M. G and Kimwaga, R. Gakwavu, R. Solms, Applied Ecology and Environmental Research 10 4 : refereed. Kibena, J. Chisadza, B. Bola, G. Chinyama A.

Muserere, S. Dube, F. Chisadza B. Disaster Prevention and Management , Vol. Muhonda, P. Mashauri, D. Mathew, K and Nhapi, I. Jonker, L. The lake was created in and is located 35 km south-west and downstream of Harare. Overloading and maintenance problems plague all the plants and the quality of the final effluent flowing into rivers is very poor.

Many of these researchers report that wastewater is the major problem. Research focusing on Mukuvisi River Zaranyika, ; Moyo and Worster, ; Machena, ; Kamudyariwa, revealed numerous sources and causes of pollution like industrial discharges, solid waste dumps and WTP effluent. It has been recommended that industrial effluents and wastewater discharges be tightly controlled if water quality in the rivers and Lake Chivero is to be managed effectively Moyo, ; Bethune and Roberts, Two decades ago, nitrogen and phosphorous were identified as the main nutrients limiting phytoplankton growth Robarts and Southall, ; Watts, In the past few years, a number of planned developments such as the expansion of Crowborough and Firle wastewater treatment plants have failed to take place.

A harsh economic environment, characterised by lack of foreign currency and electric power cuts also affects the proper functioning of these plants. Using the annual average inter-censal - growth rate of 2. This will lead to increased water consumption, resulting in larger water abstractions from Lake Chivero.

It will also result in an increased volume of wastewater discharges, whilst natural river flows will increase marginally because of an increase in impervious areas caused by urban construction. The current rainfall trend suggests more frequent drought years Luxemburg, and this means that rivers would increasingly be carrying more WTP effluent than natural flows.

Lake spillages would decrease with the lake increasingly becoming a major pollutant sink, leading to serious water quality problems. Figure 1. The major wastewater treatment plants Crowborough and Firle are close to the Lake, so there will be insufficient time for natural purification to play a significant role in the current set up.

The water balance of Lake Chivero Fig. The City of Harare will face increased problems in ensuring safe and sufficient water supply especially in the dry season when urban water demand, water evaporation from the lake surface, and lake productivity hyacinth and algae blooms is at its highest. The disposal of effluent and sludge on pastures in Harare is already showing limitations in removing nutrients and pollutants such as heavy metals McKendrick, ; Nyamangara and Mzezewa, ; Manjonjo, Sustainable nutrient removal via pasture irrigation requires more land, a resource that is now becoming both scarce and too costly for this purpose Furthermore, the efficiency of the land treatment of effluent decreases during wet weather McKendrick, The cumulative loading of chemicals and fertilisers on the soils and groundwater, and subsequent leaching into the lake, will be a problem in the future but urban agriculture could have positive effects by reusing nutrients and water if organised differently.

If no intervention measures are put in place, the present problems related to effluent disposal and downstream pollution in Harare will continue and increase in magnitude. The lake will become more eutrophic and the quality and quantity of urban water supplies will be adversely affected as will other uses of the lake, e. This research project was therefore motivated by the need to seek effective ways of managing water resources and pollution in the Harare sub-catchment area of Lake Chivero.

Emphasis is put on wastewater management, especially the control of water, nitrogen and phosphorus flows. The philosophy driving the research dictates smallest cycles for nutrients and water not only the end of the pipe. The problem is compounded by the fact that water release from the lake does not takes place in years of low rainfall as the dam's floodgates are permanently closed. Spillway discharges normally take place only from January to April, meaning that the lake acts as a sink for pollutants for most of the year.

As the population increases, the lake will increasingly receive a higher fraction of WTP effluent whilst raw water abstraction will also increase, posing a water quality and quantity problem. This has led to excessive primary productivity and related problems in the lake. Nitrogen and phosphorous inputs need to be controlled to avoid further deterioration in water quality. Research is required to understand the current flows of water and nutrients into and out of the catchment so that corrective measures can be based on a better understanding of the system.

The specific objectives of this research were: i. The first was the collection of background and current information on water quality and quantity in the Harare sub-catchment area of Lake Chivero. This was used to validate available information and to assess the current state of the problem. The lake was taken as a reference point for the development of sustainable measures, which meant that the impacts of recommended measures on the water quality of the lake were assessed. The second aspect dealt with the formulation of intervention measures based on the control of nitrogen, phosphorus and water flows, beginning at the lowest property level and up to the city or central level were developed.

This control could be achieved through a systematic and strategic approach that includes cleaner production approaches, treatment, reuse, and recycling. Harare presents a peculiar situation in that Lake Chivero, situated downstream, is not refreshed in drought years leading to a build-up of pollutants. This is worsened by the ever-increasing wastewater discharges.

An assessment of the long-term sustainability of this practice needs further analysis but research in Zimbabwe has focused mainly on the impacts of effluents on the rivers and the lake. A broad study of the pollution situation in the catchment was done by JICA but it was too general for this purpose as it covered a much broader perspective. Generally, the amount and quality of data available does not seem consistent and sufficient for the quantification of the problem, the trends and future projections, and does not permit the development of concrete remedial measures.

The current data cover different segments of the catchment at different times, often using different parameters. Reliable data are required to define a correct water and nutrient balance of Lake Chivero. Once the current situation is understood, projections to the future could be made. From this, solutions could be developed that would lead to a desirable balance that avoids water quality deterioration now and in the future. The nature and sources of the pollution, reuse and recycling options, and long-term sustainabllity of these options all need to be understood in order to arrive at feasible intervention measures.

This research will take into account the above considerations and contribute to: i. Thesis outline The structure of this dissertation is shown in Fig 1. The current Chapter gives a background to the dissertation, including the problem statement, the objectives to be addressed, and the scope. Chapter 2 covers an inventory of the existing wastewater management practices, focusing on Harare and the Lake Chivero.

A detailed monitoring of water quality and the pollution situation in the Chivero catchment area, including the contribution of wastewater discharges, and current nutrient balances, are covered in Chapter 3. A conceptual framework for the sustainable management of wastewater in Harare based on the so-called "3-Step Strategic Approach" is presented in Chapter 4.

Chapters 5, 6, and 7 are based on this strategic approach and look in detail into options for managing wastewater at various stages of onsite, decentralised and centralised levels, with the overall aim of reducing the generation of wastewater and to recover and reuse nutrients. Chapter 1 : Introduction 16 1. Options for pollution reduction and prevention are considered as a first step for reducing water consumption, wastewater generation, and phosphorus use. Chapter 6 explores the decentralised concept of wastewater management and how it could be applied to Harare. Chapter 7 looks at the centralised management of wastewater in Harare focusing first on the current situation and its limitations.

Alternative solutions based on treatment and reuse, and on disposal with stimulation of river self-purification capacity are offered. A detailed case study on pasture irrigation was carried out Chapter 8 to understand the extent to which nutrients may be removed via land irrigation. Chapter 9 synthesised the findings in previous chapters and presents an integrated approach aimed at reducing nutrient flows into Lake Chivero and an improved water supply for Harare.

Chapter 1: Introduction 17 zyxwvutsrqp 1. Cosgr ov e , W. Fr ij ns, J. Sei: Tech. Haar hof f. Khour i, N. Manj onj o, M. Mat hut hu, A. Mbi ba, B. W K eds. Moyo, N. Robar t s R. Chapter 1 : Introduction 21 zyxwvutsrq T hor nt on, J. T hor nt on, J. In: Wat e r Pollut ion Cont r ol.

Wat t s, C. This paper looks at the sustainability of current practices of the urban water cycle in relation to water quantity and quality m anagem ent in the Chivero catchm ent. Data on population, water supply, water and wastewater treatment and river flows were obtained from urban councils and governm ent departm ents. The data were used to assess water consum ption, wastewater generation, treatment and disposal practices, river flow trends, raw water abstractions, and water dem and patterns. The results showed that the current situation is not sustainable as water quantity in the catchm ent will soon be a problem at current levels of consum ption whilst water quality already is a problem , especially with regards to nutrient levels.

Water and nutrient m anagem ent strategies, which include water use efficiency, treatment and water recycling, and nutrient reuse in controlled urban agriculture are recom m ended as matters of urgency. Urban wat er m anagem ent UWM involves the qualit at ive hygienic and quantitative aspect s of all wat er in urban set t ings Siebel and Gi j zen, 2 0 0 2. The convent ional m et hods of urban wat er m anagem ent m ainly f ocus on the cont rol of wat erborne d i sea ses and prevent ing t he degradat ion of the urban environm ent and surf ace wat ers.

This syst em is charact erised by high wat er consum pt ion and large cent ral treatm ent work s. The convent ional m et hods are supply-driven and suit an era of abundance Wright, 1 9 9 7. Fresh wat er as a resource is renewable and t herefore unlim it ed in quantity but it is lim ited in t erm s of quality and flow. Wat er dem and will cont inue to increase worldwide because of populat ion growt h, increased irrigation requirem ent s and indust rialisat ion. The availabilit y of an adequat e wat er supplyzyxwvutsrqponmlkjihgfedc i. Zimbabwe has a population of 2 about 12 million people and a land area of , km CSO, With an annual 3 population growth of about 1.

The situation could be worse if the effects of the Aids scourge on population growth are removed via the discovery of an Aids cure. Harare, the capital city of Zimbabwe, is one of the areas where water quantity and quality issues are receiving increased attention. Harare is located upstream of Lake Chivero and the lake's catchment area also includes the towns of Chitungwiza, Epworth, and Ruwa. The lake receives large amounts of wastewater effluent and pollution discharges from urban and agricultural runoffs McKendrick, ; Moyo, ; Nhapi and Tirivarombo, The rapid increase of population in the lake's catchment area and a shortage of funds to extend and rehabilitate the water supply and wastewater treatment infrastructure threaten Harare's water supply in terms of both quality and quantity.

It is now essential to correlate the results of these studies to produce an integrated picture of water issues quantity and quality in the catchment. The present Chapter forms part of a broader study on the management of water quality and quantity in the Chivero catchment.

It describes various components of the urban water cycle and systematically reviews their contribution to potential problems in the lake now or in the near future. The components are water abstraction, treatment and distribution, wastewater production and its management, and river flows.

The extent to which these individual components can be influenced in order to arrive at a sustainable situation needs further study. The Chivero catchment has an estimated 2 population of about 2. The latter consists of communal and commercial farming lands in nearly equal proportions Magadza, The lake, created by a dam constructed in , is located about 35 km south-west and downstream of Harare. It has a full capacity surface area of Its full supply level is 1, m above mean sea level. It receives water from the Manyame, Mukuvisi and Marimba Rivers.

The Marimba and Mukuvisi Rivers drain most parts of Harare. Five wastewater treatment works and two water treatment works are found in the catchment Fig 2. Figure 2. These are: 1 the water resource rainfall and runoff, dams , 2 water treatment plants and water supply system, 3 wastewater generation, collection and treatment, 4 wastewater disposal systems river discharge, pasture irrigation. However, the nutrient flow is strongly interconnected with the urban water cycle, so urban agriculture is also discussed as a major user and source of nutrients.

The study considered mainly quantitative data, focusing on water security and technologies used. On urban agriculture, the study dealt with the possibility of channelling water and nutrients present in wastewater to food production. This would be an alternative to the current practice of discharging it into the river and pasture irrigation.

Surface water quality is assessed and discussed on the basis of available data in the literature. The current Chapter shows the impact of existing water and nutrient management practices on the environment, and on various water uses, especially the impacts on Lake Chivero as a major source of water supply. The question of sustainability is discussed in the context of water scarcity and water quality, now and in the future. Data were collected from the City of Harare, the Meteorological Office, and the Zimbabwe National Water Authority ZINWA and they included the volume of raw water abstracted and treated, the volume of wastewater produced and treated, rainfall, and river flows.

Population data were obtained from the Central Statistical Office and city planning reports. The water quality situation is based on literature data. Rainfall over the Chivero catchment varies in time and in space Fig 2. The Marimba and Mukuvisi rivers, with sub-catchments of km and km respectively, have fairly similar flow patterns and magnitudes Fig 2. The catchment area of the Manyame River 2 2 is 1, km and about km of the catchment area is not gauged. Runoff in the Marimba and Mukuvisi Rivers is increased by effluent discharged from Crowborough and Firle wastewater treatment plants.

The value for Manyame reflects upstream water losses via abstractions for irrigation and urban water supply and evaporation, especially from the Seke and Harava dams. Human activities, mostly boating clubs, lodges and National Parks premises, around the lake are not significant consumers of water and have no impact on its quantity and quality. In severe periods of droughts, zyxwvutsrqponmlkjihgfedcbaZYXW e. The level of Lake Chivero water volume falls considerably during years of low rainfall Fig 2. Extrapolation from Fig 2.

Some industries have pre-treatment facilities whilst others discharge into stormwater drains.

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The major part of the industrial areas is also located in the sub-catchment of these two rivers. The Manyame River collects tertiary effluent from Chitungwiza, upstream of the lake. This has serious implications for downstream water quality. Both Mazowe and Gwebi Rivers are outside the Chivero catchment area. Of this, Crowborough Farm has a total area of ha of which only ha are under irrigation Manjonjo, , suggesting that the area under irrigation could be increased.

Kikuyu grass Pennisetum clandestinum and star grass Cynodon plectostochus are grown as pasture and flood-irrigated about once every three weeks. These pastures could produce enough to feed 13, head of cattle. The disposal of effluent by pasture irrigation has a number of problems. Secondly, this practice is increasingly losing its capacity to remove nutrients e. There is a risk that some pollutants could enter the food chain. Another disadvantage is that this practice requires more land, a resource that is gradually becoming scarce and costly. In fact, some areas of suitable land between the wastewater treatment plants and the lake are now being converted into housing developments.

During the dry season, the effluent is drained into the soil releasing its nutrients, mixing with the subsoil water and thus raising the water table forming springs and streams. Ur b a n agr i cul t ur e Urban agriculture is a notable economic activity in Harare. Bowyer-Bower ef zyxwvutsrqponmlkjihgfedcba al. A rough estimate, based on an assessment of unpublished reports and newspapers, puts it at more than 16, ha by year The cultivation of open spaces has been passively encouraged in Zimbabwean towns since the great drought of Marongwe, It could also be related to economic hardships caused by an economic structural adjustment programme in the early 's Mbiba, The crops that are grown are mainly maize, sweet potatoes, cassava, pumpkins, and sweet canes.

On-plot cultivation is quite common for almost all residential areas. There are no institutions or mechanisms to control these practices and to ensure that appropriate conservation measures are taken to minimise nutrient runoff and soil loss Masoka, Urban agriculture therefore poses a potential threat to water quality and this practice is likely to continue without regulation into the future Zanamwe, Pesticides and fertiliser applications are thought to be generally higher than in normal farming ENDA-Zimbabwe, This will result in cumulative fertiliser loading on the soils, leading to groundwater pollution and subsequent leaching into the lake.

Studies on the water quality of the Mukuvisi River reported by Zaranyika and Kamudyariwa showed that it was significantly affected by human activities, especially wastewater discharges that adversely affected its water quality. Moyo and Worster also studied the effects of organic and other pollutants on the Mukuvisi River. They reported total nitrogen and total phosphorus concentrations of In other studies, Machena and Magadza recommended the management of rivers so that they become part of pollutant reduction systems through increased self-purification.

There are a number of options to achieve this. Magadza suggests constructed wetlands and the control of activities that degrade streambeds and stream banks, such as landfills and urban agriculture on open spaces. The approach is to reduce flow velocity and increase biological and chemical activities. Algal growth raises the pH of the water and causes volatilisation of ammonia and the sedimentation of heavy metals and phosphates. The levels of virtually all water quality variables monitored went up downstream of wastewater discharge points.

Manjonjo also found that wastewater effluent, irrigation seepage and runoff were significantly increasing the concentrations of nutrients and heavy metals in the river. Partially treated trickling filter effluent from Crowborough wastewater treatment plant was the major polluter of the Marimba River Nhapi and Tirivarombo, Lake Pollution. Numerous hydro-biological investigations carried out in Lake Chivero in the 's showed that the lake was eutrophic and that wastewater from the city of Harare was the major cause Thornton, ; Jarvis e f al.

Robarts and Southall showed that nitrogen was the primary growth-limiting nutrient for phytoplankton. Besides some work reported in Moyo , there appears to have been no comprehensive research programme in Lake Chivero in the s and there is limited recent information on limnological aspects that might be useful in managing the lake's problems. Problems related to water quality reported in Lake Chivero include extensive fish deaths that occurred in from the deoxygenation of the water compounded by ammonia and algae toxicity Moyo and Mtetwa, ; Magadza, The growth of invasive water weeds like the water hyacinth Eichhornia crassipes , and blue-green algae, principally Microcystis aeruginosa and Anabaena sp.

Marshall, is another example of water quality-related problems in the lake. Excessive amounts of algae and other organic matter seriously affect raw water abstraction and water treatment by requiring extra filtration measures and through the production of unpleasant tastes and odours, as well as the possible production of toxins McKendrick, ; Moyo and Mtetwa, The high organic content often taints the water with a distinctive taste and affects pH, making additional treatment necessary to make it potable.

The lake is also losing its value as a recreational area e. These are water scarcity, inadequate wastewater treatment, and surface water pollution. The Lake Chivero catchment receives a total 9 3 3 9 3 of about 1. Some of the water is abstracted, treated and used in towns after which it returns to the lake as treated wastewater Table 2.

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The rest either evaporates or flows downstream where some of it is abstracted for agricultural irrigation. Raw water is also abstracted from the immediately downstream Lake Manyame giving a combined storage volume for the two lakes of about 3 3 ,, m. Abstraction levels are approaching the limit for Lake Chivero so the abstraction system needs to be optimised for Lake Manyame to avoid drawing down Lake Chivero.

At current abstraction levels, the lake level is rapidly going down in the dry season. The hydraulic overloading of wastewater treatment plants is an urgent problem in Harare and is related to high water consumption. This could be achieved via demand management measures centred on cleaner production approaches Gijzen ; Siebel and Gijzen, The measures include water saving measures at plant and distribution network, pollution prevention or reduction at household level, reuse and recycling of wastewater components, installation of water saving devices like low water use flush toilets, water saving faucets and showerheads.

If demand management is extended to commercial, industrial and other water consumers, even larger savings would result. The lack of spillway outflows from the lake in the dry season when inflows are perennial and increasingly contaminated suggest that the lake could be acting as a sink for pollutants. This situation will worsen in low rainfall years.

The overloading and breakdowns of wastewater treatment plants are also detrimental to lake water quality. Since that time, Zimbabwe has had extreme rainfall seasons droughts and cyclones and Harare has experienced high population growth due to rural-urban migration. The rapid increase in population in the Chivero catchment 2. Water scarcity is not a problem now in Harare but it will be a major problem after year Urbanisation and increased abstraction of water from Lake Chivero has resulted in changes in the lake inflow and outflow regimes meaning that the lake is receiving pollution throughout the year.

Lake water outflows are reduced in the dry season and this could result in the accumulation of nutrients and other pollutants in the lake. An urgent investigation on water quality is therefore necessary. The high water consumption in Harare is costly in terms of treatment infrastructure both for water and wastewater and needs to be reduced as a matter of urgency.

There are good prospects for the application of cleaner production concepts water conservation, pollution prevention and reuse to urban water management in the Chivero catchment. The presence of uncontrolled urban agriculture in the area is a potential threat to water quality in the long-term. On the other hand, when properly managed, urban agriculture offers a logical alternative for the reuse of nutrients in wastewater. References Abeysinghe, D. Bowyer-Bower, T. Urban Agriculture in Zimbabw e: Realities and Prospects. Jar vi s, M. F, Mit che ll, D.

Junk Publ i she r s. In Moyo, N. In Moy o, N. Mar shal l , B. Mcke ndr i ck, J. McKe ndr i ck , J. Lake Chivero: A Polluted Lake. Munr o, J. Nhapi , I. Proceedings ofthe Zimbabw e Institution of Engineers, Vol. Asse ssm e n t of maj or w at e r an d nut rient f low s in t he Ch i ve r o cat chm e nt ar e a, Zi m ba bw e , Physics and Chemistry ofthe Earth, Vol. Agriculture, Ecosystems and Environment, Vol. Hydr obi ol ogi a. In T hor nt on, J. Ecological Effects of W aste W ater.

Wr ight , A.

1st Edition

Zar anyi k a, M. Nhapi, I. Keywords: Eut r ophi cat i on, La k e Ch i ve r o, m a ss ba l a n ce s, nut r ie nt s, pollut ion, w ast e w at e r Introduction Eutrophication of water bodies is undesirable because of deterioration of water quality, interference with most of the beneficial uses of water, and corresponding economic losses.

Eutrophication may severely alter the recreational value of water bodies and impair related activities swimming, fishing, etc. Its impact on drinking water supply may be serious, leading to reduced final quality OECD, Furthermore, water treatment is made more difficult and costly McKendrick, Eutrophic waters usually have to be extensively chlorinated during treatment and transportation in distribution networks. High levels of both chlorine and organic substances could lead to significant concentrations of organochlorinated compounds in drinking water.

These substances are, on the long-term, considered potentially hazardous to human health carcinogenic risk. Water for potable use should thus be protected from eutrophication through better control of nutrients. The catchment has a total population of about 2.

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The lake is located about 35 km downstream of Harare and receives the bulk of wastewater treatment plant effluent from Harare, Chitungwiza, and Ruwa via upstream rivers. Numerous hydro-biological investigations carried out in Lake Chivero in the 's showed that the lake was eutrophic and wastewater from the City of Harare was cited as the major cause Robarts and Southall, A number of water quality-related problems have been reported In Lake Chivero. In deoxygenation of water compounded by ammonia toxicity caused massive fish mortality Magadza, Excessive amounts of algae and other organic matter in the lake have seriously impacted on raw water abstraction and water treatment McKendrick, '.

Besides, the growth of water hyacinth Eichhornia crassipes and blue-green algae, principally Microcystis aeruginosa and Anabaena sp. There is no comprehensive research program that has taken place in the Lake Chivero catchment since to define the major sources of pollution and their impacts on the lake. The pollution situation has undoubtedly changed over the years because of population growth in Harare, Chitungwiza and Ruwa.

The ageing of existing wastewater treatment works and the reported numerous breakdowns are also contributing factors. The current economic decline has affected the implementation of pollution control measures, including the rehabilitation and extension of wastewater treatment works, enforcement of legislation, and controlled urban agriculture. Zimbabwe has faced low rainfall years in the s' and this pattern is likely to affect the nutrient balance of the lake.

This Chapter reports on a study of the current status of pollution carried out from June to December through a monthly water quality monitoring program. The aim was to assess the water pollution situation in the Chivero catchment rivers and lake and to update water and nutrient nitrogen and phosphorus balances. This covers all areas drained by the Marimba and Mukuvisi Rivers.

The Mukuvisi and Marimba Rivers drain most parts of Harare. The catchment area of the Manyame River includes the town of Chitungwiza, Ruwa, the eastern parts of Harare, and part of Seke communal lands. The lake's total catchment is about 2 2 3 2, km. Lake Chivero has a surface area of It stratifies in summer and overturns in late February or March of each year Marshall and Falconer, ; Thornton, In addition, lake water was analysed for ortho-phosphates, and secchi disc measurements taken. A secchi disc is a round, flat disc with alternating black and white quadrants that is lowered into a lake to visually measure water clarity.

The depth at which a secchi disc disappears provides an indication of the level of nutrients and algal growth in the lake. Other physical observations were also noted for purposes of data interpretation. River samples were collected upstream and downstream of WTP effluent discharge points stations 15 - 18A on Marimba River, and 22, 23 on Mukuvisi River and at continuous gauging stations just before discharge into Lake Chivero 12 - 14 19, 20, and 21 Fig 3. River stretches 16A and A were monitored for the influence of seepage and runoff from pasture irrigation on the Marimba River. Point 14A monitored a small stream that consists entirely of runoff and seepage from Ingwe Pasture Irrigation Farm.

Six points, as indicated in Fig 3. Samples from the lake were collected at depths of 0. Samples were collected monthly from June to December Samples were collected in plastic containers that had been washed with phosphorous- free detergents, rinsed with distilled water and left to stand overnight in 1M HCL. Containers were rinsed again with distilled water and twice with sample water on site.

Samples were analysed according to standard procedures Standard Methods, The TP was determined by digesting samples with concentrated sulphuric and nitric acids followed by the analysis of phosphate content using the vanado molybdophosphoric acid method. The lake algal-available phosphorus was estimated as the soluble ortho-phosphates as measured by the vanado molybdophosphoric acid procedure after the sample has been filtered on site through a 45 mm GFC filter.

The tests for ortho-phosphates were done on the same day of collection. Samples for TKN analysis were digested according to the micro-Kjeldahl method followed by distillation with sodium hydroxide and sodium thiosulphate solution. The released ammonia was collected in excess boric acid and determined by titration with 0. Wastewater flows and raw water abstractions were determined from the continuous metering and pumping records of the City of Harare. The flow data were analysed statistically using the Microsoft Excel computer package.

Chapter 3: The Impact of Urbanisation on the Water Quality of Lake Chivero, Zimbabwe 45 zyxwvutsr A water and nutrient balance for Lake Chivero was developed based on average annual flow data and sampling results of this study. Precipitation was considered over the entire lake surface area assuming no water losses from rainfall falling directly on the lake surface area. Activities around the lake were not considered in the mass balances, as the population is negligibly small and, therefore, their impact was considered insignificant.

The amount of time needed for the lake's planktonic algal biomass to respond to reductions in available P load was estimated as three times the phosphorus residence time of the water column Lee and Jones, This empirical determination does not directly include the lake internal P-cycle.

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The phosphorus residence time was defined as the average in-lake water column phosphorus mass divided by the annual phosphorus input load. Equation 3. After validation of the equation, by getting figures close to observed concentrations of phosphorus in the lake, the same equation was used to assess alternative scenarios.