The sustainability of urban water management poses serious challenges in the face of rapid urbanisation taking place in most developing countries. In these countries, extreme climatic conditions of drought and cyclones, coupled with rapid population growth, result in significant financial resources being channelled into the provision of clean drinking water and sanitation services. In many cases, these resources are scarce and are only applied in a reactive manner to mitigate the effects of droughts and to combat disease outbreaks. The planning authorities are rarely given enough time and space to properly plan decent settlements with sustainable and adequate water and sanitation facilities.
Recent initiatives, notably the Dublin Principles and Vision21, have acknowledged the shortcomings of current approaches to water services management and hence the need for lasting solutions. The importance of improved water supply and sanitation provision was again emphasised in the Millennium Development Goals. The Environmental Resources Department at the UNESCO-IHE Institute for Water Education in the Netherlands has, in line with Vision21 and the Bellagio statement, proposed a cleaner production approach to urban water managemen. This involves pollution prevention/minimisation, and resource recovery, reuse and recycling at all levels (household, onsite, decentralised and centralised) and in all sectors (domestic, commerce, industry).
The capital city of Zimbabwe, Harare, is one of the towns that are facing serious water management problems. Harare drains into Lake Chivero while also abstracting raw water from the same reservoir. In this way, the lake serves as a sink for pollutants that are not effectively removed via wastewater treatment or reuse. Wastewater is believed to be the major direct and indirect source of pollution in Lake Chivero. This paper is based on the results of a monthly water quality monitoring study in Harare, covering wastewater treatment plant (WTP) effluent, pasture irrigation runoff and seepage, and river and lake water quality, conducted from June 2000 to December 2001. The monitoring part focused on water flows, and nitrogen and phosphorus concentrations. Possible solutions for limiting nutrient inflows into Lake Chivero are discussed. This paper develops a composite solution for the sustainable use of water and nutrients in the Chivero catchment. The overall strategy consists of three important components: (i) a three-step strategic approach (how to approach the problem), (ii) a differentiated approach (where to do what: residential, commercial and industrial areas) and (iii) a staged approach (when to do what: that is, the short-, medium- and long-term action plan).
Wastewater generation
Harare has a high water consumption in low- and medium-density residential areas. Available figures of water consumption are 320–630 L/capita (cap.) day for high-income residential areas, which were recently confirmed. In these areas, 20% of the total population of Harare accounts for 32% of the total domestic water consumption. The water consumption figure for high-density areas is reasonably low at 80 L/cap. day. Here, 80% of the population uses only 31% of the total water consumption; in other words, the 20% rich uses about the same quantity of water as the 80% poor. In comparison, the average water consumption in the second largest city of Zimbabwe, Bulawayo, is 36 L/cap. day for high-density areas and 75 L/cap. day for low-density residential areas). The sources of wastewater in Harare for the year 2002 are shown. High-density areas produced the bulk of the nutrients (70% of total) because of the high population number. An estimated 304 000 m3/day of wastewater was produced in Harare in the year 2002. A major challenge in Harare lies in what to do with the increasing volume of wastewater produced, especially the discharge of nutrients.
Wastewater treatment
The WTPs in Harare are overloaded owing to rapid population growth and a halt in the construction of new treatment plants since 1996. Frequent breakdowns have also contributed to untreated or partially treated wastewater discharges into receiving rivers. The treatment technologies and capacities of the five WTPs in Harare are shown in it. From the table, it can be deduced that 55% of the wastewater is ‘treated’ in systems that are heavily overloaded (i.e. more than 36% over the design capacity). The treatment plants have a combined nutrient removal efficiency of 24% for total nitrogen (TN) and 8% for total phosphorus (TP).
The estimated total wastewater flows differ slightly from the average measured figures by 6%, and this is attributed to malfunctioning meters and estimation errors. (The estimates were used to validate the per capita wastewater production figures that were used in making future projections.) Infiltration also plays a significant role in water inflows at WTPs (6%). The trickling effluent and primary/secondary sludge are mixed and pumped to pasture irrigation farms. The effluent from activated sludge plants modified for biological nutrient removal (often referred to as BNR plants) is discharged into rivers while waste stabilisation pond (WSP) effluent is supposed to be pumped to farms for irrigation. Plant failures and insufficient land for irrigation, however, have resulted in the WSP effluent being discharged straight into rivers.
Wastewater disposal
The disposal of wastewater effluent and sludge in Harare poses serious problems. According to, the BNR effluent from Crowborough WTP is often of good quality in terms of nutrient concentration (4.6±3.0 mg/L TN, 0.7±0.6 mg/L TP). On the other hand, the BNR effluent from Firle WTP is poor (13.7±10.7 mg/L TN, 1.9±3.4 mg/L TP), mainly because of plant breakdowns and poor maintenance. The applicable Zimbabwean nutrient limits for effluents discharged into sensitive waters are 10 mg/L TN and 0.5 mg/L TP (Government of Zimbabwe Statutory Instrument 274 of 2000). The bulk of the BNR effluent from Firle WTP (87%) is used for pasture irrigation whereas the remainder is discharged into the Mukuvisi River. All BNR effluent at Crowborough WTP is discharged into the Marimba River. About 4000 m3/day of sludge from Crowborough and Firle are used for pasture irrigation. This practice is questionable, as it allows previously separated nutrients and other contaminants to enter again into the water bodies via seepage and runoff from pastures.
The current land area reserved for pasture irrigation is very small compared with the effluent and sludge quantities pumped to the pastures. Only about 1400 ha are available for pasture irrigation compared with about 6730 ha required for handling the current irrigation load of about 196 000 m3/day of effluent. Extra land has to be acquired beyond the city boundaries. Because of overloading, nutrient removal efficiency via pasture irrigation decreased to 84% for TN and 54% for TP, resulting in increased nutrient inflows into Lake Chivero. The practice of pasture irrigation needs to be managed within the capacity range of the available land in terms of water and nutrient requirements. Also, when using cattle, improved management of supplementary feed and manure collection could significantly improve the situation. High-value crops with high nutrient uptake rates like maize could be used instead of pasture grass [kikuyu grass (Pennisetum clandestinum) and star grass (Cynodon plectostochus)]. On the other hand, significant improvements in current BNR units would enable the safe discharge of part of the effluent into rivers, thereby reducing the overloading via land irrigation.
Receiving water quality
The water quality in the major effluent receiving rivers of Marimba and Mukuvisi has been deteriorating over the years. The flows in these rivers have become perennial, meaning that the lake is receiving pollution throughout the year. Although wastewater appears to be the major polluter of these rivers, it has also been observed that nutrient levels upstream of WTP effluent discharge points mostly exceed allowable levels for rivers. This is due to nonpoint source pollution, especially from stormwater and some illegal point source discharges. Small formal and informal industries and poor garbage collection could be major causes of pollution upstream of WTP effluent discharge points. It shows water and nutrient flows for the Harare subcatchment area and Lake Chivero. It can be deduced from that the maximum allowable limits of 0.1 mg/L TN and 0.03 mg/L TP for avoiding excessive plant growth in rivers are being exceeded. Moreover, the allowable limits of 0.3 mg/L TN and 0.01 mg/L TP for drinking water in lakes are also being exceeded. Lake Chivero is now hypertrophic, have established that nutrients are accumulating in the lake. About 59% of the incoming TP were retained in the lake, and this could lead to further eutrophication problems for some time even after phosphorus inflows have been stopped. This situation is very dangerous for a major water supply source of over 2 million people. It could result in serious fish kills, loss of beneficial uses of the lake (fishing, boating), water treatment difficulties, clogging of irrigation pipes and health problems (e.g. gastroenteritis).
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