Options for wastewater management in Harare, Zimbabwe

The capital city of Zimbabwe, has adopted an urban water cycle that is geared towards high level service provision. Water supply and sewerage/sanitation coverage amounts to over 98%, which makes Harare with the highest coverage. The city's high volume of water abstraction from its main water resource, Chivero, however, can no longer be sustained. The lake has been seriously polluted by large volumes of (partially) treated effluents from wastewater treatment plants in Harare and the neighbouring town of Chitungwiza. It also receives pollution from agricultural, solid waste, industrial, and natural sources. Most of the wastewater treatment plants in the lake's catchment are overloaded and they experience frequent breakdowns. This situation has been worsened by repeated years of drought, resulting in the accumulation of nitrogen and phosphorous in the lake. The negative impacts of this have been reflected in periodic fish kills, proliferation of algae and water hyacinth, and the reduction in biological diversity. Other related problems are difficulties in potable water treatment and clogging of irrigation pipes.There is now an urgent need to control pollution loads and to remove contaminants that have<span class=GramE>accumulated</span>in LakeChivero over many years. A great deal could be achieved through rational management of the urban water system and the associated nutrient cycle. This should be based on an integrated approach that includes reduction of water consumption, and the wise use of water through pollution prevention/reduction measures. On the water supply side, available options include reduction of water losses (now at ± 30%), water-saving installations (in households, commerce, and industry), direct reuse ( e.g ., greywater), and alternative water resources ( e.g. , rainwater harvesting and groundwater). On the wastewater side, options available include onsite, decentralised and centralised treatment plus reuse.The general objectives of this research were to assess the contribution of wastewater from Harare to the nitrogen and phosphorous inflows into Lake Chivero and, based on this assessment, to formulate feasible sanitary engineering solutions to the problem of excessive nutrient inflows into the lake. The research specifically targeted nutrients because these are the major problem parameters. BOD is largely taken care of via current wastewater treatment and river self-purification processes. The general strategy was to intervene at various levels; i.e. , property, decentralised and centralised levels, with various options aimed at reducing water use and limiting wastewater production and reusing or recycling water and nutrients. This strategy would reduce nitrogen and phosphorous flows to the lake, whilst increasing water availability.An extensive water quality and quantity monitoring study in the Chivero catchment was carried out from June 2000 to December 2001 to assess the current situation in terms of water use, treatment and reuse levels, and flow balances. In addition, current contributions of wastewater discharges to nutrient flows in the rivers and Chivero were assessed. Intervention strategies were developed based on an approach, referred to as the " 3-Step Strategic Approach " to wastewater management. The steps include: 1) pollution prevention/reduction at source, 2) treatment in the direction of reuse, and 3) disposal with stimulation of self-purification capacity of the receiving water body. The three steps should be considered in this chronological order. Options considered include source control by the users (residents, industries, etc) using various strategies such as greywater separation and reuse, implementation of toilets with urine separation, and other ecological methods of wastewater management. Other possible options are invoking better behaviour through fees and information, and user responsibility through education, legislative changes and stricter controls over industry. Options for boosting the self-purification capacity of water bodies include introducing wetlands into the river system via natural overflow, land irrigation, reducing retention time in the lake, etc. Flexible and differential solutions were developed for each landuse category (commercial, industrial and residential).The results of this study confirmed that wastewater plays a major role in the pollution of Chivero. Wastewater contributed over 50% of the annual water flows in the major inflow rivers of Marimba and Mukuvisi. Water quality was found to be an urgent problem that requires immediate action whilst water scarcity was considered a medium-term problem. The river water quality for points upstream and downstream of wastewater discharge points were far above the 0.03 mg/l TP required for avoiding excessive plant growth in rivers. The lake nutrient concentrations were 2.01.3 mg/l TN and 0.6 ±3 mg/l TP ( standard deviation), reflecting a hypertrophic status. Nearly 70% of the annual phosphorus inflows were retained within the lake, which had a hydraulic retention time of 1.6 ±1.1 years based on rainfall years 1981/2 to 2000/1. However, for the monitoring period, the hydraulic retention time reduced to 0.4 years due to the heavy rains received in that period. Besides the need to substantially reduce nutrient inflows in Lake Chivero, adequate water inflows are also essential for the flushing out of nutrients from the lake, especially phosphorus. The continued accumulation of phosphorus in the lake sediments leads to an internal phosphorus cycle, further complicating remedial measures. The effective reduction of nutrient inflows into Lake Chivero hinges on solutions related to wastewater management. It is in this context that the "3-Step Strategic Approach" was applied, focusing on wastewater treatment and reuse options at onsite, decentralised, and centralised levels. An aggregation of these options led to the development of short-term, medium-term, and long-term solutions. It was estimated that significant improvements in the lake water quality (to about 0.4 mg/l TN and 0.07 mg/l TP in the medium-term) could be achieved by applying the measures suggested in this dissertation. In addition, the treatment of part of the effluent to tertiary standard and subsequent discharge into Lake Chivero could also reduce the lake hydraulic retention time to below 0.5 years, thereby enhancing the flushing out of nutrients. It was concluded that both water quality and quantity problems in the Chivero catchment could be significantly reduced via improvements in wastewater management in combination with the control of other point and non-point sources of pollution