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Wageningen Universiteit (2004)

Options for wastewater management in Harare, Zimbabwe

Nhapi, I.

Titre : Options for wastewater management in Harare, Zimbabwe

Auteur : Nhapi, I.

Université de soutenance : Wageningen Universiteit

Grade : PhD thesis 2004

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 haveaccumulatedin 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.

Mots clés : waste water / water management / waste water treatment / urban areas / zimbabwe

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