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close this bookSourcebook of Alternative Technologies for Freshwater Augmentation in Africa (International Environmental Technology Centre - United Nations Environment Programme, 1998, 182 p.)
close this folderPart B - Technology profiles
close this folder2. Domestic water supply
close this folder2.3 Wastewater treatment technology and reuse
View the document(introduction...)
View the document2.3.1 Direct reuse of treated municipal wastewater
View the document2.3.2 Indirect reuse
View the document2.3.3 Regeneration water

2.3.1 Direct reuse of treated municipal wastewater

Technical Description

Direct reuse involves the abstraction of effluent from sewage treatment works and, after further treatment (e.g., tertiary treatment or retention in maturation ponds), mixing it with raw water at the inlet of a water treatment works.

Extent of Use

This technology is extensively utilized in Southern Africa, especially in South Africa, Namibia, Zambia and Zimbabwe, as well as in Mauritania and Burkina Faso in West Africa.

Operation and Maintenance

Catchment quality control is essential. This involves the segregation of industrial effluents from the catchment of the reclamation plant to avoid contamination with persistent organic contaminants, heavy metals, and other substances deleterious to human health. The wastewater should undergo the both biological and physico-chemical treatments: chemical coagulation and flocculation; solids separation; disinfection; activated carbon filtration; reverse osmosis filtration; and stabilisation. These steps may be considered to be routine water reclamation stages. Further, because of the flocculation and solids separation stages, sludge management practices are required when using this technology. There is a need for a steady supply of wastewater entering the reclamation process. This is usually in contrast to the irregular urban flows. Balancing inflows is normally accomplished through the use of maturation ponds.

Also, quality analysis is essential since each reuse application has its own quality requirements. Specific approaches to reclamation technologies vary depending on the quality of the wastewater. This, in turn, dictates the specific operation and maintenance requirements for each approach. It is essential to the proper operation and maintenance of these systems that the correct procedures be adopted. Regular and frequent monitoring is required for the safe use of this technology, including flow measurement, continuous monitoring of selected parameters, sampling for quality control, maintenance of instrumentation and operating systems, and visual observation and bio-monitoring of the product water.

Conventional System

The simple conventional system is described here, although this could be operated as activated sludge system.

Technical Description

The technology consists of

(i) the preliminary treatment stage which has a screen chamber, a grit chamber and flow recorders similar to the waste stabilization system;

(ii) primary sedimentation basins for me settlement of organic solids;

(iii) biological reactors which in Africa are commonly trickling filters for the biodegradation of soluble organics;

(iv) secondary sedimentation basins for settlement of biomass;

(v) sludge treatment systems such as digesters or drying beds.

Extent of Use

The technology is widely used in Africa.

Operation and Maintenance

Maintenance consists primarily of inspecting and repairing pumping systems and screens, and sludge removal.

Level of Involvement

These systems may be constructed by local artisans, but designed by engineers.

Costs

Costs are relatively high compared to ponds.

Suitability

This technology is suitable in all countries, but for reasonably large settlements.

Effectiveness

The technology is effective in removal of BOD.

Environmental Benefits

The use of this technology usually improves the water quality of the discharged wastes but can potentially result in nutrient enrichment of surface and groundwater.

Advantages

The technology is effective in removal of BOD5.

Disadvantages

This technology needs skilled manpower to operate, is costly to run, and does not remove nutrients leading to potential pollution problems.

Cultural Acceptability

Poor operation results in odour problems which are objectionable to community. Also, Africans are not usually at ease with seeing their own excreta.

Information Sources

Standard text books and research organizations in Africa

Level of Involvement

The major player is the wastewater collection and treatment agency, normally the local authority. The user receiving secondary effluent becomes responsible for its tertiary treatment, even though he would have to depend on the local authorities for the implementation of the necessary catchment quality control.

Costs

Costs vary from moderate to very high depending on method. Major factors are the capital costs of treatment facilities; labour, spares and energy; reticulation systems; and land. In Bulawayo, Zimbabwe, the costs of sewage treatment using coagulation, clarification, rapid sand filtration and chlorination stages is about $0.05/m3

Effectiveness of the Technology

The group of technologies is extremely effective as for each unit of wastewater recycled an equivalent amount of freshwater is saved.

Suitability

This technology is appropriate in regions experiencing severe water shortages, and where wastewater is collected in a sewerage system. Direct recycling is most appropriate for use in towns with modified activated sludge (MAS) plants since these plants have the capacity to remove nutrients. However, effluent used for irrigation need not undergo MAS treatment since the nutrients a beneficial for plant growth.

Modified Activated Sludge (MAS) System

The MAS system is a wastewater treatment system aimed at polishing the effluent to remove nitrates and phosphates which can contribute to eutrophication of surface waters.

Technical Description

The MAS system has a preliminary stage similar to conventional wastewater treatment systems, including the primary sedimentation process. MAS treatment involves me passage of settled wastewater through a series of anoxic and oxic zones, la die oxic zone, nitrification of ammonia nitrogen compounds take place, and, in the oxic zone, the redaction of nitrates take place. Microorganisms in both zones utilize soluble phosphorus for biomass production (growth). The excess biomass thereby generated is settled in the sedimentation basin. Oxygen in oxic zone is provided by electrically drive aerators. The effluent produced in this way can be discharged to a receiving water body with low dilution potential, and in situations where effluent from conventional systems would result in nutrient enrichment of the water body.

Extent of Use

MAS systems are used in South Africa, Zimbabwe, and Namibia, and are being used experimentally in other countries.

Operation and Maintenance

Electric motors and rotors need regular inspection. All other operation and maintenance requirements for conventional wastewater treatment plants apply.

Level of Involvement

Use of this technology requires skilled operators and support staff.

Costs

MAS systems are expensive to operate, especially given the electric power input required.

Suitability

The technology is suitable for urban centres that need to, or may need in future to, recycle water.

Effectiveness of die Technology

The technology is very effective in removal of BOD, suspended solids and nutrients

Environmental Benefits

MAS treatment reduces the dangers of pollution of surface water bodies.

Advantages

MAS treatment removes nutrients, and produces a product water that may be recycled immediately.

Disadvantages

The technology requires high energy inputs, making it expensive to operate.

Cultural Acceptability

There are no known cultural problems recorded for the specific technique of nutrient removal, but communities object to having a wastewater treatment plant close to residential areas. There are religious restrictions on direct reuse of wastes.

Information Sources

City Engineer, City of Harare, Zimbabwe.
City Engineer, City of Pretoria, South Africa.
Design details may be found in standard wastewater engineering textbooks.

Environmental Benefits

Poor or absent control of effluent quality can have serious health problems for the users. However, most countries using this technology have both water quality and public health standards in place. Use of wastewater for irrigation can enhance crop or plant production and improve surface water quality.

Advantages

It is a proven technology that is effective in water resources management. Costs and production efficiencies are predictable. Moderate skill levels are required. Use of this technology typically reduces pollution problems by turning wastewater into an economically attractive substitute water source for irrigation and non-potable industrial use at reasonable cost. It therefore increases water availability.

Disadvantages

Wastewater reuse may be culturally and aesthetically unacceptable. Increased nutrient loads may lead to enhanced algal growth in surface waters and the need for higher rates of chemical usage in water treatment. There is also a possibility of ground water pollution. As noted above, health problems can occur if the effluent has been poorly treated. Poor or incomplete treatment can also lead to a risk of contamination of potable water with heavy metals and organic compounds. Because salts are not significantly affected by these treatment techniques, there is the risk of gradual build up of proportions of dissolved salts to unacceptable levels with direct reuse.

Cultural Acceptability

The acceptability of this technology depends on the region. Some cultures do not accept the handling and direct reuse of wastewater. It is essential to determine an appropriate balance between cost and efficiency.

Further Development of the Technology

There is a need for legislation and regulations for the control of both treatment and use, where these do not exist, and, where they do, for their consistent application. Studies need to be undertaken to determine the variations in effluent quality and its effect on the raw water being reused.

Information Sources

Odendaal, P.E. 1991. Wastewater reclamation technologies and monitoring techniques. Water Science and Technology, 24(9): 173-184.

Odendaal, P.E. and L.R.J. Van Vuuren 1979. Reuse of wastewater in South Africa - Research and application. Proceedings of the Water Reuse Symposium I, 25-30 March 1979, Washington, DC. p. 886-906.

Department of Water Affairs 1986. Management of the Water Resources of the Republic of South Africa, Government Printer, Pretoria.

Holland, J.R. and S.M. Holland 1994. Urban Water Supplies Conservation Study for MLGRUD. Emergency drought recovery and mitigation programme.

Binnie and Partners Consulting Engineers, in association with Burrow Binnie Limited 1993. Bulawayo, Water Conservation Study, Final Report. Vol. 2. Overseas Development Agency, London.

Meiring, P.G.J., P. Rose, and O. Shipin 1994. Algal aid puts a sparkle on effluent. Water Quality International, 1994(2): 30-32.

Meiring, P.G.J., R.J.L.C. Drews, H. Van Eck, and G. Stander 1968. A guide to the use of pond systems in South Africa for the purification of raw and partially treated sewage. National Institute for Water Research, CSIR, Pretoria.