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1.3 WASTEWATER TREATMENT TECHNOLOGIES AND REUSE

1.3.1 Wastewater Reuse

Technical Description

Wastewater from a conventional or wastewater stabilization pond system is directed by a system of pipes or canals to a night storage reservoir from which it is used for pasture or crop irrigation. The crop will strip the wastewater of its excess nutrients (nitrates and phosphates), while many pathogens that may be in the wastewater die due to the inhospitable environment at the soil surface or are filtered out by the vegetation. Nevertheless, the ability of some pathogens to survive suggests that this technology be applied cautiously, with full knowledge of the characteristics of the wastewater source being used. Appropriate infrastructure is required to transport the effluent from wastewater treatment works to the site of use.

This technology may be combined with aquacultural operations. In which case, ponds may need to be designed for use in fish culture and additional infrastructure will be needed for this purpose. If the effluent is to be applied for irrigation of crops and the production of foodstuffs, special pre-treatment steps may need to be applied prior to the transportation of the effluent to the storage site.

Extent of Use

Wastewater reuse is practised in Tunisia, South Africa, Zimbabwe, and Burkina Faso. Over 15 schemes are operational in Tunisia. In Tunis, for example, irrigation of citrus trees with wastewater has been done since 1964. There are plans to extend the wastewater irrigation scheme to an additional 40 000 ha around Tunis.

Operation and Maintenance

The treatment system that provides the wastewater is usually maintained by the local authority. The conveyance system and wastewater distribution system may be operated by these same local authorities, but is generally operated by governmental agricultural development authorities from the point of effluent discharge to the end user. The on-farm distribution system is typically operated by the individual farmers. Maintenance of the pumping systems, delivery canals, pipes and furrows is required. It is recommended that a system for monitoring the health of workers be established to minimise the risk of spreading disease, given the nature of the raw water.

Level of Involvement

The transfer system from the treatment works to the farmer is usually managed by trained or experienced personnel from the local authority. The application of the effluent onto the lands is done by the farmer, and does not require a lot of technical know-how since it mimics traditional or conventional practises. However, additional health precautions should be used, and rural health workers should participate in training schemes established to promote this technology. It is also important for the farmers to know the application rates of the effluents to minimise the occurrence of phosphorus poisoning of grazing animals and avoid over-fertilisation. Thus, agricultural extension workers also should participate in the establishment of this technology.

In Tunisia, wastewater is distributed to farmers by the local Agricultural Development Authorities, which are responsible to the Ministry of Agriculture. These Authorities construct and maintain the wastewater distribution system. They distribute the wastewater to the farmers according to an organised delivery schedule and collect revenues from the sale of the wastewaters. The farmers are responsible for on-farm distribution.

Costs

In Tunisia, the cost to the farmer is $0.031/m3 of wastewater supplied. Costs are influenced by the necessity of pumping, provision of infrastructure for pipelines, distance from the source to point of application, and type of application.

Effectiveness of the Technology

Irrigated agriculture benefits from the high nutrient levels present in wastewater, thus reducing the need for fertilizer applications. In aquacultural operations, reported yields of 0.8 to 4.0 tonnes/ha/year of fish have been achieved from effluent ponds.

Suitability

This technology is suitable wherever proximity to a wastewater disposal system coincides with a need for irrigation water or where the nutrient content can beneficially replace fertilisers or fish food. Applications of wastewater in agriculture should conform to World Health Organisation guidelines.

Environmental Benefits

The reuse of wastewater reduces the need to further exploit available freshwater resources which may be limited. There is, however, a danger of polluting the environment, especially the groundwater, if the wastewater is not properly treated initially.

Advantages

Use of wastewater for agricultural purposes does not require special skills, and can reduce the amount and use of other artificial fertilizers. Wastewater reuse serves as a polishing ground for the removal of nutrients, providing those nutrients to farmers at low cost. This technology may offer job opportunities to farmers who cannot afford conventional irrigation systems.

Disadvantages

Wastewater reuse can cause pollution of surface and ground waters if not properly managed. Further, the effluent may contain pathogens which can be harmful to farmers and consumers of edible crops irrigated with wastewater. Use of this technology requires significant investment in effluent transfer mechanisms such as pumps and pipes. The availability of irrigable land often limits the volume of wastewater that can be treated; however, in some cases, the volume of wastewater may limit the extent of irrigated agriculture.

Cultural Acceptability

A general social aversion to close association with excreta makes the use of wastewater problematic in Africa. Fish from sewage ponds are too closely associated with the excreta to be acceptable in many cultures. However, irrigated crops may be fully acceptable. Religious, especially Islamic, restrictions may also further limit the application of wastewater reuse technologies in parts of Africa.

Further Development of the Technology

There is need for further promotion of this technology. However, the bacteriological quality of the crops requires further investigation to ensure the public health. The technology, although relevant and quite widespread, is now being challenged by forthcoming technologies which seek to remove nutrients from effluents and therefore recycle wastewaters directly back to receiving waterbodies.

Information Sources

Contacts

Centre Regional Pour l'Assainissement a Faible Cout (CREPA), 03 BP 7112, Ouagadougou 03, Burkina Faso, tel (226) 310359/60, fax: (226) 310361

Ecole Inter-Etats d'Ingenieurs, de I'Equipment Rural (EIER), BP 7023 Ouagadougou, Burkina Faso.

Bibliography

Edwards, P. 1992. Reuse of Human Wastes in Aquaculture. A Technical Review. World Bank, Water and Sanitation Report No. 2.

Mara, D. and S. Cairncross 1987. Wastewater and Excreta Use in Agriculture and Aquaculture in Developing Countries. UNEP/WHO, Geneva.

Tournakara Mistandia, C.T. and C. Toure 1994. Wastewater Reuse for Irrigation; Studies of the Fertilizing Qualities of Treated Wastewater for a Stabilization Pond Treatment Plant in Climate Sahelian Conditions. Proceedings of the HYDROTOP 94 Conference, Marseille.

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