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United Nations Environment Programme
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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

8.5 Scenarios for Sound Practices

General scenarios can be sketched based on population density to illustrate integration of technology, environmental, economic and social factors. For a low population density and where land is available around dwellings, on-site systems with on-site reuse provide householders with options which are a function of water availability, toilet type and desired reuse of blackwater and greywater. Use of a double vault composting toilet (2 (4.1.2)) and greywater for subsurface irrigation is shown in Figure 2.50. Maintenance requirement will be emptying the vault (say, every 6 months), windrow-composting the content with garden waste and diverting blackwater from a full vault to the one just emptied. Irrigation system for greywater need to be checked weekly.


Figure 2.50: Composting toilet for blackwater and sub-surface irrigation of greywater (Lange and Otterpohl, 1997)

A system requiring less householder maintenance is a septic tank with an inverted leach drain or evapotranspiration trench (2 (4.1.5)). The septic tank needs to be de-sludged every 3 to 5 years. This is done by calling a sludge contractor. This service should be available in the community for this option to operate satisfactorily including the safe disposal of the sledge by the contractor.

For a high population density, community ablutions blocks with payment for use can work well. The wastewater can be conveyed to a location where land is available for land-based treatment (2 (4.2.4)) and reuse through grazing grasses irrigated by treated wastewater. The operator of the ablutions facilities needs to ensure public health requirements for the wastewater reuse are met.

Toilet facilities in individual dwellings are an option with wastewater collected using simplified sewerage (2 (3.2)). This can be condominial sewers or with street connections depending on community choice. Collected wastewater is treated using a series of lagoons (2 (4.2.3.)), with the final lagoon employed for aquaculture (2 (6.1.2.)). Depending on land use downstream of the lagoons, wastewater can be reused further for agriculture, horticulture or tree plantation.

The requirement of planning a sewerage system within a catchment basin (to use gravity flow), the environmental requirement for reuse of wastewater nutrients (to prevent pollution), the economic requirement of balancing economy of scale of treatment and the cost of the sewer pipes, and the social requirement for community consultation point to planning for a community-scale collection, treatment and reuse of wastewater. The optimum size of the population served for a community-scale systems will depend on local conditions, which in turn are determined by local geographical (topography, climate, soil), environmental, economic and social/institutional considerations.

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