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2.2.4 In-stream Water Quality Upgrading

Technical Description

A typical water quality enhancement unit consists of a low diversion dam, infiltration gallery and clear water reservoir. A low diversion dam is constructed using a pile of rocks bound together with cement or mortar. The filter box is constructed of concrete according to designed specifications. There could be one or two filter boxes depending on the quality of the stream water. The filter box is then filled with a filter media which is comprised of 20 cm of gravel and 1.2 m of sand packed on top of the gravel. The inlet end of the intake pipe is covered with nylon mesh to prevent large suspended particulate matter from entering the intake pipe. The portions of the pipe within the filter boxes are perforated. Using such an arrangement, water could flow from a raw water source onto a filtration medium through which it is filtered and advanced to the collection well, all under gravity. The clear well reservoir, in which the filtered water is collected, is constructed of concrete. The size of the reservoir depends on the size of the population.

Having been provided with a design, the construction of this system can be accomplished by a community with the help of local artisans living in the community. A typical unit, constructed on the Nima Creek in Accra, Ghana, was designed to deliver a minimum of 30 litres/person/ day into the filtered water well (Figure 40). This well was 5 m deep and 2 m wide, extending 2 m below the water table in depth. The filtered water in this system was accessed by using a handpump fitted onto the collection reservoir. However, for small communities with populations of 1 000 or less, outlet pipes with valves fitted to the clear water well are cheaper alternatives.

Extent of Use

This technology has been used widely in some rural communities in Ghana. In the last twenty years, however, the technology has not been actively used, having been replaced by other technologies for rural water supply, augmented by the electrification scheme started by the government. Nevertheless, the technology has recently been revisited and improved for application in rural water supply.

Operation and Maintenance

The local residents operate and maintain the system using simple tools. Preventive maintenance involves cleaning the nylon mesh cover of the intake pipe two to three times per week.

Level of Involvement

This technology was adopted as a community built and managed system. The planning and design of the system are the only aspects of the project which require the involvement of expert personnel. Figure 40. Infiltration gallery on the Nima Creek, Accra, Ghana.

(larger image)

Costs

A community can develop this technology with very little guidance. The approximate costs of construction are as follows: Intake civil works $ 15/m Filtration gallery and clear water well civil works $600.

Suitability

This technology is suitable for low income, rural communities that cannot finance and sustain conventional water supply systems. It is also suitable for refugee camps where electric power supplies are not available, or in other areas where chemicals and equipment for water purification are difficult to obtain.

Advantages

The technology is low cost and needs no power supply, no chemicals, and few spare parts to function. It can be constructed, operated and maintained by semi-skilled people in local communities.

Disadvantages

It is difficult to backwash the filter without a power source; thus, the quality of the filtered product water and a satisfactory filtration rate may not be sustained. In such situations, the filtration media may have to be replaced on a frequent basis for best efficiency and quality of product water.

Further Development of the Technology

Development of methods for backwashing the filter material is necessary to maintain the sustainability of the system.

Information Sources

S. Mintah-Boateng Asare, Construction and Operation of Infiltration Galley System, Water Resources Research Institute (CSIR), Post Office Box M, Accra, Ghana.

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