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2.2.4 In-stream Water Quality Upgrading
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
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used, having been replaced by other technologies for rural water supply,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.
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
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.
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.
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.
S. Mintah-Boateng Asare, Construction and
Operation of Infiltration Galley System, Water Resources Research
Institute (CSIR), Post Office Box M, Accra, Ghana.