4.2 Pond sand filtration

Technical description

In Bangladesh, Pond Sand Filters (PSFs) are built around artificially constructed ponds, locally known as "sweet water ponds" which are replenished by rainwater during the monsoon season. In these systems, rainwater collected in these ponds is pumped by hand into a storage tank through a filter chamber. The filter chamber is constructed in two parts, the first of which is a pre-filter packed with coconut fibres. This pre-filter reduces the turbidity of the raw water as the raw water flows into the filter chamber. The outflow from the pre-filter flows into the main body of the filter chamber through two overflow pipes. The main filter chamber consists of a layered, sand filter bed, through which the water trickles and in which impurities, including bacteria, are removed in a manner similar to slow sand filtration. The quality of the raw water is further protected through the reservation of the ponds feeding into the filtration system solely for potable water use. Details of this technology are provided in Part C.

Extent of Use

In Bangladesh, numerous Pond Sand Filters have been constructed. About 90 PSFs, each serving about 50 to 60 households on the average, have been constructed in Dacope thana since the start of the PSF programme in 1984. In Kaliganj thana, there are about 24 PSFs, all constructed during 1993-94. The average life of a PSF is a minimum of 10 years. The use of PSFs has the potential to revolutionize the drinking water systems in the saline areas in the southern belt of the country, covering the Greater Khulna, Patuakhali, Barisal and Noakhali districts.

Operation and Maintenance

Operation and maintenance requirements relate primarily to the handpump used to transfer the water from the ponds to the filtration units. The expenses associated with this are borne by the community. Initially, the Bangladesh Department of Public Health Engineering (DPHE) supplies necessary tools to the caretaker family to maintain the pump as required, and the users make minor repairs themselves. If major repairs are needed, and the related maintenance expenses are large, the community can apply to the DPHE for assistance. Operation of the PFS pump is generally by the users who pump and filter enough water for their own use.

Routine cleaning of the PSF is required. The rate of filtration gradually decreases over time, with the length of run resulting in increasing head loss. In order to maintain a constant rate of filtration, the height of water above the sand bed can be increased; however, a time will come when the filter bed must be cleaned to retore a reasonable rate of filtration. When the turbidity of the pond water is less than 8 NTU, the usual time between cleanings is about five months. When the turbidity increases to 30 NTU, it may be necessary to clean the filter every one and half months. The length of run also depends on the number of users drawing water from the system. Cleaning of the PSF is very simple and can be accomplished by two persons in under 45 minutes.

Level of Involvement

Adoption of this technology is at the community level. In Bangladesh, prospective users of this technology apply to the Department of Public Health Engineering (DPHE) for technical assistance, acknowledging their willingness to build and operate a PSF. This undertaking a willingness not only to transport the required materials to the site from pick-up points, but also to provide labour. Brick-makers, as well as a mason, plumber and carpenter, are provided by the community as needed during the construction of the PSF system. The community also nominates two men and two women (caretaker families) to be trained in the maintenance of the PSF when it is handed over to them by the DPHE. The community then takes full responsibility for the cleaning, maintenance and repair of the PSF.

Costs

The PSFs in Bangladesh cost about $ 1 500 each to construct, depending on size. This cost is currently met to varying extents by the DPHE, depending on the degree of need within the community and the ability of the community to fulfill the DPHE criteria governing provision of these systems.

Suitability

This technology is suitable for use in areas where there is adequate (seasonal) rainfall. In Bangladesh, the use of this technology is limited to those areas lacking access to adequate groundwater sources which can be accessed using tubewells are found to be successful in the location. In the areas where PSF systems have been developed, tubewells are not successful as suitable fresh water aquifers are not available at reasonable depths. Groundwater is saline down to depths of 200 m to 350 m, and naturally-occurring surface water sources are saline and also polluted.

Effectiveness of the Technology

The present version of the Pond Sand Filter (PSF) system is a great improvement over the older design, which did not incorporate the pre-filter nor did it protect the source water from contamination. The PSF is capable of producing a potable product water from a pond water source, removing both harmful organisms and impurities, and turbidity. The iron content of the product water is also reduced. This has contributed to a considerable reduction in the incidences of water-borne diarrhoeal diseases. The technology also provide a convenient platform for drawing water, washing utensils, etc. A 1989 survey of PSF users in Dacope by the DPHE and UNICEF indicated that:

- users were satisfied with the quality of the product water,

- 84% of users were female,

- users travelled an average of 0.5 miles, with a maximum distance travelled of 1.5 miles, to obtain potable water,

- 100% of users used the water for drinking, 80% used it for cooking, and 13% used it for washing; other water sources used were area ponds and rivers, which were used mainly for washing and cooking purposes,

- 36% of users used the water throughout the year, and 59% use it only in the dry season,

- during the dry season, users spent up to 20 minutes waiting to obtain water during periods of peak demand; longer delays discouraged water users, who tended to use nearby unprotected sources of water instead of the PSF treated water.

Advantages

This technology can be built to serve a large community. Use of this technology induces community cooperation in the provision of safe drinking water supplies.

Disadvantages

The major limitation of this technology is raw water storage. The pond must be large enough to ensure that it will not dry out in the dry season. It is also important to ensure that the salinity and iron content of the pond water not exceed 600 ppm and 5 ppm, respectively, at any time of the year.

Cultural Acceptability

This technology is very well accepted. The active involvement of the community demonstrates an high degree of community acceptability at the local community level.

Further Development of the Technology

This technology may be considered fully developed.

Information Sources

Contacts

Mohammed Aslam, Saleh Ahmed Chowdhury, Alamgeer Faridul Hoque, and S.R. Sanwar, Intermediate Technology Group, House 32, Road 13A, Dhanmondi, Dhaka, Bangladesh, tel. 880 2 811 934, fax: 880 2 813 134, E-mail: [email protected].

Bibliography

DPHE and UNICEF 1989. A Report on the Development of Pond-Sand Filter. Department of Public Health Engineering, Dacca.