Performance of existing pond sand filters and design modification

Abstract

Pond'Sand Filter (PSF), a simple option has been used mainly in the southern fringes of Khulna, Bagerhat, Satkhira, Jhalokathi, Noakhali, Perojpur and Barguna districts to treat the water from rain-fed ponds where ground water is mostly saline and suitable freshwater aquifers are not available. However, recent reports have shown that the performance of the existing PSFs are not satisfactory. Out of 477 PSFs in Pathorghata Upazila, Barguna only 36 (i.e. about 8%) are working (DPHE-Danida, 2000). In this study the performance of the existing PSFs in Pathorghata and Mathbaria Upazila were investigated. The major problems of the existing PSFs include slow production, poor performance in removing fecal coliform, shorter filter run and poor operation and maintenance. This study also reveals that performance of PSFs are declining due to some design faults including insufficient depth of filter bed, inadequate pre-filter chamber, insufficient outlet structure to prevent the negative pressure in the filter bed, lack of awareness among the beneficiaries. A survey was carried out among the beneficiaries about their opinion towards the existing PSFs and their participation in operation & maintenance. After critical examination of the technical, social and water quality issues of the existing PSFs, a modified community type PSF was developed on a pilot basis. Design modifications were made by providing sufficient depth of filter bed and incorporating a horizontal flow-roughing filter following vertical up-flow roughing filter with intermediate buffer zone. These were done as a method of extending runtime before cleaning by reducing raw water turbidity for trouble free operation of sand filter bed and effective capture of Fecal Coliform. The community type pilot PSF was constructed at Dhanisafa Munshi Bari, Mothbaria Upazila under Perojpur District. Another household type model PSF was also developed which was installed in the Concrete Laboratory of BUET. The study also examined the effectiveness of various treatment units of the community type pilot PSF and household type model PSF and, the findings were analyzed. Some important parameters including total coliform, fecal coliform, turbidity, salinity, pH, color, COD, TS, TDS, SS and arsenic were tested for the raw water and treated water at vmious steps of treatment process. Extensive laboratory investigations were done to determine the effectiveness of the treatment units of the community PSF and household type model PSF. Removal of bacteria particularly Fecal Coliform and time interval of washing the filter bed were also analyzed based on observational and experimental findings. In combination with roughing filters, the average turbidity and faecal coliform removal efficiency of sand filters were found to be 93.97% and 99.93% respectively. Filter run of the modified model PSF has also increased from about 30 days (in case of existing PSF) to 128 days. Performance assessment of the community type pilot PSF and household type model PSF were done in terms of quality of filtered water, ease of operation & maintenance, monthly check up of production capacity/efficiency of filter, observation of people's participation and acceptability of the modified PSF.