Study of performance of existing pond sand filters in different parts of Bangladesh

Abstract

Pond Sand Filter (PSF), a small scale filtering unit has been widely used in the saline problem areas of Bangladesh. Recently it has been tried in arsenic problem areas of the country too. But performance of the units is not up to the mark. Recent study shows that the performance of existing PSFs is not satisfactory. Out of 61 visited PSF in study areas only 48 (78.68%) PSF are functioning. In this study the performance of the existing PSF in 13 upazilas with arsenic and saline problem areas were investigated. The major problems of the existing PSFs include slow production, poor fecal coliform removal, shorter filter run and poor operation and maintenance. The study reveals that performance of PSF is declining due to design faults including wrong placement of intake pipe, insufficient depth of filter bed, inadequate pre.filter chamber, weak top lid, absence of overflow pipe in clear water reservoir. A survey was carried out among the beneficiaries about their opinion towards the existing PSFs to evaluate the social aspect of PSF. The study also compared the DPHE, NGO Forum and Danida model PSFs. It covers pond selection, intake system, pre-filter, sand bed depth, out let structure. turbidity, color, fecal coliform removal efficiency. The study examined the effectiveness of various treatment units of three model PSFs and analyzed the findings. Turbidity, color, fecal coliform were tested for the raw and treated water at various steps of treatment processes. Removal of bacteria, particularly fecal coliform, time interval of washing, filter depth and filter run lenh>1hwere also analyzed based on observational and experimental findings. The average turbidity and color removal of the PSFs under the study were compared. Turbidity removal of DPHE, NGO Forum and Danida model was 93.69%, 95.62% and 92.12% respectively. Color removal of DPHE, NGO Forum and Danida model was 88.16%, 93.76% and 88.22% respectively Beneficiaries participation in O&M for NGOs installed PSFs were better than others. Turbidity removal was satisfactory in all thee model PSFs. Effluent water color and fecal coliform did not meet drinking water standards. Only 6% of PSF treated water found coliform free. Performance assessment of PSF's was done in terms of quality of filtered water, ease of operation and maintenance, efficiency of filter and acceptability of existing PSFs.