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.
