Effect of shading on performance of slow sand filters

This research investigated the effect of shading on three slow sand filters, each 1.65 m in diameter. One filter (designated “sun”) served as the control and was open to the sun and sky, the second filter (designated “shaded”) was partly shaded to exclude direct sunlight, and the third (designated “covered”) was completely shaded. All three filters were operated at filtration rates of 0.1 and 0.2 m/h using the same influent raw water. Research was conducted in the summer season when algal activity is at its peak. The performance of the filters was assessed on the basis of filtrate quality and length of filter run. Statistical evaluation of the physicochemical quality of filtrate was performed. Quantitative estimation was made of phytoplankton and zooplankton in the raw and filtered waters. Results indicated that in a tropical country, shading of slow sand filters may help reduce the algal activity in the filters without materially affecting the filter performance.