Pre-treatment for slow sand filters

Slow sand filters are being used for the treatment of surface waters in many areas of Pakistan. Most slow sand filters draw water from irrigation canals. To ensure supplies during the canal closure period, storage tanks with a capacity equivalent to 21 days of supply are provided. The raw waters are highly turbid and cannot be treated effectively with slow sand filters without proper pre-treatment. The results of the studies of the operating slow sands filters in selected communities show that many of these filters have short filter run lengths besides producing turbidity in excess of the WHO guideline value for drinking water (Lillah, 1987; Durrani, 1992). The evaluation of the results shows that in most cases the storage tanks are unable to produce water suitable for treatment by slow sand filters suggesting additional pre-treatment (Ali and Tariq, 1987). This paper evaluate the effectiveness of Horizontal Flow Coarse Media Filter (HFCME) as a pre-treatment method for slow sand filters treating different types of raw waters.