evaluation of horizontal - flow roughing filteration performance in turbidity removal

slow sand filtration (s.s.f) is a main process in the water treatment technology and generally considered as most appropriate for application in rural and small urban areas in developing countries. however, performance of the s.s.f is very sensitive to high turbidity levels. therefore, pretreatment is necessary for such waters. horizontal - flow roughing filtration (hfrf) and direct horizontal - flow roughing filtration (dhrf) have been porposed as two effective pre-treatment technologeis, prior to ssf. in this study two par all el lab - scale pilot plants were used. each pilot consisted of three units: raw water supply unit , coagula tion un it and horizontal filter. the filter was made of p. v. c. tubes, each 20 cm in diameter. each filter consisted of a 1.5 m long first compartment (15 to 25 mm grains) and 1.5 m second compartment (3-5 mm grains in firs t filter and 7-9 mm grains in second filter) . initial turbidity was 300 ntu. the coagulant dose in dhrf was 2.5 mg/l fecl3. the results of the experiments indicated that the efficiency of filtration decreases as the filtration rat e is increased. also, a fine graded filter, has a higher efficiency than a coarse graded one. in the first compartment of the filter in a dhrf procosses, floculation and sedimentation are efficiently achieved. the amount of wash water in the fast drain age method is acceptable. the scour velocity increases with the increased sludge age .