In this study, a shallow-bed traveling-bridge (SBTB) filter was used as a contact filter, to investigate the effect of coagulant addition to the filtration efficiency. The filter bed was 25 cm deep, containing 0.55 mm quartz sand. 10–15 m3/hr. secondary effluents were filtered at 3.5–5 m/hr. For different alum doses effective removal of particulates up to (70–80%), turbidity (65–70%) and phosphates up to (80–90%) was obtained. The headloss data indicated an increasing surface straining with the addition of alum which, by changing the (aluminum/TSS) ratio, increased the floc volume. For this reason an optimum alum dose range would be 10–15 mg/l. The efficiency of using high molecular weight, medium to high charge density cationic polymers as primary coagulants was also investigated. The medium cationic high molecular weight polymer used as a primary coagulant at a 0.5 mg/l dose was able to efficiently remove > 10 μm particulates but did not significantly improve turbidity. The same polymer used at a 3 mg/l dose improved the removal of the whole range of particles. By using a high cationic high molecular weight polymer as a secondary coagulant it was possible to decrease the alum dose necessary for an efficient filtration.

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