Comparing plain and coagulated horizontal flow roughing filtration for high turbidity water

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In this work, a multi-compartment, horizontal flow roughing filter (HRF) was designed, constructed and operated to evaluate its effectiveness against high turbidity surface water with and without coagulation. The objective was to evaluate the efficacy of the HRF in removing turbidity, total organic carbon (TOC), total coliform and particulate matter. The HRF was operated at filtration rates of 1.3, 1.0 and 0.7 m/hr. At 0.7 m/hr, plain HRF (PRF) reduced turbidity by 35%, TOC by 31%, coliform by 90% and particles with average diameter >4 μm. For coagulated roughing filtration (CRF), the optimum alum dose of 110 mg/L (L100) was determined using jar tests. In-line, CRF of the raw water using 25% (L25) and 50% (L50) of L100 reduced turbidity by 87 and 98%, respectively. TOC removal at L50 was 41% and total coliform removal up to 99%. CRF was more effective in removing particles with average diameter >1 μm. CRF did not only help attenuating the turbidity shock loads but also consistently produced high quality effluent of 5–10 NTU at 55 mg/L vs 176 mg/L used by the plant. Over 98% reduction in coliform and removal of most particles of >1 μm diameter demonstrated that CRF would also be effective in removal of protozoa such as Giardia.

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