Design and Performance Evaluation of Horizontal Flow Roughing Filter for High Turbidity Water

Abstract

In this work a pilot scale horizontal flow roughing filter (HFRF) was designed and operated at Chakwal water treatment plant (CWTP) in order to evaluate its effectiveness, to pre-treat highly turbid water. In Chakwal, highly turbid rainwater runoff and seasonal stream flows are collected in "Khoker Zer Dam" which serves as a major source of raw water supply to CWTP. Highly turbid influent of CWTP requrres high dose of coagulant (i.e. alum, 244 mg/l used by CWTPs management) which produce large volume of sludge. In addition, in wet weather, raw water requires higher coagulant dose (> 500 mg/l) yet coagulated and settled water turbidity remains above 30 NTU, resulting in shorter filter run length. Keeping in view the concept of multistage filtration that consists of roughing filtration followed by slow sand filtration which can overcome treatment limitation posed by highly turbid water. Major objective of this study was to design, construct and operate a pilot plant HFRF at different hydraulic loading rates, with and without addition of coagulant. The HFRF consisted of five compartments having identical dimensions but different media (gravel) size. Media size decreased in the direction of flow. The HFRF was operated at three filtration rates (1.3, 1.0 and 0.7 m/hr), for a 20+ days duration each. Jar test experiments were conducted on daily basis to determine the optimum coagulant dose. Later, the HFRF was operated with 50% and 25% of the optimum coagulant dose in in-line filtration mode at 0.7 m/hr filtration rate.