EVALUATION OF ROUGHING FILTER AS A PRETREATMENT SYSTEM FOR THE WATER TREATMENT PLANT IN CHAKWAL

Abstract

Slow sand filtration is a proven and sustainable technology for drinking water treatment in small communities. The process, however, is sensitive to high raw water turbidity that can lead to premature clogging of the filter and frequent cleaning requirements, resulting in increased risk of poor quality drinking water. Multistage filtration, consisting of roughing filtration followed by slow sand filtration, can overcome treatment limitations posed by highly turbid water and serve a robust treatment supplement. In conditions where the turbidity of raw surface water varies considerably between dry and wet seasons, roughing filtration offers a viable pretreatment solution to prevent early exhaustion of slow sand filters. The existing treatment train at Chakwal Water Treatment Plant (CWTP) consists of online coagulation and sedimentation followed by slow sand filtration. To meet the challenge of increased turbidity (varying from 200 — 3300 NTU) a very high dosage of coagulant (102 mg/l in dry season and 272 mg/l in wet season) is added prior to sedimentation. This, not only raises the treatment cost (approximately Rs 1.8 M per month in the rainy season) but also results in a large volume of chemical sludge (110 ml/l). The primary goal of this research was to examine the efficiency of roughing filters as pretreatment system for surface water treatment plant at Chakwal. In this research, tests were conducted on bench scale as well as laboratory scale model roughing filters that were subjected to raw water from Chakwal Water Treatment Plant (CWTP) and synthetically prepared clay suspension. The laboratory scale model featured a treatment train composed of three columns of 15 cm depth packed with three grades of locally available gravel and the bench scale model featured three I m deep column packed with the same three grades of gravel. Independent variables within this experimental work included raw water turbidity, media size and depth, suspended solids and hydraulic loading rate. Dependant variables were the effluent turbidity, coagulant dose and sludge volume. Preliminary experiments comprised determination of the effective direction of flow and settling characteristics of the raw as well as the synthetic suspension. Experiments were carried out using both laboratory and bench scale models. Tests were initially carried out using laboratory scale model and by passing laboratory prepared clay suspension as influent. Results of these experiments were used to configure a laboratory scale treatment system of Upflow Roughing Filters in Series (URFS) using locally available media. After determination of the most efficient configuration i.e column depth, media size, hydraulic loading rate and direction of flow; a bench scale model was manufactured. Samples from the CWTP were obtained for both; wet and dry seasons, and later passed through the bench scale model. The performance of both multistage filtration models was highly dependent on the hydraulic loading rate and the amount of settle-able solids in the raw water. In water samples with high settle-able solids; i.e.(> 40 mg/l), higher turbidity removal was observed as compared to waters with low settle-able solids (< 8 ml/). Similarly raw water collected after a rainy spell gave a better removal result which was attributed to presence of settleable solids. Sample of water collected in dry season; having more colloidal matter, gave only 40-50 % turbidity removal efficiency. Turbidity removal was also found to be dependant on the direction of flow and the hydraulic loading rate. Test using synthetic clay suspension indicated that upward direction of flow results in 92% turbidity removal while downward flow provides only 80% removal. Similarly the hydraulic loading rate of 0.5 m/h resulted in more than 80% turbidity removal whereas a hydraulic loading rate of I .5 m/h provided less than 60% removal efficiency.