Effect of hydrogeological factors on removal of turbidity during river bank filtration: Laboratory and field studies

Abstract

Turbidity removal from surface water was investigated in laboratory columns to identify critical hydrogeological factors during bank filtration such as fine content, soil bed depth, hydraulic gradient, grain size (d50) of aquifer materials as well as detention and operation time. Effect of hydrogeological factors on electrical conductivity was also evaluated. With increase in detention and operation time increase in turbidity removal efficiency and electrical conductivity was observed for both aquifer material as well as modified aquifer materials. The turbidity removal efficiency for aquifer material increased from 70% to 87.6% and 62.4%–79.1% with increase in detention time and operation time from 1 h to 7 h respectively. It was found that removal of turbidity increased from 84.1% to 100% with increase in percentage fine content from 7.5% to 17.5%, increased from 90% to 99% with increase in soil bed depth from 25 cm to 85 cm and increased from 74.1% to 92% with decrease in grain size i.e. D50 from 3 mm to 0.17 mm considering detention time of 7 h. Multiple linear regression analysis was used to demonstrate the vital hydro-geological factors affecting turbidity removal. Out of different hydro-geological factors, detention time and fine content were directly proportional to turbidity removal efficiency while turbidity removal efficiency was inversely proportional to hydraulic gradient and mean grain size. Validation of model was done using field study data, done at two river bank filtration sites, which gave satisfactory results and thus ensuring the precision of model developed.