Reduction of sedimentation and water turbidity at intakes of drinking water treatment plants

ABSTRACTSedimentation at intakes of drinking water treatment plants (DWTPs) clogs intake pipe inlets causing interruption of water supply and serious pump abrasion. It occurs due to river flow velocity (FV) decline which causes flow inability to carry sediment particles. On the other hand, high velocities increase water turbidity because they induce flow turbulence that diffuses bed sediment particles over the water depth. The diffusion causes turbid water withdrawal and consequent high treatment running cost. This paper aims to adjust FVs near DWTP intakes using river training works (TWs) to control sedimentation and turbidity. Road El-Farag DWTP intake was taken as a case study. Groin construction and riverbed dredging were proposed as effective TWs. A 2-D mathematical model (SMS) was used to simulate different scenarios of the TWs. The simulations aimed to increase the FVs near the intake area provided that they neither allow sedimentation nor bed scour. The study area with FVs ranging from 0.025 to 0.40 was simulated in a physical flume to investigate the corresponding water turbidity (WT) values. A Jar test was conducted to find a relationship between WT and alum doses at different FVs. This relationship determines the optimum alum dose that can minimize turbidity. SMS and flume simulations revealed that positioning of groins on the river bank opposite to the intake area combined with dredging near the intake managed to increase FVs that can prevent sedimentation and bed scour. Finally, a mathematical relationship between alum doses and FVs was established.