EFFECT OF COAGULATION-FLOCCULATION AND SEDIMENTATION PRE-TREATMENT STEPS ON FLOC CHARACTERISTICS IN DRINKING WATER TREATMENT

Abstract

Humic substances are major part of NOM (Natural Organic Matter) contamination in surface water. It adds color to water, transports heavy metals, precursor of carcinogenic DBPs formation and also act as a substrate for bacterial growth in water distribution system. Many advance technologies and disinfection procedures are being practiced for removing humic acid and pathogens from drinking water, but they are costly and posing severe environmental effects. Conventional water treatment process includes coagulation-flocculation using alum as chemical coagulant due to its cost effectiveness and easy availability. To improve efficiency of coagulation-flocculation and sedimentation, it is necessary to study floc properties in detail. In this study two synthetic feed waters (SFWs) have been prepared, one with humic acid (SFW-1) and other with humic acid incorporating E. coli (SFW-2). Optimum coagulant dose of 70

and 60 mg/L has been found for SFW-1 at pH 10.25±0.5 before coagulation and SFW- 2 at pH 7.5±0.5 respectively. Floc characteristics i.e., floc size and fractal dimensions

(Df) have been studied on coagulation at 150 rpm for 2 minutes with three flocculation regimes of high, medium and low Gtmix conditions for both SFWs i.e., 117660, 57564, 16713 respectively. After that settling time of 30 minutes has been provided in each condition. Largest average floc size was 395.1± 44.8 μm for SFW-1 and 498.2±29.7 μm for SFW-2 and highest Df was 2.64±0.04 for SFW-1 and 2.70±0.05 for SFW-2 achieved at medium Gtmix condition before 30 minutes of settling for both SFWs. This study may ultimately beneficial for improving water quality in treatment plants that plays a main role in achieving SDG 6 ‘clean water and sanitation’ target 6.1 ‘safe and affordable drinking water for all’.