Impact of alkalinity on the removal of natural organic matter from raw waters by enhanced coagulation

Abstract Natural organic matter (NOM) in all soils, ground and surface waters cause negative effects in potable water quality (undesired colour, taste, odour, and bacterial re-growth in distribution systems). Numerous studies have also found that the reaction of NOM with oxidative chemicals during drinking water treatment processes can result to the formation of carcinogenic disinfection by-products (DBPs). Many countries including South Africa, have therefore established regulations to control and minimise NOM and its effects. Enhanced coagulation (EC), a multiple-objective chemical dosing strategy, offers a viable option for NOM removal, and this study explores its use for typical South African raw waters. A consistent and reproducible jar test procedure, simulating the actual coagulation and flocculation pre-treatment steps, was developed and used to investigate the treatability of NOM (measured as UV 254 nm) in all the source waters. Ferric chloride was used as the coagulant due to its extensive application in South Africa. Raw water samples representing the various water types found in the country were seasonally collected for investigation, thus corresponding to a year-long data collection period. Since the removal of NOM is linked to strict control of pH, the coagulant dosage for the jar tests aimed at specific pHs (pH 7.0, 6.0, 5.5, 5.0 and 4.5) with the use of titration curves. The response parameters for the tests were temperature, turbidity, pH and UV 254 nm. Algorithms of finding the optimum dosage for both turbidity and UV 254 nm removal were developed from jar tests and consistently applied to subsequent batch tests. The results of the study suggested that low-alkalinity waters are more amenable to coagulation than high-alkalinity waters. The results also led to the conclusion that the alkalinity and pH of a water are key factors influencing coagulation performance. The optimum pH for the waters fell within the range of 5.0 to 6.5