Turbidity removal and trihalomethanes (THMs) formation in turbid surface water coagulation using ferric coagulant from natural iron in groundwater reacted with chlorine

Abstract

This research aimed at studying the possibility of using ferric coagulant which was produced from natural iron in groundwater reacted with chlorine for removing turbidity from surface water. Ping River water, which is currently the main water source utilized to produce water supply for Chiang Mai city, was selected as turbid surface water for the experiments. A selected groundwater in this study was from the shallow well near Ping River sampling point that naturally contains more or less 21 mg/L of total iron. Coagulations were experimented in Jar-test unit by using various sodium hypochlorite concentrations as chlorine sources added into different ratios of groundwater and surface water. The results shows that chlorine dosage of 30 mg as Cl2 per one liter of groundwater was the optimal chlorine dosage for producing maximum ferric coagulant of about 21 mg/L(as total iron). The ratio of groundwater to surface water of 35:65 under the previous optimal chlorine dosage were found to be the most appropriate coagulation condition for turbidity removal which could reduced turbidity in surface water to be below 10 NTU. In addition, it was observed that coagulation operated by dosing chlorine into a mixture of groundwater and surface water (called as post-chlorine adding method) gave more efficient turbidity removal than that of dosing chlorine into groundwater before mixing with surface water (called as pre-chlorine adding method). This study was also interested on trihalomethanes (THMs) formation in the coagulated water under the above-mentioned operational conditions. The results shows that total THMs concentrations at ratios of groundwater to surface water of 35:65 and 50:50 in raw water were 2.98 µg/L and 2.67 µg/L, respectively, whereas those in coagulated waters were increased to be 51.17 µg/L and 51.47 µg/L, respectively, which were higher than the Maximum Contaminant Level (MCL) for THMs of 40μg/L for stage 2 but were lower than that of 80μg/L for stage 1 of drinking water standard issued by U.S. Environmental Protection Agency (USEPA, 1998). Furthermore, chemical cost of sodium hypochlorite used in the coagulation process proposed by this study and chemical cost of the coagulation by commercial ferric chloride were compared, it could be stated that coagulation by using sodium hypochlorite added into a mixture of groundwater and surface water to produce ferric coagulant was cheaper than that of utilizing commercial ferric chloride.