Arsenic removal from water supplies in northern Chile using ferric chloride coagulation

Many raw waters in the and North of Chile contain high concentrations of arsenic (0.1-1.0 mg/l) and, during the 1970s, drinking-water treatment using coagulation was introduced in an attempt to comply with the Chilean standard of 0.05 mg/l. The new World Health Organization recommendation of 0.01 mg/l for drinking water has led to efforts to enhance arsenic removal. This paper describes pilot-plant experiments which were carried out to optimize removal by varying the ferric chloride coagulant dose (3-9 mg/l Fe) and pH value (pH 5.5-8.0) in a raw water which contained an average arsenic concentration of 0.44 mg/l. At pH 5.5, arsenic adsorption was best; however, a pH of 6.5 was considered to be the most suitable for treatment when considering flee elimination. An empirical formula to predict residual arsenic under different operational conditions was obtained and this was confirmed by data collected at a full-scale water-treatment plant.