Enhanced coagulation: its effect on NOM removal and chemical costs

hlorine is the most widely used oxidant in the United States for water disinfection. But chlorine reacts with the natural organic matter (NOM) that remains after treatment and forms halogenated by-products. Only some of these by-products have been identified, such as trihalomethanes (THMs) and haloacetic acids (HAAs), which are suspected to be human carcinogens. In 1979 the US Environmental Protection Agency (USEPA) set a maximum contaminant level (MCL) of 0.10 mg/L for total THMs in drinking water. More recently, USEPA proposed the Disinfectants/Disinfection By-products (D/DBP) Rule, which will reduce the THM MCL to 0.080 mg/L. In addition, HAAs will be regulated along with other DBPs at a proposed MCL of 0.060 mg/L. USEPA has also announced that as additional health risk data are collected, Stage 2 of the D/DBP Rule will be developed, which will revise the DBP MCLs. Long-term standards for THMs and HAAs may be Several bench-scale studies were conducted to evaluate the effectiveness of enhanced coagulation for controlling chlorination by-products and to assess the impact of coagulation pH preadjustment on total organic carbon (TOC) removal and plant operating costs. Tests were conducted on a variety of surface water sources with TOC values ranging from 2 to 11 mg/L. An iron-based coagulant, such as ferric chloride, was consistently more effective than alum in removing natural organic matter (NOM). Coagulation pH appeared to be a determining factor for maximum NOM removal when ferric chloride was used as a primary coagulant. Typically, preadjustment of pH at a value of 6.0 ± 0.2 increased NOM overall removal to as much as 65 percent and reduced the coagulant dose by as much as 60 percent. Enhanced coagulation led to higher overall operating costs, but preadjustment of pH with sulfuric acid reduced costs by lowering the coagulant dosage as well as sludge production.