Application of Magnetized Anion Exchange Resin for Removal of DOC at the Coldiron Watkins Memorial Water Treatment Plant in Danville, KY
According to data gathered by the Information Collection Rule1, surface water treatment plants (SWTPs) can expect treated-water total haloacetic acid concentrations to be less than total trihalomethane (TTHM) concentrations. However, many SWTPs are plagued with distribution system haloacetic acid (HAA5) concentrations that are higher than TTHM concentrations. The 10 million gallons per day (MGD) Coldiron Watkins Memorial Water Treatment Plant in Danville, Kentucky, is one of these utilities. Distribution system HAA5 concentrations are up to two times higher than TTHM concentrations.
To comply with the Disinfectant/Disinfection By-product Rule2 (DBPR), the Danville Water Treatment Plant (WTP), like many surface water utilities, has tried to reduce disinfection by-product formation through enhanced coagulation and minimal chlorination. This technique has made only a small difference in TTHM and HAA5 concentrations.
Because of lack of contact time at this facility, disinfection must be applied at the rapid mix to maintain adequate CT (concentration X time) credits3. The current distribution system four-quarter running annual average (4QRAA) for TTHMs is 0.06 mg/L while the HAA5 4QRAA is 0.08 mg/L. Even though the annual average total organic carbon (TOC) removal ratio is 1.7, this utility will not comply with the Stage 1 DBPR total haloacetic acid maximum contaminant level (MCL) of 0.06 mg/L without additional treatment measures.
The source of supply for the Danville WTP, Lake Herrington, is a man-made impoundment that demonstrates TOC characteristics similar to sources not likely to be amenable to enhanced coagulation. Source water TOC is typically between 2-4 mg/L with the dissolved organic carbon (DOC) making up 98 percent or better of the TOC. Source water specific ultraviolet absorbance (SUVA)3 is slightly greater than 2 L/mg-m. Lake Herrington is highly buffered water with alkalinity typically greater than 100 mg/L and total hardness of about 170 mg/L.
To meet maximum contaminant levels (MCLs) established in Stage 1 of the DBPR2, the Coldiron Watkins Memorial WTP must improve the removal of DOC, a disinfection byproduct (DPB) precursor. One process considered for enhanced removal of DOC is continuous ion exchange using a magnetized anion exchange resin (MIEXÒ )4.