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.