Challenge

This municipality disinfects 1-1.5 million gallons per day of drinking water, and is currently transitioning from a small system serving <10,000 people to a large system serving >10,000 people. Chlorine gas was used as the primary disinfectant for the raw water entering the plant. TTHM’s levels consistently averaged 64 μg/L, the trigger limit for disinfection profiling. HAA5’s averaged 48 μg/L, and TOC reduction was consistently between zero and the required removal rate of 25%. Reducing the levels of TTHM’s, HAA5’s and TOC and meeting compliance standards for both the Stage 1 D/BPR and the LT1 Enhanced Surface Water Treatment Rule prompted the municipality to consider different pre-oxidants and alternative disinfectants.

Solution

A complete survey of the system was conducted to evaluate water quality and other factors such as consumption rate, maintenance, economics and compliance goals. A recommendation was made to the municipality to treat the raw water with chlorine dioxide at an initial feed rate of 0.2 mg/L. This dose rate was gradually increased after eight weeks to 0.8 mg/L until a trace amount of chlorine dioxide was measured in the clear well. As the dose rate of chlorine dioxide was increased an improvement in water quality was expected allowing for a reduction in the dose rate of the primary disinfectant chlorine gas.

Results

Chlorine dioxide effectively lowered TTHM and HAA5 levels by 30% over the same period a year ago. TOC levels have consistently been reduced by 25% or more using the new process which includes enhanced coagulation. The applied chlorine dose rates have also been reduced from 3.4 mg/L to 1.8 mg/L during the same month in consecutive years.