DELL RAPIDS, S.D. – When the town of Colton started testing positive for trihalomethanes the utility called the South Dakota Association of Rural Water Systems for assistance.
“The water has shown the potential to produce DBPs because it contains a significant amount of organic carbon,” explained SDARWS Training Specialist James Zeck. “Since they started using chloramination several years ago, they haven’t reported any problems with DBPs.”
Disinfection By-Products are the result of disinfecting agents like chlorine and ozone with material in the water. These byproducts are regulated and have maximum levels that can be present in drinking water.
Zeck worked with the systems bulk water supplier to possibly identify problems that could lead to high DBP levels. During the inspection Zeck noticed the in-line chlorine monitor responsible for dosing the ammonia for chloramination did not match the measured chlorine residuals leaving the plant or in the distribution system.
“Because the system was registering a lower chlorine residual, not enough ammonia was being fed and breakpoint chlorination was occurring in the clearwell leaving them with a free chlorine residual,” Zeck explained.
The utility replaced the chlorine monitor and recalibrated the system to accommodate the new chemical levels. The changes reduced the THMs. During a later follow-up visit, the system was operating properly and the town should show a significant decrease in the THM levels on their next round of quarterly sampling.