Bristol County Water Authority

July 31, 2013

The Bristol County Water Authority is in the process of installing treatment systems to reduce the amount of Trihalomethanes (THM) in the water, with the installation of a chlorine injection and monitoring system and a THM removal system in the 2 million gallon Bay View water storage tank in Bristol.

Trihalomethanes are formed when chlorine, used for disinfection, comes in contact with the natural organic compounds present in water. The entire water supply comes from the Scituate reservoir, one of the highest quality reservoir systems in the country. However, that system, like all others, contains organic compounds that form when vegetation in the watershed area decays. The EPA regulation for amount of THMs allowed in drinking water has recently been made more stringent. A running annual average of below 80 parts per billion is now required. (THMs may be cancer causing agents if high levels are ingested over a period of 70 years.)

The THMs are testing close to the limits in the summer, when higher levels of chlorine are used and the water temperatures are elevated. Both of these factors increase the amount of THMs in the water.

Reducing the amount of THM’s can be accomplished by removing natural organics during the water treatment process, minimizing the amount of chlorine added for disinfection, and by removing THM’s by aeration within the distribution system.

Providence Water has worked to optimize the removal of natural organic compounds at their Scituate treatment plant, but they are dealing with very low levels in the parts per billion range. Removal rates in this range are not efficient enough to preclude the formation of THMs from surface water supplies.

The BCWA has installed a new automated chlorine monitoring and injection system on the main connection to Providence to closely control the amount of chlorine added to the water. In addition, water quality monitoring efforts have been increased to include analysis of background bacteria in water samples to provide better indication of when chlorine levels need to be adjusted, allowing the operators to optimize treatment.

The installation of the THM removal system in the Bay View storage tank is the next step in the course of actions to reduce THMs. Since THM’s are volatile, they can be removed by aeration. This project involves a 3 step process: 1) provide a thorough mixing of the water in the tank; 2) pump water through sprayers at the top of the tank to form very small droplets to transfer the THMs to the air in the tank; and 3) blow fresh air into the top of the tank to promote the removal of the THMs.

These projects, included in the BCWA Strategic Plan and Capital Plan, should maintain THM levels well below the regulated levels.

Additional information is available from the RI Department of Health and the EPA website:

Contact: Pamela M. Marchand
Executive Director
245-2022, ext 18

Tagged on: