Disinfection

Controlling the Byproducts

Joe Honner, PE

Disinfection is a key step in the water treatment and distribution process to maintain safe drinking water to the customers of the system. Until recently, the Stage 1 Disinfectant/Disinfection Byproducts Rule, or Stage 1 D/DBP Rule, has dictated how rural and regional water systems manage one aspect of water quality: controlling disinfection byproducts. Most systems currently use free chlorine as the primary or secondary disinfectant. Primary disinfection occurs at the treatment plant as part of the treatment process to comply with the requirements of the Surface Water Treatment Rule (SWTR). Secondary disinfection provides a residual disinfectant in the finished water that is transmitted to the distribution system and aids in maintaining safe water.

Byproducts are a natural part of the disinfection process. They result as a reaction between the disinfectant and the constituent(s) in the source water. Different disinfectants produce different byproducts. Treatment with free chlorine, for example, results in the presence of two compounds: total trihalomethanes (TTHMs) and haloacetic acids (HAAs). Acceptable limits of each are mandated by Stage 1 D/DBP Rule based on the system’s running annual average (RAA) of water samples. The acceptable limits established under Stage 1 D/DBP Rule are maintained by Stage 2 D/DBP Rule. However, the method on which the compliance is determined has been shifted from a system-wide running annual average to a locational running annual average (LRAA). Thus, trouble spots or areas of high concentrations that were previously average out will be considered individually. This requires additional action by the system for changes in process.

Systems that use chlorine dioxide or ozone to disinfect their water must be conscious of the concentrations of chlorite (byproduct) and chlorine dioxide, or bromate, respectively in the finished water. These, too, are subject to Stage 1 D/DBP limits, which remain in the Stage 2 D/DBP Rule.

Regardless of the disinfectant used (free chlorine, chlorine dioxide, or ozone) for primary or secondary disinfection, Stage 1 and Stage 2 D/DBP Rule are the regulations that set the levels of acceptable byproducts.

It is important for systems to take a close look at their disinfection and treatment practices. There are several ways to reduce byproduct contamination. In older plants, for instance, chlorine added to raw water that undergoes long periods of detention in clarifiers has ample time to develop byproducts. The byproducts may be reduced if the detention time in which the free chlorine is added is shorter. The solution then may focus more on moving the disinfection to another point in the overall process, where the chlorine would have less time to develop byproducts. If the chlorine feed point is relocated, it is critical that the operator verify the system is still in compliance with the disinfection requirements of the SWTR regarding proper contact timer. An alternative secondary disinfectant, such as chloramines, may be considered when systems are having difficulty meeting the Stage 1 or Stage 2 requirements.

In many situations, operators try to control THM and HAA byproduct contamination by reducing the levels of total organic carbon (TOC) in the source water, thus reducing the concentration of organics available for the disinfectants to react with to create the byproducts. This is often done by adding coagulants to the treatment process. Another method treats the water with powdered activated carbon (PAC).The goal in both cases is the same: to reduce the amount of total organic carbon present with which the disinfectant interacts to create byproducts. Recent experience with a rural water system incorporated relocating the chlorine feed points to later in the treatment process in conjunction with focusing on additional TOC removal. Initial by-product analyses indicate a beneficial impact (lowering of the byproducts) without a significant input in capital costs. The result is the system will not have to implement modifications to their secondary disinfection practices in the short-term.

The Stage 2 Disinfectant/Disinfection Byproducts Rule, or Stage 2 D/DBP Rule enacted December 15, 2005,, places new demands on systems. Specifically, Stage 2 D/DBP changes the way systems account for levels of byproducts in their water and presents new challenges to water quality management. As a portion of the rule there are required Initial Distribution System Evaluations (IDSE) which require the system (rural water system or municipality) to analyze existing water quality data or create hydraulic models that predict where the highest levels of byproducts occur. Stage 2 D/DBP require running annual averages to meet not only overall system targets but individual/locational site limits as well.

The compliance schedule for systems that are a part of a combined distribution system, which includes a wholesale system and consecutive system(s), is based on the population of the largest entity within the combined system. Thus, a rural water system that purchases water from a large community is required to meet the compliance schedule of the large community. Each entity within the combined distribution system is required to submit a monitoring plan, complete the IDSE, submit the IDSE report, and conduct compliance monitoring.

It is increasingly more important for systems to examine what they have done previously to control byproduct contamination and conduct testing to determine what will work in the future to continue to meet the stated requirements.