The primary reason we measure the level of various disinfectants in water is to ensure complete disinfection in order to protect public health. Disinfectants are measured in potable water applications as well as in pools, spas and wastewater applications. Under the Safe Drinking Water Act (SDWA), public water supplies are required to monitor the levels of disinfectants and disinfection byproducts to ensure they do not exceed the standards.
Amendments to the SDWA in 1996 required the U.S. Environmental Protection Agency (EPA) to develop rules to balance the risks between microbial pathogens and disinfection byproducts (DBPs). The rule is intended to strengthen protection against microbial contaminants, especially Cryptosporidium, and at the same time reduce potential health risks of DBPs. Prior to the 1996 amendments, the only regulation in regards to disinfection was from 1979, and established maximum contaminant levels (MCLs) of 100 ppb for the trihalomethanes.
In December 1998, the Stage 1 Disinfectant and Disinfection Byproduct Rule (DDBR) came out as the first rule under the 1996 amendment to the SDWA. The Stage 1 DDBR established maximum residual disinfectant level goals (MRDLGs) and maximum residual disinfectant levels (MRDLs) for three commonly used chemical disinfectants: chlorine, chloramine and chlorine dioxide. This rule also established MCL goals for the disinfection byproducts including: trihalomethanes, haloacetic acids, chlorite and bromate (Table 1).
In January 2006, Stage 2 DDBR was published, and strengthens public health protection for those being supplied water that is disinfected by requiring systems to meet the MCLs for trihalomethanes and haloacetic acids as an average at each sampling point instead of the system-wide average required in Stage 1. Depending on the size of the system and the amount of construction needed to comply, systems will begin the first year of compliance monitoring between 2012 and 2016 and must be within compliance with the Stage 2 DBP rule’s MCLs at the end of a full year of monitoring.
Samples & Sampling
Per the regulations, chlorine and chloramine monitoring requirements are the same in terms of number of samples and sampling point. They require as a minimum community water supplies and nontransient, noncommunity water supplies measure for residual chlorine and chloramines at the same point in the distribution system and at the same time as total coliform samples are collected.
Coliform sampling is based on the population served; therefore, the larger the population, the more disinfectant testing that needs to take place. Systems using chlorine as the residual disinfectant and operating booster chlorination systems must take three samples within the distribution systems. The samples should be collected as follows: one as close as possible to the first customer; one that is representative of the average residence time; and one as close as possible to the end of the distribution system.
In order to comply with the MDRL, systems running an annual average of monthly averages of all samples, computed quarterly, must be less than the established MDRL listed in Table 1. The EPA has approved several methods for measuring free and total chlorine based on ASTM and Standard Methods. Some states may have approved DPD colorimetric testing kits as a method to meet compliance of this rule.
Chlorine dioxide is mostly used to control taste and odor issues, but it is also used to oxidize iron and manganese and finally as a disinfectant and algaecide. Only systems using chlorine dioxide are required to monitor for this disinfectant.
Systems that are required to monitor must take daily samples at the entrance to the distribution system. Unlike chlorine and chloramines, the MDRL for chlorine dioxide cannot be exceeded for short periods of time due to potential health effects from short-term exposure to levels of the MCL of 0.8 mg/L. If any daily sample exceeds the MCL, the system is required to take three additional samples in the distribution system the following day.
The EPA has also approv