What are disinfection by-products (DBP)?

Human epidemiology studies have suggested a weak association exposure to chlorinated surface water and certain cancers, reproductive and developmental effects. Because hundreds of millions of people drink chlorinated water, the EPA takes the health risks associated with DBPs very seriously. Here are some ways NTMWD and Allen comply with standards.

EPA’s DBP Stage 1 Rule

The EPA required compliance with what is termed their Stage 1 Disinfection and Disinfection Byproduct Rule by January 1, 2002.  The rule established the highest levels allowed for the disinfectants chlorine, chloramines, and chlorine dioxide represented as the running annual average (RAA) concentrations of these disinfectants. The rule also set maximum levels as RAAs across all sample points for groups of DBPs called Trihalomethanes (THM) and Haloacetic Acids (HAA), as well as some other dangerous contaminants.  The Stage 1 rule also set water plant removal requirements for organic materials called Total Organic Carbon (TOC). The lessening of TOCs in the distribution system reduces the amount of materials that interact with residual disinfectants to from DBPs.

EPAs DBP Stage 2 Rule

The EPA later tightened the Stage 1 Disinfection and Disinfection Byproduct Rule by implementing a Stage 2 Rule that added more THMs and HAAs to the regulated contaminant lists. It also changed the RAA rule to establish contaminant concentration levels that apply at each sample point rather than the RAA across an entire distribution system. Allen and NTMWD implemented DBP Stage 2 rules in 2012. Though still an annual rolling average, many water systems expected meeting concentration limits at each sample point would be challenging.


Due to DBP Stage 2 rule changes, NTMWD fast-tracked a plan to use ozonation to disinfect water processed at its Wylie plant. The original purpose envisioned for ozonation implementation was to minimize adverse tastes and odors caused by annual summer algae blooms at Lake Lavon. However, it was known that ozonation plus the use of biologically active filters would help reduce TOCs and thus reduce DBP formation. The $125 million conversion helped the district and their customer cities meet DBP Stage 2 TOC limits, improved disinfection at the plant, and dropped DBP levels in the City distribution systems.

Biologically Active Filters (BAF)

The water plant is still undergoing the major construction activity associated with implementation of biologically active filters (BAF). By 2020, NTMWD will complete construction of BAF to treat the water at the Wylie plant. These filters allow beneficial microorganisms living on the treated water filters to consume and further reduce TOCs. After ozonation, water will be processed through the BAF before residual disinfectants like chloramine are introduced. Both ozonation and BAF represent state-of-the-art water treatment processes that reduce the TOCs that, when combined with residual disinfectants, form DBPs. NTMWD continues looking for emerging treatment technologies and processes to improve water quality.

Show All Answers

1. What is temporary change in disinfectant (TCD)?
2. What is a "chlorine burn" and is it different than "chlorine maintenance" or "TCD"?
3. Can you reduce the amount of chlorine in the water so the taste and odor isn’t as noticeable and disinfection byproducts (DBPs) are reduced?
4. What are disinfection by-products (DBP)?
5. Does Allen comply with DBP limits? How do we know tests are accurate?
6. Does our water system contain any lead pipes?
7. How does Allen regulate levels of lead and copper?
8. Where can I learn more about lead and copper in drinking water?
9. Where does the list of regulated contaminants and allowed concentration limits or ranges come from?
10. How are new contaminants selected for the list of contaminants and how are the limits established?