Secondary disinfection’s role, drawbacks, and regulations in distribution systems

In this IQ&A, Garver’s leading experts answer the most pressing questions concerning secondary disinfection's impact on water quality, utilities, and regulations.

Why do distribution systems need to maintain a disinfectant concentration?

Disinfectant residuals serve multiple purposes in drinking water distribution systems, from curbing microorganism growth to indicating water quality within the distribution system. Since disinfectants react with substances within the water and on distribution materials over time, gradual decreases in disinfectant residuals can tell us where water is oldest and of lower quality, while sudden decreases can alert us to contamination events.

What would happen to distribution systems if there was no form of secondary disinfection?

Without a disinfectant residual, microbial activity would likely increase, possibly increasing the regrowth of pathogens because the water entering many distribution systems contains organic carbon and nutrients that serve as food for microorganisms. 

We would also lose a tool for monitoring water quality in our distribution systems, potentially allowing problems to go unnoticed and unaddressed. Many distribution systems in the United States have aging infrastructure that is at increased risk for leaks and contamination or may have corrosion products that harbor microorganisms. 

How do utilities address the drawbacks to secondary disinfection?

Utilities flush distribution system pipes and operate water storage tanks in a way that minimizes water age and mitigates the formation of “dead zones,” where water becomes stagnant due to the formation of regulated disinfection byproducts (DBPs).  DBPs form over time from reactions between disinfectants and natural organic matter in systems that use free chlorine and nitrification in systems that use chloramines. Nitrification is a biological process that converts free ammonia, usually from the breakdown of chloramines, to nitrite and nitrate. DBPs and nitrification are typically problematic when water age is high.

How have secondary disinfection practices influenced regulations on water quality?

Some states have implemented numeric standards for disinfectant residuals to increase protection against pathogens. The Surface Water Treatment Rule (SWTR) requires water systems to maintain a “detectable” disinfectant residual, rather than a numeric residual standard. Free chlorine has been used to provide that residual for many years.

Other states require water systems that use chloramines to implement nitrification monitoring and control plans to maintain water quality and prevent residual loss. Research in the 1970s linking DBPs to drinking water disinfection prompted many water utilities to either curb free chlorine usage or switch to chloramines, which form fewer regulated DBPs. The Stage 1 and 2 Disinfectants/DBP (D/DBP) Rules are examples of these changes. 

Currently, the EPA is considering revisions to the SWTR and D/DBP Rules, such as implementation of numeric residual standards, which may impact secondary disinfection practices in the next few years.

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