Distribution system water quality

Matt Singleton
Director of Public Works
City of Grapevine, Texas
Member, APWA Water Resources Management Committee

The Environmental Protection Agency (EPA) implemented a Disinfectant and Disinfection Byproduct Rule (DBPR) to reduce disease incidence associated with disinfection byproducts that form when public water supply systems add disinfectants. Stage 1 established maximum contaminant levels (MCLs) of contaminants. Recently, Stage 2, which supplements existing regulations by requiring water systems to meet the disinfection byproduct MCLs at each monitoring site in the distribution system, was adopted. The rule also contains a risk-targeting approach to better identify monitoring sites where customers are exposed to high levels of disinfection byproduct (DBP). This proposed regulation will reduce DBP exposure and provide more equitable health protection, and will result in lower cancer and reproductive and developmental risks.

Chlorine and other chemical disinfectants have been widely used by public water systems as a principal barrier to microbial contaminants in drinking water. DBPs are formed when certain disinfectants interact with organic and inorganic materials in source waters. Many public water systems rely on surface water as the raw water source for drinking water production and contain relatively high levels of organic materials that combine with chlorine to produce DBPs.

In an effort to minimize DBP formation, many water purveyors utilize the combination of free chlorine and ammonia to produce chloramine to achieve disinfection. Chloramination is commonly used for disinfection purposes to control microbial growth in finished water. Reliance on chloramines for disinfection particularly over a long period of time can have adverse consequences in the distribution system. When chloramines are introduced into the drinking water system, the residual naturally begins to decay as the water ages and the demand exerted by various organic, inorganic and microbiological constituents in the water increases. In chloraminated systems, rapid decay of residuals can be caused by nitrification occurring in the distribution system or in the ground or elevated storage tanks.

Public water systems are required to maintain a minimum chloramine residual of 0.50 mg/l to ensure micrological safety. The City of Grapevine, TX as well as a number of other cities utilizing chloramine for disinfection purposes have observed seasonal decay of chloramine residual, typically occurring during the warm summer months. The decay has become more apparent as utilities comply with Phase II of the DBP rule that requires sampling from areas of the distribution system where high DBP concentrations exist.

Nitrification has been found to be a leading candidate for causing chloramine residual decay. Nitrification is a microbial process by which reduced nitrogen compounds (primarily ammonia) are sequentially oxidized to nitrite and nitrate. Ammonia is present in drinking water, either through naturally occurring processes or through the addition of ammonia during the disinfection process to form chloramines. The nitrification process is primarily accomplished by two groups of autotrophic nitrifying bacteria that can build organic molecules using energy obtained from inorganic sources, in this case ammonia or nitrite. Free ammonia either from overdosing of ammonia to form chloramines or from decay can cause nitrifying bacteria to form in the distribution system causing disinfectant decay.

Water age is also a factor in water quality deterioration within the distribution system. In addition to meeting current water demands, many water systems are designed to maintain pressures and quantities needed to meet future demands or to provide extra reserves for fire fighting, power outages and other emergencies. Impacts of these design practices lead to water age that can cause disinfectant deterioration. The design of many elevated and ground storage tanks is such that mixing of the influent water and stored water is poor. This leads to disinfectant residual decay, nitrifying bacteria formation and degraded water quality.

Operators of public water systems can avoid the pitfalls of degraded water quality, violation of public health standards and the risk of inadequate disinfection by adhering to a prevention/mitigation program that addresses the causes of disinfection decay. Understanding and awareness of the various unique characteristics of each distribution system is critical. Flow modeling, field testing to evaluate water quality, tracer studies, and operation practices will impact your ability to optimize and meet water quality standards.

The City of Grapevine identified areas of low disinfectant residuals. The primary problem areas were elevated storage tanks, dead end water mains and lower water velocity points in the distribution system. Testing revealed nitrifying bacteria as a probable cause. The heterotrophic plate count method was utilized to identify the problem. An integrated approach to improving water quality was implemented as follows:

  • A unidirectional flushing program was implemented. Valves are utilized to isolate sections of the system to increase water velocity. This will scour the biofilm (nitrifying bacteria) from pipe walls and subsequently dumped from the system.

  • Chemical dosages at the water treatment plant were adjusted. Ammonia dosage will be paced directly off of the free chlorine concentration to reduce the free ammonia concentration entering the system.

  • Elevated storage tank levels were fluctuated diurnally to improve mixing. The use of tank mixing devices will be investigated.

  • A nitrification response plan was developed and will be implemented when testing reveals disinfection residual problems.

Additional methods of combating nitrification include breakpoint chlorination, utilization of ozone or chlorine dioxide for disinfection, mechanical cleaning of lines, or chlorite amendments. Each of these methods is fraught with its own challenge; nevertheless, these are options to be considered.

No utility system wants the publicity associated with a mandatory "boil" notice. More importantly, proper disinfection and maintaining a disinfectant residual to ensure deactivation of microbial contamination is paramount in our responsibility as water purveyors.

Matt Singleton is a member of APWA's Water Resources Management Technical Committee and a Past President of the Texas Chapter. He can be reached at (817) 410-3328 or matts@ci.grapevine.tx.us.