Distribution Systems

Operational Concerns

Water quality can deteriorate in a distribution systems for many reasons. The main reasons are excessive growth of biofilm, disinfection by-product (DBP) formation, nitrification, and improper storage of finished water.

Biofilm Growth

Biofilm can be found on virtually any surface that comes into contact with the water in a distribution system, and biofilm grows until it sloughs off into the water.

Biofilm is formed in distribution system pipelines when microbial cells attach to pipe surfaces and multiply to form a film or slime layer on the pipe. Biofilm is a complex and dynamic microenvironment that includes processes such as metabolism, growth, and product formation, and finally detachment, erosion, or "sloughing" of the biofilm from the surface. The pieces of biofilm released into the water may continue to provide protection for the organisms that live in it until they can colonize a new section of the distribution system. In addition, biofilm may increase microbial-induced pipe corrosion, adversely affect pipe hydraulics, and reduce the utility of total coliforms as indicator organisms. All this leads to deterioration of water quality; generation of bad taste, color, and odor; and proliferation of macroinvertebrates.

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Disinfection By-Product Formation

Although water treatment is intended to remove all disease-causing bacteria, treatment does not produce sterile water. In fact, some organisms survive the disinfection process and may lead to disease in some individuals. Therefore, a disinfection residual in the distribution system is necessary to inactivate pathogens, maintain water quality, and protect the system against bacteria regrowth.

However, an excessive amount of disinfectant residual may itself pose a threat to health by contributing to the increased formation of harmful DBPs, such as chloroform and haloacetic acids. Many of these DBPs are suspected of causing cancer and reproductive and developmental problems in humans.

To minimize the formation of DBPs, EPA has promulgated regulations that specify maximum residual disinfectant level goals. In order to meet these requirements, public utilities may need to remove the DBP precursor material from the water prior to disinfection by applying appropriate treatment techniques.

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Nitrification

Ammonia is added to drinking water during secondary disinfection in order to form chloramine (used to remove DBPs from drinking water). But ammonia can oxidize into nitrite and nitrate. The formation of nitrite and nitrate, called nitrification, reduces alkalinity, pH, dissolved oxygen, and promotes bacterial regrowth in our drinking water, a direct threat to public health.

The use of chloramine is expected to increase in the near future as a result of more stringent regulations.

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Improper Storage of Finished Water

Finished water is that which has gone through all the processes in a water treatment plant and is ready for delivery. But long detention times in finished-water storage facilities before delivery may lead to water quality deterioration.

Water storage facilities are designed to meet temporary surges in water demands, reduce pressure fluctuations in the distribution system, and provide reserves for fire fighting, power outages, and other emergencies. However, if the built-in extra storage capacity is not properly maintained and used, the water will degrade. For example:

In addition to under use, short circuiting within a storage reservoir can cause long detention times, which results in excessive water aging.

Poor mixing (including stratification) can exacerbate water quality problems by creating zones within the storage facility where water age significantly exceeds the average water age throughout the facility. For larger distribution systems that contain storage facilities where water cascades from one facility to another (such as pumping up through a series of pressure zones), poor mixing can result in exceedingly long water storage in the most distant tanks and reservoirs.

Long detention times can allow the disinfectant residual to be completely depleted, thereby not protecting the finished water from additional microbial contaminants that may be present in the distribution system downstream of the storage facility. Although the loss of disinfectant residual within a storage facility does not necessarily pose a direct public health threat (many systems throughout the world are operated without use of a disinfectant residual), disinfectant decay can contribute to biofilm growth and related problems.

The rate of residual disinfectant decay can be affected by external contamination, temperature, nitrification, exposure to ultraviolet light (sun), and the amount and type of chlorine-demanding compounds present, such as organics and inorganics. Chlorine decay in storage facilities can normally be attributed to bulk water decay rather than wall effects due to the large volume-to-surface area ratio.

Sediment accumulation may also occur within storage facilities due to motionless conditions that promote particle settling. Potential water quality problems associated with sediment accumulation include increased disinfectant demand, microbial growth, DBP formation, and intermittent increased turbidity within the bulk water.

Uncovered finished-water reservoirs provide the greatest opportunity for contaminant entry into the distribution system. Reservoirs are subject to contamination from bird and other animal excrement that can transmit disease-causing organisms to the finished water. Microorganisms can be introduced into open reservoirs from windblown dust, debris, and algae. Organic matter, such as leaves and pollen, is also a concern in open reservoirs. Even reservoirs with floating covers are susceptible to bacterial contamination and regrowth from untreated water that collects on the cover surface: if the cover rips or is otherwise damaged, any untreated water on the cover would mix with the stored water, potentially causing health problems.

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