Drinking water distribution systems are unique long and complex structures and dynamic microbial and chemical environments. The water will contact different equipment (including pumps, valves and pipes) of different ages, sediments, tuberculation, biofilms, temperature differences, and stagnation times during its transit. The water that arrives at the consumer’s tap and glass is not identical to the water that left the water treatment plant. The challenge to the water treatment engineer and operator is to ensure that the water entering the system is safe and palatable, and that it has not been significantly degraded by its storage and travel throughout the distribution system before its use. Numerous opportunities for water quality degradation occur during its transit from the plant to the consumer. This article discusses several of them.

Waterborne disease

Reported waterborne disease outbreaks have been declining since the implementation of the Safe Drinking Water Act, starting around 1976 to 1980. However, the causes have shifted from what was previously inadequate water treatment to distribution system deficiencies. The 2011-2012 Morbidity and Mortality Weekly Report (MMWR) data from the Centers for Disease Control (CDC) are similar to the previous 2009-2010 report. The MMWR listed 431 cases of 10 microbial diseases, plus one chemical incident from a cross connection, and 102 hospitalizations from about 150,000 public water supplies. Of 32 total outbreaks, 21 were caused by inhalation of aerosols containing Legionella pneumophila that can proliferate at warm water temperatures and in biofilms, resulting in 111 cases and 14 deaths. Reporting for drinking-water-related legionellosis began in 2001.

Legionella was the only disease among the outbreaks that resulted in deaths. Legionella cases occurred in hospitals, long-term care facilities, guest lodging and apartment/condo locations. Hospital/health care and long-term care facilities accounted for 80 cases and 12 deaths. The guest lodging (hotel/motel/lodge/inn) category had 35 cases and two deaths, while 10 cases and one death occurred in apartments/condos.

Disinfectant residuals

Primary disinfectants include chlorine, ozone, chlorine dioxide, and ultraviolet (UV) light. Chloramines are secondary disinfectants. Disinfectant residuals provide some protection from post treatment contamination and microbial regrowth and keep the oxidation potential of the water in distribution positive (for example, nitrate instead of nitrite). Chlorine/hypochlorite is the most common primary disinfectant with chlorine dioxide, ozone and UV as distant but growing methods. Free chlorine and chlorine dioxide can provide residuals, but ozone and UV do not, making a secondary disinfectant necessary. Chloramines are weak disinfectants, but they are chemically less reactive and more persistent than free chlorine. Chloramines are probably more effective at suppressing Legionella regrowth because they can penetrate biofilms.

Uncovered reservoirs

Until recently, many uncovered, finished water reservoirs were in use. Regulations now require coverage. Uncovered finished water reservoirs allowed recontamination by runoff and animals. Even a disinfectant residual might not be sufficient to kill some microorganisms introduced into the reservoir or chemical contamination. For example, several years ago an uncovered finished water reservoir in Los Angeles had significant bromate formation from sunlight acting upon the chlorine residual in the presence of bromide.

Transit time & water age

Transit time and water age can vary from hours to days depending on the system configuration, size and length, and water demand. Low use extremities and dead ends provide opportunities for a loss of disinfectant residual, disinfectant byproduct (DBP) formation, microbial regrowth and water degradation, including nitrification.

Storage tanks

Elevated or on-ground cylindrical storage tanks provide water storage and continuous flow and pressure. Flat cylindrical tanks are especially prone to accumulated sediments from inches to feet in depth. The sediment can be a growth site for microbes and protect them from disinfectants. Tanks must be regularly inspected using divers or cameras and cleaned, as needed, by power washing or chemical methods. Recommendations include spray-on and rinse-off formulations that remove biofilms and scale.

Distribution pipes

Distribution pipes range from pressurized large pipes several feet in diameter to smaller transmission pipes to water service laterals from a few inches to 1 inch. Their composition could be coated or uncoated ductile iron or cast iron; some asbestos cement pipe in the large diameters; copper and polyethylene or polyvinyl chloride plastics; and some lead in older, smaller diameter home services.

Tuberculation (the development of small mounds of corrosion products on the inside of iron pipes) that reduces flow, biofilm formation, pipe leaks and breaks, and corrosion are common problems requiring constant attention. Total investments in water distribution systems are in the trillions of dollars, and many are deteriorating and not being maintained and replaced before their useful service life has expired.

Distribution systems are aging with many miles in the U.S. more than 100 years old. Replacement rates are often much less than 0.5 percent per year, so risks of breaks are high. Hundreds of them occur each month/year in a large system. Leaks and breaks can cause major disruptions of service and damage to property, but they also provide opportunities for water contamination. Lines are disinfected before they are placed back into service, but inflows because of loss of line pressure, for example, during high demand such as during a fire in the area, cannot be disinfected.

Maintenance flushing

Distribution systems accumulate sediment, tuberculation and biofilms, so regular maintenance is essential. This is usually accomplished by unidirectional flushing between fire hydrants. The dislodged material often colors the water reddish brown, and customers are usually notified to anticipate the temporary changes, and frequently also assured that the water is safe to drink. It seems unlikely that safety can be proven, given that a bacterial sample would usually require about 24 hours before results are known.

Lead service lines

Millions of lead service lines were installed in many cities in the early part of the 20th century, and later in a few places because of low-cost and long service lives. However, as lead toxicity concerns grew, it was recognized that corrosive waters can leach significant amounts of lead. Lead service lines are being replaced with other materials by attrition, and removal is sometimes required when the U.S. Environmental Protection Agency’s (EPA) Lead and Copper Corrosion Rule monitoring detects excessive corrosion that cannot be resolved by water conditioning.

A notable example is the increased lead detections in Washington, D.C., about 14 years ago when the system shifted from free chlorine residual to chloramine to reduce disinfection byproduct formation. Another example is the 2016 dilemma in Flint, Michigan, when the source of raw water was changed without managing the different corrosion characteristics. Insoluble lead salt coatings were dissolving because of the changed water chemistry. Old deteriorating galvanized iron plumbing is a factor because not only is the water rust colored, but lead can also accumulate on the suspended iron oxides. Rather than lead, the more significant Flint problem could well be a spike in legionellosis and related deaths that occurred after the source water change.

The problem was ultimately resolved in Washington by adding a few milligrams per liter of phosphate at the treatment plant. The Flint problem has abated many months after  the system reverted to the original source. That issue is ongoing and complex. It resulted from a breakdown of standard water treatment practice and inadequate regulatory oversight.

Disinfection byproducts & nitrification

Disinfection byproduct formation continues when organic carbon and a chemically active disinfectant residual are present. The DBP formation chemistry continues until the precursors or the disinfectant has been exhausted. Trihalomethanes (chloroform, bromoform, bromodichloromethane and dibromochloromethane), five haloacetic acids, and DBP indicators are monitored and regulated in the distribution system, and treatment processes are adjusted to control them.

Nitrification is the conversion of ammonia nitrogen species to nitrate and nitrite and possibly nitrosamines during distribution. It occurs especially in long distribution systems when water is derived from surface sources, chloramine is used as the residual disinfectant, and nitrosomonas bacteria are present.

Regular monitoring throughout the system and especially in dead ends and remote regions with longer retention times is necessary to determine whether nitrification is occurring.

Microbial regrowth

Legionella risk from regrowth in warm water systems and showerheads is prevalent. Numerous additional pathogenic or opportunistic microbes such as Pseudomonas aeruginosa, Naegleria fowleri and mycobacteria can regrow in biofilms. Regrowth microorganisms, especially Legionella, have caused disease and death to patients in hospitals and also to guests in hotels from exposure to aerosols from showering and cooling system heat exchangers as well as from spa use. Other microorganisms could also be inhalation or ingestion risks.

Biofilms are reservoirs where microbes can congregate and multiply in an environment protected from the disinfectant residual. The at-risk population that includes people who have immune deficiencies, the elderly and smokers is increasing and is not confined to hospital settings. This is an unanticipated consequence of plumbing and potentially endemic in all plumbed water systems throughout the world.


Some plumbing configurations can permit the backflow of contaminated water resulting from pressure drops in a system. Pressure drops can occur in a building on different floors, or when water is pumped for fighting fires or other high-demand events in a service area. Many plumbing codes require the installation of backflow preventers, but this is often observed in the breach. A regular maintenance of the backflow preventers is also required but is usually lacking.

Cross connections

Cross connections between potable and non-potable plumbing systems in homes and buildings are not uncommon. They can occur from errors in original installations and during repairs. When more than one type of water is plumbed in a facility (such as potable drinking water and recycled water for irrigation or sanitation purposes), this becomes a particular concern, but it also occurs from crossing drinking water and waste lines.


Distribution systems and plumbing are vulnerable to deliberate contamination. Most toxicants require such large additions to high volume water supplies that contamination of sources and storage is an unlikely risk. However, contamination in a building system requires only a small amount of material and a small pump for its introduction.


It is an anomaly that as drinking water standards become more stringent, demanding more sophisticated water treatment, the distribution systems continue to age and are more likely to degrade that water as it transits to the consumer. Water quality changes during distribution, and the need for rehabilitation of distribution systems should be among the EPA’s highest drinking water quality and infrastructure priorities, rather than regulating a few more chemicals of probably little public health consequence. Numerous distribution system problems exist, and waterborne disease incidences relating to distribution are increasing.

Drinking water is usually well-treated and safe when it leaves the plant. However, in the distribution process, more problems can occur, and some of them are difficult and expensive to control. Distribution systems were designed to provide sufficient water during peak demand such as fire events. That results in longer water retention times and opportunities for degradation of microbial and chemical quality. Even so, drinking water in the U.S. is almost always safe.

Distribution systems and plumbing are expensive, fixed investments and are continuously aging and costly to repair, replace and maintain. However, the cost of neglect is substantial in money, public health and public acceptance. Tuberculation, biofilms that harbor pathogens, leaks and breaks, and corrosion are common major problems. Concerns about water quality and off taste and odor contribute to a loss of consumer confidence in the drinking water supplier and greater use of bottled water and home treatment.

Water terms

DBPDisinfection byproducts are THMs and HAAs among others.

HAAs Haloacetic acids; the five most common are monochloroacetic, dichloroacetic, dichloroacetic, monobromoacetic and dibromoacetic.

MMWR Morbidity and Mortality Weekly Report

THM Trihalomethanes, which are chloroform, bromoform, bromodichloromethane and dibromochloromethane

SDWA Safe Drinking Water Act

Dr. Joe Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants and technical editor of Water Technology. He is a former director of the EPA Drinking Water Standards Division.