Q: What is the most vulnerable aspect of providing public drinking water?
A: The distribution system.
The water distribution system consists of the path that finished drinking water takes after it leaves the treatment plant to the consumers’ taps. It can include storage tanks and finished water reservoirs, water mains and laterals, service lines to the home, plumbing and taps and shower fixtures. Water encounters many hazards along the way that can significantly alter its quality and safety.
Total waterborne disease outbreaks have been in general decline resulting from the changes in regulations and practices that have occurred since the implementation of the Safe Drinking Water Act, starting about 1976-1980. Microbial indicator monitoring is enforced, several filtration rules require surface waters, and groundwaters under the influence of surface waters, to be filtered and disinfected. Vulnerable groundwaters must be disinfected, and lead and copper corrosion potential is being assessed and corrected. Numerous disinfection byproducts and chemicals have been regulated.
While reported waterborne disease outbreaks have been declining, there has been a shift of the origin of disease outbreaks to distribution system deficiencies. In the CDC MMWR for 2009-2010, waterborne disease outbreaks totaled 33 and they resulted in 1,040 cases, 85 hospitalizations and 8 deaths. Most notably, 19 of the 33 outbreaks were caused by legionella bacteria, resulting in 72 cases and 8 deaths, and it was the only waterborne illness that resulted in deaths. Six more legionellosis deaths were associated with other drinking water originated aerosol exposures, such as from fountains or cooling systems. Reporting for drinking water-related legionellosis began in 2001.
There are numerous opportunities for water quality degradation to occur during its transit from the plant to the consumer. We will briefly discuss several of them.
Primary disinfectants include chlorine, ozone, chlorine dioxide and UV light. Chloramines are secondary disinfectants. Disinfectant residuals provide some protection from post treatment contamination and microbial regrowth, and keep the oxidation potential positive (e.g. nitrate, instead of nitrite). Chlorine 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, so a secondary disinfectant is also necessary. Chloramines are weak disinfectants, but they are chemically more stable and persistent than free chlorine. Chloramines are probably more effective at suppressing legionella regrowth than chlorine and chlorine dioxide because they can penetrate biofilms.
Until recently, there were many uncovered finished water reservoirs in use. Regulations now require coverage. Uncovered reservoirs allowed recontamination by runoff and animals. Even a disinfectant residual might not have been sufficient to provide adequate kill of some microorganisms.
Transit time and water age
Transit time and water age can vary from hours to days depending upon the system configuration, size, length and water demand. Low use extremities and dead ends provide opportunities for loss of disinfectant residual, regrowth and water degradation, including nitrification.
Distribution pipe ranges from pressurized large pipes, which are several feet in diameter, to laterals, several inches in diameter, to service lines from a few inches to one inch. Their composition could be coated or uncoated ductile iron or cast iron and a few asbestos cement in the large diameters, to copper and polyethylene or PVC plastics and some lead in the smaller diameters. Tuberculation that reduces flows, biofilm formation, pipe leaks and breaks, and corrosion are common problems. Total investments in distribution systems are in the trillions of dollars, so many are deteriorating and not being maintained and replaced as soon as necessary. Distribution systems are aging with many miles in the U.S. over 100 years old. Water suppliers often delay replacements. Replacement rates are often much less than 0.5 percent per year, so risks of breaks are high, and hundreds of breaks occur each year in large systems. Small leaks occur and breaks can cause major disruptions of service and damage to property, but they also provide opportunities for contamination to occur. Lines are disinfected before they are placed back into service, but inflows due to loss of line pressure cannot be readily disinfected.
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 also assured that the water is safe to drink. It seems unlikely whether that can be assured.
Hundreds of thousands of lead service lines were installed in some cities in the early part of the 20th century because of low cost and very long service life. However, as lead toxicity concerns grew it was recognized that corrosive waters can leach significant amounts of lead from them. Since that time, lead service lines are being replaced with other materials by attrition and sometimes directed when the Lead and Copper Corrosion Rule monitoring detects excessive corrosion. A notable example was the increase of lead measurements in Washington, D.C. about 14 years ago, when the system shifted from free chlorine residual to chloramine to reduce disinfection byproduct formation. Insoluble lead salt coatings were dissolving because of the change of water chemistry. The problem was ultimately resolved by addition of a few mg/l of phosphate at the treatment plant.
Disinfection byproducts and nitrification
Disinfection byproduct (DBP) formation continues when there is available organic carbon and active disinfectant residual present. Trihalomethanes (THM) and haloacetic acids (HAA) DBP indicators are monitored and regulated in the distribution system, and treatment processes are adjusted to control them.
Nitrification is the conversion of 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 and capable of converting ammonia or organic amines to nitrite, and nitrite to nitrate. About 25 percent incidence of several parts per trillion levels of nitrosamine has been detected in surveys, especially in surface supplies using chloramine disinfection. This is a minute portion of total daily exposure to nitrosamines from endogenous production, and food.
We have provided several prior articles in Water Technology on legionella risk from regrowth in warm water systems and showerheads. There are numerous additional pathogenic or opportunistic microbes such as pseudomonas aeruginosa and mycobacteria that can regrow and that have caused disease in high risk populations, such as in hospital patients.
Some plumbing configurations can permit backflow resulting from pressure drops in a system. Contaminated water can enter the drinking water by that route. Pressure drops can occur in an area when water is being pumped for fighting fires or other high demand events in a service area. Many plumbing codes require backflow preventers to be installed, but this is usually observed in the breach; a regular maintenance of the backflow preventers is also required but usually lacking.
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.
Distribution systems and plumbing are vulnerable to deliberate contamination. Most toxicants require such large additions to high volume water supplies that source and storage contamination is an unlikely risk. However, contamination in a building system requires only a small amount of material and a small pump for its introduction.
What can a person do?
Apart from encouraging their water suppliers to do their best and invest in the needed improvements, consumers have the option to add POU or POE treatment as barriers to some of those potential problems. POU for chlorine taste and ion exchange water softening are common treatments for aesthetic water problems, but some have suggested a “Final Barrier” concept, where treatment in the home is intended to remove contaminants of concern that might possibly occur. A complete “Final Barrier” approach would be expensive and require microbial filters and chemical removal treatment, probably RO plus activated carbon, or microfiltration, ultrafiltration and UV. RO is wasteful of water and carbon will grow heterotrophic bacteria. Maintenance is important.
Many would say that water quality changes in distribution and the need for rehabilitation of distribution systems should be EPA’s highest drinking water and health priorities, rather than regulating a few more chemicals of marginal concern that get headlines, but are of little public health consequence. Numerous distribution system problem opportunities are itemized here.
Drinking water is usually well treated and safe when it leaves the plant. However, the distribution process is clearly where more problems can occur and some of them are very difficult and expensive to control. Distribution systems were designed to provide sufficient water during peak demand such as fire events. That results in longer retention times and opportunities for degradation of microbial and chemical quality.
Other problems occur in the users’ plumbing system. Distribution systems and plumbing are expensive fixed investments and continuously aging, and costly to repair, replace and maintain. Backflow preventers should be installed and maintained. Tuberculation, biofilms that harbor pathogens, leaks and breaks and corrosion are common. Some people have decided to use POU or POE treatment to give them more control over the quality of their drinking water supply. Of course, that means that they are taking responsibility for controlling some of those problems, so they must be diligent in selecting the appropriate technologies and providing proper maintenance of their system.
Dr. Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the U.S. EPA Drinking Water Standards Division.