National Drinking Water Clearinghouse
West Virginia University
PO Box 6893
Running Out of Water?
Dual Systems May Be Answer for Small Communities
by Daniel A. Okun,Ph.D.
Kenan Professor of Environmental Engineering, Emeritus,
University of North Carolina
On Tap readers dont need anyone to make a case about small community problems. No one knows better than people in small communities that their tap water is likely to be of poorer quality than that of people in large cities and that their wastewater disposal is often unsatisfactory. For many small communities, confronting these trials is a daily chore. And if that isnt enough, during dry periods, small communities know all too well that they will face water shortages long before large communities.
Recognizing that something needed to be done, the U.S. Environmental Protection Agency (EPA) financed a National Research Council study, Safe Water from Every Tap: Improving Water Service to Small Communities in 1997.
Photo Caption-In Irvine,California,farmers have been using reclaimed water to irrigate crops for decades.Population growth has become so great that the Irvine Ranch District installs dual systems in all new areas,while retrofitting older areas of the community.
This report addresses the difficulties that small systems face in meeting drinking water standards. Further, it describes appropriate technologies for small system water treatment as well as operator training. However, the report pays little attention to management issues, such as how regionalization can be more cost effective and make the systems involved more efficient. And the report does not adequately address the problems that small communities have in assuring adequate quantities of water.
During droughtswhen groundwater levels drop and small streams slow to a tricklesmall systems are the first to be without water. Seeking water from a distant source is seldom a feasible solution because such a source may not be available or its distance from the community may make the costs prohibitive
But there is good news. Small communities that face water shortages now or in the future have another option: dual distribution systems, which means one system for drinking water and another for nonpotable purposes.
Photo Caption-This chlorine tank in Cocoa Beach,Florida, serves a population of approximately 10,000 people. Chlorinating reclaimed water and maintaining chlorine residuals throughout the nonpotable system is necessary to protect against infectious disease should reclaimed water be inadvertently ingested.
Dual distribution systems reclaim a communitys wastewater and reuse it for nonpotable purposes. This practice is hardly revolutionary. The first such system was built in 1926 to serve Grand Canyon Village because water has always been scarce there according to E.C Garthe and W.C. Gilbert in "Wastewater Reuse at the Grand Canyon.
Hundreds of cities in the U.S. from the very large to the very small have adopted dual systems. They are also popular in Japan and Australia. California adopted the first state standards in 1968, and 40 states have since adopted reclaimed water guidelines or standards. California andFlorida have the greatest number of communities with dual systems.
EPA published the second edition of Guidelines for Water Reuse in 1992, while the American Water Works Association published the second edition of its Manual M24 Dual Water Systems in 1994. Many organizations regularly hold water reuse conferences in the U.S. and internationally.
Photo Caption-Travelling bridge filters,such as this rapid sand filter in cocoa Beach,Florida,are economical but difficult to maintain.Most operators prefer the more conventional deep-bed filt.
While adopting dual systems requires new thinking, it does not require new technology. The treatment practices are all in common use. And best of all, the public has adopted dual systems with enthusiasm. A big selling point is that dual systems assure customers of water no matter how serious the drought.
Reclaimed Wastewater for Nonpotable Uses
The quality parameters that characterize the reclaimed water for unrestricted nonpotable reuse, as recommended in EPA guidelines, are:
- per 100 milliliters (ml)
- Monitored dailyChlorine residual
- Minimum; 1 milligram per liter (mg/L) in the system
- Monitored continuously
- 2.0 nephelometric turbidity units average (NTU); 5 maximum
- Monitored continuously
Dual systems comprise two separate distribution systems: one for water for potable purposes and the other for nonpotable purposes. The United Nations Economic and Social Council in 1958 promulgated the principle that: No higher quality water, unless there is a surplus of it, should be used for a purpose that can tolerate a lower grade. In other words, why use limited, high-quality water resources to flush toilet when reclaimed wastewater can be made readily available for that purpose?
In communities that have opted for dual systems, the drinking water system is already in place, and the reclaimed water system only needs to be retrofitted. Of course, installing the two systems simultaneously, along with the sewerage system, is much more economical, but this is only possible for new communities, which are few in number. Dual systems can easily be introduced in the growth areas of existing communities. A common practice, used by the Irvine Ranch Water District, California, is to install the dual system in new areas, while retrofitting the reclaimed water lines in the older areas of the community.
All U.S. communities that discharge wastewater to surface water are obligated, under the Clean Water Act (CWA), to provide secondary treatment. Secondary treatment generally involves primary sedimentation followed by some type of biological treatment, most commonly, activated sludge treatment. But other processes are available. To reclaim wastewater, sand filtration and chlorination must follow secondary treatment, and thereby provide tertiary treatment. All communities that draw water for drinking from surface sources are obligated under the Safe Drinking Water Act (SDWA) to provide filtration and chlorination. These water and wastewater treatment processes are so common that many small communities can purchase them in off-the-shelf package plants.
Package plants for wastewater treatment and disposal have a bad reputation because regulatory agencies often do a poor job in monitoring them, and the plants often fail to meet the effluent quality requirements for disposal. Communities have tolerated these failures because poor quality effluent is usually only a problem to communities downstream. However, if the wastewater treatment plant is to serve as a reclamation plant and produce water for nonpotable purposes throughout the community, the community will want to be assured that treatment plants are adequately operated and maintained.
What are nonpotable uses for reclaimed water?
In many U.S. communities, large and small, the growing interest in golf seemed to parallel and even spur the growing demand for reclaimed water. Golf courses represent a major demand for water in many communities, and reclaimed wastewater is an economical approach to meeting that need. However, this perception has its down side because planning to use reclaimed wastewater only for golf courses is shortsighted.
Photo Caption-This car wash uses reclaimed water.When a drinking water shortage occurs,these kinds of commercial enterprises are the first to be cut off.
A wide range of urban nonpotable uses needs to be considered when siting facilities and designing distribution systems. They include:
These uses vary in importance in different parts of the country and among communities.
- landscape irrigationpublic and private;
- recreational irrigationgolf courses, sports fields, and playgrounds;
- agricultural irrigation for market crops in and near the community;
- industryprocess, cooling tower make-up, boiler feed, and sanitation;
- commercial and residential air-conditioning;
- toilet flushing, particularly in high-rise commercial, public, and multifamily residential buildings;
- cleaning vehicles, buildings, and streets;
- fire protection;
- constructionconcrete and dust control; and
- environmental enhancementmaintaining fountains, ponds, urban streams, and wetlands.
Customers readiness to pay for reclaimed water is an important consideration. Service charges around the country range from a small fraction to an equal amount of the charge for potable water. Often customers are ready to connect to a reclaimed water system because it ensures that they will not be cut off during periods of shortage, such as those with valuable shrubbery or car washes, often the first to go during drought.
Landscape irrigation is a major nonpotable water use in the U.S. An additional benefit of irrigating with reclaimed water is that it is rich in dissolved nutrients, such as nitrogen and phosphorus, which perform better and at far less cost than commercial fertilizers. In many locations, if the wastewater is not reclaimed, communities are obliged to resort to costly treatment measures to remove the nutrients.
What about fire protection?
Providing fire protection is a special case. From the earliest days of public water supplies, communities have used drinking water supply systems for fire protection. Using reclaimed water for fire fighting only saves a small amount of water. St. Petersburg, Florida, one of the earliest cities to adopt dual systems some 30 years ago, designed its retrofitted reclaimed water system for fire protection to add to the potable water systems capacity to fight fires.
Because conventional drinking water distribution systems provide for fire protection, pipe sizes and service reservoir storage capacities in small communities must be based on needs for fire protection, such as minimum pipe diameters of six or eight inches. The residence time between the water treatment facility and the consumers tap extends to many hours, which results in the loss of chlorine residuals, the growth of microorganisms in the pipelines, the deposition of aftergrowths on the pipe walls, and the degradation of water quality. Tuberculation of the pipe walls and the accumulation of sediment reduce the carrying capacity of the pipe lines.
Tuberculation is the accumulation of metal growths on the inside of pipe walls resulting from corrosion of metal pipes. It creates pockets that form adhesions on the pipe walls that reduce the carrying capacity of the pipe lines and permits aftergrowths on pipe walls that encourage microbial growths that reduce water quality as water leaves the treatment plants on the way to the customers tap.
If the reclaimed water system is designed for fire protection, the potable water piping can be very small diameter and investments can be made in higher quality pipe materials, which, with much shorter residence time in the system, would vastly improve the quality of the water at the tap. With this in mind, where retrofitting one of the two systems is necessary, it might be wiser to use the existing potable water system for the reclaimed water and retrofit with new, high-quality, smaller, potable water lines.
What water quality is required for urban nonpotable reuse?
The principal difference between potable and nonpotable water quality is that no effort is made to remove trace organic chemical contaminants in the reclaimed water nor is it necessary to perform costly monitoring to assure that they are absent. These contaminants are a health risk only when ingested over long periods of time. On the other hand, reclaimed water must be free of microorganisms that, if ingested inadvertently, cause infectious disease.
Photo Caption-High-rise commercial buildings in irvine,California,use reclaimed water for toilet nad urinal flushing as well as air conditioning.The plumbing fixtures in the above photo are in a high-rise building in Irvine.The potable water lines are on the left and the reclaimed water lines are on the right.
Reclaimed water is not visibly distinguishable from drinking water. The water may well be high in chlorination byproducts, which need to be tightly controlled in drinking water, but are of little significance in water that is not to be ingested over long periods of time. Online monitoring serves to assure that if the chlorine residual at the reclamation plant falls below a certain level, or if the turbidity exceeds a set value, the reclaimed water will be diverted from the reclaimed water distribution system to be run through the plant again.
Tertiary treatmentconventional sand filtration and chlorinationis how to attain water quality good enough for a variety of purposes. In some instances, conventional chemical coagulation precedes filtration. In addition, microfiltration membranes may replace the filters in some applications.
How do the two distribution systems differ?
When reclaimed water lines are retrofitted, most communities do not provide fire protection from them. Otherwise they are much the same. They need to be distinguished everywhere from the potable water pipes and fittings by color (for example, purple) and by lettering on the pipes. Both systems need wet wells, high-lift pumps, covered storage tanks for daily variations in supply and demand and for emergency. Finally, both systems need customer meters. Just as some water utilities have adopted rising block rates to conserve conventional water supplies, they have adopted these rates for some reclaimed water supply systems.
Communities need to develop plumbing codes that include measures for residential, commercial, public, and industrial properties for reclaimed water and to avoid cross-connections. Many states provide guidance for communities in all aspects of water reclamation, including providing financial incentives for adopting such conservation measures.
Who should manage dual systems?
Managing reclaimed water in a community may be a problem, depending upon how a communitys water and wastewater utilities are currently managed. If the systems are combined, management is simpler than where the two utilities are managed separately. In the latter instance, which utility should take the lead? If the driving force for the dual system has been the need for additional water, the water utility might well take the lead, but close cooperation is necessary.
A more ticklish problem exists with the state regulators, who are generally organized in separate water supply and pollution control agencies. This is the case with EPA, where SDWA administrators are responsible for water supply while CWA administrators are responsible for pollution control and, therefore, the treatment and discharge of excess reclaimed water to receiving waters. Thus far, the regulatory problems have not proved to be a significant barrier to adopting dual systems.
What is the conclusion?
Small communities that face water shortages for whatever reason should examine the potential for wastewater reclamation and nonpotable reuse through dual distribution systems as one of the many options for meeting future water demands. While the idea of dual systems may appear too costly, the fact that many communities, large and small, have adopted this approach is evidence that other options were either not available or were, in fact, more costly.
American Water Works Association. 1994. Dual Water Systems. AWWA Manual M24. AWWA: Denver, CO.
U.S. Environmental Protection Agency. 1992. Guidelines for Water Reuse.
EPA/625/R-92/004. EPA: Washington, DC.
Garthe, E.C., Gilbert, W.C. 1968. Wastewater reuse at the Grand Canyon. Water Pollution Control Federation (Water Environment Federation). 40:1582-85.
United Nations. 1958. Water for industrial use. Economic and Social Council: New York, NY.
National Research Council.1997.Safewater from Every Tap:Improving Water Service to Small Communities.National Academy Press:Washington, D.C.
About the Author
Okun is Kenan Professor of Environmental Engineering, Emeritus, University of North Carolina, Chapel Hill, NC 27599-7400. You may contact him at firstname.lastname@example.org.