Download PDF Technologies for Clean Water September 1, 2008 Volume 38 Issue 3 The Bridge, Volume 38, Number 2 - Fall 2008. The papers in this issue of The Bridge describe some recent advances in the search for water, in the distribution and treatment of water and wastewater, and in the modeling of complex water systems. Water-Distribution Systems: The Next Frontier Monday, September 1, 2008 Author: Vanessa Speight Water-distribution systems, the last barriers in the water-treatment process, are vital to protecting public health. Technologies for treating drinking water have advanced significantly over the past century. Today the availability of abundant, clean, safe drinking water, on demand at every location, is taken for granted in the developed world. However, even this highly treated water is subject to degradations in quality once it leaves the treatment plant and enters the distribution system. By the time water reaches the consumer, its quality might be very different from what it was when it left the plant. Thus distribution systems, the last barriers in the water-treatment process, are vital to protecting public health. And, because pipes are buried and not subject to the direct control of water utilities, the management of distribution systems has become one of the most difficult challenges to providing safe drinking water. In the past few decades, researchers have increasingly focused their efforts on water-distribution systems. Maintaining a continuous water supply to customers, providing adequate fire flow (enough pressure to put out fires) to all parts of the system, maintaining water pressure, and ensuring water quality is a difficult balancing act. Because distribution systems must grow as cities grow, they usually are made up of a variety of pipes of different ages, materials, configurations, and quality. In most U.S. systems at least some of the pipes are more than 80 years old (EPA, 2005). The National Academies recently convened a panel to review the public-health risks from distribution systems (NRC, 2007). The report focused on three areas: physical integrity of the system (i.e., the quality of the pipes); hydraulic efficiency (i.e., the quantity and pressure of delivered water); and water quality (i.e., maintenance of a high level of quality throughout the system). All three of these factors must be addressed to ensure public health. The study panel also identified high-priority areas for risk reduction, including improvements in cross-connections, new and repaired water mains, and water storage (NRC, 2005). Distribution systems represent the next frontier for the drinking-water industry. In addition to the challenges of managing and replacing infrastructure, the industry is subject to detailed, highly critical scrutiny by the public and the media. Tools are emerging to address these challenges, but considerable work remains to be done. _________________________ Aging infrastructure and microbial or chemical contamination are major threats to public health. ________________________ The Infrastructure Crisis It is difficult to calculate the extent of buried infrastructure in the United States, but based on water-industry surveys, there are approximately 1 million miles of piping, 24,000 storage tanks, 6.8 million fire hydrants, and 14.6 million valves in water-distribution systems (AWWA, 2007). The average rate of water-main breaks is estimated to be on the order of 23 to 27 breaks per 100 miles of pipe. More than half of these breaks are attributable to the deterioration of materials in the pipes or fittings. However, one-quarter of them are associated with construction activities, which are generally not under the control of water utilities (AWWA, 2007). The cost of repairing water mains averages $3,000 per break for the repairs themselves; indirect costs, such as the impact on businesses that lose water service and damage to property, are much higher (American Water Works Service Company, 2002). Estimates of required infrastructure replacement by several industry groups and the Environmental Protection Agency (EPA) vary. But they all agree that maintaining the current level of service will require a very large investment in drinking-water infrastructure. The 2005 Report Card for America’s Infrastructure prepared by the American Society of Civil Engineers gives drinking-water infrastructure a grade of D–, down from its 2001 rating of D (ASCE, 2005). ASCE estimates that the country faces an annual shortfall of $11 billion, above and beyond the existing investment levels for replacing aging facilities and complying with drinking-water regulations. This report cites the conclusion of the Congressional Budget Office that “current funding from all levels of government and current revenues generated from ratepayers will not be sufficient to meet the nation’s future demand for water infrastructure” (CBO, 2003). The latest Drinking Water Infrastructure Needs Survey and Assessment estimates that a total investment of $276.8 billion will be required for drinking-water infrastructure in the next 20 years (EPA, 2005). This total includes both the installation of new infrastructure to meet growing demand and the rehabilitation or replacement of aging infrastructure. Of the $276.8 billion, approximately 75 percent would be related to distribution-system infrastructure: $183.6 billion for piping and $24.8 billion for storage systems. Rising energy costs adversely affect water utilities, which rely on extensive pumping to maintain pressure and deliver water. Despite some attempts to minimize costs by pumping during off-peak energy times, the majority of these costs cannot be avoided without compromising water-pressure levels and fire protection. Challenges to Maintaining Water Quality Microbial contamination in distribution systems is a potential threat to public health (Craun and Calderon, 2001). Pathways for the entry of contaminants into distribution systems include: organisms that survive the treatment process; contaminated ground-water that flows in from outside when pressure in a pipe drops; contamination during the installation or repair of water mains; and backflow from non-potable systems connected to potable plumbing. Chemical contamination can occur in the distribution system as a result of corrosion reactions, the accumulation of contaminated sediments, and the intrusion of chemical compounds into the pipes. Intentional contamination is also a potential threat. The Centers for Disease Control and Prevention (CDC), which tracks outbreaks of waterborne diseases related to drinking water, found that from 1971 to 2004, 21 of 168 reported outbreaks were attributable to deficiencies in distribution systems. The primary deficiency was cross-connection or back-siphoning from a contaminated water source (Roy, 2007). Degradation of Disinfectant Residual One key to protecting public health is maintaining a residual amount of disinfectant, typically in the form of free chlorine or chloramine (also known as combined chlorine), in the distribution system. However, as water travels through the pipes, the disinfectant oxidizes material in both the bulk water and on the pipe wall, thereby reducing the amount of disinfectant available to ensure continued disinfection (Figure 1). The decrease in the disinfectant residual can leave water vulnerable to contamination by microbes. FIGURE 1 Schematic drawing of chlorine decay reactions in a distribution system. Source: Adapted from Speight, 2003. At the pipe wall, chlorine can react with corrosion products, sediments, and biofilm, which forms when bacteria adhere to a surface and excrete slimy, glue-like polymers that form a protective barrier; biofilm is also a site for colonization by a variety of microbes (Montana State University Center for Biofilm Engineering, 2008). Biofilm has been shown to grow on most common pipe materials, but the quantity of attached bacteria is several orders of magnitude higher in unlined cast-iron pipes (Camper et al., 2003). Drinking-water regulations specify the minimum and maximum amounts of disinfectant allowable in water. Thus the problem of maintaining sufficient disinfectant residual throughout the distribution system cannot be solved by simply adding more chlorine at the treatment plant. In addition, chlorine not only disinfects, but can also produce several classes of harmful disinfection by-products. These compounds, which form when chlorine reacts with natural organic matter present in the water, are suspected carcinogens and have potential reproductive health effects (EPA, 2006). By-product compounds continue to form in the presence of free chlorine as water travels to the consumer, forcing water utilities to balance the need for disinfection with the need to minimize disinfection by-products. Therefore, it is important that treated water be delivered as quickly as possible to end users to minimize the formation of disinfection by-products and disinfectant decay. Contamination in Storage Facilities Water is stored in elevated tanks, ground tanks, and sometimes in the pipes themselves. The design and operation of storage systems varies widely across the country, mostly as a reflection of regional preferences and architectural influences. In addition, the need to provide enough water to maintain fire flow often means that storage tanks must be larger than is optimal for water-quality purposes. Depending on the design and operation of a storage facility, water might remain in a tank for an extended period of time. The longer the retention time in the tank, the greater the potential for the decay of residual disinfectant, the formation of disinfection by-products, and microbial regrowth. Tanks have also been shown to be the sites of contaminant entry into distribution systems, either through broken hatches or sediment accumulation (Clark et al., 1996). Therefore, the management of storage systems is a critical issue for the operation of distribution systems. Problems in End-User Systems Every location where water is available to consumers is connected to a distribution system. Typically a water utility’s jurisdiction ends at the customer’s meter, and the remaining plumbing is the responsibility of the building owner. All of the reactions that can degrade water quality in a distribution system can also occur in household plumbing. In fact, many incidents that gain media attention, such as the problems with lead in Washington, D.C., in 2004, are linked to household plumbing (EPA, 2007). New Tools Because access to distribution systems is extremely limited, the water industry relies on a variety of tools to operate, maintain, and continually improve them. Water quality is sampled daily in a variety of ways to meet regulations and to inform operational decisions. The traditional sampling method is “grab samples,” but the use of continuous (also known as online) monitors is increasing. Continuous water-quality monitors can measure simple parameters, such as pH and turbidity, or provide more sophisticated analyses, such as the concentration of residual disinfectant and total organic carbon. Considerable research is being done to determine the optimal number and placement of both grab samples and continuous monitors, the best parameters to monitor for different purposes, and the most accurate analytical methods (Speight et al., 2004). Hydraulic and Water-Quality Models Along with the collection of continuous data on water quality, data analysis and event detection are emerging fields (Hart et al., 2007). Because each continuous monitor can generate several data points per minute per water-quality parameter, sophisticated techniques must be used to sift through these data to identify meaningful information on the status of the distribution system. Much of the work on data analysis was first done in the security arena, but water utilities are looking for (1) benefits beyond security to justify investing in expensive continuous-monitoring equipment; (2) basic operational information; and (3) indications of contamination, intentional or otherwise (ASCE, 2004). Hydraulic models, which have been used for decades, provide reliable simulations of flows and pressures when appropriately calibrated to real-world conditions (Ormsbee and Lingireddy, 1997). Newer models can link hydraulic and water-quality parameters in a single simulation. Figure 2 shows the input data required for a hydraulic and water-quality model of a distribution system with chlorine disinfectant. FIGURE 2 Schematic drawing of model-input requirements to simulate chlorine in a distribution system. Source: Adapted from EPA, 2002. Accurate modeling of hydraulic and water-quality behavior in distribution systems is heavily dependent on the accuracy of the input data. For water utilities with very old pipes, substantial efforts are necessary just to collect and verify information about pipe diameters, materials, and locations. However, with the advent of geographic information systems (GIS), data required to build a model of a distribution system are becoming more readily available and more accurate (Figure 3). GIS is also a helpful tool for understanding the spatial relationships between water-quality measurements at different locations in the distribution system over time. A major challenge in hydraulic modeling is determining customer water usage at all points in the distribution system over time. Because meters are generally read on a monthly or quarterly basis, they do not provide real-time data. Innovations in automated meter reading may improve the collection of real-time data, but there are still significant challenges to data management. FIGURE 3 Example of a hydraulic model using GIS data. Therefore, real-time mod-els today are primarily used for energy management and for detecting situations that differ from the baseline, such as water-main breaks or large fires (Jentgen et al., 2003). The field has not yet advanced to the level of sophistication necessary to fully automate the operation of a water-distribution system, so very few utilities have created real-time models of their systems, which require linking operational data with the model and running repeated simulations. Even though water-quality models are still considered an emerging technology, they are beginning to be used to inform operational decisions and for developing plans for sampling or system changes. In addition, a relatively simple first-order decay model for chlorine, the most commonly modeled parameter for water quality, has been shown to provide good simulations of field data (Vasconcelos et al., 1997). Modeling of microbial contaminants in distribution systems is limited by the lack of knowledge about biofilm processes, the attachment and detachment of microbes to particles, growth mechanisms in low-nutrient environments, concentrations of microbes that enter the system via different pathways (e.g., low-pressure transient versus a cross-connection with a sewer), and the presence of pathogens. Particle-transport and deposition modeling are particularly challenging because of the highly variable flow caused by the highly variable use of water by individuals. Using deterministic models for simulating disinfectant by-products has been even less successful (Speight et al., 2000). However, probabilistic models of water quality look promising because they include mechanistic formulations that can account for uncertainties. However, because probabilistic models are computationally intensive, they are currently limited to research. Infrastructure Models Researchers in asset-management are developing tools for modeling the status of infrastructure based on data inputs, such as the age of pipes, the materials used in pipes, soil conditions, and the number of water-main breaks (Davis and Marlow, 2008). Asset-management analyses are used to help prioritize infrastructure investments by balancing the cost of replacing pipes, the likelihood of pipe failure, the consequences associated with failure of a given pipe, and other variables. The development of reliable, in situ condition-assessment technologies for water mains is an emerging field, as is the development of methods of quickly rehabilitating water mains that are out of service (Deb et al., 2002). Given the large financial burden of replacing the deteriorating water-distribution infrastructure in the United States, cost-effective approaches to asset management, rehabilitation, and replacement will be essential to the integrity of distribution systems in the future. Alternative Water-Delivery Systems Alternative water-delivery systems, such as dual-distribution systems that carry potable water and non-potable water separately, are also being investigated. In some areas of the United States where water is scarce, such as Florida and California, the use of reclaimed water (highly treated wastewater effluent) is common-place, and dual-distribution systems are being installed throughout cities to meet the demand for non-potable water. The challenges associated with dual-distribution systems are parallel to those of current systems; they include the cost of installation, maintaining the integrity of the buried infrastructure, maintaining water quality, and ensuring adequate supplies and pressures. Point-of-use treatment devices are also being considered in areas where water quality at the tap is inconsistent or cannot be guaranteed. Conclusions Research on water-distribution systems presents exciting challenges for the engineering community. Solving the problems facing distribution systems will require research in a number of interrelated fields, including infrastructure materials, water treatment, hydraulics, water chemistry and microbiology, data management, computer modeling, human behavior, public health and education, and risk management. Emerging tools include advanced hydraulic and water-quality modeling software, continuous monitors for water-quality parameters, and data-management systems. In addition, we still need basic data about the physical characteristics of buried infrastructure and fundamental research on water-quality reactions in distribution systems. Finally, the consequences of the infrastructure crisis are likely to be felt by consumers as water utilities struggle to find economic and political backing to repair, maintain, and replace aging structures. References ASCE (American Society of Civil Engineers). 2004. Interim Voluntary Guidelines for Designing an Online Contaminant Monitoring System. Available online at http://www.asce.org/static/1/wise.cfm. ASCE. 2005. Report Card on America’s Infrastructure. Available online at http://www.asce.org/files/pdf/reportcard/2005_Report_Card- Full_Report.pdf. AWWA (American Water Works Association). 2007. Distribution System Inventory, Integrity, and Water Quality. Prepared for the Environmental Protection Agency. Available online at http://www.epa.gov/safewater/disinfection/tcr/pdfs/ issuepape r_tcr_ds-inventory.pdf. 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Randomized stratified sampling methodology for water quality in distribution systems. Journal of Water Resources Planning and Management 130(4): 330–338. Speight, V.L., J.R. Nuckols, L. Rossman, A.M. Miles, and P.C. Singer. 2000. Disinfection by-product exposure assessment using distribution system modeling. Proceedings of AWWA Water Quality Technology Conference, Salt Lake City, Utah, November 5–9, 2000. Denver, Colo.: American Water Works Association. Vasconcelos, J.J., L.A. Rossman, W.M. Grayman, P.F. Boulos, and R.M. Clark. 1997. Kinetics of chlorine decay. Journal of the American Water Works Association 89(7): 54–65. About the Author:Vanessa Speight is practice leader for distribution-system modeling and master planning for Malcolm Pirnie Inc.