The Case for Using Reclaimed Water

Issue 11 and Volume 112.

Using water reclaimed from waste water treatment plants to cool power plants isn’t new, but the trend is likely to increase.

By: Ethan T. Smith, Ph.D., Sustainable Water Resources Roundtable, and Harry X. Zhang, Ph.D., CH2M HILL

If climate change is found to have significant impacts, one area may be in how precipitation changes will affect how much water is available for power production. Climate change can affect water availability due to changes in air temperature, precipitation and their interactions. Impacts depend on annual and seasonal changes in air temperature and precipitation and also the form of precipitation. Other factors include the frequency and magnitude of extreme events such as heat waves, storms and droughts.

While data exist to indicate a general warming, information concerning current trends in precipitation change is not definitive. The Intergovernmental Panel on Climate Change published a map showing precipitation trends from 1900 to 2000, which shows increases and decreases on different continents (Figure 1). Much of the United States seems to show an increasing precipitation trend. However, one should be careful about drawing conclusions based on relatively short periods. Among global circulation models, a much higher variation in regional precipitation exists than in temperature. For some regions, the models even disagree as to whether future carbon emission scenarios will lead to an increase or decrease in precipitation.

Figure 1 World Precipitation Trends from 1900 to 2000
Source: IPCC (2001), www.ipcc.ch/present/graphics.htm
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Two Problems

Power plant operators face two major problems. The first is physical: whether or not possible climate change adversely affects the amount of water available for power generation. The second is economic: how to determine the most cost-effective way to find water for power generation, regardless of what the physical effects turn out to be. This article will show how waste water treatment plants (WWTPs) may be part of the solution to both problems.

Climate change could affect energy production and supply in any of the following cases:

  • If extreme weather events become more intense
  • If regions dependent on water for hydropower and/or thermal power plant cooling face reductions in water supplies
  • If rising temperatures decrease overall thermoelectric power generation efficiencies
  • If thermoelectric plants cannot meet their thermal discharge permits.

In December 2006, the U.S. Department of Energy published a report to Congress titled “Energy Demands on Water Resources” that detailed many interdependencies. High on the list was the effect of power plants, both once-through and evaporative cooling. The report called for federal action in regulation, collaborative planning and scientific research.

A 2007 report, “Effects of Climate Change on Energy Production and Use in the United States,” identified a range of possible impacts for fossil, nuclear and renewable sources. Of those impacts listed, we will address only the possible impacts due to cooling water demands compared to available supply. Because of the great disagreement about forecasting techniques, we have chosen here to rely on historical data. Opinions can differ about how to interpret the statistics, but at least we are dealing with recorded facts.

Power plants represent a significant component of U.S. water withdrawals. Data from the U.S. Geological Survey indicate that from 1950 through 2000, demands for cooling water (along with irrigation demands) consistently outrank all other withdrawals of water in the nation. Figure 2 shows this trend. Much of the past cooling withdrawal was caused by operating once-through cooling systems, which actually consume a small fraction of what is withdrawn. How will closed cycle cooling systems used in new plants, increased demands for electricity and climate change alter this large withdrawal demand? What options are there to alter cooling water use efficiency and reduce cooling water demands? What indicators would assist policy makers in addressing this issue?

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Water Availability for Cooling

Changes in U.S. streamflow appear favorable. A 1999 study by the U.S. Geological Survey analyzed domestic streamflow trends for 395 climate sensitive streamgaging stations in the coterminous U.S. between 1944 and 1993. Generally, streamflow has increased across broad sections of the nation. Decreases appear only in parts of the Pacific Northwest and the Southeast. The results indicate that the country will have fewer extremes in streamflow amounts. This is exactly what’s needed for additional cooling water.

This effect is illustrated in Figure 3, which shows two important characteristics. First, the number of increasing streamflow trends is high. The number is approximately equal across the lower half of the flow distribution, but falls sharply across the upper half. Second, downtrends decrease in number from the Q0 to Q50 flow, but increase from the Q50 to Q100 flow. This pattern indicates that baseflows are increasing (which suggests that drought is decreasing), median or average streamflow is increasing, but annual maximum flows (including floods) are neither increasing nor decreasing.

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Hydrologically speaking, this means that the nation appears to be getting wetter, but less extreme. If these conclusions and those of the IPCC are correct, the first problem of physical availability of water may be solved, at least for some parts of the country.

The Southeast’s current severe drought has threatened the cooling water supplies of more than two dozen of the nation’s 104 nuclear power reactors. Not surprisingly, proposals to add additional power plants in the region are meeting increased public resistance. The growing scarcity of freshwater has started to raise prices. Therefore, managing and using all available water capital might both limit future water costs and maximize the health, social and economic benefits of water use, thereby increasing freshwater “productivity.” To some extent this is already happening: waste water reuse in the United States is growing at 15 percent a year, driven by water scarcity and higher prices for freshwater in some regions.

Rising Demand for Cooling Water

As economic development continues, state policy makers will be faced with increasing power demands, which means expanding water use at existing power plants or constructing new facilities. Even if demands are met by importing power from out of state, increased use will occur somewhere. Water withdrawal for power generation has recently been more modest because of greater use of recirculating systems, which need only make-up water. Still, the growth rate from 1995 to 2000 is about 3 percent and this is likely to continue.

However, the Statistical Abstract of the United States includes trend data from the Energy Information Administration that show a 1995 to 2000 growth rate of about 7 percent. Given our ever-greater domestic and industrial needs for electricity, this appears to be a good forecast, too.

Detailed plant level statistics from the Electric Power Research Institute (EPRI) show the regression lines for different technologies and might well be important when plants are sited in a specific region. The detailed graph is shown in Figure 4. Evidently, we can depict the impact of power plants on the available water supply. Interpreting the log-log graph can be difficult, but as a rough guide it might be thought that use of the recirculating technologies can save between 3 and 7 times the water demand compared to once-through technologies. Of course, this also assumes that the authorities have complete control over what kind of plant will be asked to meet the new demand. It may be the case that only older once-through facilities are available for expansion.

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The National Energy Technology Laboratory (NETL) has attempted to connect both freshwater needs and power demands in a 2007 report (DOE/NETL-400/2007/1304). To do this, NETL has used generating capacity projections from the EIA Annual Energy Outlook 2007 and constructed a series of alternative scenarios depending on technological assumptions. An important result of this study concerns what happens regardless of which scenario is used; on a national basis, consumption through 2030 is expected to increase, even if withdrawal decreases. There will be less water returned to the river from which it is taken. When regional patterns are examined, one plausible scenario shows freshwater consumption increases of 274 percent for California, 250 percent for Florida, and 396 percent for New York. With such large impacts, some form of coping action seems mandatory.

Wastewater for Cooling

Two studies show policy options that might use technology to address any shortfalls in water availability to meet additional power generation. An EPRI study dated July 2007 includes the use of wastewater from treatment plants as among the options to meet cooling water needs. With some technical caveats, possible sources could include treated urban wastewater, storm water, mine drainage and similar sources. The cost for recycled wastewater might be less than $0.25/kgal. This illustrates a way to address the second major problem we are addressing: that of cost-efficient power generation. It appears that significant cost savings result from the use of reclaimed water, without respect to possible climate change effects.

Furthermore, Argonne National Laboratory has conducted a study on the use of reclaimed water for power plant cooling (August 2007). About 50 power plants are currently using reclaimed water for cooling. Most are in Florida, California, Arizona and Texas, states that have dealt with freshwater shortages for many years. Use of reclaimed water is not a new practice. Results to date suggest that we have the information and can show water-energy indicators to offer this option to policy makers.

A wide range of water is used at several types of power plants. The amounts are often large, but in the range customary for WWTPs. Even plants using cooling towers at fossil-fueled facilities would require about 300 to 600 gal/MWh. Regardless of fuel type, steam electric power plants require a great deal of water. Looking at more detail about where this kind of technology is used, Argonne documents that Florida has 17 such power plants and California has 13. But even states with ample water have found the technology useful. For example, Massachusetts has three, Maryland two and New Jersey two. By far the greatest use in such plants is for cooling tower makeup water.

The Argonne report notes that reclaimed water must meet at least secondary treatment standards. Sometimes the state agency requires tertiary treatment or some form of filtration or disinfection. Process reasons also exist for treating reclaimed water; chemical parameters may cause scaling, corrosion, biofouling or other problems. Although use of reclaimed water is not really new, it is likely to become increasingly common in many parts of the country, either because of water scarcity or for economic reasons.

Case Study

The Argonne report includes a case study to show practicality of the concept. The 230 MW Panda Brandywine combined cycle gas-fired plant owned by independent power provider Panda Energy is in Maryland, near Washington, D.C. The Mattawoman wastewater treatment plant provides reclaimed water to Brandywine, which went into service in 1996. Brandywine uses about 1.5 million gallons a day of tertiary-treated water from Mattawoman, delivered through a 17-mile-long pipeline. In addition to chlorination, chemicals are added to the water flow for corrosion control. The Mattawoman treatment plant continues to upgrade its operation to further reduce contaminants in the ultimate receiving waters. The result is a mutually beneficial partnership between the two facilities.

Because of the lack of research to date, prospects for energy providers, energy users and society at large to adapt to climate change are speculative, but the potentials are considerable. To improve adaptive management by policy makers, we have presented a series of indicators in graph or map format that provides options about how to respond to possible climate- induced changes in water supply when attempting to meet increased demands for electric power generation. We have demonstrated that the major problems of physical availability and economic cost-effectiveness can both be addressed. Water resources indicator development is a long-term goal of the Sustainable Water Resources Roundtable.

Authors: Ethan T. Smith is coordinator of the Sustainable Water Resources Roundtable and spent 36 years with the Water Resources Division of the U.S. Geological Survey. He holds a Ph.D. in planning and policy development, a master’s degree in city and regional planning from Rutgers University and a B.S. in physics from the Polytechnic University of New York.

Harry Zhang is principal technologist and industrial water resources lead with CH2M HILL, where he consults on water quality and resource issues for industrial, municipal and federal projects. He also chairs the Hydrology & Watershed Management Committee at the American Water Resources Association and is on the steering committee of the Sustainable Water Resources Roundtable. He received his Ph.D. from the University of Virginia and is a registered professional engineer in Virginia.

The authors wish to acknowledge the continuing support and encouragement of Robert Goldstein of EPRI, a co-chair of the Sustainable Water Resources Roundtable. This article contains many of his ideas and suggestions. The authors also wish to acknowledge the support provided by the National Energy Technology Laboratory, U.S. Department of Energy.