Article pubs.acs.org/est
Geographic, Technologic, And Economic Analysis of Using Reclaimed Water for Thermoelectric Power Plant Cooling Ashlynn S. Stillwell*,† and Michael E. Webber‡ †
Department of Civil and Environmental Engineering, University of Illinois at Urbana−Champaign, Urbana Illinois, United States Department of Mechanical Engineering, The University of Texas at Austin, Austin Texas, United States
‡
S Supporting Information *
ABSTRACT: Use of reclaimed watermunicipal wastewater treatment plant effluentin nonpotable applications can be a sustainable and efficient water management strategy. One such nonpotable application is at thermoelectric power plants since these facilities require cooling, often using large volumes of freshwater. To evaluate the geographic, technologic, and economic feasibility of using reclaimed water to cool thermoelectric power plants, we developed a spatially resolved model of existing power plants. Our model integrates data on power plant and municipal wastewater treatment plant operations into a combined geographic information systems and optimization approach to evaluate the feasibility of cooling system retrofits. We applied this broadly applicable methodology to 125 power plants in Texas as a test case. Results show that sufficient reclaimed water resources exist within 25 miles of 92 power plants (representing 61% of capacity and 50% of generation in our sample), with most of these facilities meeting both short-term and long-term water conservation cost goals. This retrofit analysis indicates that reclaimed water could be a suitable cooling water source for thermoelectric power plants, thereby mitigating some of the freshwater impacts of electricity generation.
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INTRODUCTION AND BACKGROUND Thermoelectric power plants commonly use water for cooling to condense process steam. In the United States, thermoelectric power generation is responsible for about 41% of freshwater withdrawals1 and 3% of freshwater consumption.2 As freshwater supplies become increasingly strained due to population growth and economic development, thermoelectric power plants have attracted scrutiny for their use of water.3 Despite this highly visible freshwater use, cooling water is not required to be potable water quality. Consequently, reclaimed watertreated effluent from municipal wastewater treatment plantscould be a plentiful, safe, and suitable cooling water source.4−10 Such alternative sources of water become attractive when conventional sources become scarce, expensive, and/or strictly regulated in terms of acceptable uses. In 2010, 46 thermoelectric power plants in the United States reported using reclaimed water as a cooling water source,11 and Li et al.7 estimate that 50% of the existing thermoelectric power plants in the United States could obtain sufficient cooling water from publicly owned treatment works within 10 miles of the facility. Reclaimed water is generally accepted as an alternative water source of considerable volume. An estimated 30−40 billion gallons per day (bgd) of wastewater are treated in the United States, making reclaimed water a generally plentiful source.7 (For reference, estimated freshwater consumption in the United States is about 100 bgd.2) However, recent reports indicate increasing demand and competing uses for reclaimed water that could strain supplies in the future.5 Reclaimed water © 2014 American Chemical Society
is also a drought-resistant water supply as wastewater is generated from indoor uses of water, which are rarely curtailed. In many areas, reclaimed water use is exempt from droughtinduced water use restrictions. Many safety concerns regarding reclaimed water use can be addressed through the current regulatory environment. Federal standards require at least secondary wastewater treatment12 and guidelines exist for water reuse.13 Water reuse standards are left to states, such as the Texas Administrative Code, Title 30, Part 1, Chapter 210,14 which sets limits for biochemical oxygen demand, turbidity, and fecal coliform based on different uses and possible human exposure. The bacterium Legionella pneumophilia (which causes Legionnaires’ Disease, a type of respiratory infection) raises concerns about the safety of aerosol emissions from power plants operating cooling towers since plumes and airborne water droplets can travel beyond the immediate vicinity of the power plant.7 However, risks can be minimized through conscious operational management and appropriate water treatment. Despite the benefits of reclaimed water use for power plant cooling, some operational trade-offs exist. Use of reclaimed water in power plant cooling equipment can lead to some additional operational challenges, such as corrosion, scaling, Received: Revised: Accepted: Published: 4588
June 21, 2013 February 27, 2014 March 13, 2014 March 13, 2014 dx.doi.org/10.1021/es405820j | Environ. Sci. Technol. 2014, 48, 4588−4595
Environmental Science & Technology
Article
and biofouling,5−7,15,16 often requiring pretreatment such as ion exchange or softening.17 Typically, control of corrosion, scaling, and biofouling in a cooling tower is manageable with 4−6 cycles of concentration;17,18 however, these challenges present trade-offs because low-scale water has high corrosivity, and antiscalants are compromised with use of free chlorine to control biofilm growth.18,19 Consequently, in existing thermoelectric power plants that use reclaimed water for cooling, operations are usually based on the maximum concentration of the limiting constituent (e.g., nitrogen, calcium, etc.) to prevent fouling.20 Despite challenges, some facilities that conducted pilot scale tests observed no additional water use and no appreciable change in chemical water treatment cost when switching to reclaimed water as a cooling source.21 Others estimate costs on the order of 1−2% of electricity sales revenues to manage fouling, condenser cleaning, and reduced generating efficiency.15 The economic feasibility of using reclaimed water to cool power plants depends on the distance between facilities, the price of water, and safety and operational factors. Literature suggests “rule of thumb” distance limits of 10−25 miles between the power plant and reclaimed water sources;18 however, notable exceptions exist, such as the Palo Verde Nuclear Generating Station in Arizona, which pumps reclaimed water over 36 miles from the greater Phoenix area.20 This analysis focused on Texas as a test bed, and applied the 10- to 25-mile range as a filter on the possibilities. Texas is a suitable location for this analysis due to its geographic, climate, and land use variability; independent electricity grid that covers most of the state; and existing and projected water constraints. Municipal wastewater treatment plants and thermoelectric power plants are often located within close proximity, as shown in Figure 1, with the opportunity for wastewater treatment plant effluent (2.1 bgd; see Figure 1) to satisfy all thermo-
electric consumptive demands (total consumption is 0.36 bgd; see Figure 1) and about 1/10 of withdrawal demands (total withdrawal is 28 bgd; see Figure 1). Additionally, several thermoelectric power plants in Texas currently use reclaimed water for at least some portion of their cooling water needs, including Austin Energy’s Sand Hill Energy Center; CPS Energy’s J K Spruce, J T Deely, and O W Sommers plants; Xcel Energy’s Nichols, Harrington, and Jones facilities; and the Spencer Generating Station near Denton, among others.6,22 For this analysis, we hypothesize that reclaimed water could be a geographically, technologically, and economically suitable cooling water source for many more existing thermoelectric power plants in Texas beyond the few identified above. We define this geographic, technologic, and economic feasibility to include facilities with sufficient reclaimed water resources within 25 miles that can meet cooling needs at an annualized cost less than current water conservation cost targets. Beyond Texas, many areas are projected to experience increasing water stress along with expansions in thermoelectric power generation (at both existing and new facilities), making consideration of alternative water sources like reclaimed water a pertinent and timely topic concerning water and energy sustainability.
■
MATERIALS AND METHODS Our methodology uses spatially resolved data layers on land use, elevation, water stress, and existing facility locations to determine the feasibility of using reclaimed water to cool existing thermoelectric power plants in Texas. Fundamentally, our analysis represents a specific type of geographic information systems-based multicriteria decision analysis (GIS-MCDA)23 because we use spatial raster data to estimate areas where use of reclaimed water for power plant cooling might be suitable in current facilities. GIS-MCDA has been applied to many different types of resource analyses, including, for example, colocation of algal biomass cultivation with wastewater treatment plants,24 water availability for algal biofuels production,25 and siting of power generation facilities.26 For this analysis of using reclaimed water to cool power plants, we consider both 10- and 25-mile distances between a power plant and sources of reclaimed water, based on typical feasibility limits.18 The overall approach for our retrofit analysis was adapted from the methodology presented in Lee27 and included the modeling steps shown in Figure 2. These modeling stepsdata compilation, least cost path analysis, and optimizationare described below. We performed a retrofit analysis considering switching cooling water sources at existing thermoelectric power plants. Although we apply this methodology to the state of Texas in this analysis, our approach easily translates to other locations worldwide with sufficient spatially resolved data on thermoelectric power plants and municipal wastewater treatment plants (or other alternative water sources). Data Compliation. Wastewater treatment plant data from the U.S. Environmental Protection Agency (EPA) Clean Watershed Needs Survey28 and Texas Commission on Environmental Quality (TCEQ) wastewater outfall database29 were joined to compile a data set of domestic (
Geographic, Technologic, And Economic Analysis of Using Reclaimed Water for Thermoelectric Power Plant Cooling Ashlynn S. Stillwell*,† and Michael E. Webber‡ †
Department of Civil and Environmental Engineering, University of Illinois at Urbana−Champaign, Urbana Illinois, United States Department of Mechanical Engineering, The University of Texas at Austin, Austin Texas, United States
‡
S Supporting Information *
ABSTRACT: Use of reclaimed watermunicipal wastewater treatment plant effluentin nonpotable applications can be a sustainable and efficient water management strategy. One such nonpotable application is at thermoelectric power plants since these facilities require cooling, often using large volumes of freshwater. To evaluate the geographic, technologic, and economic feasibility of using reclaimed water to cool thermoelectric power plants, we developed a spatially resolved model of existing power plants. Our model integrates data on power plant and municipal wastewater treatment plant operations into a combined geographic information systems and optimization approach to evaluate the feasibility of cooling system retrofits. We applied this broadly applicable methodology to 125 power plants in Texas as a test case. Results show that sufficient reclaimed water resources exist within 25 miles of 92 power plants (representing 61% of capacity and 50% of generation in our sample), with most of these facilities meeting both short-term and long-term water conservation cost goals. This retrofit analysis indicates that reclaimed water could be a suitable cooling water source for thermoelectric power plants, thereby mitigating some of the freshwater impacts of electricity generation.
■
INTRODUCTION AND BACKGROUND Thermoelectric power plants commonly use water for cooling to condense process steam. In the United States, thermoelectric power generation is responsible for about 41% of freshwater withdrawals1 and 3% of freshwater consumption.2 As freshwater supplies become increasingly strained due to population growth and economic development, thermoelectric power plants have attracted scrutiny for their use of water.3 Despite this highly visible freshwater use, cooling water is not required to be potable water quality. Consequently, reclaimed watertreated effluent from municipal wastewater treatment plantscould be a plentiful, safe, and suitable cooling water source.4−10 Such alternative sources of water become attractive when conventional sources become scarce, expensive, and/or strictly regulated in terms of acceptable uses. In 2010, 46 thermoelectric power plants in the United States reported using reclaimed water as a cooling water source,11 and Li et al.7 estimate that 50% of the existing thermoelectric power plants in the United States could obtain sufficient cooling water from publicly owned treatment works within 10 miles of the facility. Reclaimed water is generally accepted as an alternative water source of considerable volume. An estimated 30−40 billion gallons per day (bgd) of wastewater are treated in the United States, making reclaimed water a generally plentiful source.7 (For reference, estimated freshwater consumption in the United States is about 100 bgd.2) However, recent reports indicate increasing demand and competing uses for reclaimed water that could strain supplies in the future.5 Reclaimed water © 2014 American Chemical Society
is also a drought-resistant water supply as wastewater is generated from indoor uses of water, which are rarely curtailed. In many areas, reclaimed water use is exempt from droughtinduced water use restrictions. Many safety concerns regarding reclaimed water use can be addressed through the current regulatory environment. Federal standards require at least secondary wastewater treatment12 and guidelines exist for water reuse.13 Water reuse standards are left to states, such as the Texas Administrative Code, Title 30, Part 1, Chapter 210,14 which sets limits for biochemical oxygen demand, turbidity, and fecal coliform based on different uses and possible human exposure. The bacterium Legionella pneumophilia (which causes Legionnaires’ Disease, a type of respiratory infection) raises concerns about the safety of aerosol emissions from power plants operating cooling towers since plumes and airborne water droplets can travel beyond the immediate vicinity of the power plant.7 However, risks can be minimized through conscious operational management and appropriate water treatment. Despite the benefits of reclaimed water use for power plant cooling, some operational trade-offs exist. Use of reclaimed water in power plant cooling equipment can lead to some additional operational challenges, such as corrosion, scaling, Received: Revised: Accepted: Published: 4588
June 21, 2013 February 27, 2014 March 13, 2014 March 13, 2014 dx.doi.org/10.1021/es405820j | Environ. Sci. Technol. 2014, 48, 4588−4595
Environmental Science & Technology
Article
and biofouling,5−7,15,16 often requiring pretreatment such as ion exchange or softening.17 Typically, control of corrosion, scaling, and biofouling in a cooling tower is manageable with 4−6 cycles of concentration;17,18 however, these challenges present trade-offs because low-scale water has high corrosivity, and antiscalants are compromised with use of free chlorine to control biofilm growth.18,19 Consequently, in existing thermoelectric power plants that use reclaimed water for cooling, operations are usually based on the maximum concentration of the limiting constituent (e.g., nitrogen, calcium, etc.) to prevent fouling.20 Despite challenges, some facilities that conducted pilot scale tests observed no additional water use and no appreciable change in chemical water treatment cost when switching to reclaimed water as a cooling source.21 Others estimate costs on the order of 1−2% of electricity sales revenues to manage fouling, condenser cleaning, and reduced generating efficiency.15 The economic feasibility of using reclaimed water to cool power plants depends on the distance between facilities, the price of water, and safety and operational factors. Literature suggests “rule of thumb” distance limits of 10−25 miles between the power plant and reclaimed water sources;18 however, notable exceptions exist, such as the Palo Verde Nuclear Generating Station in Arizona, which pumps reclaimed water over 36 miles from the greater Phoenix area.20 This analysis focused on Texas as a test bed, and applied the 10- to 25-mile range as a filter on the possibilities. Texas is a suitable location for this analysis due to its geographic, climate, and land use variability; independent electricity grid that covers most of the state; and existing and projected water constraints. Municipal wastewater treatment plants and thermoelectric power plants are often located within close proximity, as shown in Figure 1, with the opportunity for wastewater treatment plant effluent (2.1 bgd; see Figure 1) to satisfy all thermo-
electric consumptive demands (total consumption is 0.36 bgd; see Figure 1) and about 1/10 of withdrawal demands (total withdrawal is 28 bgd; see Figure 1). Additionally, several thermoelectric power plants in Texas currently use reclaimed water for at least some portion of their cooling water needs, including Austin Energy’s Sand Hill Energy Center; CPS Energy’s J K Spruce, J T Deely, and O W Sommers plants; Xcel Energy’s Nichols, Harrington, and Jones facilities; and the Spencer Generating Station near Denton, among others.6,22 For this analysis, we hypothesize that reclaimed water could be a geographically, technologically, and economically suitable cooling water source for many more existing thermoelectric power plants in Texas beyond the few identified above. We define this geographic, technologic, and economic feasibility to include facilities with sufficient reclaimed water resources within 25 miles that can meet cooling needs at an annualized cost less than current water conservation cost targets. Beyond Texas, many areas are projected to experience increasing water stress along with expansions in thermoelectric power generation (at both existing and new facilities), making consideration of alternative water sources like reclaimed water a pertinent and timely topic concerning water and energy sustainability.
■
MATERIALS AND METHODS Our methodology uses spatially resolved data layers on land use, elevation, water stress, and existing facility locations to determine the feasibility of using reclaimed water to cool existing thermoelectric power plants in Texas. Fundamentally, our analysis represents a specific type of geographic information systems-based multicriteria decision analysis (GIS-MCDA)23 because we use spatial raster data to estimate areas where use of reclaimed water for power plant cooling might be suitable in current facilities. GIS-MCDA has been applied to many different types of resource analyses, including, for example, colocation of algal biomass cultivation with wastewater treatment plants,24 water availability for algal biofuels production,25 and siting of power generation facilities.26 For this analysis of using reclaimed water to cool power plants, we consider both 10- and 25-mile distances between a power plant and sources of reclaimed water, based on typical feasibility limits.18 The overall approach for our retrofit analysis was adapted from the methodology presented in Lee27 and included the modeling steps shown in Figure 2. These modeling stepsdata compilation, least cost path analysis, and optimizationare described below. We performed a retrofit analysis considering switching cooling water sources at existing thermoelectric power plants. Although we apply this methodology to the state of Texas in this analysis, our approach easily translates to other locations worldwide with sufficient spatially resolved data on thermoelectric power plants and municipal wastewater treatment plants (or other alternative water sources). Data Compliation. Wastewater treatment plant data from the U.S. Environmental Protection Agency (EPA) Clean Watershed Needs Survey28 and Texas Commission on Environmental Quality (TCEQ) wastewater outfall database29 were joined to compile a data set of domestic (
