Environ. Sci. Technol. 2009, 43, 2680–2687
Energy and Air Emission Effects of Water Supply JENNIFER R. STOKES AND ARPAD HORVATH* Department of Civil and Environmental Engineering, 215 McLaughlin Hall, University of California, Berkeley, California 94720-1712
Received June 29, 2008. Revised manuscript received December 15, 2008. Accepted February 17, 2009.
Life-cycle air emission effects of supplying water are explored using a hybrid life-cycle assessment. For the typically sized U.S. utility analyzed, recycled water is preferable to desalination and comparable to importation. Seawater desalination has an energy and air emission footprint that is 1.5-2.4 times larger than that of imported water. However, some desalination modes fare better; brackish groundwater is 53-66% as environmentally intensive as seawater desalination. The annual water needs (326 m3) of a typical Californian that is met with imported water requires 5.8 GJ of energy and creates 360 kg of CO2 equivalent emissions. With seawater desalination, energy use would increase to 14 GJ and 800 kg of CO2 equivalent emissions. Meeting the water demand of California with desalination would consume 52% of the state’s electricity. Supply options were reassessed using alternative electricity mixes, including the average mix of the United States and several renewable sources. Desalination using solar thermal energy has lower greenhouse gas emissions than that of imported and recycled water (using California’s electricity mix), but using the U.S. mix increases the environmental footprint by 1.5 times. A comparison with a more energy-intensive international scenario shows that CO2 equivalent emissions for desalination in Dubai are 1.6 times larger than in California. The methods, decision support tool (WEST), and results of this study should persuade decision makers to make informed water policy choices by including energy consumption and material use effects in the decision-making process.
1. Introduction Drinking water scarcity is an issue in many parts of the world. By 2025, 1.8 billion people will be living in areas likely to experience absolute water scarcity (1). More than 40% of the world’s population may face serious water shortages if they must rely solely on locally available freshwater (2). Some of these places experience scarcity due to climate and others because infrastructure is unavailable; however, in some places, both issues are problematic. The western United States is especially impacted by scarcity. For example, California’s population is expected to increase by 14 million people by 2030. If we assume water use rates for the year 2000 are representative values, demand will increase by 40% in the same period (3). Much of this growth will occur in the more arid areas of the state, where the scarcity will be acute (4). Currently, most water in arid * Corresponding author phone: 510-642-7300; fax: 510-643-8919; e-mail:
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areas is imported via major conveyance networks that comprise more than 4800 km of pipelines, tunnels, and canals, and dozens of pump stations, e.g., the State Water Project (SWP, from the Sacramento-San Joaquin River delta) and the Colorado River Aqueduct (CRA). The CRA and SWP supply more than 18% of the water for California’s urban water use as well as a significant volume of water for agricultural and environmental uses (Supporting Information). Both sources may be adversely affected by climate change (5-7). Water and energy are tightly connected. In California, 19% of the state’s electricity is consumed providing waterrelated services, e.g., water and wastewater services as well as industrial and agricultural applications (processing and pumping) (8). This connection and the amount of electricity consumed will increase as desalination or other energy intensive sources are adopted in areas of water scarcity. Desalination is considered a realistic water source in arid, coastal regions, including California, Florida, the Mediterranean islands, and the Middle East. However, desalination is not without critics (9). Growth in desalination will occur at a considerable energy and environmental cost. Electricity used to supply water is the main source of greenhouse gases (GHG) from water provision, thereby contributing to the climate change problem. Finding feasible alternative water sources is a major challenge that will stress society economically, environmentally, and politically. The political implications of water scarcity have been discussed (ref 10) and can include water wars and transboundary conflicts between states. Some conflicts occur between water providers, e.g., between the agriculture sector and urban utilities in the western United States. Economically, obtaining water in dry areas is already expensive, and costs will increase with scarcity. For example, brackish groundwater desalination can range in cost from $110 to $1000 per 1000 m3 of water [$130-$1250 per acrefoot (AF)], and ocean desalination can cost $650-$1200 per 1000 m3 ($800-$1500 per AF) (3). Figure 1 in the Supporting Information depicts costs and potential volumes available for water sources in southern California. This paper focuses on the material and energy consumption associated with water provision systems and related air emissions using typical U.S. conditions. These effects are considerable but have not been explored fully using lifecycle assessment (LCA) in order to inform decision making. Previous environmental assessments of water provision systems (11-21) are summarized in Table 1, including a summary of the study results and distinctions between prior studies and the work described in this paper. Previous articles, with one exception, have relied solely on process LCA and commercial databases or on economic input-output (EIO)LCA, while our study uses a hybrid approach. One study from Australia (17) primarily used commercial software but used an EIO-LCA-based inventory tool to estimate construction effects without explicit analysis. That paper, a hybrid LCA, described a process-based approach, supplemented with EIO-LCA; our research took the opposite approach. These studies are discussed in detail in the Supporting Information. Table 1 indicates that comprehensive U.S. LCA studies are lacking. Other studies evaluated the urban water cycle, including wastewater treatment and collection and, in some cases, the effects of consumer end uses (22-24), but because their scope was much broader than this study and they are not United States-based, they were not included in the table. 10.1021/es801802h CCC: $40.75
2009 American Chemical Society
Published on Web 03/20/2009
2. Research Scope and Method We have developed a comprehensive, hybrid LCA-based decision-support tool to facilitate the implementation of LCAs of U.S. water provision systems in order to inform designers, policymakers, utility managers and operators, and others about energy, material, and air emission effects. The Water-Energy Sustainability Tool (WEST) can evaluate the construction, operation, and maintenance of water systems and compare the direct and indirect (supply chain) energy and environmental effects of alternative water sources in terms of material production (e.g., concrete, pipe, and chemicals), material delivery, construction and maintenance equipment use, energy production (electricity and fuel), and sludge disposal. LCA is a systematic methodology that allows the user to comprehensively and quantitatively evaluate the inputs and outputs, both direct and indirect, of a process, product, or service. The Supporting Information describes the hybrid LCA methodology employed in this research. Descriptions of the original framework and design and an early version of WEST have been previously described (in a paper (25) and in an unpublished dissertation (26)). This paper is based on an updated framework, design, and database of WEST. In particular, WEST now embeds results of a hybrid LCA for all life-cycle phases customizable to any U.S. state, combines inventory data from EIO-LCA (www. eiolca.net) as well as from a commercial LCA database (GaBi (http://www.gabi-software.com/)), includes specific treatment chemicals in its database and additional treatment alternatives for comparison (conventional pretreatment versus emerging membrane pretreatment for desalination), allows for specification of custom electricity mixes, and includes alternative sludge management options. WEST incorporates water utility designs and typical operational practices (electricity mixes, transportation distances, and sludge disposal methods) of U.S. water utilities, which are herein studied for the first time as a comprehensive system, using hybrid LCA and U.S. conditions. Alternative water sources from a hypothetical southern California case study are compared using the best available information and WEST. The hypothetical water system is based on data from several California utilities and is representative of a U.S. water utility in size. Four water sources are analyzed: imported water, seawater desalination, brackish groundwater desalination, and recycled water. A sensitivity analysis of the results based on alternative choices of electricity mixes is also performed. For international comparison, a desalination analysis in Dubai is described. A few limitations of this analysis should be noted. Only targeted sources available in urban coastal California were analyzed, other possible sources were excluded. In particular, groundwater is not commonly useable because of the local geology or salt water intrusion. Furthermore, the water sources analyzed in this study are not strictly equivalent. The recycled water system produces nonpotable water, while all other sources are treated to a higher standard. However, to the extent that recycled water meets the customer’s needs, it can be considered functionally equivalent. Recycled water has the advantage of being drought resistant and locally available, which are important issues in arid California. In spite of these advantages, it is perceived as a lower quality product, and finding customers can be difficult. Recycled water, though beneficial when viewed from a life-cycle perspective, is unlikely to meet the needs of significant growth in water demand. Furthermore, not all environmental effects associated with water supply are captured by this study. Surface water extraction can affect the water quality of a river and aquatic species’ habitat downstream. Seawater desalination raises concerns about entrainment and impingement of fish and
other animals at the intake and the effects of discharging concentrated (and perhaps chemically contaminated) brine into the ocean (27). Brackish groundwater desalination can cause ground subsidence. Water-related environmental effects are extremely important, and other authors are addressing these issues elsewhere (18, 28-30). However, more information is available about these water-related issues than about the life-cycle energy, material use, and associated air emissions reported in this study. As a result, we have chosen to focus on air emissions. Water quality issues are not addressed herein.
3. Case Study To analyze energy and emission implications of alternative water sources, we developed a hypothetical case study in southern California because no utility was found that considered all possible alternatives. However, while the case study is hypothetical, it realistically combines data from several utilities. This utility provides 36 million m3 of water annually from imported water sources (IMP): 75% of the water from the CRA and the rest from the SWP. This water supplies 175000 residents over 325 km2. It is ideally sized for analysis because while systems serving over 100000 people represent only about 1% of public utilities, these serve 45% of the U.S. population. The current system relying on imported water is evaluated to establish baseline energy use and air emissions. Because of rapid population growth, local industrial changes, imported supply reduction, and other factors, the utility will likely need to increase its water provision over the next 20 years. Imported water supplies are likely to be less reliable in the future due to demand growth and climate change. This utility is evaluating these potential sources to meet their needs: (1) desalinating seawater with conventional pretreatment (DC), (2) desalinating seawater with pretreatment by micro- and ultrafiltration (MF/UF) membranes (DM), (3) desalinating brackish groundwater (DBG), and (4) recycling wastewater for nonpotable use in irrigation and commercial and industrial applications (REC). Seawater desalination is analyzed with two different pretreatment processes to evaluate the advantages of emerging membrane pretreatment processes versus more conventional processes. Studies have shown that membrane pretreatment provides a more reliable, higher quality product (31, 32). The functional unit of the analysis is one cubic meter (m3). Table 2 summarizes the details associated with each of these alternatives.
4. Results of Water Source Comparisons Table 3 summarizes the results of analyzing the current imported water scenario and the four expansion alternatives. Sample calculations are in the Supporting Information. The current IMP source is preferable to all desalinated water alternatives. Regardless of the pretreatment alternative, seawater desalination results are 1.5-2.4 times higher than IMP results, depending on the parameter. DBG is significantly better than conventional seawater desalination (53-66% of DM) and is comparable to IMP for NOx and PM results. For other parameters, DBG results are approximately 150% higher than IMP. However, REC is preferable to IMP for all parameters except PM. For energy, greenhouse gases, and SOx, the improvements for REC are slight. For PM, REC produces 20% more emissions per functional unit. On a unit basis, the life-cycle energy results are 1.6 to 2.6 times higher than the direct electricity use reported in Table 2 due to the contribution of life-cycle supply chain effects associated with electricity and fuel production as well as manufacturing concrete, pipe, chemicals, and other system materials. VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of Water LCA Literature reference
Herz and Lipkow, 2002 (11)
Friedrich, 2002 (12)
Filion et al., 2004 (13)
Raluy et al., 2005 (14-16)
Tangsubkul, et al., 2005 (17)
Landu and Brent, 2006 (18)
Friedrich, et al., 2007 (19)
Racoviceanu, et al., 2007 (20)
Vince, et al., 2008 (21)
summary Results: compared dig and no-dig installation for a variety of sewer and distribution pipe materials; no-dig installation reduced CO2 emisisons by 20-30%; for water, lining pipes with mortar extended life and improved results Distinctions: Germany focus; process-based LCA; evaluated only distribution system Results: compared treatment by conventional filters and membranes; either could be preferred depending on the indicator; electricity generation is a dominant contributor to effects from both Distinctions: South Africa focus; GaBi based; considered only treatment; did not consider alternative electricity sources or water sources Results: compared life-cycle energy use of various pipeline replacement rates; 50-year pipe replacement rate was recommended Distinctions: EIO-LCA based; evaluated only distribution system; did not compare electricity mixes Results: compared desalination processes and importation; reverse osmosis (RO) is preferred to multistage flash and multieffect desalination; environmental effects of importation were lower than RO given current technology Distinctions: Spain focus; SimaPro based; does not analyze distribution system or recycled water; compared four European electricity mixes and wind, solar, and hydropower generation Results: compared treatment for nonpotable reuse by continuous microfiltration (CMF), membrane bioreactor (MBR), and wastewater stabilization pond (WSP); for all indicators, WSP produced the least emissions and CMF the most Distinctions: Australia focus; GaBi with EIO-based analysis for construction; considered only water recycling treatment; did not compare electricity mixes Results: evaluated water used for manufacturing; surface water withdrawals created most significant effects, followed by electricity generation Distinctions: South Africa focus; process-based LCA; if present, analysis of infrastructure construction phase was not well-described; did not compare water sources or electricity mixes Results: emphasized significant contribution of energy and electricity use; recommended electricity use as an indicator of environmental performance of South African water systems Distinctions: South Africa focus; inventory source is not specified; considered surface and recycled water; did not compare electricity mixes Results: evaluated water treatment focusing on chemical production, chemical transport, and plant operation; operational components were responsible for 94% of energy and 90% of GHG; 60% of operational burden was due to on-site pumping Distinctions: Canada focus; EIO-LCA based; evaluated only treatment operation phase; did not compare water sources or electricity mixes Results: compared groundwater treatment, ultrafiltration, nanofiltration, ocean RO, and thermal distillation; electricity use for plant operation is the main cause of impacts; chemical production (lime, ozone, etc.) contribute significantly to results Distinctions: Europe focus; GaBi based; evaluated treatment processes only; did not specifically analyze infrastructure construction or compare electricity mixes
Pretreating seawater using membranes prior to RO is a recent advancement in desalination. Because the MF/UF systems generally consume less electricity and chemicals, they are an improvement over conventional pretreatment. Surprisingly, the environmental difference between the DC and DM scenarios is small. Energy use and GHG results are slightly lower (
