Energy-Water Nexus: Amount of Energy Needed to Supply, Use, and Treat Water Is Location-Specific and Can Be Reduced by Certain Technologies and Approaches
Published: 2011-03-23T00:00:00

Fast Facts:

Highlights:
section_title: Why GAO Did This Study
paragraphs: Providing drinking water and wastewater services are two key functions needed to support an urban lifestyle. To provide these services, energy is needed to extract, use, and treat water and wastewater. As the demand for water increases, the energy demands associated with providing water services are similarly expected to grow. GAO was asked to describe what is known about (1) the energy needed for the urban water lifecycle and (2) technologies and approaches that could lessen the energy needed for the lifecycle and barriers that exist to their adoption. To address these issues, GAO reviewed scientific studies, government-sponsored research, and other reports and interviewed specialists from a variety of organizations, including drinking water and wastewater utilities; federal, state, and local government offices responsible for water or energy; and relevant nonprofit groups, about the energy needed to move, use, and treat water. GAO also selected three cities--Memphis, Tennessee; San Diego, California; and Washington, D.C.--as illustrative case studies to help understand the energy demands of the lifecycle in different areas of the country. GAO is not making any recommendations in this report. A draft was provided to the Departments of Defense, Energy (DOE), and the Interior, and the Environmental Protection Agency (EPA). DOE and EPA provided technical comments, which we incorporated as appropriate..
section_title: What GAO Found
paragraphs: Comprehensive data about the energy needed for each stage of the urban water lifecycle are limited. In particular, few nationwide studies have been conducted on the amount of energy used to provide drinking water and wastewater services, and these studies do not consider all stages of the lifecycle in their analysis. Specialists GAO spoke with emphasized that the energy demands of the urban water lifecycle vary by location. Considering location-specific and other key factors is necessary to assess energy needs. The specialists mentioned such factors as the topography of the area over which water is conveyed, the level and type of treatment provided, and the quality of the source water. For example, systems relying on groundwater as their source for drinking water generally use less energy than systems relying on surface water because groundwater usually contains fewer contaminants and, therefore, requires less treatment before distribution to customers. A variety of technologies and approaches can improve the energy efficiency of drinking water and wastewater processes, but barriers exist to their adoption. Installing more efficient equipment, adopting water conservation measures, and upgrading infrastructure are among some of the approaches that can decrease energy use, according to specialists GAO spoke with and studies GAO reviewed. For example, technologies to identify potential pipeline leaks throughout water systems can reduce water loss and the energy required to pump and treat that "lost" water. However, according to specialists, adoption of technologies and approaches to improve energy efficiency may be hindered by the costs of retrofitting plants with more energy-efficient equipment and competing priorities at treatment facilities, among other barriers.

section_title: Letter
paragraphs: According to the U.S. Census Bureau, in 2005, 83 percent of the U.S.  population lived in metropolitan areas, up 6 percent from 2000. Two key  resources necessary to support an urban lifestyle are drinking water and  the infrastructure necessary to treat wastewater. The average American is  estimated to use about 90 gallons of water and produce 66 to 192 gallons  of wastewater each day, according to the U.S. Environmental Protection  Agency (EPA). As the demand for drinking water and wastewater  treatment in urban areas grows, it is expected that water utilities will have  to increasingly seek out alternative sources of water and treatment  methods to increase the water supply, especially in areas of water scarcity  where demand outpaces supply. However, treating and using these  alternative sources, such as seawater, come with a cost because, in  addition to other factors, they tend to be heavily energy dependent.
Providing drinking water and wastewater services to an urban  environment involves extracting, moving, and treating water—referred to  as the urban water lifecycle (see fig. 1). Energy plays a crucial role  throughout this lifecycle in the following ways:    Drinking water processes. Energy is needed to extract raw water from the  source—such as lakes, rivers, and underground aquifers—and convey it to  the drinking water treatment facility, treat the water to certain drinking  water standards established under the Safe Drinking Water Act, and  distribute the treated drinking water to customers.
Customer use. Energy is needed to circulate, pressurize, and heat water  for use inside households and businesses, and for outdoor water-related  uses by customers, such as watering lawns.
Wastewater processes. Energy is needed to convey wastewater to  treatment facilities, treat the wastewater to levels required under the  Clean Water Act, and discharge the treated effluent into a receiving body  of water.
As urban populations increase and the demand for water grows, the  energy needed for the urban water lifecycle is also expected to grow. In  this context, you asked us to review the energy needs of providing  drinking water and wastewater treatment services to urban users.  Specifically, the objectives of this review were to describe what is known  about (1) the energy needed for each stage of the urban water lifecycle  and (2) technologies and approaches that could lessen the energy needs of  the urban water lifecycle, as well as any identified barriers that exist to  their adoption.
To address both of these objectives, we conducted a systematic review of  studies and other documents that examine the energy required to extract,  move, use, and treat water, including peer-reviewed scientific and industry  periodicals, government-sponsored research, and reports from  nongovernmental research organizations. We also selected a  nonprobability sample of three cities to examine in greater depth and  better understand regional and local differences related to urban water  lifecycles: Memphis, Tennessee; San Diego, California; and Washington,  D.C. We chose these cities as illustrative case studies based on criteria  such as type of water source; water availability; type of wastewater  system; unique characteristics, such as potential to treat seawater to help  meet drinking water demands; and economic factors, such as energy costs.  While the information derived from our analysis of these cities cannot be  generalized to all U.S. cities, these examples provide valuable insights  regarding the complexities of assessing the energy needs for the urban  water lifecycle. For each of these case studies, we analyzed  documentation from, and conducted interviews with, a wide and diverse  range of specialists from organizations involved in all stages of the urban  water lifecycle. These organizations included drinking water and  wastewater treatment facilities, and state and local agencies responsible  for water or energy.
In addition to specialists associated with the illustrative case studies, we  interviewed a range of other knowledgeable individuals whom we  identified as having expertise related to the energy needs of all stages of  the urban water lifecycle throughout the United States. We selected these  specialists using an iterative process, soliciting additional names from  each person we interviewed. From among those identified, we interviewed  specialists who could provide us with a broad range of perspectives on the  energy needs of the urban water lifecycle. We also interviewed specialists  whom we identified during our systematic review of studies who have  analyzed (1) the energy needed in one or more stages of the water lifecycle  at the national or local level or (2) techniques available to reduce the  energy demands for water. These specialists represented a variety of  organizations, including drinking water and wastewater treatment  facilities; state and local government offices responsible for water or  energy; officials from EPA and researchers from some of the U.S.  Department of Energy’s (DOE) national laboratories, such as Sandia  National Laboratory; university researchers; water and energy industry  representatives from groups such as the American Water Works  Association and the Water Research Foundation; and relevant  nongovernmental organizations, such as the Pacific Institute, a  nonpartisan research institute that works to advance environmental  protection, economic development, and social equity. The specialists also  included individuals with knowledge of the energy demands for water in  other states, including Arizona, Colorado, Florida, New York, and  Wisconsin, to provide a better understanding of water and energy issues in  other regions around the United States.
We also interviewed other federal agency officials, scientists, and  researchers and analyzed data and information from federal agencies that  have responsibilities related to the energy needs of the urban water  lifecycle—the Department of Defense’s U.S. Army Corps of Engineers,  DOE, the Department of the Interior’s U.S. Geological Survey (USGS) and  Bureau of Reclamation, EPA, and the National Science Foundation. We  performed our work from January 2010 to January 2011 in accordance  with all sections of GAO’s Quality Assurance Framework that are relevant  to our objectives. The framework requires that we plan and perform the  engagement to obtain sufficient and appropriate evidence to meet our  stated objectives and to discuss any limitations in our work. We believe  that the information and data obtained, and the analysis conducted,  provide a reasonable basis for any findings and conclusions in this  product.
subsections: section_title: Background
paragraphs: According to EPA, about 52,000 community water systems use energy to  treat and deliver drinking water to over 290 million Americans. In a  typical drinking water treatment plant, large debris and contaminants are  physically removed from the raw water using screens (see fig. 2). Next,  dirt and other particles suspended in the water are removed through the  addition of alum and other chemicals during the processes of coagulation  and sedimentation. After these particles have separated out, the water  passes through filters made of layers of materials such as sand, gravel, and  charcoal to remove even smaller particles. At this point, the water is  stored in a closed tank or reservoir, allowing time for disinfection which  kills many disease-carrying organisms. The treated water is pressurized for  distribution to consumers. The distribution infrastructure consists of  pumps, pipes, tanks, valves, hydrants, and meters that support delivery of  water to the customer and control flow and water pressure.
Once water is delivered, residential consumers use it for a variety of  purposes, including for drinking; bathing; preparing food; washing clothes  and dishes; and flushing toilets, which can represent the single largest use  of water inside the home. Energy is needed to accomplish many of these  activities. For example, energy is used in homes to filter and soften water  and to heat it for use in certain appliances, which accounts for 12.5  percent of a typical household’s energy use, according to DOE. In addition  to residential water users, commercial, industrial, and institutional  customers use energy for water-related purposes. For example, energy is  used to produce hot water and steam for heating buildings, to cool water  for air conditioning buildings, and to generate hot water needed to  manufacture or process materials, such as food and paper.
After water is used by customers, energy is needed to collect and treat  wastewater, and to discharge effluent into a water body. Wastewater  service is provided to more than 220 million Americans by about 15,000  municipal wastewater treatment facilities. During a typical wastewater  treatment process, solid materials, such as sand and grit, organic matter  from sewage, and other pollutants, are removed before the treated effluent  is discharged to surface waters. Systems for collecting, treating, and  disposing of municipal wastewater vary widely in terms of the equipment  and processes used, and wastewater may go through as many as three  treatment stages—primary, secondary, and advanced treatment—before  water is discharged (see fig. 3).
Preliminary and primary treatment. As wastewater enters a treatment  facility, it is screened to remove large debris and then passes through a grit  removal system to separate out smaller particulate matter. After  preliminary screening and settling, primary treatment removes solids from  the wastewater through sedimentation. Solids removed during the  treatment process may be further treated and used for other applications,  such as fertilizer; incinerated; or disposed of in landfills.
Secondary treatment. After primary treatment, the wastewater undergoes  secondary treatment to remove organic matter and suspended solids  through physical and biological treatment processes. Activated sludge is  the most commonly used biological treatment process in secondary  treatment of wastewater. This process relies on micro-organisms to break  down organic matter in the wastewater. More specifically, aeration— whereby blowers or diffusers inject oxygen into the wastewater—enables  the micro-organisms to digest the organic matter. After being pumped into  an aeration tank to allow time for digestion, the wastewater is next  pumped to a secondary settling tank for removal of digested material.  After secondary settling, the effluent either is disinfected and discharged  into a water body, or it undergoes advanced treatment.
Advanced treatment. Most wastewater goes through at least secondary  treatment. However, before treated wastewater can be released in some  receiving waters, it may need to be further treated to reduce its effect on  water quality and aquatic life after discharge. Over 30 percent of  wastewater treatment facilities provide this kind of advanced treatment,  which can remove additional contaminants.
Two key pieces of federal legislation—the Safe Drinking Water Act and the  Clean Water Act—govern the treatment of drinking water and wastewater.  Each municipality or water utility generally may choose amongst  technologies for achieving a given standard. Under the Safe Drinking  Water Act, EPA has established National Primary Drinking Water  Standards for specified contaminants and has the authority to regulate  additional contaminants that the agency determines may have adverse  health effects, are likely to be present in public water supplies, and for  which regulation presents a meaningful opportunity for health risk  reduction. EPA’s regulations establish a limit, or maximum contaminant  level, for specific contaminants and require water systems to test the  water periodically to determine if the quality is acceptable. EPA has  regulations in place for 89 contaminants, including disinfectants,  byproducts of disinfectants, and microbial contaminants, but has not  issued a regulation under the Safe Drinking Water Act for a new  contaminant since 2000.
The Clean Water Act governs the discharge of pollutants into the waters of  the United States, including the treatment of wastewater discharged from  publicly owned treatment facilities. Specifically, industrial and municipal  wastewater treatment facilities must comply with the National Pollutant  Discharge Elimination System (NPDES) permits that control pollutants  that facilities may discharge into the nation’s surface waters. The act  requires that municipal wastewater treatment plants provide a minimum  of secondary treatment prior to discharge. In some cases, modification of  secondary treatment requirements may occur, however, for discharges  into marine waters under certain conditions. For example, the discharge  may not interfere with that water quality which assures protection of  public water supplies and the protection and propagation of a balanced,  indigenous population of shellfish, fish, and wildlife and allows  recreational activities on the water. In 2000, Congress amended the Clean  Water Act to require permits for discharges from combined sewers— sewers that transport both wastewater and stormwater to the municipal  wastewater treatment plant—to conform with EPA’s Combined Sewer  Overflow Control Policy, which requires systems to demonstrate  implementation of certain minimum pollution control practices.  Combined sewers may overflow when there is heavy precipitation or  snowmelt, resulting in the discharge of raw sewage and other pollutants  into receiving water bodies.
section_title: Comprehensive Data about the Energy Needed for the Urban Water Lifecycle Are Limited, However Energy Needs Are Influenced by Location-Specific Factors
paragraphs: Comprehensive data about the energy needed for each stage of the urban  water lifecycle are limited, and few nationwide studies have been  conducted on the amount of energy used to provide drinking water and  wastewater treatment services to urban users. However, specialists with  whom we spoke emphasized that the energy demands of the urban water  lifecycle vary by location; therefore, consideration of location-specific and  other factors is key to assessing the energy needs of the urban water  lifecycle. These factors include the source and quality of the water, the  topography of the area over which water is conveyed and the distance of  conveyance, and the level and type of treatment required.
subsections: section_title: Comprehensive Data on the Energy Needed to Support the Urban Water Lifecycle Are Limited
paragraphs: Providing a reliable and comprehensive estimate of the total energy  requirements for moving, treating, and using water in urban areas is  difficult, in part, because comprehensive data on the energy demands of  the urban water lifecycle are limited and few nationwide studies have been  conducted to quantify the amount of energy used throughout the lifecycle.  Two studies most often cited by the specialists we spoke with were  conducted by the Electric Power Research Institute (EPRI) on the energy  needs of the urban water lifecycle. These studies concluded that 3 to 4  percent of the nation’s electricity is used to move and treat drinking water  and wastewater. While some specialists noted that these studies provide  reasonable estimates of the energy demands of the urban water lifecycle,  other specialists raised a number of concerns with the studies. In  particular, according to several specialists, the EPRI studies are outdated.  The first study dates back to 1996, and the more recent study was  conducted in 2002 but relied on projections of future water use based on  statistics compiled in 2000. Some specialists also told us these studies do  not reflect the treatment processes that have been implemented over the  last decade, which have increased the amount of energy needed to treat  water. In addition, the studies do not include all stages of the urban water  lifecycle—specifically, they omit energy used by customers. Because they  exclude end use, the EPRI studies underestimate the energy demands of  the entire lifecycle because customer end use, including use by residential  customers, can be the most energy-intensive stage of the entire lifecycle,  according to some specialists we spoke with and studies that we reviewed.  Some specialists also added that the studies underestimate total energy  demands because they include only electricity, excluding other fuel types  that can be used throughout the lifecycle. For example, the studies do not  assess the use of natural gas, which can be a primary energy source at  wastewater treatment plants for certain processes. Furthermore, some  specialists explained that the studies do not use actual measured data,  relying instead on previously published estimates of energy used for  portions of the water lifecycle.
Furthermore, some specialists noted that there are limited data on the  amount of energy associated with customer water use. Federal agencies  like DOE’s Energy Information Administration collect some data on  energy used to heat water in residences and in the commercial sector, but  these data are reported on a national level and do not allow for analysis at  the local level. In addition, data needed to get a full picture of the energy  needs for water in an urban setting may not be readily available at the  local level. Specifically, water utilities may not have detailed data on their  facilities’ energy use, may not have conducted audits to understand how  their facilities use energy, or may be reluctant to share data, according to  specialists we spoke with.
section_title: Consideration of Location- Specific and Other Factors Is Key to Assessing the Energy Needs of the Urban Water Lifecycle
paragraphs: Many of the specialists told us that efforts to assess the energy needs of  the urban water lifecycle on a national scale can be difficult, and the  majority of the specialists we spoke with emphasized that to obtain a more  accurate picture, one needs to consider location-specific and other factors  that influence energy use. The specialists identified the following as key  factors that must be considered for such an assessment.
Type of water source. Drinking water systems that rely on surface water  are often designed to take advantage of gravity and use little to no energy  to extract water from the source and convey it to the treatment facility. In  contrast, systems that rely on groundwater require more energy for  extraction because water must be pumped to the surface from  underground aquifers, especially if they rely on deep underground  aquifers. For example, Washington, D.C., which relies on surface water,  withdraws its water from two locations—Great Falls Dam and Little Falls  Dam—on the Potomac River. Most of the water is withdrawn at Great  Falls Dam and conveyed via gravity to the treatment plant, using little  energy during the extraction and conveyance process. In contrast,  extraction of water is an energy-intensive process for Memphis, which  relies on groundwater that is extracted from over 160 wells that draw  water from aquifers including the Memphis Sand Aquifer, located 500 to  600 feet below ground.
Quality of water to be treated. The quality of water also impacts the  amount of energy needed for treatment, with higher-quality water  containing fewer contaminants and, therefore, requiring less treatment  than lower quality water. For example, treating groundwater generally  uses less energy than treating surface water because groundwater is  typically of higher quality than surface water. As a result, cities that rely on  groundwater as the source for their drinking water, such as Memphis,  generally use less energy for treatment than cities that rely on surface  water, such as Washington, D.C. However, the type of contaminants in  water can also affect the energy required for treatment. For example, as  one specialist noted, if groundwater contains arsenic, treating this type of  contamination can require the use of more energy-intensive treatment  technologies than treating surface water that is extracted from a protected  watershed or clean snowmelt.
Topography and distance. Pumping water is one of the most energy- intensive aspects of the urban water lifecycle, accounting for 80 to 90  percent of the energy used to supply drinking water in some systems, and  most of this energy is used to distribute water to customers. The energy  demand of pumping is affected by the topography over which the water  must be moved and the distance the water must travel to treatment plants  after extraction and to customers after treatment. For example, San  Diego gets a large amount of its water from northern California.  Transporting this imported water to southern California is energy  intensive because the water must be conveyed hundreds of miles and  lifted 2,000 feet over the Tehachapi Mountains. Furthermore, because of  the hilly terrain in some parts of the city and the great expanse over which  the customers are distributed, additional energy is needed to pump water  from the treatment plants to consumers.
Condition of water system. The age of a system and the condition of its  pipes and equipment can also impact the energy demands of providing  drinking water and wastewater treatment services. Specifically, older  systems can be less energy efficient if the equipment and infrastructure  have not been properly maintained. The American Society of Civil  Engineers recently evaluated America’s drinking water and wastewater  infrastructure and assigned both systems a grade of a D-. The assessment  noted that these systems contain facilities that are nearing the end of their  useful lives and need upgrades to meet future regulatory requirements. In  addition, the condition of pipelines also has energy implications.  According to some specialists we spoke with, up to 50 percent of water is  lost through leaking pipes, which results in a loss of the energy that was  used to extract, convey, and treat the water. Furthermore, if pipelines are  not routinely cleaned, blockages can lead to friction in the pipes, requiring  additional energy to push water through these pipes.
Required treatment level. Energy needed for drinking water and  wastewater treatment is affected by the treatment levels required to meet  existing water quality standards, with each additional treatment level  increasing energy demands. In the case of wastewater treatment,  characteristics of the water body into which treated effluent is discharged  can impact the required level of treatment. For example, San Diego  officials told us the city’s wastewater treatment facility has been granted a  modified permit by EPA. According to these officials, this permit allows  San Diego to treat its wastewater only to an advanced primary level in part  because years of ocean monitoring have shown that the plant discharges  have no negative impact to the Pacific Ocean. If the city had to treat its  wastewater to secondary treatment levels, city officials estimate that its  energy usage would increase six to nine times as a result of having to use  more energy-intensive technologies to meet these higher standards.
Type of treatment process. The type of treatment process used at drinking  water and wastewater facilities also influences the energy demands of  providing drinking water and wastewater services to urban users. For  example, treatment plants that use the activated sludge process for  secondary treatment use more energy than plants that use other  processes, such as trickling filters or lagoon systems. The activated  sludge process can account for 70 percent of a wastewater treatment  plant’s energy consumption because of the energy needed to power the  blowers that pump oxygen into the wastewater to sustain the micro- organisms. Furthermore, according to many of the specialists we spoke  with, a number of the new technologies used in drinking water treatment  plants are more energy intensive than traditional treatment technologies.  For example, some treatment plants are installing ultraviolet light  disinfection processes that are more energy intensive, accounting for 10  to15 percent of a plant’s total energy use, than traditional disinfection with  chlorine. Other energy-intensive technologies that are increasing energy  demands for water treatment include filtration using membranes and  ozonation, a process that destroys bacteria and other micro-organisms  through an infusion of ozone.
Water use and type of customer. Characteristics related to customer water  use, such as how and where water is consumed, can also influence the  amount of energy needed to provide water and wastewater services to  urban users, according to specialists we spoke with. Large amounts of  household energy are consumed by heating water for showering,  dishwashing, and other uses. These uses would require more energy than  other household uses, such as flushing toilets. In addition, some specialists  told us that where the water is used influences the amount of energy  consumed. For example, water used in tall apartment buildings or  skyscrapers requires energy-intensive pumps to move the water to the top  floors. Furthermore, according to some specialists we spoke with, the type  of customer, such as whether the customer is residential or industrial,  affects the energy demands of providing water and wastewater services.  For example, Memphis has two wastewater treatment plants, one of which  is located in an industrial section of the city and receives a higher  percentage of its wastewater from industrial sources than the other  facility, which receives a higher percentage of its wastewater from  residential sources. Because the industrial wastewater contains increased  levels of organic contaminants and thus requires more energy for  treatment, the two facilities consume different amounts of energy on a per- gallon basis.
Water availability. As current water supplies diminish, some cities,  especially those in areas that are already water stressed, are moving  toward alternative water supply sources that will require more energy for  treatment than processes used for surface water and groundwater. For  example, to help meet future demands for water and reduce dependence  on imported water supplies in San Diego, the region is pursuing energy- intensive seawater desalination, which can be 5 to 10 times more energy  intensive than conventional processes to treat surface water and  groundwater. Other areas, such as Tucson, Arizona, that do not have ready  access to seawater are pursuing desalination of brackish groundwater— water that is less saline than seawater but that contains higher saline  levels than found in freshwater. Although treating brackish water is less  energy intensive than seawater desalination, it still can use two to three  times more energy than conventional water treatment processes for  freshwater supplies. Furthermore, San Diego is studying the viability of  treating a portion of its reclaimed water—wastewater effluent that is  treated to an advanced level and suitable for nonpotable water  applications such as irrigation—for potable water use. To implement such  a system, San Diego would need to add energy-intensive advanced  treatment processes to its current wastewater treatment system. However,  because this additional energy use would offset the energy demands for  imported water, city officials told us the project is expected to result in a  net reduction in San Diego’s energy profile. Using reclaimed water can  also increase energy demands for pumping, depending on the design of the  existing wastewater system. That is, many wastewater collection systems  were designed with treatment plants located in low elevation areas to take  advantage of gravity in conveying the wastewater to the plant. However, if  wastewater is recycled, energy could be needed to pump this water against  the flow of gravity into the distribution system, but such increases may  actually be less energy intensive than reliance on imported water.
Future regulatory changes. To address growing concerns about emerging  contaminants and nutrients in the nation’s water bodies, according to  many specialists, additional or more stringent regulatory standards could  increase the energy demands of treatment processes in the future.  Specifically, any more stringent standards that are promulgated would  most likely require additional levels of treatment, and energy-intensive  technologies, such as ozonation and membrane filtration, may be  necessary to meet such new standards. More stringent regulations in the  future could also increase energy demands even for facilities that have  already implemented such technologies. For example, according to  officials of the Washington, D.C., wastewater treatment plant, while the  facility already must meet the nation’s most stringent permit requirements  and uses advanced treatment processes, stricter standards are expected to  increase the plant’s energy demands, in part, because new energy- intensive technologies may need to be added to the plant’s treatment  process. Regulatory changes could also increase energy demands at other  stages of the urban water lifecycle. For example, higher standards for  effluent discharge from wastewater treatment plants could increase the  energy required for treatment. Furthermore, stricter water quality  standards for receiving waters could necessitate more plants to employ  advanced treatment standards, resulting in increased energy use for the  additional treatment or to pump effluent farther away to other waters.
Complexity of water systems. In addition to location-specific factors, the  complexity of some urban water systems can make assessing the energy  demands of the urban water lifecycle challenging. For example, some  urban water systems like San Diego’s are highly complex, involving a  number of different entities that have responsibility for different parts of  the system. Specifically, the City of San Diego currently imports 85 to 90  percent of its water from the Colorado River and northern California. In  addition, the city’s regional drinking water, wastewater, and recycled  water systems are managed by a number of different organizations  responsible for conveying drinking water, wastewater, and recycled water  to multiple treatment facilities with over 160 pumping stations spread over  400 square miles within the City of San Diego’s service territory alone. As a  result, collecting consistent data on energy use from each of these  organizations is challenging, according to San Diego water officials we  spoke with.
section_title: Certain Technologies and Approaches Can Reduce Energy Use, but Barriers Could Impede Their Adoption
paragraphs: Specialists we spoke with and studies we reviewed identified a variety of  technologies and approaches that can improve the energy efficiency of  drinking water and wastewater processes associated with the urban water  lifecycle, and determining the appropriate solution depends on the  circumstances of a particular system. However, adoption of these  technologies and approaches may be hindered by costs; inaccurate water  pricing; barriers associated with operational factors, such as limited  staffing levels at water utilities; competing priorities at drinking water and  wastewater facilities; and lack of public awareness about the energy  demands of the urban water lifecycle.
subsections: section_title: Certain Technologies and Approaches Can Reduce the Energy Use of the Urban Water Lifecycle
paragraphs: Several key technologies and approaches are currently available that can  improve the energy efficiency of drinking water and wastewater  processes, but determining the most appropriate solution depends on the  circumstances of a particular system and requires an understanding of the  system’s current energy use. Many studies that we reviewed and  specialists we spoke with identified process optimization, equipment and  infrastructure upgrades, water conservation, and improved energy  management as approaches that can help reduce the energy demands for  water. In addition, the increased use of renewable energy could offset the  energy purchased by water utilities from energy providers.
subsections: section_title: Process Optimization
paragraphs: According to some studies we reviewed, energy consumption by water and  wastewater utilities can comprise 30 to 50 percent or more of a  municipality’s energy bill. Optimizing drinking water and wastewater  system processes, including energy-intensive operations like pumping and  aeration, was identified in many studies that we reviewed as an approach  to reducing the energy demands of the urban water lifecycle.  Implementing monitoring and control systems and modifying pumping and  aeration operations are some ways to reduce energy use through process  optimization.
Implementing monitoring and control systems. Monitoring and control  systems, also known as supervisory control and data acquisition systems,  can be used to optimize drinking water and wastewater operations. Such  systems provide a central location for monitoring and controlling energy- consuming devices and equipment, which provides plant operators with  the ability to schedule operations or automatically start and stop devices  and equipment to manage energy consumption more effectively and  improve overall operations.
Modifying pumping operations. A variety of modifications could increase  the efficiency of pumping systems. For example, operating constant speed  pumps as near as possible to their most efficient speed, using higher  efficiency pumps as opposed to lower efficiency pumps, and operating  multiple smaller pumps rather than a few large pumps to better match  pumping needs can help maximize pumping efficiency. In addition, using  devices to monitor and control pump speeds—known as variable  frequency drives (VFD)—may allow facility operators to accommodate  variations in water flows by running pumps at lower speeds and drawing  less energy when water flows are low. Potential energy savings from the  use of VFDs can range from 5 to 50 percent or more, according to studies  we reviewed. However, these studies and some specialists we spoke with  also noted that VFDs are not necessarily well suited for all applications— such as when flow is relatively constant—and that potential benefits of  VFDs should be evaluated based on system characteristics, such as pump  size and variability of flow.
Modifying aeration operations. According to many studies we reviewed  and specialists we spoke with, aeration in wastewater treatment consumes  a significant amount of energy, and systems can be reconfigured and  better controlled to improve energy efficiency. Specifically, blowers and  mechanical aerators are typically powered by a large motor, and installing  variable controls on blowers to enable operators to better match aeration  with oxygen requirements can reduce energy demands. Likewise several  studies noted that dissolved oxygen control systems can be used to match  oxygen supply with demand by monitoring the concentration of dissolved  oxygen in the wastewater and adjusting the blower system or mechanical  aerator speed accordingly. In addition, probes can be installed to monitor  dissolved oxygen levels within the wastewater and signal operators when  the system may need adjustment.
section_title: Equipment and Infrastructure Improvements
paragraphs: According to many studies and specialists we spoke with, installing more  efficient equipment—motors, pumps, blowers, and diffusers—for energy- intensive processes such as aeration and pumping can reduce energy use.  In addition, ensuring the proper sizing and maintenance of equipment and  infrastructure can improve energy efficiency.
Upgrading equipment. Replacing less efficient equipment with more  energy-efficient equipment can reduce energy use. For example, installing  more efficient motors could reduce energy use by 5 to 30 percent,  according to studies we reviewed. In addition, blower and diffuser  technologies, including high-speed “turbo” blowers and fine or ultra-fine  bubble diffusers, could decrease the energy demands of aeration. High- speed turbo blowers use less energy than other blower types, although,  because these blowers are a new technology and relatively few are in use,  efficiency claims are not yet well documented, according to a 2010 EPA  report. Energy-saving estimates for fine bubble diffusers, which have  higher oxygen transfer efficiencies than coarse bubble diffusers, range  from 9 to 50 percent or more, but some specialists and studies expressed  concerns about maintenance requirements as well as the durability of this  technology.
Right-sizing equipment. Many wastewater treatment systems were  designed to handle greater capacity in the future because of anticipated  population growth. However, this growth has not always occurred and, as  a result, existing equipment may be oversized and consume more energy  than is needed to treat current flows, according to some specialists we  spoke with. Proper sizing and selection of pumping and aeration  equipment to more closely match system needs can help maximize  efficiency. For example, in Washington, D.C., the operators of the  wastewater treatment plant replaced a 75-horsepower motor with a 10- horsepower motor in one facility to better meet actual energy demands.
Improving maintenance and leak detection technology. Periodic  inspections to assess pump performance and the need for replacement or  maintenance of electrical systems and motors can increase the energy  efficiency of the overall system, according to studies we reviewed. In  addition, leak detection technologies can identify leaks throughout water  systems, thereby reducing water loss and the related energy required to  pump and treat that “lost” water. For example, acoustic leak detection  systems use sensors to monitor for sounds that may indicate potential  leaks and relay the data back to a central control room, which helps water  utility staff identify actual leaks and schedule maintenance accordingly.  The San Diego County Water Authority, which provides water to San  Diego and other areas in southern California, has fiber optic lines in place  to monitor its pipeline 24 hours a day to detect evidence of leaks.
section_title: Water Conservation and Efficiency
paragraphs: Many studies we reviewed and specialists we spoke with also identified  water conservation as an approach to reducing the energy needed for the  urban water lifecycle. Several studies noted that decreased customer  water use could directly translate into energy savings. Furthermore, water  conservation also reduces the amount of energy used to convey, treat, and  distribute drinking water to the customers. Studies we reviewed and  specialists we spoke with identified a variety of tools that utilities can use  to promote water conservation, including enhanced metering, increased  water prices, public education, and incentives to install water-efficient  appliances. For example, San Diego is implementing advanced metering  tools to better manage its system and to provide real-time information to  customers regarding their water use in order to help them make choices  that conserve water. In addition, EPA has developed water efficiency and  performance criteria for several product and program categories through  WaterSense, a federal water efficiency program.
section_title: Energy Management
paragraphs: While many technologies and approaches have been identified to reduce  the energy demands for water, determining the most appropriate solution  depends on the circumstances of a particular system—including the type  of facilities and treatment processes in place—and requires an  understanding of current energy use. Several studies we reviewed  identified improved energy management, including conducting energy  audits of treatment facilities or systems, as a necessary first step to  reducing energy demands. Specifically, specialists told us that by  providing utility managers with information about their facilities’ energy  use, energy audits can help managers identify opportunities to change  plant operations in ways that will save energy. For example, the energy  supplier for one wastewater treatment plant in Memphis conducted an  energy audit of the blower system, which used about 75 percent of the  plant’s total energy. As a result of this audit, operators changed their  practices to run blowers at the lowest levels possible while still ensuring  they continued to meet the effluent discharge standards required by the  plant’s permits.
Similarly, in 2000, San Diego established an in-house energy management  program, which includes an audit team that looks for technologies and  approaches to lessen the energy demands of the city’s drinking water and  wastewater systems. The team studies the efficiency of existing equipment  and treatment processes and considers upgrading or replacing equipment  with available energy-efficient technologies. For example, the energy audit  team identified over a dozen energy conservation measures that could be  applied to reduce energy consumption at two of the city’s sewer pump  stations, including installing timers to turn off lighting and upgrading,  resizing, and replacing motors and blowers.
In addition, EPA’s Energy Star program provides energy management  tools and strategies to support the successful implementation of energy  management programs. Officials told us that EPA also works with  municipal drinking water and wastewater utilities to provide information  on potential energy efficiency opportunities. EPA’s online benchmarking  tool, known as the Portfolio Manager, offers wastewater treatment plant  managers the opportunity to compare the energy use of their plants with  that of other plants using the EPA energy performance rating system. EPA  has also published a variety of educational materials for drinking water  and wastewater utilities to help identify, implement, measure, and improve  energy efficiency and renewable energy opportunities.
section_title: Other Approaches
paragraphs: Specialists we spoke with and studies we reviewed identified two  additional approaches for reducing the energy required to treat and  distribute water: improving advanced treatment technologies and  redesigning a city or region’s water system.
Improving advanced treatment technologies. According to EPA officials,  and as previously noted by specialists, improving energy-intensive  advanced treatment technologies—such as ultraviolet disinfection, ozone,  and membrane technologies—is important because plants are increasingly  using them. For example, the use of membrane materials that require less  pressure to push water through to remove contaminants could decrease  the energy demands of that technology. In addition, some specialists we  spoke with told us that newer technologies are being developed, such as  forward osmosis, that may offer alternative treatment approaches that are  more efficient than the technologies currently used for desalination.  Several specialists told us the federal government should conduct  additional research to understand and improve the energy efficiency of  water supply, treatment, and water use—for example, by conducting more  research on energy-efficient desalination technologies.
Redesigning water systems. Some specialists noted that redesigning  water systems in ways that better integrate drinking water, wastewater,  and stormwater management could improve the energy efficiency of water  systems overall. Decentralizing treatment systems, implementing  approaches to better manage stormwater, reusing wastewater, and using  less energy-intensive processes for biological treatment can help reduce  energy needed for providing drinking water and wastewater services. For  example, current water systems primarily rely on a few plants with large  capacities to treat drinking water and wastewater. Some specialists told us  that systems could be redesigned to incorporate more treatment plants  with smaller capacities and to locate these plants closer to the point of  water use by customers, thereby reducing some of the energy required for  pumping to the treatment site. In addition, some specialists identified  improvements in stormwater management through strategies such as low- impact development—which involve land use planning and design to  better manage stormwater—as a way to reduce the energy required for  treatment. For example, by decreasing stormwater infiltration into some  wastewater systems through low-impact development activities such as  the capture and use of rainwater, flows into treatment plants would also  be reduced, thereby decreasing the energy needed for treatment. In  addition, reusing wastewater for purposes that may not require potable  water, such as industrial processes or landscaping, may reduce overall  energy use by decreasing energy used currently to pump, treat, and  distribute potable water to these customers, according to some studies we  reviewed. However, the potential for energy savings from reuse depends  on the energy intensity of a given system’s water supply as well as the level  of treatment needed for potential uses. Furthermore, some studies we  reviewed and specialists we spoke with noted that relying more on  biological treatment processes that do not require aeration, such as using  lagoons or trickling filters, may be an option to reduce energy demands.  However, these approaches may be limited by available space in urban  areas and therefore may not be applicable everywhere.
section_title: Renewable Energy
paragraphs: Many studies we reviewed and specialists we spoke with stated that  drinking water and wastewater utilities could adopt renewable energy  projects to reduce energy purchased from energy providers. Renewable  energy projects may include solar, wind, and hydroelectric power as well  as the recovery and use of biogas from wastewater treatment processes.  In addition, some studies we reviewed and specialists we spoke with  identified hydro turbines as an option for recovering energy in the  distribution system. For example, water systems with changes in  topography that have pressure-reducing valves in place can install turbines  that generate electricity as water flows past. This energy could then be  recovered for use in powering equipment.
The city of San Diego has adopted a variety of renewable energy projects  to power its drinking water and wastewater treatment operations. For  example, the city installed a 945-kilowatt solar power system at the Otay  Water Treatment Plant that produces enough electricity to meet the power  needs of the plant’s pumping operation (see fig. 4). In addition, at the city’s  Point Loma Wastewater Treatment Plant, both methane and hydroelectric  power are recovered from wastewater processes. The plant uses digestion  processes to treat organic solids resulting from its wastewater treatment  processes. Methane, a by-product of the digestion process, is removed  from the digesters and used to power two engines that supply all of the  plant’s energy needs, making it energy self-sufficient. In addition, the plant  recovers hydroelectric power from the treated effluent that it discharges  into the ocean. The effluent drops 90 feet from the wastewater treatment  plant to the ocean, powering a 1,350 kilowatt hydroelectric plant. The city  can sell any excess energy produced by the plant back to the electric  utility.
While renewable energy projects have the primary benefit of reducing the  energy needed by water treatment facilities from outside providers, such  projects could also reduce overall energy use. For example, solar power  systems co-located at treatment facilities in San Diego may result in the  offset of slightly more electricity than they produce, since electricity  generated by the energy provider off-site and transferred over a greater  distance results in some loss of energy during transmission.
section_title: Key Barriers Could Impede Adoption of Technologies and Approaches
paragraphs: Specialists we spoke with identified a number of key barriers to adopting  the available technologies and approaches that could reduce the energy  demands of the urban water lifecycle. These barriers fall into five  categories: (1) costs associated with these technologies, (2) inaccurate  water pricing, (3) barriers associated with how water utilities operate, (4)  competing priorities at drinking water and wastewater facilities, and (5)  the lack of public awareness about the energy demands of the urban water  lifecycle.
subsections: section_title: Costs Associated with Energy- Saving Improvements
paragraphs: Energy-saving technologies may lessen the energy demands of the urban  water lifecycle, but such improvements are often expensive to adopt.  Many specialists told us that, as a result, utilities may not be able to justify  the costs necessary to install energy-efficient equipment. For example,  some specialists told us that upgrading to VFDs, higher-efficiency pumps,  and ultra-fine bubble diffusers may lessen a water facility’s energy  demands, but the costs of installing these technologies can be prohibitive  for some systems, and it can take years to realize the full energy-saving  benefits. As a result, some utility operators may choose to wait until there  is an immediate need to upgrade equipment because the costs can be  justified more easily at that point. Similarly, some specialists told us that  the cost of installing renewable energy projects, such as solar panels, can  be a barrier to adoption for some treatment facilities. According to an  energy specialist we spoke with, it may take over 30 years to fully realize  the cost savings from such projects. However, a DOE official noted that  while expensive in the past, the cost of solar panels has been decreasing in  recent years. Furthermore, installing energy-efficient equipment and  infrastructure upgrades, such as replacing leaking pipelines, can be  particularly challenging for smaller water utilities because they often  compete for limited funds against other municipal services, such as fire  and police protection. In addition, in areas where energy costs are low,  there may be little incentive for water utility operators to implement  capital-intensive practices to save energy. To help overcome the barriers  associated with the costs of upgrading facilities, some specialists told us  that utilities should conduct cost analyses to account for the total savings  incurred over the life of the energy-saving projects, not just focus on the  short-term returns on investment. Some specialists we spoke with also  suggested that utilities should take advantage of the federal funding  available through the Drinking Water and Clean Water State Revolving  Fund programs, which can be used to fund a variety of projects that  improve water and energy efficiency. These programs provide financial  assistance for drinking water and wastewater infrastructure projects,  respectively, and for certain other purposes, such as installing water  meters, installing or retrofitting water-efficient devices, and promoting  water conservation. In addition, the American Reinvestment and Recovery  Act of 2009 and EPA’s fiscal year 2010 appropriation encourage states to  use a portion of those funds for such energy and water efficiency  projects.
section_title: Inaccurate Water Pricing
paragraphs: According to many specialists with whom we spoke, the true cost of water  is often not reflected in rates customers are charged. Specifically,  specialists told us that water subsidies have kept water rates artificially  low and do not reflect the actual cost, including energy costs, of pumping,  treating, and moving drinking water and wastewater. The effect of this  situation is two-fold. First, there may be little incentive for customers to  use water more efficiently if they are not paying the true cost of it. Second,  because these reduced water rates generally do not cover the actual costs  incurred by drinking water and wastewater facilities, some utilities do not  generate enough revenue to implement upgrades that could lessen their  facilities’ energy demands. Some specialists noted, however, that rate  increases are not a politically popular approach and may be met with  public and political resistance.
section_title: Operational Challenges
paragraphs: Other barriers to adopting energy-reducing technologies and approaches  are operational in nature. Specifically, specialists we spoke with noted a  number of such challenges, including utilities not having staff with  adequate knowledge about technologies and access to energy-use data,  reluctance to change, and lack of coordination between water and energy  utilities. For example, several specialists told us that smaller utilities lack  staff with knowledge about the energy-efficient techniques or may only  have operators in place part time to manage or oversee new technologies.  Because operators generally are the advocates for energy-efficiency  upgrades, the specialists believe it could be difficult to gain support for  such investments without knowledgeable operators. Further, operators  may be unaware of the amount of energy their facilities use because, in  many municipalities, these bills are received and paid by other  departments and operators may not have access to these data.  Consequently, operators may be unaware of the potential for energy  savings from upgrades. Moreover, many specialists told us that operators  are often resistant to alter the practices that they have employed for years  to move and treat water and may be reluctant to adopt new technologies  or approaches, especially if the effectiveness of such changes has not yet  been adequately proven. Some specialists also told us that drinking water  and wastewater utilities do not coordinate as closely as they could with  energy utilities to identify opportunities to optimize their operations and,  thereby, lessen their energy demands.
section_title: Energy Usage Considerations Are Secondary to Complying with Water Quality Regulations
paragraphs: Considering the energy demands of treatment can be an afterthought to  complying with water quality regulations for treatment plant operators,  according to some specialists with whom we spoke. One drinking water  utility operator told us that energy is considered to the extent possible  when decisions are being made about altering treatment processes to meet  regulatory requirements but that the safety of the water supply is his  primary concern. For example, when the city of San Diego’s Public  Utilities Department was considering which disinfection technology to  employ, it chose to use ozonation because it would provide more effective  disinfection for the plant and also reduces disinfection by-products, even  though it is a more energy-intensive technology than the current  disinfection process. In addition, to ensure that minimum effluent  discharge standards are met, water utility operators may over-treat  wastewater by, for example, running aeration blowers at higher levels than  necessary to meet regulatory requirements. In light of the potential for  more stringent standards in the future, some specialists noted that  regulators should consider the energy demands associated with these  increased water quality standards.
section_title: Lack of Public Awareness about the Energy Demands of the Urban Water Lifecycle
paragraphs: Many specialists told us that many customers are not aware of and do not  understand the energy demands of drinking water and wastewater  services. While some customers may be aware of their total energy use, it  may not be clear to them how much of that energy use is for heating water  and other water-related uses. In addition, customers may not be aware  that water conservation saves not only water but also energy. Some  specialists told us that federal programs such as EPA’s WaterSense and  Energy Star and some state efforts, such as in California and New York,  have begun to educate the public on the energy demands of the urban  water lifecycle; however, additional efforts may be needed to increase  awareness of the energy-water nexus for providing drinking water and  wastewater to urban users.
section_title: Agency Comments
paragraphs: We provided a draft of this report to the Departments of Defense, Energy,  and the Interior and EPA for review and comment. DOE and EPA provided  technical comments that we incorporated into the final report as  appropriate.
As agreed with your office, unless you publicly announce the contents of  this report earlier, we plan no further distribution until 30 days from the  report date. At that time, we will send copies of this report to the  appropriate congressional committees; the Administrator of EPA; the  Secretaries of Defense, Energy, and the Interior; and other interested  parties. In addition, the report will be available at no charge on the GAO  Web site at http://www.gao.gov.
If you or your staff have questions about this report, please contact us at  (202) 512-3841 or mittala@gao.gov or gaffiganm@gao.gov. Contact points  for our Offices of Congressional Relations and Public Affairs may be found  on the last page of this report. GAO staff who made key contributions to  this report are listed in appendix II.
section_title: Appendix I: Objectives, Scope, and Methodology
paragraphs: Our objectives for this review were to describe what is known about (1)  the energy needed for each stage of the urban water lifecycle, and (2)  technologies and approaches that could lessen the energy needed for the  urban water lifecycle, as well as any identified barriers that exist to their  adoption. We focused our work on community drinking water systems and  publicly owned wastewater facilities located in the United States. We also  focused on residential customers and, to the extent possible, commercial,  industrial, and institutional customers.
To address both of these objectives, we conducted a systematic review of  studies and other documents that examine the energy required to extract,  move, use, and treat water, including peer-reviewed scientific and industry  periodicals, government-sponsored research, and reports from  nongovernmental research organizations. In conducting this review, we  searched databases such as ProQuest, EconLit, and BioDigest, and used an  iterative process to identify additional studies, asking specialists to  identify relevant studies and reviewing studies from article bibliographies.  We reviewed studies that fit the following criteria for selection: (1) the  research was of sufficient breadth and depth to provide observations or  conclusions directly related to our objectives; (2) the research  demonstrated the energy demands of water supply systems in the United  States; (3) the studies typically were published between 2000 and 2010;  and (4) the studies were determined to be methodologically sufficient. We  examined key assumptions, methods, and relevant findings within the  studies related to drinking water processes, customer end use, and  wastewater processes. We believe we have included the key studies and  have qualified our findings, where appropriate. However, it is possible that  we may not have identified all of the studies with findings relevant to these  two objectives.
We also selected a nonprobability sample of three cities to examine in  greater depth to better understand regional and local differences related to  urban water lifecycles: Memphis, Tennessee; San Diego, California; and  Washington, D.C. We chose these cities as illustrative case studies based  on criteria such as their type of water source; water availability; type of  wastewater system; unique characteristics, such as potential for  desalination; and economic factors, such as energy costs. The results from  our visits to these cities cannot be generalized to all U.S. cities, but they  provide valuable insights as illustrative case studies. For each of these  case studies, we analyzed documentation from and conducted interviews  with a wide range of specialists to gain the views of diverse organizations  covering all stages of the urban water lifecycle. These groups included  relevant drinking water and wastewater treatment facilities, and state and  local agencies responsible for water or energy. We requested interviews  with representatives from electrical utilities in each location. In San Diego  and Washington, D.C., the utilities did not meet with us or told us they did  not have relevant data. In Memphis, however, which has a combined water  and energy utility, an energy official was present at our meeting with the  utility, but the utility told us it does not track data on energy for water- related uses for some customer types. In addition, we conducted site visits  to drinking water and wastewater treatment facilities in each of these  locations to better understand the role that energy plays in their operation.
In addition to the specialists we interviewed as part of our illustrative case studies, we also interviewed a range of specialists whom we identified as  having expertise related to the energy needs of all stages of the urban  water lifecycle in general. We selected these specialists using an iterative  process, soliciting additional names from each person we interviewed.  From among those specialists identified, we interviewed those who could  provide us with a broad range of perspectives on the energy needs of the  urban water lifecycle. We also interviewed specialists that we identified  during our systematic review of studies who have analyzed (1) the energy  needed in one or more stages of the water lifecycle at the national or local  level or (2) techniques available to reduce the energy demands for water.  These specialists represented a variety of organizations, including drinki water and wastewater treatment facilities; state and local government  offices responsible for water or energy; officials from the EPA; researc from some of the Department of Energy’s national laboratories, such as  Sandia National Laboratory; university researchers; water and energy  industry representatives from groups such as the American Water Works  Association and the Water Research Foundation; and relevant  nongovernmental organizations, such as the Pacific Institute, a  nonpartisan research institute that works to advance environmental  protection, economic development, and social equity. The specialists also  included individuals with knowledge of the energy demands for water in  other states, including Arizona, Colorado, Florida, New York, and  Wisconsin, to gain a better understanding of water and energy issues in  other regions around the United States. We also interviewed other federal  agency officials and analyzed data and information from federal agencies  that have responsibilities related to the energy needs of the urban water  lifecycle—the Department of Defense’s U.S. Army Corps of Engineers, th Department of Energy, the Department of the Interior’s U.S. Geological  Survey and Bureau of Reclamation, the Environmental Protection Agency, and the National Science Foundation.
To analyze information gathered through the interviews with specialists  and the scientific studies, research, and other key documents reviewed,  we conducted content analyses. Specifically, to conduct the content  analysis of information gathered through interviews with specialists, we  reviewed each interview, selected relevant statements, and identified and labeled these statements using a coding system that identified the topicarea. Once relevant statements from the interviews were extracted and  coded, we used the coded data to develop key themes. An independentreviewer then verified that the codes were accurately applied to the  statements and the key themes were correctly developed. During the  course of our review, we conducted over 60 interviews with over 100  specialists. For the purposes of our interview analysis, each interview  represents the views of one specialist even if more than one specialist  present at the interview. We used the following categories to quantify  responses of experts and officials: “some” refers to responses from 2 to specialists, “several” refers to responses from 6 to 10 specialists, and  “many” refers to responses from 11 or more specialists. We used a similar  coding scheme to identify key themes resulting from our analysis of the  scientific studies, research, and other key relevant documentation.
section_title: Appendix II: GAO Contacts and Staff Acknowledgments
subsections: section_title: GAO Contacts
section_title: Staff Acknowledgments
paragraphs: In addition to the contact named above, Elizabeth Erdmann, Assistant  Director; Colleen Candrl; Antoinette Capaccio; Janice Ceperich; Nancy  Crothers; Abbie David; Angela Leventis; Katherine Raheb; Ellery Scott;  Rebecca Shea; Jena Sinkfield; Kevin Tarmann; and Lisa Vojta made  significant contributions to this report.