Thermoelectric generating plants and many manufacturing facilities withdraw large volumes of water for use in production and, especially, to absorb waste heat from their industrial processes. Water withdrawals by electric generating plants, used primarily for cooling, are the largest category by sector in the United States—201 billion gallons per day (BGD)—while water withdrawals by industrial facilities, used both for production and cooling, are 18 BGD. Together, water withdrawals by manufacturers and electricity generators represent more than one-half of the 410 billion gallons of water withdrawn daily for various uses in the United States. Withdrawing water from streams, rivers, lakes, and coastal waters is a necessity for most electricity generating and manufacturing facilities, but facilities that require water for cooling also present special problems for aquatic resources. For example, the direct release of heated water after circulation through a power plant or manufacturing facility can harm aquatic life in a stream or lake. And the process of drawing surface water into the plant or facility can simultaneously pull fish, shellfish, and tiny organisms into the plant, generally killing them. In recognition of such impacts, Congress included Section 316 in the 1972 Federal Water Pollution Control Act (the law is commonly known as the Clean Water Act, or CWA) specifically to address the potential problems of heat discharges and cooling water intake structures. Cooling water intake structures (CWIS) can cause two types of adverse environmental impacts. First, impingement occurs when fish, invertebrates, and other aquatic life are trapped on equipment such as screens at the entrance to the CWIS. The force of the intake water traps the organisms against the screen, and they are unable to escape. Second, entrainment occurs when small aquatic organisms, eggs, and larvae pass through the intake screening system and are drawn into the cooling mechanism, travel through the cooling system pumps and tubes, and then are discharged back into the source water. Impingement and entrainment injure or kill large numbers of aquatic organisms at all life stages. In turn, reducing impingement and entrainment mortality rates is likely to increase the number of fish, shellfish, and other aquatic organisms in affected waters. Effects of these manmade stressors on local biota may contribute to or compound the local impact of CWIS impingement and entrainment mortality. CWA Section 316(b) authorizes regulation of CWIS in order to protect such organisms from being harmed or killed. The Environmental Protection Agency (EPA) has been engaged in regulatory efforts to implement Section 316(b) for more than 35 years. In March 2011, EPA proposed national requirements to be implemented through CWA discharge permits to minimize adverse environmental impact of cooling water intake structures at existing electricity generating and manufacturing facilities. Even before release, the proposed regulations were highly controversial among stakeholders and some Members of Congress. The issue for Congress has been whether a stringent and costly environmental mandate could jeopardize reliability of electricity supply in the United States. Many industry stakeholders feared, while environmental groups hoped, that EPA would require installation of technology called closed-cycle cooling that most effectively minimizes impingement and entrainment, but also would be the most costly technology option. However, as discussed in this report, the EPA proposal declined to mandate closed-cycle cooling universally and instead proposed a less costly, more flexible regulatory option. EPA's recommended option would require uniform impingement mortality standards at all affected facilities and case-by-case determination of entrainment controls for all facilities. Based on its review of public comments and additional data, in June 2012, the agency announced that it was considering certain changes to the impingement standards in the proposed rule (see " Impingement " below). Under the original court-ordered schedule, EPA was to issue a final rule on July 27, 2012. EPA did not meet that deadline, which was extended six times before EPA announced the final rule on May 19, 2014. Delays reportedly resulted from time needed to assess possible changes to the proposal and consultation with federal wildlife services for an Endangered Species Act (ESA) review of the rule. In the remainder of this report, provisions of the final rule are highlighted in text boxes that are titled "Final Rule." The primary goal of the CWA is "to restore and maintain the chemical, physical, and biological integrity of the Nation's waters." To further this goal, the act prohibits the "discharge of any pollutant by any person" unless a statutory exception applies; the primary exception is procurement of a CWA permit. Under the law, EPA or an authorized state agency can issue a permit for the discharge of any pollutant provided that the discharge complies with the conditions of the CWA. The act requires technology-based solutions to minimize adverse environmental impacts of pollutant discharges, and Sections 301 and 306 require EPA to develop technology-based effluent limitation guidelines for existing sources and performance standards for new sources, respectively, that are used as the basis for restrictions specified in discharge permits. Section 316, the provision concerned with thermal discharges and cooling water intake structures, cross-references both Sections 301 and 306. Section 316(b) provides in full— Any standard established pursuant to section 301 or section 306 of this Act and applicable to a point source shall require that the location, design, construction, and capacity of cooling water intake structures reflect the best technology available for minimizing adverse environmental impact. EPA's efforts to implement Section 316(b) have a long and complicated history, including legal challenges at every step by industry groups and environmental advocates (for details, see the Appendix ). In summary, however, CWIS regulation involves three rulemaking phases. These regulatory requirements are applied to individual facilities through discharge permits issued by EPA or an authorized state agency. Phase I, issued in 2001, covers CWIS at new facilities, Phase II, issued in 2004, covers large existing electric generating plants, and Phase III, issued in 2006, covers certain existing facilities (manufacturing facilities and small electric generating plants) and new offshore and coastal oil and gas extraction facilities. EPA's March 2011 proposal stemmed from legal challenges to the Phase II rule (including a Supreme Court ruling in 2009) and the Phase III rule. Eventually, EPA determined that the most efficient regulatory approach would be to consolidate the regulations for existing facilities in a single proposal. In November 2010, EPA signed a settlement agreement with environmental group plaintiffs regarding rulemaking dates for establishing CWIS technology-based standards for existing facilities. EPA agreed to propose standards by March 14, 2011, and to take final action by July 27, 2012. EPA proposed the regulations on March 28, 2011. As noted above, EPA and environmental litigants agreed to several extensions of the initial deadline for a final rule, which EPA announced on May 19, 2014. It was published in the Federal Register on August 15, 2014, with an effective date of October 14, 2014. Most power generators and manufacturing facilities use various types of water-based systems to cool power production processes or manufacturing equipment. On average, existing power generators use 85% of withdrawn water for cooling, while manufacturing facilities on average use 52% of withdrawn water for cooling. As defined in the proposed and final rule, "cooling water" is water used for equipment cooling, evaporative cooling tower makeup, and dilution of waste heat. Most facilities use either once-through cooling or recirculating cooling (also known as closed-loop or closed-cycle). The basic designs are illustrated in Figure 1 . Once-through cooling refers to cooling systems in which untreated water is withdrawn from a source, circulated through heat exchangers, and then returned to a surface-water body. Large amounts of water—typically in the range of tens of millions to billions of gallons per day—are needed for once-through cooling. The vast majority of it is returned to the stream or lake, but at higher temperatures. The discharged water also may contain chemicals or pollutants from the cooling process. Once-through systems are used by the majority of electric generating units (63%) and about one-half of manufacturers (47%) subject to EPA's rule. Recirculating, or closed-cycle, cooling systems receive their cooling water from and return it to a cooling tower and basin, cooling pond, or cooling lakes. Closed-cycle cooling systems allow facilities to transfer waste heat to the environment using much less water, relative to once-through cooling systems. In a closed-cycle system, water is withdrawn from a source, circulated through heat exchangers, cooled using ponds or towers, and then recirculated. Some water is removed from the recirculating system as a blowdown stream to control the buildup of suspended and dissolved solids. Amounts of water that are lost to evaporation, blowdown, and leakage are replaced with new withdrawal, usually from surface water bodies. Closed-cycle systems are used by 27% of electric generating units and 20% of manufacturers subject to the final rule. There also are combination systems, facilities with multiple cooling water systems that use both once-through and closed-cycle cooling; combination systems account for another 9% of electric generators and 22% of manufacturers subject to the final rule. There are two main types of closed-cycle recirculating systems. Both reduce the quantity of water that must be withdrawn, compared with once-through cooling systems. In a wet cooling tower system, water that has absorbed waste heat transfers that heat through evaporation into the surrounding air and recirculates the water to continue the cooling process. In dry cooling tower systems, waste heat is transferred completely through convection and radiation, rather than evaporation. Dry cooling towers are much larger and therefore more expensive—reportedly 5 to 10 times more expensive—than wet cooling towers. Because dry cooling towers virtually eliminate the need for cooling water withdrawals, they have been used as part of newly constructed cooling systems at smaller electric generating units and in areas with limited water supplies, such as the arid southwest of the United States.