Many power plants and manufacturing facilities use water from nearby sources, such as lakes and rivers, to produce steam to generate electricity, or to cool high-temperature machinery. Cooling water intake may take the form of a once-through system, in which water enters a facility, is used for cooling, and then is sent directly back into the water body. Another form is closed-cycle cooling, often in the form of cooling towers, where water is recirculated several times in the facility, thereby using only two to five percent the amount of water required for once-through cooling.
Large amounts of water many used for these cooling purposes, and the intake structures for water may have adverse environmental impacts. One significant problem is impingement, where fish or other animals are trapped against and mortally wounded by intake structures. Another environmental problem is entrapment, where animals, especially small organisms, eggs, and larvae, are sucked into the facility and killed by high pressures and temperatures.
Sec. 316(b)  of the Clean Water Act (CWA) requires that the location, design, construction, and capacity of cooling water intake structures reflect the best technology available for minimizing these adverse environmental impacts.
EPA has proposed a new rule complying with these mandates of the CWA for existing facilities. New facilities are not covered by this rule and remain subject to applicable 2001 cooling water intake regulations. The new rule has three main components. The first component is to establish a limit to the number of fish that can be killed by impingement at a facility and allow the facility to determine what technology to use to meet that standard. The second component applies only to the largest consumers of water (more than 125 million gallons per day), and they must undertake an evaluation, with public comment, on if reductions in the entrapment rate need to be made. The third component requires all new construction of intake structures that are part of increased electric generating capacity to meet the low rates of entrapment achieved by closed-closed cycle intake structures.
This new rule on cooling water intake structures covers about 1,065 existing facilities  that each withdraw at least 2 million gallons per day of cooling water (544 power plants and 521 manufacturing plants). EPA estimates that nearly 40 percent these facilities already employ technologies that are likely to comply with the impingement requirements of the rule. Manufacturers of aluminum, iron, steel, petroleum, paper, chemicals, and food processing are likely to be the most affected by this rule.
EPA first promulgated regulations to implement Sec. 316(b) in 1976 following mandates of the Clean Water Act, but the court remanded those regulations in 1977 after legal challenges from industry. Instead of issuing new rules, EPA undertook relevant studies and directed state permitting authorities to determine the best technology available to reduce impingement and entrapment on a case-by-case basis.
In 1995, EPA entered into a consent decree establishing a schedule for taking final action on regulations following litigation. In November 2001, EPA finalized regulations for new facilities, which it referred to as Phase I of the regulations. Phase II of the regulations, released in 2004, applied to certain power plants built before 2002 – those that withdrew more than 50 million gallons per day from waters of the United States, 25 percent of which was used for cooling purposes. In 2006 offshore oil and gas facilities were given a categorical exclusion from additional rules, which essentially subjected them to the same regulation as Phase I facilities.
This regulation is considered to be the last set following the consent decree, Phase III, and the proposed rule was issued in April 2011 with the public comment period lasting through August 2011. The final rule was issued on May 19, 2014.
Read more from EPA  on cooling water intake structures.