Cooling Water Resources

Conventional water use involves extracting the nearest available resource, typically rivers, lakes, or freshwater aquifers. Freshwater aquifers are considered to be those with a total dissolved solids content of less than 10,000 parts per million (ppm). Both surface and groundwater resources are susceptible to depletion as a result of droughts and/or overallocation.

Water that is treated at an industrial plant such as a refinery, food-processing plant, chemical plant, etc., is considered treated industrial wastewater. These plants may possess a National Pollutant Discharge Elimination System discharge permit for direct discharge to a receiving body and must meet discharge criteria for the state in which the plant is located. In some cases, industrial wastewater treatment plants may discharge their wastewater to a municipal wastewater treatment plant.

Municipal sewage that has undergone secondary treatment and meets the discharge criteria for the state in which the plant is located is defined as treated municipal wastewater. Every municipal wastewater treatment plant possesses a U.S. Environmental Protection Agency National Pollutant Discharge Elimination System permit, which allows for treated water discharge to a receiving body of water. Typical effluent water quality requirements are biochemical oxygen demand (BOD) and total suspended solids (TSS) values, typically not to exceed 30 mg/L for a 30-day average or 45 mg/L for a 7-day average for both BOD and TSS.

Produced water comes from the process of extracting hydrocarbons from subsurface formations. As oil and/or gas are lifted to the surface, water is often brought along with it. The quality of produced water in the United States is highly variable, depending on the type and location of hydrocarbon being produced, and typically contains total dissolved solids levels ranging from 100 to 400,000 mg/L. In some cases, such as with coalbed methane extraction, produced water quality can be quite high and the water can be reused with little or no treatment. In other cases, the salinity of the water can be ten times that of seawater, often rendering treatment uneconomical. Produced water often contains hydrocarbons and dissolved salts and sometimes smaller concentrations of naturally occurring radioactive material and/or chemical additives used in drilling and production. In 2007, more than 20 billion barrels (one barrel = 42 gallons) of produced water was generated in the United States as a result of onshore oil and gas production.

Mine drainage commonly refers to the water that is pumped out of a mining area. The water typically originates as rainwater or, if the level of the mine is below that of the surrounding water table, groundwater. Mine drainage is often acidic because of the reaction of water and newly exposed sulfide minerals.

Saline aquifers contain formation waters or brines with total dissolved solids concentrations greater than 10,000 ppm. With appropriate treatment, saline groundwater can provide a freshwater resource for an array of applications, including thermoelectric power plant cooling.

Waste heat utilization in coal-fired power plants includes using low-grade power plant waste heat to dry lignite and subbituminous (low rank) coals prior to firing in a boiler. The heat source for drying the coal is the hot water generated from the steam condenser. The heat removed from the steam condenser cooling water improves cooling tower performance. Because water is removed from the coal, boiler heat rate and efficiencies are improved.

Restored wetlands are being evaluated by the U.S Department of Energy National Energy Technology Laboratory as a potential water source for power plant cooling. Goals of the project include the demonstration of economic and ecological benefits such as enhancing carbon sequestration, avoiding low-surface-water flows for power plant cooling when electricity demand is the greatest, and providing a suite of biological and physical mechanisms to help treat wastewater effluent.

Flue gas from coal-fired power plant boilers is a potential water resource that can be recovered using advanced technologies, such as condensing heat exchangers, membrane separation, and liquid desiccant-based dehumidification.