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Thermal Effect.doc (Size: 91 KB / Downloads: 118)
INTRODUCTIONThermal pollution is the degradation of water quality by any process that changes ambient water temperature. A common cause of thermal pollution is the use of water as a coolant by power plants and industrial manufacturers. When water used as a coolant is returned to the natural environment at a higher temperature, the change in temperature (a) decreases oxygen supply, and (b) affects ecosystem composition. Urban runoff--storm water discharged to surface waters from roads and parking lots--can also be a source of elevated water temperatures. When a power plant first opens or shuts down for repair or other causes, fish and other organisms adapted to particular temperature range can be killed by the abrupt rise in water temperature known as 'thermal shock'.
Environmental engineers and chemical engineers take a narrow view of thermal pollution, unfortunately. Their jobs are to remove heat from waste streams so that discharge regulations are satisfied. The regulation may be stated as the volumes and temperatures that are permissible for discharge or as the thermal rise that is that is tolerable for the receiving water.
Almost half of all water withdrawn in the United States each year is for cooling electric power plants. The cheapest and easiest method is to withdraw water from a nearby body of surface water, pass it through the plant and return the heated water to the same body of water.
Warmer temperatures lower dissolved oxygen content by decreasing the solubility of oxygen in water. Warmer water also causes aquatic organisms to increase their respiration rates and consume oxygen faster, and it increases their susceptibility to disease, parasites, and toxic chemicals. Discharge of heated water into shallow water near the shore of a lake also may disrupt spawning and kill young fish.
Fish and other organisms adapted to a particular temperature range can also be killed from thermal shock
While some scientists call the addition of excess heat to aquatic systems thermal pollution, others talk about using heated water for beneficial purposes, calling it thermal enrichment. They point out that heated water results in longer commercial fishing seasons and reduction of winter ice cover in cold areas.
The production of energy from a fuel source can be direct, such as the burning of wood in a fireplace to create heat, or by the conversion of heat energy into mechanical energy by the use of a heat engine. Examples of heat engines include steam engines, turbines, and internal combustion engines.
Heat engines work on the principal of heating and pressuring a fluid, the performance of mechanical work, and the rejection of unused or waste heat to a sink. Heat engines can only convert 30 to 40 percent of the available input energy in the fuel source into mechanical energy, and the highest efficiencies are obtained when the input temperature is as high as possible and the sink temperature is as low as possible. Water is a very efficient and economical sink for heat engines and it is commonly used in electrical generating stations.
The waste heat from electrical generating stations is transferred to cooling water obtained from local water bodies such as a river, lake, or ocean. Large amounts of water are used to keep the sink temperature as low as possible to maintain a high thermal efficiency. The San Ono re Nuclear Generating Station between Los Angeles and San Diego, California, for example, has two main reactors that have a total operating capacity of 2,200 megawatts (MW). These reactors circulate a total of 2,400 million gallons per day (MGD) of ocean water at a flow rate of 830,000 gallons per minute for each unit. The cooling water enters the station from two intake structures located 3,000 feet offshore in water 32 feet deep. The water is heated to approximately 19°F above ambient as it flows through the condensers and is discharged back into the ocean through a series of diffuser -type discharges that have a series of sixty-three exit pipes spread over a distance of 2,450 feet. The discharge water is rapidly mixed with ambient seawater by the diffusers and the average rise in temperature after mixing is less than 2°F.
3. EFFECT OF THERMAL POLLUTION
Warm water typically decreases the level of dissolved oxygen in the water. The decrease in levels of dissolved oxygen can harm aquatic animals such as fish, amphibians and copepods. Thermal pollution may also increase the metabolic rate of aquatic animals, as enzyme activity, resulting in these organisms consuming more food in a shorter time than if their environment were not changed. An increased metabolic rate may result in food source shortages, causing a sharp decrease in a population. Changes in the environment may also result in a migration of organisms to another, more suitable environment, and to in-migration of organisms that normally only live in warmer waters elsewhere. This leads to competition for fewer resources; the more adapted organisms moving in may have an advantage over organisms that are not used to the warmer temperature. As a result one has the problem of compromising food chains of the old and new environments. Biodiversity can be decreased as a result.
In the 1970s there was considerable activity from scientists in quantifying effects of thermal pollution. Hydrologists, physicists, meteorologists, and computer scientists combined their skills in one of the first interdisciplinary pursuits of the modern environmental science era. First came the application of gaussian function dispersal modeling that forecasts how a thermal plume is formed from a thermal point source and predicts the distribution of aquatic temperatures. The ultimate model was developed by the U.S. Environmental Protection Agency introducing the statistical variations in meteorology to predict the resulting plume from a thermal outfall.
Coal-burning power plants are known producers of thermal pollution in nearby bodies of water that they use as cooling ponds. This research focused on the effects that thermal pollution caused by the Marshall Steam Station had on Lake Norman, North Carolina. It was found that dissolved oxygen in the steam station's discharge cove was decreased by approximately four mg/L as compared to a site ten miles upstream, and was decreased by about three mg/L as compared to a cove several hundred yards downstream. Temperatures of the surface water in the discharge.