Chapter-16
Thermal pollution
Thermal pollution is the accumulation of unwanted heat energy in rivers, lakes and estuaries due to human activities. Thermal inefficient energy conversion processes are responsible for heat pollution. That part of the energy content of the fuel input which is not converted into useful work appears in the form of heat and must somehow be dissipated. Quantitatively, the most significant thermal polluters are the relatively fuel-inefficient energy converters in the generation of electricity by conventional and nuclear power plants, which are estimated to account for over 70 per cent of thermal plants.
Causes of Thermal Pollution
There are different sources of thermal pollution:
1. Working of Thermal Stations:
The thermal power stations are the backbone of power generation. But these power stations are contributing to enormous environmental degradation problems due to the use of poor quality of coal in underdeveloped countries. Discharge from thermal power stations include solid and gaseous pollutants like suspended particulate matter (SPM), oxides of sulphur, and carbon oxides of sulphur and nitrogen. When mixed with air and moisture, they result in formation of sulphuric and nitric acid and may result in formation of acid rain under certain adverse conditions.
2. Nuclear Power Stations:
Radioactivity is not the only waste that nuclear power plants produce. There is another unseen waste. For cooling purposes, nuclear reactors and thermal power stations use large quantities of water. Many times, water from the source of lower temperature is taken and the hot water is let into the river causing rise in temperature.
3. Industrial Power Wastes:
Similarly, industries generating electricity require huge amounts of cooling water for removal of heat. Industries release gases and high ash content. They not only create ash disposal problems but also cause many hazardous diseases like asthma and tuberculosis etc.
Effects of Thermal Pollution
The following are the main effects of thermal pollution:
(i) Changes in temperature have a disastrous effect on aquatic life, plants and animals. It is estimated that a rise of 3.6°c in the earth temperature would make ice caps to melt in the Antarctic and Arctic.
(ii) It reduces the dissolved oxygen content of water.
(iii) It influences reproductive cycle, digestion rate, respiration rate and enzymatic activities of living organisms.
(iv) It favours the growth of certain bacteria and pathogens
(v) Toxicity of pesticides and chemicals in the effluents increases with increase in temperature.
(vi) The composition of flora and fauna changes because of the species sensitive to increased temperature due to thermal shocks.
(vii) Many species may move towards suitable temperature to avoid hot conditions.
Slowing of Metabolism
Water temperatures usually change gradually as the seasons change. During cold seasons, when fewer food sources are available, fish metabolisms slow. When industrial discharges heat up the water, fish metabolisms also speed up, causing malnutrition due to insufficient food sources. Mass fish kills can occur as a result.
Oxygen Depletion
Organic matter decomposes faster in warmer temperatures, depleting dissolved oxygen from the water and increasing bacteria levels. This causes an overabundance of organic nutrients. Aquatic plants grow at a faster rate in warmer waters, resulting in overpopulation and shorter life spans. Algae blooms occur, choking fish gills and clouding the water. As the algae growth increases, other aquatic plants are unable to perform photosynthesis because of a lack of light. Corals and other marine life may die because of oxygen depletion.
Forced Migration
When water temperatures change, local marine life may be forced to migrate from the area, relocating to a more suitable location. Aquatic life from other areas may move into the vacated waters, giving rise to changes in biodiversity in both locations.
Cool Water Discharges
Most thermal pollution is caused by the discharge of warmer water, but cooler water discharges also have a detrimental effect on aquatic life. Cold waters from the bottoms of deep reservoirs are emptied into warmer rivers. Plankton cannot survive the cooler temperatures. Many fish, their eggs and their fry may die. As some organisms die, others reproduce and take over, potentially changing the area's ecosystem.
Industrial wastewater
In the United States, thermal pollution from industrial sources is generated mostly by power plants, petroleum refineries, pulp and paper mills, chemical plants, steel mills and smelters.
Some facilities use once-through cooling (OTC) systems which do not reduce temperature as effectively as the above systems. For example, the Potrero Generating Station in San Francisco, which uses OTC, discharges water to San Francisco Bay approximately 10° C (20° F) above the ambient bay temperature.
Urban runoff
During warm weather, urban runoff can have significant thermal impacts on small streams, as stormwater passes over hot parking lots, roads and sidewalks. Stormwater management facilities that absorb runoff or direct it into groundwater, such as bioretention systems and infiltration basins, can reduce these thermal effects. Retention basins tend to be less effective at reducing temperature, as the water may be heated by the sun before being discharged to a receiving stream.
Measures to Control Thermal Pollution
The following measures are suggested to control thermal pollution:
First, for controlling thermal pollution, factories and power houses should make their own cooling towers and spray ponds. This way the water can be used again and again and the thermal pollution can be eliminated.
Heated water from these sources may be controlled with:
1- cooling ponds, man-made bodies of water designed for cooling by evaporation, convection, and radiation
2-cooling towers, which transfer waste heat to the atmosphere through evaporation and/or heat transfer
3- cogeneration, a process where waste heat is recycled for domestic and/or industrial heating purposes.
Second, alternative sources of energy should be used.
Third, minimize the use of electricity so that thermal and nuclear power plants can use less amount of water.
Fourth, the techniques used in the industrial sector should be those which reduce the generation of hot water. Last, there should be complete ban on the discharge of heated water directly into water sources.
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