29 Water Pollution-III (Thermal Pollution)

Anjali Malan and Hardeep Rai Sharma

Introduction

Water is able to absorb large quantities of heat without changing from its liquid state. The high heat capacity means that it can be extensively used as a coolant in many industries. Heat can be considered a pollutant when its release into an environmental system adversely affects the optimal temperature ranges or indirectly alters other conditions that harm organisms, including humans.

Thermal pollution can be defined as “an accumulation of unusable heat from human activities that disrupts ecosystems in the natural environment” or “as the degradation of water quality by any process that changes ambient water temperature.” One major physical stressor on aquatic ecosystems is thermal pollution; it is not only problematic in itself, but can also exaggerate the impacts of chemical pollution. Much of the heat produced by industries is in the form of condenser cooling water. The principal user of water as a coolant include the electricity generating industry, thermal power plants, nuclear power plants, petroleum refineries, steel mills, chemical plants, paper and pulp mills etc. Future thermal stress on aquatic systems is likely to increase due to intensive use of water bodies as sources and sinks of anthropogenic heat as well as due to ongoing climate warming. The coolant water required by industry is drawn directly from water bodies, frequently rivers which after use, is often directly discharged back into the original water body, resulting in thermal pollution.

The increase in heat contributes to the physical, chemical and biological changes in the receiving water bodies. Soil erosion and shoreline deforestation also contribute to thermal water pollution but to a limited extent. The soil erosion makes the water muddy, which in turn increases the light absorbed and thus the water temperature is raised. Deforestation of shorelines further contributes to the problem in two ways. First, it increases soil erosion and secondly, it increases the amount of light that strikes the water, both of which increase the temperature of water.

Sources of thermal pollution: This pollution is caused due to release of waste heat into water bodies and sources of such kind are:

1.  Nuclear power plants (NPPs): Nuclear power plants including drainage from hospitals, research institutions, nuclear experiments and explosions, discharge a lot of heat along with traces of toxic radio nuclides into nearby water streams. Emissions from nuclear reactors and processing installations are also responsible for increasing the temperatures of water bodies. Heated effluents from power plants are discharged at temperature 10˚C higher than the receiving waters and affect the aquatic animals and plants. The cooling water discharge from NPPs is among the greatest local sources of thermal pollution due to the high levels of energy produced per plant. In addition, nuclear power plants require 30–100% more cooling water than other types of plant with a comparable power output [Electric Power Research Institute (EPRI, 2008)]. The warm water discharge from the NPP superimposes the regional climate warming, which provides an additional heat stress at the water surface. For example river Mississippi receives the highest total amount of heat emissions (62% and 28% of which come from coal-fuelled and nuclear power plants, respectively) and presents the highest number of instances where the commonly set 3°C temperature increase limit is exceeded (Raptis et al. 2016).

2.  Coal-fired power plants: Water from nearby lakes or rivers is used to cool the condenser coils in coal fired power plants. That heated water is discharged into lakes and streams thereby raising the water temperature by 15˚C. Heated effluent decreases the dissolved oxygen content of water resulting in death of aquatic organisms. The sudden fluctuations in temperature also lead to “thermal shock” that can kill aquatic life. Globally, over 46% of the thermal emissions into rivers are due to coal-fuelled power plants and almost one third due to nuclear power plants. The countries like United States, China and France are with the highest combined rates of riverine thermal emissions, occupying shares of 26%, 16%, and 12% of the global thermal emission rate, respectively (Raptis et al. 2016).

3.  Thermoelectric power plants: Thermoelectric power plants are one of the main causes of thermal pollution. Such plants pump water directly from rivers, lakes or the ocean, to cool the turbine condensers. During the process, which usually involves once-through cooling, the water becomes warmer than the source water, so that the wastewater is returned to its source at temperatures significantly higher than the freshwater that originally entered the electric generation station.

4.   Industrial effluents: Industries like textile, paper, sugar and pulp manufacturing discharge large amounts of cooling water along with effluents into nearby water bodies. In urbanized watersheds, heat contained in effluents from wastewater treatment plants can also lead to significant increase in stream temperature (Kinouchi et al, 2007). Similar is the case with effluents of other industries. The water gets polluted by release of sudden and massive organic loads resulting in drop in levels of dissolved O2 which affect aquatic life.

5.Sewage: Domestic sewage is discharged into rivers, lakes, or streams with minimal or without any treatment. These wastes having a higher temperature and organic load leads to decrease in dissolved O2 content in the receiving waters. Eventually, this leads to the development of anaerobic and anoxic conditions resulting in sudden death of aquatic organisms.

6.  Hydro-electric power: Generation of hydroelectric power sometimes leads to negative thermal loading in water systems. Dams may change a river habitat into a lake habitat by creating a reservoir (man-made lake) behind the dam. The reservoir water temperature is often colder than the original stream or river. On the contrary, downstream of dams and Hydropower Plants (HPPs) has generally warm water than upstream because of passing of water from pipelines, penstock, turbine and cooling system (Bobat, 2015). Despite the change in water temperature emerging from construction and operation of HPPs not as high as that in fossil-fuel and nuclear power plants, it is too important to affect lifecycle and survival of aquatic organisms.

7.   Soil erosion: Soil erosion is another major factor that causes thermal pollution. Consistent soil erosion causes water bodies to rise, making them more exposed to sunlight. The high temperature could prove fatal for aquatic biomes as it may give rise to anaerobic conditions.

8. Deforestation: Trees and plants prevent sunlight from falling directly on lakes, ponds or rivers. When deforestation takes place, these water bodies are directly exposed to sunlight, thus absorbing more heat and raising its temperature. Deforestation is also a main cause of the higher concentrations of green house gases i.e. global warming in the atmosphere.

9.  Runoff from paved surfaces: Urban runoff discharged to surface waters from paved surfaces like roads and parking lots can cause warming of water. The pavement gets quite hot during summers, which creates warm runoff that gets into the sewer systems and water bodies.

10. Natural causes: Natural causes like volcanoes and geothermal activity under the oceans and seas can trigger warm lava to raise the temperature of water bodies. Lightening can also introduce vast amount of heat into the oceans. This means that the overall temperature of the water source will rise, having significant impacts on the environment.

Effects of thermal pollution: There are many impacts of thermal pollution on physic-chemical properties of water as well as on the aquatic organisms. Some of these effects are as mentioned in Figure 1:

Temperature influences the viscosity, density, vapour pressure, surface tension, gas solubility and gas diffusion rates. Heated water has low density and spreads on the surface of water bodies causing them to stratify thermally. The stratification is barrier to the oxygen penetration into the deeper layers. At elevated temperature, the sedimentation of suspended materials increases due to reduction in density and viscosity of water. Evaporation rate of water also gets increased at high temperature. Rate of chemical reactions normally increases with rise in temperature which is about two-fold with every rise of 10ºC. BOD is also increased with temperature. The rates of photosynthesis and plant growth also increase with rise in temperature. An increase in plant growth may seem to be a good thing at first glance but more live plant means more dead plants. The pile up of dead plants leads to an increase of bacterial population which consumes oxygen along with dead plants leading to more demand of oxygen while availability of oxygen less.

1.  Reduction in dissolved oxygen (DO): Concentration of DO decreases as well as solubility of gases decreases with increase in temperature. The decrease in DO can create suffocation for plants and animals such as fish, amphibians and copepods, which may give rise to anaerobic conditions. Also, warmer water favors algae to flourish on surface of water. Increased temperature may not be tolerable for aquatic organisms as it increases microbial growth, which in turn decreases DO, makes metals more bioavailable, or in other ways increases the harm from nutrients and toxins. Even if the adult fish can survive at the reduced DO levels, their reproductive capacities decrease. In addition, the reproduction is not adversely affected, but the survival of juvenile fish can be reduced (Vallero, 2011).

2.  Increase in toxicity: The increase in the metal compounds combined with the reduced DO and the increased temperatures can act synergistically to make the conditions toxic for higher animals, for example, a fish kill (Vallero, 2007). With the constant flow of high temperature discharge from industries, there is a huge increase in toxins that are being discharged into the natural water bodies. These toxins may contain chemicals or radiation that may have harsh impact on the local ecology and make them susceptible to various diseases. For example: A 10˚C increase in temperature of water doubles the toxicity effect of potassium cyanide, while 80˚C rise in temperature triples the toxic effects of o-xylene causing massive mortality to fish. Moreover, concentrations of mercury and other toxic metals will become more with elevated temperatures. The lower DO concentrations will lead to a reduced environment where the metals and compounds could form sulfides and other compounds that can be toxic to the fish. The disease resistance in fish decreases and pollutants become more toxic at elevated temperature making the species become more vulnerable to parasites.

3.   Disturbance in biological activity: Temperature is considered to be of vital significance to physiology, metabolism and biochemical processes that control respiratory rates, digestion, excretion and overall development of aquatic organisms. Temperature changes cause total disruption to the entire ecosystem. Many aquatic species are sensitive to small temperature changes such as one degree celsius that can cause significant changes in organism metabolism and other adverse cellular biology effects. Changes in the environment may cause certain species of organisms to shift to other place because of warmer waters. For example in Southeastern Brazil, thermal discharges from nuclear power plants decrease the benthic cover and consequently, changes the associated rocky reef fish assemblage structure (Teixeira et al., 2012).

4.  Interference in reproduction: Thermal pollution can also interfere with the natural reproductive cycles of fish. In fishes, several activities like nest building, spawning, hatching, migration and reproduction depend on optimum temperature. Due to thermal pollution, the temperature gradient of water bodies gets disturbed and hence affects aquatic organisms. Excessive temperature can cause the release of immature eggs or can prevent normal development of certain eggs. For example, premature hatching of eggs by artificially raised temperatures may lead to mass mortality of the young fish through starvation. Mass killing of fish and other aquatic organisms can occur when there is a very rapid change in water temperature and this is known as ‘thermal shock’.

5.  Ecological impacts: The increased temperature can also increase the solubility of substances toxic to organisms. Thus, the change in temperature, the resulting decrease in DO and increase in metal concentrations, and the synergistic impact of combining the hypoxic water and reduced metal compounds is a cascade of harm to the stream’s ecosystems. A continuous exposure to heat leads to the development of a new ecosystem comprising of thermally adapted species. Sudden stoppage of the industrial plants will again disrupt the system. Thousands of warm water fishes and other animals were found dead when a thermal power plant in New Jersey, U.S.A. was stopped for repairing for one day in February 1972 (Laws, 2000). Natural migration of fish is also affected due to the formation of thermally polluted zones which act as barrier to the migration.

6. Increase in metabolic rate: Thermal pollution increases the metabolic rate of organisms as increase in enzyme activity occur and as a result, the organisms consume more food than normal requirement, if their environment is not changed. It disrupts the stability of food chain and alters the balance of species composition and may also lead to faunal migration, as species attempt to adapt to changed thermal conditions. As a result, original species may migrate away and alien species may enter a local aquatic system. In some cases significant loss of biodiversity can arise. Jesus et al (2017) observed that under current water pH and increasing warming (+3°C) caused differential interspecific changes in lactate dehydrogenase (LDH) activity in glycolytic pathways by increasing its activity in Squalius carolitertii fish and decreasing in S. torgalensis, respectively.

7.  Direct mortality: Thermal pollution is directly responsible for mortality of aquatic organisms. Increase in temperature of water leads to exhaustion of microorganisms thereby shortening the life span of fish. Above a certain temperature, fish die due to failure of respiratory system and nervous system. The most readily observable phenomenon is that of mass fish kills in a surface water body; in this case, there are often large numbers of dead fish seen floating in the water or washed up on the water banks. Juveniles or fish fry are particularly vulnerable to small changes in water temperature.

8.  Loss in biodiversity: The species composition changes as species tolerant to warmer water gets replaced with those that are unable to adapt. This transition is often accompanied by an overall decrease in species richness. For example, attached algae in heated effluents were reported to show an increase in biomass but decrease in the number of species.

9.  Food shortage for fish: Abrupt changes in temperature alter the seasonal variation in the type and abundance of lower organisms leading to shortage of right food for fish at the right time. The warmer water also increases the metabolic rate of fish, which leads to, a sharp decrease in the life expectancy of aquatic insects. The enhanced metabolism requires more oxygen. However, the amount of dissolved oxygen present in water is inversely related to its temperature. On the other hand with the lack of aquatic insects, fish faces shortage of food.

10.  Lake stratification: Thermal pollution in the winter months has more drastic consequences for the lake thermal regime than the same pollution in the summer months. The heat released into the lake in the winter at environmental temperatures below 4˚C mixes down through the water column and remains stored in the lake hypolimnion throughout the subsequent summer, altering the seasonal mixing regime. In turn, the heat added to the lake in the summer is effectively released into the atmosphere by evaporation and infrared radiation at the lake surface. These seasonal aspects in the design of cooling water discharge practices could help in minimizing the ecological threats of thermal pollution (Kirrilin et al. 2013). The main effects of thermal pollution on the summer temperature regime – warming of the hypolimnion, weakening of the summer stratification and decrease of the surface mixed layer (epilimnion) thickness – suggest appreciable consequences for the lake biological productivity and water quality.

11.        Impact on coastal areas: The horizontal dispersion of thermal plumes is mainly driven by wind-induced currents. Near-shore regions close to the emitting source are therefore especially sensitive to thermal pollution since heat plumes can be trapped by coastal currents without being dispersed across the water body (Salgueiro et al., 2015).

Standards for thermal pollution

The Central Pollution Control Board (CPCB, 2015) in India has specified the standards for thermal discharges from thermal power plants. The condenser cooling water should not have temperature more than 10ºC higher than the intake water temperature. Thermal water pollution can be avoided by pre-cooling the warm water prior to its discharge. The major principles involved in heat loss are conduction, convection, radiation and evaporation. For example, cooling ponds and cooling towers are often used for cooling water in the electricity generating industry. In cooling ponds, the water from condensers is stored in earthen ponds where natural evaporation brings down the temperatures. The water after cooling is recirculated or discharged to the nearby water body. Alternatively, the warm waste water can be effectively used by other industries. The potential uses of waste heat may be in green houses, agriculture, aquaculture and space heating beside others.

The following methods can be adopted to control high temperature caused by thermal discharges:

1.   Cooling towers: Use of water from water systems for cooling purposes, with subsequent return to the water way after passage through a condenser, is called cooling process. Cooling towers transfer heat from hot water to the atmosphere by evaporation. In a cooling tower setup, most of the absorbed heat is removed via evaporation and dissipated into the atmosphere. Cooling towers are of two types:

i)  Wet cooling tower: Hot water coming out from the condenser (reactor) is allowed to spray over baffles. Cool air, with high velocity, is passed from sides, which takes away the heat and cools the water.

ii)    Dry cooling tower: Here, hot water is allowed to flow in long spiral pipes. Cool air with the help of a fan is passed over these hot pipes, which cools down hot water. This cool water can be recycled.

2.   Cooling ponds: Cooling ponds are the best way to cool thermal discharges. Heated effluents on the surface of the water in cooling ponds maximize dissipation of heat to the atmosphere and minimize the water area and volume.

3.    Spray ponds: The water coming out from condensers is allowed to pass into the ponds through sprayers. Here water is sprayed through nozzles as fine droplets. Heat from the fine droplets gets dissipated to the atmosphere.

4.   Artificial lakes: Artificial lakes are manmade water bodies that offer once-through cooling. These lakes are very helpful for normalizing the temperature of hot water. This way, the hot water will not be disposed back to the lakes, rivers, etc., and will be used in other suitable tasks. Actually, the artificial lakes or ponds use evaporation or convection technique for cooling down the water. The heated effluents can be discharged into the lake at one end and water for cooling purposes may be withdrawn from the other end. The heat is eventually dissipated through evaporation.

5.    Recycling used water: If the recycling of the used water is started in plants and factories, the problem of thermal pollution will definitely be lessened to a significant extent. Every industry or plant should commit themselves that water used as coolant will not be spilled back into water bodies. Rather, it will be recycled for further tasks.

6.   Afforestation along the banks of rivers, seas and other water bodies: The trees around sources of water help in absorbing the harsh sun rays and prevent them from falling directly upon the water and inhibit heating of water bodies. The problem of soil erosion can also be controlled by planting more because the strong roots of trees hold the soil firmly and prevents erosion.

7.    Public awareness: Making more and more people aware about the problem of thermal pollution, will be very beneficial in the long run. Group discussions of people with different plants and industries can be done to talk about effects of thermal pollution on aquatic life and our environment.

8.    Suitable arrangements in urban places: Places like parking spaces, drainage pipes, sewerage tanks, etc., should have proper arrangements so that the water does not get collected at those spaces. When the water gets accumulated, it gets heated up and gets mixed with seas, ponds, lakes, etc., thus making way for thermal pollution. Hence, by making appropriate arrangements, we can stop water from getting accumulated.

9.   Co-generation: Co-generation is also a wonderful idea to combat thermal pollution. In the process of co-generation, the useless heat from hot water can be recycled and used smartly in many tasks by industries.

10.   Use of alternative cooling agents other than water: The primary reason behind use of water as a coolant is its easy availability and disposal, once the cooling purpose is fulfilled. However, the heated water disturbs the ecosystem of water bodies. Hence, emphasis should be laid on using coolants other than water. Air-cooled systems are the best alternative to water-based cooling systems. For smaller units, oil-based cooling systems would fulfill the task of cooling. Well, the oil is reusable and can be used for multiple cooling cycles.

11.      Water temperature threshold values: In order to protect aquatic ecosystems, there are regulations in place in both the United States and Europe, which impose thresholds on surface water temperatures (European Parliament and Council of the European Union, 2006). Several U.S. states impose an upper temperature threshold of 32°C, which is often exceeded (Madden et al., 2013). Many water management authorities have enforced three kinds of limitations to heat use: (i) a maximum temperature of water used for cooling, (ii) a maximum temperature increase in the natural waters receiving the thermal effluents, and (iii) a maximum temperature in the receiving waters (Vinna et al. 2017). Among some options, recirculating (tower) cooling overall increases water consumption, dry cooling results in reduced power output (or increased fuel consumption), and once-through cooling with seawater can be problematic when heat dilution at the coast is limited or where reefs are concerned (Raptis, 2016).

Summary

In this module the students learnt about:

Basic concept of thermal pollution

Causes of thermal pollution

Effects of thermal pollution

Control of thermal pollution

References

• Bobat, A. 2015. Thermal pollution caused by hydropower plants. Energy Systems and Management, Springer International Publishing Switzerland, pp. 19‒32.
• Central Pollution Control Board (CPCB). 2015. Pollution Control Implementation Division–II, Thermal Power Plants.Available at:http://www.cpcb.nic.in/divisionsofheadoffice/pci2/ThermalpowerPlants.pdf Electric Power Research Institute (EPRI). 2008. Water Use for Electric Power Generation, EPRI, Palo Alto, CA, 1014026.
• European Parliament and Council of the European Union. 2006. Directive 2006/44/EC of the European Parliament.
• Golosov, S., Maher, O.A., Schipunova, E., Terzhevik, A., Zdorovennova, G. and Kirillin, G., 2006. Physical background of the development of oxygen depletion in icecovered lakes. Oecologia, 151, 331–340.
• Jesus, T.F., Rosa, I.C., Repolho, T., Lopes, A. R., Pimentel, M. S., Almeida-Val, V.M.F., Coelho, M.M., Rosa, R. 2017. Different ecophysiological responses of freshwater fish to warming and acidification. Comparative Biochemistry and Physiology, Part A 216, 34–41.
• Kinouchi, T., Yagi, H. and Miyamoto, M. 2007. Increase in stream temperature related to anthropogenic heat input from urban wastewater, Journal of Hydrology, 335, 78–88.
• Kirillin, G., Shatwell, T. and Kasprzak, P. 2013. Consequences of thermal pollution from a nuclear plant on lake temperature and mixing regime. Journal of Hydrology, 496, 47–56.
• Laws. E. A. 2000. Aquatic Pollution: An introductory text. Third edition. John Wiley & Sons Inc. Canada.
• Madden, N., Lewis, A. and Davis, M. 2013. Thermal effluent from the power sector: an analysis of once-through cooling system impacts on surface water temperature. Environmental Research Letters, 8, 035006.
• Raptis, C. E., Vliet, M. T. H. and Pfister, S. 2016. Global thermal pollution of rivers from thermoelectric power plants. Environmental Research Letters, 11, 104011.
• Smythea, A. G., Sawyko, P. M. 2000. Field and laboratory evaluations of the e€ects of “cold shock” on fish resident in and around a thermal discharge: an overview. Environmental Science & Policy 3, S225–S232.
• Teixeira T. P., Neves, L. M. and Araujo, F. G. 2012. Thermal impact of a nuclear power plant in a coastal area in Southeastern Brazil: effects of heating and physical structure on benthic cover and fish communities. Hydrobiologia, 684:161–175.
• Vallero, D. A. 2011. Thermal pollution. Pratt School of Engineering, Duke University, Durham, NC, USA. Elsevier Academic Press. Pp. 425-442.
• Vallero, D.A., Reckhow, K.H. and Gronewold, A.D. 2007. Application of multimedia models for human and ecological exposure analysis. International Conference on Environmental Epidemiology and Exposure. Durham, NC, USA.
• Vinna, L. R., Wuest, A. and Bouffard, D. 2017. Physical effects of thermal pollution in lakes. Water Resources Research, 53 (5): 3968–3987.