Thermal pollution is the degradation of water quality by a sudden change of its temperature. It can be a change that makes it suddenly hotter, for example from a discharge of very hot water from a manufacturing facility into a river or makes it dramatically colder from for example, releasing much colder water from behind a dam into a warmer water below.
The primary source of thermal pollution is the discharge of heated water from power plants. Manufacturing facilities are another significant source. This is because heat engines work by heating and pressuring fluid for the energy needed to perform the mechanical work.
The unused heat, generally around 60% of the input energy which is unable to be used in the process, is then returned to a sink, such as water. Water is commonly used in power generating station. The waste heat from electrical generating stations is transferred to cooling water obtained from local water bodies such as a river, lake, or ocean.
The danger of thermal pollution
Many aquatic animals, such as fish and amphibians are unable to sustain dramatic temperature changes. Even only one or two degrees Celsius can adversely affect their metabolic activities, for example by rendering their cellular walls less permeable to necessary osmosis and otherwise altering enzyme metabolism.
This affects their abilities to digest, properly metabolize and reproduce. It can increase their metabolic rate so that they need to consume more food in a shorter time than if the environment was not changed. And chances are there will be less food available as the change in water temperature also affects their food source, whether it is other aquatic life or plants. The warm water can cause particular species of organisms to migrate to a more suitable environment. This can result in loss for those species that depend on them for their daily food.
Aquatic species that can adapt more easily to warmer temperatures will move in and the others will move out or die. The food chains of the old and new environments will further change and may be compromised with an imbalance. Biodiversity will decrease as fewer organisms are able to flourish in the warmer environment.
Higher water temperatures increase plant growth rates as well, resulting in a shorter lifespan and species overpopulation. This can cause algae bloom, which further reduces oxygen levels.
It has also been suggested that a systematic increase in water temperature contributes to harmful algal blooms. An algal bloom can produce toxins harmful to other organisms. They are often associated with large-scale marine mortality events and have been associated with various types of shellfish poisoning.
Prevention of thermal pollution
Measures can be taken to reduce the water temperature before discharge and prevent the toxic shock of a sudden increase in water temperature that kills aquatic life.
For example, the San Onofre Nuclear Generating Station between Los Angeles and San Diego, California, has two main reactors circulating a total of 2,400 million gallons per day 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. It 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 so that the average rise in temperature after mixing is less than 2° Celsius.
Heated water can also be controlled with cooling ponds, i.e. man-made bodies of water designed for cooling by evaporation, convection or radiation; cooling towers, which transfer waste heat to the atmosphere through evaporation or heat transfer, and, cogeneration, where waste heat is recycled for other heating purposes.
During warm weather, urban runoff can have significant thermal impacts on small streams, as storm water passes over hot parking lots, roads and sidewalks. Storm water management facilities that absorb runoff or direct it into the groundwater, such as bioretention systems and infiltration systems can reduce the thermal effects.
Soil erosion and deforestation also cause thermal pollution. Soil erosion causes water bodies to rise, exposing more surface area to sunlight and deforestation exposes more water to sunlight by removing adjacent shade. In each case, the water absorbs more heat, raising its temperature.