Upload
sneha-parab
View
588
Download
48
Tags:
Embed Size (px)
Citation preview
Thermal pollution and its remedies
BY
MS. SNEHA PARAB
Thermal pollution
Introduction
Thermal pollution is basically the form of water pollution that refers to
degradation of water quality by any process that changes ambient water temperature. It is
the act of altering the temperature of a natural water body, which may be a river, lake or
ocean environment. While a degree or two of difference may sound minor, warming of an
aquatic or marine environment even by a small amount can result in devastating
alterations to the habitats of fish, insects, plants, and animals. Increases in ambient water
temperature also occur in streams where shading vegetation along the banks is removed
or where sediments have made the water more turbid . Both of these effects allow more
energy from the sun to be absorbed by the water and thereby increase its 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. Thermal pollution is one parameter of the broader subject of water
pollution. There can be significant environmental consequences of thermal pollution with
respect to surface receiving waters such as rivers and lakes; in particular, decrease in
biodiversity and creation of an environment hospitable to alien aquatic species may
occur.
Illustration of thermal pollution
Sources
There are several main causes of thermal pollution, each contributing to what some
environmental experts call a possible environmental catastrophe. Two major sources that
lead to thermal pollution can be given as
Electric power plants and use of water as a cooling agent
deforestation of the shoreline
1. Electric power plants and use of water as a cooling agent
The major sources of thermal pollution are electric power plants and industrial factories.
In most electric power plants, heat is produced when coal, oil, or natural gas is burned or
nuclear fuels undergo fission to release huge amounts of energy. This heat turns water to
steam, which in turn spins turbines to produce electricity. After doing its work, the spent
steam must be cooled and condensed back into water. To condense the steam, cool water
is brought into the plant and circulated next to the hot steam. In this process, the water
used for cooling warms 5 to 10 Celsius degrees (9 to 18 Fahrenheit degrees), after which
it may be dumped back into the lake, river, or ocean from which it came. Similarly,
factories contribute to thermal pollution when they dump water used to cool their
machinery.
A major example of that proves this point is the Coal-burning power plants that 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
Deforestation Of The Shoreline
The second major cause of thermal pollution which is much more widespread is deforestation of the shoreline. Streams and small lakes are naturally kept cool by trees and other tall plants that block sunlight. People often remove this shading vegetation in order to harvest the wood in the trees, to make room for crops, or to construct buildings, roads, and other structures. Left unshaded, the water warms by as much as 10 Celsius degrees (18 Fahrenheit degrees). In a similar manner, grazing sheep and cattle can strip stream sides of low vegetation. Even the removal of vegetation far away from a stream or lake can contribute to thermal pollution by speeding up the erosion of soil into the water, making it muddy. Muddy water absorbs more energy from the sun than clear water does, resulting in further heating. Finally, water running off of artificial surfaces, such as streets, parking lots, and roofs, is warmer than water running off vegetated land and, thus, contributes to thermal pollution.
EFFECTS OF THERMAL POLLUTION
1. Ecological Effects — Warm Water
Elevated temperature typically decreases the level of dissolved oxygen (DO) in
water. The decrease in levels of DO 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
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. It is known that temperature changes of even one to two degrees
Celsius can cause significant changes in organism metabolism and other adverse cellular
biology effects. Principal adverse changes can include rendering cell walls less permeable
to necessary osmosis, coagulation of cell proteins, and alteration of enzyme metabolism.
These cellular level effects can adversely affect mortality and reproduction.
Primary producers are affected by warm water because higher water temperature
increases plant growth rates, resulting in a shorter lifespan and species overpopulation.
This can cause an algae bloom which reduces oxygen levels.
A large increase in temperature can lead to the denaturing of life-supporting
enzymes by breaking down hydrogen-and disulphide bonds within the quaternary
structure of the enzymes. Decreased enzyme activity in aquatic organisms can cause
problems such as the inability to break down lipids, which leads to malnutrition.
In limited cases, warm water has little deleterious effect and may even lead to
improved function of the receiving aquatic ecosystem. This phenomenon is seen
especially in seasonal waters and is known as thermal enrichment. An extreme case is
derived from the aggregational habits of the manatee (Sea Cow), which often uses power
plant discharge sites during winter. Projections suggest that manatee populations would
decline upon the removal of these discharges.
2. Ecological effects — cold water
Releases of unnaturally cold water from reservoirs can dramatically change the
fish and macro invertebrate fauna of rivers, and reduce river productivity. In Australia,
where many rivers have warmer temperature regimes, native fish species have been
eliminated, and macro invertebrate fauna have been drastically altered.
3. Impact Of Thermal Pollution On Aquatic Ecology
All plant and animal species that live in water are adapted to temperatures within
a certain range. When water in an area warms more than they can tolerate, species that
cannot move, such as rooted plants and shellfish, will die. Species that can move, such as
fish, will leave the area in search of cooler conditions, and they will die if they can not find
them. Typically, other species, often less desirable, will move into the area to fill the
vacancy.
In general, cold waters are better habitat for plants and animals than warm ones
because cold waters contain more dissolved oxygen. Many freshwater fish species that
are valued for sport and food, especially trout and salmon, do poorly in warm water.
Some organisms do thrive in warm water, often with undesirable effects.
As water temperature rises, the rate of photosynthesis and plant growth also increases.
More plants grow and die. As plants die, they are decomposed by bacteria that consume
oxygen. Therefore, when the rate of photosynthesis is increased, the need for oxygen in
the water (BOD) is also increased.
The metabolic rate of organisms also rises with increasing water temperatures, resulting
in even greater oxygen demand. The life cycles of aquatic insects tend to speed up in
warm water. Animals that feed on these insects can be negatively affected, particularly
birds that depend on insects emerging at key periods during their migratory flights.
Most aquatic organisms have adapted to survive within a range of water temperatures,
Some organisms prefer cooler water, such as trout, stonefly nymphs, while others thrive
under warmer conditions, such as carp and dragonfly nymphs. As the temperature of a
river increases, cool water species will be replaced by warm water organisms. Few
organisms can tolerate extremes of heat or cold.
Temperature also affects aquatic life's sensitivity to toxic wastes, parasites, and disease.
For example, thermal pollution may cause fish to become more vulnerable to disease,
either due to the stress of rising water temperatures or the resulting decrease in dissolved
oxygen.
The hot waste water released from power plants and industries disrupts the biodiversity
of aquatic ecosystems by disturbing metabolic functions of living organisms. In addition,
sever thermal shocks distort the food web by killing fish and other animals. All plant and
animal species that live in water are adapted to temperatures within a certain range. When
water in an area warms more than they can tolerate, species that cannot move, such as
rooted plants and shellfish, will die. Species that can move, such as fish, will leave the
area in search of cooler conditions, and they will die if they can not find them. Typically,
other species, often less desirable, will move into the area to fill the vacancy. In general,
cold waters are better habitat for plants and animals than warm ones because cold waters
contain more dissolved oxygen. Many freshwater fish species that are valued for sport
and food, especially trout and salmon, do poorly in warm water. Some organisms do
thrive in warm water, often with undesirable effects. Algae and other plants grow more
rapidly in warm water than in cold, but they also die more rapidly; the bacteria that
decompose their dead tissue use up oxygen, further reducing the amount available for
animals. The dead and decaying algae make the water look, taste, and smell unpleasant.
(Eutrophication ).
4.Physical and chemical effects
Water is naturally constituted of chemical components (metals, salts, trace elements,
gases, carbonates,) in balance. A change in temperature can modify it and accelerate
chemical and biochemical reactions: it is a catalyst. For example, the hotter the
temperature of rejected water is, the more its effects on benthic organizations are
harmful.
An increase in temperature can also cause a reduction of solubility. It's an important
consequence because the amount of dissolved oxygen (DO) in water decreases. This
makes it more difficult for fish and other aquatic organisms to obtain the necessary
oxygen for cellular respiration. But, in the first part, we could see that the seaweeds rate
increase, so, they use up more O2: there is a lack of oxygen so aquatic living beings are
asphyxiated. Another repercussion of this phenomenon is taste and odor. In fact, products
which are in water become more volatile, it is why there are impacts on the surrounding
air.
Salinity is an important measurement in biology because salt is dissolved in the bodily
fluids of all living things. The level of dissolved salts in a fluid controls many of the
processes of life. Most animals are adapted to a narrow range of salinity and cannot live
in water that is outside that range. But, when the temperature of water increases, salinity
decreases so, there are important repercussions on aquatic life.
A particular problem is the stratification in lakes. Because of density difference,
layers of different temperature are formed in accordance with the depth of water. So the
temperature of water changes suddenly with the depth. These have consequences on
fauna and flora, which modifies the landscape.
Increase in temperature = Reduction of solubility
Increase in temperature = Change of salinity
Stratification in lakes
5. Biological effects of this pollution
Temperature is an important factor for life. So, an increase, or a reduction, of this one can
destroy ecologic niches, which are very sensitive to little variation of their environment.
Their disappearance is followed by that of the fauna and flora living there.
Producing or catching heat in or from a source of water can involve the death of micro-
organisms by thermal shock. This problem also affects other living beings that feed on it,
and so on and so forth. So, the tropic chain is broken.
We have presented you the problem of the destruction of nature, but this pollution can
also generate the proliferation of bacteria that occupy the niches of other types of living
beings. So, almost the same result is achieved through another way ... Secondly, with a
higher temperature, algae grow much more rapidly. They use up dissolved oxygen (DO)
in water, block the sunlight from getting to the submerged aquatic vegetation (SAV), and
the entire ecosystem suffers.
Change in temperature = Stratification
Increase \ reduction in temperature = Destruction of fauna and flora
Proliferation of algae
Measure Of Thermal Pollution
Guidelines for testing for thermal pollution vary from place to place. One way to test
thermal pollution in water is to measure the difference in the water temperature between
the entry site and a point 1 mi upstream. A change of 0°C to 2°C is evaluated as
excellent. A change of 2.2°C to 5°C is evaluated as good. A change of 5.1°C to 9.9°C is
fair, and more than 10°C difference is considered poor.
Prevention & Control Of Thermal Pollution
There are a number of ways to minimize the harmful effects of Thermal Pollutions:
Prevention
Following are the means to reduce thermal pollution:
Theoretically, when efficiency of any heat engine is equal to 1.0 then it will
convert 100% of heat energy to mechanical energy. So there will be no loss of
heat to the environment. This is practically impossible. Rather, we should aim at
Increase in temperature = Proliferation of bacteria \ reduction of O2
maximizing the efficiency of heat engines (steam, IC, nuclear etc) so that heat
loss is minimum.
Reduce mechanical friction in any rotating parts.
Avoid consuming energy more than necessity. Burn less coal, oil or gas.
Temperature signal conditioners accept outputs from temperature measurement
devices such as resistance temperature detectors (RTDs), thermocouples, and
thermostats. They then filter, amplify, and/or convert these outputs to digital
signals, or to levels suitable for digitization.
Industrial fans and industrial blowers and commercial fans and blowers are
designed to move air and/or powders in industrial and commercial settings.
Typical applications include air circulation for personnel, exhaust or material
handling
Limiting the amount of heated water discharged into the same body of water.
Control By Dilution
Returning the heated water at a point away from the ecologically vulnerable shore
zone.
Transferring the heat from the water to the atmosphere by means of wet or dry
cooling towers.
Discharging the heated water into shallow ponds or canals, allowing it to cool,
and reusing it as cooling water. This method is useful where enough affordable
land is available.
Control of thermal pollutionIndustrial wastewater
Thermal pollution from industrial sources is generated mostly by power plants,
petroleum refineries, pulp and paper mills, chemical plants, steel mills and smelters.
Heated water from these sources may be controlled with:
cooling ponds, man-made bodies of water designed for cooling by evaporation,
convection, and radiation
cooling towers, which transfer waste heat to the atmosphere through evaporation
and/or heat transfer
cogeneration, a process where waste heat is recycled for domestic and/or
industrial heating purposes.
Cooling pond:
A cooling pond is a man-made body of water primarily formed for the purpose of
supplying cooling water to a nearby power plant or industrial facility such as a petroleum
refinery, pulp and paper mill, chemical plant, steel mill or smelter. Cooling ponds are
used where sufficient land is available, as an alternative to cooling towers or discharging
of heated water to a nearby river or coastal bay, a process known as "once-through
cooling." The latter process can cause thermal pollution of the receiving waters. Cooling
ponds are also sometimes used with in large buildings as an alternative to cooling towers.
Air conditioning systems The pond receives thermal energy in the water from the plant's
condensers and the energy is dissipated mainly through evaporation. Once the water has
cooled in the pond, it is reused by the plant. New water is added to the system ("make-
up" water) to replace the water lost through evaporation. Many such ponds have
secondary outdoor recreational purposes that include fishing, swimming, boating,
camping and picnicking. The warm waters are frequently used as a fish hatchery.
Cooling Tower
Cooling towers are heat removal devices used to transfer process waste heat to the
atmosphere. Cooling towers may either use the evaporation of water to remove process
heat and cool the working fluid to near the wet-bulb air temperature or in the case of
"Close Circuit Dry Cooling Towers" rely solely on air to cool the working fluid to near
the dry-bulb air temperature. Common applications include cooling the circulating water
used in oil refineries, chemical plants, power stations and building cooling. The towers
vary in size from small roof-top units to very large hyperboloid structures that can be up
to 200 metres tall and 100 metres in diameter, or rectangular structures that can be over
40 metres tall and 80 metres long. Smaller towers are normally factory-built, while larger
ones are constructed on site. They are often associated with nuclear power plants in
popular culture. A hyperboloid cooling tower was patented by Frederik van Iterson and
Gerard Kuypers in 1918.
Cogeneration (Combined heat and power, CHP)
Cogeneration (Combined heat and power, CHP) is the use of a heat engine or a
power station to simultaneously generate both electricity and useful heat. All power
plants must emit a certain amount of heat during electricity generation. This can be
released into the natural environment through cooling towers, flue gas, or by other means.
By contrast CHP captures some or all of the by-product heat for heating purposes, either
very close to the plant, or as hot water for district heating with temperatures ranging from
approximately 80 to 130 °C. This is also called Combined Heat and Power District
Heating or CHPDH. Small CHP plants are an example of decentralized energy.
In the United States, Con Edison distributes 30 billion pounds of 350 °F/180 °C steam
each year through its seven cogeneration plants to 100,000 buildings in Manhattan—the
biggest steam district in the United States. The peak delivery is 10 million pounds per
hour (corresponding to approx. 2.5 GW). This steam distribution system is the reason for
the steaming manholes often seen in "gritty" New York movies. Cogeneration is a
thermodynamically efficient use of fuel. In separate production of electricity some energy
must be rejected as waste heat, but in cogeneration this thermal energy is put to good use.
Bioretention systems and infiltration basins
Storm water 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
Bioretention System
Bioretention is the process in which contaminants and sedimentation are removed
from stormwater runoff. Stormwater is collected into the treatment area which consists of
a grass buffer strip, sand bed, ponding area, organic layer or mulch layer, planting soil,
and plants. Runoff passes first over or through a sand bed, which slows the runoff's
velocity, distributes it evenly along the length of the ponding area, which consists of a
surface organic layer and/or groundcover and the underlying planting soil. The ponding
area is graded, its center depressed. Water is ponded to a depth of 15 cm (5.9 in) and
gradually infiltrates the bioretention area or is evapotranspired. The bioretention area is
graded to divert excess runoff away from itself. Stored water in the bioretention area
planting soil exfiltrates over a period of days into the underlying soils.
Infiltration Basin
An infiltration basin (also known as a recharge basin or in some areas, a sump), is
a type of best management practice (BMP) that is used to manage stormwater runoff,
prevent flooding and downstream erosion, and improve water quality in an adjacent river,
stream, lake or bay. It is essentially a shallow artificial pond that is designed to infiltrate
stormwater though permeable soils into the groundwater aquifer. Infiltration basins do
not discharge to a surface water body under most storm conditions, but are designed with
overflow structures (pipes, weirs, etc.) that operate during flood conditions. It is
distinguished from a detention basin, sometimes called a dry pond, which is designed to
discharge to a downstream water body (although it may incidentally infiltrate some of its
volume to groundwater); and from a retention basin, which is designed to include a
permanent pool of water.
Retention Basin
A retention basin is a type of best management practice (BMP) that is used to
manage stormwater runoff to prevent flooding and downstream erosion, and improve
water quality in an adjacent river, stream, lake or bay. Sometimes called a wet pond or
wet detention basin, it is an artificial lake with vegetation around the perimeter, and
includes a permanent pool of water in its design. It is distinguished from a detention
basin, sometimes called a dry pond, which temporarily stores water after a storm, but
eventually empties out at a controlled rate to a downstream water body. It also differs
from an infiltration basin which is designed to direct stormwater to groundwater through
permeable soils. Wet ponds are frequently used for water quality improvement,
groundwater recharge, flood protection, aesthetic improvement or any combination of
these. Sometimes they act as a replacement for the natural absorption of a forest or other
natural process that was lost when an area is developed. As such, these structures are
designed to blend into neighborhoods and viewed as an amenity.
Conclusion
Thermal pollution is a problem that we have to deal with more and more. This
pollution is principally located in rivers and waterways nearby industries. Even if we
can't see immediately the effects of thermal pollution it has an impact on our
environment. So, scientists have to research solutions to reduce it (heat!!).
When prevention is inefficient, industries use some techniques to solve thermal
pollution. The first technique is the control by dilution; it consists in diluting warm water
in cool water to decrease the impact on water life. Another solution is to use cooling
towers to transfer heat from water to the atmosphere. The last one is the discharging of
heated water in shallow ponds and canals. The last two are very much used even if the
last one is the least costly.
References
Brown, Richard D.; Ouellette, Robert P.; and Chermisinoff, Paul N. (1983). Pollution
Control at Electric Power Stations: Comparisons for U.S. and Europe. Boston:
Butterworth-Heinemann.
Langford, Terry E. (1990). Ecological Effects of Thermal Discharges. New York:
Elsevier Applied Science.
Slovic, Paul. (2000). The Perception of Risk. London: Earthscan Publications Ltd.