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Marine Pollution : |
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Marine
Pollution
Marine pollution can be defined as the introduction of substances
to the marine environment directly or indirectly by man resulting
in adverse effects such as hazards to human health, obstruction
of marine activities, and lowering the quality of sea water.
While the causes of marine pollution may be similar to that
of general water pollution, there are some very specific causes
that pollute marine waters.
- The most obvious inputs of waste is through pipes directly
discharging wastes into the sea. Very often, municipal waste
and sewage from residences and hotels in coastal towns are
directly discharged into the sea.
- Pesticides and fertilizers from agriculture, which are
washed off the land by rain, enter watercourses and eventually
reach the sea.
- Petroleum and oils washed off from the roads normally
enter the sewage system but storm water overflows carry
these materials into rivers and eventually into the seas.
- Ships carry many toxic substances such as oil, liquefied
natural gas, pesticides, industrial chemicals, etc., in
huge quantities sometimes to the capacity of 350,000 t.
Ship accidents and accidental spillages at sea can, therefore,
be very damaging to the marine environment. Shipping channels
in estuaries and at the entrances to ports often require
frequent dredging to keep them open. This dredged material
that may contain heavy metals and other contaminants is
often dumped out at sea.
- Offshore oil exploration and extraction also pollute
the seawater to a large extent.
Pollution due to organic wastes :
The amount of oxygen dissolved in the water is vital for the
plants and animals living in it. Wastes, which directly or indirectly
affect the oxygen concentration, play an important role in determining
the quality of the water. Normally, the greatest volume of waste
discharged to water courses, estuaries and the sea is sewage,
which is primarily organic in nature and is degraded by bacterial
activity. Using the oxygen present in the water, these wastes
are broken down into stable inorganic compounds. However, as
a result of this bacterial activity the oxygen concentration
in the water is reduced. When the oxygen concentration falls
below 1.5 mg/l, the rate of aerobic oxidation is reduced and
replaced by the anaerobic bacteria that can oxidize the organic
molecules without the use of oxygen. This results in end products
such as hydrogen sulfide, ammonia and methane, which are toxic
to many organisms. This process results in the formation of
an anoxic zone which is low in its oxygen content; from which
most life disappears except for anaerobic bacteria, fungi, yeasts
and some protozoa; and renders the water foul smelling.
Control measures: One
way of reducing the pollution load on marine waters is through
the introduction of sewage treatment plants. This will reduce
the biological oxygen demand (BOD) of the final product before
it is discharged to the receiving waters. Various stages of
treatment such as primary, secondary or advanced can be used,
depending on the quality of the effluent that is required to
be treated.
Primary treatment:
These treatment plants use physical processes such as screening
and sedimentation to remove pollutants that will settle, float
or are too large to pass through simple screening devices. This
includes stones, sticks, rags or any such material that can
clog up pipes. A screen consists of parallel bars spaced 2-7
cm apart followed by a wire mesh with smaller openings. One
way of avoiding the problem of disposal of materials collected
on the screens is to use a device called a comminuter, which
grinds the coarse material into small pieces that can then be
left in the wastewater. After screening the wastewater passes
into a grit chamber. The detention time is chosen to be long
enough to allow lighter, organic material to settle. From the
grit chamber the sewage passes into a primary settling tank
(also called as sedimentation tank), where the flow speed is
reduced sufficiently to allow most of the suspended solids to
settle out by gravity. If the waste is to undergo only primary
treatment it is then chlorinated to destroy bacteria and control
odors after which the effluent is released. Primary treatment
normally removes about 35% of the BOD and 60% of the suspended
solids.
Secondary treatment: The main objective of secondary
treatment is to remove most of the BOD. There are three commonly
used approaches: trickling filters, activated sludge process,
and oxidation ponds. Secondary treatment can remove at least
85% of the BOD.
A trickling filter consists of a rotating distribution arm that
sprays liquid wastewater over a circular bed of 'fist size'
rocks or other coarse materials. The spaces between the rocks
allow air to circulate easily so that aerobic conditions can
be maintained. The individual rocks in the bed are covered with
a layer of slime, which consists of bacteria, fungi, algae,
etc., which degrade the waste trickling through the bed. This
slime periodically slides off individual rocks and is collected
at the bottom of the filter along with the treated wastewater
and is then passed on to the secondary settling tank where it
is removed.
In the activated sludge process, the sewage is pumped into a
large tank and mixed for several hours with bacteria-rich sludge
and air bubbles to facilitate degradation by microorganisms.
The water then goes into a sedimentation tank where most of
the microorganisms settle out as sludge. This sludge is then
broken down in an anaerobic digester where methane-forming bacteria
slowly convert the organic matter into carbon dioxide, methane,
and other stable end products. The gas produced in the digester
is 60% methane, which is a valuable fuel and can be put to many
uses within the treatment plant itself. The digested sludge,
which is still liquid, is normally pumped out onto sludgedrying
beds where evaporation and seepage remove the water. This dried
sludge is potentially a good source of manure. Activated sludge
tanks use less land area than trickling filters with equivalent
performance. They are also less expensive to construct than
trickling filters and have fewer problems with flies and odor
and can also achieve higher rates of BOD removal. Thus, although
the operating costs are a little higher due to the expenses
incurred on energy for running pumps and blowers, they are preferred
over trickling filters.
Oxidation ponds are large shallow ponds approximately 1-2 m
deep, where raw or partially- treated sewage is decomposed by
microorganisms. They are easy to build and manage, accommodate
large fluctuations in flow, and can provide treatment at a much
lower cost. However, they require a large amount of land and
hence can only be used where land is not a limitation.
Advanced sewage treatment:
This involves a series of chemical and physical processes that
removes specific pollutants left in the water after primary
and secondary treatment. Sewage treatment plant effluents contain
nitrates and phosphates in large amounts. These contribute to
eutrophication. Thus, advanced treatment plants are designed
to specifically remove these contaminants. These plants are
very expensive to build and operate and so are rarely used.
Pollution due to oil:
Oil pollution of the sea normally attracts the greatest attention
because of its visibility. There are several sources though
which the oil can reach the sea.
Tanker operations : Half the world production of crude
oil, which is close to three billion tones a year, is transported
by sea. After a tanker has unloaded its cargo of oil, it has
to take on seawater as ballast for the return journey. This
ballast water is stored in the cargo compartments that previously
contained the oil. During the unloading of the cargo a certain
amount of oil remains clinging to the walls of the container
and this may amount to 800 t in a 200,000 t tanker. The ballast
water thus becomes contaminated with this oil. When a fresh
cargo of oil is to be loaded, these compartments are cleaned
with water, which discharges the dirty ballast along with the
oil into the sea. Two techniques have substantially reduced
this oil pollution. In the load-on-top system, the compartments
are cleaned by high-pressure jets of water. The oily water is
retained in the compartment until the oil floats to the top.
The water underneath that contains only a little oil is then
discharged into the sea and the oil is transferred to a slop
tank. At the loading terminal, fresh oil is loaded on top of
the oil in the tank and hence the name of the technique. In
the second method, called crude oil washing, the clingage is
removed by jets of crude oil while the cargo is being unloaded.
Some modern tankers have segregated ballast, where the ballast
water does not come in contact with the oil. Thus, with the
introduction of these new methods of deballasting, the amount
of oil entering the sea has been considerably reduced.
Dry-docking : All ships
need periodic dry-docking for servicing, repairs, cleaning the
hull, etc. During this period when the cargo compartments are
to be completely emptied, residual oil finds its way into the
sea.
Bilge and fuel oils
: As ballast tanks take up valuable space, additional
ballast is sometimes carried in empty fuel tanks. While being
pumped overboard it carries oil into the sea. Individually,
the quantity of oil released may be small, but it becomes a
considerable amount when all the shipping operations are taken
into consideration.
Tanker
accidents : A large number of oil tanker accidents happen
every year. Sometimes this can result in major disasters, such
as that of the Exxon Valdez, described in the section on water
pollution.
Offshore oil
production : The oil that is extracted from the seabed
contains some water. Even after it is passed through oil separators
the water that is discharged contains some oil, which adds to
marine pollution. Drilling muds, which are pumped down oil wells
when they are being drilled, normally contain 70-80% of oil.
They are dumped on the seabed beneath the drilling platform,
thus heavily contaminating the water. In addition, the uncontrolled
release of oil from the wells can be catastrophic events resulting
in oil pollution.
Control
measures for oil pollution: Cleaning oil from surface
waters and contaminated beaches is a time-consuming and labor-intensive
process. The natural process of emulsification of oil in the
water can be accelerated through the use of chemical dispersants,
which can be sprayed on the oil. A variety of slick-tickers
in which a continuous belt of absorbent material dips through
the oil slick and is passed through rollers to extract the oil
have been designed. Rocks, harbor walls can be cleaned with
high-pressure steam or dispersants after which the surface must
be hosed down.
Effects of marine pollution: Apart from causing eutrophication
a large amount of organic wastes can also result in the development
of 'red tides'. These are phytoplankton blooms of such intensity
that the whole area is discolored. Many important, commercially
important marine species are also killed due to clogging of
gills or other structures.
When liquid oil is spilled on the sea, it spreads over the surface
of the water to form a thin film called an oil slick. The rate
of spreading and the thickness of the film depend on the sea
temperature, winds, currents, and the nature of the oil.
Oil slicks damage marine life to a large extent. Salt marshes
and mangrove swamps are likely to trap oil and the plants, which
form the basis for these ecosystems, thus suffer. For salt-marsh
plants, oil slicks can affect the flowering, fruiting and germination.
If liquid oil contaminates a bird's plumage, its water-repellent
properties are lost. Water then penetrates the plumage and displaces
the air trapped between the feathers and the skin. This air
layer is necessary as it provides buoyancy and thermal insulation.
With this, the plumage becomes waterlogged and the birds may
sink and drown. Even if this does not happen, the loss of thermal
insulation results in exhaustion of food reserves in an attempt
to maintain body temperature, often followed by death. Birds
often clean their plumage by preening and in the process consume
the oil, which, depending on its toxicity, can lead to intestinal,
renal or liver failure.
Drill cuttings dumped on the seabed create anoxic conditions
and result in the production of toxic sulphides in the bottom
sediment thus eliminating the benthic fauna.
Fish and shellfish production facilities can also be affected
by oil slicks. However, the most important commercial damage
can come from 'tainting', which imparts an unpleasant flavor
to fish and seafood and is detectable even at extremely low
levels of contamination. This reduces the market value of seafood. |
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