WATER POLLUTION
Water pollution occurs when undesirable and unwanted
substances are added to the water bodies that can cause physical, chemical and
biological changes in water quality.
The
major sources of water pollutants are as given below:
The major pollutants are:
1.
Pathogens: Bacteria
and other micro-organisms come under this category. They get into water bodies
through domestic sewage and animal excreta. Bacteria like Escherichia
coli which are found in human excreta are known vectors of
gastrointestinal diseases.
2.
Organic wastes: They are generally bio-degradable wastes that pollute the
water bodies by mixing with them during their runoff phase. An excess amount
of organic matter in water bodies suck up the dissolved oxygen in water thus
inhibiting aquatic life in these water bodies.
3.
Chemical pollutants: Heavy metals like mercury, cadmium and nickel come under
this category. These chemicals when present above the allowable limit can
result in damage of the central nervous system, kidneys and liver.
Sources of water pollution
Point source
When contamination originates from a single source, it’s
called point source
pollution. Examples
include wastewater (also called effluent) discharged legally or illegally by a
manufacturer, oil refinery, or wastewater treatment facility, as well as
contamination from leaking septic systems, chemical and oil spills, and illegal
dumping.
Nonpoint source
Nonpoint source pollution is contamination derived from
diffuse sources. These may include agricultural, surface runoffs or stormwater
runoff or debris is blown into waterways from the land. Nonpoint source
pollution is the leading cause of water pollution as per the studies.
Causes of Water Pollution
The key causatives of water pollution in India are:
·
Industries
·
Urbanization
·
Social and Religious Practices
·
Agriculture
Effects of Water Pollution
The
effect of water pollution depends upon the type of pollutants and its
concentration. Also, the location of water bodies is an important factor to
determine the levels of pollution.
- Water bodies in the vicinity of urban areas are
extremely polluted. This is the result of dumping garbage and toxic
chemicals by industrial and commercial establishments.
- Water pollution drastically affects aquatic life. It
affects their metabolism, behaviour, causes illness and eventual death.
Chemicals such as this travel up the food chain before entering the human
body.
- The effect of water pollution can have a huge impact on
the food chain. It disrupts the food-chain. Cadmium, Mercury and lead are
some toxic substances, these pollutants upon entering the food chain
through animals(fish when consumed by animals, humans) can continue to
disrupt at higher levels. Eg:
Minamata disease caused
by severe mercury poisoning.
- Humans are affected by pollution and can contract
diseases such as hepatitis through faecal matter in water sources. Poor
drinking water treatment and unfit water can always cause an outbreak
of infectious diseases such as cholera etc.
- The ecosystem can be critically affected, modified and
destructured because of water pollution.
* Biochemical oxygen demand is the amount of oxygen required
by bacteria to decompose organic matter in a certain volume of a sample of water.
Clean water will have a BOD of below 5 ppm whereas a highly polluted water the source will have a BOD of more than 17 ppm.
Eutrophication
The excessive growth (or bloom) of algae and plankton in a
water body due to enrichment of nutrients like Nitrogen and Phosphorus is known
as Eutrophication. Eutrophication is considered to be a serious environmental concern since it often results in the deterioration of water
quality and the depletion of dissolved oxygen in water bodies. Eutrophic waters
can eventually become “dead zones” those are incapable of supporting life.
Causes of Eutrophication
The
availability of nutrients such as nitrogen and phosphorus limits the growth of the plant
life in an ecosystem. When water bodies are enriched with these nutrients, the
growth of algae, and phytoplankton.
The
enrichment of water bodies with the nutrients include:
Among
these sources, the primary contributors to eutrophication include agriculture
and industrial wastes.
The excessive
growth of algae in eutrophic waters is accompanied by the generation of a large
biomass of dead algae. These dead algae sink to the bottom of the water body
where they are broken down by bacteria, which consume oxygen in the process.
The overconsumption of oxygen leads to hypoxic conditions
(conditions in which the availability of oxygen is low) in the water. The
hypoxic conditions at the lower levels of the water body lead to the
suffocation and eventual death of larger life forms such as fish.
Effects of
Eutrophication
- The algal blooms limit the amount of dissolved oxygen required for respiration by other animal and plant species in the water leading to oxygen depletion.
- The growth of phytoplankton also causes reduced light penetration into the lower depths of the water. This can bring about aquatic dead zones, loss of aquatic life and it also lessens biodiversity.
- Extensive deterioration of water quality and decline in the availability of clean drinking water is resulted.
- The toxic compounds that get accumulated due to excess plant growth and anaerobic conditions can also make their way up the food chain, contributing to various negative health impacts such as cancers.
- It can also cause death in humans and animals even at the least concentration when ingested in drinking water.
- High nitrogen concentration in drinking water is associated with the ability of inhibiting blood circulation in infants, a condition known as blue baby syndrome.
WASTE
WATER TREATMENT (OR) SEWAGE
WATER TREATMENT
Wastewater treatment is any
process, operation or combination of process and operations that make the
wastewater less dangerous and repulsive to man. It is a combination of
physical, chemical and biological process. There are three types of operation
and process:
(a) Physical unit operation:
It is an operation in which the application of physical forces predominates.
Eg: screening,
flocculation, sedimentation and floatation.
(b) Chemical unit process:
It is a process in which removal of contaminants are brought about by chemical
activity.
Eg: Chemical
precipitation, adsorption, ion exchange, electrodialysis.
(c) Biological unit process:
It is a process in which removal of contaminants is brought about by biological
activity.
Eg: Activated sludge process, trickling filtration.
Conventional
wastewater treatment process:
It
consists of a combination of physical, chemical and biological process and
operations to remove solids and organic matter. General terms used to describe
different degree of waste water treatment are preliminary, primary, secondary
and tertiary treatment.
PRIMARY TREATMENT
Sedimentation: It is a physical water treatment process
used to settle out suspended solids in water under influence of gravity. It is
after used as a primary stage in modern waste water treatment plant, reducing
the content of suspended solids as well as the pollutants embedded in the
suspended solids.
It is an operation by which the
suspended particles in liquid settle by gravity to the bottom of the tank, from
which they may be removed as sludge. After some time, lighter solids and oil
floats on the surface of liquid and can be separated as sludge.
Coagulation
and Flocculation: It is a chemical water treatment
techniques typically applied prior to sedimentation and filtration to enhance
the ability of a treatment process to remove particles.
Coagulation
is a process where water are treated with reagents to form coagulates.
Coagulants with charge opposite to that of suspended solids are added to the
mass to trap particles thus forming a mass charge enough to settle or be
trapped in the filter.
The
substances that are used as coagulants. The most common ones are: Alum (Al2(SO4)3.H2O),
ferrous sulphate, ferric chloride.
Flocculation: It is gentle stirring or agitation to encourage the particles thus formed to agglomerate into masses large enough to settle and be filtered from the solution.
Equalization:
Flow equalization is a method used
to overcome the operational problems and flow rate variations to improve the
processes.
Its objective is by providing
storage to hold water when it is arriving too rapidly and to supply when it is
arriving less rapidly.
The main function of equalization
is to act as buffer. To collect the incoming raw effluent that comes at widely
fluctuating rates and maintain at steady (average) flow rate.
Neutralisation:
· Industrial wastes often
contain acidic or alkaline components which require neutralisation before
discharge or treatment.
- For waters entering
biological treatment processes the “pH” should be maintained between
“6.5 - 9” for optimum growth of the micro-organisms.
- Acidic waste are
commonly neutralised with alkaline streams like ammonia, caustic soda / soda
ash.
- Lime is the most widely
used alkaline material for neutralisation of acid wastes.
- Alkaline wastes require
treatment with acidic streams like Sulphuric acid or Hydrochloric acid.
Ion Exchange :
H2O --> H+ + OH-
NaCl --> Na+ + Cl-
Ion exchange is a treatment method
where one or more undesirable ionic contaminants are removed from water by
exchange with another or less objectionable ionic substance.
Both the contaminants of the
exchanged substance must be dissolved and have the same type of electric charge
(+ve or –ve). The use of ion-exchange aims to reduce “Calcium and Magnesium”
content in water softening.
Adsorption
Adsorption is a waste water
purification technique for removing a wide range of compounds from industrial
waste water. Adsorption works on the principle of adhesion. The process of
adhesion involves separation of a substance from one phase accompanied by its
accumulation or concentration at the surface of another.
The process can take place in any
of the following system:
·
Liquid-Gas,
·
Liquid-Liquid,
·
Solid-Liquid,
·
Solid-Gas.
- The adsorbing phase is the ‘adsorbent’ and the material concentrated or adsorbed at the surface of adsorbing phase is the ‘adsorbate’.
- Activated carbon was
the first widely used adsorbent.
- Activated carbon is an
excellent adsorbent because it has a strong affinity for binding organic
substances even at low concentration.
- It is considered to be a refining treatment and as such is applied at the end of the most common treatment systems especially after a biological treatment.
Electro-dialysis
Electro-dialysis
water treatment process is one of the water treatment method that employ
reverse osmosis membranes which are semi-permeable to ions based on their
charge, under the influence of electric field to reduce the ionic content of
water.
Electro-dialysis
involves passing water between two plates with opposite electrical charges. The
+vely charged ions in the water are
attracted to the plate with negative charge
(cathode) while the –vely charged ions are attracted to the plate with
the positive (+) charge. Both the types of ions can be removed from the plates
and discarded.
Chemical Precipitation
Chemical
precipitation is the most common method for removing dissolved metal from waste
water solution containing toxic metals. To convert the dissolved metals into
solid particle, a precipitation reagent is added to the mixture.
Filtration can then be
used to remove the particles from the mixture.
Eg: (a) Softening
of water with milk or lime (removed by “Ca” or “Mg”)
(b) Removal
of phosphorous from waste water using Iron Chloride.
(c) Removal
of heavy metals as Hydroxides.
SECONDARY
TREATMENT
Aerobic
Treatment
Waste water treatment process is
governed by aerobic organisms which need oxygen for the breaking process.
Aerobic waste water treatment tanks are constantly supplied with oxygen. It has
been done by circulating air through the tanks. For effective functioning of
aerobic organisms sufficient amounts of oxygen should be present in the aerobic
tanks at all times. Therefore, aeration is properly maintained throughout
aerobic treatment.
Anaerobic waste water treatment is
a biological treatment process where organisms, especially bacteria, break down
organic material in the waste water in oxygen absent environment. Anaerobic
digestion is well known anaerobic waste water treatment process. The
degradation of organic material is done anaerobically. For the effective
anaerobic digestion of organic material, the entry of air into anaerobic tanks
is prevented.
During anaerobic digestion, Methane and Carbon dioxide are
produced. Methane is a biogas. Hence, anaerobic digestion process can be used
to produce biogas which can be utilized as electricity.
Biological
Treatment
Activated Sludge
Microorganisms such as bacteria and
protozoa can use the small particles and dissolved organic matter, not removed
in primary treatment, as food. Secondary or biological treatment is performed
in a tank containing a “soup” of starved microbes called activated sludge. Like
us, these microbes require air to live (they are aerobic organisms) and thus
air is pumped into the tank. Microorganisms in this aeration tank use the
dissolved and particulate organic matter as food, producing more microorganisms
which can be collected and separated from the water in the next step.
It then remains to separate out the
microorganisms (activated sludge) so
that just clean water is left. This is done in a secondary clarifier which
operates in the same manner. Some of the solids collected in the secondary
clarifier (return activated sludge) are sent back to the aeration tank to treat
more wastewater and the excess (waste activated sludge) is pumped to another
location in the pant for further treatment. The clean water that flows out the
top of the clarifier is sent along for disinfection.
Trickling filters
Trickling filters
are conventional aerobic biological wastewater treatment units, such as active sludge systems or rotating biological contactors which are used to remove
organic matter from wastewater. The TF is an aerobic treatment system that
utilizes microorganisms attached to a medium to remove organic matter from wastewater.
This type of system is common to a number of technologies such as rotating
biological contactors and packed bed reactors (Biotowers).
TFs enable organic material in the
wastewater to be adsorbed by a population of microorganisms (aerobic,
anaerobic, and facultative bacteria; fungi; algae; and protozoa) attached to
the medium as a biological film or slime layer (approximately 0.1 to 0.2 mm
thick). As the wastewater flows over the medium, microorganisms already in the
water gradually attach themselves to the rock, slag, or plastic surface and
form a film. The organic material is then degraded by the aerobic
microorganisms in the outer part of the slime layer.
As the layer thickens through
microbial growth, oxygen cannot penetrate the medium face, and anaerobic
organisms develop. As the biological film continues to grow, the microorganisms
near the surface lose their ability to cling to the medium, and a portion of
the slime layer falls off the filter. This process is known as sloughing. The
sloughed solids are picked up by the underdrain system and transported to a
clarifier for removal from the wastewater.
Adequate airflow
is important to ensure sufficient treatment performance and prevent odours. To
evenly distribute the water on the filter, a “rotary sprinkler/distributor” is
most often used.
The primary factors that must be considered in the design of
trickling filters include
·
the type of filter media to be used
·
the spraying system, and
·
the configuration of the under-drain system
TERTIARY
TREATMENT
The final step before releasing the
treated wastewater to the environment is to kill microorganisms that might be
health hazards. This process is called disinfection.
Disinfection
The process of killing or
inactivating pathogens or other microorganisms is known as disinfection. This
is done by the following ways:
(a)U.V Treatment: Exposing water to UV rays will kill the germs
and microorganisms in water.
(b) Ozonisation: Ozone is passed into water will kill the
germs and microorganisms. It produces powerful disinfectant.
(c) By
Passing Chlorine: Chlorine gas is
passed to the water. It produces hypochlorous acid, which acts as a disinfectant.
Liquid Chlorine is a powerful disinfectant.
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