Wastewater that are produced
by industries are needed to treat in a proper manner before discharge into
public sewer or receiving water bodies because industrial wastewater contain
lots of harmful chemicals and pathogens. International and national standards
must be followed at the discharge point of industries into receiving water
bodies or public sewer.
Untreated
wastewater or effluent cause severe environmental, social and economic problems
when they are directly discharged without pretreatment. The release of wastewater
without pretreatment stresses the public sewers. Untreated wastewater contain
high pollution load which upset the biological processes. It exceeds the
capacity of aerobic repertory microorganisms which involve in the degradation
of organic matter in the aeration tank. It has to be given more retention time
to reduce the undesirable qualities of the wastewater but it increases the cost
of the treatment. It needs storage tank with high capacity which makes more
land requirements to handle large wastewater volumes. Therefore pretreatment of
industrial wastewater is essential before the discharge into public sewers.
Small
scale industries and some of large scale industries tend to discharge their
wastewater without treatment into receiving water bodies. These wastewater are
able contaminate the ground water table which threats health of human. Release
of untreated wastewater into surface drinking water sources discharge
considerable amount of toxic metals and dyes which affects the ecosystems and
human health. Industrial wastewaters that bring sewage as well contain loads of
phosphate and nitrate which are capable of accelerating the growth of algae. With
time, higher growth of the algae makes the aquatic ecosystem completely die. Some
algal species are capable of producing harmful substances that are toxic to
animals and human. This kind of algal
growth is known as harmful algal broom.
Most industries have a combined system for process both industrial
wastewater and sewage wastewater which is generated by the sanitation
activities of workers in the factories due to the cost effectiveness.
Wastewater is collected from the all over the industry and
passed through a metallic bar screen.
It retains the large particles such as straws, glass pieces and sanitary
napkins that can damage the pumps and other equipment in the treatment plant.
Oil
from incoming wastewater is removed in the oil separation tank. API (American Petroleum Institute)
separators can be used for better efficient oil separation. Here specific
gravity difference between oil and wastewater is the main principle behind the
separation. Oil is collected form the surface. Sediment that is settled down at
the bottom of the tank is directed to the central sludge management facilities.
Then
wastewater is transferred to flow
equalization tank. As the production cycle of industries varies, water is
stored for certain period of time in the flow equalization tank. It produces
stable wastewater for downstream process. Temperature of the wastewater is
reduced by giving enough retention time in this tank. The variation of the concentration of
wastewater throughout the whole tank is avoided by continuous mixing.
Coagulant
agents such as lime, alum, ferrous Sulfate and ferric Sulfate are added to
remove the dissolved solid matter. After addition of coagulant agent, water is rapidy
mixed to disperse the coagulant throughout all of the water in the coagulation tank.
Then
water is transferred into flocculation
tank. Here water is gently mixed to allow flocs to form. These flocs are removed by using a sedimentation tank. Chemical flocs are
settled down under gravity. They are collected from the bottom of the tank and
send for central sludge management facilities.
Wastewater
is passed into a floatation tank to
remove solid particle from the water. It helps to remove very tiny light
particles from the water. Here a gas (air bubbles) is introduced and to form
scum which is collected from the surface. The sediments that are collected at
the bottom are directed to the central sludge management facilities.
The
biodegradable organic matter in the wastewater is removed by using aeration tank. Here aerobic respiratory
microorganisms are employed under aeration. They oxidize organic matter
completely into carbon dioxide and water without leaving any residues. External
nutrient sources such as canteen waste or cow dung can be added to enhance the
efficiency of the microorganisms. Aerobic respiratory microorganisms are also
added externally to maintain high F/M
(Food to Microorganisms) ratio because when microorganisms are higher than
the available food, they tend to form flocs via attaching to each other. These biological flogs are removed by using
another settling tank. Retention time of wastewater is 4-8 hours in the
aeration tank. The amount of sediments can be reduced by extending the
retention time from 1-2 days.
Then
wastewater is transferred to secondary clarifier or settling tank. Here
biological flocs are settled down under gravity. They are collected from
bottom. A portion of sediment is returned back to the aeration tank to as activated sludge to enhance the
treatment of incoming wastewater. Remaining portion is disposed as sludge. It
can be degraded completely in an anaerobic digester. Unoxidized sludge mostly
includes high strength organics such as lignin and cellulose which is unable to
degrade by aerobic respiratory bacteria. Anaerobic digestion of organic matter produces
methane which combustible gas which can be used as the alternative fuel for
fossil fuel. This sludge also can be send to central sludge treatment
facilities to treat with the sludge which is generated by other process.
After
biological treatment of wastewater, it is transferred to another tank for disinfection. Disinfection is used to
kill disease cause microorganisms. Chlorine treatment, ozonation and UV
treatments are the available method for disinfection. Although use of chlorine
is cost effective, excess amount of chlorine produce disinfection by products
(DBP) which are carcinogen chemicals. Both UV and ozonation do not leave any
chemical residue. But ozonation require highly specialized equipment to produce
ozone and contact with water. So UV treatment is relatively cost effective and
environmental safe.
These
levels of treatment is not enough to release wastewater into public sewers or
receiving water bodies because wastewater still contains non degraded organic
matter and excess nutrient such as phosphate and nitrate. They can be removed
by using different chemical, physical and biological methods. But it has to be
combined several method to achieve better tertiary treatment. It is expensive
and labor intensive. Therefore construction
of artificial wetland solves these problems and gives relatively efficient
and environmentally friendly tertiary treatment.
Wetland system consists mainly there components such as
surface substrate, macrophytes
and microbial communities.
A gavel layer is applied as the top layer of surface substrate. Beneath
the gavel layer a coarse media is applied.
Rooted emergent macrophytes such
as cattle tails are planted in the coarse media. They produce aerial stem with
underground extensive fibrous root system. These the macrophytes support the growth of
microbial communities via association with their root systems and remove
certain pollutants from the water. The plants uptake the excess nutrient which
is able accelerates their growth from the wastewater. Some plants are capable
of absorbing toxic metal of the water such as Ipomoea aquatic.
The microorganisms help to further removal of organics. They plays
important role for the removal of excess nutrients. Ammonium ions are removed
from the wastewater by denitrification. Oxygen leaks from the rhizomes into the
nearby waster where the aerated zone is located. In aerated zones, ammonium
ions are oxidized into nitrite by Nitrosomonas then nitrite is further
oxidized intro nitrate by Nitrobactor. Then these nitrates are reduced into
nitrogen gas which expels from water into the atmosphere by Thiobacillus
dentrificans in the anaerobic zone.
In
addition above biological process, wetland use physical and chemical methods to
remove the pollutants from the wastewater. Main physical processes are
sedimentation and filtration. Above gavel and coarse media of the wetland traps
the remaining sediment in the incoming wastewater through straining. Volatile
organic compounds (VOC) such as phenolic compounds which are resistant to
remove during primary and secondary treatment are subjected to volatilization.
Those VOCs diffuses from the wastewater from atmosphere due to the action of
the photo energy. Chemical process includes the sorption, redox reactions,
chemical precipitation and hydrolysis.
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