WASTEWATER TREATMENT
The purpose of waste water treatment is to remove contaminant from water so
that the treated water can meet the acceptable quality standard. The quality
standard usually depends whether the water will be reused or discharged into river.
Methods of waste water treatment depends on composition of waste water and
required quality for treated water. Treatment process are broadly classified as
physical, chemical and biological treatments.
1. Physical treatment methods utilizes physical separation of pollutant such
as by filtration etc.
2. Chemical treatment methods utilizes chemical characteristics of pollutant
for purification. For eg. Coagulation etc
3. Biological treatment methods utilizes biological characteristics of
pollutants such as bacteria, viruses by purification.
Other purpose of waste water treatment includes;
• To reduce strength of sewage
• To make waste water less offensive
• To prevent public health from toxic effect of pollutant
• To conserve nature
Steps of sewage treatment process:
1. Preliminary treatment of wastewater
▪ The main objective of preliminary treatment is to remove gross solids
(such as plastics, cloths, cans, dead body of animals etc), grits and
fats from waste water.
▪ Some of the treatment technique applied for preliminary treatment
purpose are-
Screening:
▪ Screening is used to remove gross solid waste like plastics, cloths,
dead animals from waste water.
▪ For this purpose waste water is passed through a metal screen which
consists of vertical or inclined steel bars usually set 5 cm apart.
Grit removal:
▪ Grits are small, non-biodegradable particles which are heavier than
suspended organic matters.
▪ Grits are removed by carefully regulating the flow velocity of sewage
in grit removal tank.
Skimming:
▪ Skimming is the process of removal of fatty and oily material from
sewage.
2. Primary treatment of wastewater
After removal of gross solids, grits and fats, next step in treatment is removal of
remaining suspended solids as much as possible.
Sedimentation:
▪ Sedimentation tank is used for removal of suspended solids and some
organic matters.
▪ In sedimentation tank sewage flow very slowly (1-2 feet/min) such
that solids present is waste water settle at bottom.
▪ Settled solids are periodically removed by sludge scrapper.
▪ This technique removes about 90% of suspended solids and about
40% of organic matters from sewage.
Mechanical flocculation:
▪ In this method sewage is paced in a flocculation tank, then sewage is
rotated at an speed of 0.43m/sec with the help of rotating paddles
▪ While sewage rotates in circular motion, small size dissolved solids
attached to each other to form large size solids and settles at the
bottom which is then removed out.
Chemical flocculation:
▪ In this method, sewage is placed in coagulation tank and then some
precipitating agents such as alum is added.
▪ Alum forms precipitate of Al(OH)3, suspended solids attached to the
precipitate such that size of precipitate increase gradually to settle
down at bottom.
3. Secondary treatment of waste water:
▪ In secondary treatment, dissolved or colloidal organic matters are
present in sewage are removed by utilizing microorganisms. In this
steps, microorganisms utilizes organic matter and converts them into
inorganic minerals.
▪ Following changes occurs in sewage during secondary treatment;
▪ Organic matter (carbon) is oxidized into CO 2 and H2O
▪ Organic nitrogen compounds are first converted into NH 3
and then into NO3
▪ Colloidal matters are coagulated or precipitated out.
▪ Thus main purpose of secondary treatment of sewage is to reduce
BOD level.
▪ Various techniques are used in secondary treatment of sewage. Some
of them are;
Trickling filter
▪ Effluent or sewage from primary treatment tank is sprayed uniformly
over the filter bed. During filtration a gelatinous layer of bacteria,
algae, protozoa and some fungi is produced on the surface of filter
bed.
▪ As the water trickles through the filter bed, organic matter present in
it are oxidized by microorganism.
▪ Trickling filter can reduce BOD of sewage by about 65-85%
depending on the rate of filtration.
Oxidation ditch:
▪ In this method, sewage from primary treatment plant is placed in
oxidation ditch and then it is agitated with the help of mechanical
rotator and then left for a period of about 12-24 hours.
▪ During the period of oxidation, microorganism present in sewage
oxidize the organic matter.
▪ Finally the sewage is removed from oxidation ditch through outlet for
tertiary treatment.
Oxidation pond
▪ In this treatment method, sewage from primary treatment plant is
placed in an oxidation pond and left there for 10-40 days.
▪ During this period in oxidation pond, microorganisms oxidize the
organic matter present in sewage. Oxygen released by algae during
photosynthesis is utilized by microorganism for oxidation of organic
compounds.
▪ Advantage of oxidation pond;
▪ It is very simple and easy technique
▪ Treated sewage can be utilized for irrigation
▪ Limitation of oxidation tank:
▪ Holding time is very long (10-40 days)
▪ It require large area
▪ It creates bad odor. Furthermore it may become breading
place for mosquitoes and other vectors
▪ It is influenced by seasonal temperature. It is effective
only in warm climate but not in cold and rainy season.
Activated sludge system:
▪ Activated sludge system, consists of aeration tank, settling tank and
sludge return system.
▪ At first sewage from primary treatment plant is mixed with sludge
drawn from previous batch, which is known as activated sludge.
▪ The activated sludge contains large number of microorganisms and
serves as inoculum of microorganisms.
▪ After mixing of activated sludge, sewage is placed in aeration tank.
In aeration tank. Sewage is continuously aerated for 6-8 hours.
During this period microorganisms oxidizes the organic compounds
to form CO2, H2O and NO3 etc.
▪ After oxidation, sewage is passed to settling tank and left undisturbed
for 2-3 hours. Sludge settle to the bottom. This sludge is called
activated sludge.
▪ Most of the sludge is removed and some is returned to aeration tank
for next round of treatment.
▪ By sludge digestion process, BOD of sewage is reduced by 5-15%.
4. Tertiary or final treatment of waste water:
▪ Tertiary treatment of waste water is final treatment process in which
all the chemical and biological agents are completely removed from
sewage before disposal into river.
▪ Suspended solids are removed by chemical coagulation and filtration.
▪ Dissolved solids are removed by adsorption by activated carbon and
reverse osmosis.
Removal of nitrate and phosphate
▪ If sewage after treatment is to be discharge into river, nitrate and
phosphate should be removed from sewage before disposal. It is
because nitrate and phosphate causes eutrophication.
▪ These plant nutrients are removed by biological process. At first
sewage is placed in a tank containing nitrifying bacteria. These
bacteria converts ammonium salt and nitrite into nitrate
▪ Then the sewage is placed into second tank containing denitrifying
bacteria. These bacteria converts nitrate into Nitrogen gas that leaves
the sewage.
▪ Phosphate is also removed by bacteria by microbial assimilation
process.
Killing of microorganisms
▪ Finally microorganisms in sewage are killed by disinfection like
chlorination.
