Hari Prasad Kafle
Lecturer, Public Health
The four things that have spoiled your life
are: “I” and “mine”, “You” and “Yours”.
Sewage is waste water from a community,
containing solid and liquid excreta, derived
from houses, street and yard washing, factories
The term sullage is applied to waste water
which does not contain excreta, e.g. waste
water from kitchen and bathrooms.
a. Creation of nuisance, unsightness and
b.Breeding of flies and mosquitoes.
c. Pollution of soil and water supplies.
d.Contamination of food.
e. Increase in the incidence of disease,
especially enteric and helminthic disease.
0rganic and inorganic solid: 0.1%
One gram faeces may contain about 1000
million E, coli, 10-100 million faecal
streptococci, and 1-10 million spores of Cl.
To stabilize the organic matters so that it can
be disposed off safely and to convert the
sewage water into an effluent of an acceptable
standard of purity which can be disposed of in
to land, rivers or sea.
A standard test which is an indicator of the
organic content of the sewage is biochemical
BOD is defined as the amount of oxygen
absorbed by a sample of sewage during
specified period, generally five days, at a
specific temperature, generally by living
BOD value range from 1mg/liter for natural
water to 300 mg/liter for untreated sewage.
Aerobic process: in presence of oxygen
organic matter is broken down into CO2,
water, ammonia, nitrites, nitrates and sulfates
by action of bacterial action including fungi
Anaerobic process: anaerobic bacteria
decompose organic waste into methane,
ammonia, CO2, and H2.
Sludge drying beds
Screens Rivers, lakes,
◦ solids are separated
◦ dissolved biological matter is converted into a solid
mass by using water-borne bacteria
◦ 95% of the suspended molecules should be removed
◦ biological solids are neutralized then disposed, and
treated water may be disinfected chemically or
◦ Influx (Influent)
◦ Removal of large objects
◦ Removal of sand and grit
◦ Primary Sedimentation
◦ Trickling bed filter
◦ Activated sludge
Sewage first passed through metal screen to
remove large floating objectives such as pieces
of woods, rags, plastics paces, masses of
garbage and dead animals.
The screen consist of vertical steel bars usually
set 5 cm apart.
After screen sewage is then passed through a
long narrow chamber called as grit chamber or
The chamber is about 10-20 meter and
velocity of sewage flow is about 1 foot per
second taking a detention period of 0.5-1
The grit chamber allows to settle down heavier
solids such as sand and gravel while
permitting the organic matter passing through.
The grit collected at the bottom of camber is
removed periodically or continuously.
After passing through, sewage is then admitted
to a huge rectangular tank called as Primary
Sewage is made to flow very slowly across the
tank at a velocity of 1-2 feet/minute and
retained 6-8 hours in tank.
Nearly 50-70% organic solids settled down
due to gravity.
The sludge is removed mechanically operated
devices and subjected to sludge dryer.
Due to biological action certain treatment
Scum at the uppermost layer should be
The effluent from the primary sedimentation
tank still contains a proportion of organic
matters in solution and numerous living
It has high oxygen demand and subjected to
further treatment ‘aerobic oxidation’ by one
of the following methods:
Trickling bed filter method
Activated sludge process
Spread wastewater over microorganism.
Made of coke (carbonized coal), limestone
chips or specially fabricated plastic media.
Optimize their thickness by insect or worm
To remove suspended solids, nitrogen, and
About 90 % removal of BOD5 and SS
About 20 % removal of phosphate
Variable removal levels of viruses, 20-80 %
removal of bacteria and >90 % removal of
Subjected to aeration chamber for aeration for
The aeration is accomplished by either
mechanically agitation or by forcing
compressed air continuously from the bottom
of aeration tank.
The organic matter of the sewage get oxidized
into Co2, nitrates and water with the help of
Best suited for mega and metro cities because
it occupies small space the trickling filter.
The oxidized sewage from the trickling filter
or aeration chamber is led into the secondary
sedimentation tank for 2-3 hours.
The ‘aerated sludge' is pumped back into the
aeration tank for activated sludge process and
finally into the sludge digestion tank.
To inactivate pathogens in wastewater
◦ Free chlorine and combined chlorine
◦ Chlorine dioxide
Use for Irrigation
Disposal in river, sea, lakes, ponds etc.
3-4 weeks complete digestion takes place and
it is carried out in sludge drying bed.
Digested sludge is used as organic fertilizer for