1) Modern sewage treatment plants use biological and multi-step processes to purify sewage water. 2) The primary steps are screening, grit removal, and primary sedimentation to remove solids. 3) Secondary treatment further breaks down organic matter, using methods like trickling filters or activated sludge processes. 4) Tertiary treatment can further polish the water before disposal into waterways, irrigation, or potentially for drinking water.

1. MODERN
SEWAGE
TREATMENT
DR. ANCHU R NATH
1ST YEAR PG RESIDENT
DEPARTMENT OF COMMUNITY MEDICINE

2. SEWAGE TEATMENT PLANT

3. •The Aim of sewage treatment is to stabilize the organic
matter.
•To convert the sewage water into an effluent of an
acceptable standard of purity which can be disposed of
into land, rivers or sea.
•Modern sewage treatment plants are based on biological
principles of sewage purification.
•Purification is brought about by the action of aerobic and
anaerobic bacteria.

4. Sewage treatment is a multistep process.
1. Primary treatment
2. Secondary treatment
3. Tertiary treatment

5. 19m

6. PRIMARY TREATMENT
 SCREENING
 GRIT CHAMBER
 PRIMARY SEDIMENTATION

7. SCREENING
• Sewage arriving at a disposal work is first passed through a metal screen
which intercepts large floating objects such as pieces of wood, rags,
masses of garbage and dead animals.
• Their removal is necessary to prevent clogging of the treatment plant. The
screen consists of vertical and inclined steel bars usually set 5 cm(2in)
apart.
• In some plants the screens are of the fixed type while in others ,the screens
are of moving type.
• The screenings are removed from time to time manually or mechanically,
and disposed off by trenching or burial.

8. GRIT CHAMBER
 Also called as Detritus chamber
 It is a long narrow chamber approximately
10-20mts in length
 It is so designed as to maintain a constant
velocity of 1foot/sec , with a detention period of
30sec -1 min.
 Function: allow the settlement of heavier
solids such as sand and gravel, while permitting
the organic matter to pass through.
 The grit which collects at the bottom of the
chamber is removed periodically or continuously
and disposed off by plain dumping or trenching .

10. PRIMARY SEDIMENTATION
• Sewage is now admitted into a huge tank called the primary
sedimentation tank. It is very large tank , holding from 1/4 to1/3
the dry weather flow.
• Commonest tank used is the rectangular tank .
• Sewage is made to flow very slowly across the tank at a velocity
of 1-2ft per minute.
• The sewage spends about 6-8 hrs in the tank . During this long
period considerable amount of purification takes place mainly
through sedimentation of suspended matter.
• Nearly 50-70 per cent of the solids settle down under the
influence of gravity.

11. PRIMARY SEDIMENTATION TANK

12. • A reduction of between 30-40% in the number of coliform
organisms is obtained .
• The organic matter which settles down is called sludge.
• A small amount of biological action also takes place in which the
micro organisms attack complex organic solids and break them
down into simpler soluble substances and ammonia.
• A certain amount of fat and grease rise to the surface to form
scum.
• Organic trade ways are treated with chemicals such as lime,
aluminum sulphate and ferrous sulphate.

13. SECONDARY TREATMENT
• The effluent from the primary sedimentation tank still contains a
proportion of organic mater in solution or colloidal state , and
numerous living organisms.
• It has a high demand for oxygen and can cause pollution of soil
or water.
It is subjected to further treatment by :-
• TRICKLING FILTER METHOD
• ACTIVATED SLUDGE PROCESS
• SECONDARY SEDIMENTATION
• SLUDGE DIGESTION
• DISPOSAL OF EFFLUENT

15. TRICKLING FILTER
• Also called percolating filter.
• It is a bed of crushed stones or cinker, 1-2m (4-8ft) deep and 2-30m
(6-100ft) in diameter depending upon the size of population.
• The effluent is sprinkled uniformly on the surface of the bed by a
revolving device. The device consists of hollow pipes each of which
have a row of holes.
• The pipes keep rotating, sprinkling the effluent in a thin film on the
surface of filter.
• A complex biological growth consisting of algae, fungi, protozoa and
bacteria of many kinds occurs- Zoogleal layer

17.  This bacterial flora oxidizes the effluent percolating through the
filter bed thus the action of the filter is purely a biological one.
 They do not need rest pauses, because wind flows freely through
the bed supplying the oxygen required by the flora.
 The flora lives, grows, and dies. The dead matter sloughs off,
breaks away and is washed down the filter.
 It is a light green, flocculent material and is called Humus.
 The oxidized sewage is now led into secondary
sedimentation tanks or humus tanks.

19. ACTIVATED SLUDGE PROCESS
• Modern method of purifying sewage, in place of trickling filter.
• The ‘heart’ of activated sludge process is aeration tank.
• The effluent from the primary sedimentation tank is mixed with
sludge drawn from final settling tank( also known as activated
sludge or return sludge)
Primary
sedimentatio
n
Aeration
Tank
6-8hrs
detention
Final
settling
Sludge to
digester
Excess sludge Return and excess sludge
20-30%
alternate
Excess sludge to
digester/thickener

21. AERATION TANK
 Aeration is accomplished either
by mechanical agitation or
Forcing compressed air continuously
from the bottom of the tank.
- ‘Diffuse aeration’, is considered
a better method.
 During this process organic
matter gets oxidized into CO2,
Nitrates and water with the help of
aerobic bacteria in the activated
Sludge.
 Typhoid and cholera organisms-
definitely destroyed and coli
forms-greatly reduced.

23.  Activated sludge plants occupy less space, require skilled
operations.
 One acre of this plant does the work of 10acres of percolating
filter.
 This process is therefore best suited for larger cities and the
percolating filter for smaller towns because they are cheaper to
install and easier to operate.

24. SECONDARY SEDIMENTATION
• Oxidized sewage from trickling filter or aeration chamber is led into
secondary sedimentation tank where it is detained for 2-3hrs.
• The sludge thus collected is called Aerated or Activated sludge.
• It is inoffensive and is rich in bacteria, nitrogen and phosphates.
• It is a valuable manure, if dehydrated.
• Part of it is pumped back into aeration tanks and the rest into the
sludge digestion tanks for treatment and disposal.

25. SECONDARY SEDIMENTATION TANK

26. SLUDGE DIGESTION
• One million gallons of sewage produces 15-20tons of sludge.
• SLUDGE- thick, black mass containing 95% water, and has a
revolting odor.
• Methods of sludge disposal:
a. Digestion
b. Sea disposal
c. Land disposal

27. Process of Sludge Digestion:
• Sludge digestion is carried out in sludge digestion tanks.
• On incubation at optimum temp and pH, sludge undergoes auto-
digestion.
• Complex solids are broken down into H2O, CO2,CH4,NH3
• It takes 3-4wks or longer for complete digestion.
• The residue is inoffensive, sticky and tarry mud that dries readily
and forms excellent manure.
• Methane gas, a by-product, can be used for heating and lighting
purposes.

28. DISPOSAL OF EFFLUENT
DISPOSAL BY DILUTION: Disposal into water courses such as rivers
and streams is called “disposal by dilution”.
• The diluting capacity and dissolved oxygen contents of the receiving body
of water are important before discharging the effluent.
• Since river water is used for drinking purpose the effluent must be
rendered free from pathogenic organisms by adequate chlorination.
• The royal commission in England in its 5th report(1908) recommended
that a sewage treatment plant should not have more than 30mg/ltr of
suspended solids and 5 day B.O.D should not exceed 20mg/ltr.
• As per these standards the receiving body would provide an 8:1 dilution.

29. • The effluent may contain substances toxic to man, that can kill fish,
damage agriculture or interfere with normal flowing of a stream.
• In many places in UK, effluent standards have been raised to
10mg/Litre
• The WHO is seized with this problem, and is fostering research in
tertiary methods of treatment or polishing the effluent further.
DISPOSAL ON LAND: If suitable land is available, the effluent
can be used for irrigation purposes (e.g:the Okhla Sewage Treatment
Plant in Delhi).

30. ADVANCED (TERTIARY) SEWAGE TREATMENT
Tertiary Treatment (Physicochemical Process)
• Precipitation
• Filtration
• Chlorination
• Treated water is discharged to waterways
• Used for irrigation
• Recycled into drinking water
Expensive process, sharply reduces
Inorganic nutrients (PO4,NO3)

31. OTHER METHODS OF SEWAGE DISPOSAL
• SEA OUTFALL
• RIVER OUTFALL
• LAND TREATMENT
• OXIDATION PONDS
• OXIDATION DITCHES

32. SEA OUTFALL:
•Sea coast towns and cities.
•Purification takes place by dilution and the solids get slowly oxidized.
•Draw back : the offensive solid matter may be washed back to the
shore. In order to prevent this, the sewage outfall is designed to
discharge into deep water at many points.
RIVER OUTFALL:
•Raw sewage should never be discharged into rivers.
•How far the sewage should be purified depends upon the dilution the
river provides to carryon aeration and self purification.

34. LAND TREATMENT:
• Also known as sewage farming or broad irrigation
• An acre of land would be required to treat the sewage of 100-300
persons.
• Sewage is fed into furrows intermittently and crops are grown on
bridges.
• During the rainy season it may not be possible to operate the
sewage farms.
• Badly managed farms stink , a condition called sewage sickness
because of lack of sufficient aeration and rest pauses to the land

36. OXIDATION POND:
• Also known as waste stabilization pond , redox pond, sewage
lagoons etc.
• Over 50 ponds are working at present in India.
• The first large scale installation was the one at Bhilai where it
serves a population of a lakh.
• To qualify as an oxidation pond , there must be the presence of
algae , certain types of bacteria, sunlight.
• The sewage purification in oxidation pond is brought about by a
combination of aerobic and anaerobic types of bacteria.

39. OXIDATION DITCH
Oxidation ditches and aerated lagoons
make use of mechanical rotors for
extended aeration.
For treatment of the wastes of a
population between 5000-20,000 an
oxidation ditch requires.
An area of 1 acre and aerated lagoon
2.5 acres.
These are low cost treatment methods
for purification of sewage.