SEWAGE TREATMENT PLANT
Sewage is waste water from a community, containing solid and liquid excreta, derived
from houses, street and yard washings, factories and industries..(Park 25th ed)
Dry Weather Flow:The average amount of sewage which flows through the sewerage
system in 24 hours
a) unsightliness and unpleasant odours.
b) breeding of flies and mosquitoes
c) soil and water pollution
d) contamination of food
e) increase in the incidence of disease
Composition of Sewage
a) 99.9% Water+ 0.1% solids (organic/inorganic in suspension/solution).
b) 1g faeces: 1000 million E. coli
10-100 million faecal streptococci
1 to 10 million Cl. perfringens spores
Biological Oxygen Demand Chemical Oxygen Demand
dissolved oxygen required by aerobic
organisms to breakdown organic
materials, during a specified period days,
at a specified temperature.
oxygen equivalent of that portion of
organic matter in a sample which is
susceptible to oxidation by strong
Slow : five days required Fast: takes just two hours to complete
300 mg/L and above : "strong“
100 mg/L: "weak"
*COD and BOD can be indicated as mg/L or ppm (parts per million).
Expression of strength of sewage
Suspended Solids :
a) The amount of suspended solids in domestic sewage
b) vary from 100 to 500 mg/L
c) 100 mg/L, the sewage is said to be weak : if the amount is 500 mg/L the sewage is said
to be strong.
Decomposition of organic matter
In sewage, decomposition of organic matter takes place by 2 ways:
1. Aerobic: Most efficient method
Require continuous supply of free dissolved oxygen
Organic matter Bacteria, fungi & protozoa CO2, H2O, Ammonia,
nitrates & sulphate
2. Anaerobic: Effective when sewage is highly concentrated with plenty of solids
Slower Extremely complex mechanism
Methane, ammonia, CO2 and H2
Treatment divided into 2 stages:
screening and sedimentation Aerobic oxidation
& Anaerobic digestion
1 . SCREENING
a) Sewage passed through a metal screen
b) Vertical or inclined steel bars usually set 5 cm (2 in) apart
c) Necessary to prevent clogging
d) Fixed type and moving type.
e) The screenings are removed from time to time and disposed off by trenching
2 . GRIT CHAMBER
a) Passed through grit chamber or detritus chamber.
b) 10 to 20 metres in length
c) Two important types of Grit Chambers
(i) Horizontal rectangular flow
(ii) Aerated Grit Chamber.
d) Allow the settlement of grits sand, gravel and other heavy solid
e) The grit is removed periodically or continuously, and
disposed off by plain dumping or trenching.
f) Grit chamber
i. prevents unnecessary abrasion and wear of mechanical equipment
ii. grit deposition in pipelines and channels
iii. accumulation of grit in anaerobic digesters and aeration basins.
3 . Primary sedimentation
a) flow very slowly at a velocity of 1- 2 feet per minute
b) spends about 6-8 hours in the tank
c) purification by sedimentation of suspended matter.
d) 50-70% solids settle down
e) reduction of 30 to 40 % of coliform organisms
g) sludge is removed mechanically
h) Complex organic matter broken down to
simpler soluble substances and ammonia.
i) Fats and grease rise on the surface, Scum is
a) Trickling filter method
b) Activated sludge process
b) ACTIVATED SLUDGE PROCESS
I. The heart, Aeration tank
II. Effluent mixed with sludge from final settling tank (activated sludge)
III. Activated sludge: effluent : 20-30%
IV. Mixture in aeration chamber for 6-8 hrs.
V. Aeration by Mechanical agitation or by compressed air from bottom
VI. Organic matter CO2, nitrates and water
2. Secondary sedimentation
I. It is detained for 2-3 hours.
II. 'Aerated sludge’ or activated sludge
III. It is a valuable manure, if dehydrated.
IV. Rest pumped into the sludge digestion tanks
for treatment an disposal.
I. One million gallons of sewage produces 15- 20 tons of sludge.
II. methods of sludge disposal :
1. incubated under favourable conditions of temperature and ph
2. anaerobic auto-digestion
3. complex solid water, CO2, methane and ammonia.
4. Takes 3-4 weeks to complete.
5. In-offensive, sticky and tarry mud, dry readily & form excellent manure
b) Sea disposal: By pumping into sea
c) Land: composting with town refuse
Disposal of effluent
1. By dilution: Rivers and streams
a) Diluting capacity depends on dissolved oxygen contents of receiving bodies.
b) Effluent should not have more than 30 mg/l suspended solids and BOD should not
exceeds 20 mg/l ( 8:1 dilution)----Royal commission of England 5th report.
2. Disposal on land: irrigation
• There are 1,631 STPs (including proposed STPs) with a total capacity of
36,668MLD covering 35 States/UTs*.
• Out of 1,631 STPs, 1,093 STPs are operational, 102 are Non-operational,
274 are under construction and 162 STPs are proposed for construction.
• Sewage generation from urban centers estimated as 72,368 MLD.
STPs in India