Sewage Treatment : How do they work ?

Sewage :- It is waste water from a community, containing solid and liquid excreta, derived from houses, street and yard washing, factories and industries

Composition of Sewage :-

99.9% water
0.1% solids (organic and inorganic)

Treatment :- the manner in which someone behaves towards or deals with someone or something.

Sewage treatment is the process of removing contaminants from municipal wastewater, containing mainly household sewage plus some industrial wastewater.

Strength of Sewage :-

Biological Oxygen Demand (BOD) :- is amount of Oxygen absorbed by a sample of sewage, during a specific period (usually 5 days), at a specific temperature of (usually 20°C) for use of organic matter by living organisms.
- Strong sewage has BOD>300g/L
- Weak sewage has BOD<100g/L
Chemical Oxygen Demand (COD) :- measures Oxygen equivalent of that portion of organic matter in a sample which is susceptible to oxidation by a strong chemical oxidizer.
- Potassium dichromate is best for COD extimation
- Strong Sewage has suspended solids amount>500mg/L
- Weak Sewage has suspended solids amount<100mg/L
Suspended solids :- varies from 100-500 ppm
- Weak sewage :- 100ppm
- Strong sewage :- 500ppm

So as our topic says, In this post we will talk about sewage and it’s treatment/ how we deal with it.

Water carriage system/ sewage system :-

It implies collection and transportation of human excreta and waste water from residential, commercial and industrial areas, by network of underground pipes.

Components :-

House hold sanitary fittings
- Water closet
  - Indian squatting type
  - Western commode type
- Urinal
- Wash basin
House drains :- 10cm in diameter, 15 cm below the ground under a bed of cement sufficient to empty house drain to sewage
Street sewers or trunk sewers :- 22.5cm – 2-3 m in diameter, 3m below ground, with self cleansing velocity if 2-3 feet/second
Sewer appurtenances
- Manholes ( opening) :- at a distance of 100m or metting of 2 or more sewers, allows man to enter for checking, repair and cleaning
- Traps :- to prevent foul smell, dirt from sewage

Sewage treatment Plant:-

It is used to convert Sewage to manure, consists of 2 parts aerobic and anaerobic process. It is also known as Waste Water Treatment plant.

Process of sewage Disposal :-

Based upon biological principles of sewage purification (treatment brought by anaerobic/aerobic bacteria)

Primary treatment :- Solids are separated and subjected to anaerobic digestion
- Screening :- consists of vertical steel bars which stops floating objects which are later removed and buried
- Grit chamber :- it’s a long narrow chamber (10-20m), velocity of 1 foot/sec is maintained, allows heavier solids to settle and organic matter to pass
- Primary sedimentation :- consists of large tank, ¼-⅓ dry weather flow, velocity of 1-2 feet/min for 6-8 hours, organic matter settles down (sludge) is removed, microorganisms break down into simple nh³ compounds.
  - Organic trade waste are treated with lime, Al2SO4 and Fe2SO4
Secondary treatment :- effucent is subjected to aerobic oxidation
- Aerobic oxidation :-
  - Trickling filter method :- contains bed of crushed stones, effucent is sprinkled, down to bed consist of zoological layer to oxidise effucent.
  - Activated sludge process:- effucent is mixed with sludge ( sludge is rich in aerobic bacteria), kept for 6-8 hours.
- Secondary sedimentation :- tank where allowed to sedimentate for 2-3 hours. Aerated sludge part is again sent for zoological layer.
- Sludge digestion :- here sludge is thick, black mass containing 95% water, goes through auto anaerobic digestion in which compounds are broke to water, ammonia, methane and carbon dioxide
  - 15-20 tons of sludge is produced 1 million gallon of sewage
  - Residue is in-offensive, tarry mud can be used as manure.
  - Takes 3-4 week for complete digestion
Tertiary treatment :- Treated water is sometimes disinfected chemically or physically (for example, by lagoons and microfiltration) prior to discharge into a stream, river etc
Biological nutrient removal (BNR) :- used for removal of nitrogen and phosphorus/ eutrophication and eventually algal bloom. It is also known as tertiary treatment.

Disposal of effluents :-

Disposal by dilution :- disposal into water such as rivers and streams. The effluent should not have more than 30mg/L suspended solution, BOD at 5th day should not be more then 20mg/L
- Sea outfall
- River outfall
If suitable land is available the effluent can be used for irrigation purposes e.g. Okhla treatment plant in Delhi.
Oxidation ponds :- shallow pond with algae/bacteria and sunlight. (Aerobic during sunlight and anaerobic during night time).
Oxidation ditches

Terms used :-

Zoogleal layer :- complex biological growth consisting of algae, fungi, protozoa and bacteria.
Effluent :- sewage part separated from sludge.
Sludge :- organic matter that settles down.
Influent :- sewage is also called influent.
Sullage :- is waste water from community which does not contain human excreta.

To Know About Solid Waste Management, CLICK HERE.