Municipal sewage treatment systems carry out various steps involved. These steps are primary treatment, secondary (or) biological treatment, and tertiary treatment.
2. Liquid water is a mixture of sewage; water from
household use; water used in commercial and
industrial applications etcโฆ
Wastewater contains a variety of biological and
chemical pollutants which make it unsafe for
humans and the environment.
The objective of wastewater treatment is to
remove or modify these pollutants, so that it can
be safely discharged into the environment.
3.
4. 1. Primary treatment
Physically removes large solids using grates, screens, and
settling tanks.
Large pieces of debris are removed by screening, and
suspended particles are removed from the water through
settling processes.
The sewage is passed through a series of filters of graded
openings and then allowed to flow through sedimentation
units.
5.
6. Typical materials that are removed during primary
treatment include,
fats, oils, and greases
sand, gravels and rocks
larger settleable solids including human waste
floating materials
Screening : to remove large objects, that could plug
lines or block tank inlets
Grit chambers : slows down the flow to allow grit to fall
out. (horizontal, aerated and vortex).
Sedimentation tank ( settling tank or clarifiers) :
Settleable solids settle out & are pumped away,
while oils float to the top & are skimmed off.Settleable
solids are then passed for aeration in aeration tanks.
7. 2. SECONDARY TREATMENT
( BIOLOGICAL TREATMENT)
This is purely a biological treatment of mechanically
treated sewage and concerns microbial activity which
biodegrades organic substrates and oxidizable
inorganic compounds
This treatment accomplishes two important phases,
namely, aerobic phase and anaerobic phase.
โขTrickling filters
โขOxidation ponds
โขActivated sludge
9. โขTrickling filter consists of generally 6-10 feet deep bed of
crushed stone, gravel, slag, or similar material.
โขThe sewage effluent is sprayed over the surface of the bed; the
spraying saturates the effluent with oxygen.
โขThe bed surface becomes coated with aerobic microbial flora
consisting of microalgae, microfungi, bacteria, and protozoa.
โขAs the effluent seeps over, the aerobic microbes degrade the
organic matter.
โขHowever, the treated effluent collected at the bottom of the tank
is passed to sedimentation tank and like activated sludge process,
the effluent follows tertiary treatment.
โข80-85 % BOD removal.
11. โขOxidation pond sewage treatment is recommended for small communities in rural
areas where suitable and sufficient land is available.
โขOxidation ponds are generally 2-5 feet deep shallow ponds designed to allow
direct wind action and algal growth on the sewage effluent.
โขOxygen supplied from air and produced as a result of algal photosynthesis fulfils
biochemical oxygen demand (BOD) of sewage effluent and thus helps in
maintaining aerobic condition in sewage effluent.
โขAlgae use CO2- photosynthesis- produce O2-inturn encourage aerobial activity in
the sewage.
โขBacterial action โ decomposing organic matter & generate CO2.
โขConsiderable amount of organic matter in the algal form accumulates is not a
problem. since pond is nutrient-rich.
โขIn such condition the aerobic microbes grow rapidly and digest organic matter,
Chlorella pyrenodosa is a common algal representative grown in oxidation ponds.
13. Aerobic sewage treatment in which flocculated biological
growth is circulated & in contact with organic waste in
oxygen presence is called activated sludge process.
Activated sludge with its living organisms has the property
of absorbing or adsorbing colloidal and dissolved organic
matter.
In this process, the mechanically treated sewage effluent is
pumped into a sedimentation or settling tank wherein the
sewage flocs and settles out.
Thus, in the presence of plentiful oxygen, oxidation of
sewage effluent is brought about by aerobic
microorganisms which break down organic matter to CO2
and H2O.
14. Secondary treatment is entirely dependent upon the activity of microbes.
Eg: Biofilters (Trickling filters) , Activated sludge etc..
Microbes frequently present in activated sludge:
Achromobacter,
Flavobacterium,
Nitrosomonas,
Nocardia,
Mycobacterium,
Geotrichum,
Nitrobacter.
Some microorganisms are used during the secondary treatment to remove
pollutants.
However, most microorganisms will be removed from the wastewater during the
disinfection stage of the treatment.
15. 3. TERTIARY TREATMENT (CHEMICAL
TREATMENT)
Removes disease-causing organisms from wastewater .
3 different disinfection process are :
โขChlorination
โข UV light radiation (a physical treatment)
โข Ozonation
The final step of tertiary treatment is disinfection which is
commonly accomplished by chlorination using either
sodium or calcium hypochlorite respectively or chlorine.
Now the effluent is clean water and is considered
microbiologically safe even for human consumption.
16. Chlorination
โขChlorine is used in 2 forms โ Cl2 gas form or hypochlorite tablets
โขCl react with water to form HOCl, which rapidly dissociate to form
hypochlorite ion.
โขCl is effective against enteric bacteria.
โขAdvantages:
low cost & effective
โขDisadvantages:
chlorine residue could be harmful to environment.
