Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environment or is reused for various purposes (called water reclamation).

1. Wastewater Engineering
OXIDATION PONDS, OXIDATION DITCHES AND ROTATING
BIOLOGICAL CONTACTORS
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
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2. Stabilization Ponds or Oxidation Ponds
•Open flow through earthen basins
•Rural Areas
•Completely mixed biological reactors
•Natural Process- Wind, Heat and Fermentation
•Long detention periods extending from a few
to several days.
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
MODI
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3. Classification of Stabilization Ponds
Stabilization ponds may be aerobic, anaerobic or facultative.
Aerobic ponds are shallow ponds with depth less than 0.5 m and BOD loading of 40-120
kg/ha.day so as to maximize penetration of light throughout the liquid depth. Such ponds develop
intense algal growth.
Anaerobic ponds are used as pre-treatment of high strength wastes with BOD load of 400-3000
kg/ha.d Such ponds are constructed with a depth of 2.5-5m as light penetration is unimportant.
Facultative pond functions aerobically at the surface while anaerobic conditions prevail at the
bottom. They are often about 1 to 2 m in depth. The aerobic layer acts as a good check against
odour evolution from the pond.
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
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4. Construction
o Earthen Pit with side sloping
o Long detention periods extending from a few to several days.
o Imperious soil- Clay
o Pervious Soil - Lining has to be provided
o Side walls - 2:1 Slope (inside) and 3:1 (outside)
o Free board - 0.6 m
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
MODI
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5. Mechanism of Purification
symbiotic relationship in the pond are shown
below.
Sewage organics are stabilized by both aerobic and
anaerobic reactions.
In the top aerobic layer, where oxygen is supplied
through algal photosynthesis, the non-settleable and
dissolved organic matter is oxidized to CO2 and
water.
In addition, some of the end products of partial
anaerobic decomposition such as volatile acids and
alcohols, which may permeate to upper layers are
also oxidized periodically.
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
MODI
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6. Mechanism of Purification
The settled sludge mass originating from raw waste
and microbial synthesis in the aerobic layer and
dissolved and suspended organics in the lower
layers undergo stabilization through conversion to
methane which escapes the pond in form of
bubbles.
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
MODI
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7. Advantages and Disadvantages of
Oxidation Ponds
Advantages of oxidation Ponds
1. Suitable for hot climate, where 200 sunny days are expected per year
2. Best suited for small cities and towns where land is cheap
3. Capital cost is only 20 to 30% of conventional plant
4. Maintenance cost is minor
5. No skilled supervision is required
6. Flexible and will get upset due to sudden fluctuations in organic loading
Disadvantages of oxidation Ponds
1. Mosquito nuisance
2. Bad odors
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
MODI
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8. Oxidation Ditches (Extended Aerated
Lagoons):
Modified form of the activated sludge system.
Treatment of wastewater using an oxidation
ditch is relatively similar to wastewater
treatment in a packaged plant.
But the oxidation ditch replaces the aeration
basin and provides better sludge treatment.
This system achieves both high BOD
reduction and some nutrient removal.
Pretreatment typically used in an oxidation
ditch system is the bar screen.
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
MODI
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9. Process – Oxidation Ditch
After passing through the bar screen, wastewater flows
directly into the oxidation ditch.
The OD consists of a "ring or oval shaped channel"
equipped with mechanical aeration devices.
Activated sludge is added to the oxidation ditch so that the
microorganisms will digest the B.O.D. in the water.
This mixture of raw wastewater and returned sludge is
known as mixed liquor.
Screened wastewater, which enters the ditch is aerated and
circulated.
ODs typically have long detention times and are capable of
removing between 75% and 95% of the Biological Oxygen
Demand (BOD).
Once the B.O.D. has been removed from the wastewater, the
mixed liquor flows out of the oxidation ditch. Sludge is
removed in the clarifier.
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
MODI
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10. Rotating Biological Contactors
Cylindrical Media
Closely mounted thin flat plates
3-3.5 cm dia, 10 mm thickness & 30-40 mm
spacings
8 m length and can be placed in series or
parallels
40% of diameter is immersed
1-2 RPM
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
MODI
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11. Principles of the Rotating Biological
Contactor
Wastewater is purified using microorganism
membranes which are attached to disks.
The disks slowly rotate with approximately
40% of surface area submerged in the
wastewater.
By absorbing oxygen from the air and
pollutants from the wastewater, the pollutants
are decomposed aerobically.
While new microorganisms are continuously
increase on the disks, old microorganisms
whose activation has declined drop off the
disks.
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
MODI
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