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
ONSITE SANITATION-SEPTIC TANKS, SOAK PIT &
IMHOFF TANKS
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
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2. What is Septic tank?
Primary sedimentation tank
large detention time (12 to 36hrs against a period
of 2hrs in an ordinary sedimentation tank).
Suitable for disposal of night soil.
But sufficient water should be available as water
is required for flow of the night soil from latrine
to the septic tank
Size - Sewage is retained in the tank for 24hrs
Biological decomposition by the action of
anaerobic bacteria takes place which liquefies
and breaks the night soil leaving small quantity
of soil which is known as sludge and settles at
the bottom of the tank
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
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3. What is Septic tank?
Clear water known as effluent flows out of the
tank.
The effluent from the septic tank is usually
disposed by absorption in the soil through soak
pit
If municipal drainage line exists in the area, the
effluent is discharged to the drain.
It is to be noted that disinfection agent such as
bleaching powder, phenyl etc. should not be used
in cleaning latrines as disinfectant entering the
septic tank kills the bacteria growth as a result of
which rate of biological decomposition is
retarded.
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
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4. Design of Septic Tank
The capacity of septic tank depends on number of users and interval of sludge removal.
Normally sludge should be removed every 2 years.
The liquid capacity of tank is taken as 130 liters to 70 liters per head.
A septic tank is usually provided with brick wall in which cement mortar [not less than 20cm (9
inch)] thick and the foundation floor is of cement concrete 1:2:4.
Both inside and outside faces of the wall and top of the floor are plastered with minimum thickness
of 12mm (one-half inch) thick cement mortar 1:3 mix.
All inside corners of septic tank are rounded.
Water proofing agent such as Impermo, Cem-seal or Accoproof etc. is added to the mortar at the rate
of 2% of the cement weight.
For proper convenience in collection and removal of the sludge, the floor of septic tank is given a
slope of 1:10 to 1:20 towards the inlet side.
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5. Dimensions of Septic Tank Components
i) Length, Width and Depth of Septic Tank
Width = 900 MM
Length = 2 to 4 times width
Depth = 1000 to 1300mm. (min below water level) + 300 to 450mm free board
Maximum depth = 1800mm + 450 mm free board
ii) Detention period
Detention period of 24hrs (mostly) considered in septic tank design. The rate of flow of effluent must
be equal to the rate of flow of influent.
iii) Inlet and outlet pipes
An elbow or T pipe of 100mm diameter is submerged to a depth of 220-600mm below the liquid
level. For outlet pipe an elbow or T type of 100mm diameter pipe is submerged to a depth of 200-
500mm below the liquid level. Pipes may be of stone ware or asbestos.
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
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6. Dimensions of Septic Tank Components
iv) Baffle Walls of Septic Tank
For small tanks, RCC hanging type scum baffle walls are provided in septic tanks. Baffle walls are
provided near the inlet. It is optional near the outlet.
The baffle wall is generally extended 150mm above to scum level and 400-700mm below it.
Scum being light, generally floats at the water level in the tank. Thickness of the wall varies from
50mm to 100mm. for large tanks lower portion are having holes for flow of sludge.
v) Roofing Slab of Septic Tank
The top of the septic tank is covered with a RCC slab of thickness of 75-100mm depending upon the
size of the tank. Circular manholes of 500mm clear diameter are provided for inspection and
desludging. In case of rectangular opening clear size is kept as 600X450mm.
vi) Ventilation Pipe
For outlet of foul gases and ventilation purpose cast iron or asbestos pipe of 50-100mm diameter is
provided which should extend 2m (min) above ground level. Top of the ventilation pipe is provided
with a mosquito proof wire mess or cowl.
REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
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7. REFERENCE – SEWAGE TREATMENT DISPOSAL & WASTE WATER ENGINEERING – P.N
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Fig: Sectional plan ZZ shoes the
typical layout of the septic tank.
Section XX shows the Cross-
Sectional detail of septic tank.
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8. Why do we Need Soak Pit Tank ?
The water in the septic tank is not
pure, it is called grey water
because it still contains organic
materials that need to be filtered
out.
A Soak Pit is a covered, porous-
walled chamber that allows water
to slowly soak into the ground.
Pre-settled effluent from septic
tank is discharged to the
underground chamber from where
it infiltrates into the surrounding
soil.
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9. Design
The following considerations for the design of a soak-pit such as:
It should be between 1.5 m and 4 m deep but as a general rule, never less than 2 m above the
groundwater table.
From a drinking water source, it should be located at a safe distance ideally more than 30 m and it
should also be kept away from high-traffic areas.
To provide support and prevent collapse, it can be left empty and lined with a porous material.
From collapsing, the rocks and gravel will prevent the walls but for wastewater, we will still provide
adequate space.
A layer of sand and fine gravel should be spread over the bottom to help disperse the flow in both
cases.
A removable lid should be used to seal the pit until it needs to be maintained to allow for future
access.
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10. Working & Maintenance
Working
As waste-water percolates through the soil from the soak pit , small particles are filtered out by the
soil matrix and organics are digested by micro-organism.
Soak pit are best suited to soils with good absorptive properties;
Maintenance
• The effluent should be clarified or filtered well to prevent excessive build up of solids.
• The Soak Pit should be kept away from high-traffic areas.
• Particles and biomass will clog the pit so need to be cleaned or moved.
• For future access a removable lid should be used to seal the pit.
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11. Aadvantages & Disadvantages
Advantages
• Can be built and repaired with locally available materials.
• Small land area required.
• Power conservative.
• Simple technique for all users.
Disadvantages
• Pre-treatment is required to prevent clogging, although eventual clogging is inevitable.
• Negatively affects soil and groundwater properties.
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12. Imhoff Tank
Improvement over Septic Tank
Sewage is not allowed to mix with Sludge and
outgoing effluent is not allowed to carry with it
large organic load
Mr. Karl Imhoff - Designer
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13. Constructional Details:
Double Chamber Rectangular Tank
Upper Chamber - Sedimentation Chamber or Flow through Chamber
Lower Chamber - Digestion Chamber (Anaerobic Decomposition -
Sludge)
Sloping Bottom at the sides of Sedimentation Chamber
Slope 1.25 vertical to 1
Entrance Slot - 15 cm (Overlapped) prevent direct contact of Gases
Gas Vent/Scum Chamber -gases to escape
To prevent entry of Sludge into sedimentation Chamber 45 cm
clearance has to be kept (Neutral Zone)
Digestion Chamber - 3 to 4 interconnected Compartments
Hopper Bottom (Sloping 1:1)
Cleaned after 1 to 1.5 months
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14. Advantages and Disadvantages of Imhoff
Tanks:
Advantages
• Small Sized treatment plants
• Economical
• Do no require Skilled Supervision
• 60 - 65 % removal of Solids
• 30 - 40 % removal of BOD
Disadvantages
• More depth - Construction Costlier
• Offensive odours
• Unsuitable for Acidic Waste
• Foam or Boil
• No Control over operation
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15. Treated Effluent Disposal
Natural Evaporation
• large impoundments with no discharge.
• Depends on the climatic conditions
• Net evaporation, storage requirements, and possible percolation and groundwater pollution.
• Beneficial where recovery of residues is desirable such as for disposal of brines.
Groundwater Recharge
• Rapid infiltration by effluent
• Impoundment, intermittent percolation, and direct injection.
• Risks for groundwater pollution exists.
• High costs of treating effluent and injection facilities.
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16. Treated Effluent Disposal
Irrigation
• Substitute for scarce natural waters or sparse rainfall in arid areas.
• In most cases food chain crops (i.e. crops consumed by humans and those animals whose
products are consumed by humans) may not be irrigated by effluent.
• However, field crops such as cotton, sugar beets, and crops for seed production are grown
with wastewater effluent.
• Wastewater effluent has been used for watering parks, golf courses and highway medians.
Recreational Lakes
• Stored in a lagoon for approximately 30 days.
• Chlorinated and then percolated through an area of sand and gravel, through which it
travels for approximately 0.5 km and is collected in an interceptor trench.
• Discharged into a series of lakes used for swimming, boating and fishing.
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17. Treated Effluent Disposal
Aquaculture
• Production of aquatic organisms
• Production of food, fiber and fertilizer.
• Lagoons are used for aquaculture, although artificial and natural wetlands are also being
considered.
• Uncontrolled spread of water hyacinths is itself a great concern because the flora can clog
waterways and ruin water bodies.
Municipal Uses
• Meet drinking water quality standards.
• Emergency basis.
• Many natural bodies of water that are used for municipal water supply are also used for
effluent disposal which is done to supplement the natural water resources by reusing the effluent
many times before it finally flows to the sea.
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18. Treated Effluent Disposal
Industrial Uses
• Used as a cooling water or boiler feed water.
• Deciding factors for effluent reuse by the industry include
○ availability of natural water
○ quality and quantity of effluent, and cost of processing
○ pumping and transport cost of effluent
○ industrial process water that does not involve public health considerations.
Discharge into Natural Waters
Discharge into natural waters is the most common disposal practice. The self-purification or
assimilative capacity of natural waters is thus utilized to provide the remaining treatment.
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