2. Septic Tanks
• Septic tanks are generally provided in the
area where there is no sewerage System.
Highway Hotels, Guest Houses, Isolated
Colonies, Rural Areas, etc. are several
examples where septic tanks are used for
treatment of wastewater and subsequently
their disposal in Soak Pits.
5. Septic Tank
• Septic tanks are Suitable under following
Situations :
• There is no Sewerage System
• It is Costly and Uneconomical to carry
wastewater to sewerage system which is far
away.
• Soil is relatively Porous and it is possible to
dispose wastewater in underground disposal
facilities like Soak Pit dispersion trenches etc.
8. Septic Tank
• A Septic tank is defined as a Sedimentation
tank with detention Period of 12 to 36
Hours and with extra provision for anaerobic
digestion of settled sludge. Septic tank is
anaerobic treatment unit and therefore foul
gases will be evolved, and that is the reason
why Septic tanks are kept away from the
buildings.
11. Advantages & Disadvantages of
Septic Tanks
Advantages
• It is Cheaper
• There is no need of skilled supervision and
hence maintenance is negligible
• There is no need of sewerage system if the soil is
pervious. The septic tanks effluent can be
disposed in Soak Pit or Distribution trenches.
• Large Cleaning Period Can be allowed.
• It Removes Considerable amount of BOD & SS
13. Advantages & Disadvantages of
Septic Tanks
• Disadvantages of Septic Tanks
• It has to be provided at some distance from buildings
because it being anaerobic Treatment Unit generates
foul gases like H2S, Methane, etc. Which are
Hazardous.
• The effluent from septic tank contains high BOD
and needs proper disposal.
• If the soil permeability is poor, It is difficult to
dispose effluent of septic tank, larger distribution
trenches are required which again involves large land
requirement and thus greater cost.
15. Construction Details
• Following are features of the Septic Tank:
• It is rectangular in plan the length is usually 2 to 4
times breadth.
• For smaller tanks 100 cm of wastewater depth is
provided. For larger tanks, it may be up to 180 cm.
Freeboard is 30 to 45 cm.
• T shaped outlet as well as inlet pipe ends are
provided
17. Construction Details
• In smaller tanks one baffle of hanging type is
enough and is provided at about 30 cm from inlet
pipe and remains 30 cm above and 30 cm below the
wastewater surface.
• Tank is covered with RCC Slabs having Thickness
of not less than 10 cm
• Ventilation pipe can be 10 cm dia plastic pipe with
a cowl at top. Cowl of Plastic is also easily available
in the market.
19. Design Criteria for Septic Tanks
Capacity of Tank
• The Volume of Waste water a septic tank
can accommodate is called its Capacity. A
septic tank should be capable of storing
wastewater flow during detention period and
an additional volume of sludge accumulated
for period of 6 months to 3 years depending
upon frequency of cleaning.
20. Design Criteria for Septic Tanks
• If only water closets are connected the per capita
sewage shall be around 40 to 70 litres per day.
But if sullage is also to be discharged per capita
wastewater may be 90 to 150 liters per day.
• The sludge accumulation rate is normally taken as
30 litres/capita/year. This means that every
person shall contribute 30 litre sludge in a year.
• Free board can be taken between 30 to 45 cm.
• Detention Period= 12 to 36 hours
• Length/ breadth ratio can be taken as 2 to 4.
22. Design of Septic Tank
• Design the dimensions of a septic tank for a
small colony of 150 persons. The rate of
water supply is 125 litre/capita/day
23. Design of Septic Tank
Solution:
• Quantity of Water Supplied
• = 150 x 135
• = 20250 litres/day
• Assume 80 % of water supplied goes to the
house sewer
• Waste water Quantity
• = 20250 x 0.8
• = 16200 litres /day
24. Design of Septic Tank
• Assume Detention time 24 hrs. (one day) the
capacity of tank
• = volume of wastewater per day
• = 16200 litres
• Assume Sludge deposition rate= 30 l/c/day
• Therefore Sludge Produced in a year
• = 150 x 30
• = 4500 litres
25. Design of Septic Tank
• Assume that the Cleaning Period is one Year
• Total Capacity of Tank
• = Volume of Wastewater formed in 24 hrs. +
Volume of Sludge formed in one year
• = 16200 + 4500 litres
• = 20700 litres
• = 20.7 cubic meters
26. Design of Septic Tank
• Assume Depth of Tank = 1.8 m
• Surface area of the tank= Volume= 20.7
Depth 1.8
27. Design of Septic Tank
= 11.5 m2
Now let us keep L/B = 3
Therefore L= 3 B
Therefore 3 B x B = 11.7
Therefore B2 = 11.5 / 3 = 3.833
Therefore B= 1.957 say 2 m
Now as L= 3 B
L= 2 x 3 = 6 m
Therefore Provide tank with dimensions
6m x 2m x 1.8m
28. Soak Pits
• Septic tank effluent is having foul odour and
contains biodegradable organic Matter. The
BOD 5 of the Septic effluent may be from 200
to 250 mg/l It is therefore necessary to dispose
septic tank effluent carefully so that there is
minimum nuisance and risk to the health of
people.
31. Soak Pits
• There are three methods of disposal of septic
tank effluent, they are :
• Soil Absorption System
• Biological filters
• Up flow anaerobic filters
32. Soil Absorption
• Soil absorption System is based on the
characteristics of the soil to absorb water or
wastewater because of its porosity.
Wastewater is poured or applied on wide
area of land and it slowly percolates and
disappears. This type of system is only
applicable when soil is pervious and has a
high rate of percolation
34. Soak Pits
Soil absorption system are classified as :
• Seepage Pit or Soak Pit
• Dispersion Trenches
35. Seepage Pit or Soak Pit
• It is a Circular Covered Pit. The effluent is
discharged in the pit. Because of porosity soil
absorbs the wastewater.
• Lined & Unlined Soak Pits are Provided in
the Permeable Soil. It is not advisable to
provide Soak Pit at the sites having percolation
rate more than 30 min.
37. Disposal in Absorption Trenches
• Septic tank effluent is allowed to enter in to
a masonry chamber in this method. The
Masonry chamber distributes the effluent
into various chambers through porous pipe
lines
39. Disposal in Absorption Trenches
• Dispersion Trenches may be adopted in areas where
percolation rate is not exceeding 60 minutes.
• The minimum absorption area of the trenches can be
calculated on the basis of maximum allowable effluent
discharge q. The value of q
• Q= 130 𝒕
• Where,
• Q= Maximum rate of effluent applied in litres /day/
sq. m of leaching area.
• t= Standard Percolation rate in minutes
41. Disposal in Absorption Trenches
• The standard percolation rate is the rate of
percolation determined by excavating a hole up to
the depth of the trench and measuring fall in level
of water by 1 cm and measuring time taken. The test
hole diameter can be 10 to 30 cm.
42. Disposal in Absorption Trenches
• Is 2470 Part-II 1985 gives another formula for
determining maximum discharges and is given by
• Q = 204
𝒕
Where,
Q= max rate of discharge of effluent applied in
l/m2/min
t= Standard Percolation Rate in Minute
From the above equation we can find out the area of
trench if we know the maximum rate of effluent applied to
the trench.
43. Example
• Calculate the bottom area of trenches for
septic tank effluent discharges (Maximum)
equal to 1.0 litre/ Sec and Standard
Percolation Rate of 36 minutes.
• Q = 204 , Q is in l/m 2/day
𝑡
• Where t= Standard Percolation rate= 2
45. Disposal in Absorption Trenches
• We can Provide trenches Having one meter
width length of trenches
• = 604 = 604 m
1
Provide 51 m long 12 trenches having width =
1.0 m
