low cost sanitation systems

Low Cost Sanitation Systems

Unit-III

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.

Septic Tanks

Septic Tank

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.

Septic Tank

Dispersion Trenches

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.

Septic Tank

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

Septic Tanks

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.

Septic Tanks

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


• 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.

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.


• 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.

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


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


• 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


• 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


• Assume Depth of Tank = 1.8 m• Surface area of the tank= Volume= 20.7 Depth 1.8


= 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 mTherefore Provide tank with dimensions6m x 2m x 1.8m

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.

Soak Pits

Soak Pits

• There are three methods of disposal of septic tank effluent, they are :

• Soil Absorption System• Biological filters• Up flow anaerobic filters

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

Soil Absorption

Soak Pits

Soil absorption system are classified as :• Seepage Pit or Soak Pit• Dispersion Trenches

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.

Seepage Pit or Soak Pit

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


• 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 • Where,• Q= Maximum rate of effluent applied in litres /day/ sq.

m of leaching area.• t= Standard Percolation rate in minutes

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.


• Is 2470 Part-II 1985 gives another formula for determining maximum discharges and is given by

• Q = 204 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.

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 • Where t= Standard Percolation rate= 2


• Q = 204 = 143 litres/day/m2

Maximum effluent Discharge= 1 lit/ sec= 60 lit/ minTherefore Area= Maximum Effluent Discharges Effluent Discharge Rate = 86400 lit/day 143 lit/min/m 2

• = 604 m2


• We can Provide trenches Having one meter width length of trenches

• = 604 = 604 m 1Provide 51 m long 12 trenches having width = 1.0 m

References• Environmental Engineering : By Prof B.R.Shah Prof A M Malek• Internet Websites

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