Septic Tank – Types, Design Calculation
What is a septic tank?
Septic tank is the underground, onsite, small scale sewage treatment setup, which collects the sewage for the decomposition activity by bacterial action. Sewage is a collection of waste water from domestic purposes or commercial purposes or industrial purposes. When we mention the domestic purpose sewage it refers to the human body wastes such as urine, faeces and wastes from bathroom/washroom/kitchen. When we consider commercial or industrial sewage, a combination of domestic sewage and wastes of human harmful substances such as metal, hospital wastes, factory wastes are included in it. These sewage produce environmental pollutions such as marine environmental distract, bad odour to the living environment, ground & surface water pollution and also many diseases. As to overcome this sewage should be treated. Also when a personal (domestic or commercial) sewage is not connected to the public sewage system then there is a need for septic tank to treat the sewage and those septic tanks are need to be drained using an equipment from public or private sewage cleaning sectors (although the septic tanks are fully water tight, there is a man hole for the cleaning purpose) unless it treated with soakage pit. The treated sewage of septic tank still impure, which means only 60 -70% of sewage get treated in septic tank. That impure water then shifted to soakage pit for the additional treatment.
Structural design of a septic tank
As septic tank is a basic sedimentation tank, it is in a rectangular or cylindrical shape with minimum of two chambers. The first chamber should be at least half of the total length – in case of more than two chambers and a length of 2/3 of the total length should be considered, when there is only two chambers exist.
Baffle is present in a septic tank to prevent the spread and flow of sewage within the tank. Commonly a septic tank possesses two baffles attached with the inlet and outlets of the septic tank. The inlet and outlet “T” or baffle of a septic tank helps to achieve a good sewage treatment. The inlet baffle helps the incoming sewage to reach the septic tank without disturbing the first layer or formed crust. The outlet baffle helps in the additional treatment of effluent or grey water before reaching the soakage pit. The baffles can be made of reinforced concrete and nowadays the PVC “T” sockets are widely used. When installing baffles, the outlet baffle should be placed lower than inlet baffle as to stop passing of large particles with grey water or effluent to the soakage pit.
The process within the septic tank is a chemical reaction. That chemical reaction produces toxic and non toxic gases which result harmful effects as well as bad odour. The bacteria and micro – organism helps to digest the collected sludge anaerobically will produce heat and gasses like Hydrogen, Sulphide, Ammonia, Methane, Carbon mono oxide, Sulphur-dioxide and nitrogen oxides. These gasses should be removed from the septic tank as to reduce damages caused by pressure in the tank. To solve these all a vent pipe is necessary on a septic tank otherwise the odour will reach you back..! The vent pipe from septic tank is attached to the vent pipe line of house which located at the roof top. Due to the low pressure within the pipe line the odour will escape from septic tank to the roof top.
Man holes or inspection ports or inspection risers
The man holes or inspection ports or inspection risers are easy access ports that exist to inspect or clean out the septic tanks. Normally these ports are located near the inlet pipe and outlet pipe. These ports are covered by lids which are visible at the ground surface. Some install PVC pipes straight vertical to the inspection ports of their septic tank. This helps in the sludge suck out process, in which any unknown could open up the lids without any hard digging process to access the man hole or inspection ports. The covers or lids should be durable to withstand even with the weight of a vehicle you park …!
The depth of septic tank can be change from 1-2 m but the length and width vary in a ratio of 2:1 or 4:1. Also the time required for a septic tank to pass the sewage at an average rate flow is 24 to 48 hrs. This time period is known as detention time which is not fixed for all tanks and this time period is not necessary to consider in designing a septic tank. According to Indian standard the sedimentation rate, sludge digestion rate, sludge storage period and inflow rate are the factors which decide the capacity of a septic tank. And additionally a 0.3meters depth should be provided in design consideration. But the British standard uses the number of people and some constants to calculate the capacity of septic tank.General equation to calculate the volume of septic tank is,
C = A + P (rq + nS)
A is a constant
P is the number of persons
r is the minimum retention time of sewage within the setic tank (in days)
q is the sewage flow in litres per person per day
n is the number of years the sludge will stored
S is the rate of sludge accumulation in litres per person per year.
In British code of practice A = 2000liters, rq+nS = 180 liters. Which can re written in a format of C=2000 + 180P.
Calculating septic tank volume for 20 persons (According to Indian standard)
Consider the peak flow of total sewage as 30 L/min for 20 persons. The area required for sedimentation is 0.92 x 30/10 = 2.76 m2 . (It is provided that 0.92m2 area and 0.3meters depth is needed for sedimentation when the sewage inflow rate is 10litres per minute).
The volume needed for sedimentation is 2.76 m3 X 0.3m = 0.828m3.
Digestion rate is 0.032m3 X 20 = 0.64 m3 (Provided that the digestion rate per user at 25degree Celsius is 0.032m3).
Sludge storage volume is 0.073m3 X 20 = 1.46 m3 for one year. (Provided that the sludge clearance rate per person in one year is 0.073m3).
Space above liquid level is 0.3m depth. So the volume of space above liquid level is 2.76m2 X 0.3 = 0.828m3.
Hence, total volume of septic tank for 20 person with a sludge clearance period of one year is = 0.828 + 0.64 + 1.46 + 0.828 = 3.756 m3 .
Calculating septic tank volume for 20 persons (According to British standard)
C = A + P (rq + nS)
C = 2000 liters + 180 X 20 liters
C = 5600 liters / 5.6 m3.