In this article we will discuss about the design of septic tanks.
An septic tank is a settling-cum-digestion tank, it requires space for:
(i) Settling of incoming sewage,
(ii) Digestion of the settled sludge, and
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(iii) Storage of digested sludge till it is taken out.
Design for space for settling:
This is calculated for the average flow and detention period. Smaller tanks are designed on the basis of average flow and 24 hours detention period, while larger tanks are designed for 12 hours detention period.
Both surface area and detention of depth are important factors in the settling of flocculent particles such as sewage solids. For average Indian conditions at a temperature of 25°C, the surface area required will be 0.92 nr for every 10 lpm peak rate flow.
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This is based on 75% removal of sewage particles of 0.05 mm size and above with a specific gravity of 1.2. A minimum depth of sedimentation of 25-30 cm is necessary. The length of the septic tank is kept 2-4 times the breadth.
If only the discharge from the latrines flow in the septic tank, the average flow per capita per day may be taken as 45 litres. On the other hand, if all the waste water of the houses is to be treated in septic tank the average flow should be taken per capita per day depending on the water supply.
Design for digestion space. In the septic tank, the operation goes in natural way and there is no control over all it such as mixing, heating, etc. a provision of 0.0425 m3 per capita should be done for it.
The fresh sludge stay in the tank long enough to undergo satisfactory anaerobic digestion so that as much of the organic matter as possible may be destroyed and the sludge may become innocuous and suitable for dewatering or drying. The time required for digestion depends on the temperature.
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The per capita suspended solids entering the septic tank may be taken as 70 gm/day. Assuming that 60% of the solids are removed along with fresh sludge, of which 70% is volatile, with a solid content of 5% or moisture 95%, the volume of fresh sludge works out to 0.00083 m3/cap/day.
Now considering that 2/4 of the volatile matter is destroyed of which 1/4 is mineralized during digestion and solid content of 13% in the digested sludge, the volume of the digested sludge works out to 0.0002 m3/cap /day.
The digestion zone contains both fresh and digesting sludge. Therefore, the digestion space should provide for the average volume of the mixture of fresh and digested sludge which works out to 0.000515 m3/cap./day. Now based on the period of digestion, the capacity required for the digestion zone can be worked out at 25°C, the capacity for sludge digestion works out to 0.032 m3/capita.
Design of space for storage of digested sludge:
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The digested sludge produced per capita in different periods is as follows:
The design of space for storage of digested sludge is done on the basis of period of cleaning and the number of persons using the tank.
Adequate space should be provided in the septic tank for the storage of digested sludge and scum, otherwise their accumulation interferes with the efficiency of the tank by encroaching upon the space provided for sedimentation and digestion. A sludge storage capacity of 7.3 m3/100 persons for an interval of cleaning of one year is provided below the sedimentation zone.
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Total Capacity:
The tank should also provide for a free board of atleast 30 cot, which should be sufficient to include the scum depth above the liquid surface. Addition capacity for seed sludge is not required. Care should be taken to leave 20-50 mm depth digested sludge for seed purpose.
When the cleaning is yearly, at 25°C for 10 persons the tank capacity shall be 2.15 m3 as per details below:
(i) Sedimentation = Probable Peak flow 320 lpm
Providing a depth of 30 cot, volume = 1.84 × 3 = 0.55 m3
(ii) Digestion space = 0.32 m3
(iii) Space for sludge storage = 0.73 m3
(iv) Space for free board including = 0.25 m3
Total = 2.15 m3
A septic tank designed on the criteria given above normally provides a detention period of 24-48 hours, based on an average daily flow of sewage. But as the average daily flow varies so widely from one installation to another, detention period should not be considered as an important criteria for design of septic tanks.
Example:
Design a septic tank for 50 users, assuming the rate of water supply as 60 litres/head/day.
Solution:
Assuming the detention period as 24 hours and the time of cleaning the sludge as 3 years.
