What are the Design features of septic tank?
A septic tank is a primary sedimentation tank with a long retention period (12 to 36hrs. against a period of 2hrs. in an ordinary sedimentation tank).
Septic tanks are useful for the disposal of night soil in un-sewered rural and urban regions. However, enough water should be present to allow the night soil to move from the latrine to the septic tank and for the septic tank to work properly.
The septic tank is structured in such a way that the sewage is held in the tank for 24 hours, during which time, anaerobic bacteria act to liquefy and break down the night soil, leaving a tiny amount of sludge that settles at the bottom of the tank and clear water known as effluent that flows out. If there is no municipal drainage infrastructure in the region, the effluent from the septic tank is normally disposed of through absorption in the soil through a soak hole. If municipal drainage line exists in the region, the effluent is dumped to the drain.
Design aspects of a septic tank
1: Capacity: The volume of a septic tank is determined by the amount of flow and the duration of the detention period. It can also be configured on a per capita basis, with septic tank capacity ranging from 60 to 110 litres per person. Sludge storage space is normally retained at a rate of 15 to 45 litres per capita per year.
2: Detention period: Detention periods range from 12 to 72 hours, with 24 hours being the most usual.
3: Freeboard: The freeboard should be between 40 and 60 cm.
Shape: The shape of a septic tank is usually rectangular. The length-to-width ratio is approximately 2 to 4.
Design features of a septic tank
1. Sewage flow:The number of plumbing fixtures discharging at the same time determines the maximum sewage flow into a septic tank. Various sanitation facilities are equivalent in terms of fixture units for this purpose. A fixture unit is a unit of measurement that expresses the load-producing effect of various plumbing fixtures on the plumbing system on a scale that is arbitrarily chosen. A fixture unit of 9 litres per minute is used in septic tank design.
2. Detention period: The minimum holding period for sewage in a septic tank should be 24 hours in order to create adequately quiet conditions for successful sedimentation of suspended solids. A septic tank, on the other hand, is normally intended for a detention time of 24 to 48 hours, due to the volume required for sludge and scum accumulation.
3. Tank capacity: The following methods can be used to determine a septic tank's capacity: Rational method and Detention Period Method.
4. Dimensions:Septic tanks are typically rectangular, with a minimum width of 0.75 m and a length of 2 to 4 times the breadth. The liquid depth is 1 to 2 m with a minimum value of 1 m.
5. Sludge withdrawal and disposal:Half-yearly or yearly sludge removal from a septic tank is possible. De-sludging may be done once every two years for small household tanks for economic reasons, providing the tank is not overloaded owing to use by more than the number for which it was designed. The anaerobic process in the tank is inhibited by frequent desludging. When the depth of collected scum and sludge is detected to exceed half the depth of the tank, the tanks are normally cleaned. A part of the sludge with a depth of at least 25 mm should be left in the tank bottom as seeding material for the new sludge.
6. Constructional details: A single compartment septic tank can be built for lesser capacity (typically up to 2000 litres), but for greater capacities, a two-compartment tank with the partition wall around two-thirds the length from the intake provides superior performance than a single compartment tank. Septic tanks can be made of brick, stone, or cement concrete, which can be cast in place or precast.
7. Disposal of septic tank effluent: The septic tank effluent is foul-smelling and contains a significant amount of dissolved organic content and pathogenic organisms, therefore it should be disposed of carefully to avoid becoming a nuisance or posing a health concern to the public. Septic tank effluent is typically disposed of either land disposal or soil absorption.
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