Guidelines & References

Guidelines for Soakaway System


iv) Septic Tank

Purpose: To facilitate sedimentation and decomposition of small solid particles. (Septic tanks are normally not required for poultry farms because dry muck-out at poultry farms can achieve very high levels of solids removal. Soakaway pits/trenches are however necessary.)

Design Considerations:

(a) Sufficiently large to provide a minimum capacity for 3 days' storage of wastewater, and to hold all the sludge likely to accumulate between desludging. The required size is therefore determined by the amount of water used during washing, the solids content of the wastewater entering the tank (and therefore the amount of solids remaining after dry muck-out) and the frequency of desludging. Table 1 illustrates how the required capacity for a pig farm can be affected by different operational practices. For example, if the amount of water used in a pig farm is 15L/pig/day, with 80% of solids removed during dry muck-out, and the tank is desludged once every 3 months, the required capacity is about 0.15m3 per pig. Nevertheless, a septic tank should not be smaller than 2.5m3 to enable ease of maintenance.

(b) Well vented with a vent pipe to avoid accumulation of methane or other obnoxious gases.

(c) Leak-proof.

(d) Covered to prevent external deposition of solids or ingress of rain.

(e) Access for mechanical desludging.

Table 1 Size of Septic Tank for Pig Farms with Different Operational Practices

Quantity of washwater used (L/pig/day) Desludging interval (month) Septic tank size (m3 / pig) for different percentage removal of solids during dry muck-out
70% 80% 90%
15 3 0.20 0.15 0.10
6 0.35 0.25 0.15
10 3 0.18 0.13 0.08
6 0.32 0.23 0.13
5 3 0.16 0.11 0.07
6 0.30 0.21 0.11

Maintenance Requirement: Periodic sludge removal, at least once every 6 months but once every 3 months is normally expected.

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User review date: 
Friday, 12 October, 2018