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Guidelines & References

Guidelines for Soakaway System

THE SOAKAWAY SYSTEM

v) The Soakaway Pit/Trench

Purpose : Percolation of wastewater to surrounding subsoil.

Design Considerations:

(a) Soakaway pit should be sufficiently large to avoid flooding and overflow. The minimum capacity of the pit should accommodate all the wastewater produced during one washing or in one day, whichever figure is the greater.

(b) Permeability of Soil. If the percolation rate is too high, the wastewater might drain into the nearby watercourses before any effective treatment. If it is too low, the pit/trenches might soon clog up and wastewater would overflow. A site percolation test should be conducted to determine the soil permeability. Table 2 illustrates the maximum allowable loadings of soakaway pits and trenches. The common simplified procedures of percolation test for determining the absorption capacity of soil are given below:

  • Excavate a hole 300mm square to the proposed depth of the pit and trench.
  • Fill the hole with approximately 150mm of water and allow it to seep away completely; no need to measure the time.
  • Refill the hole with water to a depth of 150mm and observe the time, in minutes, for water to seep completely away.

Table 2 Soakaway Area Requirements at Different Soil Percolation Rates

Time for water to fall 150mm in test pit (minutes) Required trench bottom area (m2) per 1,000L/day of wastewater Required pit percolation area (m2) per 1,000L/day of wastewater
6 or less 31 23
12 38 29
30 51 38
60 72 53
180 152 111

Note
1. This table is only applicable to small inland duck farms or those poultry or pig farms intending to carry out dry muck-out of livestock waste.

2. A septic tank is required for pig farms.

(c) Sufficient Soakaway Area. A reasonable percolation rate which is neither too high nor too low is about 60 minutes for water to fall 150mm. With such a percolation rate, a pig farm using 15L/pig/day of washwater and removing 80% of solids during dry muck-out requires a minimum soakaway trench area of 0.9m2 for every pig (ie. 0.75m of trench if it is 1.2m wide). Table 3 illustrates how this requirement can be affected by different operational practices. For a chicken farm using 0.4L/chicken/week of washwater and achieving 97% removal of solids, a minimum of about 0.4m2 of trench is required for every 100 chickens (ie. 0.33m of trench if it is 1.2m wide). Table 4 illustrates how this requirement can be affected by different operational practices. Similarly, a duck farm generating 1,000L/day of wastewater requires 72m2 of trench (ie. 60m of trench if it is 1.2m wide). Table 2 illustrates how this requirement can be affected by different soil percolation rates.

Table 3 Soakaway Trench Area Requirement for Pig Farms with Different Operational Practices

Percentage removal of solids during dry muck-out 70% 80% 90%
Minimum soakaway trench area requirement (m2/pig) 1.1 0.9 0.7

Note
The above is based on

1. a soil percolation time (rate) of 60 minutes for water to fall 150mm (N.B. The trench area requirement may be reduced if the percolation rate is increased, pro rata to the requirements shown in Table 2; on the other hand, the trench area would need to be increased if the percolation rate is slower.);

2. a septic tank is provided; and

3. a washwater usage rate of 15L/pig/day. (N.B. the trench area requirement can be reduced slightly by reducing the amount of washwater used.)

Table 4 Soakaway Trench Area Requirement for Chicken Farms with Different Operational Practices

Quantity of washwater used per wash (L/chicken) Wash interval (day) Minimum soakaway trench area (m2/100 chicken) for different percentage removal of solids during dry muck-out
95% 97% 99%
0.4 3 0.70 0.46 0.22
7 0.64 0.40 0.16
28 0.61 0.37 0.13

Note
The above is based on a soil percolation time (rate) of 60 minutes for water to fall 150mm. (N.B. the trench area requirement may be reduced if the percolation rate is increased, pro rata to the requirements shown in Table 2; on the other hand, the trench area requirement would need to be increased if the percolation rate is slower.)

(d) A soakaway pit or trench should be located sufficiently far away from building foundations, watercourses and wells, in order to safeguard public health and maintain the structural integrity of nearby buildings. A safe distance is generally 30m from watercourses and wells, and 3m from structures.

Maintenance Requirement : Periodic removal of any sludge accumulated.

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User review date: 
Friday, 12 October, 2018