*Example 8.6*

*Determine the size of soakpit required in porous silty clay to dispose of the effluent from the septic tank considered in Example 8.5.*

From Example 8.5, the sewage flow is 200 litres per day.

From Table 5.4, the infiltration rate for sewage is 20 l per m^{2} per day.

Therefore, the wall area required is _{}

If the pit is 1.5 m in diameter, then the depth required from the bottom of the pipe from the septic tank to the bottom of the pit is:

_{}

*Example 8.7*

*Determine the size of drainage field required in porous silty clay to dispose of the effluent from the septic tank considered in Example 8.4.*

From Example 8.4 the sewage flow is 1440 l per day.

From Table 5.4, the infiltration rate for sewage is 20 l per m^{2} per day.

So the wall area required is _{}

If the effective depth of the trench (the depth from the bottom of the pipe to the bottom of the trench) is 0.6 m, the length of trench required is:

_{}

This allows for infiltration on both sides of the trench.

If the plot is large enough, the drainage field should consist of two trenches, each 30 m long, connected in series.