Combined Services UK Ltd

A Septic Tank is typically a two or three chamber system in which the solid matter is retained for sufficient time and under appropriate conditions to partially break down (Anaerobic Decomposition) and from which an effluent discharge takes place. It is important to remember that effluent from a septic tank is not suitable for discharge to a watercourse without further treatment (see treatment plants).

Effluent from a septic tank may be disposed of by soakage into the ground provided this disposal does not generate a pollution risk t surface or groundwater resources.

Before the effluent can be disposed of by soaking away into the ground, the area of land required has to be established by means of a percolation test. Areas of heavy clay or steeply sloping sites are not normally suitable.

If the septic tank is undersize or the separation of solids poor (less than two chambers) the suspended solids will be carried through into soak away forming a non-permeable seal thus preventing the effluent from soaking away. Inevitably, the life span of the Soakaway will be shortened considerably.

Surface water entering the septic tank can have the same effect as poor separation. The septic tank would have been designed to manage effluent discharged by the population of the premises, so an additional volume created by surface water would have the same result as increasing the population.

The main difference being that an increase in population can be calculated and allowed for and because this is impossible with rainfall the solution is to eliminate it from the equation.

Size of the Septic Tank

The size of the tank should be calculated using this formula

C = (180P + 2000)

Where C = Capacity of the tank in litres. P = Population served (180 litres per person)

A septic tank should be de-sludged and serviced at least every 12 months. The septic tank and Soakaway should not be sited within 10 metres of any ditch, drain or watercourse or preferably within 15 metres of any dwelling. Septic tanks and Soakaways should not be installed in the vicinity of nay well or bore hole. The minimum distance required will depend on specific site conditions.

Percolation Tests

This test is for soak away's, taking water from a septic tank or treatment plant. Avoid carrying out this test in extreme weather conditions such as drought, frost and heavy rainfall.

Excavate the bore holes 300mm square depth of 250mm below the proposed invert level (bottom of pipe) of the land drain and spaced evenly along the proposed line of the sub-surface irrigation system (soak away). Fill each hole with water and allow to seep away overnight.

The next day refill each hole with water to the depth of no more than 300mm and observe the time in seconds for the water to seep away completely. Divide each figure by the depth of water in millimetres placed in the hole.

This information gives the time required, in seconds, for the water to drop 1mm. This is the percolation value (V) in seconds; adding all three values and dividing by three, obtains the average figure for the percolation value.

If the percolation value exceeds 100 sec/mm, then the ground conditions may be unsuitable for discharge from a septic tank and if pursued, may cause pending of septic effluent due to ineffective soakage. In this case an alternative will have to be considered. The floor area of land drains required may be calculated from:

= P x V x 0.25
Where P = number of persons served by the tank V = percolation value described above