HS Bothma Property Developers

Percolation Tests for Septic Tanks

A percolation test (colloquially called a perc test) is a test to determine the water absorption rate of soil (that is, its capacity for percolation) in preparation for the building of a septic drain field (leach field) or infiltration basin.

Percolation Tests for Soakaway Design

Why do a Percolation Test?

A soil percolation test is required to be carried out on the land where the septic tank system or sewage treatment plant soakaway is intended to be built. It is a test that determines the rate in which soil absorbs a known volume of water. This is to make sure that the land (soil) is suitable for a soakaway, and to make sure that the soakaway is designed properly.

A percolation test is ALWAYS required for septic tanks.

IMPORTANT The Councils insist that Percolation Tests are carried out by 'qualified persons' and do not accept your own test results. You will be required to contact a contractor to carry out the percolation test.

A percolation Test fails if the results are under 15 or over 100 seconds/mm. drop.

The Soakaway Design

The Building Regulations: Drainage and waste disposal states that

A drainage field or mound serving a wastewater treatment plant or septic tank should be located:

at least 10m from a watercourse or permeable drain.
at least 50 m from the point of abstraction of any groundwater supply
Not in any Zone 1 groundwater protection zone.
at least 15m from any building.
sufficiently far from any other drainage fields, drainage mound soakaways or soakaways so that the overall soakage capacity of the ground is not exceeded.
At least 1.2 metres above the highest winter water table level.

Remember, the purpose of a soakaway is not only to dispose of the effluent into ground, but also to further treat it, using aerobic soil bacteria, before it mixes with groundwater.

Differences between septic tanks and conservancy tanks

There a few subtle yet important differences between septic tanks and conservancy tanks; the article below outlines some of the differences.

In South Africa, sewage is dealt with in 3 ways-

1)it can sent to a septic tank where bacteria work on it and reduce the volume;

2)it can be sent to a conservancy tank where the sewage is periodically removed by a ‘honey sucker’ or

3)it is sent to a municipal sewerage reticulation system where it is eventually treated at a plant.

Many people get confused between a septic tank and a conservancy tank. When planning a building (and where municipal sewerage systems cannot be connected to); the builder must plan for either a in situ treatment system, i.e. a septic tank or for a conservancy tank (or tanks) where the contents will be periodically be pumped out. It must be ascertained whether private contractors or municipalities offer a sewage removal service in the area.

Generally, a polyethylene septic tank is less costly than a conservancy tank. Although a poly septic tank usually has two internal chambers, in other respects it is less robust and with fewer connectors/outlets when compared to an equivalent polyethylene conservancy tank. A conservancy tank is generally built with more strengthening ribs and is heavier; this is to ensure that when it is completely pumped dry, the tank can still hold its shape and not collapse due to the surrounding earth applying pressure on it. These factors mean that conservancy tanks are usually slightly more expensive than equivalent capacity septic tanks.

When positioning the septic or conservancy tank, remember that a sewage removal truck will need to access the tank in case of a conservancy tank.

The advantage of a septic tank is that, correctly sized, it may last for 10 years or more without any intervention. Disadvantage is that if for any reason the household increases in number of people, the septic may be overloaded with unpleasant results.

Conservancy tanks can be regularly pumped out, preventing any chance of overloading which is an advantage.