Absorption disposal systems may be one of two types as shown below. Location and details must be shown on plans submitted to Council with a development application.

Absorption trenches

These must be fully lined with geotextile fabric, filled with 30 mm coarse gravel and topped with a 300 mm soil and grass layer for aesthetics that restrict the entry of silt to the system via a geotextile underlay.

The connection into the absorption trench from the rainwater tank must be via a 90 mm diameter inflow pipe passing through a 250 mm diameter covered PVC inspection trap to a 90 mm perforated distribution pipe allowing stormwater to percolate to the gravel. The base of the trench must not exceed a slope of 3%.

Infiltration cells

These are modular plastic systems that can be used in an absorption trench instead of gravel fill. As with absorption trenches, infiltration cells are surrounded with geotextile fabric and placed under a 300 mm layer of sand or loam. An infiltration cell generally has a greater volume of void space than a conventional gravel-filled retention trench; consequently it can provide a greater storage volume per unit of area.

These systems must be installed in accordance with the manufacturers’ recommendations, particularly in relation to the required volume which is dependent on site soil conditions.

Details must be submitted to Council with a development application. The connection into the infiltration cells from the rainwater tank must be via a 90 mm diameter distribution pipe allowing stormwater to fill the cells. The base of the trench must not exceed a slope of 3%.