Water

Following septic tank/AWTS/ASTS treatment, effluent is moved by gravity or pump via a subsoil drainage system to a land-application disposal area where aerobic bacterial action carries out the final treatment as it filters through the soil.

The effluent from an ASTS will have had significant treatment and may be used for irrigation, but it may not be used for crops for human consumption.

Land-application disposal systems include:

• gravity soakage trenches/beds
• low pressure effluent distribution (LPED)/dose loading systems
• drip-line irrigation systems
• evapo-transpiration systems (ETS)
• sand mound systems.

Find out about site investigation.

Gravity soakage trenches/beds (septic tank system only)

Perforated dose lines, 100 mm in diameter, are laid in trenches or beds filled with aggregate and covered with a layer of topsoil. Effluent trickles through the aggregate into the surrounding soil.

Gravity soakage can only be used with a septic tank system. It can work well in reasonably flat, good draining soils, but a common problem is that the effluent flow does not spread evenly over the disposal area, and most of the effluent will discharge at the beginning of the trench. If a trench is too deep, aerobic bacterial treatment of effluent will not occur.

Low pressure effluent distribution (LPED)/dose loading (septic tank system only)

Effluent may be discharged more evenly across the disposal area by pump or dose loading. A controlled dose is pumped through a doseline at regular intervals over a 24-hour period, ensuring the effluent is spread over the whole area and also gets a rest period between soakings. It also eliminates the chance of disposal surges that may occur during periods of high household use.

Alternatively, even soakage can also be achieved by nesting the perforated dose line within a drainage coil installed in the trench. The effluent moves along the drainage coil, spreading more evenly across the whole land-application area. This distribution system is also used with a septic tank.

A distribution or diverter box can be used to different parts of the field and allow the trenches to be periodically ‘rested’ to prevent drains becoming clogged by saturated conditions.

As the soil filtering process provides the secondary treatment, the disposal area for both gravity soakage and LPED disposal should be fenced off to prohibit access.

Drip-line irrigation systems

Several options of drip-line irrigation are available including:

• sub-surface drip – where lines are buried in topsoil 100–150 mm deep
• surface drip – where lines are laid on the surface and covered in bark or mulch
• spray system – where treated and disinfected effluent is sprayed over the ground surface.

Drip-line systems are only suitable for secondary treatment effluent. Effluent is pumped, distributing it over the whole of the effluent field each time it operates.

Evapo-transpiration systems (ETS)

In evapo-transpiration, effluent is dispersed into beds planted with selected, shallow-rooted plants. The plants absorb effluent through the roots and release water through the leaves into the atmosphere in a natural process of transpiration. Effluent not taken up by plants will be absorbed into the soil.

Sand mound systems

Where it is not possible to achieve a suitable depth of trench due to a high natural water table or poor percolation, soil or sand can be mounded to provide a suitable filtering depth for the effluent treatment. The effluent can trickle through the mound into the underlying soil.