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Land-application disposal system

Following treatment in a septic tank or other treatment system, effluent is disposed of on land. There are several ways this can be done.

On this page:

  • gravity soakage trenches/beds
  • low pressure effluent distribution (LPED)/dose loading systems
  • proprietary aerobic treatment system
  • drip-line irrigation systems
  • evapo-transpiration systems (ETS)
  • sand mound systems
  • site investigation

Following primary or secondary treatment, effluent is moved by gravity or pump via a subsoil drainage system to a land-application disposal area, where bacterial action carries out the final treatment as the effluent 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.

The various land-application disposal systems (listed above) each have their own advantages and disadvantages.

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.

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.

Proprietary aerobic treatment system

A proprietary dispersal system is available that treats effluent from a septic tank, releasing much cleaner water into the soil than standard dispersal systems. The US-designed system passes effluent into 300 mm diameter perforated underground pipes on a sand bed. The pipes have a geotextile and fibre wrap which microorganisms colonise. Passive aerobic breakdown follows, which is effectively a secondary treatment. The process is passive – there are no pumps and no filters to change.

By putting much cleaner water into the soil, the system is far less likely to see progressive failure and the soils are less likely to be compromised.

The system is consented as an alternative solution.

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.

Typical piped land application trench system

Traditional distribution of effluent from a septic tank has been through gravity-fed perforated pipes in an aggregate bed.

Evapo-transpiration trench

Treated effluent from an on-site domestic waste-water system may be discharged through an evapo-transpiration trench. The effluent is absorbed into the topsoil, taken up by plants, or may evaporate.

Site investigation

Site requirements for an effluent disposal system must consider the:

  • nature of the subsoil, including permeability (the rate at which water can percolate through it) and stability
  • characteristics of the site such as:
    • slope
    • natural drainage characteristics
    • water table levels
    • water course location
    • tendency to flood
    • area available for land application
    • vegetation and planting
  • potential effects on:
    • downstream neighbouring properties
    • natural water courses the sea
    • local ecology
    • the field location i.e. fields must not be grazed or driven over.