ETA Trenches

A trench system suitable for sites with heavy or clay soils (such as Category 4-6 soil profiles).

A viable alternative for heavier or clayey soils

An Evapo-transpiration-absorption (ETA) trench system comprises a pipe distribution system, sand and aggregate, and a suitable soil for the long-term application of effluent.

The trenches are designed to be covered by plants; typically a lawn grass is used. They are approved under AS 1547:2012 and disperse the treated water via three natural water processes: sub-surface infiltration into the soil; evaporation from solar and wind influences; and transpiration by plants into the atmosphere.

The ETA trenches themselves provide additional treatment through bio-mat filtration, soil adsorption, and the ability of plants to assimilate nutrients and water into their biomass. An ETA trench has improved performance and Long Term Acceptance Rate (LTAR) in comparison to conventional trenching.

In heavier or clayey soils an ETA system can be a viable alternative. Evapo-transpiration-absorption beds are described in AS1547:2000 and exploit the three natural water removing dynamics by way of sub-surface infiltration of effluent into the soil, evaporation from solar and wind influences and transpiration by plants to assimilate nutrients into plant biomass and remove the hydraulic load.

They are most suited to category 4-6 soil profiles. As discussed above, all of the above design advances can be applied to ETA trenches to improve their performance and Long Term Acceptance Rate (LTAR). As for conventional trenching, grass can be planted over the top of the beds.

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Tim Woods

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