Reference Document - OPW25 0085 (AS Published)

categorisation has been derived via the method described in Table M1 of AS1547:2012 and accounts for
the following factors:
    Variable silty/clayey sands and sandy silt topsoil (Loams, Soil Category 3) were found to depths of
     0.4 m to 1.1 m across proposed Lots 1 and 2.
    The underlying subsoils generally comprise silty/ and sandy clay soil (Soil Category 5 light clay).
    Natural gradients exist which increase likelihood of runoff.
    Vertical drainage beyond about 1 m depth may be affected by the presence of weathered rock in
     some areas.
Where the natural ground slope exceeds 6° (10%), wastewater irrigation areas must incorporate a slope
reduction rate, as described in Table M2 of AS1547:2012, or shall be effectively terraced as necessary to
reduce slope to < 10% negating the effect of natural slopes. As it is not currently known whether
terracing of slopes will occur at the building stage, a slope reduction factor of 20% has therefore been
applied to all sites to account for slopes of up to 20%.


7.3          Recommendations
7.3.1        System Type
It is recommended that wastewater from the future residences be directed to a suitably sized (max. 10
EP) aerated Sewage Treatment Plant (STP). The STPs must be licensed to produce at least Secondary
standard effluent, with Chief Executive approval from the Department of Housing and Public Works.
However, to maximise available space and to reduce setback requirements, it may be necessary/desired
to use an Advanced Secondary treatment system.
The STPs should be connected to either shallow subsurface drip irrigation system or surface irrigation. It
is noted that surface irrigation is not supported for areas with greater than 10% slope.
Table M1 of AS1547:2012 recommends a Design Irrigation Rate (DIR) of 3 mm/day for Category 5 soil.
Therefore, a minimum irrigation area of about 280 m² (840 L/day / 3 mm/day) would be required.
However, an additional 20% slope reduction rate for application areas with slopes between 10-20% shall
be applied in accordance with Table M2 of AS1547:2012. Therefore, a minimum irrigation area of about
350 m² will be required. This may be reduced at final design stage depending on the final location of the
building and land application area and associated slope configuration.
The need for a reserve area is generally applied to septic tank systems, producing low quality (Primary
Standard) effluent, due to increased potential for fowling and failure of the trenched land application
area to occur. As the use of an aerated wastewater treatment system is proposed, which can deliver high
quality (Secondary Standard or better) effluent, the risk of fouling and system failure is negligible.
Shallow subsurface drip or surface irrigation can also be easily repaired in-place as required, therefore
the addition of a separate reserve area is not warranted.

7.3.2        System Positioning
Positioning of the land application systems must be in accordance with the horizontal and vertical
‘setback’ distances recommended in Tables T2, T3 and T5 of the Queensland Plumbing and Wastewater
Code 2019, as summarised in Tables 6, 7 and 8 below and over page.




Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0                                                                           24