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Septic Tank - Absorption Trench Disposal System

Required Information for the installation of Trenches

When lodging your application, the following information must be supplied in order for Council to assess your application.

  1. Manufacturer’s plans and specifications indicating tank dimensions, capacity and brand; plus the licensed plumber’s name, address, phone number and licence number.
  2. An accurately drawn, detailed site plan drawn to scale and showing:
    1. The position of the house, including aspect.
    2. Floor plans for the dwelling showing all bedrooms, study, rumpus room, etc.
    3. The sanitary fittings of the house (including toilet(s), bathroom(s), kitchen and laundry wastewater disposal points), internal and external drainage lines leading from the dwelling to the wastewater tank(s).
    4. The position of the wastewater tank(s), ensuring at least 1.5m distance between tank/s and dwelling, property boundaries, driveways and any impervious surfaces.
    5. Location of property boundaries, watercourses, easements, bushland, recreation areas, etc. (Point 5, Page 6)
    6. A contour plan of the disposal area indicating trench position and dimensions. Note that the trench must be constructed parallel to the natural contours of the land. Trench location must comply with setbacks in accordance with AS1547:2012 OR:
      • 12 metres if up-slope and 6 metres if down-slope of property boundary
      • 6 metres if up-slope and 3 metres if down-slope of swimming pools, driveways and buildings
      • 100 metres from permanent surface waters (e.g. rivers, streams, lakes, etc)
      • 250 metres to domestic groundwater wells
      • 40 metres to other waters (e.g. farm dams, intermittent waterways and drainage channels, etc)
      • 20 metres to natural bushland
  3. Provide details on topography, soil type, soil composition and vegetation within the intended disposal area (this includes presence of rocky outcrops, slope, potential drainage problems, etc).
  4. Position of all roof, surface and subsoil drainage pipelines to their points of discharge.

Calculation to Determine Absorption Trench Length

Calculations for the required length of absorption trenches are to be made in accordance with the Australian Standard AS1547:2012. The following table contains a guide for trench length for wastewater disposal by sub-soil absorption.

LENGTH OF ABSORPTION TRENCH (METRES) FOR WASTEWATER DISPOSAL
BY SUB-SOIL ABSORPTION *

NUMBER OF
BEDROOMS
SOIL TYPE
SANDLOAMCLAY LOAMLIGHT CLAY
2 50m 66m 98m 190m
3 62m 82m 121m 240m
4 74m 98m 146m 288m
5 87m 115m 170m 330m

* A reserve area equal to the primary trench area is also to be shown on the site plan

Please Note:

  1. Failure to supply the above information may result in processing delays.
  2. Council requires that all wastewater applications are prepared in accordance with recommendations in Environment & Health Protection Guidelines - Australian Standard 1547:2012.
  3. The length referred to is in accordance with standard dimensions outlined in AS 1547:2012 and is only intended as a guide.
  4. Due to low permeability, trenches are not suited to clay soils.
  5. The position of the trenches relative to the wastewater tanks may result in the installation of a pump and secondary tank.
  6. Should you require information on tank sizes, contact the Environmental Compliance Team on 9847 6829 or compliance@hornsby.nsw.gov.au

Download Septic and Trench System Information