Natural groundwater levels may be an important factor when determining the appropriate position and type of onsite Effluent Disposal System. The property will be located in the Resource Zone in most cases where these levels are close to the natural ground level. The Resource Zone includes Banjup, Aubin Grove and some parts of Jandakot.
Within the Resource Zone, a minimum vertical separation of 2.0 metres is required between the base of a conventional onsite effluent disposal system and the highest known groundwater level. If this minimum separation cannot be achieved, either an Aerobic Treatment Unit (ATU) or Nutrient Retentive leaching system must be used.
Where the property is outside the Resource Zone a minimum vertical separation of 1.2 metres between the highest known groundwater level and the base of the leach drain/soak well applies.
The highest known groundwater levels are the estimated maximum levels in Australian Height Datum, which are determined using the Waters and Rivers Commission Groundwater Atlas.
Each effluent disposal system must:
- Be thirty (30) metres from a potable bore or other potable water source (used for drinking and domestic purposes)
- Six (6) metres from any subsoil drainage system or any open drainage channel
- 100 metres from any wetland area.
Conventional septic systems have the following standard requirements:
- Installations are comprised of a 1520mm and a 1220mm diameter septic tank
- Wastewater flows from the secondary tank to the onsite effluent disposal system via a diverter box
- The onsite effluent disposal system will consist of a pair of either leach drains or a number of soak wells in two series (see diagram below)
- Insert diagram from here:
- Where estimated wastewater flows may be above 1400L per day, the size of tanks and disposal systems requires individual assessment and approval
- All leach drains or soak wells are to be situated a minimum of 30m from any well or bore intended for human consumption, a minimum of 6m from any sub-soil or open drainage system, and a minimum of 100m from any wetland
- or public water supply bore.
Minimum clearances required are:
- 1m between septic tanks
- 1.2m from the septic tank to any building, footings, retaining wall, or boundary
- 1.8m from any leach drain or soak well to any septic tank, any other leach drains or soak wells, any building, footing or retaining wall, any sealed or paved area or any boundary.
Reduced systems may be considered where written confirmation from Water Corporation is provided confirming that sewer will become available to the property within the next three years. A statutory declaration stating that the owner will connect to sewer may be required.
The City of Cockburn can approve most applications for on-site effluent disposal systems on single residential or small commercial premises (producing no more than 540 litres of waste water per day).
When an Approved System has been installed, the property owner or applicant must ensure that the following processes are completed so that the system may be approved for use by the City and a Permit to Use may be issued:
- Arrange for an on-site inspection of the system by the City’s Health Services
- Provide confirmation of the disposal system base height where required
- Provide an ‘as-constructed’ site plan of the system (please see below for details)
- Provide the City of Cockburn copy of the AS 3500 “Certificate of Completion & Compliance”.
You must submit a completed application form (under Documents below) with the following:
- Two copies of site plans to scale (1:100), where the application may be approved by the City of Cockburn’s Health Service
- Three copies of site plans to scale (1:100), where Department of Health (WA) approval is required
- Schematic plans of the proposed plumbing layout (please see details below)
- An application and permit to use fee (see application form for details)
- A cheque made out to the Executive Director of Public Health (if Department of Health WA approval is required).
Effluent disposal system site plans
Site plans must be drawn to a scale of 1:100, be labelled with all dimensions and include:
- Location of the effluent disposal system and all drains and pipe work
- Distance of the system from all buildings, boundaries, trafficable areas, bores, waterways and water bodies
- Site contours or spot heights in Australian Height Datum
- Volume of spa (if provided). Where the volumes of the spa exceed 350L, a separate septic tank and effluent disposal system is required.
Approval of on-site effluent disposal for all larger residential or commercial premises must be assessed and approved by the Department of Health (WA). These applications should first be lodged with the City's Health Services, who will forward it onto the Department. Please note that additional Department of Health fees are incurred, refer to the back of the application form for further details.
Property owners must:
- Pump out the tanks on a regular basis. This equates to every four years for an average four person household or every eight to nine years for a two person household
- Alternate leach drains each year. All domestic systems approved from 1990 and onwards have two leach drains and an alternating device to allow systems to rest.
Property owners must not:
- Dispose of materials that do not break down readily (e.g. plastic bags, kitchen sponges, sanitary napkins, tampons, disposable nappies).
- Dispose of old medicines, large amounts of disinfectant or other strong chemicals down the pipes leading to the septic tank. (Bleaches and detergents are acceptable if used in moderation).
Please be advised that only licensed liquid waste contractors can pump out septic tanks. Contractors are listed in the Yellow Pages under Septic Tank Cleaning Services.