Septic and On-Site Sewerage for Residential Builds in Australia

What it is

On-site sewerage is the treatment and disposal of all household wastewater on a single allotment. It applies whenever a dwelling sits outside the reticulated sewer network. Most rural lots, semi-rural lots and acreage subdivisions across Australia run on some form of on-site system. The two main system types are conventional septic tank with absorption trench and aerated wastewater treatment system, often called an AWTS.

For a builder, the on-site sewerage approval is one of the earliest gates on a rural job. It is usually a condition of the building permit. Get the soil assessment wrong or pick the wrong system type and the build stalls before slab pour.

Septic tanks

A septic tank is a two chamber primary treatment vessel. Sewage enters the first chamber where solids settle and grease floats. Anaerobic digestion reduces the sludge volume. Clarified effluent flows to the second chamber and out to a land application area, usually an absorption trench, evapo-transpiration bed or sand mound.

Septic tanks suit lots with good soil percolation and enough setback distance from waterways, bores and neighbours. Tank capacity is set by the number of bedrooms, with a four bedroom dwelling typically requiring a tank of around 3,000 litres. State health regulations list the minimum capacities and the absorption area sizing tables.

Aerated wastewater treatment systems

An AWTS adds an aeration chamber, a clarification chamber and disinfection. The system produces secondary treated effluent that can irrigate to surface or sub-surface dripper lines. AWTS units suit smaller lots, poor soils, sloping ground and sites near sensitive waterways where a septic absorption trench would not work.

The trade off is mechanical complexity. AWTS units need power, a service contract every three months, an alarm and an annual report to council. The lifetime running cost is higher than a septic system. The system must hold a state health accreditation, with the model number listed on the council approval document.

AS/NZS 1547

AS/NZS 1547 is the design standard for on-site domestic wastewater management for flows up to 14,000 litres per week from up to 10 people. The standard covers system selection, tank sizing, land application area design and setback distances. It groups soils into six categories from sand through to clay and sets the design hydraulic loading rate for each.

Designers use AS/NZS 1547 with the relevant state guideline. NSW has the On-site Sewage Management Code, Victoria has the EPA Code of Practice for Onsite Wastewater Management, Queensland uses the Plumbing and Drainage Act with the On-site sewerage facilities guideline. Each state guideline tightens or expands the standard for local conditions.

Council approval pathway

Council approval for on-site sewerage starts with a soil and site assessment by a qualified designer. The report covers soil category, slope, distance to waterways and bores, available land area and groundwater depth. The report recommends a system type, tank size and land application area.

The application then goes to council for an Approval to Install. This is separate from the building permit but usually required as a precondition of it. After installation, the plumber lodges a certificate of compliance and the council issues an Approval to Use. The building cannot reach occupation certificate until the Approval to Use is in hand.

Setbacks and constraints

Standard setback rules under most state guidelines include 6 metres from any building, 12 metres from a property boundary, 100 metres from a drinking water bore and 100 metres or more from a watercourse depending on flow class. The setbacks compound. A small lot with a creek and a neighbour bore can leave very little usable area for the land application zone.

Builders should run a setback overlay over the site plan before any earthworks. A driveway, garage or shed placed in the wastewater zone is expensive to undo.

Service obligations

Septic systems have light service obligations: pump out every three to five years and inspect the absorption area annually. AWTS units carry a regulated service obligation. The homeowner must hold a current service contract with an accredited technician and the council can issue notices if the service log lapses.