Septic tank or aerated wastewater system on a Baw Baw block

A conventional septic tank gives primary treatment and usually sends effluent into buried absorption trenches. An aerated wastewater treatment system (AWTS) adds oxygen to give secondary treatment, which allows irrigation, but it runs on power and needs regular servicing. On a rural Baw Baw Shire block the choice is rarely yours alone: the land capability assessment and the council permit decide which system the site can take.

This page compares the two types. For permits, servicing and failure signs on an existing system, see the septic systems service page.

What is the difference between a septic tank and an AWTS?

EPA Victoria’s Guideline for onsite wastewater management (publication 5001, May 2024) sorts treatment into levels. A septic tank is listed as primary treatment: solids settle and scum floats, and bacteria that work without oxygen break down part of the waste. The liquid that leaves the tank has no set quality standard and still carries a high load of organic matter and germs.

An aerated system belongs to the secondary treatment group. The EPA guideline describes secondary treatment as aerobic biological processing and settling or filtering after primary treatment. In practice an AWTS has a primary chamber, then a chamber where a blower pushes air through the wastewater so oxygen-loving bacteria can do more of the work, then usually a disinfection stage using chlorine or ultraviolet light. Sand and media filters placed after a septic tank are another way to reach secondary treatment.

How do the two systems compare side by side?

Point	Conventional septic tank	Aerated wastewater treatment system
Treatment level	Primary	Secondary, often with disinfection
Where effluent usually goes	Absorption trenches or beds below ground	Subsurface or surface irrigation, or trenches
Surface irrigation allowed	No	Only with secondary treatment plus disinfection
Needs mains power	Usually not, unless a pump is fitted	Yes, for the blower and pumps
Moving parts	Few	Blower, pumps, alarm, disinfection unit
Routine servicing	Outlet filter cleaning, sludge checks, pump-outs when due	Service contract with a trained technician recommended
Land needed for dispersal	Often more, because effluent is weaker	Can be less, depending on the assessment
Sandy soils with groundwater risk	Primary effluent not recommended	Usually the safer choice

The surface irrigation row comes straight from the EPA guideline, which says surface irrigation needs secondary treatment with disinfection because people and animals can come into contact with the water. The sandy soil row comes from the same document, which warns against dispersing primary treated effluent into sands and gravels where groundwater is at risk.

Why does the land capability assessment decide?

Baw Baw Shire Council says on its getting a septic system page that a land capability assessment (LCA) shows whether wastewater can be treated and kept inside the property boundary. It looks at soil type, block size, distance to waterways and climate. Council assesses the LCA against EPA guidelines, the Victorian Land Capability Assessment Framework and its own Domestic Wastewater Management Plan.

Two points from that council page matter for the choice of system:

• An LCA is required for any application inside a declared water supply catchment. Council’s septic pages also list a Tarago Reservoir Catchment Project, so ask whether your lot falls inside a catchment.
• Land application areas are sized with a monthly water balance because of excess rainfall in West Gippsland.

That second point is a local one. A wet winter can leave clay soil saturated for weeks, and trenches that would cope on a dry inland block may struggle here. The EPA guideline says some secondary treatment plants have been tested for nutrient reduction and can be useful in special water supply catchments. If your assessor recommends secondary treatment, the usual reasons are the soil type and the catchment. The Neerim South area page and the Nilma area page cover the local setting for two of those rural pockets.

Who approves the system and the permit?

Two separate approvals are involved.

1. The treatment plant itself. Under the Environment Protection Regulations 2021, the EPA guideline says an onsite treatment plant must be certified by an accredited body as conforming to the relevant Australian and New Zealand standard. A brand or model without a current certificate of conformity cannot be installed in Victoria. EPA keeps a list of certified plants, using details supplied by the makers.
2. The installation on your land. Baw Baw Shire Council approves systems for flows up to 5,000 litres a day, which covers ordinary homes. Larger systems go to EPA. Council issues a Permit to Install and later, after inspection, a Permit to Use.

Design of land application systems follows AS/NZS 1547, the onsite domestic wastewater management standard, which the EPA guideline refers to throughout for trench sizing and loading rates.

How much daily wastewater does a house produce?

Baw Baw Shire Council gives the example that a four-bedroom house typically produces around 900 litres a day. The design flow, the number of bedrooms and the soil category together set the size of the trenches or irrigation area. A larger house, a granny flat or a planned extension can push a block from one system type to another, so mention future plans before the design is drawn up. If you already know the bedroom count and lot size, you can ask a local plumber to size a system for your block at this stage.

What does each system need to keep running?

Both types need care. The EPA guideline recommends fitting outlet filters to every treatment plant and cleaning them at least quarterly, checking inspection points quarterly for odour or high water, and keeping sludge and scum at or below half the tank’s working capacity. It also says the owner must keep records of all maintenance, including pump-outs and service reports.

For a septic tank the main jobs are filter cleaning, periodic sludge checks and pump-outs. For an AWTS the EPA recommends a service contract with an accredited, trained technician who services the unit to its maintenance schedule and writes a service report each time. Owners should not service an AWTS themselves unless they are a registered plumbing practitioner or wastewater engineer, or have been certified by the maker and shown that to council.

A published Gippsland range for a septic or AWTS service visit, covering inspection, pump check and report, is $290 to $660 per visit. An AWTS needs that visit more often than a plain septic tank, so over the life of the system the servicing cost is part of the comparison. More price detail sits on the plumbing cost guide.

What can go wrong with each type?

A septic tank fails quietly. Signs include soggy ground over the trenches, effluent pooling on the surface, smells, and toilets or drains running slowly. An AWTS has more to break: a blower that stops, a tripped circuit, an alarm sounding, or chlorine tablets that have run out or are seated wrongly in the dispenser. The EPA guideline lists a duty to notify council as soon as practicable if a system poses a risk of harm or is otherwise not in good working order.

Holiday homes and weekenders need a thought too. The EPA guideline notes that powered systems may need a low-flow switch to recirculate effluent when nobody is home, because the bacteria suffer under stop-start use.

What does a careful plumber check before recommending one?

A licensed plumber who designs a system in Baw Baw will usually want to see, or will arrange:

• The land capability assessment, or advice on whether one is needed for your lot.
• The soil category and slope, and where the trenches or irrigation area can go clear of the house, the bore, the dam and the boundaries.
• Whether the block is in a declared water supply catchment.
• The number of bedrooms now and any planned additions.
• Whether power is available near the tank site, and how far the blower would be from bedrooms.
• Whether the chosen treatment plant model holds a current certificate of conformity.

A plumber who skips the assessment, or offers to install without a council permit, is a provider to avoid. The local plumbing businesses that answer enquiries through this site are independent firms, and each one assesses and prices the job on its own terms.

Next step

Before you ask for a design or a replacement, gather the lot size, the number of bedrooms, whether the property is in a water catchment, any existing permit or LCA report, and photos of the current tank lids and the area where effluent goes. Then send a septic or AWTS enquiry for your rural block with those details and your locality, and an independent provider can advise which system the site suits.

Send a job enquiry