Septic Tank vs AWTS in NSW: Which System Your Soil, Your Block and Your Council Will Actually Allow

A conventional septic tank with absorption trenches costs $12,000 to $18,000 to install and almost nothing to run; an aerated wastewater treatment system (AWTS) costs $15,000 to $25,000-plus, needs a $90 to $150 service every three months and a blower that draws power around the clock. On price alone the septic tank wins. The reason so many Hunter Valley properties have an AWTS anyway is that a septic tank only works where the soil can absorb primary-treated effluent, and on the coalfields clay, the floodplain, the coastal sands and the small village lots, it often cannot. This guide compares the two on how they work, what they cost, what happens when the power goes off, and where in the Hunter each is still approvable.

How each one works

Conventional septic tank and trench. Wastewater settles in a watertight tank (in NSW, at least 2,300 litres for five people under NSW Health’s accreditation rules, with a partition where the tank is over 2,050 litres). Solids sink as sludge, fats float as scum, and the clarified liquid in between flows out through an outlet tee to a land application area: absorption trenches, an evapotranspiration bed or a mound. The soil does the treatment. Nothing moves except by gravity. The tank is desludged when sludge and scum reach one-third of its depth, commonly every 3 to 5 years.

AWTS. The Easy Septic Guide describes it as two tanks: a septic tank first, then an aeration tank where a blower feeds air to bacteria that break the waste down, a clarifier, and chlorine (or UV) disinfection, after which the treated effluent is pumped to a garden irrigation area, subsurface drip preferred. The NSW Health certificate for every accredited AWTS requires a service every three months by a manufacturer-authorised agent, a report to council after each service, and effluent in any grab sample under 30 mg/L BOD, 45 mg/L suspended solids and 100 cfu/100 mL E. coli with 0.2 to 2.0 mg/L free chlorine. Current NSW designs share a pattern: a single 4,150 to 7,200 litre tank, a partitioned primary chamber, an 80 to 120 litre-a-minute blower, tablet chlorination and a submersible irrigation pump, with an alarm and about 1,000 litres of emergency storage.

The difference that matters: a septic tank sends primary effluent into the ground and asks the soil to finish the job; an AWTS finishes the job in the tank and gives the soil disinfected secondary effluent that can be applied over a lawn.

Cost over ten years

The ten-year lines are arithmetic on the published prices, not quotes; a blower or pump replacement adds $600 to $1,400 in the year it happens. The septic tank’s numbers assume a trench that works. A trench that fails from solids carry-over costs $1,500 to $4,500 to rejuvenate or $8,000 to $14,000 to rebuild, which is the figure that closes the gap.

What happens in a blackout

A septic tank does not notice. Gravity keeps working. An AWTS stops treating when the blower stops and stops discharging when the pump stops; every manufacturer says never switch the power off, and Ozzi Kleen’s manual says plainly that during a power failure no effluent is pumped out and the system will overflow once its roughly 1,000-litre storage is used. Fuji Clean’s ACE 1200 holds 1,104 litres of emergency storage and its alarm panel has battery backup so it can tell you the power is off; Aqua Nova’s older alarm will not sound in a general power failure. In practice a day’s outage on a normal household is inside the storage; a week is not, and a house on a stand-alone solar system needs an inverter sized for the blower (Fuji Clean says its unit is compatible with a suitable one). On the Hunter’s storm-prone coast and floodplain that is a real consideration, and one more reason the OLG guidelines want AWTS electrical components above the 1-in-100-year flood level.

Where in the Hunter each is still approvable

No council bans conventional septic tanks and none mandates a brand. What decides the outcome is the site and soil assessment against AS/NZS 1547:2012 and the NSW Office of Local Government’s 2025 guidelines, plus each council’s risk framework.

Soil category. The OLG’s loading-rate table gives trench rates for soil Categories 1 to 5 (gravels and sands through light clays) and none for Category 6, medium to heavy clays; for those the design will need to rely on more processes than just absorption by the soil, evapotranspiration systems are suitable only with secondary treated effluent, and drip irrigation of secondary effluent is the preferred method. Dispersive (sodic) or shrink-swell clays are treated as Category 6 whatever their texture. Much of the Cessnock coalfields, the Maitland floodplain and the Upper Hunter black soils falls there, which is why secondary treatment is the usual answer on those soils and a plain trench is a hard sell. The OLG hedges rather than bans, and so should you: a well-drained loam on a large block in the Singleton hills can still take a trench.

Lot size and water table. Hunter council risk tables put lots under 2,000 m², groundwater within 0.6 m, slopes over 20 per cent and flood-prone sites in the high-risk class, and Cessnock’s LEP requires any new unsewered lot to be at least 4,000 m². Small lots and shallow water tables push toward secondary treatment because there is no room or depth for a trench to work.

Catchments. Inside the Hunter Water special areas (Grahamstown, Tomago Sandbeds, Nelson Bay, Williams River, Chichester) an on-site system is lawful only with a council approval, council must refer applications to Hunter Water, and Port Stephens will not accept new unsewered lots with under 4,000 m² of useable land in those catchments. On sand, the problem is not absorption but treatment: Hunter Water notes contamination moves through sand to the groundwater quickly, so disinfected secondary effluent is what the catchment managers want.

Council preferences. Maitland City Council’s policy names AWTS as the preferred method for new applications in unsewered subdivisions and says a pump-out system will not be approved unless there is no other alternative. Upper Hunter Shire bans surface irrigation for new systems on blocks under 4,000 m². Port Stephens and Dungog put a serviced secondary treatment system in their lightest inspection category, managed by exception. Singleton lists sand filters and reed beds as accepted secondary options alongside AWTS.

Put together: on a large block with good loam and no water or catchment constraint, a conventional system is approvable and cheaper to own; on clay, sand, small lots, floodplain or catchment land, expect the assessment to point to secondary treatment. Our replacement planning service runs the assessment before anyone chooses a tank.

Other things to weigh

• Smell. A serviced AWTS irrigating disinfected effluent should not smell; a septic trench that is failing does. Odour is a maintenance symptom on either system.
• Vegetables. Neither. NSW Health permits AWTS effluent for garden irrigation but not internal reuse, and the OLG model conditions prohibit vegetable gardens on the effluent application area; Aqua Nova’s manual says do not let treated wastewater contact crops used for food.
• Land needed. An AWTS with subsurface drip needs a designed irrigation area; a trench needs trench length set by the loading rate. On heavy clay the trench area gets very large, which is often what tips the decision.
• Keeping an old septic. The OLG guidelines say existing systems need not meet current standards unless they fail, the buildings are altered or the system is altered. A working old trench can stay; add a bedroom and the assessment starts again.
• Other options. Sand filters, reed beds, composting toilets with greywater treatment, and pump-out tanks (last resort) all exist in the OLG’s list, and Singleton names the first two as accepted.

Frequently Asked Questions

Which is cheaper over ten years?

The conventional septic tank, by a wide margin, if the soil can absorb its effluent: roughly $1,300 to $3,300 in pump-outs and council charges against $7,000 to $14,000 for an AWTS’s services, power, parts and desludges, on the published prices above. A failed trench ($8,000 to $14,000 to rebuild) is what erases the difference.

Does council decide for me?

The site and soil assessment does, and council approves what it supports. Category 6 clays, small lots, shallow groundwater, flood-prone land and drinking-water catchments all push toward secondary treatment; Maitland names AWTS as its preferred method for new unsewered subdivisions. A good loam on a large block can still take a conventional system.

Does an AWTS smell?

Not when it is working and serviced; the effluent is aerated and disinfected before it is irrigated. A smell from either system means something is wrong: sludge over the one-third mark, a dead blower, chlorine run out, or a saturated application area.

Can I water vegetables with AWTS effluent?

No. NSW Health permits garden irrigation only, the OLG model approval conditions keep vegetable gardens off the effluent application area, and manufacturers say the same. Lawn and ornamental shrubs, signposted where surface spray is used.

What happens to an AWTS in a blackout?

It stops treating and stops pumping. The common units hold about 1,000 litres of emergency storage, so a short outage on normal use is fine and a long one is not. Reduce water use, do not switch the unit off yourself, and expect the alarm when power returns. A septic tank is unaffected.

Which is better in clay?

Neither is good in Category 6 clay without design help. The OLG gives no trench loading rate for medium to heavy clays and says evapotranspiration beds suit them only with secondary effluent, with drip irrigation of secondary effluent preferred. In practice that means an AWTS or another secondary system with a designed application area.

Sources and further reading

• NSW Office of Local Government, Onsite Wastewater Management Guidelines (April 2025): soil categories and loading rates (Table 6-4), land application options, flood positioning, existing systems.
• NSW Office of Local Government, The Easy Septic Guide: plain descriptions of septic and aerated systems and the quarterly contract.
• NSW Health, Secondary Treatment System Accreditation Guideline (2018): AWTS conditions, effluent standards and permitted uses.
• NSW Health, Sewage Management Facility Vessel Accreditation Guideline (2016): septic tank minimum capacities and partitions in NSW.
• Hunter Water Regulation 2024 (NSW): on-site systems in the drinking-water special areas.
• Maitland City Council, Sewage management and Upper Hunter Shire Council, OSSM inspections: council preferences on system type and irrigation.
• Fuji Clean Australia, ACE 1200 information pack: emergency storage, power and solar compatibility statements.