How a septic tank actually works — and why the soil does most of the job (Australia)

Almost everything written about septic tanks describes the tank. That is the wrong object. A correctly sized septic tank removes roughly a quarter to a third of the organic load in your wastewater — WaterNSW puts it at 25 to 35 per cent of BOD5 — and then hands the rest to the soil. The tank is a settling chamber with a slow biological hobby. The treatment plant is your backyard.

Understanding that one fact reorganises every other decision you will make: why the trench costs more than the tank, why a soil report decides your budget, and why nothing you pour into the toilet matters as much as what you drive over the lawn.

Inside the box: three layers and a rule about time

1. Inlet tee — drops incoming sewage below the surface so it cannot punch into the settled sludge.
2. Scum — fats, oils and grease, floating.
3. Effluent — the clarified middle layer. Only this should ever leave.
4. Sludge — settled solids and inert matter like sand.
5. Baffle — splits the tank into two chambers, roughly 2:1 by capacity.
6. Outlet tee — draws from the middle layer and blocks scum from escaping.
7. Effluent filter — the last thing between your trench and a bad decade.

The anaerobic bacteria at the bottom digest some of the sludge, converting organic matter into carbon dioxide, methane and hydrogen sulfide — the last of which is why the vent exists. They also remove some nitrogen and phosphorus. What they do not do is clean the water.

The one operating parameter that matters is time. Wastewater must remain in the tank for at least 24 hours, long enough for the light things to rise and the heavy things to fall and for a burst of flow — someone’s shower plus the washing machine — not to push raw sewage straight out the outlet. Every sizing rule in this article exists to protect that 24 hours. (A commercial tiny-house guide recommends three days’ retention; the government minimum is one, and the difference is a marketing preference, not a rule.)

The baffle, the tees and the filter

NSW Health does not treat the baffle as optional. Every tank larger than 2,050 litres must be baffled, the two chambers must sit at roughly a 2:1 capacity ratio, and the top of the baffle must stand at least 150 mm above the water line. The reasons are mechanical rather than biological: a longer path means more settling time, incoming surges cannot disturb the sludge bed, and the water near the outlet stays still.

The inlet tee moderates that surge. The outlet tee — an inverted pipe — draws liquid from the clarified middle band while holding back the scum above it. And the effluent filter, which many older Australian tanks simply do not have, is the difference between primary effluent at 150–250 mg/L of BOD5 and the same effluent at 100–140 mg/L. On suspended solids the gap is starker still: 40–140 mg/L without a filter, 20–55 mg/L with one.

That filter costs almost nothing and protects a trench that costs $3,000–8,000 to replace. It is the single most under-specified component in Australian on-site wastewater.

What actually leaves the tank

Read the first and second rows together. Raw sewage at 340, primary effluent at 160: that is the whole contribution of the tank, and it matches WaterNSW’s 25–35% figure almost exactly. Everything between the tank’s outlet and the water table — an order of magnitude of treatment — happens in soil.

The trench, and the living filter you cannot see

Effluent leaves the tank, runs down perforated pipes or arches into a bed of gravel and sand, and enters the soil through the trench floor and walls. Three things then happen: the media and soil filter out fine solids; native soil microorganisms break down organic matter and eat the pathogens; and grass over the trench takes up water and nutrients while sun and wind remove the rest.

At the interface between effluent and soil, a biomat forms — a layer of microorganisms, microbial debris and fine particles roughly 25 mm thick that grows on the trench floor and side walls. Every owner who reads about it assumes it is the enemy. It is not. The biomat is the filter: it strips pathogens, and by restricting flow it keeps the soil below it unsaturated, which is what keeps that soil aerobic and able to treat anything at all.

The biomat only becomes the problem when it thickens beyond usefulness, turns anaerobic and stops passing water. Australians have a name for the slow version of this: creeping failure.

That is a better diagnostic than most published ones. Lush grass over the trench means effluent is arriving and plants are using it. Grass over the trench that stops being lusher than the rest of the lawn means effluent has stopped arriving there — because the biomat has sealed, or the pipe has broken, or the trench is ponding somewhere upstream. Another owner in the same thread put the failure signal the other way round: “if the trench is always soaked and green it’s definitely on its way out”

Why your trench is the length it is

Nothing about trench length is negotiable, and nothing about it is arbitrary. AS/NZS 1547 sets it with one equation:

L = Q ÷ (DLR × W)

Q is the daily design flow in litres, DLR the design loading rate of your soil in millimetres per day, W the trench width in metres, and L the length in metres. The design flow allowance is 150 litres per person per day on reticulated or bore water. The design loading rate comes from your soil category — and that is where the money is. EPA Victoria’s tables put a massive sandy loam (category 2b) at 15 mm/day for primary effluent, and a strongly structured light clay (category 5a) at 5 mm/day.

Put a four-person household on a 600 mm wide trench and the arithmetic writes itself:

Same house, same tank, same family. Three times the trench, three times the excavation, and quite possibly a block that cannot fit it — at which point the council will tell you to install an aerated system instead, and the $15,000–25,000 quote you did not expect arrives.

This is also why an installer who quotes before the soil report exists is quoting a number he has invented. As one owner put it bluntly on Reddit: “no you cant just go and put a septic in they have to be engineered.”

The reserve area, and why half your yard is spoken for

AS/NZS 1547 §4.2.3.4 generally requires a 100% reserve area for primary effluent absorption systems: a duplicate patch of land, the same size as the trench, held back so the first can be rested if it fails.

Victoria makes it mandatory in every Special Water Supply Catchment and for all trench and bed systems unless the council is satisfied the risk is low on the basis of a land capability assessment. Clarence Valley and Camden in New South Wales require it. So do Douglas Shire in Queensland and Sorell in Tasmania. One technical guideline suggests 50%, which contradicts both the standard and the Victoria Government Gazette.

The reserve must be separate land, excluded from traffic and from future building works, protected from anything that would stop it ever receiving effluent. Which means the shed, the pool and the second driveway all have to go somewhere else — and that the block you are looking at is not as large as the fence suggests.

Run the arithmetic before you buy the land. Four people at 150 litres a day on a 600 mm trench need about 67 metres of trench on a well-structured sandy loam, and about 200 metres on a moderately structured light clay. Double each of those for the reserve. On clay, that is 400 metres of trench-shaped land, unbuilt on, forever. Our trench length calculator does the sum with the AS/NZS soil categories.

Everything in this article is one idea told four ways. The tank buys time; time buys separation; separation buys the trench a chance; and the trench, in the end, is the treatment plant. Get the order right and every number falls out of it. Get it wrong and you will spend money at the top of the system to fix something happening at the bottom.

How big the tank has to be

Design flow is 150 litres per person per day, but the minimum capacities are set by state and they do not agree. NSW Health mandates 2,300 litres for an all-waste septic tank and 2,050 litres for a greywater-only or blackwater-only tank. Western Australia requires a minimum of 3,180 litres for an all-waste tank serving five bedrooms or fewer. WaterNSW and EPA Victoria both work to 3,000 litres as the practical residential minimum.

Meanwhile a commercial tiny-house guide asserts that 1,500 litres is the regulatory minimum for a permanent dwelling in Australia. No government source we read supports that number. If you are building small, check your own state before you buy the tank, not after.

Work out your own capacity with the tank size calculator, and see whether your soil supports a trench at all with the which system calculator. The cost calculator prices the whole system, and what a septic system really costs breaks the quote into its eight lines. Once it is running, the pump-out guide explains the sludge line, and septic safe products explains what closes a trench from the laundry.