How long does my absorption trench have to be?
The AS/NZS 1547 formula, with the real design loading rate for your soil category.
Trench length is not a guess and not a quote. It is an equation: L = Q ÷ (DLR × W). Daily flow divided by the design loading rate of your soil, divided again by the trench width.
The only number you cannot read off a table is your soil category, and that comes from a land capability assessment. Everything else is here. Change the soil category and watch the same household need three times the trench — which is why the soil report, not the tank, decides your budget.
From your land capability assessment. If you do not have one, this is the number to go and buy.
The design loading rates below are the AS/NZS 1547:2012 values as reproduced by EPA Victoria and Central Goldfields Shire. Sands and weakly structured sandy loams (Categories 1 and 2a) are not permitted for primary effluent unless there is no high perched or seasonal watertable — the soil drains too fast to treat anything.
At the other end, light clays that are weakly structured or massive (5c) and medium to heavy clays (6) are not recommended for primary effluent at all. If that is your soil, the conversation is about secondary treatment, not about a longer trench.
A commercial Queensland guide quotes a maximum DLR of 25–30 mm/day for primary effluent. Government tables and AS/NZS put similar soils at 8–15 mm/day. Use the conservative number: an under-length trench fails silently, years later.
Whatever length you land on, AS/NZS 1547 §4.2.3.4 expects a 100% reserve area — a duplicate patch of land, the same size, kept free of traffic and buildings so the first one can be rested if it fails.
FAQ
What is a design loading rate?
The number of millimetres of effluent per day that a square metre of your soil can accept and treat. It comes from soil texture and structure, and AS/NZS 1547 tabulates it by soil category. A well-structured sandy loam takes 15 mm/day of primary effluent; a moderately structured light clay takes 3.
Why does the same house need a much longer trench on clay?
Because L = Q ÷ (DLR × W) and only the DLR changed. Four people at 150 litres a day on a 600 mm trench need about 67 m at DLR 15, and about 200 m at DLR 5. The tank is identical. The land is not.
What is a reserve area and do I really need one?
A duplicate land application area of the same size, held in reserve so the first can be rested if it fails. AS/NZS 1547 §4.2.3.4 generally requires 100% for primary effluent absorption systems, and Victoria, Camden, Clarence Valley, Douglas Shire and Sorell all require it. One technical guideline suggests 50%, which contradicts the standard.
Where do I find my soil category?
In a land capability assessment, done by a soil scientist or wastewater consultant. Councils in Victoria require one; most councils elsewhere will ask for it with a new installation. It is the cheapest document in the project and the only one that changes the answer.