Drainage field
Also called a soakaway or infiltration field
What it is & how it works
A drainage field is not a treatment unit — it is the part that disperses the water back to the environment. It is a set of perforated pipes laid in gravel-filled trenches, spread out underground.
Liquid from a septic tank (or a treatment plant) trickles out through the pipes and soaks into the surrounding soil. Bacteria in the top layers of soil provide the final stage of treatment before the water reaches groundwater.
Strictly, a "soakaway" is for rainwater and a "drainage field" is the version designed for sewage effluent — but people often use the words interchangeably. It is the most common place for an off-mains system to fail, almost always because the tank upstream wasn't looked after.
The rundown
At a glance
- Size & capacity
- Sized from a percolation test — digging trial holes to measure how fast the soil drains — together with the number of people. Poorly-draining or waterlogged ground may not be suitable at all.
- Coping with busy periods
- Surges and any solids escaping the tank overwhelm it and form a clogging "biomat" that stops the soil absorbing water.
- Bleach & chemicals
- Harsh chemicals from the house pass through the tank and can harm the soil bacteria the field relies on.
- Wipes, sanitary items & fats
- Very vulnerable. Solids, fats and wipes that escape a neglected tank are the number-one cause of a blocked, failed drainage field.
- Land footprint
- Large. It needs an open area of suitable soil, kept at least 10 m from any watercourse, 50 m from a well or borehole, and 15 m from a building.
- Water quality & where it goes
- This is where the final treatment happens — a working field is what makes a septic tank acceptable. A failed field means raw-ish effluent surfacing or reaching water.
- Care & servicing
- Protect it: no driving, parking, building, paving or tree-planting over it, and keep the upstream tank de-sludged so solids never reach it. Where possible, rest or rotate fields.
- Signs it is failing
- Boggy, wet or unusually lush ground, effluent surfacing, smells, and the whole system draining slowly because the water has nowhere to go.
Compare
Other systems
Septic tank
An underground tank that settles out solids, then sends the liquid to a drainage field to soak away.
Learn more → Treats the wastePackage treatment plant
Like a septic tank, but with a powered stage that fully treats the waste — clean enough to discharge to a watercourse.
Learn more → Stores onlyCesspool
A sealed tank that just stores sewage with no treatment and no outlet — it has to be emptied by tanker.
Learn more → Treats the wasteFilter / clinker bed
A bed of stone or clinker that settled water trickles over, where bacteria on the surfaces clean it — often added after a septic tank.
Learn more → Treats the wasteReed bed
A planted gravel bed where reeds and the life around their roots polish the water — low-energy, with a wildlife benefit.
Learn more →