Drain field for a septic tank

How a drain field works

The liquid flowing out of the septic tank enters a distribution box, which spreads it evenly across several perforated drain pipes. The drains lie in a layer of crushed stone or gravel on geotextile, and the liquid slowly seeps through the holes into the soil.

The treatment happens in the unsaturated soil layer between the drains and the groundwater: the sand filters mechanically, and soil bacteria break down organic matter in the presence of oxygen. That is why there are ventilation pipes at the ends of the drains, and why a drain field cannot be laid in groundwater, as there is neither oxygen nor a filtering layer there.

Septic tank with a drain field: what the regulations require

Cabinet Regulation No. 384 states that wastewater from a septic tank may only be discharged through a purpose-built infiltration system. The systems listed are filtration fields, underground drains, sand-gravel filters, infiltration trenches and wells, and reed beds. A septic tank with an outlet straight into a ditch or a pipe running towards the neighbour's side does not meet this requirement.

Drain field, filtration field and biofilter

• Drain field, also called an infiltration or filtration field: perforated drains in a crushed-stone layer; the classic solution in sandy soil.
• Infiltration tunnels (chambers): plastic arches with a large internal volume; they take up less space than drains.
• Filtration well: a well with a gravel filter at the bottom; suitable only for a small flow, for example a summer house or a sauna.
• Sand-gravel filter: an artificially built-up filtering layer used where the natural soil is clay; the treated water is collected in drains and led further away.
• Septic tank with a biofilter: a module with filter media installed after the septic tank, on which a bacterial film forms; it gives the water additional treatment before infiltration.

Is your soil suitable for infiltration

Sand and gravel let water through well, loamy sand more slowly, and clay and peat hardly at all. The soil's filtration capacity is checked with a simple test: dig a hole at drain depth, fill it with water and measure how fast the level drops. The drain length depends on the result.

The second condition is groundwater. What matters is not the summer level but the highest spring level: a sufficiently thick unsaturated soil layer must remain beneath the drains. If the groundwater is high, the field is raised on a mound or a different solution is chosen: a septic tank for high groundwater, a septic tank in clay soil or a biological wastewater treatment plant (BIO plant) with an outlet to a ditch.

Size and location

• The drain length is set by the amount of wastewater and the soil's filtration capacity; it is calculated in the design or from the septic tank manufacturer's table.
• The field must be in a place where cars do not drive and no building is planned, because compacted soil does not let water through.
• Keep it as far as possible from trees and shrubs: roots grow into the drains.
• Distances to a drinking water well, the house and the boundary must follow the municipality's building regulations and the Protection Zone Law. We check them before the work starts.
• The liquid from the septic tank must flow to the field with a fall; if the field has to be placed higher, you need a pump.

How the field is built

1. Survey and test Soil filtration test, spring groundwater level, location for the field and distances.
2. Trenches We dig trenches or an area to the required depth and do not level the bottom with heavy machinery, so that it is not compacted.
3. Base Geotextile and a layer of crushed stone or gravel.
4. Drains and distribution box Perforated drains with a slight fall, a distribution box for an even flow, ventilation pipes at the ends of the drains.
5. Backfilling Crushed stone over the drains, geotextile, soil and lawn. We mark the field on the site plan so that nothing is built over it later.

Why a drain field clogs

A field is clogged by what should never get into it: sediment and grease from a septic tank that is not pumped on time. Connecting rainwater, compaction by machinery, tree roots and a field that is too small also do damage.

• Wet, greener or sunken patches above the field.
• A smell in the garden.
• The liquid level in the septic tank is above the outlet pipe.
• The septic tank fills up faster than it used to.

What we do: we pump out the septic tank, check the distribution box and the drains, and flush them. If the crushed-stone layer and the soil around it are completely clogged, the field is rebuilt next to the old one. If the septic tank fills up quickly, see septic tank fills up fast.

How much a drain field costs

We calculate the price of a drain field from the drain length, the soil, the amount of digging and the materials, so there is no single starting price for it. As a guide: septic tank installation including the tank and the works from €2,200, sewer pipes from €25/m, septic tank pumping before the field is renewed from €75.

The whole system together: septic tank installation. All prices: prices.