Plastic septic tank installation

What matters for a plastic septic tank

• Flotation: an empty plastic tank in wet soil behaves like a boat. If the groundwater rises to the tank, without anchoring it can be pushed upwards and torn off the pipes.
• Soil pressure: plastic is strong only when it is evenly supported by sand. Stones, lumps of clay or voids in the backfill can push in or split the wall.
• Load from above: you must not drive a car over a standard plastic septic tank. If there will be a driveway above the tank, you need a load-bearing slab that transfers the load to the ground.
• Depth: the manufacturer sets the permitted backfill depth for each model. If the tank has to go deeper, a neck extension is used rather than simply piling on more soil.

How a plastic septic tank is installed

1. Location and depth The tank depth is set by how deep the pipe leaves the house and by the fall of the pipe to the tank. The location is chosen so that the vacuum tanker can get close enough.
2. Pit and base The pit bottom is levelled and covered with a compacted layer of sand or crushed stone. If the groundwater is high or the soil is clayey, a concrete anchor slab is laid.
3. Placing the tank The tank is lowered, levelled and, if necessary, strapped to the anchor slab. We check that the inlet and outlet are at the correct height.
4. Connection The connection pipe from the house and the outlet to the drain field are connected. Ventilation is through the house's soil stack.
5. Backfilling The tank is backfilled with sand in layers while it is filled with water at the same time, so that the internal and external pressure stay in balance. Compaction is done carefully without damaging the walls.
6. Drain field Drains in gravel or crushed stone with geotextile, topped with soil and lawn. The size of the field depends on the soil and the number of occupants.

The most common mistakes in plastic septic tank installation

• Backfilled with the excavated clay instead of sand, so the tank is crushed after the first winter.
• No anchoring, even though the water stands close to the surface in spring, and the tank floats up at the first pumping.
• The tank is not level: the outlet ends up higher than the inlet, and the septic tank does not flow.
• A car drives over the tank, so the cover and neck cave in.
• The outlet is connected to a ditch or land drainage without infiltration. This is prohibited, and it pollutes.

If the plot has high groundwater or clay soil, installing a plastic septic tank involves additional work: see septic tank for high groundwater and septic tank in clay soil.

Documents and registration

For a septic tank with infiltration, as a small wastewater treatment structure of up to 5 m³ per day, an explanatory memorandum to the building authority is usually enough; the exact scope of documents is set by the municipal building authority. After installation, the system has to be registered in the municipality's decentralised sewage system register; in Riga, this must be done within a month of construction or commissioning. Septic tank wastewater may only enter the environment through a purpose-built infiltration system.

How much plastic septic tank installation costs

A septic tank including the tank and installation costs from €2,200. If you already have a tank, we calculate the installation work in an estimate. As a guide: installation work for a biological wastewater treatment plant (BIO plant) is from €800, and for a septic tank with a drain field the scope of work is similar or larger. A 2 m³ tank on its own costs from €800. The price goes up with an anchor slab, pumping water out of the pit, a long connection pipe from the house (from €25/m) and a large drain field.