The answers determine the type and volume of the septic tank and how the drain field should be built. Septic tank installation including the tank and the installation work costs from €2,200.
How a septic tank works and why it needs infiltration
A septic tank is a tank with two or more chambers. In the first, heavy solids settle and begin to break down without oxygen; in the next ones the liquid clarifies, and then it flows on. This liquid is not yet clean: the treatment is completed by the soil, where it is filtered by sand and soil microorganisms.
That is why Cabinet Regulation No. 384 allows septic tank effluent to be discharged only through a purpose-built infiltration system: a filtration field, underground drains, a sand-and-gravel filter, an infiltration trench or well, or a reed bed. A septic tank with a drain field is a single system, and both parts have to be calculated together. More: drain field.
Cabinet Regulation No. 384 of 27.06.2017, sub-point 2.3 (definition of a septic tank) and sub-point 4.4 (discharge only through an infiltration system), likumi.lv/ta/id/291947. Checked on 30 October 2026.
Types of septic tank
There are three main types of septic tank at private houses. The material and the number of chambers determine how well the septic tank settles the wastewater, how long it lasts and how complicated it is to install.
| Type | Advantages | What to watch out for |
|---|---|---|
| Plastic septic tank (polyethylene, polypropylene, fibreglass) | Factory-made, watertight, light; installed without a crane, chambers already built in | Where groundwater is high, it must be anchored so that it cannot float up; backfilling needs sand, not construction rubble |
| Concrete-ring septic tank | Withstands soil pressure, does not float up, can have a large volume | The joints must be sealed; a crane is needed; old concrete-ring septic tanks often let groundwater in and wastewater out |
| Three-chamber septic tank | Better settling, so less sediment reaches the drain field and the drains last longer | Larger and more expensive; all chambers must be accessible for pumping |
A plastic septic tank is a practical choice for a new house: it is light, watertight and comes with the manufacturer's installation instructions. More: plastic septic tank installation.
Concrete-ring septic tanks are built less often today, but they are still in use at many older houses. If the rings have crumbled, tilted or leak water, consider replacing the old septic tank.
A three-chamber septic tank can be made of plastic or of concrete rings. The third chamber gives the sediment more time to settle, so clearer liquid reaches the drain field.
What size septic tank you need
The volume is calculated from the number of people and their water consumption, not from the floor area of the house. A commonly used calculation is about 150 litres per person per day and at least three days, so that the wastewater has time to settle. This is a calculation assumption, not a standard. The final volume is set by the specific septic tank manufacturer's table and the calculation of the infiltration system.
| People in the house | Calculation (150 l × 3 days) | Typical choice |
|---|---|---|
| 1–2 | up to 0.9 m³ | 1.5–2 m³ |
| 3–4 | 1.35–1.8 m³ | 2–3 m³ |
| 5–6 | 2.25–2.7 m³ | 3 m³ |
| 7–8 | 3.15–3.6 m³ | 4 m³ or more |
By this calculation, a 3 m³ septic tank is enough for up to six people and leaves a reserve for guests or a sauna. A larger volume also means less frequent pumping: a small septic tank fills with sediment faster, and the sediment starts to flow out to the drain field.
If the house uses a lot of water at once (a bath, a jacuzzi, a sauna) or you expect the family to grow, choose the next size up.
When a septic tank is not the best choice
- Groundwater comes close to the surface in spring, so infiltration will not work and a plastic tank can be pushed up out of the ground. Solutions: septic tank for high groundwater or a biological wastewater treatment plant (BIO plant) with an outlet to a ditch.
- Clay or peat soil that water does not soak into: you need a sand-and-gravel filter or a biological treatment plant. More: septic tank in clay soil.
- A small plot with no room for a drain field: the solution is installing a holding tank or a BIO plant.
- There is a mains sewer in the street, so connecting to it is usually better value: mains sewer connection.
A comparison with a holding tank: holding tank or septic tank.
How septic tank installation works
- Site survey We assess the soil, the groundwater level, the distance to the house, the well and the boundary, the space for the drain field and access for the vacuum tanker.
- Documents Before the work starts we find out which document the building authority requires; usually it is an explanatory memorandum.
- Digging With an excavator we dig the pit for the septic tank and the trenches for the pipes and drains; if groundwater flows into the pit, we pump it out.
- Base and placement Compacted sand or a concrete slab; where groundwater is high, anchoring is needed. The tank is set level so that the chambers overflow correctly.
- Connection An inlet pipe from the house with an even fall, an outlet to the infiltration system, ventilation. If the pipes are shallower than frost depth, we insulate them.
- Backfilling We backfill a plastic tank with sand in layers while filling it with water, so that the external pressure does not crush the walls. The hatches stay above ground and accessible.
- Handover We explain how to use the septic tank and how often to pump it out. The septic tank must be registered in the municipality's register of decentralised sewage systems.
Septic tank price with installation
Septic tank installation including the tank and the installation work costs from €2,200. A 2 m³ plastic septic tank on its own costs from €800; larger and three-chamber tanks cost more. The total depends on:
- the tank volume and material;
- the digging depth, the soil and the groundwater (pumping out, anchoring);
- the type of infiltration system and the length of the drains;
- the length of the pipe from the house (from €25/m);
- pumping out and dismantling the old septic tank;
- access for machinery and restoring the surface.
All prices and calculation examples: prices.
Documents and registration
A septic tank with an infiltration system is a small wastewater treatment structure (capacity up to 5 m³ per day). For such Group I engineering structures, construction is usually carried out with an explanatory memorandum accepted by the municipal building authority; the building authority confirms the exact list of documents for the specific site.
After construction, the septic tank must be registered in the municipality's register of decentralised sewage systems; in Riga this must be done within one month of construction or commissioning. How to do it: registering a decentralised sewage system.
Sources: Cabinet Regulation No. 253 of 09.05.2017 “Construction Regulations for Certain Engineering Structures”, sub-points 6.2.1 and 94.2, likumi.lv/ta/id/291197; LV portāls, e-consultation of 06.12.2024 (small wastewater treatment structure up to 5 m³/d: first group, explanatory memorandum); Cabinet Regulation No. 384, point 15; Riga City Council Binding Regulations No. 66 of 19.12.2018, point 4.2, likumi.lv/ta/id/304471. Checked on 30 October 2026.
Special cases
- Replacing an old septic tank: pumping out, dismantling or filling in, and a new system.
- Septic tank repair: joints, lids, pipes, ventilation.
- Septic tank for high groundwater: anchoring and a raised drain field.
- Septic tank in clay soil: a sand-and-gravel filter or a BIO plant.
- Septic tank installation in winter: what to watch out for in frost.
- Sewage system for a new build: from design to registration.
- Holding tank installation and septic tank for a summer house.