They are chosen when there is no mains sewer, but the soil is unsuitable for a septic tank with a drain field or the groundwater is too high. We supply and install plants for 4–6 people from €1,750; the installation work costs from €800.
How a biological treatment plant works
Unlike a septic tank, a BIO plant does not just settle wastewater; it also treats it. This is done by activated sludge (a community of microorganisms that feed on the organic matter in the wastewater). The microorganisms need oxygen, so the plant has a compressor that blows air through aerators at the bottom of the tank.
- Primary settling As the wastewater flows in, sand, paper and heavy solids settle, and grease collects on the surface. This chamber protects the next ones from clogging.
- Aeration Air bubbles mix the water with the activated sludge, and the bacteria break down organic matter and some of the nitrogen compounds.
- Secondary settling In a calm chamber the sludge flakes settle, and the clear water flows out through the outlet.
- Sludge return An airlift (an air-driven pump) returns some of the sludge to the aeration chamber. Excess sludge gradually builds up and is pumped out during maintenance.
Some plants work with a continuous flow, others in cycles (SBR) run by a control unit: filling, aeration, settling, discharge of the treated water. Both types need an uninterrupted power supply and regular maintenance.
Bio sewage system, septic tank or holding tank
| Septic tank with infiltration | Biological treatment plant | Holding tank | |
|---|---|---|---|
| How it treats | Settles; the soil completes the treatment | Aeration with activated sludge | Does not treat, only stores |
| Where the water goes | Only into an infiltration system | Into a ditch, the ground or a storm sewer | Removed by a pumping contractor |
| Electricity | Not needed | Needed continuously | Not needed |
| Soil | Must filter (sand, gravel) | Clay is fine too, if there is an outlet | Any |
| Routine work | Sediment pumping | Maintenance and sludge pumping | Frequent full pumping |
The term “bio septic tank” is often used for a BIO plant, but an ordinary septic tank without aeration is not a biological plant, because it only settles the wastewater. When choosing, compare not just the price but also the soil and where the water will go. On the simpler solutions: holding tank or septic tank.
How to choose the capacity of a wastewater treatment plant
The capacity is chosen by the number of people living in the house permanently (the model name often contains a number: for 4, 6 or 8 people) and by the largest simultaneous load. A sauna, a large bath or weekend guests produce a lot of water at once, and in a plant that is too small this washes out the sludge.
A plant that is too large is not better either: if there is little wastewater, the bacteria lack food and treatment gets worse. Larger buildings, guest houses and groups of houses need different calculations (see treatment plants for businesses and treatment plants for villages).
Where to discharge the treated water
- Into a ditch, if there is one nearby and the discharge has been agreed with the ditch's owner or manager.
- Into the ground through a drain field, an infiltration well or tunnels, if the soil filters.
- Into a storm sewer, but only with its owner's permission.
- Through a pump chamber, if the outlet is higher than the plant.
In Riga, water discharged into the environment must meet quality requirements (BOD5, COD, suspended solids, total phosphorus and nitrogen), and samples must be tested in an accredited laboratory once every two years. That is why the plant must not only be installed correctly but also maintained regularly.
Cabinet Regulation No. 384 of 27.06.2017 applies to factory-made treatment plants with a total capacity of less than 5 m³ per day that discharge treated wastewater into the environment (point 3.1), likumi.lv/ta/id/291947. In Riga, the tests once every two years and the quality requirements are set by Riga City Council Binding Regulations No. 66 of 19.12.2018, point 5.5 and Annex 1, likumi.lv/ta/id/304471. Checked on 30 October 2026.
Plants made in Latvia and imported plants
The Latvian market has both biological treatment plants made in Latvia and imported ones. In private houses, the most common are plants from AUGUST, PURA, BIO-VIDE, Sotralentz and other manufacturers.
The choice should be driven not by the brand but by capacity, the strength of the tank body, the technology (continuous flow or SBR), the availability of spare parts and servicing, and whether the plant may be installed where groundwater is high. A neutral comparison: BIO plant comparison.
Installation step by step
- Site survey We find out the number of people, the soil, the groundwater level, the outlet location, the power supply and where the excavator and, later, the vacuum tanker will get in.
- Documents A small wastewater treatment plant (up to 5 m³ per day), as a Group I engineering structure, usually needs an explanatory memorandum accepted by the municipal building authority.
- Pit and base We dig the pit and make a compacted sand or concrete base; where groundwater is high, the plant is anchored so that it cannot float up.
- Connection We connect the inlet and outlet pipes with a fall, run a cable to the compressor and control unit, and set up ventilation.
- Backfilling We backfill with sand in layers while filling the tank with water, so that the soil pressure does not deform the tank body.
- Start-up and registration We switch on the compressor, check the aeration and the airlifts, and explain how to use the plant. The plant must be registered in the municipality's register of decentralised sewage systems.
Small wastewater treatment structures (capacity up to 5 m³/d) are listed in sub-point 94.2 of Cabinet Regulation No. 253 of 09.05.2017 “Construction Regulations for Certain Engineering Structures”; explanatory memorandum for Group I engineering structures: sub-point 6.2.1, likumi.lv/ta/id/291197; explanation of the first group: LV portāls, e-consultation of 06.12.2024. The exact list of documents is confirmed by the municipal building authority. Checked on 30 October 2026.
How much a bio sewage system with installation costs
A biological treatment plant for 4–6 people costs from €1,750, and the installation work from €800. The total depends on the digging depth and soil, the type of outlet (ditch, infiltration or pump), the length of the sewer pipe (from €25/m) and whether an old septic tank needs to be dismantled.
After that, allow for maintenance once a year (from €160) and electricity for the compressor. An example with a total: prices.
What damages a BIO plant
- Switching off the compressor: without air the aerobic bacteria die, the plant starts to smell and releases untreated water.
- Wet wipes, hygiene products and cooking oil clog the airlifts and pipes.
- Aggressive chemicals and large amounts of chlorine suppress the activated sludge.
- Connecting rainwater or drainage washes out the sludge.
- Long periods of non-use without preparation according to the manufacturer's instructions.
- Overdue maintenance: excess sludge flows out with the treated water and clogs the outlet.
Regular maintenance: biological treatment plant maintenance. If the plant raises an alarm: BIO plant alarm.
Other cases
- Converting a septic tank into a biological treatment plant, if the septic tank no longer meets the requirements.
- Treatment plants for businesses: guest houses, cafés, production sites.
- Treatment plants for villages: for several houses together.