One larger biological wastewater treatment plant (BIO plant) replaces several septic tanks or small plants, but it needs a shared network, a clear owner and a maintenance arrangement. We calculate the load, plan the network and the plant, and help put the paperwork in order in Riga and the Riga region.
When a shared plant is better than separate ones
- The houses are in a compact group, and the shared network to the plant isn't long.
- The individual plots have no room for infiltration, or the groundwater is high.
- The whole village has one suitable discharge point: a ditch or a watercourse.
- A new village is being developed by a single developer who can build the network straight away.
- The neighbours are ready to agree in writing on how to share the costs.
A shared plant also has a technical advantage: the load is more even. If one house stands empty for a while, the bacteria in the plant still get fed by the others.
Separate plants are simpler if the houses are far apart or if nobody is prepared to take responsibility for a shared plant. If a mains sewer is planned for the street soon, ask the water utility first, because connecting to the mains sewer may work out better.
How the capacity of a shared plant is calculated
Capacity is calculated in population equivalents, not in the number of houses: permanent residents, guests, seasonal occupants and non-residential buildings, if any will be connected. In manufacturers' data, the nominal flow is about 0.15 m³ per person per day. For example, the PURA-30 is designed for 4.5 m³ and the PURA-35 for 5.25 m³ per day.
Small factory-made plants comply with the EN 12566-3 standard, which covers plants of up to 50 population equivalents; in the AUGUST AT and PURA ranges, the largest models are for exactly up to 50 people. A larger village needs a custom-designed plant or several plants working in parallel.
A village usually grows, so we decide at the start how the next houses will be connected: spare capacity in the plant, space for a second plant next to it, or a network that can be extended.
The network: gravity or pumps
If the terrain allows, the houses are connected to a gravity collector with inspection chambers. The LBN 223-15 building code requires inspection chambers where the pipe changes direction, gradient or diameter, and on straight runs every 35 m for pipes up to 200 mm.
On flat ground, or if some houses are lower than the plant, you need pumps: either one shared pump station before the plant, or small stations at each house with pressure pipes. The first option has fewer units, while the second has thinner pipes and less digging.
Documents and permits
A factory-made plant with a capacity of up to 5 m³ per day is a decentralised sewage system. It is registered with the municipality, it is serviced at least once a year (unless the manufacturer specifies otherwise), and within a month of each service a copy of the document confirming the maintenance is submitted to the register.
From 5 to 20 m³ per day, if the treated water is discharged into the environment, it is a category C polluting activity. The notification is submitted to the State Environmental Service at least 30 days before the activity starts. From 20 m³ per day, a category B permit is needed. According to the manufacturers' flow data, the 5 m³ threshold lies between a 30-person and a 35-person plant.
In Riga, the effluent must meet the limit values in the binding regulations, and it must be analysed in an accredited laboratory once every two years. If the network crosses several plots, you need to agree with their owners, usually by establishing an easement. The construction documents are submitted through BIS (the Construction Information System).
Sources, checked on 30 October 2026: Cabinet Regulation No. 384 of 27.06.2017 (points 3.1, 4.3, 15.3); Cabinet Regulation No. 1082 of 30.11.2010 (point 6, Annex 1 point 8.9, Annex 2 point 6.3); Riga City Council Binding Regulations No. 66 (point 5.5, Annex 1); LBN 223-15 “Sewerage Structures” (point 73); PURA 4-50 technical passport (EN 12566-3, flows).
Who manages the shared plant
With shared plants, the most common problem isn't the technology but the fact that nobody is clearly responsible. Before construction, agree in writing:
- who owns the plant and the network (joint ownership, an association or the developer);
- who pays for the compressor's electricity and where its meter is;
- who orders maintenance and the removal of excess sludge, and keeps the maintenance log;
- how the costs are shared (by the number of people or by water meters);
- who receives the alarm (a GSM module makes this easy) and what happens in the event of a fault.
We can carry out maintenance regularly by arrangement with the responsible person, recording everything in a single maintenance log.
How we work
- Site visit and data The layout of the houses, the terrain, groundwater, the discharge point, electricity, and the number of residents now and in the future.
- Load and layout We calculate the population equivalents and the flow, and choose the plant and the network solution.
- Estimate and documents We prepare an itemised estimate and the information for the construction documents and registration.
- Construction and start-up We build the network, install the plant, start it up and adjust the air supply.
- Maintenance We agree a maintenance schedule and issue the documents for the register after each service.
How much does it cost?
The price of a shared plant comes from the calculation, so we give the total after a site visit. The price is most affected by the number of people, the length and depth of the network, the number of pumps, groundwater and the discharge solution. As a guide, use our published prices (see prices).