Converting a septic tank into a biological treatment plant

When conversion makes sense - and when it doesn't

Conversion is usually chosen when:

• the drain field is clogged with septic sludge: water stands on it and the area around it smells;
• a concrete-ring septic tank lets groundwater in or wastewater out;
• more people live in the house than when the septic tank was built;
• the plot is small, and there is no room for a new drain field;
• the municipality requires the decentralised system to be brought into compliance.

Conversion isn't needed if the septic tank is simply full; then pumping is enough. If the septic tank is sound and only the field is clogged, restoring the drain field may be enough. If there is a mains sewer in the street, connecting to the mains sewer is often the better-value option.

Maintenance differs too. In Riga, a septic tank must be pumped out at least once a year, whereas for a biological treatment plant the frequency of excess sludge removal is set by the manufacturer's instructions, and maintenance takes place at least once a year.

Three ways to do the conversion

1. A new plant in place of the old septic tank or next to it The most common and safest option. The old tank is pumped out, cleaned, and then removed or backfilled; the new plant is factory-made, with tested treatment performance.
2. The old tank as a pre-treatment or buffer tank Only possible if the tank is watertight and large enough, and the plant manufacturer allows this layout. The benefit: coarse particles stay upstream of the plant.
3. An aeration kit in the existing tank A less common solution. The tank must be watertight, with a suitable volume and compartments. We will only recommend it after an inspection, because the result depends heavily on the condition of the old tank.

What to check before the work

• The depth of the sewer outlet from the house. For some models the manufacturer sets a maximum inlet depth; if the outlet is deeper, you need a pump station.
• The groundwater level. In high groundwater, the plant is anchored to a concrete slab, and the tank must not be completely emptied during maintenance.
• Electricity. The compressor needs a permanent 230 V supply, preferably through a separate circuit breaker.
• The discharge point. The existing drain field can only be used if it still absorbs water; otherwise you need new infiltration or a discharge into a ditch, if this is permitted.
• Vehicle access. A vacuum tanker must be able to reach the plant to pump out the excess sludge.

How the conversion works

1. Site visit and choice of solution We assess the septic tank, the field, the groundwater and the load, and choose the plant model and the discharge solution.
2. Construction documents The documents are submitted through BIS; the building authority decides what type of documents are needed, depending on the plant and the site.
3. Pumping out and cleaning the old septic tank We pump out all the contents and the sludge, and clean the tank.
4. Removal or backfilling We take the tank out or backfill it with sand. We decide this based on the site and on whether the tank gets in the way of the new plant.
5. Installing the plant We prepare the base, lower the plant in, connect the pipes and backfill in layers, gradually filling the tank with water at the same time, as the manufacturers require.
6. Compressor and start-up We connect the compressor and the controls, add activated sludge and adjust the air supply. The plant reaches its full treatment performance gradually (one manufacturer states up to four months).
7. Register Changes must be reported to the decentralised sewage system register within a month. We issue the documents needed for this.

Requirements after conversion in Riga

In Riga, changes are reported to the Riga Housing and Environment Department. If the treated water is discharged into the environment, the effluent must meet the limit values in the binding regulations (for example, total phosphorus 1 mg/l, total nitrogen 10 mg/l), and it must be analysed in an accredited laboratory once every two years. That is why, for Riga, we recommend plants whose tests have also shown nitrogen and phosphorus removal.

After each scheduled or unscheduled service, the owner submits a copy of the document confirming the maintenance to the register keeper within a month. In the Riga region, the procedure is set by each municipality's binding regulations (see registering a decentralised sewage system).

How much does it cost?

The total for a conversion is made up of the plant, the installation work and decommissioning the old septic tank. The price depends on whether the tank is removed or backfilled, whether you need a pump station or new infiltration, the groundwater and the distance from the house. The full price list is on the prices page.