During maintenance we pump the excess sludge out of the settling tank, clean the blower filters, check the oxygen and set the airlifts. We work with the PURA-4, PURA-6, PURA-9 and larger models in Riga and the Riga region.
How a PURA plant works
PURA plants are made by the Lithuanian company UAB “Plastic Technology”. They are polypropylene tanks whose performance characteristics are declared under the EN 12566-3 standard (small plants of up to 50 population equivalents). The plant has four chambers:
- the anaerobic chamber (AN): here the sludge prepares the phosphorus compounds that the bacteria later take up during aeration;
- the denitrification chamber (DN): nitrates are converted into nitrogen gas;
- the nitrification or aeration chamber (N): air through a diffuser at the bottom, oxidation of organic matter and ammonium;
- the secondary settling tank (NUS): the sludge settles and the treated water leaves.
Airlifts move the sludge from the settling tank to the AN chamber and from the N chamber to the DN chamber. The airflow to each airlift, the mixers and the aeration is set with six manual valves. The blower is either built into the lid (standard version) or housed in a separate container up to 3 m from the plant (small-lid version, -MD).
| Model | Flow | Organic load | Residents |
|---|---|---|---|
| PURA-4 | 0.60 m³/d | 0.24 kg BOD5/d | 2–4 |
| PURA-6 | 0.80 m³/d | 0.36 kg BOD5/d | 4–6 |
| PURA-9 | 1.35 m³/d | 0.54 kg BOD5/d | 6–9 |
The flows and loads are taken from the manufacturer's passport, and the number of residents from the Latvian distributor's page. The range goes up to the PURA-50 (7.5 m³/d).
What the PURA passport specifies for maintenance
| Frequency | Tasks |
|---|---|
| Every 2 weeks | How the blower is running (not hot, no vibration, not louder than usual), the pipes, the look of the effluent, the sludge volume after 30 min (usually 200–400 ml/l) |
| Once a month | Electrical wiring, airlifts (cleaned if needed), the level of settled sludge in the settling tank (the clear water layer must be at least 0.5 m) |
| Excess sludge | At least every six months, ideally every two months |
| Once a year | Professional maintenance with the plant switched off |
| In total | Servicing at least 2 times a year |
The manufacturer intends the fortnightly and monthly checks to be done by the owner. If that isn't convenient for you, we can agree on more frequent visits, for example excess sludge removal every couple of months and full maintenance twice a year.
How our maintenance works
- Assessment We open the hatches and assess the smell, the colour, any foam and how clear the effluent is.
- Blower off, 30 minutes We switch off the blower and wait for the sludge to settle.
- Excess sludge removal We pump the excess activated sludge out of the secondary settling tank, leaving about 20% of the sludge so that the process can continue.
- Blower filters We clean the filters and check the blower; if needed, we replace worn parts.
- Adjustment We switch the blower back on and use the valves to set the airlifts and the mixing. The oxygen in the aeration chamber should be about 2 mg/l. We measure it with a portable oxygen meter.
- Log and document We record the work in the plant's inspection log and issue a document, a copy of which must be submitted to the decentralised sewage system register within a month.
What harms a PURA plant
- Wet wipes, hygiene products, paper towels, grease and oils: the passport prohibits putting these into the sewer.
- Paints, varnishes, solvents, medicines, pesticides and other toxic substances.
- Very salty water and wastewater with a pH outside 6.5–8.5.
- Industrial wastewater, if its composition isn't close to that of domestic wastewater.
If something from this list has got into the plant and a smell or cloudy effluent has appeared, an unscheduled service or a repair is needed.
Warranty and the manufacturer's conditions
The PURA passport states a 2-year warranty for the electrical part and a 10-year warranty for the tank body, provided that installation and use follow the instructions. Commissioning, adjustment and periodic maintenance may only be carried out by qualified staff, and the manufacturer publishes a list of recommended partners.
We are not the manufacturer's representatives. If the plant is still under warranty, check the seller's warranty terms before the maintenance.
Sources, checked on 30 October 2026: PURA 4-50 technical passport (manufacturer, EN 12566-3, chambers, flows and loads, valves, maintenance frequency and procedure, restrictions, warranty); the Latvian distributor's page (number of residents for the PURA-4/6/9); Cabinet Regulation No. 384 of 27.06.2017 (points 4.3, 15.3).
How much does it cost?
PURA maintenance costs from €160. The price depends on the size of the model (the PURA-9 and larger have more sludge), how much sludge has built up since the last visit, whether blower parts need replacing, and the distance outside Riga.