Until quite recently, a classic, two-stage (mechanical and biological) treatment process was used at the wastewater treatment plants in St. Petersburg.

• Mechanical treatment is designed for wastewater clarification. This block comprises an inlet chamber, mechanized screens, grit channels and primary clarifiers.
• The biological treatment block includes aeration tanks and post-precipitation.

The biological treatment process occurs due to vital activities of activated sludge in aeration tanks in continuous contact with atmospheric oxygen injected into the aeration tank. Activated sludge is a biocenosis inhabited by different bacteria, protozoa and multicellular organisms which transform contaminants in wastewater and treat them.

Nevertheless, the combination of only these two blocks did not provide the quality of treated effluents stipulated in HELCOM (Convention on the Protection of the Marine. Environment of the Baltic Sea) recommendations concerning nutrients – total nitrogen and total phosphorus (when entering the Baltic Sea water they create a nutrient medium for blue-green algae, that take in oxygen from water and cause the death of living organisms in the ponds).

That is why chemical and biological wastewater treatment – a combination of enhanced removal of nutrients by means of biological treatment and chemical phosphorus precipitation - is widely implemented at the wastewater treatment plants of Vodokanal. Iron sulfate is used as a chemical.

Since 2010 HELCOM has introduced more stringent standards for nitrogen and phosphorus concentrations in the effluents - 10 mg/l and 0.5 mg/l, respectively. In 2008 Vodokanal already managed to achieve phosphorus concentrations lower than 0.5 mg/l and nitrogen concentrations – 8 mg/l in the treated effluents at the South-West WWTP.

One more level of wastewater treatment is UV disinfection of treated effluents. In 2008 this technology was applied at the South-West WWTP and Sestroretsk WWTP.