Wastewater treatment technologies

Until quite recently, a classic, two-stage treatment was used at the wastewater treatment plants of St. Petersburg comprising two main processes:

  • mechanical treatment
  • biological treatment

Mechanical treatment is designed for wastewater clarification. This block comprises an inlet chamber, mechanized screens, grit channels and primary clarifiers.

The biological treatment includes aeration tanks and secondary sedimentation tanks. The biological treatment process occurs due to vital functions 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 microorganisms which transform contaminants in wastewater and treat them.

  • All this was found in wastewater

  • Sludge Incineration Plant at the Northern WWTP

  • At the WWTP

  • Central WWTP on Bely Island

  • South-West WWTP

Nevertheless, the combination of only these two treatment stages did not ensure 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 the water bodies living organisms).

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 Wastewater Treatment Plant.

In summer 2011, the International Project "The Clean Baltic Sea" was completed. And now St.Petersburg fully complies with new Helsinki Commission Recommendations on Baltic Sea protection – phosphorus content in total municipal treated effluent does not exceed 0.5 mg/l.

One more stage of wastewater treatment is UV disinfection of treated effluents. Nowadays this technology is applied at the South-West WWTP as well as WWTP in Sestroretsk, Repino and Petrodvorets.