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).

Therefore, today, chemical and biological wastewater treatment is introduced at the Vodokanal wastewater treatment plants, which combines enhanced biological nutrients removal with the accompanying chemical phosphorus precipitation. Today, a chemical method for phosphorus removal has been introduced at all the city wastewater treatment plants, using aluminum sulphate, which is the most effective and economical chemical.

The wastewater chemical treatment principle is that chemical dosing results in formation of insoluble compounds of aluminum with phosphates, which are then removed from the system along with the sludge.

Wastewater chemical treatment method implementation at all the city facilities enabled sustainable compliance with the HELCOM effluent recommendations – “total phosphorus” parameter not exceeding 0.5 mg/l.

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.

The SUE “Vodokanal of St.Petersburg” facilities wastewater treatment efficiency in 2016 for household and municipal wastewater amounted to more than 97% for suspended solids and BOD; 95.4% for total phosphorus and 72.5% for total nitrogen.

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.

Wastewater treatment technologies improvement at wastewater treatment plants comprises:

  • Pilot operation of rehabilitated treatment facilities of the 2nd stage of the Northern Wastewater Treatment Plant (1st stage of the rehabilitation provided implementation of the UCT technology for nutrients enhanced removal by SWECO (Sweden)).
  • Pushkin WWTP sludge thickeners reconstruction with the implementation of the separate excessive activated sludge and raw sludge thickening technology.
  • Kolpino WWTP two secondary sedimentation tanks reconstruction.