Side Stream Hydrolysis for WWTPs

Biological phosphorous removal

Introducing side stream hydrolysis on a part stream of the return sludge, an easily degradable carbon source is obtained for biological nitrogen and phosphorus removal.

  • The hydrolysis produces easily degradable organic material (VFA) on a small flow of the return sludge with long retention time (20-40 hours)
  • The hydrolysis activates the biological phosphorus removal process
  • The hydrolysis tank can be used for pretreatment of reject water containing high ammonia concentrations´, concentrated industrial waste water, external sludge or carbon sources (e.g. molasses)
  • The side stream hydrolysis is run with a fixed hydraulic retention time regardless of the return sludge flow to obtain a very stable process condition
  • The process has been used in full scale applications in Denmark since 1996 and is in operation on 50 Danish wastewater treatment plants.


  • A small amount of return activated sludge is introduced eliminating problems related to high content of nitrate in the return sludge
  • Reduction of operational cost as chemicals for precipitation of phosphorous is decreased
  • No internal load of phosphorous under storm water conditions
  • Less chemical sludge reduces operational costs related to excess sludge handling

About author

EnviDan is a full service environmental company offering professional solutions within the drinking water, wastewater, biogas and environment.

Primary contact
Gitte Nabe Kristensen

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