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Biological Nutrient Removal (BNR)

BNR plants arrange anaerobic, anoxic and aerobic zones so bacteria remove nitrogen and phosphorus as well as BOD. The zones only work if each one stays in its intended condition.

The three conditions

ZoneDONitrateMain purpose
AnaerobicnonenonePAOs take up volatile fatty acids and release phosphorus
AnoxicnonepresentDenitrification
Aerobic (oxic)presentforms hereBOD removal, nitrification, phosphorus uptake
Three-stage BNR layoutAnaerobic, anoxic and aerobic zones in series with a nitrate recycle and RAS. Phosphorus accumulating organisms release phosphorus in the anaerobic zone and take up more in the aerobic zone.Anaerobicno DO, no nitrateAnoxicnitrate, no DOAerobicDO 1.5 to 2.5ClarifierNitrate recycleRASP release, VFA uptakeDenitrificationNitrification, P uptakeA2O style layout: biological nitrogen and phosphorus removal
A typical A2O BNR train: anaerobic, anoxic and aerobic zones with internal recycle and RAS.

Nitrogen removal

Nitrogen removal combines nitrification in the aerobic zone with denitrification in the anoxic zone. See Nitrification and Denitrification. Total nitrogen limits around 8 to 10 mg/L are usually reachable with MLE; 3 to 5 mg/L typically needs a second anoxic zone (four stage Bardenpho) and often supplemental carbon.

Enhanced biological phosphorus removal (EBPR)

PAOs take up volatile fatty acids (VFAs) in an anaerobic zone, storing them and releasing phosphate. In the aerobic zone they use the stored carbon to grow and take up more phosphate than they released. Phosphorus leaves with the WAS.

EBPR needs:

  • A truly anaerobic zone: no DO and no nitrate. Nitrate in RAS is the most common problem.
  • Enough VFAs: a common guide is at least about 10 to 20 mg/L of readily degradable COD or VFA per mg/L of phosphorus removed. Fermenting primary sludge can supply VFAs.
  • Aerobic conditions after the anaerobic zone.
  • Careful solids handling: digesters and thickeners can release phosphorus back to the plant.

Common configurations

  • MLE: anoxic then aerobic, nitrogen removal. White River WRRF uses MLE with alum for phosphorus.
  • A/O (Phoredox): anaerobic then aerobic, phosphorus removal without nitrification.
  • A2O: anaerobic, anoxic, aerobic. Nitrogen and phosphorus.
  • Four and five stage Bardenpho: extra anoxic and reaeration zones for low total nitrogen (and phosphorus in five stage).
  • UCT and modified UCT: RAS goes to the anoxic zone to protect the anaerobic zone from nitrate.
  • Oxidation ditches and SBRs can create zones in time or space.

Chemical phosphorus removal

Many plants add alum or ferric to meet low phosphorus limits, alone or as a trim to EBPR. White River doses 48.5% liquid alum to the mixed liquor channel at about 35 mg/L as alum. Metal salts consume alkalinity and add sludge.

Control points

  • ORP and DO in each zone.
  • Nitrate in RAS and at the end of anoxic zones.
  • Ortho-P profile: release in the anaerobic zone, uptake in the aerobic zone.
  • VFA supply and primary sludge fermentation.
  • Recycle streams from solids handling.
  • SRT: long enough to nitrify, not so long that PAOs decline.