A free process control resource from Wastewater Solutions, Inc.help@ActivatedSludge.com  |  503-701-2766
ActivatedSludge.comPractical wastewater process control

Denitrification

In an anoxic zone, bacteria breathe nitrate instead of oxygen and release nitrogen gas. Done on purpose, it removes nitrogen and saves alkalinity and air. Done by accident in a clarifier, it floats sludge.

How it works

Many heterotrophic bacteria can use nitrate in place of oxygen when DO is absent. They oxidize organic carbon and reduce nitrate (NO3-) through nitrite to nitrogen gas (N2), which escapes to the air.

Denitrification and the MLE processNitrate is reduced stepwise to nitrogen gas. In a Modified Ludzack-Ettinger layout, an internal recycle returns nitrate from the aerobic zone to an anoxic zone at the front where influent carbon drives denitrification.NO₃⁻NO₂⁻NON₂ON₂ gasHeterotrophs use nitrate instead of oxygen when DO is near zero and carbon (BOD) is availableReturns 3.57 mg/L alkalinity per mg/L nitrate-N and saves 2.86 lb O₂ per lb NAnoxic zoneDO < 0.2 mg/L, mixedAerobic zonenitrificationClarifierInternal recycle (nitrate), 200 to 400% of QRAS
Denitrification in an anoxic zone: nitrate from internal recycle and carbon from the influent are converted to nitrogen gas.

What it needs

  • Nitrate: from nitrification, returned to the anoxic zone by internal recycle or RAS.
  • No DO: DO in the anoxic zone should be near zero (below about 0.2 to 0.3 mg/L). Oxygen carried in by recycle from a highly aerated zone robs the zone.
  • Carbon: readily degradable BOD from the influent. A common rule of thumb is about 4 to 6 lb BOD per lb nitrate-N removed. When influent carbon is short, plants add methanol, glycerin or other carbon.
  • Mixing: submersible mixers keep solids suspended without adding air.
  • Time: anoxic detention long enough for the reaction.

Credits

ItemValue per lb (mg/L) nitrate-N denitrified
Alkalinity recovered3.57 lb (mg/L) as CaCO3
Oxygen credit2.86 lb O2 saved

Denitrifying about half the nitrate formed recovers about a quarter of the alkalinity used in nitrification and cuts total oxygen demand significantly.

The MLE process

The Modified Ludzack-Ettinger (MLE) process places an anoxic zone ahead of the aerobic zone. Influent provides carbon; an internal recycle pump returns nitrified mixed liquor (often 2 to 4 times influent flow) from the end of the aerobic zone. RAS also carries some nitrate.

White River WRRF is an MLE plant: each of four trains has a two cell anoxic zone (1.21 MG total) with mixers, followed by a three grid aeration basin. Internal recycle pumps (8.0 MGD each) run 200 to 400% of influent flow, with a target of less than 1.0 mg/L nitrate-N at the end of the anoxic zone.

Theoretical nitrate removal in an MLE depends on total recycle: with internal recycle ratio IR and RAS ratio R, the fraction of nitrate that can be returned is about (IR + R) / (1 + IR + R). At IR = 3 and R = 0.5, that is 3.5 / 4.5 = 78%. Real removal is lower because of DO carried in and limited carbon.

Common problems

  • DO carryover into the anoxic zone from high DO at the end of aeration. Lower the last grid DO setpoint if ammonia allows.
  • Not enough carbon, especially after strong primary treatment or in dilute wet weather flow.
  • Too much recycle, which shortens anoxic time and carries more DO.
  • Poor mixing, solids settling in corners.
  • Rising sludge in clarifiers when nitrate is high and blankets are deep. See the rising sludge guide.

Checking performance

Measure nitrate at the end of the anoxic zone, end of aeration and effluent; ORP in the anoxic zone (often -50 to -150 mV); and DO in the recycle. If the anoxic zone ends with high nitrate and low ORP is not reached, look at DO and carbon first.