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

Nitrification

Nitrifying bacteria turn ammonia into nitrate. They need oxygen, alkalinity and time, and they slow down sharply in cold water.

The two steps

Nitrification pathwayAmmonium is oxidized to nitrite by ammonia oxidizing bacteria and then to nitrate by nitrite oxidizing bacteria. The process needs oxygen, alkalinity, adequate SRT and temperature.NH₄⁺ammoniumNO₂⁻nitriteNO₃⁻nitrateAOB(Nitrosomonas)NOB(Nitrobacter, Nitrospira)Needs: oxygen (4.57 lb O₂ per lb N), SRT, warm water, pH 7 to 8and alkalinity (7.14 mg/L as CaCO₃ consumed per mg/L N)Slow growers: doubling time of a day or more in cold waterWashout happens if SRT drops below what the temperature requires
Nitrification: ammonia-oxidizing bacteria convert ammonia to nitrite, and nitrite-oxidizing bacteria convert nitrite to nitrate. Both use oxygen and consume alkalinity.
  1. Ammonia oxidation: NH4+ to NO2- by ammonia-oxidizing bacteria.
  2. Nitrite oxidation: NO2- to NO3- by nitrite-oxidizing bacteria.

Normally the second step keeps up with the first and nitrite stays low (below about 0.5 mg/L). Nitrite buildup is a sign of stress: low DO, inhibition, a recent temperature drop, or a recovering population.

Key numbers

ItemValue per lb (or mg/L) of ammonia-N nitrified
Oxygen used4.57 lb O2
Alkalinity consumed7.14 lb (mg/L) as CaCO3
Biomass producedabout 0.1 to 0.2 lb VSS

What nitrifiers need

Oxygen. Rates drop below about 1.5 to 2.0 mg/L DO. In plug flow basins, provide enough DO along the whole length.

Alkalinity and pH. Keep effluent alkalinity above about 50 to 70 mg/L as CaCO3 and pH above about 6.8 for reliable rates. Low alkalinity water needs denitrification credit or chemical addition.

SRT. Nitrifiers grow slowly. As a rough guide, aerobic SRT of 3 to 5 days may be enough at 20 C; 8 to 12 days or more may be needed at 10 C, plus a safety factor. White River targets 10 to 14 days aerobic with a minimum of 8 in summer and 12 to 15 in winter.

Temperature. Growth rate roughly halves for every 8 to 10 C drop. White River's mixed liquor ranges from about 11 to 24 C over a year.

No inhibitors. Metals, cyanide, some solvents, high free ammonia or free nitrous acid, and some disinfectants inhibit nitrifiers at low concentrations.

Worked example: ammonia load and oxygen

White River design: 8.0 MGD primary effluent, TKN 36 mg/L. Allowing for nitrogen used in cell growth, assume 30 mg/L ammonia-N is nitrified.

Ammonia nitrified = 8.0 x 30 x 8.34 = 2,002 lb/d

Oxygen for nitrification = 2,002 x 4.57 = 9,148 lb/d

Alkalinity consumed = 30 x 7.14 = 214 mg/L. White River's influent alkalinity of 260 mg/L plus recovery in the anoxic zones keeps residual alkalinity comfortable.

Keeping it stable

  • Set SRT by season and build inventory before cold weather.
  • Hold DO through the peaks; check end of basin DO.
  • Trend effluent alkalinity and pH weekly (daily for soft water plants).
  • Manage centrate and other high ammonia returns.
  • Profile ammonia along the basin when in doubt.
  • Know your pretreatment program and industrial dischargers.

When ammonia does rise, follow the high effluent ammonia guide.