Nitrite lock occurs when the ammonia (NH₄) doesn’t complete the full nitrification process, which typically involves the following steps:
- Ammonia Oxidation (NH₄ → NO₂): Ammonia is oxidized to nitrite by ammonia-oxidizing bacteria (AOB).
- Nitrite Oxidation (NO₂ → NO₃): Nitrite is further oxidized to nitrate by nitrite-oxidizing bacteria (NOB).
High nitrite levels can lead to issues like:
- Interference with disinfection processes
- Violations of effluent bacteria limits
- Whole effluent toxicity (WET) failures
- Effluent pH violations
In most cases, nitrite conversion to nitrate is not the rate limiting steps in full nitrification. However, under certain conditions (such as a poorly established or inhibited NOB population), the nitrite concentration can increase.
Causes of Nitrite Lock
- Free Ammonia (FA) & Free Nitrous Acid (FNA) levels
- FA inhibits NOB at 0.1 – 1.0 mg/L
- FNA inhibitions NOB at 0.22 – 2.8 mg/L
- pH – FA more toxic to NOB, so pH < 8.0 promotes NOB growth
- Time lag between AOB & NOB populations
- BOD5/COD loading
- MCRT, DO, Alkalinity
What to do if experiencing nitrite lock
- Use FA/FNA spreadsheet to check for potential problems (to get the spreadsheet email us at info@asterbio.com)
- Make sure you are in optimal range on the key nitrification parameters (DO, Alkalinity, pH)
- Can run a qPCR check on AOB/NOB populations – but must use the correct primers for wastewater. Aster Bio has an in-house molecular lab with primers developed from data collected in hundreds of wastewater treatment facilities.
- After determining the cause, you can develop a plan to increase NOB growth rates and activity.