Nitrification - inhibitors of nitrite oxidation

Nitrification, the conversion of ammonia into nitrate, is a key metabolic process in many different wastewater treatment systems.

The nitrogen cycle annotated with genes used in qPCR assays. © 2023 Paul Campbell.

The first step, the conversion of ammonia to nitrite, is carried out by ammonia-oxidizing bacteria (AOBs), which in most systems are generally members of the family Nitrosomonadaceae or the genus Nitrosomonas. The second step, the conversion of nitrite to nitrate, is carried out by nitrite-oxidizing bacteria (NOBs), with Nitrospira the most common genus. Although Nitrobacter has been well studied in the lab, in most systems it is a minor contributor to NOB activity.

When nitrite begins to creep up in a nitrifying WWTP, it's very likely that something specific is inhibiting the NOB population. We have seen this happen in a few different facilities recently, so we've been looking into potential inhibitors. I thought it would be good to gather this information into one place, along with links to references.

If you find other compounds, please let me know - I'll add them to the table.

Compound	Inhibitory Conc. (if reported)	Reference(s)
Free Nitrous Acid (FNA)	1.7 - 6.8 µM	Reference 1 Reference 2
Sodium Azide	24 µM	Reference
Hydroxylamine	< 10 mg/L	Reference 1 Reference 2
Sulfides	2.4 - 6.8 mg/L as S	Reference
Chlorate	10 - 30 mM	Reference
Chloroxylenol	65 mg/L	​Reference
Cyanate	2- 10 mM	Reference
Benzethonium Chloride	0.023 g BZC / g MLSS	Reference

Sulfide Note

NOBs appear to be more sensitive to sulfides than AOBs. The same reference cited above suggests that AOBs are inhibited by sulfides between 7.8 and 14 mg/L as S.

Quaternary Amines Note

Quaternary amines in general seem to inhibit ammonia oxidation, but NOBs are more sensitive to benzethonium chloride. This compound has been used to inhibit nitrification in shortcut nite-denite processes.