In wastewater treatment, the biological conversion of ammonia to nitrate (nitrification) is a two-step process performed by Ammonia-Oxidizing Bacteria (AOB) and Nitrite-Oxidizing Bacteria (NOB). Because these organisms are chemolithoautotrophic and have slow growth rates, they are highly susceptible to inhibition by various influent compounds.
Nitrogenous Compounds (Substrate Inhibition)
Both AOB and NOB can be inhibited by their own substrates or products, specifically in their unionized forms: Free Ammonia (FA) and Free Nitrous Acid (FNA).
- Free Ammonia (NH3): AOB Inhibition: Typically begins at 10–150 mg/L.
- NOB Inhibition: Much more sensitive; starts at 0.1–1.0 mg/L.
- Note: This sensitivity difference is often exploited in “sharon” or “shortcut” nitrogen removal to bypass NOB.
- Free Nitrous Acid (HNO2):
- AOB Inhibition: Roughly 0.1–0.5 mg/L.
- NOB Inhibition: Roughly 0.02–0.2 mg/L.
Heavy Metals
Heavy metals interfere with the enzymatic activity (such as ammonia monooxygenase) or disrupt the cellular membrane. NOB are generally considered more robust against metals than AOB, though both are impacted at high concentrations.
| Metal | Critical Concentration (Approx. 50% Inhibition) | Target Organism |
| Copper (Cu) | 1.0 – 5.0 mg/L | Both (AOB more sensitive) |
| Zinc (Zn) | 5.0 – 10.0 mg/L | AOB |
| Nickel (Ni) | 0.25 – 2.0 mg/L | Both |
| Cadmium (Cd) | 2.0 – 5.0 mg/L | AOB |
Organic Compounds & Specific Inhibitors
Industrial influents often introduce xenobiotic compounds that are toxic to nitrifiers.
- Phenols: Concentrations as low as 2–5 mg/L can cause significant respiratory inhibition in AOB.
- Sulfur Compounds: Hydrogen sulfide (H2S) is a potent inhibitor. Concentrations above 0.5 mg/L can drastically reduce nitrification rates.
- Cyanide (CN–): Extremely toxic; even 0.1–0.5 mg/L can lead to a complete collapse of the nitrifying population.
- Thiourea: Often used in laboratory settings as a specific AOB inhibitor; influent concentrations above 0.1 mg/L are critical.
Operational Factors Influencing Toxicity
The “critical level” is rarely a fixed number because the toxicity of these compounds is heavily dependent on:
- pH: Determines the fraction of NH3 vs. NH4+ and HNO2 vs. NO2–.
- Temperature: Higher temperatures generally increase the toxicity of unionized compounds.
- Sludge Age (SRT): Systems with a longer Solids Retention Time can sometimes acclimate to higher levels of inhibitors.
Technical Note: Because NOB are generally more sensitive to NH3 and HNO2 than AOB, unstable systems often experience nitrite buildup, which can be toxic to other aquatic life if discharged.