Engineering
Nitrification
Biological oxidation of ammonia to nitrate. The critical process for meeting nitrogen discharge limits in secondary treatment.
Nitrification Biology
Nitrification is a two-step autotrophic process where ammonia-oxidizing bacteria (primarily Nitrosomonas) convert ammonium (NH₄⁺) to nitrite (NO₂⁻), and nitrite-oxidizing bacteria (primarily Nitrobacter) convert nitrite to nitrate (NO₃⁻).
Nitrifiers are slow-growing organisms with doubling times of 12–36 hours (compared to 2–6 hours for heterotrophic bacteria). This means nitrification is typically the rate-limiting biological process and dictates the minimum SRT required.
Key Parameters
Dissolved Oxygen
Minimum for reliable nitrification
SRT
Temperature-dependent; longer at <12°C
pH Range
Optimal for nitrifying bacteria
Alkalinity Consumed
Per mg NH₄-N oxidized
Temperature Dependency
Nitrification rate approximately halves for every 8–10°C drop in temperature. At 10°C, the required SRT may exceed 25–30 days. This is often the critical design condition for plants in temperate climates.
Engineering Note
Inhibition
Nitrifying bacteria are sensitive to several inhibitory conditions:
Low pH (<6.5) — reduces nitrification rate significantly
Free ammonia (>10–150 mg/L at high pH) — inhibits Nitrobacter
Heavy metals — copper, zinc, nickel at low concentrations
Organic toxicants — industrial discharges, solvents, biocides
Low dissolved oxygen — nitrifiers outcompeted by heterotrophs at <1.0 mg/L
Nitrification Troubleshooting
| Condition | Potential Issue | What to Investigate |
|---|---|---|
| Effluent NH₄-N > 5 mg/L | Incomplete nitrification | Check SRT, DO profile, temperature, alkalinity |
| SRT < 10 days at 15°C | Nitrifier washout risk | Reduce WAS rate, verify SRT calculation |
| DO < 1.5 mg/L in aeration basin | Oxygen limitation | Increase blower output, check diffuser condition |
| pH < 6.8 | Alkalinity depleted | Monitor alkalinity; consider lime or soda ash dosing |
| Nitrite accumulation > 1 mg/L | Partial nitrification | Check free ammonia, DO, Nitrobacter inhibition |
Frequently Asked Questions
Nitrification is the two-step biological oxidation of ammonia. Ammonia-oxidising bacteria (such as Nitrosomonas) convert ammonium to nitrite, and nitrite-oxidising bacteria (such as Nitrobacter) convert nitrite to nitrate. It is the key process for meeting ammonia and nitrogen discharge limits.
Nitrifiers are slow-growing and highly temperature-sensitive. The nitrification rate approximately halves for every 8–10°C drop in temperature, so at around 10°C the required SRT can exceed 25–30 days. Plants in temperate climates are often designed for this winter condition; too short an SRT causes nitrifier washout.
A dissolved oxygen concentration of at least about 2.0 mg/L in the aeration basin is generally needed for reliable nitrification. Below roughly 1.0 mg/L, nitrifiers are outcompeted by faster-growing heterotrophic bacteria and ammonia removal deteriorates.
Nitrification consumes alkalinity — about 7.14 mg of alkalinity (as CaCO₃) per mg of ammonia-nitrogen oxidised. If alkalinity is depleted, pH falls below the optimal 6.8–8.0 range and nitrification slows. Monitoring alkalinity and, if needed, dosing lime or soda ash helps maintain stable performance.
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