Glossary term
Free Nitrous Acid
Undissociated HNO2 fraction of nitrite, controlled by pH and used to assess biological inhibition in nitrification, denitrification and deammonification.
Definition
metricFree nitrous acid is the undissociated HNO2 fraction associated with nitrite in water or wastewater, controlled mainly by pH.
In wastewater treatment, free nitrous acid is used to interpret nitrite inhibition, nitrification instability, denitrification upset and sidestream deammonification control. It is not the same as nitrite nitrogen concentration. For a given nitrite concentration, lower pH increases the HNO2 fraction. Interpretation depends on nitrite reporting basis, pH, temperature where relevant, biological process state, exposure time, sampling location, analytical uncertainty and the inhibition or control decision being made.
Free nitrous acid is the undissociated HNO_2 fraction associated with nitrite in water or wastewater. It is important because small free-nitrous-acid concentrations can affect biological nitrogen-removal processes even when the nitrite concentration looks moderate.
In wastewater engineering, free nitrous acid is usually calculated from nitrite nitrogen and pH. It is used in nitrification, denitrification and sidestream deammonification reviews where nitrite accumulation or inhibition is a credible risk.
Engineering Meaning
Nitrite and nitrous acid are related through acid-base equilibrium:
A simple fraction for free nitrous acid is:
For wastewater screening near ordinary temperature, a practical value is:
Lower pH increases the undissociated HNO_2 fraction sharply.
FNA From Nitrite Nitrogen
If:
then:
The free nitrous acid concentration on an as-nitrogen basis is:
This small number can still matter because biological inhibition thresholds for nitrite-related species are often low and process-specific.
Reporting Basis
If a threshold is reported as molecular HNO_2 rather than as nitrogen, convert the basis:
Using:
gives:
Mixing as-N and as-HNO2 units can create a factor-of-3.36 interpretation error.
pH Sensitivity
At the same nitrite nitrogen concentration but:
the fraction becomes:
and:
A 0.6 pH-unit decrease raises the calculated free nitrous acid by about four times in this example.
Process Use
Free nitrous acid is relevant when nitrite accumulates. In sidestream deammonification, too much FNA can inhibit key organisms or indicate unstable nitrite control. In nitrification, nitrite accumulation may show partial oxidation or nitrite-oxidizer inhibition. In denitrification, nitrite buildup may point to carbon limitation, pH effects or step imbalance.
FNA is not a replacement for nitrite nitrogen, ammonia nitrogen, nitrate, pH and total nitrogen. It is a derived risk indicator that helps explain why the same nitrite concentration may behave differently under different pH conditions.
The operating meaning depends on the process. In a conventional nitrifying basin, elevated FNA may be a symptom of nitrite accumulation and pH depression. In a sidestream deammonification reactor, a controlled nitrite residual may be expected, but excessive FNA can destabilize the organisms that the process depends on. In denitrification, FNA-related inhibition can appear as nitrite accumulation, slower nitrate removal or incomplete nitrogen conversion.
Control Boundaries
FNA should be interpreted across a defined sampling and process boundary. A grab sample at the end of a nitrite pulse may not represent the exposure seen by biomass over several hours. A low daily composite can hide short high-FNA periods if pH drops during peak sidestream loading.
Useful control actions may include reducing nitrite accumulation, adjusting pH or alkalinity, changing aeration, smoothing sidestream return, improving carbon availability for denitrification or protecting biomass retention. The right action depends on whether FNA is the cause, a symptom or only a correlated indicator.
Interpretation Limits
The calculation is sensitive to pH because the exponent contains pH-pK_a. Small pH errors can therefore create meaningful differences in calculated FNA, especially near operating thresholds. Field pH should be measured at the same location and time as the nitrite sample whenever possible.
Temperature, ionic strength and the selected equilibrium relation can also affect precise work. For many wastewater troubleshooting decisions, the simple pK_a\approx3.25 screen is useful, but compliance reports, research comparisons or vendor guarantees should state the exact method and basis.
Validation Evidence
Useful evidence includes nitrite nitrogen, pH, temperature if a temperature-specific relation is used, ammonia nitrogen, nitrate nitrogen, total nitrogen, alkalinity, dissolved oxygen, sidestream loading, reactor configuration, biomass retention, inhibition symptoms, sampling time, laboratory method and the threshold being applied.
Validation should connect the calculated FNA to the observed process response: nitrite accumulation, ammonia removal loss, nitrate production change, denitrification slowdown, deammonification instability or recovery after pH and loading adjustment.
A strong diagnosis checks trend alignment. FNA should rise before or during the observed inhibition window, and process recovery should follow a credible change in nitrite, pH, load or exposure time. If the timing does not match, other causes such as low oxygen, insufficient carbon, toxicity, low temperature or solids loss may be more plausible.
Common Mistakes
Common mistakes include treating nitrite nitrogen as free nitrous acid, ignoring pH, comparing as-N values with as-HNO2 thresholds, calculating FNA from non-representative grab samples, assuming one inhibition threshold applies to every biomass and blaming FNA without checking oxygen, carbon, alkalinity, temperature and sampling evidence. A strong review states nitrite basis, pH, formula, concentration basis, threshold and validation status.