Ammonia (NH₃) and its ionised form ammonium (NH₄⁺) are key species in the nitrogen cycle of wastewater treatment plants. The relative distribution between these two species is governed by the acid‑base equilibrium NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, which directly influences odor, toxicity, and the efficiency of biological nitrogen removal processes.
The position of the equilibrium is highly sensitive to both pH and temperature. As pH rises, the reaction shifts toward the un‑charged NH₃, while higher temperatures lower the acid dissociation constant (pKa), also favouring NH₃ formation. This relationship is captured by the Henderson‑Hasselbalch expression, which can be visualised in the formula band below.
pKa = temperature‑dependent acid dissociation constant (°C)
Understanding this equilibrium enables engineers to predict the concentration of free ammonia, which is the toxic species for many microorganisms, and to design pH‑adjustment or temperature‑control strategies that optimise nitrogen removal while minimising odor emissions.
How does pH affect the equilibrium between NH₃ and NH₄⁺?
What happens to the equilibrium when temperature rises?
Why is this equilibrium important in wastewater treatment?
Can you explain the chemical reaction involved?
What factors determine the position of this equilibrium?
Results are for informational purposes only and do not constitute professional advice.