Nitrite pathway in A²/O WWTPs: Modelling organic matter reduction, operational cost and N₂O emissions

Deciding whether the implementation of the nitrite pathway is useful for the treatment of a given wastewater depends on several criteria such as organic matter availability, N and P removal, N₂O emissions and operational costs. This work is a simulation-based study with a conventional Anaerobic/Anoxic/Oxic (A²/O) WWTP where the nitrite pathway can be implemented. The outcomes are general correlations to calculate the minimum COD requirements for a certain influent and a practical decision tree on the opportunities of nitrite pathway in A²/O WWTPs as a function of the influent wastewater composition (for P and N concentrations in the ranges 3–11 mgPO₄ ⁻³-P·L⁻¹ and 20–60 mg NH₄ ⁺-N·L⁻¹). This study shows that the implementation of the nitrite pathway reduces the COD requirements (depending on the influent, between 9 and 68%). It also can lead to a reduction in aeration costs. For an equivalent COD in the influent, the implementation of the nitrite pathway compared to the nitrate pathway results in a reduction of 41-47% in aeration costs and similar sludge production, leading to a reduction in the operational cost index of 10-16%. However, it is essential to note that this strategy can also lead to increased N₂O emissions, with an emission factor for the nitrite pathway in the range of 2.5–17 times that of the nitrate pathway.