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.