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 (A(2)/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 A(2)/O WWTPs as a function of the influent wastewater composition (for P and N concentrations in the ranges 3-11 mgPO(4-)(-3)-P center dot L-1 and 20-60 mg NH4+-N center dot 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.

Automated summary

Deciding whether the nitrite pathway is useful for wastewater treatment depends on criteria such as organic matter availability, N and P removal, N2O emissions, and operational costs. This study simulated the implementation of the nitrite pathway in a conventional A(2)/O WWTP, providing correlations to calculate minimum COD requirements and a decision tree for its opportunities. The results show that the nitrite pathway can significantly reduce COD requirements (9-68%), aeration costs (41-47%), and operational costs (10-16%), but it may also lead to increased N2O emissions (2.5-17 times higher than the nitrate pathway).