Theoretical understanding of the optimum conditions for a mainstream granular nitritation-anammox reactor coupled with anaerobic pretreatment

Mainstream nitritation-anammox processes with anaerobic pretreatment may provide a paradigm-shifting strategy for achieving more energy-efficient and cost-saving wastewater treatment, and most efforts in this area were through bench studies. To better understand the performance of bench systems and make possible predication of key parameters beyond experiments, a mathematical model was developed in this study. The model inputs include hydraulic retention time (HRT), influent chemical oxygen demand (COD), sulfate, ammonium, and nitrate, as well as the bulk dissolved oxygen, while the outputs include the COD degradation, nitrogen removal pathways, and biomass partitioning in the reactor and along the granule radius. The simulated results suggest that the COD/N ratio of anaerobic effluent is highly related to the HRT of the anaerobic pretreatment, and it can significantly impact the fractions of AOB, NOB, anammox bacteria and denitrifiers, as well as their contribution to nitrogen conversion in the subsequent nitritation-anammox reactor. The optimal conditions for achieving a reliable and robust anaerobic pretreatment system is predicted to have an HRT _HAR of 11 h that can provide an effluent COD/N ratio of 2 for the subsequent nitritation-anammox reactor to achieve ~93% total nitrogen removal (~76% by anammox) at HRT _AMX of 8 h. The proposed model can be valuable to the optimization of the anaerobic pretreatment - mainstream anammox system.