A critical review of the mainstream anammox-based processes in warm climate regions: Potential, performance, and control strategies

Conventional biological nitrogen removal (CBNR) process is widely employed due to its efficiency. However, its high operating and capital costs have led to the exploration of more cost-effective solutions. The anaerobic ammonium oxidation (anammox) pathway has proven to be a sustainable technology for energy-neutral wastewater treatment plants (WWTPs). This pathway can occur through two approaches: partial nitritation-anammox (PN/A) and partial denitrification-anammox (PD/A). Nevertheless, large-scale application of these processes in mainstream municipal WWTPs is scarce. This review evaluates the mainstream PN/A and PD/A processes for their suitability as biotechnological solutions while identifying potential challenges. Key process parameters, including the carbon/nitrogen ratio, free ammonia, and hydraulic retention time, were evaluated to identify adequate conditions for achieving stable and high-efficiency nitrogen removal. In comparison to the CBNR process, PN/A and PD/A processes demonstrate significant reductions of 54 % and 36 % in aeration, with corresponding sludge generation reductions of 83 % and 62 %, respectively. Although nitrite accumulation appears to be more feasible and stable in PD/A against PN/A, these results indicate that the PN/A process is more economical and environmentally sustainable than PD/A. It has been shown that reducing COD beforehand is crucial for both deammonification processes, as it decreases competition between anammox bacteria and heterotrophic denitrifiers for nitrite. In warm climate regions, this can be achieved through integration of the deammonification process with anaerobic technology consolidated in mainstream WWTPs. Overall, this critical review comprehensively addresses the outcomes of PN/A and PD/A processes, contributing to the understanding of these alternatives and addressing gaps in warm regions.