Recent developments of the mainstream anammox processes: Challenges and opportunities

Anaerobic ammonium oxidation (anammox) processes are gaining attention owing to their advantages over the conventional biological nitrogen removal processes. Although technologies related to sidestream anammox processes are being adopted as the latest advanced solutions, the application of these processes to mainstream wastewater is still under investigation. Here, we have comprehensively reviewed the challenges and opportunities of mainstream anammox processes. Mainstream processes face a variety of challenges that limit their stable operation such as the difficulty of out-selection of nitrite-oxidizing bacteria, high organic carbon to nitrogen (C/N) ratio, retention of anammox bacteria, and the influence of high concentrations of ammonia and nitrite compounds. For widespread applications of mainstream anammox processes in full-scale reactors in wastewater treatment plants (WWTPs), efficient strategies are necessary to manage high carbon to nitrogen ratios, improve performance in low-intensity wastewater, and retain anammox bacteria. Key elements in the design, operation, and maintenance of mainstream anammox processes in full-scale WWTPs are also suggested. Overall, this review provides a critical perspective on the challenges and opportunities associated with mainstream anammox processes, which will be of use to researchers and engineers in future.