A review of biomass immobilization in anammox and partial nitrification/anammox systems: Advances, issues, and future perspectives

Two biomass immobilization techniques; entrapment and carrier-based, attract increasing attention in anammox and partial nitrification/anammox (PN/A) systems. This paper provides a comprehensive review of the advances, outstanding issues, and future research directions in this field. The application of both entrapment and carrier-based biofilm immobilization for reactor start up, improving the nitrogen removal performance, and protecting autotrophic bacteria from environmental fluctuations in anammox and partial nitrification/anammox systems are summarized and discussed. The key characteristics of carriers for biomass immobilization are biocompatibility for supporting microbial growth, permeability for effective mass transfer, and physical/chemical stability for long-term use. Carriers without these characteristics must be improved and re-evaluated for their feasibility in applications. Lab-scale, pilot, and full-scale studies are needed to overcome the potential obstacles of preliminary studies, and to investigate the long-term performance of biomass immobilization techniques, especially using real wastewater as influent, which may introduce more complexity and threaten the carrier's immobilization. In addition, calculating the ‘nitrogen removal rate normalized by the packing ratio of carriers (NRR-C)’ in the immobilization system is strongly suggested to obtain a direct comparison of immobilization performance/limitations from different studies. This review will improve understanding of the major challenges of immobilization technology in anammox and PN/A systems and provide insights into the next-stage of research and full-scale applications.