Gel immobilization: A strategy to improve the performance of anaerobic ammonium oxidation (anammox) bacteria for nitrogen-rich wastewater treatment.

•Cell immobilization maintain a critical mass of bacterial cells in the system.•The property, behavior, and immobilization process of gel materials are dissimilar.•Immobilized anammox can be avoided from the unwanted environmental conditions.•The cost and life of gel beads are main issues to commercialize cell immobilization.