Revealing roles of immobilization in microalgae-bacteria symbiosis system for nutrient removal from wastewater

Limited information is available on immobilization roles in the microalgae-bacteria system for pollutant removal. In this work, nutrient removal performances and pathways were investigated in suspended microalgae-bacteria, immobilized microalgae-bacteria and co-immobilized microalgae-bacteria systems. Alginate immobilization enabled efficient biomass recovery with remarkable settling velocity at 1.88 cm s^-1. Co-immobilization achieved the highest total nitrogen (TN) removal (50 %) despite the lowest ammonium removal (84 %). Bacterial nitrification dominated ammonium removal, despite contribution variations from 58 % in suspended group to 70 % in immobilized microalgae-bacteria due to decreasing microalgal assimilation, and to 51 % in co-immobilized group ascribed to encapsulation-induced mass transfer limitation. Co-immobilization enhanced bacterial denitrification (17 %) for TN removal. All systems achieved complete phosphorus removal via microalgal assimilation (>90 %), despite slight chemical oxygen demand increase in co-immobilized group originating from polymer hydrolysis. Considering lower separation cost ($0.47 m^-3) and comparable stability, immobilization would provide alternatives realizing efficient wastewater treatment and resource recovery simultaneously.