A sustainable algal-bacterial symbiosis system based on completely autotrophic nitrogen removal over nitrite: Efficient nitrogen removal, biofilm formation, and microbial analysis

A single completely autotrophic nitrogen removal over nitrite (CANON) process still faces technical challenges such as aeration energy consumption for nitrite accumulation and residual nitrate in the effluent. To address these issues, an algal-bacterial (AB) symbiosis system was developed based on CANON by adjusting light/dark cycles, wherein oxygen could be provided through algal photosynthesis during the light period, and nitrate could be eliminated via algal assimilation during the dark period. After 150 days of operation, the average total inorganic nitrogen removal efficiency of the bioreactor increased from 85.0 ± 1.3 % to 96.7 ± 3.0 % after the formation of AB biofilm. Compared to the CANON biofilm, the specific ammonia oxidation rate increased by 7.9 % in the AB biofilm, whereas the specific nitrite oxidation rate and specific anammox rate decreased by 11.9 % and 61.1 %, respectively. The protein and polysaccharide contents of extracellular polymeric substances increased by 53.5 % and 45.4 %, respectively. Typical cycles and batch experiments revealed that algal assimilation was the main pathway for nitrate removal in the AB symbiosis system. Microbial community analysis found that the algae and light had little effect on the relative abundance of ammonia-oxidizing bacteria represented by Nitrosomonas (2.6 %–2.4 %), but had certain negative effects on anaerobic ammonia-oxidizing bacteria (AnAOB) represented by Candidatus_Jettenia (11.1 %–4.2 %), Candidatus_Brocadia (3.3 %–2.3 %) and Candidatus_Kuenenia (1.0 %–0.6 %). In addition, Tetradesmus sp. (2.3 %–59.4 %) replaced Chlorella sp. (93.0 %–7.3 %) as the dominant algal genus in the AB system. The results of this study can simultaneously solve the problems of aeration energy consumption and residual nitrate in the traditional CANON process and provide a low-carbon and energy-saving biological wastewater treatment technology.