Comparison of nitrogen and VFA removal pathways in autotrophic and organotrophic anammox reactors

Organotrophic anammox is a promising process for treating both nitrogen and organic containing wastewater than that of the traditional autotrophic anammox for sole nitrogen removal. However pathways of nitrogen removal particularly at metagenomic level in both processes are still unknown. Here we report, metabolic pathways of nitrogen removal in two lab-scale sequencing batch reactors (SBR), one autotrophic and another organotrophic (TOC/TN = 0.1) anammox bacteria incubated over 220 days. Both reactors showed satisfactory nitrogen removal with 840.31 mg N/L.d and 786.81 mg N/L.d for autotrophic and organotrophic anammox reactors respectively. Four anammox species namely Candidatus B. fulgida, B. sinica, J. caeni and Candidatus K. stuttgartiensis were identified in both reactors. The Candidatus K. stuttgartiensis (4%) was dominant in autotrophic reactor whereas Candidatus J. caeni (10%) in the organotrophic reactor. The supply of organic promoted the growth of anammox bacteria more than three times higher than that of the autotrophic anammox reactor. The functional genes related to the DNRA pathway was obtained in all anammox species except for Candidatus K. stuttgartiensis. The co-existence of other DNRA (Armatimonadetes and Thauera) and partial denitrifying bacteria (Chloroflexi) was also found in both reactors. Moreover, functional genes related to acetate metabolism by acetyl-CoA way were obtained in all anammox bacteria except Candidatus B. fulgida which showed alternative ackA/Pac-t pathways in organic anammox reactor. Overall current results suggest that the anammox, DNRA and partial denitrification were the key nitrogen transformation pathways, particularly in organotrophic anammox reactor. Our findings will improve understanding of the practical application of organotrophic anammox for wider wastewater treatment.