Investigating the conversion from nitrifying to denitrifying water-treatment efficiencies of the biofloc biofilter in a recirculating aquaculture system

In the current study, two nitrifying suspended growth bioreactor processes (denoted as R1 and R2) was investigated based on biofloc technology (BFT) in recirculating aquaculture systems (RAS). The conversion of nitrifying R1 to denitrifying biofilter via adding carbohydrates was studied. It was observed that the dominant functional microbes of nitrifying BFT biofilters were ammonia-oxidizing bacteria (AOB), denitrifying bacteria (DNB), phosphorus/glycogen accumulating organisms (PAOs/GAOs), and nitrite-oxidizing bacteria (NOB). The removal efficiencies of the total ammonium nitrogen and nitrite were prominent, while the treatment efficiencies respected to nitrate and soluble reactive phosphate (SRP) were insignificant. More than 16 h after the addition of carbohydrates to R1, the regular dynamics of dissolved oxygen (DO), dissolved organic carbon, and C/N ratio became conducive to the notable removal of nitrate and SRP. Furthermore, the relative abundances of AOB and NOB decreased, and that of PAOs, GAOs, and DNB increased. Redundancy analysis indicated that, among the environmental factors and functional microbes involved in converting to a denitrifying biofilter, DO was the most important. Also, Spearman correlation heatmap elucidated Flavobacterium sp. and Norank_Nitrosomonadaceae were the dominants bacteria affected by environmental factors. This study found a high abundance of functionally active microbes participated in nitrogen and phosphorus removal in the BFT biofilters. Thus, the water-treatment efficiency of the BFT biofilter could be flexibly and controlled by adding carbohydrates.