Effects of dissolved organic carbon and total ammonia nitrogen concentrations with the same DOC/TAN on biofloc performance

The regulation of the carbon-to-nitrogen ratio (C/N) is at the core of biofloc technology. This study compared the performance of bioflocs under different dissolved organic carbon (DOC) and total ammonia nitrogen (TAN) concentrations but the same DOC/TAN ratio. Dissolved organic carbon and total ammonium nitrogen were set at 200 mg/L and 10 mg/L (Group 200/10), 100 mg/L and 5 mg/L (Group 100/5), and 50 mg/L and 2.5 mg/L (Group 50/2.5), respectively. The TAN removal rates of groups A, B, and C were 1.89 & PLUSMN; 0.02 mg N/g TSS & BULL;h, 1.63 & PLUSMN; 0.03 mg N/g TSS & BULL;h, and 1.32 & PLUSMN; 0.02 mg N/g TSS & BULL;h, respectively, and the differences were signiflcant (P < 0.05). At the phylum level, the microbial community structure was dominated by Proteobacteria, Bacteroidetes, Actinobacteria, Verrucomicrobia, and Planctomycetes. At the genus level, Gemmobacter, Hydrogenophaga, and Microbacterium were the dominant genera. Most of bacteria community differed signiflcantly among the three groups. As the concentration of carbon and nitrogen sources increased, the Shannon index showed a decrease in bacterial diversity, and the number of heterotrophic bacteria signiflcantly increased. The crude protein content of flocs in group A ((29.22 & PLUSMN; 0.20)%) was signiflcantly lower than that in groups B ((30.56 & PLUSMN; 0.32)%) and C ((30.83 & PLUSMN; 0.20)%). In groups A and B with higher carbon and nitrogen concentrations, the copy numbers of genes related to nitriflcation were relatively low, but the copy numbers of genes related to denitriflcation were relatively high. Therefore, it can be concluded that with a ratio of 20 of DOC/TAN, increasing DOC and TAN concentrations led to a signiflcant increase in TAN removal capacity, heterotrophic bacteria count, denitriflcation function gene copies, and crude protein content of bioflocs, and a decrease in bacterial diversity, nitriflcation function gene copies, and C/N of bioflocs (P < 0.05). These results suggest that the influence of DOC and TAN concentrations on bioflocs performance may be more signiflcant than that of DOC/TAN, and should be fully considered in practical bioflocs technology systems.