Three aerobic granular reactors with different mixed liquor suspended solids concentration (about 6432 mg L^-1, 9053 mg L^-1, 11840 mg L^-1) were operated to assess the influence of biomass concentration on efficiency of tetracyclines and sulfonamides removal, and on the abundance of corresponding antibiotic resistance genes in the sludge. Results indicated that biomass concentration did affect effluent concentration of sulfonamides, yet no significant difference in tetracyclines removal was observed among the three reactors. Accumulated concentrations of all antibiotics had significant negative significant correlations with biomass concentration. Accumulated concentrations of total tetracyclines and total sulfonamides in sludge from the three reactors were 9.62 μg g^-1 and 1.13 μg g^-1, 6.74 μg g^-1 and 0.70 μg g^-1, and 3.70 μg g^-1 and 0.30 μg g^-1, respectively. Among investigated antibiotic resistance genes, only tetA had a significant negative correlation with biomass concentration, and the relative abundances of tetA in the three reactors at the end of this experiment were 5.75 × 10^-2, 4.34 × 10^-2 and 3.86 × 10^-2, respectively. Results from this study suggested that biomass concentration is key operational parameter of granular reactors that can affect antibiotic removal efficiency, but it was not a major influencing factor in controlling abundance of antibiotics resistance genes in sludge.

Relative abundance of tet genes and sul genes in the granular samples on 30^th day and 50^th day.