Effects of sludge retention time on water quality and bioflocs yield, nutritional composition, apparent digestibility coefficients treating recirculating aquaculture system effluent in sequencing batch reactor

•Three days was the most advisable SRT for treating RAS effluent with SBRs by biofloc technology.•Bioflocs yield could be as high as 2.30–2.54 g VSS/g C.•A type of jellylike bioflocs pellet was designed and directly made from wet bioflocs.•The jellylike bioflocs pellet could be successfully fed to tilapia, and the advisable ADCs of dry matter, protein, crude lipid were 57.28–58.64%, 67.04–69.77% and 76.90–80.80%, respectively.

Bioflocs was produced in sequencing batch reactors (SBRs) that were treating recirculating aquaculture system (RAS) effluent using biofloc technology (BFT). The effects of sludge retention time (SRT = 1 d, 2 d, 3 d, 4 d, and 6 d) on the water quality, bioflocs yield, and nutritional composition were investigated. A type of jellylike bioflocs pellet was designed, and the apparent digestibility coefficients (ADCs) in tilapia (GIFT Oreochromis niloticus) were studied. The results showed that SRTs longer than 2 d were good for water re-utilization of the SBRs effluent. The bioflocs yield could be as high as 2.30–2.54 g VSS/g C (SRTs of 1–3 d). The protein values of solid waste (19.23 ± 0.87%) in RAS effluent treated by BFT could be significantly improved, which were 22.58 ± 1.14%, 23.38 ± 1.76%, 24.08 ± 0.94%, 23.92 ± 0.72%, 22.56 ± 1.64% in SRTs of 1–6 d, and that of the bioflocs was the highest in 3 d SRT. The results indicated that the frequently used protein/nitrogen conversion factor of 6.25 causes the protein content to be overestimated in bioflocs. SRT had no significant effects on the essential fatty acid and the polyunsaturated fatty acid contents in bioflocs. Jellylike bioflocs pellets could be successfully fed to tilapia, and the advisable ADCs of dry matter, protein, crude lipid, crude ash, and nitrogen were 57.28–58.64%, 67.04–69.77%, 76.90–80.80%, 53.50–59.82%, and 73.92–77.56%, respectively. In conclusion, a SRT of 3 d was the most advisable one.