Evaluation of settling tank efficiency and water quality in a biofloc culture system for Penaeus vannamei

Total suspended solids (TSS) concentration is an important aquaculture indicator that affects the growth of farmed animals and relates to the effective utilization of dissolved oxygen (DO). To evaluate the TSS removal efficiency of a self-designed vertical-flow sedimentation tank in Litopenaeus vannamei culture, we determined its hydraulic parameters and monitored changes in water quality before and after sedimentation. The results show that under the culture conditions of this experiment, the influent flow rate of this settling tank was (779.47±18.19) m·h^–1, the hydraulic retention time was (1.09±0.03) h, and the removal rate of TSS concentration was (78.09±1.35)%, exhibiting a significant sedimentation effect. Among the water quality indicators, the TSS concentration and dissolved oxygen (DO) concentration of the influent water were significantly higher than those in the effluent water (p<0.05). At the late stage of culture, the ammonia nitrogen ( ${\mathrm{N}\mathrm{H}}_4^{+}\text{-}\mathrm{N}$) concentration increase significantly, the nitrite nitrogen ( ${\mathrm{N}\mathrm{O}}_2^{-}\text{-}\mathrm{N}$) concentration decrease and the TSS concentration decreased significantly. During the use of the settling tank, pH and total alkalinity (T-Alk) exhibited an increasing trend. Correlation analysis of water quality factors show that influent TSS concentration was significantly negatively correlated with nitrite concentration, ${\mathrm{N}\mathrm{H}}_4^{+}\text{-}\mathrm{N}$ concentration was significantly positively correlat with pH and negatively correlated with ${\mathrm{N}\mathrm{O}}_2^{-}\text{-}\mathrm{N}$ concentration. ${\mathrm{N}\mathrm{O}}_3^{-}\text{-}\mathrm{N}$ concentration was significantly positively correlated with pH and significantly negatively correlated with DO concentration (p<0.05). There was a significant positive correlation between ${\mathrm{N}\mathrm{H}}_4^{+}\text{-}\mathrm{N}$ concentration and pH in the effluent, and a significant positive correlation between ${\mathrm{N}\mathrm{O}}_3^{-}\text{-}\mathrm{N}$ concentration and both pH and T-Alk (p<0.05). The correlation coefficients of pH and T-Alk for influent and effluent water were 0.8 and 0.7, respectively. Based on water quality indicators and their correlation analysis, it is recommended to moderately increase the sedimentation tank volume, enhance airlift aeration intensity, and reduce hydraulic retention time. Meanwhile, aquatic microorganisms should be regulated in a targeted manner through the adjustment of water nutrients to further reduce the volume and frequency in aquaculture management. The sedimentation-reflux of aquaculture water using sedimentation tanks features simple operation and energy saving, which is suitable for large-scale implementation.