Study on purification effect of shellfish and algae coupling on intensive aquaculture tailwater

To explore the purification effect of shellfish and algae coupling on intensive aquaculture tailwater, we analyzed the coupling effects of oyster (Crassostrea hongkongensis) and microalgae (Spirulina platensis) on tailwater treatment of biological flocs which are rich in nitrogen and phosphorus nutrients. A total of five groups were set up. The average mass of oysters added to the small (S), medium (M) and large (L) size groups was (50.99±7.01), (100.25±8.87) and (148.81±15.61) g, respectively. The negative control group (NC) was only added with spirulina, and the blank control group (BC) was not added with oysters and spirulina. The density of spirulina was about 8×10⁵ cell·mL⁻¹ and oyster biomass was 3 kg·m⁻³. The survival rate and growth of oysters were recorded, and the cell density of spirulina in water was counted under a microscope. The concentrations of ${\rm{NH}}_4^{\text{+}}$-N, ${\rm{NO}}_2^{\text{−}}$-N, ${\rm{NO}}_3^{\text{−}}$-N, ${\rm{PO}}_4^{3{\text{−}}}$-P, TIN, TN, TP and TSS in water were detected. The results show that the survival rate of oysters in the experimental group followed a descending order of S>M>L. The highest rate of weight gain was observed in Group M. The highest density of spirulina was (1.30±0.25)×10⁷ cells·mL⁻¹ in Group NC. The highest TSS removal rate in Group M was 30.61%. The NC group had the highest TIN and ${\rm{PO}}_4^{3{\text{−}}}$-P removal rates of 89.29% and 98.93%, respectively. Group M had the highest TN and TP removal rates of 38.91% and 55.10%, respectively. The results suggest that spirulina has a significant effect on purifying inorganic nitrogen and phosphorus. Oysters can feed on spirulina in water effectively. Coupling oysters of average mass of (100.25±8.87) g with spirulina has the best effect on removing TN, TP and TSS from aquaculture tail water.