Effects of stocking density on water quality of Haliotis discus hannai Ino in recirculating aquaculture and purification effect of biofilter

Abstract: This paper focused on the wastewater discharged from the aquaculture of Haliotis discus hannai Ino (H. d. hannai), targeted at improving the comprehensive utilization rate of water circulation system, compared the changing characteristics of the aquaculture water environment between the high density (HD) group (500 m-2), the medium density (MD) group (300 m-2) and the low density (LD) group (100 m-2), and evaluated the water treatment effect of moving bed aerating biofilter. Results showed that stocking density had a significant effect on the concentration of total ammonia (TAN), nitrite nitrogen (NO2--N), total nitrogen (TN), total phosphorus (TP) and phosphate (PO43--P) (P<0.05), and the degree of effect could be ranked as: HD > MD > LD. The total number of heterotrophic bacteria in each density group increased over time; except the 2nd and 11th sampling, the total number of heterotrophic bacteria in the HD group at other time points was significantly higher than that in the MD and LD group (P<0.05). The MD and HD group showed no significant difference in the concentration of nitrate nitrogen (NO3--N) and chemical oxygen demand (COD) (P>0.05), but were significantly higher than that in LD group (P<0.05). At the 1st, 2nd, 3rd and 5th sampling, no significant difference in respect of the total number of vibrios was identified among the density groups (P>0.05), and from the 6th sampling to the end of the experiment, the total number of vibrios in the MD and HD group was significantly higher than that in the LD group (P<0.05). In current conditions (water circulation rate, temperature, hydraulic loading), the average removal rate of biofilter for TAN, NO2--N, NO3--N, TN, PO43--P, TP, COD was 16.40%, 15.81%, 2.93%, 12.22%, 2.91%, 6.48% and 9.47% respectively. At the point of the last sampling, no significant difference in respect of the removal rate of TAN was identified compared with the 8th and 14th sampling, but it was significantly higher than that in any other time point (P<0.05). At the point of the 14th sampling, no significant difference in respect of the removal rate of NO2--N was identified compared with the 5th and 13th sampling, but it was significantly higher than that in any other time point (P<0.05). The biofilter had a better treatment effect on TAN and NO2--N which produce an obvious toxic effect on abalone in the aquaculture wastewater. However, it had a relatively low treatment efficiency on the removal of NO3--N, TN and the low-concentration PO43--P and COD. Therefore, in view of the multiple factors including economic and ecological benefits, it will be more appropriate to set the density of H. d. hannai as 500 m-2 in the multi-layer, three-dimensional recirculating aquaculture systems.