Construction and operation effect of land-based recirculating aquaculture system based on pond

To explore how the land-based recirculating water aquaculture system can be organically integrated with existing freshwater ponds, this study carried out to transform the aquaculture ponds into a combined "bacteria-algae-fish-aquatic plants" water treatment area, which was combined with a land-based aquaculture unit to construct a land-based recirculating water aquaculture system for conducting experiments on the culture of Pelteobagrus vachelli. During the experiment, the physical and chemical indicators of water quality in each water treatment unit and the growth of fish in the land-based aquaculture unit were monitored, and the nitrogen and phosphorus balance in the aquaculture system as well as the relationship between the aquaculture unit and the water treatment unit area were evaluated. The results indicated that after 8 weeks of operation, the total removal rates of ammonia nitrogen (NH₄⁺-N), nitrite (NO₂^--N), total nitrogen (TN) and total phosphorus (TP) by the water treatment units were 75.69%, 64.99%, 29.1% and 43.29%, respectively. Among them, the removal rates of NH₄⁺-N, NO₂^--N, TN and TP were 43.36%, 19.18%, 12.46% and 20.61%, respectively, in the bacteria-algal-fish area, and the removal rates of NH₄⁺-N, NO₂^--N, TN and TP in the aquatic plant area were 19.53%, 10.91%, 5.26% and 7.18%, respectively. The culture tail water was able to meet the "Requirement for Water Discharge from Freshwater Aquaculture Pond" (SC/T 9101-2007) after being purified by the water treatment unit. During the experimental period, the NH₄⁺-N concentration of the test ponds ranged from 0.13 to 0.55 mg/L, with an average value of 0.13 mg/L, and the control ponds ranged from 0.14 to 0.25 mg/L. The NH₄⁺-N concentration of the control ponds was higher than that of the test ponds from the 2nd week onward (P<0.05). The NO₂^--N concentration of the test ponds ranged from 0.007 to 0.052 mg/L, with an average value of 0.020 mg/L, and the control ponds ranged from 0.023 to 0.047 mg/L, with an average value of 0.032 mg/L. The NO₂^--N concentrations in the control ponds were higher than those in the experimental ponds from the 3rd week onward (P<0.05). The trends of changes in the TP concentrations in the experimental ponds and control ponds were consistent, and the control ponds were significantly higher than the experimental ponds (P<0.05). The land-based culture unit of Pelteobagrus vachelli had a survival rate of 97.5%, a unit yield of 6.38 kg/m³, an average weight gain rate (WGR) was 71.57%, an average specific growth rate (R_SGR,m) of 0.96 %/d, and an average food coefficient (FCR) of 1.48. The results of the nitrogen and phosphorus income and expenditure showed that the feed was the main source of nitrogen and phosphorus in the system, accounting for 61.71% and 61.85% of the total nitrogen and phosphorus input, respectively. Pelteobagrus vachelli and integrated accumulation (including bottom sediment deposition, water leakage, adsorption, etc.) were the main items of N and P output, accounting for 50.26% and 38.53% of the total N and P output, respectively. The utilization rates of nitrogen and phosphorus were 29.12% and 10.65%, respectively, and the utilization rates of nitrogen and phosphorus were 0.06% and 1.14% for Hypophthalmichthys molitrix and 0.02% and 0.33% for Aristichthys nobilis, respectively. The results of multiple comparisons showed that the utilization of nitrogen and phosphorus of Pelteobagrus vachelli was significantly higher than that of Hypophthalmichthys molitrix and Aristichthys nobilis (P<0.05). Based on the calculation of the water exchange volume, the pollutant concentration discharged by the aquaculture tank, and the production and discharge coefficient of the aquaculture fish, it was determined that the proportion relationships between the aquaculture barrel and the water treatment unit were 3.125:1, 0.0067:1 and 0.16:1, respectively. However, the proportion relationship between the aquaculture tank and the actual area of the purification pond in this study was 0.043:1, and there was still potential for further optimization. Therefore, this study demonstrated that the land-based recirculating aquaculture system based on the modification of the aquaculture ponds has a good effect in the aspects of aquaculture tailwater purification and nitrogen and phosphorus utilization, which is worthy of further optimization and promotion.