Effects of aquaculture density on self-cleaning ability of circular recirculating water aquaculture tank

Recirculating aquaculture is an important aquaculture model with the advantages of high density, low environmental pollution and high economic benefit. However, recirculating aquaculture needs to add a large amount of feed to the system. Due to the problems of solid feed particles and fish feces deposition in the process of water circulation, the accumulation time is too long, which can easily lead to the decomposition of such sediments, and produce harmful substances to pollute the water body, while further consuming dissolved oxygen in the water body, and ultimately lead to the sub-health and death of fish. Therefore, how to quickly and efficiently discharge the residual bait and feces in the aquaculture tank and reduce its impact on water quality is the primary problem facing the recirculating aquaculture model, which needs to be solved urgently. In this paper, physical experiments were conducted to study the effects of fish culture density on the hydrodynamic characteristics of circular recirculating aquaculture tank and the accumulation of dirt movement, and to reveal the response relationship between culture density and self-cleaning capacity of aquaculture tank driven by different flow rates. Based on different breeding density (ρ=0, 2.65, 4.41, 6.20 kg/m³) and influent flow rate (Q=5.20, 6.54, 9.81 L/min), 12 groups of test conditions were designed for the single tube wall inlet mode of circular aquaculture tank. The Acoustic Doppler Velocimeter (ADV) was used to measure the flow velocity of the water body in the aquaculture tank, and the de-extremization operation was carried out to remove 5% of the maximum and minimum values, and then the average treatment was carried out, which approximately represented the flow velocity of the ADV measuring point. The weighted average flow rate, water resistance coefficient, turbulence intensity and other parameters of the whole breeding pond were calculated by the formula. The self-cleaning capacity of the aquaculture tank was analyzed by using these flow field parameters and combining the data of collecting time under different working conditions obtained from the pollution collection test. The results show that the low water flow rate in the circulation process is easy to lead to the deposition of part of the sewage and aggregation, and it is difficult to discharge the sewage. Increasing the density of fish culture could reduce the average flow velocity (v_avg) of the overall flow field in the pond, the attenuation amplitude was within 0.05 m/s (less than 25%), and the resistance coefficient C_t in the water was increased. Turbulence caused by fish swimming can cause the sewage in the pool to re-suspension, which helps the dirt to be discharged; The sewage collection time was affected by the breeding density and flow rate at the same time, and the sewage collection time was within 5 min under the influent flow rate of 9.80 L/min. When the influent flow rate was 6.54 L/min, the breeding density increased from 0 to 6.20 kg/m³, the turbulence intensity increased by 2.4 times, and the sewage collection time was reduced by more than 40 min. The research results can provide reference for the design and daily management of circular aquaculture ponds.