Hydraulic characteristics of waste convergence under paddle-wheel aerators for square aquaculture pond with round angle

Abstract: Intensive and semi-intensive aquaculture have attracted more and more attention in recent years in China, where various types of fish are raised at a high density, particularly relying mainly on agricultural equipment and supplementary feeds. A considerable revolution has emerged increasingly from the traditional aquaculture methods, in order to meet the growing demands for the seafood both in quantity and quality, thereby to compensate for the significantly reduced capture from over exploited fisheries. Consequently, it is also urgent to deal with the solid wastes primarily derived from the uneaten feed and fecal droppings of cultured fish in the tank, especially that in the tank of high density. Moreover, solid wastes have been classified as the most dangerous waste in fish culture systems, for that they can clog the fish gills, thereby leading to the death of fish. Therefore, it is highly recommended to effectively remove the solid wastes in the tank as quickly as possible, because the reduction of residence time in a tank can be used to prevent the wastes of decomposition, further to lessen the stress to the cultured fish. In an excellent oxygen transfer system, a paddle-wheel aerator is commonly used as aquaculture equipment. The paddle-wheel aerator can generate flow circulation via adjusting the deployment, to converge solid wastes at the vicinity of the outlet end, and subsequently the solid wastes can be discharged from the tank. However, there is no sound theoretical guidance on adjusting the deployment, depending often on the experience of farmers in aquaculture practice. This case can result in various deployment patterns for the ineffective convergence and removal of solid wastes. In this study, a laboratory experiment was conducted in a model square tank with four round cut corners, in order to investigate the effects of deployment angel θ (θ represents the acute angle between the impellers and the center line of the tank), deployment distance ratio d/a (d is the distance between the center point of the impellers and the nearest pool wall, a is the length of the pool), and the driving velocity of paddle-wheel aerator, on the efficiency of convergence and removal for solid wastes. A simplicity artificial feed was selected to simulate the solid waste in the experiment. A CCD camera was used to record the wastes distribution in the tank, while, the captured images were obtained concurrently. A quantitative analysis was conducted using the processed images from the MATLAB software. An indicator was set as the maximum distance of the waste to the outlet in a tank. An appropriate deployment pattern of paddle-wheel aerator can be determined by comparing the maximum distribution distance under various working conditions. The flow velocity was also measured to figure out the flow characteristics under an Acoustic Doppler Velocimetry. The results show that the generation of circulation in a horizontal panel in the tank was the primary requirement for converging solid wastes to the center of the tank, and it can significantly be affected by the deployment angle, deployment distance ratio d/a, and driving velocity. When the deployment distance ratio d/a was small, the removal efficiency of solid wastes gradually decreased with the increase of the angle in the range of 0??30?, and then gradually increased in the range of 40??60?, finally decreased in the range of 70??90?, peaking at the angle of 70?. When the deployment distance ratio d/a was in some range, the removal efficiency of solid wastes gradually decreased with the increase of deployment distance at the deployment angle of 0? and 70?. It gradually increased for the deployment angle of 45? with some undesired convergency of solid wastes in the round cut corners. There were the optimal deployment angle and distance ratio d/a in the paddle-wheel aerators, where the removal efficiency of solid wastes can gradually increase as the driving velocity increased. The findings can provide a sound reference for the deployment of paddle-wheel aerator in the intensive and semi-intensive aquaculture pond.