# Design and Application of New Aeration Device Based on Recirculating Aquaculture System
## 1 Introduction  
According to the Food and Agriculture Organization (FAO) of the United Nations, China is the largest producer of freshwater fish for exporting and domestic consumption [1]. China alone supplied 58% and 59% of the global aquaculture volume and value, respectively, for all categories combined in 2017 [2]. Inland pond aquaculture, which represented 12.12% of total aquatic production in 1954, has surged significantly, reaching 44.30% by 2019 [1]. Notably, recirculating aquaculture systems (RASs) have emerged as pivotal contributors to this growth trajectory. These systems are distinguished by their capacity for accommodating high fish densities and achieving a substantial amount of production outputs [2, 3]. RAS is a simplified version of the zonal aquaculture system invented by researchers at Clemson University (Clemson) [4, 5]. It consists of a water-flow-generating and oxygenation device, a breeding raceway, and a dirt collection area. The device at the front end collects fish excrete and transfers them to a downstream purification area, where they are collected and treated with water pumps and fecal collection equipment or by filter-feeding fish bred for water purification; finally, the debris is transported, through the overall pipe, to farmland for watering or other treatments. This is, thus, a feasible green aquaculture model which allows for the sustainable use of water. In 2013, [PERSON], who worked in the U.S. Soybean Export Association, introduced it into China and actively promoted its use [6]. In RASs, among the essential factors affecting the quality of water, which is directly related to the productivity of the model, is dissolved oxygen (DO) [7]. In aquaculture practice, under natural aeration conditions, when the DO level in the water column is lower than 5 mg/L at night, fish growth is hindered, and fish may exhibit a behavior known as \"floating head\" or may even suffocate and die, resulting in significant economic losses [8]. On the other hand, DO also directly affects the feed coefficients and operating costs of the aquaculture process; thus, different DO levels may lead to significant differences in the economic efficiency of the same aquaculture model. For example, [PERSON] et al. [9] suggested that the fish feeding rate is affected by changes in DO, as it was shown that this parameter significantly decreased with less than 20-30% of saturated DO. Further, the productivity of conventional ponds is limited to 0.4-1 kg fish/m\({}^{3}\) due to the dynamics of the natural DO level and dissolution efficiency of the water column; additional artificial aeration (mechanical aeration) can increase fish productivity by up to 4 kg fish/m\({}^{3}\)[10]. Therefore, mechanical aeration equipment is used to dynamically regulate the DO level in pond water in recirculating aquaculture systems, which is a common and effective method employed to improve the productivity of fish. At present, aeration devices of various sizes and types have been developed, but they are not designed to cope with high-density aquaculture, such as that based on RASs. In this regard, researchers have proposed a variety of aerator designs. The majority of paddlewheel aerators used globally, including in China, are surface aerators [8], such as impeller-type, micro- or nano-bubble, and surge-type aeration equipment. [PERSON] et al. [11] proposed an improved circular step aerator by optimizing the geometric parameters of the original device and the dynamic conditions, which significantly improved its efficiency. [PERSON] et al. [12] evaluated the standard aeration efficiency of different configurations of mechanical surface aerators; explored the effects of the number of blades, rotational speed, and submergence depth on aeration efficiency; and determined the highest aeration efficiency that could be achieved under the optimal device operating conditions.