# Design and Application of New Aeration Device Based on Recirculating Aquaculture System
## 2 Materials and Methods  
### Study Area and Principle of Operation  
The test site was an outdoor pond in Kaitian Fishing Village, Changsha, Hunan Province, China (112\({}^{\circ}\)73\({}^{\prime}\)73.2\({}^{\prime\prime}\) N; 28\({}^{\circ}\)48\({}^{\prime}\)86.51\({}^{\prime\prime}\) E), with an area of about 2.2 hm\({}^{2}\) and an average water depth of 2.5 m. In the RAS set up in the pond, the overall load-bearing system consisted of a number of floats whose front and rear ends were connected to galvanized vertical columns fixed to the bottom of the pond and which could float up and down with the change in water level. In order to facilitate measurement and observation, load-bearing walkways were installed at the front and rear ends of the system. The new aerator was mounted at the front of the system and was secured to the flexible tank at the centre. In addition, the system also included a flexible culture tank, a suction element, and a pipe. The central culture tank of the system was made of fabric material, and its overall dimensions were 22.0 m in length \(\times\) 5.0 m in width \(\times\) 2.7 m in height; its average water level and volume were 2.0 m and 220 m\({}^{3}\), respectively. (Here, the height of the border of the culture tank was slightly larger than the height of the water level in the tank in order to ensure that the cultured fish could not escape.) The main fish species was grass carp; the carrying capacity was about 15,000 fishes, and the density was 25-30 fishes/kg. The study RAS is shown in Figure 1.  
We designed an NAD whose operation could meet the high actual DO demand of fish culture in RASs. A microporous aeration component and an impeller aeration component make up the main structure of the device. The remaining assembled parts are made of galvanized steel. The structure and working principle are shown in Figure 2.  
The microporous aeration component includes a high-pressure fan, an aeration disk set at the bottom, an output pipe for the fan, and an inclined plate whose angle is adjustable. The fan, together with the aeration pipeline arranged at the bottom of the aeration plate, generates high-pressure gas in the form of micro-bubbles in the water, to achieve the effects of water flow generation and oxygenation. The angle of the inclined plate can be changed by using an adjusting lever according to the water depth and the speed of water flow in the ponds; the structure comprises a three-link rotating mechanism including a control lever, a crank, a fixed rod connected to the main frame, a guide rod connected to the inclined plate of the water-flow-generating fan, and a slider. The other component is a water flow generation and oxygenation device driven by a vertical gear motor fixed to the main frame and impeller rotor shaft through a stainless-steel chain, and the rotor shaft is connected to the side plate of the frame through a flange and bolts. Uniformly set on the rotor shaft are  
Figure 1: A schematic diagram of the RAS and the NAD constructed in the freshwater pond under study. (**a**) RAS in the freshwater pond. (**b**) NAD installation in RAS. 8 impellers of the same size, 4 of which are connected to the floating bearing seat. In the frame side plate, there is an oblong hole that can be used to adjust the height of the rotor shaft centerline; the structure allows for a certain range so as to be adapted to different pond water depths. When the two components are synchronously operated, the centrifugal fan performs air inhalation and compression; then, the high-pressure gas is delivered to the bottom of the tank through the pipeline and into the aeration disc, which produces micro-bubbles and shoots them upward, providing them with initial kinetic energy; the bubbles are further propelled forward through the reaction force generated by their impact against the baffle plate. This process drives the lower water layer constantly upward until it mixes with the surface layer.