## 4 Discussion  
In recirculating aquaculture systems, gathering the sewage in the water body to a specific area is one of the many essential steps [16]. Removing shrimp shells, dead individuals, and sewage in time are challenging tasks but essential to ensure the success of this particular aquaculture process. In order to prevent shrimp from escaping, the backwater outlet is designed to allow the opening of gaps [18], and anti-escape nets and similar tools are added [19]. However, these often lead to the clogging of outlets. A large amount of shells may be present in the water after molting, and they can block the backwater outlet, hindering water recirculation. In this study, a device to remove shrimp shells, dead individuals, and sewage was designed (CN201811548991.1) and put in place in order to avoid clogging. The backwater sewage collection pipe of the device was located at the bottom of the rectangular tanks that were once used as sea cucumber nursery tanks. The opening of the sewage collection pipe was designed as a long strip. The width of the opening could be adjusted according to shrimp size, so that only shrimp shells, dead individuals, and sewage could pass through. A backwater drainage tank equipped with a live-shrimp anti-escape frame and anti-escape nets with adjustable mesh size was installed in the upper layer of the culture tanks. The installation of anti-escape nets not only solved the problem of potential blockage of the backwater passage, but it also allowed the retention of live shrimp that entered the net through the sewage collection pipe. These were returned to the culture tanks regularly. The use of the device for the removal of shrimp shells, dead individuals, and sewage ensured the smooth functioning of water recirculation in the RAS. The adoption of a variable flow, regulated by the recirculating pump with adjustable frequency, contributed to a more energy-saving and efficient RAS. After 53 days of recirculating aquaculture (the total culture time was 92 days including the nursery period), the final culture yield of whiteleg shrimp, _Litopenaeus vannamei_, reached 12.92 kg/m\({}^{3}\), which was a considerably larger amount compared to yields obtained using other shrimp culture technologies. The culture density of _Litopenaeus vannamei_ at the time of harvest in traditional pond cultures is generally about 1-2 kg/m\({}^{2}\)[20]. It reached 4-11 kg/m\({}^{3}\) in raceway RASs [4, 5, 21, 22] and 2-5 kg/m\({}^{3}\) in another experimental set up with a land-based closed RAS [19]. The increase in the final culture density in this study marks a breakthrough in RAS technology. Overall, the operating costs (excluding site fee and equipment depreciation) of the shrimp culture in this study amounted to about 22 RMB/kg. Considering that the average market price of _Litopenaeus vannamei_ is 46 RMB/kg, the profit per kg of shrimp would equal 24 RMB, and two sets of RAS could generate an income of 210,000 RMB for one shrimp crop. Therefore, the economic benefits of using this aquaculture technology are considerable. Environmental parameters such as water temperature, salinity, dissolved oxygen (DO), and pH play an important role in the entire shrimp culture process, and changes in their values have important effects on the physiology of shrimp, in particular on growth, metabolism, reproduction, and molting [23, 24]. The experiment was carried out between late May and late July in northern China. The external air temperature gradually increased during this time also determining a gradual increase in water temperature. The water temperature increased from 24.0 to 30.8 \({}^{\circ}\)C, within the suitable temperature range for the growth of _Litopenaeus vannamei_[25]. Studies have reported that the optimal salinity range for the growth of _Litopenaeus vannamei_ is 15-25% [23]. Due to limited freshwater resources available in the nursery farm, the salinity during the entire culture process varied between 25-32%, and the shrimp still exhibited a relatively fast growth rate.