1.Overview of Recirculating Aquaculture Systems (RAS)
(1) Characteristics of Recirculating Aquaculture Systems
Recirculating aquaculture systems (RAS) are a novel aquaculture model developed on the basis of intensive aquaculture, characterized by the recirculation and reuse of culture water. In addition to the advantages of conventional intensive aquaculture, RAS offer significant benefits in wastewater treatment, reduction of water consumption, and minimization of effluent discharge. Through optimized design of the water supply system and coordinated operation of multiple facilities and devices, RAS enable the repeated recycling of the entire culture water volume. Compared with traditional intensive aquaculture, they are superior in terms of energy efficiency for temperature control, mitigation of environmental pollution, and prevention and control of diseases.
RAS require the integrated use of a comprehensive set of water purification and treatment facilities. Their process design involves the application of multiple disciplines and industrial technologies, including fluid mechanics, biology, mechanical engineering, electronics, chemistry, and automation information technology. A well-designed RAS can achieve full control of water quality parameters such as temperature, dissolved oxygen, and nutrients, and under any circumstances, more than 90% of the system water can be reused through recirculation.
(2)Essence and Advantages of RAS
The essence of recirculating aquaculture systems (RAS) lies in supporting and optimizing aquaculture production through industrialized and modernized approaches. By enabling full-process regulation of the aquatic environment, RAS can partially overcome external constraints such as temperature, water availability, and space, thereby achieving year-round, multi-batch continuous production. This allows for off-season farming and staggered market entry, providing producers with a competitive advantage and higher economic returns.
(3)Production Efficiency and Resource Use
The excellent production performance of RAS is closely tied to its highly controllable and resource-efficient characteristics. On a per-unit-water basis, the yield of aquatic products in RAS is 3–5 times higher than that of traditional flow-through intensive aquaculture and 8–10 times higher than that of pond aquaculture, while survival rates increase by more than 10%. Furthermore, the use of veterinary drugs and chemical agents is reduced by nearly 60%. These comprehensive improvements in performance indicators ensure both the economic and ecological benefits of RAS.
(4) Water Treatment and System Integration
In RAS, culture water undergoes a series of treatments, including physical filtration, biological purification, sterilization and disinfection, degassing, and oxygenation, allowing full or partial reuse of the water. At the same time, optimization of the culture environment can be integrated with automated equipment such as automatic feeders, enabling a degree of automation and intelligent management.
(5)Technological Foundations and Key Features
RAS integrate advanced technologies from fisheries engineering, mechanical equipment, new eco-friendly materials, microecological regulation, and digital management. Owing to the fully controlled production environment, which is minimally affected by external conditions, RAS demonstrate significant advantages including water and land conservation, reduced energy demand for temperature regulation, stable rearing conditions, accelerated growth rates, high stocking densities, and the production of eco-friendly, pollution-free products. As such, RAS are regarded as "the most promising aquaculture model and investment direction of the 21st century."
(6) Development and Application in China
To date, more than 900 large-scale RAS have been designed and constructed in China, spanning major coastal provinces as well as inland regions, extending even to Xinjiang. These systems, encompassing both marine and freshwater applications, have been successfully commercialized, meeting expected production targets and demonstrating excellent operational performance. Production practices confirm that RAS not only deliver superior productivity and environmental advantages, but also achieve significantly lower production costs per unit yield compared with other aquaculture models.
2.Key Processes and Technologies of Recirculating Aquaculture Systems (RAS)
Recirculating aquaculture systems (RAS) make extensive use of industrial engineering equipment and technologies. Typically, they consist of process units and facilities for solid particle removal; removal of suspended particles and soluble organic matter; elimination of toxic and harmful soluble inorganic salts such as ammonia and nitrite; pathogen control; carbon dioxide removal from the metabolism of cultured organisms and microorganisms; oxygen supplementation; and temperature regulation. The technical processes involved include thermal insulation and temperature control, solid particle removal, removal of soluble inorganic nitrogen and phosphorus, disinfection and sterilization, as well as oxygenation.
(1)Industrialized and Intensive Production Features
RAS further enhance the intensive characteristics of industrial aquaculture, offering high production efficiency and small land occupation, while overcoming the constraints of land and water resources. As a high-input, high-output, high-density, and high-efficiency farming model, RAS aligns with China's overarching goals for ecological civilization and sustainable development strategies.
(2)Ecological and Strategic Significance
With its intensive, efficient, energy-saving, emission-reducing, and environmentally friendly features, RAS has become an important direction for transforming and upgrading aquaculture in China toward low-carbon and green development. For several consecutive years, RAS has been listed by the Ministry of Agriculture and Rural Affairs of China as a major recommended aquaculture technology.
(3)Current Development and Trends
At present, this model has gained widespread recognition from both academia and industry in China. The scale of new system construction and the overall farming capacity have been increasing steadily in recent years, making RAS one of the key future development trends of China's aquaculture industry.
3.Overview of Research and Industrialization of Recirculating Aquaculture Systems (RAS)
(1)International Research and Industrialization
Early Research and Development
The earliest recirculating aquaculture system (RAS) emerged in Japan during the 1950s. Subsequently, many countries began research on water treatment and aquaculture technologies for RAS. Initially, these studies were based on municipal wastewater treatment processes and aquarium-style systems (with culture densities of only 0.16–0.48 kg/m³). However, such approaches did not account for the unique requirements of commercial aquaculture-particularly in terms of system costs, resource use, the ratio between culture and purification water volumes, and system carrying capacity (typically 50–300 kg/m³). As a result, research efforts encountered many setbacks, consumed large amounts of resources, and progressed slowly.
Recognition of Dynamic Characteristics
Early studies also overlooked an important characteristic of RAS: its dynamic nature. The production and degradation rates of fish metabolic wastes must reach dynamic equilibrium for the system to remain stable and healthy. By the mid-1980s, with growing understanding of water quality parameters-such as pH, dissolved oxygen (DO), total nitrogen (TN), nitrate (NO₃⁻), biochemical oxygen demand (BOD), and chemical oxygen demand (COD)-and their variation patterns in aquaculture water, these dynamic changes were gradually integrated into system design. For example, oxygen deficiency can be rapidly corrected by aeration, but the response of nitrifying bacteria to rising ammonia concentrations often lags significantly behind. Thus, deeper knowledge of interacting limiting factors became increasingly important for effective system design and operation.
Challenges in Early Practices
Many aquaculture practitioners had experience with flow-through intensive systems but lacked knowledge of RAS operation. As a result, they often failed to properly control stocking density, feeding amounts, feeding frequency, and water quality management, leading to imbalances in system water flow and material cycling and ultimately causing operational failures. This lack of scientific understanding and management experience was reflected in culture density levels: laboratory-scale RAS usually achieved only 10–42 kg/m³, while early commercial-scale RAS maintained as low as 6.7–7.9 kg/m³. After more than half a century of technological advancement-including process optimization, aeration and oxygenation (e.g., liquid oxygen use), automated feeding, and selection of suitable species-modern RAS have overcome many limiting factors and can now support high culture densities of 50–300 kg/m³.
Industrial Growth and Technological Innovations
As traditional pond aquaculture faced stagnation due to land competition and environmental pressures, RAS in Europe and North America experienced rapid growth between the 1980s and 1990s. This industrial expansion was accompanied by technological improvements, including the use of pressurized and non-pressurized filters for large suspended solids, ozonation for disinfection and organic matter degradation, and the development of multiple biological filters such as submerged filters, trickling filters, reciprocating filters, rotating biological contactors, drum biofilters, and fluidized bed reactors, as well as anaerobic denitrification units. With these advances, RAS gradually matured and entered commercial application.
The Case of the United States
The United States has maintained a leading position in both fundamental and applied RAS research, covering areas such as the nutrition and physiology of intensively farmed species, disease prevention, and water treatment technologies. A key feature of U.S. RAS is their high degree of automation and mechanization in water quality control. Computer-assisted systems automatically regulate dissolved oxygen, pH, conductivity, turbidity, and ammonia levels, as well as environmental conditions such as temperature, humidity, and light intensity. Leveraging its advanced industrial base, the U.S. has widely adopted high-tech equipment for oxygenation, biological purification, solids removal, grading, and harvesting. For example, the experimental RAS developed by the Center of Marine Biotechnology at the University of Maryland incorporates anaerobic treatment processes, closely resembling systems designed by Aquatec-Solutions in Denmark.
4.Challenges and Countermeasures for the Development of Industrialized Recirculating Aquaculture Systems (RAS)
(1) Insufficient Integration of Facilities and Equipment
Although China's water treatment, automatic feeding, disinfection, and aeration equipment have gradually approached the international advanced level, the overall system integration remains inadequate. The lack of large-scale enterprises capable of producing complete sets of RAS equipment has increased construction costs and complexity, thereby hindering the rapid advancement of domestic equipment.
(2) Need for Optimization of Specialized Compound Feed
At present, aquafeed formulas in China are highly homogeneous and lack specialized feed designed for RAS and specific cultured species. This increases the operational burden of water treatment systems and affects farming performance. It is necessary to develop species-specific RAS feeds with well-balanced nutrition, low leaching rates, and favorable feed conversion ratios.
(3) Disease Prevention and Control Require Greater Precision
High-density and high-efficiency farming increases the risk of disease outbreaks once system imbalances occur, and pathogens are difficult to eliminate in closed systems. System optimization should be enhanced to improve buffering capacity, while research should focus on fish physiology, stress responses, early disease indicators, and effective disease-warning mechanisms.
(4) Significant Pressure of Energy Consumption and Cost Reduction
High initial construction investment and energy consumption are unavoidable challenges of RAS. Energy-saving measures should be implemented at both the equipment and system levels, including the development of low-energy filters, CO₂ removal devices, tailwater treatment technologies, and renewable energy applications such as solar, wind, and water-source heat pumps.
(5) Lack of Standardization in Operation and Management
Currently, there are no unified technical standards or norms for RAS in China. As a result, system design, management practices, and farming performance vary widely, and operational failures are common. It is essential to establish a standardized technical framework for healthy aquaculture, improve process and management standards, and promote demonstration projects for standardized production.
(6) Need for Strengthened Basic Research
Scientific understanding of several aspects remains insufficient, including the health status of cultured species under high-density and specific water quality conditions, biofilm structural changes during system operation, nutrient cycling mechanisms, and optimal methods for the removal and harmless treatment of solid particles. These gaps hinder the further development of relevant technologies and equipment.
(7) Future Development Trends and Opportunities
Despite these challenges, RAS offers significant advantages in production efficiency, environmental sustainability, and animal welfare. As a green, ecological, circular, and efficient farming model, it aligns with global trends toward low-carbon development. With the modernization of China's fisheries, the advancement of ecological civilization, and the acceleration of carbon neutrality goals, RAS is expected to enter a new phase of rapid development.