The Application of Circular Tank RAS in Aquaculture
0. Introduction
The aquaculture industry is a vital sector for national economic growth. However, as its scale continues to expand in the pursuit of greater economic benefits, it faces numerous challenges, including environmental pollution, water resource waste, and lagging technological updates. Therefore, the introduction of Circular Tank Recirculating Aquaculture System (RAS) technology is particularly important. This technology effectively meets the need for water resource recycling and leverages its environmental advantages, helping to solve the prominent problems of traditional farming methods and thereby promoting the sustainable development of the aquaculture industry.
1. Principles and Advantages of Circular Tank RAS
1.1 Technical Principles
Circular tank RAS is a modern, ecological aquaculture technology that combines the structural characteristics of circular tanks with a water circulation and purification system. It introduces culture water into a closed-loop system, keeping it in a constant state of flow. This water undergoes multiple treatment stages, not only meeting water recycling needs but also optimizing the aquaculture environment.
During system operation, the culture water is first pre-treated using a filtration system, where physical or chemical methods remove impurities like suspended solids and organic matter. The preliminarily filtered water then enters a sedimentation tank, where larger particles or suspended substances settle further under gravity, purifying the water. The water then flows into an oxidation pond, which utilizes microbial degradation to break down harmful substances, increases dissolved oxygen (DO) content, and creates a suitable environment for the cultured species.
Compared to traditional aquaculture, the application of circular tank RAS effectively addresses issues of water waste and environmental pollution, enhances control over the farming environment, allows organisms to thrive in a healthy setting, and comprehensively improves aquaculture efficiency and quality.
1.2 Technical Advantages
(1) Efficient Water Quality Management: Water flow forms a vortex along the tank walls, causing residual feed and feces to concentrate automatically and be discharged through the central drain. This prevents the accumulation of pollutants at the bottom and reduces water pollution risk. Combined with the recirculation purification system, it enhances water stability and controllability.
(2) Suitable for High-Density Farming: The circulating water flow allows for even oxygen diffusion. Coupled with bottom aeration or jet oxygenation equipment, dissolved oxygen levels can be maintained at optimal levels. This system is more conducive to high-density farming compared to traditional ponds, increasing yield per unit volume of water.
(3) Environmentally Friendly Resource Utilization: Circular tank RAS recycles and reuses water through its system, achieving a water saving rate of over 80% compared to traditional methods. Furthermore, pollutants generated during farming can be collected and converted into valuable organic fertilizer, avoiding the risk of water body pollution caused by direct discharge.
2. Key Technical Aspects of Circular Tank RAS
2.1 Water Quality Management Technology
Efficient water quality management is a core advantage. The water circulation system is crucial, using high-efficiency pumps to achieve more than 3 full water cycles within 24 hours, combined with mechanical filtration to remove suspended solids. Additionally, adding nitrifying bacteria for biofiltration or using activated carbon to adsorb toxins helps maintain key parameters like ammonia nitrogen, pH, and DO within suitable ranges.
(1) Real-time Monitoring: Install monitoring equipment (pH meters, DO sensors, temperature sensors) around the tanks for real-time data collection. Sensors should be regularly calibrated and connected to a central control system. The system should send alerts when parameters exceed preset values.
(2) Water Circulation and Filtration: Install high-efficiency pumps per design specifications. Use mechanical filters with appropriate precision and clean/replace them regularly. Combine with biofilters and add nitrifying bacteria to enhance organic matter degradation.
(3) Dissolved Oxygen Control: Install oxygenation equipment (e.g., microporous diffusers, oxygen generators) at the tank bottom and calibrate their operating parameters to maintain optimal gas flow and DO levels.
(4) Temperature Regulation: Install heaters or chillers to maintain water temperature within a stable range (e.g., 22–26°C). Regularly calibrate temperature sensors and use the temperature control equipment to adjust the water as needed.
2.2 Feeding Management Technology
2.2.1 Feed Formulation
Formulate feed based on the nutritional requirements of the species at different growth stages to ensure a balanced diet. For example, for adult bass, feed crude protein should be 40–45% and fat 10–12%. Use high-quality ingredients like fishmeal, soybean meal, corn, fish oil, and soybean oil. Use specialized software to design scientific formulas. Mix ingredients and process them into pellets suitable for the species' consumption (e.g., max diameter not exceeding 3mm). Regularly test finished feed to ensure quality.
2.2.2 Feeding Techniques
Base daily feeding amounts on the stocking size and growth speed. Install automatic feeders at the tank edge for even distribution and scientifically adjust feeding volume and frequency based on biomass and growth stage. Adjust promptly if abnormal behavior or changes in feeding response are observed.
Install cameras to monitor the feeding process, identifying issues like uneven distribution or waste. Regular observation of feeding behavior provides a basis for fine-tuning.
2.3 Growth Monitoring Technology
Regularly sample (e.g., at least 30 fish) to measure length and weight. Record data in a management system to automatically generate growth curves and weight distribution charts. This allows for intuitive assessment of growth trends and health, enabling refined management.
Adjust feed formulas and rations based on the growth data. If growth rates are below expectations, analyze the causes and take effective measures to control feeding frequency, volume, and formula.
2.4 Disease Prevention and Control Technology
To prevent mass mortality, apply disease control strategies based on the health status of the stock.
Conduct daily quarantine of the environment, fish health, and water quality. Use microscopes, test kits, etc., to detect pathogens early for timely intervention.
Use preventive treatments (e.g., antibiotics, anti-parasitic drugs) according to instructions and the fishes' condition, strictly controlling dosage and frequency.
In case of a disease outbreak, immediately isolate affected units, diagnose the cause through detailed examination, and implement targeted treatments (e.g., adjusting water circulation, using specific therapeutics) to curb spread.
3. Application Case Study
3.1 Project Overview
A regional "Circular Tank RAS + Aquaponics" project features about 160 m³ of culture water, including 110 m³ for vertical hydroponic vegetable areas, 65 m³ for substrate planting, and 25 m³ for centralized water treatment. Compared to traditional methods, this model has advantages like smaller footprint, flexible installation, and strong self-cleaning ability, providing a superior environment for fish while reducing water quality risks.
3.2 Specific Application in the Project
(1) Water Management: Circulating water collects and settles large waste particles. A micro-screen filter removes these solids. The filtered water enters a biofilter where nitrifying bacteria on the media convert ammonia and nitrite into nitrate for plant uptake. Purified water is returned to the fish tanks, with part diverted to the vegetable hydroponics and part disinfected before re-entering the circular tanks.
(2) Feeding Management: Implement precise feeding control. For example, when fish are ~3 cm, daily feed is 8–10% of body weight; at 5–6 cm, it drops to 5–6%. Adjust frequency by growth stage. Observe feeding response after each feeding; if over 10% remains, reduce the next feeding by 10%.
(3) Growth Monitoring: Focus on growth rates for density control. Sample and weigh every 20 days. If growth is slow, check water quality or adjust feed formulation. Control density by stocking appropriate numbers initially and splitting stocks when size standards are met to prevent issues from overcrowding.
(4) Disease Prevention: Conduct daily pond checks and environmental management. Use a monitoring platform to observe fish status (e.g., abnormal color, surfacing) and water appearance (e.g., foam, dark color). Use this information for targeted prevention and treatment.
3.3 Application Results
The "Circular Tank + Greenhouse" model was optimized. Fish effluent undergoes solid-liquid separation via a micro-screen; the separated solids are fermented into organic fertilizer for vegetables. The filtered water enters greenhouses where ammonia and nitrite are absorbed and purified by the plants, before being recirculated.
The project achieved significant output: 250,000 kg/year of non-polluted celery (7 harvests) and 35,000 kg of clean ecological bass (2 harvests). Compared to traditional vegetable farming, annual profits increased by approximately 50,000 USD (a 30% rise). It created re-employment opportunities for over 100 local farmers, increasing their average annual income by about 1,100 USD. It also resolved environmental pollution and water waste issues.
Integrating land-based circular tanks with rice cultivation was also implemented. Aquaculture effluent, rich in ammonia and nitrite, is directed to rice paddies as nutrient-rich irrigation, promoting rice growth. Vegetables are grown in winter, ensuring year-round efficient use of nutrients from the effluent, highlighting the technology's efficiency, high yield, and environmental benefits.
4. Conclusion
In summary, the application of Circular Tank RAS in aquaculture leverages the combined advantages of the circular tank structure and the recirculating purification system to reduce pollutant deposition and control water quality risks at the source. By managing stocking density, creating a favorable aquatic environment, and establishing an efficient water recirculation system according to technical specifications, water resources can be utilized to the maximum extent. This achieves the dual purpose of enhancing both the economic and environmental benefits of the aquaculture industry.