Technical Summary of Recirculating Aquaculture System (RAS) for Common Carp
The global aquaculture industry is developing rapidly, while traditional farming models face challenges such as water resource shortages and environmental pollution. As an environmentally friendly aquaculture model, the Recirculating Aquaculture System (RAS) achieves the recycling of water resources through the integrated application of water treatment technologies, providing an effective solution to the environmental pressures caused by traditional farming methods. The common carp (Cyprinus carpio), an important freshwater economic fish species in China, possesses characteristics like fast growth rate and strong adaptability, showing promising application prospects in RAS. By establishing a closed water circulation system through processes including physical filtration and biological purification, the RAS model significantly reduces reliance on external water bodies during farming and minimizes the environmental impact of wastewater discharge on the surrounding ecosystem. This model offers distinct advantages in increasing yield per unit water volume and ensuring healthy fish growth, aligning with the requirements for green and sustainable development in modern aquaculture. This paper systematically elaborates on the technical characteristics and system optimization strategies of RAS for common carp, holding significant practical importance for promoting the transformation and upgrading of the aquaculture industry.
1. Overview of RAS for Common Carp
Recirculating Aquaculture for common carp, as an intensive aquaculture method, achieves the reuse of aquaculture water by establishing a closed water circulation system. This model overcomes the dependence of traditional pond culture on natural water bodies, integrating farming activities into a controllable environment. Its core lies in establishing an ecological engineering system for water purification and recycling. During system operation, the culture water undergoes multi-stage treatment processes including physical filtration, biological degradation, and disinfection, effectively removing fish metabolites, residual feed, and harmful substances, thereby maintaining water quality parameters within a range suitable for carp growth. Utilizing RAS can significantly improve water resource utilization efficiency, with the farming yield per unit water volume being several times that of traditional models, while simultaneously reducing the environmental impact of aquaculture effluent.
From an industrial development perspective, the RAS model represents an important direction for the transition of aquaculture towards resource-saving and environmentally friendly practices. This technology is not only suitable for water-scarce regions but also provides technical support for the transformation and upgrading of traditional farming areas. With the increasing intelligence of aquaculture equipment and the reduction in system operating costs, the application prospects of RAS in the large-scale production of common carp are becoming increasingly broad.
2. Components of a RAS for Common Carp
2.1 Culture Tank Design
The design of carp culture tanks requires comprehensive consideration of multiple factors such as water circulation efficiency, fish growth requirements, and management convenience. Circular or circular-polygonal tank structures have become the mainstream choice due to their dead-zone-free water flow characteristics. This design effectively promotes the accumulation of residual feed and feces towards the central drain, avoiding the sludge accumulation in vortex areas common in traditional rectangular tanks. Tank materials mostly use fiberglass reinforced plastic (FRP) or concrete structures; the former facilitates modular installation and has a smoother inner surface than the latter, but concrete structures still hold cost advantages in large, fixed farms. The tank bottom slope is typically 5%–8%; too gentle a slope leads to poor drainage, while too steep a slope may cause stress in fish.
Tank depth must balance oxygen distribution and space utilization. A general depth of 1.5–2 m ensures adequate mixing of the upper and lower water layers while avoiding oxygen deficiency at the bottom due to excessive depth. The positioning of inlet and outlet pipes creates a three-dimensional counter-current. Inlets often use a tangential design to create a stable rotational flow, while outlets are equipped with a double-screen structure to prevent fish escape. The height of the observation window should be set about 20 cm below the normal water level, facilitating real-time observation of fish feeding behavior without disturbing the operational water level.
Tank size must be strictly matched to the treatment capacity of the recirculation system. Excessively large water volume per tank can easily lead to localized water quality deterioration, while overly small volumes increase system operating costs. The anti-slip treatment on tank walls uses an epoxy resin coating with moderate roughness, preventing fish abrasion while avoiding excessive algal attachment. The light transmittance of shading canopies is adjusted to 30%–50%, sufficient to inhibit explosive algal growth while meeting the daily operational needs of managers. The design detail of installing splash guards on the tank rim is often overlooked but plays a significant role in maintaining constant humidity in the culture facility.
2.2 Water Treatment Facilities
The core of a RAS lies in the rational configuration and efficient operation of its water treatment facilities, whose design must integrate multiple functions including physical filtration, biological purification, and water quality regulation. Physical filtration typically employs mechanical filters or drum filters (microscreens) to remove large particulate suspended solids like residual feed and feces from the water; the filtration accuracy directly affects the load on subsequent treatment stages. The biological purification stage often uses submerged biofilters or moving bed biofilm reactors (MBBR), where nitrifying bacterial communities attached to the carrier media convert ammonia to nitrite and further oxidize it to nitrate. Ozone generators and ultraviolet (UV) sterilizers form the water disinfection module.
The former decomposes organic pollutants and kills pathogenic microorganisms through strong oxidation, while the latter uses specific wavelengths of UV radiation to disrupt microbial DNA structure. Their synergistic use can significantly reduce the risk of disease transmission.
The temperature regulation system uses heat pumps or plate heat exchangers to ensure water temperature remains stable within the optimal growth range for carp. The water quality monitoring system integrates multi-parameter sensors to monitor key indicators such as pH, dissolved oxygen (DO), and ammonia concentration in real-time, providing data support for system control. All treatment stages are connected via piping systems and circulation pumps to form a closed loop. The water flow velocity needs dynamic adjustment based on stocking density and feeding rates; excessively high velocity can cause biofilm sloughing, while too low velocity may lead to localized water quality deterioration. The system design must reserve interfaces for emergency treatment, allowing for rapid activation of measures like protein skimmers or chemical precipitation during sudden water quality anomalies. Material selection for water treatment facilities should consider corrosion resistance and biocompatibility to avoid the leaching of metal ions that could harm the fish.
3. RAS Technology for Common Carp
3.1 Stocking Density Control
Appropriate stocking density is a critical factor for the efficient operation of a RAS, directly influencing the growth performance of carp and the quality of the water environment. Excessively high density restricts fish movement space, intensifies competition among individuals, leading to reduced growth rates and lower feed conversion efficiency. The accumulation rate of metabolic waste in the water increases, and dissolved oxygen consumption rises, easily triggering water quality deterioration. Excessively low density leads to underutilization of facilities, reduced yield per unit volume, and impacts economic benefits. Determining the stocking density in a RAS requires comprehensive consideration of multiple factors including fish size, water temperature, flow velocity, and water treatment capacity. As carp grow, their oxygen consumption and excretion per unit body weight increase accordingly, necessitating dynamic adjustment of the stocking density. Periodic grading and separate rearing of different-sized individuals can avoid uneven feeding caused by large size disparities.
3.2 Construction of Ecological Purification Zone
The ecological purification zone, as a core component of the RAS, is directly related to water quality stability and farming profitability. This area simulates a natural wetland ecosystem, utilizing the synergistic effects of plants, microorganisms, and substrate to purify the water body. The rational combination of submerged and emergent plants can effectively absorb excess nitrogen and phosphorus nutrients from the water. Common species include submerged plants like Vallisneria natans and Hydrilla verticillata, and emergent plants like Phragmites australis and Typha orientalis. The well-developed root systems of these plants provide attachment substrate for microbial communities.
Microbial biofilms play a key role in the purification zone. Biofilm communities formed by nitrifying and denitrifying bacteria continuously convert ammonia nitrogen to nitrate and ultimately reduce it to nitrogen gas. This process significantly reduces the accumulation rate of harmful substances in the water. The substrate layer is typically designed using porous materials like volcanic rock or bio-ceramics. Their rich pore structure not only extends the water flow path but also creates alternating anaerobic-aerobic environments favorable for microbial growth. The ratio of the purification zone area to the total system area needs dynamic adjustment based on stocking density, as both excessively high and low proportions can affect purification efficiency.
3.3 Treatment of Aquaculture Waste
Effective treatment of aquaculture waste is a crucial link for the sustainable operation of a RAS. Under high-density carp farming conditions, residual feed, feces, and metabolites accumulate continuously. If not treated promptly, this leads to water quality deterioration, affecting fish health and growth. Physical filtration, as the first step in waste treatment, removes over 80% of solid suspended solids through mechanical screens or drum filters. Such equipment requires regular backwashing/cleaning to prevent screen clogging. The biological treatment unit primarily relies on the synergistic action of nitrifying and heterotrophic bacterial communities to convert dissolved ammonia nitrogen into nitrate. This process requires maintaining suitable water flow velocity and dissolved oxygen concentration to sustain microbial activity.
The design of sedimentation tanks should balance hydraulic retention time and surface loading rate. Too short a retention time prevents adequate settling of fine particles, while excessive volume increases construction costs. The collected sludge, after thickening and dewatering, can be converted into organic fertilizer using aerobic composting technology. Adding conditioning agents like straw during composting improves the carbon-to-nitrogen ratio and promotes maturation. For the removal of dissolved nutrients, constructing aquatic plant purification zones is highly effective. Emergent plants like Eichhornia crassipes and Oenanthe javanica have high phosphate absorption rates, and their harvested biomass can be used as a supplementary raw material for animal feed.
UV sterilizers installed at the system's end can effectively kill pathogenic microorganisms, but attention must be paid to matching the UV dose with the flow rate to avoid under-dosing or over-dosing affecting treatment efficacy. Ozone oxidation technology is particularly effective for removing recalcitrant organic compounds, but residual ozone concentration must be strictly controlled to prevent damage to carp gill tissues. The entire waste treatment process should establish a real-time monitoring mechanism, focusing on trends in key indicators like total ammonia nitrogen, nitrite, and chemical oxygen demand. Operational parameters of each unit should be dynamically adjusted based on monitoring data. Treated water, upon passing water quality tests, can be recirculated back to the culture tanks, forming a complete material cycle chain and achieving the resource utilization of aquaculture pollutants.