A crucial aspect of aquaculture performance is the correct hydrodynamics within the culture tanks, especially in recirculating aquaculture systems (RAS) and pond recirculating aquaculture systems (PRAS). Researchers from Shanghai Ocean University and the Chinese Academy of Fishery Sciences have studied how the location and angle of paddlewheel aerators influence the hydrodynamic conditions of large-scale (15 m side length) chamfered recirculating aquaculture tanks (RATs), with the aim of improving energy efficiency, waste collection, and uniformity in dissolved oxygen distribution.
The importance of hydrodynamics and paddlewheel aerators
In PRAS, paddlewheel aerators are vital tools. They not only increase dissolved oxygen levels, essential for the respiration of aquatic organisms, but also generate controllable water currents. These currents are fundamental for homogeneously distributing nutrients, preventing water stagnation and the accumulation of pollutants, thus reducing the risks of water quality deterioration and disease proliferation. Therefore, a deep understanding of how these aerators affect hydrodynamics is key to deg more efficient and sustainable aquaculture systems.
Previous research had focused mainly on the shape, size, and bottom slope of industrial RATs, as well as the optimization of inlet pipes. However, knowledge about PRAS, particularly with the incorporation of paddlewheel aerators, was limited, and the mechanisms by which the placement parameters of the aerators affect hydrodynamics in large-scale RATs had not been fully elucidated.
How was water flow studied?
To address these unknowns, the researchers developed a three-dimensional numerical model of water flow in the RATs. The reliability of this model was validated through comparisons with an experimental scale model. They analyzed various hydrodynamic parameters, including flow uniformity, velocity, vorticity, the Froude number (which relates inertial and gravitational forces), and the effective energy utilization coefficient.
The study focused on a square tank with chamfered corners, 15 meters per side, part of a PRAS system divided into modules with different functions: a purification zone with aquatic plants, an omnivorous fish culture zone, and a high-density aquaculture zone (composed of six tanks like the one studied). Wastewater from the high-density tank, loaded with fish feces and uneaten feed, is discharged through a central bottom drain to a sedimentation tank. Then, it es through the purification zone and the omnivorous fish zone before being pumped back into the high-density tanks, completing the cycle.
In the numerical model, the scientists used the multiple reference frame (MRF) method to simulate the rotating domain of the paddlewheel aerator impellers, a technique that offers computational efficiency and good mesh adaptability. They investigated two key geometric variables: the angle (θ) and the distance (l) of the aerator’s placement with respect to the nearest tank wall. They also tested five different angles combined with three distinct distances, generating a total of 15 computational models. The immersion depth of the aerator blades was set at 2/3 of their length.
Impact of aerator position
The study results revealed valuable information on how the configuration of paddlewheel aerators can optimize tank operation:
Velocity and water mixing:
- Increasing the angle and distance of the aerator placement increases the average water flow velocity within the RAT and strengthens water mixing. This generates a velocity pattern with lower values in the center and higher values at the periphery of the tank.
- This velocity distribution is beneficial because it improves the intensity of the secondary flow, which helps solid particles to be carried by the current, preventing their sedimentation at the bottom and promoting their discharge through the central drain. Thus, the tank’s self-cleaning efficiency is improved.
- Good mixing also ensures a uniform distribution of nutrients and oxygen, crucial for fish health.
Formation of problematic zones:
- An important warning was observed: when the aerator placement distance is L/4 (where L is the length of the tank side) and the placement angle exceeds 30∘, a turbulent low-velocity zone forms near the corner of the tank closest to the aerator.
- This low-velocity zone can hinder the discharge of waste particles, as they tend to accumulate in such areas. The formation of this zone is because the aerator creates a cavity in front of its rotation direction, blocking the flow in the area between the aerator and the chamfered corner.
Energy efficiency and optimal conditions:
- The research identified configurations that notably improve energy utilization and water mixing performance. Specifically, when the aerator placement distance is L/6 or L/4 and the angle is in the range of 20∘ to 30∘ (20∘<θ<30∘), significant improvements are observed.
- Under these conditions, both the effective energy utilization coefficient and the Froude number increase with the increment of angle and distance, reaching maximum values of 2.98 and 0.701, respectively. A higher energy utilization coefficient means that the energy supplied by the aerator for water circulation is better leveraged.
- The Froude number, in general, remained below 1 in the different scenarios, indicating a slow flow state, which is favorable for fish survival and nutrient transport.
- A strong positive correlation was found between the aerator placement angle and the effective energy utilization coefficient when the placement distance is L/8 or L/6 (Pearson coefficient > 0.9). This suggests that, at these distances, increasing the angle can effectively improve the tank’s energy efficiency. However, at a distance of L/4, this correlation is weaker (Pearson coefficient of 0.441), indicating a more chaotic internal flow and less predictability in the hydrodynamic response when adjusting the angle.
Flow uniformity:
- Flow uniformity, a key indicator of the quality of the aquatic environment, generally increases with the increment of the aerator placement angle (when the distance is fixed). Greater flow uniformity improves water circulation efficiency and, therefore, the culture environment.
- However, very large angles combined with greater distances (closer to the center of the tank) can cause the high-velocity flow generated by the aerator to interfere with the water inflow to the tank, reducing uniformity and causing kinetic energy losses.
Implications for aquaculture
The findings of this study have direct and practical implications for the design and operation of recirculating tanks in aquaculture:
- Design optimization: It provides a theoretical basis for selecting the optimal location and angle of paddlewheel aerators in large-scale RATs, which can lead to more efficient designs from an energy and waste removal perspective.
- Improved water quality: By promoting better mixing and efficient particle discharge, superior water quality can be maintained, translating into healthier fish and higher productivity. Uniform distribution of oxygen and nutrients is essential.
- Reduced operating costs: Greater energy efficiency in water circulation can mean lower energy consumption and, therefore, lower operating costs for aquaculture producers.
- Sustainability: By improving system efficiency and waste management, it contributes to more sustainable aquaculture with a lower environmental impact.
Conclusion and future perspectives
This study demonstrates that the correct configuration of paddlewheel aerators, considering both their distance from the wall and their placement angle, is fundamental for optimizing hydrodynamics in large-scale recirculating aquaculture tanks. The results indicate that while increasing the angle and distance of the aerator generally improves water velocity and mixing, there are thresholds (distance L/4 and angle >30∘) where detrimental low-velocity zones can form. Optimal ranges were identified (distance L/6 or L/4 and angle between 20∘ and 30∘) that maximize energy efficiency and mixing, favoring the tank’s self-cleaning.
The research enriches the knowledge about the interaction between paddlewheel aerators and water flow in PRAS, offering practical guidelines for the design and operation of more efficient and ecological aquaculture systems.
The authors suggest that future research should focus on developing high-precision numerical calculation methods for three-phase flows (gas-liquid-solid) within RATs. Likewise, it is proposed to investigate the impact of other design parameters of paddlewheel aerators, such as the number of impeller blades and blade geometry, on hydrodynamic characteristics, dissolved oxygen transport, and pollutant migration. These advances will continue to drive the development of more efficient, green, and sustainable aquaculture.
The study was funded by the Science and Technology Commission of Shanghai Municipality, the National Key Research and Development Program of China, and the Shanghai Municipal Education Commission.
Jun Zhang
Shanghai Ocean University, College of engineering science and technology
Shanghai 201306, China
Email: [email protected]
Reference
Zhang, J., Zhang, Y., Guo, J., Liu, Z., Cao, S., & Liu, X. (2025). Hydrodynamics of large-scale recirculating aquaculture tanks equipped with paddlewheel aerators. Aquacultural Engineering, 111, 102569. https://doi.org/10.1016/j.aquaeng.2025.102569
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
