Recirculating aquaculture: How RAS facilities make fish farming more sustainable
The demand for fish is rising worldwide, while wild-capture volumes are stagnating. Aquaculture systems are therefore increasingly helping to meet demand. A particularly dynamic development is seen in the market for recirculating aquaculture systems (RAS), that enable sustainable fish farming. How economical and reliable the operation of such systems is depends on the selection and design of the pumps and valves used.
Less environmental impact, more planning reliability
Conventional aquaculture systems such as ponds, cages and flow-through systems are simple in design or cheap to operate. All of the three methods have a more or less direct water exchange with the environment – with ecological as well as economical disadvantages.
When salmon is farmed in open off-shore cages, for example, fish excrement, feed residues and medication may have a negative impact on the surrounding ecosystems. If farmed salmon escape, reproduction between farmed and wild salmon may alter the pool of genetic diversity. Pathogens and parasites may also be transferred between wild and farmed fish. Open aquaculture systems further depend strongly on environmental conditions, such as temperature, water quality or weather events. This heightens their economic risk.
RAS facilities, by contrast, use closed water circuits. They usually consist of a series of circular tanks located in long halls. The water is continuously recirculated and filtered. Using RAS facilities brings many ecological benefits. Re-using the water minimises the amount of water required. Nutrients, excrements and medication are not released into the environment. As the farmed stock is not in contact with wild populations in this system, the risk of spreading disease is lowered considerably.
Numerous species can be farmed in recirculating aquaculture systems (RAS): The most frequent are Atlantic salmon, rainbow trout, tilapia, African catfish, whiteleg shrimp and zander.
RAS also bring some economical benefits. Large retail chains and food processing companies prefer fish and seafood from closed aquaculture systems, as – compared to conventionally farmed products – the quality is more uniform and the availability can be planned more reliably. This trend is further strengthened by the rising demand for sustainably farmed products with traceable delivery chains.
This makes the market for recirculating aquaculture systems (RAS) particularly dynamic. Recent studies estimate the global market size at around 3 to 5 billion US dollars in 2024. Projections suggest that the market could rise from about 7 to over 8 billion US dollars by the early 2030s.
Efficient pumps are crucial for RAS
The development of recirculating aquaculture systems is promoted by international organisations and public support programmes. The FAO considers RAS a key building block in their “Blue Transformation”, an initiative for more sustainable fish production. The European Maritime, Fisheries and Aquaculture Fund (EMFAF) has made about 6.1 billion euros available for the period of 2021-2027 to support fisheries, aquaculture and maritime policies – including investments in RAS.
For RAS facilities to be able to work economically and reliably, efficient pumps are essential. They keep the water moving continuously and make sure it can be consistently cleaned, degassed and oxygenated. They not only transport the water, they also provide the volume flow rates and pressures required for the individual treatment processes.
Principle of a recirculating aquaculture system (RAS): A mechanical filter removes feed residues and excrement; a biofilter removes dissolved pollutants such as ammonium. Finally, excess gases are expelled, and the water is oxygenated.
First, a mechanical filter removes feed residues and animal excrement. Then, the water flows through a biological filter, in which microorganisms convert the ammonium produced by the animals’ metabolism into the less harmful nitrate.
In a degassing unit, also known as CO₂ stripper, dissolved gases such as carbon dioxide are expelled. These gases are generated by fish respiration and bacterial processes in the biofilter. If the gas content in the water is too high, the animals become stressed and growth is slow.
In the next stage, the treated water is enriched with oxygen. Different methods can be used for his purpose, such as oxygenation cones or low head oxygenators (LHOs). Before the water is then returned to the tank by recirculation pumps, it is often disinfected, for example by UV light or ozone treatment.
More efficiency by holistic optimisation
KSB's portfolio covers the entire range of aquaculture applications, from extracting water from oceans, rivers and groundwater to recirculating water and adding oxygen right through to transporting sludge and waste water.
KSB places the focus not only on the efficiency of individual pumps but employs a holistic approach to optimise the entire hydraulic system. The aim is to minimise the energy consumption and ensure a long service life of the pumps.
Pump technology for recirculating aquaculture: The photos on the left and in the middle show Etabloc pumps used for oxygenation, the one on the right shows several Meganorm pumps for recirculation.
Energy cannot only be saved by employing high-efficiency pumps and speed-controlled motors but also by the selection of this equipment – by making sure that the pump matches the actual demand as closely as possible and by adjusting the impeller diameter. Operating the pumps as close as possible to their best efficiency point is pivotal.
As the pumps in recirculating aquaculture systems have to continuously move large volumes of water at usually low height differences, preventing unnecessary flow resistances is very important. Resistances in the piping and valves would make a higher pressure necessary, which would drive up energy costs. Adequately dimensioned piping with short and direct routes as well as flow-optimised valves lower the required pressure and, with that, the energy consumption.
Especially in recirculating aquaculture systems, this approach can deliver substantial energy savings, as these systems contain large numbers of pumps that are operated around the clock. Here, even minor efficiency increases can add up to significant energy and cost savings in the long term.
To identify such potential savings, KSB offers energy and efficiency analyses. In this way, KSB supports RAS operators in lowering their total cost of ownership, enhancing the operating reliability and reducing CO₂ emissions. This is how efficient pump technology makes recirculating aquaculture systems not only more economical but also more sustainable.
