How SWRO on wellboats helps marine salmon aquaculture stay healthy & profitable
With the world's growing hunger for protein, aquaculture has become an increasingly significant source. Farmed salmon has emerged as a nutritious and favored option for millions of consumers worldwide. To keep the salmon free from parasites in marine aquaculture, seawater reverse osmosis (SWRO) is playing a crucial and innovative role. This is achieved through freshwater treatment on specially constructed wellboats.
As the global population grows in number and wealth, we continue to add more animal protein to our diets. In fact, the global per capita supply of animal protein has more than doubled since 1960. Seafood leads the trend: fish is the fastest growing animal-based food group, and we’re eating more of it than ever before.
Increasingly, however, the fish we eat is farmed, not wild. As you can see in the graph below, the supply of wild-caught fish is stagnating while that of farmed fish continues to grow. In 2020, the world harvested 86 million tons of fish for direct human consumption and caught 76 million tons of wild fish.
Graph adapted from Salmon Farming Industry Handbook 2021
Farm-raised salmon is a popular choice for millions
The global salmon market was estimated at 3.3 million tons in 2021 and is expected to grow to 4.2 tons by 2027.
Salmon's widespread availability and popularity can be attributed to the significant role of aquaculture. In current times, farmed salmon consumption is five times greater than wild salmon consumption worldwide. While Atlantic salmon is the most commonly farmed salmonid, steelhead trout, cohos, and chinooks are also reared.
Cold water is critical to the productivity and profitability of salmon farms, and as a result, they are mostly located in countries such as Norway, Scotland, Canada, and Chile, where such conditions are available.
Tiny sea lice are a big problem for farmed salmon
Typically, farmed salmon is raised in open-net pens located in coastal areas. These pens allow for the free flow of seawater, which ensures a constantly replenished natural environment for the fish. However, this natural environment also permits parasites to enter the cages. One such parasite that poses a significant threat to farmed salmon is Lepeophtheirus salmonis.
This parasitic sea louse feeds on the skin, mucous, and blood of salmon larvae. Although the tiny larvae, which are less than one millimeter in length, do find hosts in the wild, salmon farms provide high fish densities that are more vulnerable to infestation.
Salmon farmers face a significant challenge posed by sea lice. The presence of sea lice in salmon can render them unsellable, as it adversely affects their health, and severe infestations can even result in mass mortality. Furthermore, the high concentration of sea lice in open pens, made possible by thousands of farmed salmon, increases the risk of infestation for wild fish populations located near the farms.
Consequently, delousing infected salmon is an ongoing and arduous task for farmers.
Delousing salmon: How to solve one problem without creating another?
For several years, chemical treatments have been the primary means for farmers to delouse farmed salmon from sea lice worldwide. Various chemicals, such as hydrogen peroxide, cypermethrin, deltamethrin, and azamethiphos, have been employed for this purpose, with hydrogen peroxide and azamethiphos now being the most widely used.
Despite their efficacy in delousing farmed fish, chemical treatments cause additional complications. Chemically treated salmon cannot be consumed or sold for weeks after treatment, and the chemicals can have unintended toxic effects on non-target species, making it a significant environmental concern. Furthermore, there is a genuine risk that sea lice will adapt genetically to the chemicals, rendering them ineffective in the long term. Consequently, farmers have been exploring alternative non-chemical methods of delousing.
Various methods, such as the use of lasers and cleaner fish, have been trialed, but three mechanical techniques have emerged as more sustainable lice treatments. One such method is thermal treatment, which kills lice by temporarily immersing salmon in tanks containing seawater heated to 28-34 C. However, thermal treatment can cause fish injury and mortality. Another mechanical delousing technology is flushing salmon with low-pressure water jets, which effectively removes lice but can result in scale loss, gill bleeding, and other fish injuries.
Freshwater treatment is an innovative, gentle approach to delousing
Freshwater treatment is the third mechanical delousing technique that has been found effective against sea lice in farmed salmon. The technique involves temporarily transferring salmon from open net-pens to wellboats filled with fresh water to rid them of lice. Freshwater treatment does not harm the salmon or other marine species, as sea lice cannot survive in fresh water.
However, the technique requires large volumes of fresh water, which may not be easily available near open-net pens. To address this challenge, some salmon farmers are now using specially designed wellboats equipped with tanks and seawater reverse osmosis (SWRO) plants to treat the fish. While the number of such wellboats is currently limited, their use is expected to increase in the future. Careful monitoring and control of freshwater treatments will be necessary to ensure that sea lice do not develop resistance to the treatment.
Marine SWRO requires energy-efficient, reliable, and compact SWRO plants
Designing marine SWRO plants presents various challenges for engineers.
One of the key priorities is energy efficiency, as it is essential to minimize the specific energy consumption of the SWRO plant. This is especially important in marine applications such as wellboats, where all electricity is generated by diesel fuel. Therefore, the design and components must be carefully selected to achieve the most efficient performance possible.
Reliability and ease of maintenance are also crucial factors, but they become even more critical when a vessel is at sea for extended periods. To ensure seamless operations, the SWRO plant must be highly dependable, with longer service intervals.
In addition to performance and reliability, space is a major concern on board a vessel. To address this, SWRO plants must be compact and have optimal footprint-output ratios, allowing for efficient use of the limited available space.
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