Hygienic Removal of Contamination in Recirculating Aquaculture System (RAS)

Osland Havbruk produces 8,500 tonnes of fish annually in its breeding facilities in Bjordal on the Sognefjord in Norway. Since June 2016, the new, state-of-the-art RAS (Recirculating Aquaculture System) has been in operation in the custom-built hatchery. There, salmon are grown, from when they hatch until they become smolt. To remove dead fish from the tanks, the subsidiary Osland Settefisk installed a pneumatic extraction unit from Busch Vakuumteknikk (Busch Norway) that enables hygienic removal so that the dead fish can be treated and further processed for the production of biogas.

Osland Settefisk was founded in 1987 and specializes in breeding salmon and rainbow trout. At the Bjordal site, 3,500 tonnes of trout are nurtured each year until they are ready for slaughter. While the trout eggs come from the company's own breeding stock, the salmon eggs are purchased and remain in the company's hatchery until the eggs hatch. Afterwards, the fry, which weigh up to approximately 0.2 grams, are put into smaller stock feeding tanks before they are placed in larger growing tanks; there they reach weights of 40 to 50 grams. A total of 2.5 million smolt are farmed each year at this facility and then taken by truck on a short ride to the wellboat that brings them to the net catches in the fjord. There, the salmon grow to their slaughter weight of four to six kilograms. Osland Havbruk then sells the live fish to slaughterhouses and processing plants. 

Just like many of its customers, Osland Settefisk is part of the network company Norwegian Salmon Group. This ensures a higher quality standard for the produced fish products because consistently strict quality guidelines are followed from the egg to the finished product during breeding, feeding and processing.

Fig. 1: Infeed station directly beside the collecting tank

To be able to quickly and hygienically remove dead fish from the production units, Operations Manager Kjetil Rørtveit looked for a technical solution, which was provided by Busch Vakuumteknikk in Drøbak, Norway. An extraction system developed by Busch was installed in an adjoining room. Contamination and dead fish sink to the deepest part of the tank where they are constantly removed with water and transported to a collecting tank while the water is then returned to the recirculating aquaculture system. This makes it possible to continuously check whether dead fish are in the collecting tank, and take measures if necessary. They are then manually removed with a net and placed in an infeed station directly alongside the collecting tank (Fig. 1).

Fig. 2: Extraction system at Osland Settefisk AS

The infeed stations at each tank are connected to the extraction system in a separate room using pipelines (Fig. 2). This mainly consists of a cyclon separator, a tank and an R 5 rotary vane vacuum pump. This vacuum pump from Busch constantly maintains a vacuum in the pipework. If fish are put into an infeed station and the valve opens, they are quickly removed without residue using the pipeline and they are then transported to the cyclon separator. Once this has reached a certain fill level, a valve disconnects the cyclon separator from the extraction system and ventilates it to atmospheric pressure. A valve under the cyclon separator opens and the dead fish land in the containers located underneath. The fish are shredded there and mixed with a metered mixture of formic and lignosulphonic acid. 

This enables the pH value to be controlled and oxidation can be slowed down. The resulting substance is used in biogas production and is automatically extracted from the mixer into a transport container. The company Krüger Kaldnes designed and realized the complete RAS. Vacuum experts from Busch worked closely with the engineers of Krüger Kaldnes to design the waste extraction system.

Busch offers individually designed extraction systems for fish breeding facilities of all sizes. These are connected to the RAS control so they are completely integrated into the operating control. A total of four infeed stations for dead fish are installed at Osland Settefisk. Their maximum distance to the extraction station is 50 metres. All PE extraction pipes have a diameter of 110 mil-limetres. For this plant size, Busch Vakuumteknikk uses an R 5 rotary vane vacuum pump with a pumping speed of 100 cubic metres per hour. These vacuum pumps are available in many sizes, so they can be precisely adjusted to fit the requirements and the size of the extraction systems. Thanks to the system control, it is possible to install a relatively small vacuum pump, which has a positive effect on energy consumption.

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