Innovation / Columns

Engineering R&D: Tiny bubbles

March 7, 2012
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If future generations eat seafood, it will be supplied by aquaculture. The tipping point in fish sourcing is expected to be reached this year, when aquaculture will surpass wild-caught seafood for the first time in human history.

Most aquaculture involves sea cages, where fingerlings hatched and developed in land-based tanks are later transferred to cages after they have achieved sufficient size. Completely farm-raised fish are a small part of total production, largely because of the energy costs involved. Researchers at Linde Gas attacked the problem by developing Solvox OxyStream, an oxygenation system that strips out inert gases such as dissolved nitrogen. The system also delivers an adjustable flow pattern to provide the correct hydrodynamics for the stock, dispersing oxygen evenly and giving the fish the opportunity to exercise by swimming against a mild current. A small pump suffices to deliver air to the system’s dissolver, which discharges dissolved oxygen and micro-bubbles through orifices submerged in the tank. An absence of moving parts makes the system virtually maintenance free.

The R&D effort was headed by Stefan Dullstein, a chemical engineer who has worked with industrial gases for 10 years. Dullstein first worked as an application engineer for water treatment in France before assuming the position of head of industrial segment aquaculture & water treatment at Linde’s German headquarters.

FE: When did you begin work on this oxygenation system?

Dullstein: It’s a long development story. We had systems for wastewater treatment and drinking water systems before getting involved with aquaculture. This is just the latest version, though it represents a significant advancement.

Linde had offered multiple systems for different fish species and different levels of salinity. We developed diffusers, oxygenation cones, Venturi systems combined with flow systems and other variations. Solvox OxyStream combines features of those approaches.

 FE: What distinguishes the technology?

Dullstein: There are several functional benefits, but the real key is economic. Oxygenation systems typically consume considerable energy, and the cost of that energy made fish from aquaculture too expensive. This low-pressure oxygenation system consumes only one-fifth the amount of energy, which makes fish farming much more attractive and competitive with wild-caught fish. It also offers more controlled environmental conditions than sea cages.

FE: Describe how the technology works.

Dullstein: Microbubbles that are smaller than 1mm are created and homogenously mixed before exiting through orifices in a vertical pipe extending into the fish tank. The precise size of the bubbles is less important than the velocity of the water as it exits and mixes several meters into the tank, though the smaller the bubble size, the better the technology works. It’s like a cloud when it emerges from the orifices.

The bubbles remind me of champagne bubbles that form at the bottom of a glass. As the bubbles created by the system rise, they absorb, or strip, nitrogen and other inert gases in the water. When the bubbles reach the surface, they burst, dispersing the gases into the ambient air. Nitrogen and other contaminants still must be filtered out of the tank’s water, but removing the dissolved nitrogen promotes fish growth and makes the fish less susceptible to disease. Fish go where the oxygen is, and the homogenous distribution of the bubbles prevents the fish from clumping up in one area.

Gas is mixed intensely inside the pipe. How we do it is our technical know-how and proprietary knowledge, but the key is dissolving the oxygen and distributing it evenly. The velocity of the bubbles varies, depending on conditions, but it helps create a current that the fish swim against. The fish stock benefits from the exercise, promoting health and product quality.

FE: How strong is the current?

Dullstein: Pumping pressure ranges from 0.05 to 0.2 bar (0.7-2.9 psi) and varies depending on fish species, the size of the tank and other factors. The actual pressure is customized to fit the application. This is not off-the-shelf equipment; the same modules are assembled, but each system must be sized according to species, tank size, water temperature and other factors. We try to duplicate the conditions the fish would encounter in the wild.

 FE: Have you quantified the impact of this system on fish size and health?

Dullstein: Trials conducted at Marine Harvest in Norway showed this was the only oxygenation system suitable for young salmon hatched in fresh water and gradually transitioning to sea water. Studies are on-going, and comprehensive results have not been released, but we have seen positive impact on the fish’s feed conversion rates. The general theory is that, as dissolved-oxygen concentrations approach saturation, the feed conversion rate goes down, meaning less feed is needed to produce the same amount of fish. Oxygen consumption goes up when the fish get excited, and they become excited during feeding. The system’s dynamic response to changes in oxygen levels allows it to increase or decrease availability as needed.

Marine Harvest is the largest salmon producer in the world. When trials began, they were conducted in tanks that were perhaps 15 meters wide. Two years ago, we would have thought that was the end of the story, but we now discover that there is a need for oxygenation systems for even larger tanks. Our new testing center in Alesund, Norway is surrounded by fish farms, and we expect to be testing on even larger tanks.

FE: What species have been evaluated?

Dullstein: Primarily salmon, but that reflects the popularity of salmon in Norway. Salmonid pose a challenge in fish farming because it is tricky to predict when they are ready for smoltification, the metabolic changes the fish undergo when transitioning from fresh water to salt water. The pure physics of salinity results in larger bubbles in fresh water, corrupting performance and forcing fish farmers to operate two oxygenation systems for salmonids. OxyStream works well enough in fresh water so only one system is needed, and allowing the fish to remain in the same tank means they are less stressed and less likely to die.

FE: Are specialized pumps and other equipment required to operate the system?

Dullstein: On some other systems, a side stream of water requires a second pump. This system relies on a single pump, and it can be whatever pump the farmer uses as part of the tank system. OxyStream is maintenance free, which our marketing people don’t like because we can’t sell a maintenance contract.

FE: Aren’t most farmed fish later transferred to sea cages and other confined areas for grow-out?

Dullstein: Sea cages remain a cost-effective approach. But large concentrations in cages pose environmental hazards, and large amounts of excrement can over-fertilize the ecosystem. Legislation approved in Norway in January is going to drive more land-based production by limiting the number of fish per sea cage to about 200,000 and by increasing the size of fish that can be raised in tanks from about 250g (8.8 oz.) to 1,000g (2.2 lb.). Diseases and infections are more easily handled in land-based tanks, where there’s no need to chemically treat the fish for sea lice, since these parasites can’t get into the system in the first place.

FE: Are developments in Norway a niche application or a harbinger for aquaculture?

Dullstein: Fish farming in Norway is the most industrialized in the world and produces about one million tons of salmon a year. Global tonnage for all fish species, mollusks and other seafood is as high as 50 million tons. China produces about 70 percent of the world’s farmed fish, but most production involves labor-intensive operations. Sooner or later, China will industrialize aquaculture by transitioning to higher-level technology. The same transition is underway in Norway and elsewhere to make fish farming a sustainable business. 





 

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