A Recirculating Aquaculture System (RAS). Image courtesy of Aquacare
Oxygen is one of the key factors in a RAS and often one of the first limiting factors when it comes to fish biomass. These systems are run at an intensity where neither natural gas transfer with the surface, nor aeration can supply sufficient amounts of oxygen in order to sustain the fish and bacterial biomass. This is why the systems need oxygenation.
With oxygen at a purity of ≥93%, you can over saturate the water with oxygen so that higher carrying capacities are possible. Oxygen is therefore injected into a recirculating system in low head oxygenators (LHO), oxygen cones (at high pressure) or diffusers.
Oxygen can either be bought in liquid form from a gas company or produced on site. On site production is often the better option where electricity costs are low, or the farm is in a remote location, or even where the aquaculturist doesn't want to sign a long-term contract with a gas company. If oxygen is bought in liquid form the tanks need to be cooled, and if the demand fluctuates, the tanks need a certain pressure release so that if the demand is below that, then the aquaculturist has to release that oxygen into the atmosphere. Where an aquaculturist chooses to buy the oxygen liquid, it is wise to have enough backup with onsite production too, in order to keep the fish alive, even if an unforeseen event occurs and there is no chance of oxygen delivery.
Onsite oxygen production in aquaculture is mainly done by one of two processes - pressure swing adsorption (PSA) or vacuum swing adsorption (VSA).
In PSA, compressed air is produced, then after filtering and drying, it flows through a column of zeolite. The zeolite acts as a molecular sieve, binding the nitrogen compound of air (78%) and providing oxygen at a high purity. New developments in PSA systems are now making it possible to use compressed air at lower pressure which saves energy. Furthermore, advanced zeolite can increase the efficiency of the adsorption even more. The oxygen is pressurised and can be directly injected into an LHO, oxygen cone or diffusers.
VSA systems use an oil-free blower. They operate at lower pressures than a PSA system, so therefore have lower operating costs. However, they incur higher investment costs, and depending on the application, an oxygen booster will also be needed. This is due to the low oxygen outlet pressure the VSA system provides.
Stable conditions combined with high oxygen levels are key for the fast growth of healthy and non-stressed fish, and therefore will decide the overall success or failure of a RAS venture. Compressed air plays a pivotal role within a Recirculating Aquaculture System in achieving these outcomes. But not just compressed air - a reliable and efficient supply of compressed air that has been correctly specified to meet the precise needs of the RAS in question. Maintaining high oxygen levels also equates to a lower feed conversion ratio, and that’s another real win when you consider that feed is the highest cost factor for an aquaculturist. Furthermore, the best equipment is only as good as its maintenance status. Providing a good service will minimise the risk of system failure.
Kaeser Compressors is proud to be the preferred supplier of air products to the aquaculture industry. Kaeser blowers are especially well suited to the 24/7 operation required of Recirculating Aquaculture Systems, ensuring process safety whilst saving considerable amounts of energy compared to other blowers.
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