Solving the newest old problems in RAS technology

The RAS sector is currently experiencing an unprecedented boom.

We are observing the numbers of large RAS installations growing each year, some at scales thought not possible a decade ago. With increased scale, the size of the risks and technical challenges has also grown.

The Jevon's paradox can be applied to RAS. The paradox postulates that increasing the efficiency of using a resource leads to an increase in the demand for such resource. In the end, the net use of the resource grows. With RAS, significant savings in water use have opened the map for suitable places for fish farming.

Peri-urban areas, rough coastlines and regions with limited water sources are increasingly attractive to build fish farms. Other areas with plenty of water available? Those are good for building even larger fish farms! In other words, being able to reuse more water is allowing us to build even larger farms that, due to their sheer size, will still use vast amounts of water.

Having a good source of water with excellent quality is as important as ever. In the past, water availability used to constrain our capacity to supply farmed fish with enough oxygen and to keep them safe from accumulation of wastes. Rob van de Ven explains, 'Nowadays, with RAS, water availability hinders our capacity to keep the water free from less dangerous, but still aggravating agents. Nitrate, fine solids (which are mostly bacterial flocs) and sludge (which can lead to H2S formation) are next on our laundry list of RAS problems to solve.'

'The cost, complexity and novelty of technologies to control nitrate, fine solids and microbes are often barriers to their commercial adoption', Carlos Espinal adds. For instance, the use of ozonation is almost non-existent in Norway, but more prevalent in North America and Germany, where ozonation research has been historically undertaken.

The use of denitrification reactors is also rare in Norway as, historically, enough water has been available to control nitrate by dilution and water temperature control was not a requirement. However, denitrification technology is more prevalent in countries with water scarcity, stringent water discharge regulations or system heating needs. Israel, Germany, Denmark and the Netherlands are good examples.

To push Jevon's paradox away, at least for a while, Landing Aquaculture is developing some products which, hopefully, will increase the adoption fine solids filtration and denitrification technologies of in RAS.

Improved protein skimming

Protein skimmers are a common technique for treating fine solids in RAS. However, skimmer performance can vary wildly in each application. Salinity, gas to liquid ratios, the presence (or absence) of oils, bubble size and the addition of ozone will affect efficiency.

Protein skimmer sizing and application are half science and half art, with no clear agreement among the experts on how much of the water of a RAS should be treated or what ozone doses are safe depending on the application. The use of protein skimmers in freshwater is limited and the fear of harmful ozonation residuals makes seawater RAS operators weary.

Landing Aquaculture is therefore developing a variant of the classic protein skimmer that they have named 'DAFSkim'. Instead of a classic protein skimmer vessel, DAFSkim uses a specially designed dissolved air flotation chamber.

The chamber produces smaller bubbles and holds the process water for longer, resulting in an increased filtration efficiency. Instead of ozone, DAFSkim uses a peracetic acid and hydrogen peroxide mix which achieves a similar oxidation effect without the need for expensive ozonation equipment or the risks of using ozone gas.

The liquid compound is immiscible in water, requiring minimal mixing equipment and eliminating any needs for gas to liquid transfer devices. For RAS farms that already have ozonation equipment, DAFSkim can be operated with ozone, just like a conventional skimmer.

DAFSkim comes from the development of Desline, a bloodwater treatment system developed by Landing Aquaculture in collaboration with Sandtorv Maskin (a Norwegian supplier of offshore and aquaculture solutions), Napier (a Norwegian shipping company), Innovation Norway and NOFIMA. The technology has proven to reduce color, oils and suspended solids in bloodwater. It also successfully inactivates bacteria such as Aeromonas salmonicida.

To bring DAFSkim to RAS, Landing Aquaculture is currently looking for academic and industry partners. With proper development and testing, the company expects to offer the market not only a more efficient, but also a more predictable way of controlling fine solids and microbes in both freshwater and seawater RAS.

Simplified denitrification tech

Denitrification reactors can be highly efficient and compact. They can also be somewhat complicated, requiring monitoring and control systems. Denitrification reactors using fish sludge as a carbon source offer simplified operation and relatively lower costs – although they tend to be larger and less efficient. Their main difficulty: keeping the sludge inside the reactor vessel for the required time across a range of water flows.

Carlos Espinal explains the challenge, 'We believe that cost and complexity are partly limiting the acceptance of denitrification systems in the market. To tackle this, Landing Aquaculture is developing a sludge reactor design which has no moving parts, no complex control systems and yet offers good performance'.

The reactor, coined DeSludge, is fitted with internal components that efficiently trap sludge and keep it under gentle mixing. The sludge ferments under anaerobic conditions, providing denitrifying bacteria with the energy they need to convert nitrate to nitrogen gas. A RAS fitted with a Desludge unit is not only able to decrease its daily water requirement. But will also lower the cost of sludge transportation and disposal due to the thickening of the produced sludges.

Rob van de Ven closes, 'We enjoy being creative as much as doing sound engineering work. We hope this leads us to technical solutions that the market can accept in the short term'

LandIng Aquaculture is an engineering and consultancy company specialised in land-based intensive fish-farming and focused on creative, R&D-driven problem solving. Since 2014, LandIng Aquaculture has completed more than 40 projects in 19 countries and continues to develop with a deeper focus in R&D and tailor-made solutions.