UV light can kill 99.9 percent of bacteria and viruses found in the inlet water of aquaculture facilities. But how large should the doses be?
Recent Nofima research in the CtrlAQUA Intake project “Strategies for water treatment in closed-containment aquaculture” shows that there is a lot of gain from adapting the dose to the pathogen one is dealing with.
The technology is known. The knowledge the scientists generate through trials is fresh.
In the process of disinfecting water with ultraviolet light (UV), special UV lamps are used that emit light of a certain wavelength. Low pressure or medium pressure. These UV light waves are able to interfere with the DNA found in harmful microorganisms. UV dose is another characteristic crucial for the disinfection, the harder is to kill a pathogen, the higher UV dose is required and vice-versa. According to adopted guidelines from the Norwegian authorities, there is a minimum UV dose to apply: 25 mJ/cm2
However, using ultraviolet light is costly, both in the form of spent energy and the special lamps. So, the question has to be asked; does one need to use high doses on all the different harmful bacteria and viruses – also called pathogens?
“Because UV treatment of intake water is so energetically costly, it will be more economically sustainable to use the exact dose that is necessary to eliminate a specific pathogen. Neither more nor less. We test different doses to find out which dose leads to a 99.9 percent reduction of various pathogens. We do not require particularly high doses for most pathogens”, says Nofima scientist Vasco Mota.
Six pathogens investigated
Two master students, Kari Justad (The Arctic University of Norway) and Miguel Guerreiro (University of Algarve, Portugal), conducted their work at Nofima Tromsø laboratories. Together with Nofima technicians and researchers they used two UV Collimated Beam Apparatus to investigate six different pathogens:
- Two Virus: IPNV and ISAV
- Three Bacteria: Yersinia ruckeri, Moritella viscosa and Tenacibaculum spp
- One copepodite: salmon sealice
Two different UVC technologies of distinct wavelength lengths (λ) were used; a low-pressure (254 nm), and a medium pressure (220-300 nm).
“The IPNV is the only one of the five virus and bacteria we have tested that required a very high dose, luckily there is a vaccine against it. All the other pathogens were eliminated using UV doses typically below 10 mj/cm2”, says Vasco Mota.
Seawater for the trials was collected at depths of 2 and 40 metres at two different sites: one close to a salmon farm, and one that hadn’t been influenced by fish farms. Two different UV-C techniques of distinct wavelength (λ) were used during the trials; low pressure of 254 nanometres, and medium pressure between 220-300 nanometres. In all the trials, 99.9 percent of pathogens were eliminated using a lower dose (and therefore lower cost) than the authorities require.
“These results may indicate that there is a need to assess the UV doses that are required to disinfect the intake water of aquaculture facilities, which could potentially reduce energy costs regarding UV disinfection. The exception in our trials was the IPN virus which required a high UV dose, but the existence of a vaccine for this virus reduces the need to eliminate it. All the other pathogens were eliminated using much lower doses”, says Vasco Mota.
And what about the challenging salmon lice?
“Lice is hard to kill with UV. At least without using absurdly high UV doses (> 100 mj/cm2)”, says Vasco Mota.
Relevant on land and at sea
Water disinfection using UV is relevant for inlet water in land-based farms. This includes both flow-through systems and RAS. And for semi-closed containment facilities at sea.
“However, facilities at sea pose a challenge because of the enormous volumes of water that are used. Up to 100,000 litres of water per minute. It is not possible to use low pressure UV when working with these amounts of water”, says Mota.
Aquaculture facilities are getting bigger all the time, and medium pressure UV is easier to scale up.
“Medium pressure UV also attacks the proteins and enzimes, the machinery that the harmful microorganisms use to repair cell damage. As a result, UV disinfection efficiency is further strengthened”, the scientist says.
The fish are not affected
The UV treatment takes place when the water is on its way into the facility, where the fish are located. Therefore, at no point will the salmon be exposed to the UV rays – regardless of the chosen method.
“The rays do not affect the water in a way that can harm the salmon, nor do they affect the environment. In this sense, UV is a perfectly acceptable technology to use for disinfecting water”, the scientist states.
New trials will be carried out at four commercial fish farms in 2022. UV disinfection will be tested using clear seawater – which is common during summer and winter – and with more murky seawater, which typically occurs as a result of algae growth during spring and autumn.
“We will not be adding pathogens in these trials. Instead, we will treat what is naturally found in the water”, says Vasco Mota.