Duncan Ockendon of atg UV Technology sheds some light on the microbiological problems of aquaculture.

Life on Planet Earth began in the sea, but it is only quite recently that we have begun to understand just how important the marine environment is to life on the land.

Centuries of industrial pollution and over-fishing have left our oceans in a parlous state: poisoned by microplastics and unable to absorb the carbon dioxide generated by burning fossil fuels. All this at a time when we are intensifying aquaculture to feed an ever-increasing population. If fish farming is to be profitable in the long term it has to be sustainable, and that means healthy fish in a healthy environment. We are living in an aquacultural revolution whose impact will be every bit as great as that of the agricultural revolution in the 18th century.

The key to fish health and the minimisation of mortality is the control of parasites and pathogenic bacteria, and farms and hatcheries are particularly vulnerable. Fish farms are often located in close proximity, so effluent discharges from one can pose a significant threat of contamination to another.

Chemical disinfection of wastewater prior to discharge prevents this type of cross contamination but does not address the problem of fish health within the farm. One solution is the use of specific medication like antibiotics and de-lousing agents such as diflubenzuron and teflubenzuron, to treat disease once it has been detected, but consumers increasingly demand chemical and antibiotic free products.

In any case prevention, by eliminating the pathogens that cause disease, is better than cure. Disinfection by ultraviolet (UV) irradiation has proved to be an effective, chemical-free treatment for both water supply and wastewater discharges, safeguarding brood stock, egg production and fry growth.

The process, which acts directly on cell DNA to prevent reproduction, is effective against a wide range of microbes including viruses, bacteria and protozoans and, unlike treatment chemicals, it is impossible for resistant strains to develop. This is particularly important in indoor recirculating aquaculture systems (RAS) where the only water replacement is the water lost to evaporation and cleaning, so bacteria are retained in the system for long periods.

This means that resistant strains will multiply, with numbers doubling every twenty minutes, and build up in biofilms in pipework which are very difficult to remove. Ozone was widely used to control microbes in many older RAS farms, but it is expensive in both capital and operating costs. It is also hazardous and toxic so needs careful control both in the water and the atmosphere.

Transporting fish between onshore and offshore farming facilities in well boats can expose fish to a wide range of infections. Once again, UV can replace conventional chemical disinfection systems for intake, discharge and recirculating water used during fish transportation, protecting fish, minimising vaccinations and increasing stock yields. atg UV developed their ultra-compact WF range of UV units specifically for this application, and it recently gained Norwegian Veterinary Institute (NVI) approval based on 45 weeks of field-testing on board a Norsk Fisketransport well boat used for the transport of live salmon from sea cages and for de-lousing operations as a service to fish farms on the Norwegian coastline. Water samples were collected weekly for heterotrophic bacteria and Vibrio species analysis which showed an average 2.5 log reduction.

The WF range was designed to be compact enough to fit into the limited space available on well boats and similar marine vessels and provide a high level of disinfection for intake, discharge and recirculating water used during fish transportation. The minimum required UV dose of 25 mJ/cm2 is ensured by varying the lamp power proportionally to the measured UV intensity in the lamp chamber and the flow rate. The medium pressure lamps used in the units are low in operating cost, and a low UV dose alarm function is provided which can be interlocked with overboard valves to ensure that no untreated water is discharged into the sea. For on-shore aquaculture sites, the Data Stream feature on the UV systems allows the performance to be monitored remotely via an Ethernet connection.

Erling Lorentzen of Norsk Fisketransport was impressed with the UV equipment, "We used the UV systems during smolt transport and, despite the demanding conditions on the vessel, the UV systems maintained the required UV dose", he said, "We will use these systems in the future on our boats."

Disinfection by UV is gaining rapid acceptance in the aquacultural sector because it satisfies a number of important criteria. Firstly, it is a highly effective biocide but also meets consumer pressure for chemical free production. The equipment is robust enough to meet the challenges of the marine environment and its low energy consumption contributes to low operating cost and sustainability. All in all, the future for UV is bright.

About atg UV Technology

The company has over 35 years" experience in UV disinfection covering industries from oil and gas to food and beverage and pharmaceuticals. atg UV, established in 1981, are industry experts in the design, production and maintenance of ultraviolet disinfection/treatment systems and are recognised as a market leader for the supply of high-quality UV equipment and services.

With thousands of installations worldwide, a strong commitment to research and development and an ongoing validation programme, atg UV offers a wealth of industry experience, specialised knowledge and expertise. www.atguv.com

Contact: atg UV Technology, Genesis House, Richmond Hill, Pemberton, Wigan, Lancashire WN5 8AA Tel +44 (0)1942 216161 info@atguv.com

by Duncan Ockendon, atg UV Technology, UK

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