Demand amongst consumers for fish continues to rise, which is good news for fish farmers. It will, however, require the industry to invest in new production systems at a time when the costs of traditional open sea aquaculture are rising. Pond and open cage systems are expensive because they occupy large areas in already overcrowded sites, require huge volumes of water, and allow little control over the growing environment. No surprise, then, that many farmers are turning to recirculating aquaculture systems (RAS). RAS can consume as little as 5% of the total tank water volume per day as make-up to replace losses from evaporation and cleaning. This can be drawn from a borehole or even a municipal water supply, so the system can be located almost anywhere and does not need even to be near the sea, river or lake. Recirculating aquaculture systems are typically complex, indoor constructions, so are more expensive than either pond or cage culture but, if the system is managed properly to produce fish on a year round basis, the economic returns can make it worth the increased investment. They occupy a small footprint, producing an annual yield of around 150kg of fish per m³ of rearing volume, and, perhaps most importantly, provide a high level of environmental control. Their lower water volumes mean that temperature and salinity control are simpler. In addition, chemotherapeuticals dosing is significantly reduced and more efficient, and fish health significantly improved thanks to the high level of biosecurity and disinfection of all intake water. This makes RAS a popular choice for hatcheries and for the production of salmon smolt. But such high intensity rearing has its drawbacks.

Perhaps the most important factor in successful RAS operation is control of water quality. Salinity is an obvious parameter, and that will depend on the species of fish being produced, as are dissolved oxygen and temperature. High fish densities result in in high loads of metabolic wastes – BOD and ammonia – which creates an environment capable of supporting a wide variety of microbial life. Some bacteria are desirable, helping to purify the circulating water, and they are encouraged to grow on the media in biotreatment filters in a variety of controlled environments. In aerobic conditions, bacteria like Zooglea, Achromobacter, Pseudomonas and Flavobacterium oxidise carbonaceous BOD to carbon dioxide, whilst Nitrosomas and Nitrobacter convert ammonia into nitrate. If nitrate accumulation becomes a problem, denitrifiers like Thiobacillus denitrificans, can convert nitrate to nitrogen gas in anaerobic conditions. But excessive growth of these “friendly bacteria” leads to them sloughing off from the filter media and into the circulating water. However, the recirculating water can also harbour pathogens that cause fish disease and other bacteria like Actinomycetes and Cyanobacter which produce compounds that cause off-flavours like earthy tasting geosmin. Both result in delayed harvesting and loss of sales. So it is essential to control the accumulation of these microorganisms in order to maintain good production.

The biological treatment plant and filtration will provide good control of microbiota by controlling the level of organic matter in the circulating water, and can remove some pathogens. RAS provides a range of “micro environments” with different levels of dissolved oxygen favouring different bacterial species. Some of these, including Pseudomonas, excrete extracellular polysaccharides forming adherent biofilms on pipework and other surfaces. These biofilms are another source of free bacteria which slough off the surface and maintain the population in the circulating water. Traditional chemical disinfectants like sodium hypochlorite are toxic to fish so, clearly, cannot be used. The most widely used alternative chemical biocide is ozone. This is a powerful oxidant which can destroy the bacterial cell wall, and has the advantage that it rapidly decomposes to oxygen, Although this kills the bacterium, it allows the cell contents, including endotoxins, to be dissipated into the circulating water. Further, in brackish or sea waters, and to a lesser extent in fresh waters, ozone oxidises bromide and chloride to bromate and chlorate, both of which are potential carcinogens.

There is an alternative. UV radiation is a proven disinfection process that is chemical-free and leaves no toxic residuals or by-products. It can easily achieve a 4 of 5-log reduction in bacteria count and is completely safe. Evoqua has extensive experience in the aquaculture sector and has supplied a number of UV disinfection systems for on-shore and well boat applications. Among these installations are intake disinfection systems for a 150m³/h smolt hatchery RAS in Osland and a 360m³/h smolt hatchery RAS in Helgeland

Disinfection by UV is gaining rapid acceptance in recirculating aquaculture systems because it satisfies a number of criteria of importance. These include highly effective biocidal action, completely chemical free biosecurity and sustainability resulting from low energy consumption.

^{Featured in Fish Farmer.}