Towards biocide-free recirculating aquaculture systems

Recirculating aquaculture systems (RAS) have been promoted as a sustainable supplement to net pen aquaculture and land-based flow-through systems, and RAS is currently a commonly used production concept. RAS have numerous environmental assets such as decreased water consumption, but there are challenges related to water quality control and use of biocides in some systems. As the retention time and degree of reuse of water increases, the nutrients and organic matter accumulate causing favorable conditions for micro-organism growth, which can result in decreasing water quality including blooms of harmful micro-organisms. Treatment of these blooms includes application of disinfectants such as formalin, hydrogen peroxide and peracetic acid. Recent new knowledge on the microbial dynamic and water quality in RAS has enabled development of alternative treatment methods, such as membrane filtration, UV, and ozonation, and preliminary studies have identified the potential of biological control of micro-organisms by their naturally-occurring zooplankton predators. However, these methods are not optimized and costly (physical control) or not tested in large scale (biological control), which hampers their application in aquaculture systems. Further, the sources, dynamics and environmental control of the development of micro-organism blooms are still not understood. For instance, it is conceivable that biofilter could function as a ‘ticking bomb’ and that imbalances in the biofilter communities could be the source of micro-organism blooms in treatment tanks. The main aim of our project ‘Towards biocide-free recirculating aquaculture systems’ (TOBIFREE) was to provide new knowledge of the causes and treatments of microalgal blooms in RAS systems that could result in reduction of the use of biocides in aquaculture. Specifically we wanted to investigate 1) the biofilter communities and whether they might act as a source for micro-organism blooms in growing tanks, 2) the potential alternatives to reduce the use of biocides, namely biological control by zooplankton and physical control by foam fractionation and ozone and 3) the barriers that aquaculture industry might have for the use of new treatment methods. Biofilters harbored a rich community of protozoans and invertebrates such as copepods, ostracods, nematodes, polychaetes, rotifers and diverse eggs, and appeared to function as small ecosystems with active reproduction and predator-prey interactions and high turnover times. Dominating groups or species differed between the facilities, likely depending on salinity or light conditions, but were typically similar in the different systems at the same facility. Also, abundances of most organisms did not seem to change due to maintenance cycle, suggesting that the organisms resisted backwashing and remained in the system. Experiments investigating the interacting effects of propagule size, nutrient concentrations and the presence of a zooplankton (ostracod) suggested that ostracods that are naturally present on biofilters can control the abundances of microalgae, even at high nutrient concentrations. Similarly, diverse cladocerans had high feeding rates on microalgae, and particularly individuals that were collected from lakes with cyanobacteria blooms were able to feed on toxic cyanobacteria Microcystis aeruginosa at high rates. Also, physical treatment methods were effective. Foam fractionation was a simple and effective water treatment technique to remove microparticles from freshwater RAS, and FF in combination with hydrogen peroxide and addition of salt led to significant reduction of both bacteria and turbidity. Pilot scale RAS trials documented beneficial properties of FF in terms of removal of microparticles, reduction of bacterial load, reduction of biodegradable organic matter and improvement of water clarity. FF combined with ozone led to an immediate and persistent improvement of water quality measured as bacterial load and microparticle concentrations. Both physical and biological treatment methods seem thus to be promising alternatives to chemical water treatment. Whereas biological treatments are still relatively far from application, physical treatment methods could become a viable option for freshwater RAS in near future.