Towards biocide-free recirculating aquaculture systems

22-09-2021

Recirculating aquaculture systems have been promoted as a sustainable supplement to net pen aquaculture and land-based flow-through systems.  Recirculating aquaculture systems 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.

Biofilters of the recirculating aquaculture systems 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 (FF) was a simple and effective water treatment technique to remove microparticles from freshwater recirculating aquaculture systems, 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 recirculating aquaculture systems in near future.

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