Low frequency ultrasound and UV-C for elimination of pathogens in recirculating aquaculture systems

Low frequency ultrasound (LFUS) was evaluated as a novel disinfection technique within recirculating aquaculture systems both individually and combined with UV-C. Dose-dependent inactivation rates were determined for the total viable counts and model organisms representing different taxa of common fish parasites: the ciliate Paramecium sp., second larval stage (L2) of the nematode Anguillicola crassus and metanauplii of Artemia sp. Application of LFUS up to 19 kJ/L did not reduce the number of colony forming units (CFU), whilst UV-C irradiation was highly effective. Pre-treatment with LFUS reduced the mean size of suspended solids in aquaculture water and thus increased the germicidal effect of UV-C by up to 0.6 log units. LFUS was effective against the eukaryotic organisms, and the dose-dependent inactivation could be well described by functions of an exponential decay. However, the efficiency of LFUS differed greatly between species. A LFUS dose of 1.9 kJ/L (consumed energy) was sufficient to inactivate Artemia by 99%, but a ten times higher dose was necessary to inactivate 95% and 81% of Paramecium and Anguillicola larvae, respectively. In clear water, the energetic efficiency of UV-C (emitted by a low pressure lamp) against Paramecium and Anguillicola larvae was higher compared to LFUS, but LFUS was more efficient against Artemia. However, the efficiency of LFUS against ciliates or nematode larvae would be similar or even higher than UV-C in highly turbid water or if less efficient medium pressure lamps are used. This study shows that LFUS can be applied safely at energy densities that are effective against a wide range of parasites like ciliates, nematodes and crustaceans. The combination of LFUS and UV-C could provide an appropriate water treatment with regards to all relevant pathogens in recirculating aquaculture systems.