Removal of fish pathogenic bacteria in biological sand filters

Documentation is required to evaluate the use of infiltration systems as an alternative method for removal of fish pathogenic bacteria in wastewater from fish-farms. This study was performed to investigate the removal of bacterial fish pathogens in biological sand filters. A second aim of the study was to evaluate the bacteria used in the study in order to find a suitable model organism for future experiments. Low-strength wastewater from an inland freshwater salmonid farm was intermittently loaded (70 mm/day in 24 doses) to filter columns containing either fine sand (d10=0.25) or coarse sand (d10=0.86). After a wastewater loading period of 10 weeks, separate sand columns were seeded with Yersinia ruckeri, Pseudomonas fluorescens, Aeromonas hydrophila and Aeromonas salmonicida subsp. salmonicida, respectively, for a period of 30 days. All the bacteria showed the same removal performance during the experiment, with a significantly lower removal in the beginning of the experiment (day 1–7) compared to mid- and late-phase (day 12–30). In mid- and late-phase the removal stabilized at a high level (>99.9%) for all the bacteria. The hydrophobic cell surface properties of the Aeromonads were higher than Ps. fluorescens and Y. ruckeri. This can possibly explain the significantly higher (P<0.05) removal efficiencies seen for A. hydrophila and A. salmonicida subsp. salmonicida compared to Y. ruckeri and Ps. fluorescens. Results were promising with regard to the use of low-cost infiltration systems as an alternative disinfection method for fish-farm wastewater. Following the criteria for a suitable model organism (removal efficiency, detection in filter effluent and die-off in storage tanks), Y. ruckeri was found to be a feasible model organism for use in future experiments.