This study investigates different UV doses (mJ/cm(2)) and the effect of dark incubation on the survival of the algae Tetraselmis suecica, to simulate ballast water treatment and subsequent transport. Samples were UV irradiated and analyzed by flow cytometry and standard culturing methods. Doses of ≥400 mJ/cm(2) rendered inactivation after 1 day as measured by all analytical methods, and are recommended for ballast water treatment if immediate impairment is required. Irradiation with lower UV doses (100-200 mJ/cm(2)) gave considerable differences of inactivation between experiments and analytical methods. Nevertheless, inactivation increased with increasing doses and incubation time. We argue that UV doses ≥100 mJ/cm(2) and ≤200 mJ/cm(2) can be sufficient if the water is treated at intake and left in dark ballast tanks. The variable results demonstrate the challenge of giving unambiguous recommendations on duration of dark incubation needed for inactivation when algae are treated with low UV doses.

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Disinfection of water is required for a range of applications, including ballast water treatment and land-based fish farming. Bacteria attached to or embedded in particles can be protected from the disinfectant by various mechanisms. We investigated inactivation of marine heterotrophic bacteria in the presence of biotic and abiotic particles. In one set of experiments with the planktonic rotifer Brachionus 'Nevada', water was exposed to increasing UV and ozone dose, and we examined inactivation of free-living and particle-associated heterotrophic bacteria. An estimated 99.9% inactivation of free-living bacteria was obtained compared to only 91.4% inactivation (3 of 4 experiments) of particle-associated bacteria at the same ozone dose. For the UV experiments, a 6-fold increase in disinfection dose was required to obtain 99.9% inactivation of the particle-associated compared to the free-living bacteria. In a second set of experiments we investigated the protective effect of biotic (rotifers) and abiotic (ceramic clay) particles as a function of particle concentration. Increased particle concentration resulted in reduced disinfection efficiency of free-living bacteria with both UV and ozone. Rotifers protected slightly better against UV disinfection than ceramic clay particles, while such a relationship was not evident for the ozone disinfection. The results suggest a complex bacterial inactivation mechanism in the presence of particles, and will have implications for the treatment strategy used for ballast water and land-based fish farming.

UV] irradiation, heat, and cavitation) treatment technologies (Shannon et al. 2008, Werschkun et al. 2012, 2014). Traditionally, water analysis used to assess most treatment technologies has depended on cultivation, such as the plate count or the most probable number (MPN) technique. Cultivation methods measure viability of organisms present, i.e. the ability of a cell to reproduce. Vital (live) cells can be either viable or non-viable, whereas non-vital (dead)