Use of standard test organisms for sound validation of UV-based ballast water treatment systems.

To challenge ballast water treatment system (BWTS) efficacy for organisms in the size-class 10-50 µm, intake concentration during tests must reach certain minimum requirements. Often, natural concentrations are too low to meet intake requirements and standard test organisms (STOs) are added. We tested the robustness of Tetraselmis suecica and Odontella sp. to a range of UV-treatments to explore fluences needed to meet the IMO discharge standard (<10 org. ml-1) evaluated using two viability assessment methods. To meet discharge standards, fluences of >1000 mJ cm-2 were required using vital stain whereas 135-500 mJ cm-2 were needed using regrowth assays. Besides, results suggest that T. suecica and Odontella sp. were at least as robust as natural algae towards UV-treatments. We suggest the advantageous use of these species as STOs in test water to support intake water requirements and to obtain more conservative validation of UV-based BWTS to ensure more environmental protective procedures.

UV fluences required for compliance with ballast water discharge standards using two approved methods for algal viability assessment.

This study investigates the extra UV fluence needed to meet the International Maritime Organisation's ballast water discharge standards for the 10-50 µm size-class using the approved vital stain (VS) method compared to the Most Probable Number (MPN) method for organism viability assessment. Low- and medium pressure UV collimated beam treatments were applied to natural algae collected in temperate and tropical water environments and analysed using both methods. About 10 times higher UV fluence was required to meet discharge standards when using VS compared to MPN. Implementing a dark-hold period after UV treatments decreased algal viability. Length of dark-hold period to meet discharge standards decreased with increasing UV fluence. No significant differences between temperate and tropical samples were observed. The results showed that UV treated algae assessed using the VS method could meet discharge standards by increasing fluence and/or introducing a dark-hold period.