MPN is an Appropriate Test Method for UV-based Ballast Water Management Systems
MPN is a well-established quantitative method to estimate the number of viable cells in a sample. It is a culture-based method that assesses the ability of an organism to colonize after treatment. It is useful for any treatment technology, not just UV, since neither dead nor non-reproductive organisms can reproduce. (40 CFR 141.74)
The USCG Final Rule itself uses grow-out testing to measure organisms in the <10 μm size class, i.e., all of the bacteria (E. coli, Enterococci, Vibrio cholerae, HPCs) in the discharge standard. ETV Protocol, 188.8.131.52 Organisms 10 μm
However, the USCG does not currently consider the MPN method to be an equivalent alternative to the vital stain method for the 10 – 50 μm size class. The USCG opines that the vital stain method is “applicable and practicable” for UV-based ballast water management systems.
The USCG’s current position disregards scientific data showing that unduly substantial power increases (~10x) are needed for UV-based ballast water management system to disrupt invasive organisms’ enzymatic activity to the point of measurement with the ETV Protocol (vital stain method). MacIntyre et al. (2015), Estimating phytoplankton viability using a culture-based most probable number assay, vital stains, and variable fluorescence following UV-C treatment. J. Phycol.
Also, according to a USCG-approved Independent Laboratory, dramatic increases in UV dose require substantial increases in power in combination with reductions in flow and/or operational restrictions on the water quality for which the system is approved.