USCG makes decision on use of MPN method for ballast water management systems

The USCG has declined the use of the MPN method for evaluating the biological efficacy of UV-based treatment technologies for ballast water. As a consequence, DNV GL foresees a delay when the USCG will conclude on the first type approval of any BWM system.

On 14 December 2015, the United States Coast Guard (USCG) declined the 10(b1) applications to alternatively use the most probable number (MPN) method for evaluating the biological efficacy of UV treatment technologies that had been submitted individually by four ballast water management system (BWMS) manufacturers. The rationale given by the USCG is that the wording in the regulation is live/dead, and that the MPN method does not evaluate the performance of a BWMS to that discharge standard. The MPN method evaluates the ability of an organism to reproduce and hence its ability to colonize a new environment. As such, it does not provide a result equivalent to that of the USCG’s preferred method (vital staining). 

DNV GL is of the position that the MPN method is the most relevant method and is a reliable way of evaluating the performance of UV technologies. That method has been validated to a greater extent than most of the methods described in the Environmental Technology Verification (ETV) Protocol (prescriptive guidance incorporated by reference to US regulation), and UV technologies are commonly accepted in other water treatment industries. 

DNV GL (with subcontracted laboratories) has provided much of the MPN analysis done for the four BWMS manufacturers.

DNV GL expects the manufacturers to appeal the USCG decision.

Disinfecting water while ensuring acute kill of the organisms (instead of the ability to reproduce) will require a much more conservative dosage, implying higher power consumption (three to five times) by the UV lamps compared to most systems designed today. It may also imply more operational restrictions, such as minimum holding times (in a BW tank) and UV transmittance (UV-T) limitation for the BWMS. UV-T is one of the key limiting factors that UV BWMS is tested for, indicating the ability to irradiate “murky” waters. Treatment in harbours with low UV-T will potentially require a reduction of the flow rate to increase the exposure time to the UV lamps and thus the dosage applied to the water.


Some of the likely consequences of the USCG decision are:

  • The first BWMS system to be type approved by the USCG (tested by DNV GL) will probably be available in summer 2016. This will not be a UV system.
  • BWMSs using UV technology need to apply a more conservative treatment dose to immediately kill organisms under the current USCG policy.
  • Shipowners should consider possible operational limitations of UV-T, holding time, energy consumption and reduced flow rate when discharging ballast in US waters with UV systems.
  • UV systems might be equipped with a setting for USCG mode and International Maritime Organization (IMO) mode.