Optimarin AS, a pioneer in ballast water treatment, has become the first system supplier to receive the USCG Type Approval (TA) Letter meeting the most stringent US Coast Guard test requirements. In a series of land-based tests, both the standard Most Probable Number (MPN) (regrowth) method and the more exacting technique known as FDA/CMFDA, or ‘instant kill’, benchmark was successfully assessed. Testing of the Optimarin system was carried out by DNV GL at the Norwegian Institute of Water Reseearch (NIVA) test facility in Norway. The system was also tested in parallel on a bulk carrier trading worldwide for the ship board portion of the TA testing. Optimarin's objective has been to keep the system that have been TA under the International Maritime Organization (IMO) regime as is so that existing users can continue to use it as a USCG TA system. This paper details the challenges and differences with the USCG required Environmental Technology Verification (ETV) test protocol versus the existing and new G8 test protocol used for IMO TA testing, especially the challenges using Ultraviolet (UV) to meet the CMFDA counting method for organism between 10 to 50 μm.