DNV GL use the same pH limit as the US Environmental Protection Agency (EPA) for newbuildings qualifying for their "Clean Design" notation.

Clean Marine offers the only Open Loop EGCS which complies with the most stringent Environmental Class notation offered by DNV GL: “Clean Design”. 
In order to qualify for the “Clean Design” notation the pH must be no less than 6 at the ship's overboard discharge, with the exception that during maneuvering and transit, the maximum difference between inlet and outlet of 2.0 pH units is allowed.