Interim guidance for ships operating in challenging water quality conditions (MEPC.387(81)) The purpose of this guidance is to assist ships to comply with the BWM Convention and the D-2 discharge standard when a type-approved ballast water management system (BWMS) encounters operational limitations or has difficulty meeting operational demand in challenging water quality (CWQ) conditions. This applies also when the BWMS has been properly installed, operated and maintained.
https://www.dnv.com/news/new-imo-guidance-for-managing-challenging-ballast-water-quality/