Section 1A(1)(d) in The Merchant Shipping (Control of Pollution by Noxious Liquid Substances in Bulk) Rules, 2010
(d)"ballast water" includes,-(i)"clean ballast" which means ballast water carried in a tank which, since it was last used to carry a cargo containing a substance in Category X, Y or Z, referred to in ruse 6, has been thoroughly cleaned and the residues resulting therefrom have been discharged and the tank emptied in accordance with the provisions of these rules;(ii)"segregated ballast" means ballast water introduced into a tank permanently allocated to the carriage of ballast or cargoes other than oil or noxious liquid substances and which is completely separated from the cargo and oil fuel system;