Section 6(1)(c) in The Merchant Shipping (Control of Pollution by Noxious Liquid Substances in Bulk) Rules, 2010
(c)Category Z: Category Z is the category of noxious liquid substances which, if discharged into the sea from tank cleaning or de-ballasting operations, are deemed to present a minor hazard to either marine resources or human health and justify less stringent restrictions on the quality and quantity of its discharge into the marine environment;