Section 30(6)(d) in The Merchant Shipping (Prevention of Pollution by Oil from Ships) Rules, 2010
(d)on every oil tanker at sea, dirty ballast water or oil-contaminated water from tanks in the cargo area, other than slop tanks, may be discharged by gravity below the waterline, provided that sufficient time has elapsed in order to allow oil/water separation to have taken place and the ballast Water has been examined immediately before the discharge with an oil/water interface detector referred to in rule 32, in order to ensure that the height of the interface is such that the discharge does not involve any increased risk of harm to the marine environment;