33 CFR §157.39

1. (a)A tank vessel may discharge an oily mixture from a machinery space bilge that is combined with an oil cargo residue if the vessel discharges in compliance with § 157.37.
2. (b)A tank vessel may discharge an oily mixture from a machinery space bilge that is not combined with an oil cargo residue if the vessel:
   1. (1)Is proceeding en route;
   2. (2)Is discharging an effluent with an oil content of less than 15 parts per million; and
   3. (3)Has in operation an oil discharge monitoring and control system in compliance with § 157.12 and oil separating equipment in compliance with 33 CFR 155.380.