Temporary storage in ballast tanks:
Please refer to MN59
Temporary storage in cargo hold cleaning tanks (bulk carriers)
As grey water is NOT subject to any controls under MARPOL Annex IV and treated wastewater (TWW) may be discharged anywhere, provided of course that the ship has in operation an approved sewage treatment plant and the effluent does not produce visible floating solids nor cause discoloration of the surrounding water (MARPOL Annex IV Regulation 9), cargo hold cleaning tanks can be used for temporary storage of grey water and TWW without restriction (i.e. no conditions need to be imposed by Class nor surveys required).
If GW/TWW/untreated sewage is stored in ballast tanks, para 12 of MN59 applies, or if untreated sewage is stored in the cargo hold cleaning tanks MARPOL IV applies to discharge (i.e. more than 12nm from nearest land).