Segregated Ballast Tanks
Segregated Ballast Tanks (SBT) are ballast tanks and pumping arrangements on a tanker that are entirely separate from the cargo oil and fuel oil systems. Because ballast water is only ever carried in these dedicated tanks and moved by dedicated pumps and lines, it never comes into contact with oil, so it can be discharged clean without treatment.
SBT is required by MARPOL on new oil tankers above a certain size, and on modern tankers the segregated ballast is also arranged to form a double hull protective space around the cargo tanks. It removes the old, polluting practice of carrying ballast in cargo tanks and then discharging oily water.
Before segregated ballast, tankers carried ballast in their cargo tanks and pumped oily water overboard — a major source of chronic oil pollution. Requiring dedicated, oil-free ballast tanks removed that at the source, which is why SBT is a cornerstone of the MARPOL anti-pollution regime for tankers.
Why must tankers have segregated ballast tanks?
So ballast water is never mixed with cargo oil and can be discharged clean, eliminating the oily-water discharge that used to occur when ballast was carried in cargo tanks. MARPOL requires it on new tankers above a size threshold.