As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: ON TAKEOFF BOTH AFT LAVATORY SINKS OVERFLOWED. WATER NOW BROWN AND SMELLING OF SEWERAGE CONTINUED TO FILL THE SINK. FOUND AFT DRAIN MAST CLOGGED. REMOVED BUILD UP FROM MID AND AFT SINK DRAIN LINES BY AIR PRESSURE AND BIOZIME TREATMENT PER AMM 38-31-00-100-801.