ACFT HAD SMOKE IN CABIN. CANNOT DETERMINE CAUSE. RETURN LAND.

Narrative

CLBING THROUGH 2000 MSL; THE AFTER TKOF CHKLIST HAD BEEN COMPLETED. FOR WHATEVER REASON THE TWR HAD NOT YET SWITCHED US OVER TO DEP CTL. THE FO WAS FLYING THE ACFT. I RECEIVED THE 6 BELL (EMER) CALL FROM THE 'B' FLT ATTENDANT. SHE ADVISED ME THAT DENSE SMOKE WAS FILLING THE AFT CABIN AND THAT THE SMOKE DETECTORS WERE SOUNDING. THINKING THAT A PACK VALVE HAD STUCK IN THE FULL HOT POS; I LOOKED OVERHEAD AND OBSERVED ALL NORMAL INDICATIONS ON VALVE POSITIONS AND TEMPS. AT THIS TIME MY THOUGHT WAS WE MUST INDEED BE ON FIRE. MY FEELING WAS THAT IT WAS PROBABLY COMING FROM THE LAVATORY. KNOWING THAT IF THIS WAS THE CASE; WE WOULD HAVE BUT A VERY SHORT TIME TO GET BACK ON THE GND. I ADVISED JAX TWR WE WOULD RETURN IMMEDIATELY AND ADVISED HIM OF OUR PROBLEM. WE HAD DEPARTED RWY 7 AND RETURNED TO RWY 25. AFTER THE FO TURNED THE ACFT TOWARD THE FIELD I TOOK CTL. JAX TWR HAD THE FIRE AND EQUIP STANDING BY. WE CHKED WITH FLT ATTENDANTS AND THEY STATED SMOKE WAS GETTING WORSE. I TOLD HER TO PREPARE TO EVAC WHEN ON THE GND. PRELIMINARY LNDG AND LNDG CHKLIST WERE COMPLETED. JUST BEFORE LNDG THE FLT. ATTENDANTS TOLD US THE SMOKE WAS NOW SUBSIDING AND THERE WAS NO EVIDENCE OF FLAME. I DECIDED NOT TO EVAC BASED ON THIS INFO AND KNOWING THAT SOMEONE ALWAYS GETS HURT DURING THESE EVACS. WE LANDED WITHOUT INCIDENT AND TAXIED TO OUR GATE. TOTAL AIR TIME WAS 5 MIN. AS WE TAXIED IN THE SMOKE STARTED TO CLR SLOWLY. I DID NOT DECLARE AN EMER AND ALTHOUGH OUR PROGRESS WAS NEVER SLOWED AND THE EQUIP WAS STANDING BY; I KNOW I SHOULD HAVE; THINGS WERE JUST HAPPENING TOO FAST AND IT DIDN'T GET DONE. REASON FOR SMOKE: INOP APU DUE TO BURNT OIL BEING DUMPED INTO PAX CABIN. DURING RON IN JAX; MAINT HAD WORKED ON APU AND IN DOING SO DUMPED A LOT OF OIL IN LINES AND WATER SEPARATORS AND HAD FAILED TO BURN IT OUT. WHEN WE TOOK OFF THE OIL STARTED BURNING.