What was found
STL TO MSP - AFTER TAKEOFF AT AROUND 1700FT, THE LAV SMOKE ALARM WENT OFF. THE FLIGHT ATTENDANT CONFIRMED THAT THE LAV SMOKE DETECTOR WAS GOING OFF. THERE WAS NO VISIBLE SMOKE BUT A SOOTY SMELL WAS PRESENT. AT THAT TIME AN EMERGENCY WAS DECLARED, ALL CHECK LISTS WERE ACCOMPLISHED, AND THE AIRCRAFT WAS RETURNED TO FIELD. UPON LANDING FIRE DEPARTMENT PERSONNEL INSPECTED FOR FIRE DAMAGE AND FOUND NONE. MAINTENANCE RAN THE ENGINES AND COULD SMELL SMOKE WHEN THE NR 2 BLEED WAS SELECTED. WHEN THE NR 2 ENINGE WAS VISUALLY INSPECTED, THE "0" CARBON SEAL WAS FOUND TO BE LEAKING. THE "0" CARBON SEAL WAS REPLACED ON THE NR 2 ENGINE, AND THE ENGINE WAS OPS CHECKED AND LEAK CHECKED GOOD.