A CRJ-700 developed a strong odor accompanied by the SMOKE AFT LAV Master Caution Warning after takeoff. An emergency return to land was completed. The crew was told that an engine wash the previous night that may have left a chemical residue in the engines.

Narrative

On initial climbout; we noticed a strong odor which was followed by the illumination of the 'SMOKE AFT LAV' Master Warning (Red) light. We declared an emergency and requested an immediate return to the field. I ran the checklist while the First Officer took over ATC. Flight Attendants confirmed there was a very strong odor but no significant or detectable smoke. After completing the checklist; we made a normal approach and landing at ZZZ. CFR checked and confirmed there was no indication of fire or damage to the aircraft; so we taxied to the gate for a normal deplaning. We did not detect any smoke in the flight deck. The initial Master Warning message extinguished after we completed the checklist; which included pulling several CBs. After arrival at the gate; maintenance personnel informed us that the aircraft had an engine wash overnight; and they suspected the fumes were caused by residual chemical substances in the engines. This has not been confirmed by me as the actual cause of this event.