Interesting post for purging the dehydration store:
http://www.oracle.com/technetwork/middleware/bpel/learnmore/bpeldehydrationstorepurgestrategies-192217.pdf
Also a simple explanation of High watermark:
http://www.oracle.com/technetwork/middleware/bpel/learnmore/bpeldehydrationstorepurgestrategies-192217.pdf