SUBJECT LINE: Fifty Boston-area summer recreation sites

Find out what happens after you flush the toilet. A tour of the state-of-the-art Deer Island Treatment Plant takes you to such exotic spots as the Sludge Digestion Center and Odor Control. Free tours are the first Tuesday of the month at 9 a.m., with a special 10 a.m. tour Saturday, Aug. 4. Groups can book other times. ................
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