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. 2012 Sep 6;120(11):1551–1558. doi: 10.1289/ehp.1205436

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Schematic of wastewater treatment processes at four wastewater treatment plants in the Mid-Atlantic and Midwest regions of the United States. For Mid-Atlantic WWTP1 (A) and Mid-Atlantic WWTP2 (B), 1 = influent, 2 = activated sludge reactor, 3 = post aeration, and 4 = effluent. (C) For Midwest WWTP1, 1 = influent, 2 = post aeration, 3 = secondary clarifier, and 4 = effluent. (D) For Midwest WWTP2, 1 = influent, 2 = cell B, and 3 = effluent. ^aSeasonal chlorination takes place in June, July, and August.