How a Waste Water Treatment Plant Works
can be recycled into the system for reprocessing. Eventually water flows into a final clarifier where it looks and is visibly free of sludge (right).
Digested sludge can be processed in a variety of ways, which include spinning in centrifuges (like the spin cycle in a clothes washer), and dewatering to remove as much water as possible and turn the sludge into a cake-like consistency--the drier the better. A polymer (chemical) is sometimes added to help this dewatering process along. Among the things that can be done with sludge is to incinerate it, dispose of it in landfills, and process it to make marketable fertilizer.
In the final treatment steps, chlorine is added to kill leftover bacteria. The tank shown here (left) is one of two at my local plant that provides "a lot of contact time" between the chlorine and the water to kill the pathogens (disease-carrying organisms). Most plants discharge water into a river.
(Photo credits: Ken Friedman took the egg beater aerator, chlorination tank, clarifier scum and grinder photos. All the others were taken by Karen Dancho, who works at the Bethlehem City Waste Water Treatment Plant. Karen was kind enough to contribute these photos, give me a tour of the plant so I could sniff the unique aromas, and she put an editorial pen to my description to make it more accurate. Without her help, I'd be in deep, well, you know.)
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