PETER MATULAVICH / SCIENCE PHOTO LIBRARY PETER MATULAVICH / SCIENCE PHOTO LIBRARY
Wastewater being treated in secondary aeration tank at sewage treatment plant in Ohio. Air is pumped into the water to promote the growth of bacteria, protozoa, rotifers, nematodes, and other microorganisms which eat organic material and help break it down. Sewage treatment is a multi-stage operation that includes physical, chemical, and biological processes to remove contaminates from water.
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