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component of a modern sewage-treatment system. In a large metropolitan system, waste water is passed through a series of tanks following coarse screening to remove large objects. Grit chambers settle out sand and cinders; primary settling tanks remove heavy organic solids; and aeration tanks mix the incoming sewage with activated, bacteria-rich sludge and fine air bubbles, causing the rapid oxidation of organic matter. Final settling tanks remove the remaining suspended solids from the mixture.