In aerobic secondary sewage treatment, the fluid waste is aerated and then passed through a trickling filter. In this process, the liquid waste is sprayed over a bed of crushed rocks, tree bark, or other filtering material. Colonies of bacteria, fungi, and protozoa grow in the bed and act as secondary filters to remove organic materials. The microorganisms metabolize organic compounds and convert them to carbon dioxide, sulfate, phosphates, nitrates, and other ions. The material that comes through the filter has been 99 percent cleansed of microorganisms. Liquid waste can also be treated in an activated digester after it has been vigorously aerated. Slime-forming bacteria form masses that trap other microorganisms to remove them from the water. Treatment for several hours reduces the microbial population significantly, and the clear fluid is removed for purification. The sludge is placed in a landfill or at sea.
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