How is wastewater cleaned? American Geosciences Institute

Most of the water that is used in homes and businesses is put into either municipal sewers or home septic systems. Most of that water is polluted to some extent, because it comes from clothes washing, bathing, and toilets. In earlier times, sewage was put directly into the ground, into rivers, or into the ocean, without any treatment. As population has grown, however, the need for wastewater treatment has increased as well. Home septic systems consist of a large underground tank, where anaerobic bacteria (those that do not need oxygen) gradually break down most of the solids.

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War in Afghanistan devastated the country's environment in ways that may never be cleaned up

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Metal-organic frameworks for wastewater treatment: Recent developments, challenges, and future prospects - ScienceDirect