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Water leaves our homes. It runs down sink and bathtub drains
and out of washing machines. It is flushed down toilets. Waste
water must be treated before it returns to the environment. In
some communities, waste water goes to sewage-treatment plants.
In other places, waste water enters local septic systems.
Septic tanks are usually made of concrete or metal. They are
buried outside houses. Waste water separates inside a septic tank.
Heavy materials sink to the bottom and form sludge. Lighter
materials like fats and grease rise and form scum. Bacteria break
down solids in the tank. The liquid in the middle flows through
pipes into gravel-filled trenches. The liquid in the trenches is
purified as it seeps through the gravel and soil.
Sewage-treatment plants screen waste water to remove solids.
Bacteria break down other materials. Chemicals are used to rid the
water of impurities. Treated water then discharges into streams,
lakes, or the ocean.
Water Leaving Our Homes
by David
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