A septic tank system is actually a small wastewater treatment system that's designed to treat wastewater from one home, or group of homes, rather than from a whole community. The system is generally located underground in the back yard. It consists of a tank usually connected to a series of pipes set into trenches _ the drain field. IN THE TANK Wastewater from the home flows into the tank. The tank separates solids from the wastewater, so the solids are removed before they reach the drain field. The heavier solids, called sludge, collect on the bottom of the tank. The lighter solids, like hair and grease, float to the top and form a scum layer on the water.

Both the top and bottom layers of solids are trapped in the tank and need to be pumped out periodically. In the meantime, the microorganisms in the tank begin breaking down many of these solid waste materials. continued...

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Illustrations: [The Septic Tank System] [Biodegradation] [Adsorption]