TO THE DRAIN FIELD Once the solids have been removed, the clarified wastewater, or effluent, is transported to a drain field by a pipeline. The drain field (sometimes called a tile field or leach field) consists of a series of distribution pipes set in trenches filled with gravel. These distribution pipes have holes so that the wastewater from the septic tank flows out into the gravel trenc hes, then trickles through the gravel and into the soil under the drain field. INTO THE SOIL Soil acts as a natural filter in the septic tank system. It filters out many of the bacteria that can cause diseases.

Soil can also retain certain nutrients, such as phosphate and some forms of nitrogen. The microorganisms in the soil break down many of the remaining impurities. Finally, the filtered water flows into the groundwater. continued...

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