Land Use Planning Information>Septic Systems>How Septic Systems Work

How Septic Systems Work

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The purpose of on-site disposal systems is to provide for the treatment of household wastes using natural processes. A septic system is the most common method for treating the waste from a rural residence.

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A typical system consists of three main components.

First waste exits the house and enters the septic tank where solids settle out and grease and scum floats to the top. This is the first stage of treatment.

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Next, liquid effluent flows through the distribution box. A hydraulic pump is sometimes needed if the absorption site is higher than the septic tank, or if an elevated mound is used.

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Finally, the effluent arrives at the absorption field where it is evenly distributed to the soil for treatment, Under ideal conditions microorganisms on the surface of the soil particles consume the organic pollutants in the effluent.

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The solids that settle to the bottom of the tank slowly decompose. Gas bubbles given off during this process rise, carrying with them fats, oils, and greases. The tank outlet is located between these two layers, where the clearest liquid is found.

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The absorption field (sometimes called a leach or drainage field) consists of several lateral pipes that allow the effluent to slowly flow out through holes positioned along the length of the pipe.

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The distribution of pipe laterals are located inside soil absorption trenches. The trenches provide the surface area needed for the effluent to be in maximum contact with the soil. Gravel normally supports the pipe and forms an envelope around the pipe, to protect it from roots and varmints.

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The effluent flows though the gravel and enters the soil both below and to the side of the trench.

Effluent moves downward with the force of gravity in a process called percolation. As it percolates through the soil, minute solids, bacteria, and nutrients are removed from the effluent. This percolation process is a natural biological one that can safely treat many domestic effluents before they reach groundwater sources.

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If a soil has high permeability or a high water table, the effluent can reach the groundwater before it has time to be treated. To avoid this, a shallow trench or mound system design can be used. Both usually require a hydraulic pump to ensure uniform distribution of effluent over the field.

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This section of a mound system shows the effluent treatment zone completely above the ground surface. A shallow trench is similar to the mound system design except that each trench is partially set in the natural soil and soil fill is mounded over each individual trench.

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