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The True Value of Septic Tanks

The True Value of Septic Tanks

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Producers should remember that a septic tank's primary purpose is to maximize the liquid/solid separation of the wastewater. As a waste-water treatment system component, septic tanks serve to remove more than 80% of the suspended solids out of the waste stream.

This separation process is accomplished by sedimentation. Materials that are more dense than water will tend to settle to the bottom and form a sludge layer. Likewise, materials that are less dense than water will tend to rise and form a scum layer on the liquid surface.

Septic tank design

Producers know that typically, septic tank design standards only require that the tanks must be watertight and structurally sound. But there's another important design aspect. Septic tanks must also be constructed to promote adequate sedimentation. Water must be retained long enough and still enough to allow the denser material to drop to the tank's bottom.

Producers can incorporate tank volume, tank length, and hydraulic controls in their designs to provide adequate liquid/solid separation. Tank volume provides the time needed for settling to occur. It is commonly stated that a septic tank's volume should be equivalent to two days of design flow. Many jurisdictions use 120 to 150 gallons per day per bedroom as the residential design flow.

For a three-bedroom home, a tank with an effective volume of 900 to 1000 gallons would typically be specified. The effective volume is the volume below the tank's outlet. Thus, during design flow, water should take two days to pass through the tank. During this time, most of the suspended solids should separate into the two layers of solids.

Tank length refers to the distance from the inlet to the outlet of the wastewater's flow. The longer the distance, the greater opportunity for solids to sink below (or rise above) the outlet baffle. It is generally recommended that tanks should be two times longer than their width.

Importance of baffles

Baffles are the hydraulic controls within septic tanks. Every septic tank should have an inlet baffle, outlet baffle, and a baffle wall that divides the tank into two compartments.

The inlet baffle directs the incoming wastewater down toward the tank's bottom. This directional action promotes the settling of heavy solids and helps to reduce the turbulence generated by the incoming water. Because of the design of the inlet baffle, floating solids must go down through the baffle before they can rise to the liquid surface.