Installing Baffles and Screens Correctly to Retain Solids

Installing Baffles and Screens Correctly to Retain Solids

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Septic tanks are designed to retain solids that accumulate over time. Some of these solids are byproducts of the waste treatment process, while others are materials that may not be capable of being processed, such as human hair. It is important that the solids are retained in the septic tank and not released to the downstream components. Excessive discharge of solids can cause plugging.

Baffles and screens - Septic tank baffles are critical to proper septic tank performance. 

An inlet baffle is intended to direct the incoming flow downward into the clear zone and protect the inlet piping from being clogged by the scum layer. There are typically two different kinds of inlet baffles used in tanks. One type is a plate or partial wall baffle, which is separate from the piping. These devices must be attached to the walls using appropriate fasteners (i.e., stainless steel connectors). Plate baffles may be installed by the manufacturer prior to delivery of the tank or by the installer after the delivery. Indicate who installed the device.

Another type of inlet baffle is a sanitary tee. A sanitary tee is different from a standard tee in that it has a flow line that will not catch solids. Like plate baffles, these can be installed at the site by direct attachment to the inlet piping (building sewer) on the inside of the tank. This standard PVC connection must be made using the proper materials and procedures as described in the piping section. Some tanks are delivered with a sanitary tee already installed. If the inlet baffle is installed by the manufacturer, be sure that the stub of piping is long enough to extend past the excavation so that the joint to the next pipe section is located over unexcavated soil. Care must be taken to support this pipe because any settling increases the potential for leaks or shifting of the tee out of plumb.

The outlet baffle or screen typically extends to the middle of the operating depth of the tank so that effluent is drawn from the clear zone of the outlet. The outlet baffle may be either a partial wall baffle or a pipe configuration. The outlet baffle can also be replaced by an effluent screen. An effluent screen is typically placed in the tank outlet to remove additional suspended solids that could potentially clog downstream components. Proprietary screens often include a housing that essentially serves as a tee. Alternatively, the screen is designed to be inserted into a standard tee. The screen must be installed under the tank access so it can be inspected and maintained.  

If an effluent screen is used, the following factors should be considered when selecting a screen:

  • The screen case should serve as an outlet tee.
  • The screen should allow solids of no greater than 1/8-inch to pass through the cartridge.
  • The capacity of the screen should reflect the projected organic load.
  • The screen should be secured in place and should not allow bypass of unfiltered solids if the screen openings become clogged.
  • The screen housing should be sized such that it does not interfere with periodic tank pumping.

Compartments - Many times a multiple-compartment tank may be required or recommended. The additional barrier may help slow down the effluent and retain solids. A tank that has compartments typically includes a tee, slot or center transfer hole. When septic tanks are divided into compartments, the volume of the first compartment should be equal to or larger than any succeeding compartments.

Air must flow from one compartment to another for proper ventilation of sewer gases through the plumbing stack in the facility. Verify that air can pass from one compartment to another via a gap in the top of the baffle wall. A smoke test may be used for verification or troubleshooting.

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to


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