Why Exit Filters Are Needed
It has been found that placing a filter in the exit baffle of a septic tank captures a large portion of the solids suspended in the septic tank as the effluent is delivered to the downstream dose tank or the absorption area distribution system. As new wastewater is discharge from a house into a septic tank, an equal volume of predominately liquid effluent is discharged from the septic tank to the downstream unit. In most on-lot systems the downstream unit is either a secondary treatment filter such as a sand or peat filter, a dose tank, or a gravity fed absorption area. In all on-lot systems, the septic tank serves to provide a quiescent zone where particles heavier than water can settle to the tank floor (sludge) and particles lighter than water can float to the liquid surface (scum). As long as septic tanks are properly sized, they do a pretty good job of physically separating the solids. It should, however, be remembered that the sludge and scum are just ever so slightly heavier or lighter, respectively than water. Thus the settling process is an imperfect activity that often leaves some particles partially suspended in the middle zone of the septic tank. In those cases when large volumes of wastewater are delivered to the septic tank, such as emptying a washing machine or flushing a toilet, there is a tendency for the water flowing through the septic tank to carry with it small amounts of partially settled waste material. The septic tank exit filter (or screen) located in the exit baffle of the tank serves as the final opportunity to capture and remove these partially settled particles, see Figure 1.
Figure 1. Cross-section through a Single-Chamber Septic Tank
Common Exit Filters
Septic tank exit filters usually come in two configurations. Most of the exit filters are plastic units containing small holes designed to capture particles that reach the filter (see Figure 2). The second type of exit filter looks like a brush inserted into a piece of pipe (see Figure 3). Both of these exit filters can be removed by opening the exit observation port and pulling (or lifting) the filter insert. Once removed, the internal assembly can be cleaned before it is reinserted in the exit baffle.
Figure 2. Plastic Screen Filters. Figure 3. Brush-type Exit Filter
The addition of exit filters in the exit baffles of a septic tank have improved the quality of wastewater discharged from the septic tank to the next unit downstream. If your septic tank does not have an exit filter, have one installed the next time you have your tank pumped. The exit filter will extend the useful life of your absorption area.
For additional assistance contact your local Sewage Enforcement Officer or Extension Educator.
Pennsylvania Association of Sewage Enforcement Officers (PASEO)
4902 Carlisle pike #268
Mechanicsburg, PA 17050
Pennsylvania Septage Management Association (PSMA)
Bethlehem, PA 18016