Recirculating media filters treat wastewater by recirculating effluent that has passed through the filter bed back to the septic tank or to a separate recirculation tank. In general, sewage flowing from the house enters a septic tank (or a processing tank) containing a screened pump vault in the outlet end of the tank or a separate pump tank. Effluent is pressure dosed over the media filter surface, percolates down through the media, collects at the base of the filter and flows back to the septic tank for further treatment or is diverted to the drainfield discharge basin for final dispersal. Just like a single-pass filter, the drainfield receives de facto time-dosed wastewater as long as the media filter is time dosed.

There are several potential configurations for using media filters. It is important when working on a media filter to know how that particular one is set up. They can be set up where effluent flows from the septic tank to a separate pump tank to the filter with filtrate back to the septic tank, where it is sent to another pump tank to disperse to a drainfield or back through the filter. Another common system is where a two-compartment tank is used with a pump in the second chamber to the filter and then flow is split to either the filter pump or a separate pump tank to the drainfield. A third possibility is a septic tank to a separate recirculation tank to the filter or to the drainfield.

Usually a programmable timer controls the amount of wastewater recirculated to the filter bed surface. The recirculation ratio is set so three to five times the amount of wastewater is delivered to the filter as the amount of sewage generated in the house. This means that if sand/gravel is the media it will be a little larger in size than for a single-pass sand filter because of the need to run the larger quantity of water through the filter.

Wastewater is treated by mixing effluent that has passed through the media bed with raw septic tank effluent. This mixing occurs either in a separate recirculation tank, or in the inlet end of the septic tank and is then discharged to the filter from a pump vault in the tank. Filtrate from the media filter is split so that a portion returns to the recirculation tank, and a portion goes out for final dispersal.

Splitting the flow can be accomplished in a variety of ways. One common method is the use of a divider in the bottom of the filter where the filtrate is collected. Another is the use of a recirculating splitter valve located in the septic tank. Other methods used include tipping buckets, splitter boxes and a distribution plate.

Recirculating media filters are effective in reducing BOD, TSS and total nitrogen. While recirculating filters achieve some bacterial and pathogen reductions they are generally not as effective as single-pass filters. They can be effective for nitrogen removal though, and so you see them used in areas where there are either environmental or health concerns over nitrate levels in surface or groundwater. Nitrogen removal is accomplished because the effluent is aerated as it passes through the filter accomplishing the change to the nitrate form of nitrogen and then passed back to the organic-matter-rich septic tank where denitrification can occur.

About the Author
​Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Send him questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.

This article is part of series on media filters:

• An Introduction to Media Filters
• Designing with Single-Pass Media Filters
• The Basics of Recirculating Media Filters
• Recirculation Ratios 101
• Recognizing A Few Problems with Media Filters