The term media filter covers a broad array of technologies specifically developed and used to provide additional pretreatment of septic tank effluent. This is done to address specific problems such as high-strength waste, or for nutrient or pathogen reduction. Regardless of the material or media in the filter, they consist of a watertight structure containing media of particular specifications.

After being collected in a septic tank, effluent is distributed evenly over the surface of the media, in most cases using pressure distribution to ensure equal distribution. Media filters consist of a lined or watertight structure containing media of predetermined specifications. Media can be a variety of materials that provide a surface for bacteria to colonize and for biochemical and physical treatment processes to occur. Effluent percolates through the media to an underdrain system where it is collected and sent to the soil treatment area for final dispersal and treatment.

Media should never be saturated; the presence of air promotes establishment of favorable microorganisms. Effluent percolates through the media under unsaturated conditions so oxygen is present in the pores and spaces, and aerobic treatment occurs in the media. A part of the design process is the determination of the size and type of media to accomplish the level of treatment desired. In general, the smaller the pore space between the media particles or fibers, the slower effluent percolates through the media, increasing the amount of contact time with the effluent and providing better removal of bacteria and other larger organisms or organic solids. Even distribution and small discrete doses are essential for maintaining aerobic treatment within the media.

Media filters treat wastewater through four main processes: filtration, chemical and physical sorption, assimilation, and decomposition of organic material. Filtration removes solid materials by physically straining or screening out particles in the wastewater moving through the bed. Chemical and physical sorption remove materials from the effluent through adsorption to the media surfaces and to the biological growth that adheres to the media surface. Assimilation is the process by which the microbiological communities living on the media surfaces transform the material that has been filtered or adsorbed into another chemical state. Decomposition is the process by which organic wastes break down into simpler compounds.

When discussing or considering the type of media filter to be used they can be broken into two broad categories: single pass and recirculating. Media for both types can range from sand and gravel to synthetic foams or plastic to textile sheets and organic peat. Selection of whether the filter should be single pass or recirculating is determined by the level of desired treatment and what is being treated. Single-pass filters are good at removing BOD and TSS and for reduction of bacteria and pathogens. Recirculating filters are used where single pass treatment levels do not meet permit requirements and in areas where nitrogen removal is a necessary part of the treatment required.

In upcoming articles we will take a closer look at some types of filters and their specific requirements.

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