A system with a single-pass media filter will have effluent flowing from the house to a septic tank, which can contain a screened pump vault in the outlet end of the tank or second chamber or flow to a separate pump tank. From there effluent is pressure dosed (time dosed or demand dosed) over the media filter surface, percolates down through the media, and is then collected in the discharge basin located within the filter. From the filter it is demand dosed to a pressurized drainfield. If a media filter is time dosed, then the drainfield is de facto time dosed because the first timer controls the filter; it indirectly controls flow to the drainfield discharge basin.

Effluent, after being applied to the filter surface, percolates through the filter bed only once and then flows to the next treatment step. Final treatment and dispersal is usually a series of pressure-dosed trenches.

Single-pass filters are very effective in removing BOD and TSS and pathogenic organisms. Pathogen removal is dependent upon whether it is pressure dosed or gravity dosed, and the specifications of the media. Time dosing (under pressure) achieves maximum treatment potential and lowers the risk of hydraulically overloading the filter.

Media filters can be designed using a wide variety of media including sand, foam, synthetic textile fabric or peat, as well as some other products. A comment on media: The first sand filter I worked on for a research project was called a sand filter but the media was actually made up of gravel-sized particles that were used as grit for turkeys! I mention this because selection of the type of media and the size depends on the desired level of treatment.

Although the configuration described above is common when a media filter is used, there are other single-pass variations. They include use of bottomless filters where effluent percolates through the media (usually 2 feet of media) directly into the underlying soil. This technique was first used with sand filters mimicking the operation of a typical sewage treatment mound system, which is nothing more than a covered sand filter. The difference is that the soil under the filter needs to be permeable enough to accept the effluent under the footprint of the filter since filter sidewalls are impermeable. This same approach is used with peat as the media; modules are provided and set in place over an area for infiltration directly into the soil. Use of these products depends on having suitable natural soil with adequate permeability to accept the effluent.

One other common variation on a single-pass media filter is the use of a constructed wetland for treatment. It is important to understand from the start that this is a constructed system; it does not use a natural wetland for treatment. So the system starts with an impermeable liner that is watertight. Water levels in the wetland are controlled at the outlet. Effluent is directed from the wetland to a final soil treatment area to complete the treatment and dispersal. Plants in the wetland as well as the rock media, along with controlled water levels, allow for air and gas exchange to provide treatment. Wetland plants are very important to providing oxygen for treatment processes.

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