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How Septic Media Filters Work InspectAPedia®  -

• Septic media filters
• How Septic Media Filters Work
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This document discusses the how septic media filter systems work.

When using a septic media filter system, effluent treatment is by both actual filtration and ultimately by a biochemical process as the filter "matures" and includes its own biomass. Both natural media filter septic systems (such as sand, gravel, or peat) and synthetic media filter septic systems (foam cubes, glass, slag) are used, and both single-pass and effluent recycling systems may be employed.

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How do Septic System Media Filters for Septic Effluent Treatment Work?

Media filters are not a new idea, and have been in use in various forms for more than 100 years. An early form of media filtration used simple sand to filter drinking water, though the process used pressure and worked a bit differently from media filters used to treat septic effluent.

For septic effluent treatment purposes, the filter strains the effluent, trapping small solids and debris.The principal septic effluent treatment performed by the filter is biochemical, as a biomass or film of microbes attaches to the filter surface. (I discuss biomats in the context of a conventional drainfield at Biomat Formation.) In contrast with the relatively thick biomass of a drainfield (.5cm to 10cm), a thin biomass film forms on the entire surface area of the media filter.

So it's apparent that the term "media filter septic system" fails to make clear by its name that it's not just "filtering" septic effluent. Very important is the septic effluent and suspended solids treatment that takes place by microbes which form as a biomat on the surface of the filter media.

Effluent passing through the media filter comes into contact with the biomass where the biomat's microorganisms further process and sanitize the liquid effluent by removing pathogens, nitrogen, and viruses. (Treatment that occurs in a septic tank does not remove any appreciable portion of nitrogen from the effluent.)

The septic media filter, because of its porous nature, also supplies air (oxygen) to this process to assist the microbes in digestion of pathogens and organics in the wastewater effluent. The actual treatment process (slightly simplified) is a complex of filtering, adsorption, and oxidation by microbes in the biomat.

For the media filtration system to work, effluent has to be provided from the settlement tank to the media filter at a slow rate, perhaps 5 gallons per day per square foot of media surface. Septic effluent may be passed through the media filter only once - a single pass septic media filter system - or it may be recirculated through the media filter - a recirculating septic media filter system.

Septic media filters (discussed here) are an effluent treatment method. Other types of septic filters, such as greywater filters, washing machine filters, and filters at the effluent outlet of a conventional septic tank are all designed to protect the septic effluent absorption field or leach field. These other septic filters are discussed at Septic & Graywater Filters.

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References for this Septic Media Filters Discussion

My text draws heavily on Dr. Jantrania &Gross's work and on New York State Water Treatment Standards.

New York State Wastewater Treatment Standards - Individual Household Systems, Appendix 75-A (1990), Public Health Law 201(1)(1).

New York State Alternative Septic System Design Regulations, Wastewater Treatment Standards - Individual Household Systems, Appendix 75-A, of Public Health Law 201(1)(1) 1990.

Anish Jantrania, Ph.D., P.E., M.B.A., Consulting Engineer, Mechanicsville VA, 804-550-0389
NOTE: be sure to examine Dr. Jantrania's new (2006) book,
Advanced Onsite Wastewater Systems Technologies which can be
ordered from his publisher.

The Septic System Owners's Manual, Lloyd Kahn, Blair Allen, Julie Jones, Shelter Publications, 2000. IS N 0-936070-20-X

Onsite wastewater treatment systems, Bennette D. Burks & Mary Margaret Minnis. Textbook and reference manual on all aspects of onsite treatment. This is one of the best books we've reviewed on the subject, with an excellent balance of clear simple explanation and solid engineering. Topics: Soil & Site Selection, Hydraulics, System Selection & Design, Wastewater Biology, History & Mythology of Onsite Wastewater Treatment. $49.95, Hogarth House, Ltd, 800-993-2665 x327 to order *800 -993-2665 x327 (Univ. Wisc. Bookstore)

Boxerwood Gardens is an arboretum and nature center in Lexington VA, and the generous contributor of the illustration of a gravel-bed natural septic media filter system discussed above.

More Information on Building Diagnostic Inspections and Repairs

• Septic Filters & Graywater Filters in-tank septic filters or graywater filters to keep solids out of the absorption system


• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.

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