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Using Single Pass or Recirculating Septic Media Filters InspectAPedia®  -

• Using Single Pass Septic Media Filters as Components of Alternative Septic Systems
• UsingRecirculating Septic Media Filters
• Septic filter product sources

This document discusses the single pass and recirculating septic media filter systems. Media filter septic systems use a conventional septic tank followed by any of several methods to further filter and treat septic effluent before it is discharged to the soil, soil surface, or waterway.

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Single Pass Septic Media Filter Systems

Single pass media filers using sand, other natural septic filter media or manufactured/synthetic media filter media, are usually designed for effluent to be applied (dosed) using a pressure distribution system, typically a small-diameter perforated pipe. The single pass septic media filter design permits effluent to be applied to the filter uniformly, in small, regular doses, permitting the filtering and biomat processes to do their work and avoiding overloading the system.Other systems use spray nozzles to distribute effluent over the surface of the media.

Recirculating Septic Media Filter Systems

A typical recirculating media filter system, if constructed using sand, will be constructed in a watertight excavation and filled in layers, from the bottom up, 12" of gravel, 24" of coarse sand coarse (0.05-2.0 mm), topped with another 12" of gravel. The bottom of the recirculating media sand and gravel filter bed slopes (perhaps 1" in 8 ft) to drain to the return from the media filter to the pumping station. 4" PVC pipe returns effluent by gravity to the pumping station.

Wastewater is applied to a system of this design usually at 4-5 gallons per square foot of area of the media filter system's footprint. So a family of four, using 70 gpd of wastewater, (figure 300 gpd total, a commonly-used number which I am betting is low) would need only 75 sq. ft. for the filter bed if you believe these calculations. Naturally the actual system will be built with more capacity to handle aging, variations in efficacy, and variations in loading rate.

A recirculating sand media filter of this design can remove 30% to 70% of the nitrogen in the incoming effluent, and 10 to 30% of the phosphorus in the incoming effluent. BOD may be reduced from 175 mg/L (leaving the septic tank) to 20; fecal coliform may be reduced from 1 million or much higher MPN/100mL leaving the septic tank to perhaps 5000 to 100,000 MPN/100mL.

A recirculating sand bed media filter system produces highly treated effluent and has good removal of nitrogen. In part because of the larger sand used in a recirculating system, fecal coliform is not reduced as effectively as a single pass sand media filter or a peat media filter.

The life expectancy of a recirculating media filter system of this type and the frequency with which the sand needs to be raked or replaced, is not an easy statistic to find. I'll plug that data in here when I can dig it out from the experts.

Clearly a septic media filter system like this needs regular maintenance and more frequent attention than a conventional septic tank and drainfield. Figure on a regular maintenance contract with perhaps quarterly inspections of the return media flow (slowing indicates clogging), returning effluent quality, and proper operation of the pump and controls.

The University of Minnesota Extension Service estimates an annual operating cost including inspection, maintenance, and electricity of $200.-$500./year. This figure does not appear to include, however, the cost of periodic replacement of the media.

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• References: Portions of the text in this section were based on comments in "Advanced Onsite Wastewater Systems Technologies", Jantrania and Gross, and others on an article by the University of Minnesota Extension Service on recirculating sand bed media filter systems at http://www.extension.umn.edu/distribution/naturalresources/DD7670.html. The two sources are in general agreement.

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
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