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This document discusses the differences between 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 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.
A recirculating septic media filter system, in general, may be constructed using the same materials discussed throughout this document. What's different? A recirculating media filter system will be water tight (commonly using 30-mil polyvinyl chloride plastic (PVC) - so that it can process and then recirculate the effluent back to the pumping station.
Effluent cycles through the recirculating media filter a specified number of times before being later discharged to a dispersal system. A common design to make final disposal of the effluent uses a pressure distribution system to disperse effluent into the soil.
Where high levels of treatment of the effluent are achieved
(see below), a biomat is not going to form so the absorption system can be expected to have a longer life.
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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