Sketch of a Recirculating Sand Media Filter System - EPA Sand Filter Septic System Design, Use, Maintenance, Repair
     


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Sand filter septic design details: this document discusses design and use of septic media filters using sand. 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. This is particularly true of sand-bed filter septic systems which also often use a recirculating sand bed design to move septic effluent multiple passes through the sand filter system.

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Septic Media Filters Using Sand

Illustration of a Sand Bed Media Filter System - EPA

The septic tank which receives waste from the building should have either two compartments, or two tanks in series are used.

A gas baffle on the tank outlet is recommended (NY State Wastewater Treatment Standards) to reduce the chances of a periodic bolus of gas forming in the bottom of the tank and forcing solids into the sand media.

When a sand filter system is used, effluent is periodically distributed out of the septic tank and over the surface of a constructed bed of sand through a network of perforated pipes. Collector pipes below the sand pick up effluent after it has filtered through and been treated by (the biomat formed on) the sand filter.

Effluent dosing is by pressure-fed perforated lines of 1.5" to 3" diameter, or by siphon dosing using a 3" to 4" diameter effluent line. New York requires that the system is dosed at least three times daily. Dosing should not exceed 1.15 gallons/day/sq. ft. of media area.

Various texts cite the critical importance of selecting the proper sand for a septic media filter. New York's standard specifies a sand grain size of 0.25 to 1.0 mm; if nitrification is required (which may be specified by the local health department), the grain size is larger, 0.5mm to 1.0 mm., and all sand is passed through a 1/4" sieve and must be uniform to a coefficient of 4.0.

The effluent collector pipes below the sand bed discharge the treated effluent to an absorption system such as a drain field placed below the original ground level or possibly into a raised bed or mound system. The raised bed or mound receiving the treated effluent may itself be constructed of sand or other fill material with a percolation rate of not faster than 5 minutes per inch.

Typical sand filter construction (NY State Wastewater Standards) involves an excavated bed area which is filled in layers of material as follows (bottom up):

  1. Collector pipe system set in gravel sized 3/4" to 1.5" aggregate.
  2. 4" of aggregate above the collector pipes
  3. 3" of crushed stone or clean gravel sized 1/8 to 1/4" atop the aggregate.
  4. 24" of approved sand. Jantrania points out that a geotextile fabric should not be placed below the sand and atop the gravel. Though some installers think that's a great idea to keep the sand out of the gravel, he points out that the fabric is likely to clog and lead to early sand filter bed failure.
  5. Distribution pipes placed in a layer of aggregate providing 4" or more across the whole surface of the sand filter and with aggregate of at least 2" underneath and above the distribution pipes.
  6. A geotextile, 2" of hay or straw, or untreated building paper is placed atop the bed to keep soil fines out of the filter.
  7. 6" to 12" of topsoil is placed atop the sand filter bed system, mounded to encourage surface water runoff to drain off of the system. The topsoil is seeded to grass.

Sand filter problems: keep deep rooting plants, surface runoff, and heavy traffic (that could compact the media) off of the system. Don't block air movement over the surface of the system with buildings or plantings.

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