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Re-circulating Sand Filters

Improved on-site wastewater treatment and disposal technology

A recirculating sand filter system reduces pollutants therefore minimizing the likelihood of the drainage field becoming clogged.

In many areas of the country, builders and homeowners rely predominantly on conventional septic tank-soil absorption systems for sewage disposal, even though less than a third of the land area in the United States is suitable for them. Septic-tank use in unsuitable land may lead to soil clogging within the drain field and system failure. These problems may be reduced through improved on-site wastewater treatment and disposal technologies, such as the on-site re-circulating sand filter (RSF) system. The RSF system uses a sand filter to reduce pollutants, including suspended solids so it minimizes the likelihood of the drainage field to become clogged.

With an RSF, effluent from a septic tank flows to a re-circulation tank where it is pumped to a filter. The sand filter consists of coarse sand or similar media that encourages biological growth. The filter media allows the wastewater to mix with air, as it slowly passes through the structure, giving the bacteria and other organisms time to digest the pollutants (which they consider food). Approximately 1/4 of the filtrate collected in the piping is diverted to the final soil disposal area while the remainder is returned to the re-circulation tank where it mixes with liquid waste received from the septic tank. By passing the effluent through the filter several times, the microbes eat more food, and thereby remove more pollutants, than they would in a single pass.

Diagram of a re-circulating sand filter.A single-pass sand filter consists of a septic tank, sand filter and soil absorption bed. In the septic tank, solids settle out of the waste stream and anaerobic bacteria facilitate the partial breakdown of organic matter (primary treatment). Pressured doses of clarified effluent from the septic tank are discharged to the sand filter. The sand filter, commonly referred to as a “mound in a box”, is a buried chamber containing at least 24 inches of sand between layers of gravel. It serves as the fixed porous medium on which aerobic bacteria provide much of the secondary treatment. The effluent from the sand filter is then discharged, in pressurized doses, to a soil absorption bed.

Because the effluent from the sand filter has already been treated by passage through 24 inches of sand of an approved size and consistency, the soil absorption bed is reduced to 24 inches of suitable soil (minimum 6 inches in-situ soil). And, because the sand layer is underground, the potential landscaping disadvantages of an above ground mound are alleviated. Also, since the sand filter treats wastewater within an enclosed structure, the sand can be replaced easily should the need arise. As in all systems, septic tanks require periodic pumping of accumulated solids, as well as inspection to determine that the tank remains watertight. Solids must also be removed from the pump chambers periodically.

The pollutant-removal processes in RSFs are complex. The pollutant parameters commonly analyzed in wastewater treatment systems are nitrogen, biochemical oxygen demand (BOD), total suspended solids (TSS), fecal coliform (FC), and phosphorous. RSF systems have been shown to produce effluent that is clear, odorless, and with pollutant concentrations much lower than conventional septic tank effluent. Typically, the effluent is low in nitrogen, TSS and BOD. Fecal coliform concentrations are also reduced; however, FC concentrations are still well above the acceptable effluent standards found at municipal wastewater treatment plants.

Overall, current costs of RSF systems are higher than conventional septic tank/leach field systems. Several barriers to the acceptance of RSFs are aesthetics, affordability, regulatory, competition from other types of alternative treatment systems, and long term maintenance.


Environmental Performance

Recirculating sand filters have been shown to have much lower effluent pollutant levels compared to that from conventional septic systems. They can make on-site wastewater treatment possible on soils for which treatment is marginal or not possible.


Medium

RSFs can be assembled and purchased from local septic tank manufacturers or through a national distributor. Land availability on the site may limit the applicability of recirculating intermittent sand filters. Some odors may result from these systems if they are not properly installed and maintained, depending upon the particular design. In order for sand filters to continue functioning properly, they must be adequately maintained. Electrical power is required for recirculating sand filter units. In some jurisdictions, this system must be designed by a licensed professional engineer, and requires special permitting review and consideration. Pretreatment Units: If a septic tank is used to pretreat the wastewater prior to applying the effluent to the sand filter unit, the tank should be pumped periodically to remove sludge/solids (see fact sheet for Septic Tanks under Resources).

Other types of pretreatment units and processes will require maintenance of some type. Sand Filter Units: In order to prevent clogging of the surface of the sand media, the upper layer of sand can be occasionally raked (assuming it is accessible). The upper few inches of sand may need to be removed and replaced periodically with clean sand (again, assuming it is accessible from the surface). Two persons would likely be needed at the site for some of the maintenance activities since at least some heavy lifting may be involved. The spent sand must be transported to and disposed of at a permitted landfill, or regenerated for reuse. Occasional flushing of the distribution piping through the filter may be needed.


The costs of an RSF system vary considerably due to design variations, local labor, and material costs. They are typically $3-$8K more than traditional on-site treatment systems. See Benefits/ costs section.


See Benefits/ costs section.


Approval usually comes from the state or local health department. NSF/ANSI 40 - 2004 Standard for Residential Wastewater Treatment Systems details requirements for approval of the system. The NSF/ANSI 40-2004 standard applies to on site wastewater recycling systems with capacities of up to 1,500 gallons per day, and leads to approval as a Class I or Class II plant. A Class I certification indicates performance to EPA Secondary Treatment Guidelines for three parameters: BOD, suspended solids and pH. Noise levels, odors, oily films and foaming are also measured. The Class II criteria require that not more than 10% of the effluent CBOD5 values exceed 60 mg/L and that TSS not exceed 100 mg/L.

Class I and Class II systems are required to include two years of manufacturer maintenance service and renewal options, and to have alarms to alert the homeowner of malfunctions.

If the local health department does not recognize the technology or will not allow adequate reduction or elimination of the drainage field, costs will increase. Local policy may require a drain field size that makes the total treatment system larger than if a conventional system was used.

A database including references to state by state sanitary code requirements is located at: www.nesc.wvu.edu/nsfc/nsfc_regulations.htm


Not Applicable


The typical RSF includes a septic tank with two compartments, a re-circulation tank, a pump, a sand filter, and a drainfield. Effluent flows from the septic tank to the re-circulation tank, where it is pumped to the sand filter. Effluent passes through the sand filter, where it is biologically treated, and then flows to a flow diverter. Some of the treated effluent is discharged to the drain field, and some goes back to the re-circulation tank.


Not Applicable


RSF systems have been shown to produce pollutant concentrations much lower than conventional septic tank effluent. The improved effluent quality is reported to increase soil absorption and reduce soil clogging, reducing the potential for system failure. RSFs have several advantages over single-pass sand filters. They are smaller, minimize or eliminate odors, are capable of treating higher-strength wastes, and are more effective at removing nitrogen. Potentially, RSFs offer environmental benefits by reducing harmful pollutants in wastewater effluent. The most important benefit of RSF technology is that they permit advanced on-site treatment and disposal on environmentally sensitive sites, and on soils that are typically considered marginal or unacceptable for conventional septic systems.

The costs of an RSF system vary considerably due to design variations, local labor, and material costs. They are typically $3-$8K more than traditional on-site treatment systems. These costs were estimated in a City of Austin Texas fact sheet:

Estimated initial Intermittent sand filter unit costs, installed, and including septic tank for pretreatment, pump, pump tank, control and alarm, and valves

$7,000

Equipment repair/replacement costs, estimated at $60/year (pump repair/replacement)

$5/month

O&M (does not include periodic sand removal/replacement and disposal if/as needed), with a maintenance contract of $240/year (est. two 180-day filter runs with system checks and maintenance performed twice annually) 8 hrs. @ $15/hour * 2.0, including taxes, overhead, profit, and including pump/controls servicing

$20/month

Septage and sludge pumping once every 2 to 5 years

$4.17/month

Energy costs (using 0.051 KWH/day energy use)

$0.124/month

20-year NPW (not incl. design & permitting costs)

$10,502.22

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.