S-EPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-00-034
September 2000
Decentralized Systems
Technology Fact Sheet
Types of Filters
DESCRIPTION
The primary purpose of improving the quality of the
effluent from a septic tank system is to provide a
cleaner effluent and in some cases, to improve
treatment to address local environmental
conditions. This may be necessary due to site
constraints, regulations, or other limiting factors.
Sand filters in various configurations are one of
many traditional technologies applied to
decentralized systems. These filters are located at
the effluent side of the septic tank in order to
remove solids.
Research on alternate filtration media, particularly
recycled materials, has expanded the options
available for improving effluent quality. This Fact
Sheet summarizes the research on several alternate
media materials, including crushed glass, recycled
textiles, synthetic foam, and peat.
In a traditional sand filter application, physical,
chemical, and biological transformations facilitate
the enhanced treatment of effluent. Suspended
solids are removed by mechanical straining,
through chance contact, and by sedimentation.
Aerobic conditions must be maintained to maintain
a high performance level,. Intermittent application
and venting of underdrains helps maintain aerobic
conditions within the filter.
The alternate media discussed in this Fact Sheet
generally operate in the same way as sand filters.
They provide the same treatment of wastewater and,
in some cases, enhance the treatment efficiency of
the filter. The loading rate achieved in some
alternate media filters is twice that of traditional
sand filters. The filters discussed in this Fact Sheet
are single pass filters, where wastewater passes
through the filter only once before being
discharged.
APPLICATION
Applications for alternate media filters are
emerging, with the technology still largely in the
research phase. Filtration is widely used in
conjunction with drainfield systems for septic tanks
which require enhanced effluent quality. Alternate
filter media provide an option beyond a
conventional septic tank drainfield, which consists
of several trenches with gravel beds and perforated
plastic pipes. Alternate media filters may allow a
higher soil loading rate, use less space, and use
material that is easy to obtain. For example, the
Waterloo biofilter (developed at the University of
Waterloo, Ontario, Canada) uses absorbent plastic
foam cubes as its medium. Loading rates with this
porous synthetic medium are four times higher than
which use a recirculating sand filter. These
biofilters may be followed by disinfection.
These higher loading rate filters may perform more
effectively than traditional gravel drainfields and
sand filters, especially when the drainfield must be
located on a steep slope. Alternate media filters
are suitable for lots with sizing constraints or where
water tables or bedrock limit the depth of the
drainfield. States may offer a sizing reduction
allowance for alternate media filters because of
their high loading. They are also easy to install and
repair.
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DESIGN CRITERIA
Textile
Peat
Peat is a permeable, absorbent medium used as a
filter medium for onsite wastewater treatment.
Much research has been conducted in the Northeast
where peat is widely available. Peat filters used for
onsite wastewater treatment remove 60 to 90
percent of BOD5, but no long term data yet exist.
Because peat is a natural material, significant
variations in composition have been noted. Several
manufacturers enclose the peat in fiberglass
housing.
Foam
The foam cube filter is similar in performance to an
intermittent sand filter, but has been tested at 10
times the loading rate. The filter is housed in a 1.8
meter by 1.8 meter by 1.5 meter (six foot by six foot
by five foot) container, with 1.2 meters (four feet)
of media. Wastewater is sprayed on top of the
media and withdrawn from the base of the unit.
Alternatively, filter cubes installed in pre-assembled
cylinders can be placed in a tank.
Crushed Glass
A pilot project was conducted for the City of
Roslyn, Washington, to evaluate the feasibility of
using crushed, recycled glass as a filtration medium
in slow sand filters. The study used a 38 centimeter
(15 inch) PVC pipe as the media container and
three types of sand and crushed glass. The media
were washed so that less than 0.10 percent by
weight passed a #200 mesh sieve. Wastewater was
added to the filter at a loading rate of 0.002 cubic
meters/minute/square meter (0.06
gallon/minute/square foot). The removal of
bacteriological contaminants demonstrated that the
glass filter media obtained an activity level typical
of slow rate sand filtration. The results suggest that
slow rate filtration may be an effective treatment
process for Roslyn's raw water source with the
addition of a roughing filter. All three filters had
similar removal efficiencies, although it was hard to
draw conclusions for other geographical areas.
This medium consists of textile chips known as
"coupons". The medium is placed in a filter
housing similar to a sand filter, with wastewater
applied by spraying it at the top of the filter. The
loading rate was reported at 400 liters/square
meter/day (10 gallons/square foot/day). A
modification of this design uses layers of textile
material with a break between layers, allowing
greater loading rates, up to 600 liters/square
meter/day (15 gallons/square foot/day), producing
an effluent quality that meets or exceeds advanced
treatment standards.
ADVANTAGES AND DISADVANTAGES
Advantages
Alternate media filters are moderately inexpensive,
have low energy requirements and do not require
highly skilled personnel. They generally produce
high quality effluent. The process is stable and
requires limited intervention by operating
personnel. The media may be able to withstand
higher loading rates than traditional sand filters due
to increased surface area. These filters may provide
a suitable treatment option for degraded or failed
septic systems if it is shown that they can operate
over an extended period of time at the demonstrated
efficiencies.
Disadvantages
Alternate media filters are not proven technologies
and no long term operating data for the crushed
glass and textile media are available. The cost to
operate and maintain the systems has not been
standardized. Odors from open, single pass filters
treating septic tank effluent may be a problem. The
filter medium is unique, and may not be readily
available when it must be replaced. The media may
not be consistent from supplier to supplier or batch
to batch and may require additional monitoring
costs to confirm performance across batches.
The recent arrival and continuing research into
alternate filter media do not provide a potential user
with the same performance track record as
conventional sand filters. Filter surfaces and
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disinfection equipment require periodic
maintenance, pumping and some disinfection units
require power and facilities must have state or
federal discharge permits, along with sampling and
monitoring.
Filters using alternate media have performed well in
the laboratory but have seen limited use in the field.
Frequent inspection and monitoring are required to
obtain proper functioning of filtration units and to
determine cleaning cycles.
PERFORMANCE
Effluent quality data from long term use of peat,
crushed glass, and textile media as on-site filtration
systems are not available, yet experimental filter
systems show greater treatment efficiencies at
higher loading rates than standard sand filters.
OPERATION AND MAINTENANCE
Alternate media filters require more initial
operational control and maintenance due to the lack
of long term operational data. Primary Operation
and Maintenance (O&M) tasks include filter surface
maintenance, dosing equipment servicing, and
influent and effluent monitoring. With continued
use, filter surfaces become clogged with organic
biomass and solids. Once operating, infiltration
rates may fall below the hydraulic loading rate and
permanent ponding of the filter surface may occur.
If this occurs, the filter should be taken off-line for
rest or media removal and replacement. Buried
filters are designed to operate without maintenance
for their design life. Filters exposed to sunlight
may develop algae mats controlled by surface
shading. For community systems, disinfection is
required prior to discharge, but disinfectant quantity
requirements are low due to the high quality of the
effluent.
COSTS
Detailed cost information is not available because
most systems are still under study. Alternate media
materials are not common to wastewater treatment
applications, and long term costs are difficult to
estimate. In areas where the filter materials are
commonly found (peat is easily obtained in Maine,
Minnesota, and Wisconsin) the cost of filter media
is expected to be nominal. The cost of peat in other
areas is significantly higher. One manufacturer
reports that 30 bags of peat, each weighing 30
pounds, are needed for one filter. A research paper
on crushed glass filters estimates that 10 to 20 cubic
yards per installation would be necessary. Foam,
crushed glass, and textile material are all subject to
availability and transportation cost sensitivity.
REFERENCES
Other Related Fact Sheets
Intermittent Sand Filters
EPA 832-F-99-067
September 1999
Recirculating Sand Filters
EPA 832-F-99-079
September 1999
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owmitnet/mtbfact.htm
1. Crites, R. and G. Tchobanoglous. 1998.
Small and Decentralized Wastewater
Management Systems. WCB McGraw-Hill,
Inc. Boston, Massachusetts.
2. CWC Technology Brief, 1997. Crushed
Glass as a Filter Medium for the On-site
Treatment of Wastewater. Internet site at
http://www.cwc.org/briefs/glass.html,
accessed February 2000.
3. Falling Spring Technologies, no date.
Ecoflo. Internet site at
http://www.ecoflopa.com/ecofaqdesigneri
nstaller.html accessed February 2000.
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4. Jowett, E. Craig. August 1997. Field
Performance of the Waterloo Biofilter with
Different Wastewaters National Small
Flows Clearinghouse.
ADDITIONAL INFORMATION
Task Force for Decentralized Wastewater
Treatment
c/o Marine Studies Consortium
Roger Stern
83 Chapel Street
Needam, MA 02492
David Venhuizen, P.E.
5803 Gateshead Drive
Austin, TX 78745
Waterloo Biofilter Systems Inc.
Craig Jowett
2 Taggart Court, Unit #4
Guelph, Ontario, Canada N1H 6H8
The mention of trade names or commercial
products does not constitute endorsement or
recommendations for use by the United States
Environmental Protection Agency (EPA).
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
1200 Pennsylvania Avenue, NW
Washington, D.C. 20460
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MUNICIPAL TECHNOLOGY BRANCH
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