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
                                                         »MTB
                                                         Excelence fh tompfance through optfhial tethnltal solutfons
                                                         MUNICIPAL TECHNOLOGY BRANCH

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