v>EPA
                     United States
                     Environmental Protection
                     Agency
                     Office of Water
                     Washington, D.C.
EPA 832-F-99-079
September 1999
Decentralized  Systems
Technology  Fact  Sheet
Recirculating  Sand  Filters
DESCRIPTION

A  recirculating sand  filter (RSF) system is  a
modified version of the old-fashioned, single-pass
open sand filter.  It was designed to alleviate the
odor problems associated with open sand filters.
The noxious  odors  were eliminated  through
recirculation, which increases the oxygen content in
the effluent that is distributed on the filter bed.

RSFs are a viable  addition or alternative  to
conventional methods of treatment when  soil
conditions are not conducive to proper treatment or
wastewater  disposal  through   percolative
beds/trenches.  Sand filters can be used on sites that
have shallow soil cover, inadequate permeability,
high groundwater, and limited land  area. RSFs
                    commonly serve subdivisions, mobile home parks,
                    rural  schools,  small municipalities, and other
                    generators of small wastewater flows.

                    Sand filters  remove contaminants in wastewater
                    through  physical,  chemical,  and  biological
                    processes.  Although the physical and chemical
                    processes play an important role in the removal of
                    many particles, the biological processes play the
                    most important role in sand filters.

                    Figure 1 shows the three basic components of a
                    RSF system.  These three components are a
                    pretreatment unit, a recirculation tank, and an open
                    sand filter.

                    Wastewater first flows into a septic tank (or in the
                          Pretreatment
     To disinfection/discharge
            Float valve
                                      Recirculation
                                      pump discharge
                                                              Filter media
                                                         Layered
                                                       support gravel
                FIGURE 1  TYPICAL RECIRCULATING SAND FILTER

Sources: Mines, 1998 and NSFC, 1997.

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case of a clustered or community system, a number
of septic tanks) for primary treatment.  A standard
concrete or fiberglass septic tank can be used, with
size being relative to the home/facility served.

The partially clarified effluent from the pretreatment
tank then flows into  a recirculation tank.   The
volume  of the  recirculation  tank  should  be
equivalent to at least 1 day's raw wastewater flow
(or follow local jurisdiction requirements). In the
recirculation tank, raw effluent from the septic tank
and the  sand filter filtrate  are mixed and pumped
back to the sand filter bed.

APPLICABILITY

Stonehurst Development  in Martinez,
California

The Stonehurst development  is a small residential
subdivision near the City  of Martinez in Contra
Costa County, California.  This  subdivision is
located  in a hilly, rural area that did not have a
wastewater collection system.  Thus, an innovative
decentralized wastewater system was designed to
provide  for  wastewater  collection,  treatment,
disinfection, and reuse.

The innovative system combines the use of septic
tanks, screened effluent filter vaults, high-head
effluent  pumps,  small-diameter  variable  grade
sewers,  pressure sewers, a recirculating granular
medium filter, an ultraviolet (UV) disinfection unit,
a subsurface drip irrigation system for wastewater
reuse, and  a community soil absorption field for
wintertime disposal. The  principle elements for
treatment consisted of two  sections of recirculating
granular filter followed by disinfection.

Each filter was 24 inches  deep with 3 millimeter
gravel (washed and rounded with less than  2%
fines) sandwiched between layers  of drain  rock,
which was coarse, washed  gravel approximately 1
to 2.5 inches  in diameter. The wastewater was
pumped from the recirculating tank to the filters for
five  minutes  every half hour,  and circulated
approximately  five times through the filter.  Since
one half of the filter was used during the time the
study was conducted, the hydraulic loading was 1.2
gal/ft2.
Performance  data was calculated for 28 months
from June 1994 to September 1996, based on an
average of at least two  samples per month for
five-day BOD, and at least four samples per month
for TSS, chemical oxygen demand (COD), pH, and
total coliform. Table 1 summarizes the performance
data of effluent samples  that passed through the
recirculating gravel filter and the UV system.

To date, the Stonehurst decentralized wastewater
system has exceeded all expectations by performing
beyond required standards.

   TABLE 1  PERFORMANCE DATA FOR
      STONEHURST WASTEWATER
          TREATMENT SYSTEM
Constituent
BOD5
COD
TSS
PH
Total coliform
NH4
N03
TKN
Oil and grease
TDS
EC
Range
0 - < 5 mg/L
1 -18 mg/L
2-15 mg/L
6.96 - 8.65 unitless
<2-12.5MPN/100mL
0-15 mg/L
3.55 -37 mg/L
0 - 3 mg/L
0-12 mg/L
340 - 770 mg/L
433 - 1,200 ^mhos/cm
* TDS - total dissolved solids, EC = electrical conductivity,
Aimhos/cm - micro mhos per centimeter

Source: Crites et al., 1997.
Elkton, Oregon

A RSF  system was installed and monitored for a
community in Elkton, which  is  located on  the
Umpqua River in  Southwestern  Oregon.   The
population of this  community  was 350, mostly
residential with some commercial establishments.
The wastewater generated from stores, restaurants,
schools,  and  about  100  residences  was  first
pretreated and screened in individual septic tanks.
Partially clarified  effluent was  then collected and

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conveyed by an effluent pressure sewer system to a
RSF and finally  pumped to a drainfield for final
treatment and disposal.

The sand filter was 60 feet x 120 feet with four
cells, 36 inches deep, and designed to treat 30,000
gallons per day  (gpd).   A recirculation tank  of
29,500-gallon  capacity  was   used with  four
one-horsepower pumps. Each pump dosed one cell
at the rate of 130 gallons per minute.  Two pumps
alternately dosed during  each cycle.  The actual
recirculation ratio  was  3.2:1,  and  during  low
periods,  a  motorized  valve   allowed  100%
recirculation.

Effluent quality data obtained from February 1990
through October  1997 are presented in Table 2.

It was concluded from this study that the RSF
produced a high quality effluent, thus protecting the
river nearby at an affordable cost. Capital costs for
RSFs range from $3 to $10 per treated gallon.  The
annual operating costs are very low.  For example,
at Elkton, the annual O&M cost for the RSF is less
than $5,000, which includes $780 for electricity.

Use of a smaller media  (< 3.0 nm) would have
resulted in better nitrification, but this  was not a
concern when the design was made.


   TABLE 2  ELKTON'S RSF EFFLUENT
               QUALITY DATA
Wastewater
Characteristics
BOD
TSS
NH3-N
NO3-N
Influent
(mg/L)
123
37
51
2
Effleunt
(mg/L)
4
9
10
26
 Source: Orenco Systems, Inc., 1998.
ADVANTAGES AND DISADVANTAGES

Advantages

•    No chemicals are required.

•    RSFs provide a very good effluent quality
     with  over 95% removal  of biochemical
     oxygen demand (BOD) and total suspended
     solids (TSS).

•    The treatment capacity  can be  expanded
     through modular design.

•    RSFs are effective in applications with high
     levels of BOD.

•    RSFs are easily accessible for monitoring and
     do not require a lot of skill to maintain.

•    A significant reduction in the nitrogen level is
     achieved.

•    If sand is not feasible, other suitable media
     could  be  substituted  that may  be  found
     locally.

•    Less land area is required (1/5 of the land area
     of a single-pass sand filter) for RSFs than for
     single-pass sand filters.

Disadvantages

•    If appropriate media are not available locally,
     costs could be higher.

•    Weekly  maintenance  is required for  the
     media, pumps, and controls.

•    Design  must  address   extremely  cold
     temperatures.

DESIGN CRITERIA

The RSF system is an open sand filter with a sand
media depth of 2 feet. A layer of graded gravel
(about 12 inches) is provided under the sand  for
support to the media and to surround the underdrain
system.   A  portion  of the mixture (septic tank
effluent and sand filtrate) is dosed by a submersible

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pump through a distribution system that applies it
evenly over the sand filter. The dosing frequency is
controlled by a programmable timer in the control
panels.

The filtrate from the sand filter is collected by
underdrains that  are located  at the bottom of the
bed. The filter discharge line passing through the
recirculation tank is located near the top of the tank.

Figure 1 shows a ball float valve connected to a
downturned "T"  on the discharge line, in which is
housed a rubber ball with a diameter slightly larger
than that of the pipe.  As the filter effluent rises in
the tank, it forces the rubber ball firmly against the
bottom of the downturned leg, thus discharging the
effluent for further treatment or disposal.  Other
control mechanisms may be used, but care must be
taken to ensure that the recirculation tank does not
run dry.

Table 3 gives typical design specifications for RSFs.

In very cold climates, the RSF design must include
elements that  prevent freezing of standing water.
Distribution lines must drain between  doses and
tanks, and the  filter should be insulated.

PERFORMANCE

RSFs  produce   a  high  quality  effluent  with
approximately 85% to 95% BOD and TSS removal.
In  addition,  almost   complete  nitrification  is
achieved.  Denitrification also has been shown to
occur  in RSFs.  Depending on modifications in
design and operation,  50%  or more of  applied
nitrogen can be removed.

The performance of a RSF system depends on the
type and biodegradability of the wastewater, the
environmental conditions  within the filter,  and the
design characteristics  of  the filter.  Temperature
affects the  rate  of microbial growth,  chemical
reactions,  and   other  factors   that  affect the
stabilization of wastewater within the RSFs.

Other parameters that affect the  performance and
design of  RSFs are  the  degree of wastewater
pretreatment, the media size, media depth, hydraulic
loading rate,  organic loading  rate, and  dosing
  TABLE 3 TYPICAL DESIGN CRITERIA
                  FOR RSFS
 Item
Design Criteria
 Pretreatment

 Filter medium

     Material

     Effective size

     Uniformity coefficient

     Depth

 Underdrains

     Type

     Slope

     Bedding


 Hydraulic loading

 Organic loading

 Recirculation ratio

 Recirculation tank
 Distribution and dosing
 system
 Dosing

     Time on

     Time off

     Frequency

     Volume/orifice
Minimum level: septic
tank or equivalent
Washed durable
granular material

1.0 to 3.0 mm

<4.0

24 in
slotted or perforated
pipe

0-0.1%

Washed durable
gravel or crushed
stone (0.25 -1.50 in)

3.0to5.0gpd/ft2/
(forward flow)

0.002-0.008 Ib/ft2/day

3:1 to 5:1

Volume equivalent to
at least 1 day's raw
wastewater flow

Pressure-dosed
manifold distribution
system  and spray
nozzles where
permitted
< 2-3 minutes

Varies

48-120 times/day or
more

1-2 gal/orifice/dose
Source:  Adapted from  Crites and Tchobanoglous with
permission from The McGraw-Hill Companies, 1998.

techniques and frequency.

The effectiveness of a granular material as filter
media is dependent on the size and uniformity  of the
grains.  The size of the granular media affects how

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much wastewater is filtered, the rate of filtration, the
penetration  depth of paniculate matter,  and the
quality of the filter effluent. The finer the grain, the
slower the rate and higher the quality of the effluent.

High hydraulic loading rates are typically used for
filters that receive higher quality wastewater. The
accumulation of organic material in the filter bed
affects the performance of RSFs. As with hydraulic
loading,  an  increase  in the organic loading rate
results in shorter filter life.

OPERATION AND MAINTENANCE

RSFs require routine maintenance, although the
complexity  of maintenance is  generally minimal.
Primary O&M tasks include monitoring the influent
and  effluent,  inspecting  the  dosing equipment,
maintaining the filter surface, checking the di scharge
head on the orifices, and flushing the distribution
manifold annually. The surface of the sand bed
should be kept weed free.

In addition,  the septic tank should be checked for
sludge and scum buildup  and pumped as  needed.
The recirculation tank should also be inspected and
maintained.

The  pumps  should   be  installed  with  quick
disconnect couplings for easy removal. A duplicate
recirculation pump should be available for backup.
Listed in Table 4 are the typical O&M requirements
for RSFs.

COSTS

The cost of  RSFs depends  on the labor, materials,
site, capacity of the system, and characteristics of
the wastewater. One of the most significant factors
that affects  the cost of sand filters is media cost.
Therefore, using locally available materials for the
media is usually the most cost-effective option.

Table 5 shows the costs for RSFs with sand media
and black beauty sand media used in a  facility
treating 5,000 gpd. These are typical costs, actual
costs will vary from site to site and among different
designs.  Local regulatory requirements and labor
rates will affect cost as well. The cost data in Table
5 includes the  labor and machinery necessary to
  TABLE 4  RECOMMENDED O&M FOR
                    RSFS
  Item
O&M Requirement
  Pretreatment            Depends on process;
                        remove solids from septic
                        tank or other
                        pretreatment unit

  Dosing chamber

      Pumps and controls   Check every 3 months
      Timer sequence

      Appurtenances

  Filter media
  Other
Check and adjust every 3
months

Check every 3 months

If continuous hydraulic or
biological overloading
occurs, the top portion of
the media can clog and
may need to be replaced
if not corrected in time

Weed as needed

Monitor/calibrate
distribution device as
needed

Prevent ice sheeting	
Source:  U.S. Environmental Protection Agency, 1980.

install  media,  plumbing,  and  tankage in  the
excavation and landscape, the same should be noted
for the recirculation tank (minus the media).

The  cost  of the  pretreatment  unit(s)  for a RSF
system  will  depend  on  the  waste stream
characteristics  specific to  the  site  application.
Effluent sewer systems incorporate individual or
community septic tanks to  pretreat wastewater
before  it  flows  into  the   recirculation  tank.
Developments   that  include  commercial
establishments  may   require  higher   levels  of
pretreatment in the form of additional  septic tank
storage, surge capacity, grease traps, and possibly
aerobic digestion.

Suggested maintenance for RSFs range from weekly
inspections (15  to  30  minutes)  to  monthly
inspections (for approximately 1 hour).

The  Ashco Rock Filter Storage II (RFSII) sand
filters  consists of three different gradations of

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media; high spec black beauty sand, Ashco's Bottom
Zone, and spray  grids with  spray nozzles to
distribute the recycled filtrate evenly over the media,
all contained in 75 square  foot precast concrete
cells.
 TABLE 5 COST ESTIMATES FOR A 5,000
  GPD FACILITY USING TWO DIFFERENT
                   MEDIA
                             Cost ($)
  Item
Sand1
Black Beauty
   Sand2
  Capital Costs

  Construction costs

     Pretreatment        May vary      May vary

     Recirculation        10,000         9,000
     tank and pumping
     system

     Sand filter

  Non-component costs

  Engineering

  Contingencies

  Land

  Total Capital Costs

  Annual O&M Costs

  Labor                 20/hr         20/hr

  Power                May vary      May vary

  Sludge disposalฎ 10     50/yrb         50/yrb
  cents/gal	

Note:  Non-component costs include piping and electrical.
Engineering and contingency each equal approximately 15%
of construction costs.  Costs toward land, labor, and power
may be different from site to site and system to system.

a Design does not include precast concrete cells.
b Average pumping frequency is every 5 years.

Source: (1) Orenco Systems, Inc., 1998. and (2)
10,000a
May vary
3,000
3,000
May vary
26,000
43,100
May vary
7,800
7,800
May vary
67,700
REFERENCES

1.      Anderson, D. L.; R. L.  Siegrist; and R. J.
       Otis.  1985.   Technology Assessment of
       Intermittent  Sand   Filters.   U.S.
       Environmental Protection Agency  (EPA).
       Municipal  Environmental   Research
       Laboratory. Cincinnati, Ohio.

2.      Ball, J. L. and G. D. Denn. 1997. Design of
       Recirculating  Sand  Filters  Using  a
       Standardized  Methodology.    Site
       Characterization  and  Design  of  Onsite
       Septic  Systems.  American  Society for
       Testing Materials. Fredericksburg, Virginia.

3.      Crites, R.; C. Lekven; S. Wert;  and G.
       Tchobanoglous.   Winter   1997.  A
       Decentralized Wastewater System for  a
       Small   Residential   Development  in
       California. The Small Flows Journal, vol. 3.
       no. 1.

4.      Crites, R. and G.  Tchobanoglous. 1998.
       Small  and   Decentralized   Wastewater
       Management Systems.  The McGraw-Hill
       Companies. New York,  New York.

5.      Hines, M. and R. E. Favreau. Dec. 9-10,
       1974.   Recirculating   Sand   Filter:  An
       Alternative   to    Traditional  Sewage
       Absorption Systems. Proceedings of the
       National Home   Sewage   Disposal
       Symposium, pp. 130-136. Chicago, Illinois.

6.      Hines, M. Sept. 29-Oct.  1,  1975.  The
       Recirculating Sand  Filter: A New Answer
       for an Old Problem. Proceedings of the
       Illinois   Private   Sewage   Disposal
       Symposium, pp. 68-78. Champaign, Illinois.

7.      Martin,  E. J.  and  E.  T.  Martin.  1991.
       Technologies  for   Small   Water   and
       Wastewater   Systems.   Environmental
       Engineering   Series,  pp. 285-291.   Van
       Nostrand Reinhold  (now acquired by John
       Wiley &  Sons, Inc.). New York, New York.

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8.      Orenco  Systems,  Inc.   1998.   Elkton,
       Oregon: A Case  Study. Orenco Systems,
       Inc. Sutherlin, Oregon.

9.      U.S.  Environmental Protection  Agency.
       1980. Design Manual: Onsite Wastewater
       Treatment  and Disposal Systems.  EPA
       625/1-80-012. EPA Office of Water. EPA
       Office  of  Research  &  Development.
       Cincinnati, Ohio.

The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U.S. Environmental  Protection
Agency.

ADDITIONAL INFORMATION

Infiltrator Systems Inc.
Technical Sales and Services Department
P.O. Box 768
Old Saybrook, CT 06475

Dr. Bruce J. Lesikar,  Associate Professor
Agricultural Engineering Department
Texas A&M University System
201 ScoatesHall
College Station, TX 77843-2117

David L. Lindbo
Assistant Professor, Non-Agricultural Soil  Science
Vernon G. James Research and Extension Center
NC State University, Dept of Soil Science
207 Research Station Road
Plymouth, NC 27962

Anthony Tarquin
University of Texas at El Paso
Civil Engineering Department
El Paso, TX 79968

David Vehuizen, P.E.
5803 Gateshead Drive
Austin, TX 78745
For more information contact:

Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C., 20460

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