United States June
Environmental Protection , 1984
Agency
vxEPA An Emerging
Technology
Intermittent
Sand
Filtration
A Process
Assessment
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Intermittent Sand Filtratit
Introduction
A goal of wastewater treatment processes is to
produce an environmentally acceptable effluent at
the lowest possible cost. The task of choosing a
process which fulfills this goal can be challenging
when the volume of wastewater is small. For
example, treatment of wastewater from a sparsely
populated community at a central facility may be
inappropriate due to the high cost of construction,
operation, and maintenance of long sewer lines,
numerous manholes, and lift stations. Where only
small wastewater volumes are generated, on-site
treatment is a possible cost-effective solution. One
such system - capable of effective treatment of
wastewater from residences and small commercial
establishments at a reasonable cost - is the
Intermittent Sand Filter (ISF).
This brochure discusses the technology of the ISF
system and addresses design, performance,
operation and maintenance, and cost aspects of the
process.
In an ISF system, the effluent from a septic tank or
an aerobic unit receives additional treatment
• through periodic application to a filter. ISFs can be
used to upgrade existing wastewater facilities such
as lagoons or replace failed soil absorption systems
(SAS). The ISF system can also be a low cost
alternative when site, soil, or other conditions
restrict the use of SASs.
Technology
The ISF relies primarily on biological and physical
processes to remove wastewater contaminants.
The effluent can then be discharged in an
environmentally sound manner. A simplified flow
schematic of the ISF process is shown in Figure 1.
Raw wJ
Wastewater
Septic Tank or
Aerobic Unit
Filter Media
Effluent Disposal
Figure 1 ISF Process Flow Schematic
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with 24 to 36 inches of media so that a "clogged"
top layer can be removed several times before total
bed replacement is necessary.
Compliance with local health regulations may
necessitate disinfection of the filter effluent prior to
discharge to surface water. Local codes may
stipulate the distance that an ISF must be
separated from potable water supplies (usually 100
feet).
Performance
Under normal operating conditions, ISFs will
produce high quality effluents with respect to BOD5
and suspended solids. The BOD5 concentration of
an ISF effluent is generally less than 10 mg/l,
except during filter startup and maturation. The
mean concentration of suspended solids in ISF
effluents is usually less than 15 mg/l. A
properly functioning ISF will have an organic mat
on the filter surface where microorganisms "treat"
the organics in the wastewater. As the age and
density of the mat increases, treatment improves
due to greater retention time. However, too thick a
mat results in filter clogging.
An ISF having clean sand can remove up to 50%
of a waste stream's phosphorus. Phosphorus
removal by a mature filter is low, and additional
treatment processes are necessary if discharge
permits require reduced phosphorus concentrations.
Removal of nitrogen is dependent on wastewater
type, filter loading rates, ambient temperature, and
other meteorological conditions.
ISFs can also significantly reduce the
concentrations of wastewater fecal and total
coliforms and fecal streptococci bacteria. ISF
performance will be a direct result of the system's
design, operation, and maintenance.
Operation and Maintenance
A properly designed ISF will require a minimum of
time spent on operation and maintenance (O & M).
Table 2 presents the major requirements and
suggested scheduling of O & M activities to obtain
optimum treatment efficiency. O & M requirements
for ISFs are not overly complicated, and many can
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Item
Pretreatment
Septic Tank
Aerobic Unit
Dosing Chamber
Pumps and Controls
Timer Sequence
Appurtenances
Rlter Media
Raking
Replacement
Buried
Inspect at 1 to 2 year
intervals, pump solids
if necessary (usually
once every 3 to 5
years)
Inspect at 4 to 6 month
intervals, pump solids
every 8 to 12 months
Check every year
Check and adjust every
6 months
Check every 6 months
None
None
Miscellaneous
fabie 2 ISF Operation and Maintenance Activities
be performed by an informed homeowner. Service
of pumps, electrical equipment, and pretreatment
units should, however, be performed by
experienced maintenance personnel. Communities
may wish to consider establishing a community-run
or contract maintenance program, thereby relieving
the homeowner of direct involvement in
maintenance activities.
One of the most important O & M activities for an
ISF system is the periodic pumping of solids that
accumulate in the pretreatment unit. Properly
scheduled, pumping reduces the possibility that
materials such as oils, grease, and scum will
overflow and clog the filter surface. This is
especially critical .for buried ISFs, since the filter
media is inaccessible for maintenance.
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Open
Recirculating
peck every 3 months
heck and adjust every
|3 months
heck every 3 months
> 3 inches deep when ponding
bccurs
bplace top 1 to 4" of sand when
ponded more than 12" deep,
eptic tank fed filters are rested
oximately 6 to 9 months
vhile an alternate unit operates,
terobic unit fed filters can
nediately be relumed to
fen/ice
I as required, maintain
stribution device, protect filter
ainst ice sheeting, check high
alarm
Same
Same
Check every 3 months
Check and adjust every
3 months
Check every 3 months
To 3 inches deep when
ponding occurs
Skim sand if heavy
incrustations occur, add
new sand when bed
depth falls below 24"
Weed as required, maintain
distribution device, protect
filter against ice sheeting
Costs
The expenses incurred in constructing ISFs are a
function of local labor and material costs and are
greatly influenced by the land acquisition and filter
media costs. Approximate costs (1982 dollars) for
construction of three types of ISFs are as follows:
Buried: $2400 to $4500
Open: $3900 (average)
Recirculating: $3500 (average)
These examples assume a four bedroom house
having a 1,000-gallon septic tank and a design flow
of 600 gallons/day. The costs include $300 to $600
to reflect the costs of the septic tank. An aerobic
pretreatment unit will increase the system's cost by
$600 to $3600. Annual O & M costs range from
$30 if pretreatment is a septic tank to $500 if
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Recirculating: Recirculating filters are open filters
that treat a mixture of the pretreatmeht unit effluent
and recycled filter effluent. This filter depends on
biological activity for treatment, as do buried or open
filters. When the system is operating properly,
odors from the recirculation tank and open filter are
usually not objectionable since septic tank effluent
is mixed with aerobic effluent returning from the
filter.
Design
Treatment of domestic wastewater prior to filtration
is usually provided by a septic tank or aerobic unit.
Large wastewater volumes can be pretreated in
lagoons.
Filter media particle sizes can strongly influence
final effluent quality. Media that is too coarse allows
wastewater to pass rapidly through the filter which
results in insufficient detention time for biological
decomposition. Fine media limits the quantity of
wastewater filtered and may cause premature
clogging. The most effective filter media particle
size is different for each type of ISF.
Each type of ISF requires a different method and
rate of wastewater application. Wastewater
characteristics also influence the loading rate. In
general, the higher the organic strength of the
waste, the lower will be the recommended loading
rate.
Typically recommended filter media particle sizes
and loading rates for the three different types of
ISFs are shown in Table 1.
A filter bed depth of 9 to 12 inches is sufficient to
treat most wastewaters. Additional depth of filter
media will not appreciably improve effluent quality;
however, it is recommended that ISFs be supplied
Filter Media
Particle Size
ISF Type (millimeters)
Buried ,25-1.0
Open 0.35-1,00
Rectrculattng 1.0- 1.5
Wastewater
Application Rate
(galions/day/square foot)
<1.5
2-5 for septic lank effluent
5-10 for aerobic effluent
3 to 5
Method of
Wastewater Application
Flooded 2 to 4 times daily
Flooded to 2 inches
more than 4 times daily
Dosed periodically
Table 1 Typical ISF Design Parameters
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- A Process Assessment
Raw wastewater first receives preliminary treatment
in a septic tank or aerobic unit. The effluent is then
periodically applied to a bed of granular material.
During its passage through the filter, the
wastewater is treated further by biological and
physical processes. Sand is the common choice as
filter media because of its low cost and availability.
House Sewage
A. Buried ISF
B. Open ISF
Discharge
C. Recirculating ISF
Figure 2 Three Types of ISFs
Three types of ISFs are illustrated in Figure 2 and
described as follows:
Buried: Pretreated wastewater is distributed through
a network of pipes laid above the filter. Perforated
piping beneath the filter collects and conveys the
effluent for surface disposal. Buried ISFs are best
suited for single-family residences and small
commercial establishments.
Open: Wastewater from either aerobic treatment or
a septic tank is applied to the filter surface by a
distribution pipe. A buried concrete box structure is
often used to contain the filter media and can be
operated with or without a cover depending on the
climate. When septic tank treatment precedes an
open ISF, two independent filters may be required
to allow for recovery periods. Open filters,
sometimes referred to as "free access" ISFs, can
handle larger wastewater volumes than buried ISFs
and can also be used as small community, cluster,
and commercial systems.
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pretreatment is an aerobic system requiring
pumping plus a recirculating pump and final
disinfection.
Implementation
Several factors should be assessed prior to the
construction of an ISF system. Local jurisdictions
should be consulted to determine the acceptable
methods of effluent disposal. ISF effluent can be
discharged to surface waters, subsurface to ground
water, or to land. As with any wastewater treatment
system, it will be necessary to determine the need
for Federal or State discharge permits for the
specific application.
Careful planning may allow for the implementation
of ISF systems in situations where lot size and
topography may limit disposal by other processes.
A lot of .25 acres is usually large enough for
installation of an ISF. Topography is rarely
restrictive for ISF construction, but wastewater may
be conveyed to a nearby site where an ISF system
can be installed. However, where the topography of
individual lots is not amenable to standard
construction of an ISF, special designs such as
filter trenches often overcome the problem.
Prepared by Environmental Resources Management, Inc.
For additional information contact
Small flow coordinators:
EPA-OWPO(WH-595)
401 M Street, SW
Washington, DC 20460
(202)382-7263
EPA Region 1
John F. Kennedy Federal Building
Boston, MA 02203
EPA Region 2
26 Federal Plaza
New York, NY 10278
EPA Region 3
6th & Walnut Streets
Philadelphia, PA 19106
EPA Region 4
345 Courtland Street, NE
Atlanta, GA 30308
EPA Region 5
230 South Dearbome Street
Chicago, IL 60604
EPA-MERL (489)
26 West St. Clair Street
Dncinnati, OH 45268
(513)684-7611
EPA Region 6
1201 Elm Street
Dallas, TX 75270
EPA Region 7
324 East 11th Street
Kansas City, MO 64106
EPA Region 8
1860 Uncoln Street
Denver, CO 80203
EPA Region 9
215 Fremont Street
San Francisco, CA 94105
EPA Region 10
1200 6th Avenue
Seattle, WA 98101
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