TECHNOLOGY ASSESSMENT
OF
INTERMITTENT SAND FILTERS
By
Damann L. Anderson
Robert L. Siegrist
Richard J. Otis
of
RSE, INCORPORATED
Engineers / Soil Scientists,
5708 Odana Road
Madison, Wisconsin 53719
PROJECT OFFICER
James F. Kreissl
Wastewater Research Division
Municipal Environmental Research Laboratory
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION
CINCINNATI, OHIO 452$8
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EXECUTIVE SUMMARY
Intermittent sand filtration of wastewater is not a new technology. Sand filters
were often used by sewered communities around the turn of the century. However, as
wastewater flows and land costs increased, they were replaced by mechanical treatment
processes. Recently, as the need for low cost facilities in rural areas has grown,
intermittent filters have received increased use again.
Intermittent sand filters are beds of medium to coarse sands, usually 24 to 36
inches deep and underlain with gravel containing under drains. Primary or secondary
effluent is intermittently applied to the surface and purification of the effluent occurs as
it infiltrates and percolates through the sand bed. Underdrains collect the filtrate and
convey it to additional treatment processes and/or discharge. Intermittent sand filter
design concepts include buried filters, open single-pass filters and open recirculating sand
filters.
Laboratory and field investigations have demonstrated that intermittent sand
filters can produce very high quality effluents. Concentrations of effluent BODc and TSS
are typically less than 10 mg/L with ammonia nitrogen less than 5 mg-N/L. Only limited
removal of phosphorus and fecal coliform bacteria are achieved, however. Design
considerations important to achieving this level -of treatment include pretreatment,
media characteristics, hydraulic and organic loading rates, temperature and filter dosing
techniques.
Operation and maintenance are important to achieving high levels of treatment and
to maintain long filter runs. Raking of the sand surface, resting and periodic removal of
the surface sand are commonly employed. Energy requirements of intermittent filters
are less than approximately 0.28 HP-hr per 1000 gal. (0.055 kWh per mr) of processed
flow.
Intermittent sand filters compare favorably in economics and performance with
extended aeration package plants and lagoon systems. Compared to extended aeration
units, intermittent filters possess a lower present worth cost, consume substantially less
energy, produce a more consistent and high quality effluent, but require more land area.
Compared to facultative lagoons, intermittent filters possess a lower present worth cost,
consume slightly more energy, produce a substantially higher quality effluent and require
less land area. Operational requirements for these filters are significantly less than for
extended aeration units, but more than for lagoons.
Intermittent sand filtration represents attractive wastewater treatment process
that can satisfy the significant treatment needs of small communities. Treatment needs
for communities with flows less than 0.106 MGD for which sand filtration is ideally
suited, represent 63 percent of the total national needs to the year 2000. In addition to
small community needs, many rural housing developments and business establishments
can utilize sand filtration where site and soil conditions preclude the use of subsurface
disposal systems.
Despite the long historical use of intermittent sand filters and the recent increase
in their use, their performance capabilities have not been fully optimized. Further inves-
tigation is needed to optimize relationships between design criteria and performance
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11
capabilities. The development of a data base regarding the design and performance of
full-scale plants as well as their operation and maintenance requirements and costs would
facilitate this effort and provide other needed data as well.
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CONTENTS
Page
Executive Summary i
Contents iii
Figures iv
Tables ... v
Sections ,
1. Technology Description .1
Introduction ;*....... 1
Process Description 1
Process Designs 2
2. Development Status . 5
3. Technology Evaluation 8
Process Theory 8
Process Capabilities and Limitation 9
Design Consideration 9
Filter Performance 15
Operation and Maintenance ..,.., 18
4. Comparison with Equivalent Technology 19
Costs 19
Energy Requirements 19
Performance 19
Land Area Requirements 23
5. Assessment of National Impact 25
Market Potential 25
Cost and Energy Impacts , 25
Risk Assessment ,. 25
6. Recommendations 27
Research and Development Efforts ............. 27
Process/Technology Modifications ... ^.......... 27
References 28
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FIGURES
Number p v-
---"' » ' "• i ....LuW.
1 Typical Buried Sand Filter -3
2 Open (Single Pass) Sand Filter 4'"
3 Typical Recirculating Sand Filter 4
4 Estimated Land Areas for Intermittent Sand Filters
and Comparable Processes 24
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TABLES
Number Page
1 Example Community-Scale Sand Filter System ....... 6
2 Performance Data for Community-Scale Sand
Filter Systems 8
3 Summary of General Trends Between Design and
Performance Factors , „. 13
4 Example Design Values .. 14
5 Performance of Open Intermittent Filters ........... 16
6 Performance of Recirculating Intermittent Filters
Treating Septic Tank Effluent , .. 17
7 Cost Comparison — 5,000 gpd Facility 20
8 Cost Comparison — 30,000 gpd Facility ............. 21
9 Estimated Energy Consumption of Intermittent
Sand Filters (kWh/yr) ^... 22
10 Estimated Treatment Performance by Process
Type , 22
11 Existing Number and Projected Number of Secondary
and Advanced Secondary Treatment Plants l>y Design
Capacity (USEPA, 1983a) 2$
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SECTION 1
TECHNOLOGY DESCRIPTION
INTRODUCTION
More than 23 percent of all housing units in the United States are beyond the reach
of public sewers and approximately 350,000 new homes are being built each year in
unsewered areas. Traditionally, waste waters from these homes are treated and disposed
of by septic tank-soil absorption systems. Many of these systems have failed because of
unsuitable soil and site conditions, poor design and installation and lack of maintenance.
Where failures are widespread, communities are forced to consider construction of public
collection and treatment facilities.
Due to low population densities, conventional sewage collection and treatment is
often too costly. It is not uncommon for residents of small communities to pay two or
three times as much for sewer services as residents of larger municipalities. The impact
of these user charges on family budgets can be quite severe because the average annual
incomes in rural communities are significantly lower than in more urbanized areas. As a
result, plans for construction of needed facilities are often rejected and public health
hazards, nuisances and environmental degradation from improperly functioning septic
tank systems continue while economic development is impeded. Less costly but equally
effective alternatives to conventional sewerage are needed.
Significant savings to the community can be made by reducing the operation and
maintenance costs of the treatment plant. The costs of construction are usually eligible
for grant assistance from various funding agencies, but the day-to-day costs of operating
and maintaining the facility must be borne solely by the community. Conventional
treatment plants are often highly mechanized and require substantial attention by a
skilled operator. Most small communities do not have the skilled personnel or financial
resources to provide the needed operator. Simple, low maintenance treatment processes
which can achieve required effluent standards or avoid effluent discharges into surface
waters are needed if user charges are to be kept within realistic limits.
Intermittent sand filters are one such alternative which are ideally suited to rural
communities, small clusters of homes, individual residences and business establish-
ments. They can achieve advanced secondary or even tertiary levels of treatment
consistently with a minimum of attention. They are also relatively inexpensive to
construct and have low energy requirements. Because of these advantages, their use in
rural management districts and small communities is expected to grow.
PROCESS DESCRIPTION
Intermittent sand filters are beds of medium to coarse sands, usually 24 to 36 in.
deep underlain with gravel containing collection drains. Primary or secondary effluent is
intermittently applied to the surface and percolates through the sand to the bottom of
the filter. The underdrains collect the filtrate and convey it to additional treatment
processes and/or discharge.
The treatment processes are complex, involving physical, chemical and biological
mechanisms. Straining and sedimentation of suspended solids occurs between the sand
grains and chemical sorption on the grain surfaces plays a role in the removal of some
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materials. However, it is the biological transformations that occur within the filter
which are the most significant (Calaway, 1957). Since these are most efficient under
aerobic conditions, intermittent application of the waste water and venting of the
underdrains helps to insure aeration of the sand. Biomass and associated waste byprod-
ucts develop during treatment and are retained within the filter. Biological degradation
including endogenous respiration helps to minimize solids accumulations. However, with
time, accumulations of biomass and other participate matter may build up near the filter
surface to such a degree that the sand bed must be rejuvenated to restore the hydraulic
capacity of the filter to an acceptable leveL
PROCESS DESIGNS
Buried Sand Filters
Buried sand filters are constructed below grade and covered with backfill material
(Figure 1). A 4 to 5 foot deep excavation is generally made. The underdrains are
surrounded by graded gravel or crushed rock and the upstream ends are brought to the
surface and vented. A thin layer of fine gravel is commonly placed over the larger
gravel to prevent piping of the filter sand into the underdrains. After placement of the
filter sand, another layer of washed graded gravel or crushed rock is laid over the filter
surface along with the distribution piping for wastewater application. These pipes are
vented to the ground surface at their downstream end. The entire filter is then
backfiEed. Buried filter designs are most commonly used for very small flows such as
those from _ single homes and small commercial establishments. These filters are
designed to perform for very long periods of time (up to 20 years) without the need for
operation and/or maintenance.
Open (Single Pass) Sand Filters
1
Open (single-pass) sand filters (Figure 2) are similar to buried sand filters except
that the surface of the filter is left exposed and higher hydraulic and organic loadings are
generally applied. In cold climates, removable covers may be used. In addition to
perforated distribution piping, the wastewater may be applied by flooding the surface
periodically or through spray distribution. These filters are used for individual homes as
well as larger flows from small communities or industries (up to 0.2 MGD).
Recirculating Sand Filters
Recirculating sand filters are open filters which utilize somewhat coarser media
and employ filtrate recirculation. Wastewater is dosed from a recirculation tank which
receives both settled waste (e.g., septic tank effluent) and the filtrate (Figure 3). A
recirculation rate of 3:1 to 5:1 is typical. A portion of the filtrate is diverted for further
treatment or disposal during each dose or when the recirculation tank is full. (These fil-
ters have been applied to both individual homes and small communities (up to 0.2 MGD).
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DISTRIBUTION BOX
/M... V frl. '"•* WIM
;v,--:^V-.u:^"^:^-J:V---
DISCHARGE
INSPECTION/DISINFECTION TANK-
(IF REQUIRED)
PROFILE
TOP SOIL FILL
DISTRIBUTION LATERALS
GRADED GRAVEL
3/4' TO 21/2"
PEA GRAVEL
UNDERDRAW
SECTION
Figure 1. Typical Buried Sand Filter
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INSULATED COVER
(IF REQUIRED)
DiSTRIliO*.'.'
PEA GRAVEL
DISCHARGE
GRADED GRAVEL
1/4 TO 11/2*
•COLLECTION PIPE
PERFORATED OR OPEN JOINT
Figure 2. Open (Single-Pass) Sand Filter
HAW WASTE
JTE
-1 \—
PRETREATMENT
UNIT
SEPTIC
TANK
EFFLUENT FILTRATE
FREE ACCESS
SAND FILTER
Figure 3. Typical Recirculating Sand Filter
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SECTION 2
DEVELOPMENT STATUS
The full-scale use of intermittent sand filters as a secondary wastewater treatment
process is in general not a new technology. They were frequently used by sewered
communities around the turn of the century. However, as wastewater volumes increased
and land costs rose, they were replaced by mechanical treatment processes. Only
recently, as the need for low cost facilities in small communities is growing, have
intermittent filters been re-employed. (Salvato, 1955; Teske, 1978; Evans, et. al., 1978:
Ronayne, et. al., 1982; Curran, et. al., 1983).
The design and performance characteristics for a number of community-scale
intermittent sand filters have been compiled in Tables 1 and 2. Most facilities are open
surface filters of single-pass or recirculating design serving small communities with
design flows up to approximately 120,000 gpd. Pretreatment consists of sedimenta-
tion/digestion in imhoff tanks or septic tanks. Individual filter units typically have
surface areas of less than 11,600 ft^ and media depths of 2 to 3 ft. Filter media are
exclusively sand of medium to very coarse grain size (0.25 mm - 2.00 mm). Multiple
filter units are provided with one or more standby units for use during filter maintenance
or periods of increased flows. Filter effluent is disinfected using chlorine or ultraviolet
irradiation with final disposal to infiltration basins, ditches or water courses. In some
cases effluent is discharged into subsurface absorption trenches without disinfection.
Table 1 lists several facilities for which data are available. Most of these facilities were
put into operation since 1976, although one of these intermittent filters has been in
operation since 1953.
The performance of full-scale intermittent sand filters, both single pass and recir-
culating, appear to be consistent with laboratory and field studies. A high quality
effluent is produced. Concentrations of BOD5 and TSS equal to 10 mg/L or less and
nitrification of 80 percent or more .of the applied ammonia are typically achieved (Table
2). Removals of phosphorus are limited and reductions in fecal coliform bacteria are less
than two logs, slightly less than might be expected.
Further developments in intermittent sand filter technology as generally utilized
today are likely. Several process modifications have been investigated as means of
enhancing the effluent quality produced by intermittent filters. Increased removal of
soluble organics and phosphorus has been demonstrated with mixed media of sand and
chemically active substances such as silt and clay soils, limestone fragments or activated
carbon (Schwartz, et. al., 1967; Brandes, et. al., 1975). Increased removal of coliform
bacteria may be achieved with filters comprised of multiple layers of sand of decreasing
particle size (Scherer and Mitchell, 1982). The application of modifications such as these
in full-scale facilities awaits further demonstration.
A promising development in the application of intermittent sand filters involves
their use prior to wastewater absorption in subsurface soil absorption systems, [n this
capacity, recent data suggests that sand filters may enable increased hydraulic loading
rates, as much as 300 percent higher than typically possible with conventional septic tank
effluent (Ronayne, et. al., 1982).
The equipment and hardware typically utilized in intermittent sand filters should be
available locally in most municipalities. The critical component is the media, which
often is also available locally.
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SECTION 3
TECHNOLOGY EVALUATION
PROCESS THEORY
It is known that physical, chemical, and biological treatment processes all occur to
some degree within the filter. Straining, sedimentation, inertial impaction, interception,
adhesion, flocculation, diffusion, adsorption and biological activity have all been sug-
gested as mechanisms of contaminant removal in wastewater filtration (Tchobanoglous,
1968, 1970). Straining involves a mechanical sieve action as well as lodging of particles
in crevices. Sedimentation occurs as gravity settling takes place In the interstices of the
media. Inertial impaction, interception, and adhesion occur as particles moving through
the filter strike media granules and are removed. Particles moving through the pores
will also collide with each other and flocculate causing subsequent removal by other
mechanisms. Diffusion is important in the removal of very small particles such as
viruses, and occurs because of the small interstices in porous media and the fact that
laminar flow exists. Physical adsorption of pollutants takes place on media surfaces due
to electrostatic, electrokinetic and van der Waals forces while chemical adsorption
occurs due to bonding and chemical interaction between wastewater constituents and the
filter media. Biological activity on the filter media results in removal of polluting
materials by biological assimilation and biosynthesis.
While physical and chemical processes play an important role in the removal of
many materials by filtration, successful treatment of wastewaters by intermittent filtra-
tion is dependent upon the biochemical transformations occurring within the filter.
Bacteria are the primary workers in intermittent sand filters, although there; is a broad
range of trophic levels operating within the filter, from bacteria to multi-cellular
animals including the metazoa (Calaway, 1957).
Since filters entrap, sorb, and assimilate materials in the wastewater, the
interstices between the grains may fill, and the filter may eventually clog. Clogging may
be caused by physical, chemical, and biological factors. Physical clogging is normally
caused by the accumulation of stable solid materials within or on the surface of the
sand. It is dependent on grain size and porosity of the filter media and on wastewater
suspended solids. The precipitation, coagulation, and adsorption of a variety of materials
in wastewater may also contribute to the clogging problem in some filter operations
(Schwartz, et. al., 1967). Biological clogging is due primarily to an improper balance of
the intricate biological population within the filter. Toxic components in the waste-
water, high organic loading, absence of dissolved oxygen, and decrease in filter
temperatures are the most likely causes of microbial imbalances. Accumulation of
biological slimes and a decrease in the rate of decomposition of entrapped wastewater
contaminants within the filter accelerates filter clogging. All forms of pore clogging
likely occur simultaneously. Although the dominant clogging mechanism is dependent
upon wastewater characteristics, method and rate of wastewater application, character-
istics of the filtering media, and filter environmental conditions.
PROCESS CAPABILITIES AND LIMITATIONS
Intermittent sand filtration is well adapted to small flows wastewater treatment.
The process is applicable to single homes, clusters of dwellings and small communities.
The wastewater applied to the intermittent filters should be pretreated at least by sedi-
mentation.
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Normal site contraints other than land availability should not limit the application
of intermittent sand filters, although odors from open filters receiving septic tank
effluent may require a suitable buffer zone between the system and nearby dwellings
Filters _are often partially (or completely) buried in the ground, but may be constructed
above ground when dictated by shallow bedrock or high water tables. Covered filters are
required in areas with extended periods of subfreezing weather. Excessive long-term
rainfall and runoff on submerged filter systems is detrimental to performance, requiring
appropriate measures to divert these sources away from the system.
The degree of stabilization attained by an intermittent sand filter is dependent
upon the characteristics of the wastewater applied to the filter and the environmental
conditions within the filter.
Since intermittent sand filtration is largely a biological process, the characteristics
of the applied wastewater affect the purification achieved. Domestic: wa^tewaters are
very amenable to sand filtration, whereas wastewai ;•,-:; ?\i-;&\±.fi >.•; b .\.-..-. //^..uon may
result in poor performance.
• Temperature and reaeration are two of the most important environmental condi-
tions that affect the degree of wastewater purification through an intermittent sand
filter. Temperature directly affects the rate of microbial growth, chemical reactions,
adsorption mechanisms, and other factors that contribute to the stabilization of
wastewater within the sand media. Availability of oxygen within the pores allows for
aerobic decomposition of the wastewater and alnrost complete stabilization of substances
that are readily biodegradable. Under aerobic conditions, the major end products of
biochemical stabilization of carbonaceous and nitrogenous substances are water, carbon
dioxide, bicarbonates, sulfates, phosphates, and nitrates. In the absence of oxygen,
carbonaceous material may be converted to carbon dioxide and methane, but nitrogenous
substances degrade only to ammonia, and cannot be oxidized to nitrate.
The selection of process design variables affects the degree of purification of
wastewater achieved by intermittent filters. Variables should be chosen to optimize the
previously discussed factors while providing a practical, manageable treatment system.
A discussion of design considerations is presented in the next section.
DESIGN CONSIDERATIONS
There are many variables which affect the operation and performance of inter-
mittent sand filters (ISFs). Some can be specified in design and some cannot. Although
an enormous amount of information is available in the literature regarding intermittent
filters, the confounding effects of the many variables make it difficult to come up with
simple relationships between design and performance factors.
Design considerations for sand filter systems include:
Pretreatment
Media size, uniformity and composition
Media depth
Hydraulic loading rate
Organic loading rate
Temperature
Dosing techniques and frequency •
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10
Pretreatment
The operation and performance of ISF's are directly related to the degree of
pretreatment of the applied wastewater. Schwartz, et. al. (1967) showed a direct
relationship between degree of pretreatment and both hydraulic longevity and effluent
quality m lysimeter studies with 0.20 mm Ottawa sand loaded at 5 gpd/ft\ Comparisons
of intermittent sand filtration of household aerobic unit effluent and septic tank effluent
Si Sh°Wn hlgher accePtance rates of wastewater infiltration, longer filter runs,
( filter run" is defined as the service time during which the filter successfully accepts
and treats the design flow) and equal or better effluent quality with the additional
pretreatment (Sauer, 1975; Stothoff, 1976). uiuonai
Media
The successful use of a granular material as a filter media is dependent upon the
proper choice of size and uniformity of the grains. The effective size of the granular
media affects the quantity of wastewater that may be filtered, the rate of filtration the
penetration depth of participate matter, and the quality of the filter effluent. Granular
media that is too coarse lowers the wastewater retention time to a point where adequate
biological decomposition is not attained. Too fine a media limits the quantity of waste-
water that may be successfully filtered due to early filter clogging. Effective size alone
can be misleading when describing media size. Sands of similar effective size but
different uniformity coefficient can produce significantly different performance
characteristics. Metcalf and Eddy (15) and Boyce (16) recommended that not more than
1% of the media should be finer than 0.13 mm. Recommended filter media effect sizes
fT^Vr°m a minimum of °-40 mm UP to approximately 1.5 mm. Uniformity coefficients
\ «?« . intermittent filter media normally should be less than 4.0. (PHS, 1967; Glumrb,
1960; ASCE, 1937; Salvato, 1955; WPCF, 1977; EPA, 1980).
Granular media other than sand that have been used include anthracite garnet
dmemte, activated carbon, and mineral tailings. Alternate media such as these must be
durable and insoluble in water. Any clay, loam, limestone, or organic material may
increase the initial adsorption capacity of the sand, (usually for phosphorus removal) but
may lead to a serious clogging condition as the filter ages. Any non-sand media should
conform to the same retirements discussed herein for sand and have a total organic
content of less than 1%, total acid soluble matter less than 3%, hardness of less than 3 on
the Moh's scale, and be genreally rounded in shape.
The arrangement or placement of different sizes of grains throughout the filter bed
is also an important design consideration. A homogeneous bed of one size media often
does not occur due to construction practices and variations in local materials. Abrupt
textural changes will create zones of saturation which can act as water seals and can
limit oxidation, promote clogging, and reduce the action of the filter to a mere straining
mechanism. The use of media with a UC of less than 4.0 minimizes this problem. The
media arrangement of coarse over fine appears theoretically to be the most favorable,
but it may be difficult to maintain such a filter due to internal clogging throughout the
JL1J. Iv^i*
Media Depth
_ Media depths used in intermittent sand filters were initially 4 to 10 feet. However,
studies revealed that most of the purification of wastewater occurred within the top 9 to
12 inches (23 to 30 cm.) of the bed (Clark and Gage, 1909; Emerson, 1945; Furman, et
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11
al., 1955), with the additional bed depth improving purification only slightly. Later
studies confirmed this but pointed out the need for the additional depth from a moisture
standpoint (Schwartz, et. al., 1967). The capillarity of sand causes high moisture
contents in the deeper sand limiting aeration and thus the bacterial oxidation process.
Schwartz, et. al. (1967) reported satisfactory ammonia removals (greater than 80%) only
for unsaturated depths of 4 feet (1.2 meters) or greater and showed a direct relationship
between filter depth and filter run length in 0.20 mm effective size Ottawa sand loaded
with 5 gpd/ft of septic tank effluent. These results were attributed to the fine sand
used and the high degree of capillarity of such sand. It is critical to maintain sufficient
depth of sand so that the zone of capillarity does not infringe on the zone required for
treatment. For these reasons most media depths used today range from 24 to 42 inches
(62-107 cm.). The use of shallower filter beds helps to keep the cost of installation low.
Deeper beds tend to produce a more constant effluent quality, are not affected as
severely by rainfall or snow melt (Brandes, 1970), and permit the removal of more media
before media replacement becomes necessary.
Hydraulic Loading Rate
The hydraulic loading is normally expressed as gallons per day per square foot
(gpd/ft ), or as centimeters per day (cm/day). Values of recommended loading rates for
intermittent sand filtration vary throughout the literature and range from 0.75 to 5
gpd/ft (3.1 to 20.4 cm/day). Higher hydraulic loading rates are normally applied to
filters with larger media size or those receiving higher quality waste water. Higher
hydraulic loadings of a given wastewater produce correspondingly shorter filter runs.
The relationship between hydraulic loading and effluent quality is unclear and depends on
other design factors. In general, increased hydraulic load causes a decrease in effluent
quality for a given media.
Organic Loading Rate
Organic loading rates are not often reported in the literature, however, previous
studies have indicated that the performance of ISF's is affected by the accumulation of
organic material in the filter bed (Schwartz, et. al., 1967; Clark and Gage, 1909). To
account for differences in organic strength of various waste waters, hydraulic loading
rates are often adjusted for the type of wastewater. Hydraulic loading rates may be
increased in direct proportion to the degree of pretreatment. A specific relationship
between organic loading rate and effluent quality is not clear but Schwartz, et. al.
(1967), showed that effluent COD levels as well as COD removals were directly propor-
tional to influent COD strength for 0.20 mm Ottawa sand loaded at 5 gpd/ft2 with
different waste types. Like hydraulic loading, higher organic loading rates produce
correspondingly shorter filter runs. One of the conclusions of the early ISF work
performed at the Lawrence Experiment Station from 1887 to 1908 was that the volume
of sewage that can be purified by intermittent sand filtration is dependent upon the
amount of organic matter present in the wastewater rather than the volume of waste-
water in which this organic material is held (Clark and Gage, 1909).
Temperature
Temperature directly affects the rate of microbial growth, chemical reactions,
adsorption mechanisms, and other factors that contribute to the stabilization of
wastewater within an intermittent sand filter. Somewhat better operation and perfor-
mance therefore may be expected from filters in warmer locales. For filters operated in
cold climates, it has been suggested that the temperature at which the filter is started
-------
and matured is an important consideration. Schwartz, et. ai. U367) reported tha
started m warm weather significantly outperformed those started in cold w^
regards to hydraulic longevity (filter run length) as well as effluent quality.
Dosing Techniques and Frequency
The method of application of wastewater to an intermittent sand filter is important
to the performance of the process. A dosing system should provide uniform distribution
of wastewater throughout the filter cross-section. Sufficient time must also be provided
between doses to allow reaeration of the pore space. Dosing methods used include ridge
°W apPUcation> ^ain tile distribution, surface flooding, and spray distribution
The frequency of dosing intermittent sand filters is important to their perfor-
mance. Most of the earlier studies used a dosing frequency of I/day, but studies in
Florida concluded that better performance and treatment was obtained with dosings of
?£Xy £" SandS Wlth effective sizes ranging from 0.25 to 0.46 mm (Grantham, et. al..
1948| Furman, et. al., 1955). Other studies have shown that dosing frequencies beyond
Z/day provide no additional benefit for fine to medium sand sizes (Clark and Gage 1909-
Furman, et. aL, 1955; Schwartz, et. al., 1967). For filters with media greater than' about
0.45 mm, it has been concluded that better purification is obtained when the frequency
of dosing is increased beyond twice per day. This is because the lower retention capacity
of the coarser media limits the amount of wastewater that should be applied at one time
(Clark and Gage, 1909; Furman, et. al., 1955). This multiple dosing concept is success-
fully used in recirculating sand filter systems which employ a dosing frequency of once
every 30 minutes (Hines and Favreau, 1975).
Summary of Design Considerations
While no specific relationships have been developed between design and perfor-
mance factors discussed in this section, general trends can be predicted for these
relationships based on the results of laboratory and field investigations. Table 3 sum-
marizes some of these trends between design considerations and the performance factors
effluent quality, length of filter run, and cost. Example design values for three types of
intermittent filters are summarized in Table 4.
-------
13
Table 3. SUMMARY OF GENERAL TRENDS BETWEEN
DESIGN AND PERFORMANCE FACTORS.
Design
Factors
Effluent
Quality
Performance Factors
Filter Run
Length
Capital Cost
of ISF
Increasing
Pretreatment
Increasing
Media Effective
Size
w tuc. <4.0
Dependent on
Local
Availability
Increasing
Filter
Depth
Very little
effect past 24"-
36" depending
on sand size
Very little
effect past 24"-
36" depending
on sand size
Increasing
Hydraulic Loading
Rate
Increasing
Organic Loading
Rate
Increasing
Operating
Temperature
fl
Increasing
Dosing
Frequency
Medium
to
Coarse
Sand
Very
little
effect
Fine to
Medium
Sand
Medium
to
Coarse
Sand
Very
little
effect past
Fine to
Medium
Sand
Very
little
effect
NOTE: This figure shows only general trends suggested from a review of studies on
intermittent sand filtration; however, it shouid be noted that many of the
factors shown are interrelated and therefore must be considered together on
design. Upward pointing arrows indicate an increase in the factor described,
while a downward pointing arrow indicates a decrease.
-------
14
Table 4. EXAMPLE DESIGN VALUES
Design Factor
Buried
Open
Recirculating
Pre treatment
Minimum of Sedimentation
Media
Material
Effective Size
Unif. Coeff.
Depth
Hydraulic Loading
Organic Loading
Media Temperature
Dosing Frequency
Recirculation Ratio
------ Washed, Durable Granular Material ------
0.40-1.00 mm 0.40-1.00 mm 0.40-1.5 mm
<4 <4 • " <4
24-36 inches 24-36 inches 24-36 inches
(61-91 em): (61-91 cm) (61-91 cm)
<1.5 gpd/ft2 2-5 gpd/ft2 3-5 gpd/ft2*
(<6.1 cm/day) (8.2-20.4 cm/day) (12.2-20.4 cm/day)
- - . <5 x ISO"3 Ibs. BOD5/day/ft2 - -v- - -
(<2.4 x 10"2 kg. BOD5/day/m2) - - - 7 -
>2 per'day
NA
>2 per dayv
NA /
5-10 min/30 min.
3:1 to 5:1
+ Values given are based upon past experience and.current practice. They are not
necessarily optimum values for a given performance objective. See text for
discussion.
* Based upon forward flow only.
-------
FILTER PERFORMANCE
A summary of the performance of selected intermittent sand filters treating
domestic wastewaters appears in Tables 5-6. These tables illustrate that intermittent
filters produce high-quality effluents with respect to BOD5 and suspended solids.
Normally, nitrogen is transformed almost completely to the nitrate form. Rates of
nitrification may decrease in winter months as temperatures fall. Some denitrification
can occur in single-pass filters and produce total nitrogen removals of 0-50%.
Total and ortho-phosphate concentrations can be reduced up to approximately 50%
in clean sand; but the exchange capacity and phosphorus removal of sand after matura-
tion is low. Use of calcareous sand or other high-aluminum or iron materials intermixed
within the sand may produce significant phosphorus removal. (Chowdhry, 1974, Brandes,
et. al. 1975). Intermittent filters are capable of reducing total and fecal coliforms by 2
to 4 logs, producing effluent values ranging from 1,000 to 100,000 and 100 to 3,000 per
100 ml, respectively (Schwartz, et. al., 1967; Chowdhry, 1974; SSWMP, 1978; Salvato,
1955).
-------
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-------
18
OPERATION AND MAINTENANCE
Intermittent sand filters require relatively little operational control or
maintenance. Once wastewater is applied to the filter, it takes from a few days to
several weeks before the.sand has matured (Schwartz, et. al., 1967; SSWMP, 1978). BOD
and SS concentrations in the effluent will normally drop rapidly after maturation.
Depending upon media size, rate of application, and ambient temperature, nitrification
may take from 2 weeks up to 6 months to develop. Winter start-up should be avoided
since the biological growth on the filter media may not develop properly (Schwartz, et.
al., 1967).
Clogging of the filter eventually occurs as the pore space between the media grains
begins to fill with inert and biological materials. The operational period before clogging
occurs is a function of the design factors discussed previously. Once hydraulic
conductivity falls below the average hydraulic loading, permanent ponding occurs.
Although effluent quality may not initially suffer, anaerobic conditions within the filter
result in further rapid clogging and a cessation of nitrification. Application of
wastewater to the filter should be discontinued when continuous ponding occurs.
Maintenance of the media includes both routine maintenance procedures and media
regeneration upon clogging. These procedures apply to open filters only. Buried filters
are designed to perform without maintenance for up to 20 years. The effectiveness of
routine raking of the media surface has not been clearly established, although employed
in several studies (SSWMP, 1978; Schwartz, et. al., 1967; Clark and Gage, 1909; Hines and
Favreau, 1975). Filters open to sunlight require weed removal. Cold weather main-
tenance of media may require different methods of wastewater application, including
ridge and furrow and continuous flooding. These methods are designed to eliminate ice
sheet development. Use of insulated covers may permit trouble-free winter operation in
areas with ambient temperatures as-low as -40° F (SSWMP, 1978).
Eventually, filter clogging requires media regeneration. Raking of the surface may
not in itself eliminate the need for more extensive rehabilitation (SSWMP, 1978;
Schwartz, et. al., 1967). The removal of the top layer of sand, as well as replacement
with clean sand when sand depths are depleted to less than 24 to 30 in. (61 to 76 cm.),
appears to be very effective for filters clogged primarily near the surface. This includes
filters receiving secondary effluent (SSWMP, 1978). In-depth clogging can occur which
requires oxidation of the clogging materials. Resting of the media for a period of months
has proven effective in restoring filter hydraulic conductivity (SSWMP, 1978).
A distinct advantage of intermittent sand filtration systems is the low energy
requirements in comparison to systems which offer comparable effluent quality. Open
intermittent sand filters using pumped dosing should only require approximately 0.07 HP-
hr per thousand gallons (0.013 kWh per m ) assuming a 10 foot (3.05 m) pumping head and
pump efficiency of 60%. With the same assumptions and a 3:1 recirculation ratio
(Recycle: Forward Flow) a recirculating intermittent sand filter would require
approximately 0.28 HP-hr per thousand gallons (0.055 kWh per m^).
-------
19
SECTION 4
COMPABISON WITH EQUIVALENT TECHNOLOGY
Intermittent sand filtration of partially treated wastewater is primarily a biological
wastewater treatment process. It may be further characterized as an advanced second-
ary treatment process as it achieves significant reductions in BODc and TSS as well as
nearly complete nitrification. Representative conventional treatment alternatives which
might be acceptable to authorities but not equivalent in terms of effluent quality include
extended aeration package plants and potentially facultative lagoon systems. A compari-
son was made between intermittent sand filters and these two processes in terms of
costs, energy consumption, performance and land area requirements.
COSTS
The costs to install and operate comparable wastewater facilities were estimated
to enable a cost comparison of single-pass and recirculating sand filters to facultative
lagoons and extended aeration package plants. Two different size facilities, 5,000 and
30,000 gpd, were considered. Despite the inherent inaccuracies in cost estimation com-
parisons, intermittent sand filters appear to possess present worth costs in the range of
those associated with facultative lagoons and extended aeration package plants (Tables 7
and 8).
ENERGY REQUIREMENTS
Reduced energy consumption represents a potentially significant advantage of
intermittent sand filtration over extended aeration. The estimated energy consumption
of single-pass and recirculating sand filters is generally less than 10% of that of extended
aeration (Table 9). Energy requirements of facultative lagoons are often very low, com-
paratively less than those of single-pass filters.
PERFORMANCE
Under normal operating conditions, intermittent sand filters will produce high
quality effluents, significantly better than that produced by extended aeration package
plants and definitely superior to that achieved with conventional facultative lagoons
(Table 10). Concentrations of BODg and TSS ©f 10 mg/L or less are typically achieved
through intermittent sand filtration as compared to 30 and 30 mg/L for extended
aeration units (Hinrichs, 1978). Effluent qualities from facultative lagoons are
characteristically somewhat poorer than either sand filters or extended aeration plants.
Effluent BOD5 concentrations range from 20 to 60 mg/L, but TSS concentrations fluc-
tuate even more widely (USEPA, 1983). TSS values of up to 150 mg/L are not uncommon
in warmer periods due to the presence of algal solids.
-------
Table 7. COST COMPARISON1 - 5,000 GPD FACILITY
Cost
Lagoon
Extended Single-Pass Recirculating
Aeration Filter Filter
Capital Costs
Construction Costs
Septic Tank Pretreatment
Pumping System
Sand Filters
Aeration Package Plant
Lagoon
Subtotal
Non-Component Costs2
Engineering"*
Contingencies4
Land4
Total
Annual O & M Costs
Labor @! $10/hr.
Power <§. 7
-------
21
Table 8. COST COMPARISON1 30,000 GPD FACILITY
Cost
Capital Costs
Construction Costs
Septic Tank
Pumping System
Sand Filters
Aeration Package Plant
Lagoon
Subtotal
Non-Component Costs^
Engineering3
Contingencies
Land4
Total
Annual O & M Costs
Labor @ $10/hr.
Power <§. 7<&/kwH
Chemicals
Sludge Disposal <§. 3.5iO
22,380
15,350
15,350
4,400
137,420
4,800
40
Neg.
liP.?0.
5,860
197,700
Recirculating
Filter
7,320
13,700
44,480
65,500
18,340
12,580
12,580
3,440
112,440
4,800
160
Neg.
• 1,020
5,980
173,970
Costs included for only those unit processes shown.
2
Non-componenet costs (e.g., piping and electrical, estimated to equal 28
construction costs for all processes except the lagoon system, for which
was used. *
0
Costs were each estimated to
4
percent of
14 percent
equal 15 percent of construction costs.
**•-'»•**••** VN*V» t**r *f
-------
22
Table 9. ESTIMATED ENERGY CONSUMPTION OF INTERMITTENT
SAND FILTERS (kWh/yr)
Unit Process Size Intermittent Sand Filters* Extended Aeration*
(gpd)
10,000
25,000
50,000
Single-Pass
180
455
910
Recirculating
770
1,915
3,830
15,800
39,400
49,100
*Estimated energy consumption due to pumping of effluent onto filter.
^Estimated energy consumption due to pumps and blowers (SCS Engr., 1977).
Table 10. ESTIMATED TREATMENT PERFORMANCE BY PROCESS TYPE
Process Removal Efficiency Effluent Quality
(%) (mg/L)
BOD5 SS BOD5 ss
Single-pass Sand Filter
Recirculating sand filter
Extended Aeration
Facultative Lagoon
85-95
85-95
85-90
70-90
70-90
70-90
75-90
25-85
5-10
5-10
20-30
20-60
5-10
5-10
20-50
30-150
-------
23
Intermittent sand filters are inherently very stable wastewater treatment processes
compared to biological package plants such as extended aeration. With limited
supervision and control of operating conditions, sand filters can produce consistently high
quality effluents (BOD* and TSS _ 10 mg/L). In contrast, good supervision and operating
conditions are essential for extended aeration plants to consistently maintain BODc and
TSS effluent concentrations below 30 mg/L (SCS, 1977). As a fixed film process, sand
filters should be less subject to upsets and poor effluent quality than suspended growth
processes such as extended aeration package plants. Facultative lagoon systems have a
reputation for fluctuating effluent qualities, particularly with respect to TSS, in response
to climatic influences and other factors. Careful operation of these facilities (controling
pond water levels, distribution between cells, and controlled discharge) can minimize the
fluctuations and need for post-treatment prior to surface discharge.
LAND AREA REQUIREMENTS
Intermittent sand filter units require substantial land areas as their hydraulic
loading rates, are typically 5 gpd/ft^ (20 cm/d) or less. Figure 4 summarizes the
estimated surface areas required for intermittent sand filters, facultative Lagoons,
extended aeration units and subsurface soil absorption beds. These area requirements are
for the unit process above and do not include area for standby units, pre-treatment or
post-treatment units, control rooms, access roads, fencing, etc.
Using the package plant area requirements as a baseline, the following ratios were
estimated for the areas required by the other processes: ' ;-..
Extended Aeration - 1.0 x
Recirculating Filter - 17.5 x
Single-pass Filter - 21.0 x
Buried Filter - 45.6 x
Soil Absorption Bed - 52.4 x . •' .
Facultative Lagoon - 118.8 x
Single-pass and recirculating sand filters require land areas greater than that required by
extended aeration units, but substantially less than that of a facultative lagoon. Buried
sand filters require substantially greater land areas than open filters but less than soil
absorption beds or facultative lagoons. ~
-------
24
3
OJ
u
m
8
o
80,000
£ 60,000
40,000
Jg' 20,000
Facultative Lagoon
Soil Abs r
Buried Fir
Single-Pass Filter
Recirculating Filter
Extended Aeration
50,000
Design Flow (gpd)
100,000
NOTE:
Unit process surface area based upon following:
Soil Absorption Bed: A = (Q ? 1.0 gpd/ft2)
A = (Qt 1.15gpd/ft2)
A = (Q * 3.0 gpd/ftj)
A - (Q « 5.0 gpd/ftz) x 2 parallel units
A = (Q x Id r 10 ft)
A = (Q x 90d r 5.3 ft) '
Buried Filter:
Recirculating Filter:
Single-pass Filter:
Extended Aeration:
Lagoon:
The areas shown are for the unit processes alone and do not include areas required for
other treatment units, control buildings, access roads, etc.
Figure 4. Estimated Land Areas For Intermittent
Sand Filters And Comparable Processes.
-------
25
SECTION 5
ASSESSMENT OF NATIONAL IMPACT
MARKET POTENTIAL
Intermittent sand filtration is a potentially low-cost method of wastewater treat-
ment which produces an effluent quality meeting many advanced waste treatment levels.
Maintenance requirements are less than those necessary for most mechanical plants and
can be performed by unskilled personnel. Energy costs are only those associated with
pumping of the wastewater onto the filter surface. However, areal requirements are
large in comparison to mechanical treatment methods and their application might be
constrained somewhat in severe winter climates. Therefore, intermittent sand filters are
best suited for small flows, generally less than 0.2 MGD.
Within these limitations, the potential market for intermittent sand filters is
large. The EPA 1982 Needs'Survey revealed that of the new secondary treatment plants
required by the year 2000, those treating less than 0.50 MOD (1.9 x 103 m3/d) represents
91 percent of the total number and 33 percent of the estimated $5.1 billion total capital
co|ts (USEPA, 1983a). The treatment project needs for flows less than 0.10 MGD (0.38
m d) for which sand filtration is ideally suited represent 63 percent of the total number
of new secondary treatment Dlants needed (Table H). The advanced secondary plants of
less than 0.50 MGD (1.9 x 10^ m^/d) required by the year 2000 represent 89 percent of
the total number and 40 percent of the estimated $2.4 billion total capital costs.
In addition to the small community needs identified in the survey, many rural
housing developments and business establishments can utilize sand filtration where site
and soil conditions preclude the use of septic tank-subsurface soil absorption systems.
Many state and local authorities restrict their use to publicly-owned treatment works,
however. Local regulations must be reviewed to determine what restrictions exist.
COST AND ENERGY IMPACTS
Capital and operating costs compare very favorably to conventional methods of
treatment. Land acquisition and sand media are the controlling costs of construction and
these costs are very site specific. Energy costs are primarily those associated with the
pumping of wastewater onto the filter. Therefore, energy costs associated with sand
filters are lower than most other small community processes except lagoons.
RISK ASSESSMENT
Sand filtration is a well proven process. It is a fixed growth biological reactor and
granular filtration method of wastewater treatment. It is a highly stable process able to
accept wide variations in organic and hydraulic loading with little deleterious effect on
effluent quality. Further, the effluent is extremely low in turbidity which facilitates all
methods of disinfection, if required.
-------
26
Table 11. EXISTING NUMBER AND PROJECTED NUMBER OF
SECONDARY AND ADVANCED SECONDARY
TREATMENT PLANTS BY DESIGN
CAPACITY (USEPA, 1983a)
Flow
(MGD)
0.0-0.10 \
0.11-0.50
0.51-1.05
1.06-5.01
5.02-10.56
10.51-50.19
50.2
Totals
Year
Secondary
2467
2700
843
1024
232
201
49
7516
1982
Advanced
Secondary
624
1310
588
1005
231
228
55
4041
Year
Secondary
5146
3881
1015
1178
262
220
54
11756
2000
Advanced
Secondary
1768
1775
671
1091
248
239
57
5849
-------
27
SECTION 6
RECOMMENDATIONS
RESEARCH AND DEVELOPMENT EFFORTS
Due to the long historical use of intermittent sand filters for wastewater treat-
ment, much is known of their basic capabilities. Acceptable design criteria and
operation parameters are available, but not widely used. Further research and full-scale
demonstrations would help to optimize the process. The following are suggested.
1. Development of a more defined relationship between media character-
istics, hydraulic loading rate and treatment efficiency and how this
relationship is affected by operation and environmental factors.
2. Development of operation guidelines to maximize treatment efficiency
and/or filter run length.
3. Development of a data base for performance, operation and maintenance
requirements and capital and operating costs from full-scale plants.
It would appear prudent for all communities under 10,000 population to consider
and evaluate intermittent sand filters as alternative, treatment systems, based on their
high process efficiency and reliability, low present worth cost and low operation and
maintenance requirements.
PROCESS/TECHNOLOGY MODIFICATIONS
Intermittent sand filters are customarily used to achieve secondary treatment.
However, limited data suggest that advanced secondary treatment is common and
nutrient removal is possible. Modifications in media characteristics to remove
phosphorus and changes in operation to promote denitrification are promising.
Use of intermittent sand filters may be limited in some areas where suitable sand is
unavailable. Other media may be suitable after investigation.
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