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.

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                                                                                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.

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                                                                               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.

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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^).

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                                                                                 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.

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              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
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                                                                                    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 
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                                                                          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

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                                                                                   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.                               ~

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                                                                                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.

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                                                                                  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.

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                                                          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

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                                                                                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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                                                                             29
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                                                                               30
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                                                                              31
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