United States
Environmental Protection
KAgency
Water Engineering
Research Laboratory
Cincinnati OH 45268
..V
Research and Development
EPA/600/S2-85/056  Aug. 1985
Project  Summary
Slow  Sand   Filter  Maintenance:
Costs  and  Effects  on  Water
Quality
Raymond D. Letterman and Thomas R. Cullen, Jr.
  A study was conducted to determine
the effects of scraping on slow sand fil-
ter efficiency and to quantify the labor
required to operate  and maintain a
slow sand fitter.  The data were  ob-
tained  by monitoring  scraping  and
other  maintenance operations at a
number of full-sized slow sand filtration
plants in Central New York.
  Ripening periods (the time required
for filtrate quality to improve after filter
scraping) were evident in the slow sand
filtration plants visited. Ten mainte-
nance operations were monitored in six
filtration plants. In four of the ten oper-
ations,  there was some evidence of a
ripening period. This evidence included
filtrate  turbidity and/or HIAC particle
counts that were greater for a recently
scraped filter than for an on-line control
filter. The length of the ripening period
ranged from 6 hr to 2 wk. The data also
suggest that a recently scraped filter is
less efficient than  a control filter in at-
tenuating a spike input of lower-quality
raw water. Factors such as the use of
prechlorination,  water  temperature,
scraping methodology, and frequency
of filter maintenance did not seem to be
related to the presence or absence of a
ripening period. However, the nature of
the'particulate matter in the raw water
apparently has an important effect on
filtrate quality, and a pilot plant study
should always be conducted before a
slow sand  filtration  plant  is con-
structed. Continuous monitoring of the
turbidity of each filter effluent may be
required to ensure that slow sand filter
maintenance operations do not have a
detrimental  effect on treated water
quality; the capability to waste individ-
ual filter effluent for a period of time
may be necessary in some cases to pre-
vent quality deterioration.
  Typical labor requirements for fitter
scraping  are  approximately 5  man-
hours/1000 ft2 of fitter surface. The re-
sanding operation requires  approxi-
mately 50 man-hours/1000 ft2. No clear
relationship was observed between the
frequency  of  scraping  and the raw
water quality or maintenance proce-
dures.  Operational convenience ap-
pears to be a controlling factor in the
plants visited.
  This  Project  Summary was devel-
oped by EPA's Water Engineering Re-
search  Laboratory, Cincinnati, OH,  to
announce key findings of the research
project that is  fully documented in a
separate report of  the same title (see
Project Report ordering information at
back).

Introduction
  A large proportion of the  public sur-
face water supplies  in the United States
are small and unfiltered. Many of these
systems have experienced difficulty  in
meeting the 1 nephelometric turbidity
unit (NTU) maximum contaminant level
(MCL) in the U.S. Environmental Protec-
tion Agency (EPA) Drinking Water Regu-
lations. Some  of these communities
have failed to meet the MCL for coliform
group bacteria. The slow sand filtration
process may be an appropriate treat-
ment alternative for many of these
small systems.
  When slow sand  filters are used by
water utilities, the raw water is typically
given no pretreatment.  Uncoagulated
water is applied and slowly passed
through the sand filter. As the run pro-
gresses, a layer of soil particles and bio-

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logical matter (the schmutzdecke) accu-
mulates on the top of the sand bed and
the head loss increases. When the ter-
minal head loss is reached, the water
level is drawn down to 10 cm or more
below the surface of the sand, and the
schmutzdecke and a thin layer of sand
are removed.
  The primary goal of this research was
to determine the effects of slow sand
filter scraping on water quality and op-
eration and maintenance costs. The
major objectives were as follows:
  1. To evaluate filter water quality be-
     fore and after slow sand filters are
     scraped and to compare it with the
     quality of raw water and control fil-
     ter effluent to determine how filter
     efficiency is affected by scraping.
  2. To quantify the labor required to
     operate slow sand filter plants and
     to compare the labor needed for
     routine operation  and monitoring
     with that needed for scraping fil-
     ters.
  3. To determine the frequency of fil-
     ter scraping (length of run or vol-
     ume of water filtered  per run) and
     relate this information to raw
     water quality, water treatment be-
     fore filtration (if any),  filtration
     rate, sand size, and other relevant
     design factors. A related objective
     was to determine whether and to
     what extent the frequency of filter
     scraping varies with  the depth of
                         sand removed during the scraping
                         operation.
                       Seven treatment plants were studied
                     in  New York State: Auburn, Geneva,
                     Hamilton, Ilion, Newark, Ogdensburg,
                     and Waverly. The typical study visit in-
                     volved traveling to the plant site 1 or 2
                     days before a filter was  to be scraped.
                     The plant was  toured,  and the plant
                     records were examined to determine fil-
                     ter run lengths and historical water
                     quality. The effluent from the filter to be
                     scraped was sampled,  along with the
                     raw water.
                       The  manpower,  techniques,  and
                     equipment  used in scraping (or resand-
                     ing) the filters were determined by ob-
                     servation and interview and recorded.
                       Approximately 50 samples were
                     taken during each  plant visit. When
                     water flow through the filter was started
                     after scraping, grab samples were col-
                     lected for a  period of at least 24 to 48 hr.
                     Samples were withdrawn from the
                     scraped filter effluent, a control filter ef-
                     fluent,  and  the raw water. The control
                     was a filter that had been on-line for at
                     least 1  month.
                       The water temperature and turbidity
                     were measured  immediately after the
                     sample was drawn. Standard plate
                     count and total coliform  bacteria analy-
                     ses were started within  0 to 4 hr  after
                     sampling.
                       The  samples  were transported  to
                     Syracuse University for particle count
                                                   and size analysis on an HIAC particle
                                                   size analyzer.
                                                     Samples of the filter sand were sieved
                                                   to determine the size distribution, and a
                                                   sand dissolution test was conducted
                                                   using the  procedure given  in AWWA
                                                   Standard B100-80.

                                                   Results
                                                     The average operating flow rate for
                                                   the sites visited ranged from approxi-
                                                   mately 0.3 MGD at Hamilton to 6.0 MGD
                                                   at Auburn  (Table 1). The average  raw
                                                   water turbidity was  less than 3.0 NTU
                                                   for every site except Waverly, where the
                                                   average was approximately 8 NTU. Fil-
                                                   ters are covered at all of the sites but
                                                   Hamilton, and two filters at Ilion are un-
                                                   covered.
                                                     The average operating filtration  rate
                                                   is the average operating flow rate for
                                                   the slow sand filters divided by the total
                                                   filter plan area. Filtration  rates  ranged
                                                   from 0.04 to 0.19 m/hr and had an aver-
                                                   age value of 0.15 m/hr.
                                                     Three of the plants visited (Ilion,
                                                   Newark, and Waverly)  practice prechlo-
                                                   rination. At Newark, prechlorination is
                                                   used to control biological growth in the
                                                   transmission line between the lake and
                                                   the treatment plant. Waverly uses
                                                   prechlorination to oxidize iron and man-
                                                   ganese and to decrease the filtrate tur-
                                                   bidity. The purpose of prechlorination
                                                   at Ilion  was not stated by plant person-
                                                   nel.
Table 1.    Characteristics of the Slow Sand Filtration Plants Visited
Location
Auburn
Geneva
Hamilton
Ilion
Average
Operating Flow
Rate for Slow
Sand Filtration
(MGD)
6.0
2.5
-0.3
1.5
Raw Water
Source
Owasco Lake
Seneca Lake
Woodman's
Pond
Several small
streams feeding
reservoir
Average Raw
Water Turbidity
from Plant
Records (NTU)
1.5-2.0
1.0
1.0-1.5
3.0
Total Slow
Sand Filter
Plant Area
(ft2)
74,100
30,492
12,724
19,526
Design
Filtration
Rate
(m/hr)
0.11
0.19
—
—
Average
Operating
Filtration
Rate
(m/hr)
0.14
0.19
0.04
0. 16-0. 18
Prechlorination
NO
NO
NO
YES
Covered
Filters
YES
YES
NO
2 uncovered,
3 covered, and
1 not used
Newark


Ogdensburg


Waverly
2.0


3.6


1.2
Canadaigua Lake  3.0 {summer)
                1.0 (winter)
St. Lawrence
River
1.0-1.4
Surface runoff to  7.0-9.0 (may
reservoirs        be as high as
                20-40 during
                high runoff
                periods)
21,684


33,600


12,000
0.16


0.20


0.16
0.16


0.18


0.16
YES


NO


YES
YES


YES


YES

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  The efficiency of filtration in  a  slow
sand filter is at least  partly determined
by the presence of viable microorgan-
isms within the filter  bed; thus the use
of prechlorination in  these systems
would be detrimental  to  filter  perfor-
mance.  The effluent  average,turbidity
(for the control filter)  was  compared
with the influent average turbidity at
each site and  for each monitoring pe-
riod in which a control filter was sam-
pled. The values were averaged for the
entire length of each sampling period
(using weighted averages  based on
flow volume) and used to calculate the
percent turbidity remaining in the efflu-
ent. For the three cases  in which pre-
chlorination was used  (4 sets of data),
the average and the standard deviation
of the percent turbidity  remaining were
17% and 7.9%, respectively.  For the
three cases in  which  there was no pre-
chlorination (6 sets of data), the average
and standard deviation of the percent
turbidity remaining were 21% and 7.6%,
respectively. Though  other factors may
have obscured the true significance of
adding chlorine before slow sand filtra-
tion, these results do not clearly indicate
that prechlorination  is detrimental to
performance. In fact,  it may have had a
slightly  positive effect  on turbidity re-
moval in the plants sampled.
  The effective size  of the filter sand
ranged from 0.15 mm atWaverlytoO.45
mm at Auburn.  The  average effective
size for all sites was 0.33  mm. The uni-
formity coefficient averaged 2.1  and
                         ranged from 1.7 at Newark and Ogdens-
                         burg to 2.4 at Auburn, Hamilton, and
                         Waverly.
                           Standard B100-80 of the American
                         Water Works Association states that a
                         high-quality filter sand should not lose
                         more than  5% of its weight when it is
                         treated in a prescribed way with 1:1 HCI
                         solution. At two  of the seven sites, the
                         sand meets this requirement. When the
                         sand dissolution tests were conducted,
                         significant  effervescence was noted in
                         most of the treated samples, suggesting
                         that these sands  contain  significant
                         amounts of CaC03. The significance of
                         this in terms of filter performance and
                         operation is not known.
                           Table 2  summarizes the  results that
                         pertain to the filter scraping operation.
                         The water production per filter run
                         ranged from approximately 3000 gal/ft2
                         at Ogdensburg to 16,000 gal/ft2 at
                         Geneva and Ilion. The average fre-
                         quency  of  filter  scraping ranged from
                         approximately twice a year at Geneva,
                         Hamilton, and Ilion to 12 times a year at
                         Ogdensburg. Twice a year  at Auburn
                         (usually during the colder months), the
                         filters are  raked and  no sand is re-
                         moved.  According to Auburn  person-
                         nel, raking effectively reduces the head-
                         loss across the bed without having an
                         adverse effect on  filtrate quality. The
                         frequency of 4.3 times per year listed in
                         Table 2 for Auburn includes scraping
                         (i.e., sand removal) and raking.
                           The water  production (3200  gal/ft2)
                         and scraping frequency (9.7 times/year)
                                             listed for Waverly in Table 2 are based
                                             on an estimated future filter run length
                                             of 900 hr. This estimate is based on data
                                             obtained in  a  9-mo  study in which
                                             Waverly personnel developed an opera-
                                             tional strategy for  effectively dealing
                                             with the  high raw-water turbidity and
                                             the high  iron and manganese concen-
                                             trations that  frequently occur in their
                                             reservior supply.  In the past, Waverly
                                             operators experienced filter run lengths
                                             as short  as 2 days.  If  in the future the
                                             raw water is high  in  turbidity (>12.5
                                             NTU) and/or high in total iron (>3.0 mg
                                             Fe/L) and manganese  (>1.0 mg Mn/L),
                                             the  New York State  Department of
                                             Health will require Waverly to take the
                                             slow sand filtration plant off-line and to
                                             use their well water supply exclusively.
                                               The last three  columns in Table  2
                                             summarize the methods used and man-
                                             power required for filter scraping at the
                                             sites visited.  Most of the sites remove
                                             approximately 1 in. of sand from the fil-
                                             ter surface with broad shovels.
                                               The man-hours required for scraping
                                             depend on the depth of sand scraped. In
                                             the cases where 0.5 to 1.0 in. was re-
                                             moved, the labor requirement  ranged
                                             from 2 to 9 man-hours/1000 ft2. At Ilion,
                                             where 3 to 4 in. were removed, the labor
                                             requirement  was significantly greater
                                             (23 to 42 man-hours/1000 ft2).
                                               The method used  to convey the dirty
                                             sand from the filter area also affects the
                                             labor requirement. For example,  the
                                             lowest  labor  requirement was at
                                             Newark  (2  man-hours/1000 ft2), where
Table 2.    Summary of Filter Scraping Data
 Location
Average Filter Run
 Water Production
     (gal/ft2)
Ilion

Newark
     75,487

     10,122
Average Frequency
 of Filter Scraping
Operations (Number
     per year)
       1.8

       3.3
 Amount of Sand
   Removed in
Scraping Operation
      (in.)
  Method(s) Used in
 Removing Sand from
    Filter Surface
      3-4

       1.0
Shovels, hydraulic

Shovels, motorized
buggy
 Time Required to
   Scrape Filters
(man-hours/1000 ft2)
Auburn
Geneva
Hamilton
6,844
15,718
4,302
4.3'
2.0
2.0
0.5
1.0
1.0
Shovels, hydraulic
Shovels, motorized
buggy
Shovels, 50 gal drums,
backhoe
4
4-5
8-9
      23-42

          2
Ogdensburg
Waverly
2,978
3,200*
12.0
9.7 f
1.0
1.0
Shovels, hydraulics
Shovels, wheelbarrows
4-5
5
"At Auburn, two scraping operations per year are actually occasions when the filters are raked and no sand is removed.

*Water production and scraping frequency estimated by the Waverly personnel for the future using data from a 9-month operations study.
 Waverly has had runs as short as 2 days.

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Table 3.    Estimated Operation and Maintenance Costs for Slow Sand Filters*
Location
Auburn
Geneva
Hamilton
Ilion
Newark
Ogdensburg
Waverly
Average Operational
Flow (MGD)
6.0
2.5
0.3
1.5
2.0
3.6
1.2
Labor for
Scraping
(man-hours/year)
1007
374
224
905
143
8736
582
Labor for
Resanding
(man-hours/year)
618
218
NA
563
226
t
420
Labor for
Day-to-Day
Activities
(man-hours/year)
365
365
365
365
365
365
365
Total Labor
Costs ($/year>
10,597
7,390
5,890
18,331
7,640
23,811
13,670
Total Operation
and Maintenance
Unit Cost
((/WOO gal)
0.5
1.1
5.3
3.3
1.1
2.0
3.7
"All cost figures are based on a $10/hr wage rate except at Auburn, where $3/hr was used because the workers are usually summer
 students.
tOgdensburg scrapes and resands simultaneously.
an efficient, motorized buggy was used
to haul the dirty sand from the filter. The
greatest labor requirement for the
plants that scrape 0.5 to 1 in. of sand
was at Hamilton (8 to  9  hr/1000 ft2),
where the  dirty sand removal process
involved filling  55-gal drums and haul-
ing them away  with a tractor.
  Under typical conditions (i.e., re-
moval of about 1 in. of dirty sand with
shovels and conveyance  of this sand
from the filter hydraulically), the labor
requirement was approximately 5 man-
hours/1000 ft2 of filter surface.
  Table 3 compares the estimated oper-
ating costs for slow sand filters the
treatment plants visited. Day-to-day ac-
tivities devoted  exclusively to the filters
(collecting samples, checking the filters,
etc.) were assumed to require  1 man-
hour/day. The  labor requirement for
scraping is based on the scraping fre-
quency listed in Table 2. Resanding was
assumed to require 50 man-hours/1000
ft2,  based on data from Auburn.
  The estimated operational unit costs
ranged from 0.50/1000 gal  at Auburn to
5.30/1000 gal at Hamilton. The mean
value for all plants was 2.40/1000 gal.
The exceptionally low value at Auburn
was partly because of their using low
wage summer  help ($3/hr) for most
scraping and resanding operations.
  Table 4 summarizes the results for fil-
ters that exhibit a ripening period. The
ripening period is the interval immedi-
ately following filter scraping and/or re-
sanding during  which the turbidity or
particle count  is significantly greater
than that for a control filter.
  Ripening periods were  observed at
Auburn, Ilion, Newark, and Waverly. At
Auburn, one out of the three scraping
operations monitored exhibited a short
ripening period. For a period of about
6 hr, the filtrate turbidity and particle
count data for the scraped  filter ex-
ceeded  the corresponding values for
the control filter by a factor of about 2.
However,  the turbidity values were al-
ways below the MCL of 1 NTU.
  The measurements made at Ilion are
difficult to interpret with respect to a
ripening period. The scraped and con-
trol filters yielded  very similar turbidi-
ties after  scraping, but approximately
6 hr after the scraped filter was brought
back on line, the particle counts for the
scraped filter  began to exceed the
values of the control filter by a factor of
about 2. The time required for this dis-
parity to disappear was about 12 hr.
  Two operations were monitored at
Newark. One was a typical scraping op-
eration and the other involved resand-
ing the bed. No  ripening period was ob-
served when the  scraping  operation
was monitored, but a ripening period
was clearly evident in the  resanding
case. During the ripening period, the fil-
trate turbidity of the scraped filter ex-
ceeded that of the control by a factor of
about 3. The effluent turbidity of the
control  and scraped filters  never ex-
ceeded 0.5 NTU; however, the particle
count values were  always less  than
1000/mLfor both filters.
  Ripening periods are a routine occur-
rence at Waverly. Operating personnel
are not surprised if 2 weeks  elapse be-
fore the scraped  filter turbidity de-
creases to values approaching those of
the control filter. During this study,
ripening was most apparent in the tur-
bidity results: particle counts for the
scraped and control filters appeared to
coincide after about 30 hr, whereas the
turbidity values converged after about
10 days.
  The reason for the Waverly's prob-
lems is not clear. The raw water appears
to contain submicron-sized particles
that scatter light and  increase the tur-
bidity but are not efficiently removed by
slow sand filtration. According to the
particle count data, Waverly removes
particles larger than 2 |xm as efficiently
as the other plants visited.

Conclusions
1. Four of the ten scraping and resand-
   ing  maintenance operations  moni-
   tored produced some evidence of a
   ripening period. This evidence in-
   cluded  filtrate turbidity and/or HIAC
   particle count values that were
   greater  for a filter that was  main-
   tained than for a control filter that
   had been on line for a significant pe-
   riod of time.
2. The length of  the  ripening periods
   observed ranged from 6 hr to 2 wk.
   The factor that seemed to have the
   most significant effect on filtrate
   quality was the nature of the particu-
   late matter in the raw water. The
   presence or absence of a ripening
   period does not seem to be related to
   prechlorination, water temperature,
   scraping method, or frequency of fil-
   ter maintenance.
3. The results suggest that  a  recently
   scraped filter is less efficient than a

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Table 4.    Summary of Filter Ripening Data
Location
Auburn
Auburn
Auburn
Geneva
Hamilton
///on
Type of
Operation
During Visit
11)

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Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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