flow reaching the treatment plant. Presedimentation cannot reduce the
Cryptosporidium level entering a treatment plant by 0.5-log or greater
on a continuous basis if the process is operated intermittently or
treats only a fraction of the plant flow. EPA recognizes that for some
PWSs, operating a presedimentation process intermittently in response
to high turbidity levels is preferable to continuous operation. By
establishing a requirement for continuous operation as a condition for
treatment credit, EPA is not recommending against intermittent
operation of presedimentation processes. Rather, EPA is only
identifying one of the conditions under which a 0.5-log Cryptosporidium
treatment credit for presedimentation appears to be justified.
    A second condition for presedimentation treatment credit is that
the process must operate with coagulant addition. Available data
support awarding 0.5-log Cryptosporidium treatment credit to a
presedimentation process only when a coagulant is present. The full-
scale presedimentation data reviewed in the proposal involved coagulant
addition, and literature studies indicate that Cryptosporidium removal
through sedimentation can be substantially lower in the absence of
sufficient coagulant. Further, the Stage 2 M-DBP Advisory Committee
specifically recommended 0.5-log Cryptosporidium treatment credit for
presedimentation with coagulation (USEPA 2000a). Based on these
factors, EPA concluded that coagulation is a necessary condition for
PWSs to receive treatment credit for presedimentation.
    The third condition for awarding treatment credit to
presedimentation is that the process must achieve a monthly mean
turbidity reduction of at least 0.5-log or meet alternative State-
approved performance criteria. This requirement stems from a
recommendation by the SAB, which reviewed data for awarding treatment
credit to presedimentation under the LT2ESWTR. In their report, the SAB
concluded that available data were minimal to support 0.5-log
prescribed credit for presedimentation and recommended that performance
criteria other than overflow rate be included if credit is given for
presedimentation (SAB 2003).
    In response to this recommendation by the SAB, EPA analyzed the
relationship between removal of aerobic spores (as an indicator of
Cryptosporidium removal) and reduction in turbidity in the full-scale
presedimentation processes of three PWSs. The results of this analysis,
which are shown in Table IV.D-2, suggest that presedimentation
processes achieving a monthly mean reduction in turbidity of at least
0.5-log have a high likelihood of reducing mean Cryptosporidium levels
by 0.5-log or more. Consequently, EPA concluded that turbidity
reduction is an appropriate performance criterion for awarding
Cryptosporidium treatment credit to presedimentation basins. The Agency
believes this performance criterion addresses the concern raised by the
SAB.

  Table IV.D-2.--Relationship Between Mean Turbidity Reduction and the
     Percent of Months When Mean Spore Removal Was at Least 0.5 Log
------------------------------------------------------------------------
                                                            Percent of
                                                          months with at
                                                           least 0.5 Log
        Log reduction in turbidity (monthly mean)         Mean Reduction
                                                             in spores
                                                             (percent)
------------------------------------------------------------------------
at least 0.1-log........................................              64
at least 0.2-log........................................              68
at least 0.3-log........................................              73
at least 0.4-log........................................              78
at least 0.5-log........................................              89
at least 0.6-log........................................              91
at least 0.7-log........................................              90
at least 0.8-log........................................              89
at least 0.9-log........................................              95
at least 1.0-log........................................             96
------------------------------------------------------------------------
 Source: Data from Cincinnati Water Works, Kansas City Water Services
  Department, and St. Louis Water Division.

    The proposed rule required PWSs to achieve at least 0.5-log
turbidity reduction through presedimentation in at least 11 of the 12
previous consecutive months to be eligible for presedimentation
treatment credit. EPA recognizes, however, that some PWSs will not be
able to demonstrate at least 0.5-log turbidity reduction through
presedimentation during months when raw water turbidity is lower. As a
result, these PWSs would not be able to achieve treatment credit for
their presedimentation basins. To provide more options for these PWSs,
EPA has modified this requirement in today's final rule in two
respects.
    The first modification is that in today's final rule, PWSs must
demonstrate compliance with the conditions for presedimentation
treatment credit on a monthly, rather that a yearly basis. This
requirement allows treatment credit for presedimentation in any month a
PWS can demonstrate at least 0.5-log turbidity reduction, even if the
PWS cannot achieve this level of turbidity reduction in all months of
the year.
    A PWS that meets the conditions for presedimentation treatment
credit for only part of the year must implement other microbial toolbox
options to comply with Cryptosporidium treatment requirements in the
remainder of the year. Nevertheless, achieving presedimentation
treatment credit for even part of the year may benefit certain PWSs.
For example, a PWS may be able to reduce the level of disinfection it
provides during the months it receives presedimentation treatment
credit, or this treatment credit may provide a margin of safety to
ensure compliance with Cryptosporidium treatment requirements.
    The second modification is the allowance for States to approve
alternative performance criteria to turbidity reduction that
demonstrate at least 0.5-log mean removal of micron-sized particulate
material through the presedimentation process. EPA believes that
aerobic spores are an appropriate alternative criterion. As described
earlier, studies support the use of aerobic spores as an indicator of
Cryptosporidium removal in coagulated sedimentation processes. If
approved by the State, a PWS could receive 0.5-log treatment credit for
presedimentation by demonstrating at least 0.5-log reduction in aerobic
spores. The Toolbox Guidance Manual provides information on analytical
methods for measuring aerobic spores. This may provide an option for
PWSs that are not able to demonstrate 0.5-log turbidity reduction but
have a sufficient concentration of aerobic spores in their raw water.
PWSs may work with States to identify other alternative criteria, as
well as appropriate monitoring to support use of the criteria.
c. Summary of Major Comments
    Public comments on the August 11, 2003, LT2ESWTR proposal supported
allowing PWSs to achieve 0.5-log credit towards Cryptosporidium
treatment requirements for presedimentation with

coagulation. Some commenters also supported the proposed operational,
monitoring, and performance conditions required for PWSs to receive
this credit. Other commenters, however, opposed the proposed
requirement for turbidity reduction as a condition for receiving
presedimentation treatment credit. A summary of these commenters'
concerns and EPA's responses follows.
    Commenters who opposed requiring turbidity reduction for
presedimentation treatment credit were concerned that PWSs cannot
achieve this criterion during periods when raw water turbidity is low.
Further, these commenters stated that turbidity removal does not
reflect the overall benefits of presedimentation, which improves the
performance of the primary treatment train by equalizing water quality.
Some commenters also provided data showing the reduction in turbidity
and aerobic spore levels in the presedimentation processes of several
PWSs and stated that turbidity removal may not be an appropriate
indicator of acceptable performance for presedimentation basins.
Several commenters suggested that EPA establish a limit on hydraulic
overflow rate in place of a turbidity removal requirement.
    In response, EPA continues to believes that 0.5-log turbidity
reduction is an appropriate performance indicator for 0.5-log
Cryptosporidium reduction in presedimentation processes. EPA has
reviewed the additional data submitted by commenters on the removal of
turbidity and aerobic spores (as an indicator of Cryptosporidium
removal) in full-scale presedimentation basins. These data are
consistent with data reviewed for the proposal in showing that when
turbidity removal was below 0.5-log, removal of aerobic spores was also
usually below 0.5-log. Conversely, when turbidity reduction exceeded
0.5-log, aerobic spore removal was typically higher than 0.5-log.
Consequently, while there is not a one-to-one relationship between
reduction in turbidity and reduction in aerobic spores, 0.5-log
turbidity reduction is a reasonable indicator of when Cryptosporidium
removal is likely to be at least 0.5-log.
    EPA recognizes, though, that 0.5-log turbidity reduction through
presedimentation will not be feasible for some PWSs when raw water
turbidity is low. Today's final rule contains several provisions to
address this concern. First, PWSs can receive credit for
presedimentation during any month the process achieves 0.5-log
turbidity removal. Thus, PWSs that cannot achieve 0.5-log turbidity
reduction year-round may receive credit for presedimentation in those
months when they can meet this condition. Today's rule also allows PWSs
to receive presedimentation credit using State-approved performance
criteria other than turbidity reduction. If approved by the State, a
PWS may receive credit for presedimentation by demonstrating, for
example, 0.5-log reduction in aerobic spores. Finally, if
presedimentation improves treatment plant performance by reducing and
equalizing particle loading, a PWS can receive additional treatment
credit under today's rule for achieving lower filtered water turbidity
(see section IV.D.7).
5. Two-Stage Lime Softening
a. Today's Rule
    Lime softening in drinking water treatment involves the addition of
lime and other chemicals to remove hardness (calcium and magnesium)
through precipitation. In single-stage softening, chemical addition and
hardness precipitation occur in a single clarification process prior to
filtration. In two-stage softening, chemical addition and hardness
precipitation occur in each of two sequential clarification processes
prior to filtration. In some water treatment plants, a portion of the
raw water bypasses a softening process (i.e., split softening) in order
to achieve a desired pH and alkalinity level in the treated water.
    Under today's rule, single-stage softening with filtration receives
a prescribed 3.0-log credit towards Cryptosporidium treatment
requirements, which is equivalent to conventional treatment (see
section IV.B). Two-stage softening receives an additional 0.5-log
Cryptosporidium treatment credit during any month a PWS meets the
following conditions:

    (1) Chemical addition and hardness precipitation occur in two
separate and sequential softening stages prior to filtration; and
    (2) Both softening stages treat the entire plant flow taken from
surface water sources or GWUDI (i.e., no portion of the plant flow
from a surface water source may bypass either softening stage).

    Alternatively, PWSs may apply to the State for Cryptosporidium
treatment credit for softening processes using a demonstration of
performance, as described in section IV.D.9. Demonstration of
performance provides an option for PWSs with softening processes that
do not meet these conditions for prescribed treatment credit and for
PWSs who seek greater than the prescribed Cryptosporidium treatment
credit for their softening processes.
b. Background and Analysis
    Lime softening is a common practice that PWSs use to reduce water
hardness, which is primarily calcium and magnesium. The addition of
lime elevates the pH of the raw water. Elevation to pH 9.4 or higher
causes precipitation of calcium carbonate and further elevation to pH
10.6 or higher causes precipitation of magnesium hydroxide. Soda ash
may be added with lime to precipitate non-carbonate hardness. Removal
of the precipitate occurs through clarification (e.g., sedimentation
basin) and filtration processes. Coagulants and recycled softening
sludge are often used to enhance removal. In two-stage softening, the
second stage is commonly used to precipitate magnesium, along with
increased levels of calcium.
    In addition to reducing hardness, softening processes remove
particulate material present in the raw water, including microbial
pathogens like Cryptosporidium. Particulate material flocculates with
the softening precipitate and is removed through the clarification and
filtration processes, similar to a conventional treatment plant. The
degree of Cryptosporidium removal will depend on the amount of
precipitate formation, the use of coagulants, the raw water quality,
and other factors. Available data indicate that the elevated pH used in
softening does not inactivate Cryptosporidium or Giardia (Logsdon et
al. 1994, Li et al. 2001), though it does inactivate some
microorganisms like viruses (Battigelli and Sobsey, 1993, Logsdon et
al. 1994).
    The Stage 2 M-DBP Advisory Committee recommended that lime
softening be eligible for up to 1.0-log additional Cryptosporidium
treatment credit based on a site-specific demonstration of performance,
but did not recommend any prescribed credit for this process (USEPA
2000a). After reviewing available data, however, EPA included a
prescribed 0.5-log Cryptosporidium treatment credit for two-stage lime
softening in the August 11, 2003 proposal (USEPA 2003a). This approach
reflected a recommendation by the SAB, which supported an additional
0.5-log treatment credit for two-stage lime softening if all the water
passes through both stages (SAB 2003). The proposal also allowed for
greater treatment credit through a demonstration of performance. The
following discussion summarizes the basis for the lime softening
treatment credit in today's final rule and differences with the
proposal.
    In the proposal, EPA reviewed a study by Logsdon et al. (1994) that
evaluated

Cryptosporidium removal in full-scale lime softening plants.
Cryptosporidium was detected in the raw water at 5 plants: one single-
stage plant and four two-stage plants. Based on measured levels, the
removal of Cryptosporidium across the softening clarification
(sedimentation) stages was 1.0-log in the single stage plant and ranged
from 1.1-to 2.3-log in the two-stage plants. Cryptosporidium reductions
from raw to filtered water were 0.6- and 2.2-log in the single stage
plant and ranged from greater than 2.67- to greater than 3.85-log in
the two-stage plants.
    EPA also evaluated data collected by PWSs on the removal of aerobic
spores in full-scale lime softening plants. As discussed earlier,
studies have shown the removal of aerobic spores to be an indicator for
Cryptosporidium removal, and one pilot-scale study of a softening plant
found significantly greater removal of Cryptosporidium than aerobic
spores under similar treatment conditions (Clark et al., 2001). For the
full-scale plants, average reductions in aerobic spores across the
softening clarification stages were 2.4- and 2.8-log for two plants
that practice two-stage softening and were 1.6- and 2.4-log for two
plants that practice single-stage softening (USEPA 2003a).
    The Cryptosporidium removal data from Logsdon et al. (1994) and the
aerobic spore removal data provided by PWSs indicate that a lime
softening clarification stage can achieve greater than 0.5-log
Cryptosporidium removal during routine operation. Consequently, EPA
agrees with the SAB recommendation to award an additional 0.5-log
Cryptosporidium treatment credit for two-stage softening. Today's rule
establishes two-conditions for PWSs to receive this credit.
    The first condition for 0.5-log treatment credit for two-stage
softening is that chemical addition and hardness precipitation must
occur in two separate and sequential softening stages prior to
filtration. The purpose of this condition is to ensure that plants
receiving additional credit for two-stage softening actually have
softening and associated particle removal occurring in each of two
sequential clarification stages. Plants with other types of
clarification processes in series with a softening stage are not
eligible for two-stage softening credit. Such plants may, however, be
eligible for additional treatment credit for other microbial toolbox
options, such as presedimentation, or may achieve additional credit
through a demonstration of performance.
    The second condition for two-stage softening treatment credit is
that both softening stages must treat the entire plant flow taken from
a surface water source or GWUDI. The SAB recommended this condition,
which reflects the understanding that a softening stage is unlikely to
reduce overall Cryptosporidium levels by 0.5-log or more if it treats
only a fraction of the plant flow.
    EPA recognizes that some PWSs using softening will bypass a
softening stage in order to maintain a desired pH and alkalinity level
in the treated water, and EPA is not recommending against this practice
generally. Rather, the restriction on bypassing a softening stage in
today's rule applies only to PWSs that seek additional treatment credit
for softening. Additionally, plants that soften both surface water and
ground water are eligible for softening treatment credit if they bypass
a softening stage only with ground water that is not under the direct
influence of surface water.
    The proposal also required that a coagulant be present in both
clarifiers for a PWS to be eligible for additional treatment credit for
two-stage softening. EPA is not establishing this requirement in
today's final rule. While many PWSs that practice softening add
coagulants to improve the removal of precipitates and other particles,
the SAB did not recommend coagulant addition as a condition for
receiving treatment credit. Further, available data do not indicate
that additional coagulant is necessary to achieve at least 0.5-log
Cryptosporidium removal across a softening clarification stage if
hardness precipitation is occurring.
c. Summary of Major Comments
    Public comments on the August 11, 2003, LT2ESWTR proposal supported
awarding additional Cryptosporidium treatment credit for lime softening
processes. EPA received specific comments on the types of lime
softening processes eligible for additional treatment credit, the
amount of additional treatment credit awarded, and the need for a
coagulant. A summary of these commenters' concerns and EPA's responses
follows.
    In regard to the types of lime softening processes eligible for
treatment credit, commenters recommended that EPA better define two-
stage softening. Commenters stated that two-stage softening involves
two separate reaction chambers with the addition of the softening
chemical at the beginning of each chamber. Some commenters recommended
that eligibility for additional treatment credit should be based on the
level of softening precipitate formed or the settled water turbidity
and not on whether a plant practices single- or two-stage softening.
Another commenter recommended that any plant designs with multiple,
continuously operated clarification processes in series should be
eligible for additional treatment credit.
    In response, EPA has refined the definition of two-stage softening
in today's final rule, which requires that softening processes employ
chemical addition and hardness precipitation in two sequential stages
to be eligible for the prescribed additional treatment credit. EPA
agrees with commenters that the level of precipitate formation will
influence the degree of Cryptosporidium removal. Available data,
however, indicate that two-stage softening will generally achieve more
Cryptosporidium removal than single-stage softening. Consequently, EPA
believes that two-stage softening should be eligible for the additional
prescribed 0.5-log treatment credit. Plants with single-stage softening
may receive additional treatment credit under today's rule through a
demonstration of performance. Similarly, plants that employ multiple
clarification process other than softening in series may receive
additional treatment credit either as presedimentation or through a
demonstration of performance.
    With respect to the amount of additional Cryptosporidium treatment
credit for two-stage softening, most commenters supported awarding 3.0-
log treatment credit to single-stage lime softening, equivalent to a
conventional treatment plant, and an additional prescribed 0.5-log
treatment credit for two-stage lime softening. A few commenters
requested that two-stage lime be granted an additional Cryptosporidium
treatment credit of 1.0-log, based on the level of aerobic spore
removal measured across softening clarifiers.
    EPA agrees with most commenters and the SAB that 0.5-log is an
appropriate level of additional prescribed Cryptosporidium treatment
credit for two-stage softening. Where plants are able to demonstrate a
significantly higher level of removal of Cryptosporidium or an
indicator like aerobic spores, they may apply for additional treatment
credit through a demonstration of performance.
    Commenters stated that achieving particle removal in lime softening
is not dependent on a coagulant like a metal salt or organic polymer.
Some commenters recommended that coagulant be defined to include
softening chemicals like lime and magnesium hydroxide (a softening

precipitate). EPA agrees that available data do not demonstrate the
need for a traditional metal salt or organic coagulant for effective
particle removal in softening. Accordingly, today's final rule does not
require the use of a coagulant as a condition for additional treatment
credit in two-stage softening. Instead, each stage must involve
chemical addition and hardness precipitation. EPA intends this
requirement to ensure that softening and associated particle removal
occur in each stage if a plant is to receive additional treatment
credit for two-stage softening.
6. Bank Filtration
a. Today's Rule
    Bank filtration is a water treatment process that uses one or more
pumping wells to induce or enhance natural surface water infiltration
and to recover that surface water from the subsurface after passage
through a river bed or bank(s). Under today's rule, bank filtration
that serves as pretreatment to a filtration plant is eligible for
Cryptosporidium treatment credit if it meets the following criteria:
     Wells with a ground water flow path of at least 25 feet
receive 0.5-log treatment credit; wells with a ground water flow path
of at least 50 feet receive 1.0-log treatment credit. The ground water
flow path must be determined as specified in this section.
     Only wells in granular aquifers are eligible for treatment
credit. Granular aquifers are those comprised of sand, clay, silt, rock
fragments, pebbles or larger particles, and minor cement. A system must
characterize the aquifer at the well site to determine aquifer
properties. Systems must extract a core from the aquifer and
demonstrate that in at least 90 percent of the core length, grains less
than 1.0 mm in diameter constitute at least 10 percent of the core
material.
     Only horizontal and vertical wells are eligible for
treatment credit.
     For vertical wells, the ground water flow path is the
measured distance from the edge of the surface water body under high
flow conditions (determined by the 100 year floodplain elevation
boundary or by the floodway, as defined in Federal Emergency Management
Agency flood hazard maps) to the well screen. For horizontal wells, the
ground water flow path is the measured distance from the bed of the
river under normal flow conditions to the closest horizontal well
lateral screen.
     Systems must monitor each wellhead for turbidity at least
once every four hours while the bank filtration process is in
operation. If monthly average turbidity levels, based on daily maximum
values in the well, exceed 1 NTU, the system must report this result to
the State and conduct an assessment within 30 days to determine the
cause of the high turbidity levels in the well. If the State determines
that microbial removal has been compromised, the State may revoke
treatment credit until the system implements corrective actions
approved by the State to remediate the problem.
     Springs and infiltration galleries are not eligible for
treatment credit under this section, but are eligible for credit under
the demonstration of performance provisions described in section
IV.D.9.
    Alternatively, PWSs may apply to the State for Cryptosporidium
treatment credit for bank filtration using a demonstration of
performance. States may award greater than 1.0-log Cryptosporidium
treatment credit for bank filtration based on a site-specific
demonstration. For a bank filtration demonstration of performance
study, today's rule establishes the following criteria:
     The study must follow a State-approved protocol and must
involve the collection of data on the removal of Cryptosporidium or a
surrogate for Cryptosporidium and related hydrogeologic and water
quality parameters during the full range of operating conditions.
     The study must include sampling both from the production
well(s) and from monitoring wells that are screened and located along
the shortest flow path between the surface water source and the
production well(s).

The Toolbox Guidance Manual provides guidance on conducting site-
specific bank filtration studies, including analytical methods for
measuring aerobic and anaerobic spores, which may serve as surrogates
for Cryptosporidium removal.
    PWSs using existing bank filtration as pretreatment to a filtration
plant at the time the PWS must begin source water Cryptosporidium
monitoring under today's rule must sample the well for the purpose of
determining bin classification. These PWSs are not eligible to receive
additional treatment credit for bank filtration. In these cases, the
performance of the bank filtration process in reducing Cryptosporidium
levels will be reflected in the monitoring results and bin
classification.
    PWSs using bank filtration without additional filtration must
collect source water samples in the surface water (i.e., prior to bank
filtration) to determine bin classification unless the State approves
an alternative monitoring location. This applies to systems using bank
filtration to meet the Cryptosporidium removal requirements of the
IESWTR or LT1ESWTR under the provisions for alternative filtration
demonstration in 40 CFR 141.173(b) or 141.552(a). Bank filtration
criteria for Cryptosporidium removal credit under today's rule do not
apply to existing State actions regarding alternative filtration
Cryptosporidium removal credit for IESWTR or LT1ESWTR compliance. PWSs
using GWUDI sources must collect samples from the well (i.e., the
ground water).
b. Background and Analysis
    Bank filtration is a water treatment process that makes use of
surface water that has naturally infiltrated into ground water through
a river bed or bank and is recovered via a pumping well. River bed
infiltration is typically enhanced by the pumping action of nearby
wells. Bank filtrate is water that is drawn into a pumping well from a
nearby surface water source after having traveled through the
subsurface (i.e., aquifer) and mixing with other ground water. In bank
filtration, microorganisms and other particles are removed by contact
with the aquifer materials.
    The Stage 2 M-DBP Advisory Committee recommended a prescribed
Cryptosporidium treatment credit of 1.0-log for bank filtration with
the option for PWSs to receive greater treatment credit through a site-
specific demonstration of performance (USEPA 2000a). The August 11,
2003 proposal included criteria, similar to those in today's final
rule, for PWSs to receive prescribed treatment credits of 0.5- and 1.0-
log (USEPA 2000a). The following discussion summarizes the basis for
these credits and for differences in associated requirements between
the proposal and today's final rule.
    Directly measuring the removal of Cryptosporidium through bank
filtration is difficult due to the relatively low oocyst concentrations
typically present in surface and ground water. In the proposal, EPA
reviewed bank filtration field studies that measured the removal of
Cryptosporidium surrogates, specifically aerobic and anaerobic
bacterial endospores (Havelaar et al. 1995, Rice et al. 1996, Pang et
al. 1998, Arora et al. 2000, Medema et al. 2000, and Wang et al. 2001).
These microorganisms are suitable surrogates because they are resistant
to inactivation in the subsurface, similar in size and shape to
Cryptosporidium, and present in both surface and ground water at
concentrations that allow calculation of log removal across the surface
water-

ground water interface and within the aquifer. In addition, EPA
reviewed studies of the transport of Cryptosporidium through soil
materials in laboratory column studies (Harter et al. 2000).
    Based on these studies, EPA concluded that bank filtration
processes can achieve significant Cryptosporidium removal and that
prescribed Cryptosporidium treatment credits of 0.5-log and 1.0-log are
appropriate under certain conditions. These conditions are as follows:
Only wells located in unconsolidated, predominantly sandy aquifers are
eligible
    The bank filtration removal process performs most efficiently when
the aquifer is comprised of granular materials with open pore-space for
water flow around the grains. In these granular porous aquifers, the
flow path is meandering, thereby providing ample opportunity for
microorganisms to come into contact with and attach to a grain surface.
Accordingly, only wells located in unconsolidated, granular aquifers
are eligible for bank filtration treatment credit.
    Granular aquifers are those comprised of sand, clay, silt, rock
fragments, pebbles or larger particles and minor cement. Specifically,
a PWS must extract a core from the aquifer and demonstrate that in at
least 90 percent of the core length, grains less than 1.0 mm in
diameter constitute at least 10 percent of the core material.
Laboratory column studies of Cryptosporidium transport (Harter et al.,
2000) and field studies of aerobic bacterial endospore passage in the
subsurface (Pang et al., 1998) support these criteria.

Only Horizontal and Vertical Wells Are Eligible

    A number of devices are used for the collection of ground water
including horizontal and vertical wells, spring boxes, and infiltration
galleries. Among these, only horizontal and vertical wells are eligible
for log removal credit because spring boxes and infiltration galleries
are components of engineered systems designed to speed transport
through or by-pass the naturally protective riverbed or bank.

Wells Must be Located 25 Feet From the Surface Water Source To Be
Eligible for 0.5-Log Credit and Located at Least 50 Feet From the
Surface Water Source To Be Eligible for 1.0-Log Credit

    A vertical or horizontal well located adjacent to a surface water
body is eligible for bank filtration credit if there is sufficient
ground water flow path length to effectively remove oocysts.
Specifically, the ground water flow path must be at least 25 feet and
50 feet for 0.5-log and 1.0-log Cryptosporidium treatment credit,
respectively. The ground water flow path to a vertical well is the
measured distance from the edge of the surface water body under high
flow conditions (determined by the 100 year floodplain elevation
boundary or floodway, as defined in Federal Emergency Management Agency
flood hazard maps) to the wellhead. The ground water flow path to a
horizontal well is the measured distance from the bed of the river
under normal flow conditions to the closest horizontal well lateral.
    These required flow path distances for Cryptosporidium treatment
credit are based on pathogen and surrogate monitoring data from bank
filtration field studies (Wang et al. 2001, Havelaar et al. 1995,
Medema et al. 2000). Results from these studies show that significant
removal of anaerobic and aerobic spores can occur during passage across
the surface water--ground water interface, with lesser removal
occurring during ground water transport within the aquifer away from
that interface. The ground water--surface water interface is usually
comprised of finer grained material that lines the bottom of the
riverbed. Typically, the thickness of the interface is small, ranging
from a few inches to a foot.
    These results suggest that during normal and low surface water
elevations, the surface water-ground water interface will perform
effectively to remove microbial contamination like Cryptosporidium.
During short periods of flooding, substantially lower removal rates may
occur due to scouring of the riverbed and removal of the protective,
fine-grained material. Assessing the mean Cryptosporidium removal that
a bank filtration process will achieve over the period of a year
requires consideration of both high and low removal periods. By
considering all time intervals with differing removal rates over the
period of a year, EPA concluded that 0.5-log removal over 25 feet and
1.0-log removal over 50 feet are appropriate estimates of the mean
performance of a bank filtration process (USEPA 2003a).

Wells Must Be Continuously Monitored for Turbidity

    Similar pathogen removal mechanisms are expected to occur in slow
sand filtration and bank filtration. Under the 40 CFR 141.73(b)(1), the
turbidity level of slow sand filtered water must be 1 NTU or less in 95
percent of the measurements taken each month. Turbidity sampling is
required once every four hours, but may be reduced to once per day
under certain conditions. Just as turbidity monitoring is used to
provide assurance that the removal credit assigned to a slow sand
filter is being realized, today's rule requires turbidity monitoring at
least once every 4 hours for all bank filtration wells that receive
treatment credit.
    If monthly average turbidity levels (based on daily maximum values
in the well) exceed 1 NTU, the PWS must report this result to the State
and conduct an assessment to determine the cause of the high turbidity
levels in the well. If the State determines that microbial removal has
been compromised, the State may revoke treatment credit until the PWS
implements corrective actions to remediate the problem.

Demonstration of Performance

    EPA recognizes that some bank filtration processes may achieve mean
Cryptosporidium removal greater than 1-log. Consequently, today's rule
allows PWSs to receive greater than 1.0-log Cryptosporidium treatment
credit for bank filtration through a State-approved demonstration of
performance study. This allowance is a change from the proposed rule,
which did not explicitly recognize demonstration of performance for
bank filtration (USEPA 2003a). This change reflects EPA's agreement
with public comment, described next, which recommended that EPA
explicitly recognize the option to conduct a bank filtration
performance study for greater than 1.0-log treatment credit.
    A demonstration of performance study must involve the collection of
data on the removal of Cryptosporidium or surrogates and related
hydrogeologic and water quality parameters during the full range of
operating conditions. PWSs must sample from both the production well(s)
and one or more monitoring wells that are screened and located along
the shortest flow path between the surface water and the production
well(s). This will allow determination of the removal efficiency of the
aquifer.
    Because directly measuring Cryptosporidium removal will not be
feasible for most PWSs, today's rule allows PWSs to sample for a State-
approved indicator, such as aerobic bacterial endospores. Research has
shown that aerobic spores can be very mobile in the subsurface
environment (Pang et al. 1998), and data collected by Wang et al.
(2001) indicate that aerobic spores are present in some surface waters
in sufficient quantity to allow measurement of log removal values.
    EPA has provided guidance on conducting site-specific bank
filtration

studies in the Toolbox Guidance Manual. This guidance discusses data
needs and analysis for a performance demonstration so that the State
may tailor the study plan to meet site-specific hydrogeological and
operational conditions.
    In summary, EPA believes that full-scale field data support
prescribed Cryptosporidium treatment credit up to 1.0-log for bank
filtration under the required conditions for set-back distance, aquifer
material, collection device type, and turbidity monitoring.
Demonstration of performance provides an appropriate opportunity for
States to award higher Cryptosporidium treatment credit for bank
filtration on a site-specific basis.
    For PWSs using bank filtration when they must conduct source water
monitoring for bin classification, the required sampling locations
reflect the intent for this monitoring to capture the level of
Cryptosporidium entering a PWS's primary filtration treatment process.
Where bank filtration serves as pretreatment to a filtration plant,
PWSs must collect source water samples after bank filtration but prior
to the filtration plant. In this case, the Cryptosporidium removal that
bank filtration achieves will be reflected in the monitoring results
and bin classification for the filtration plant. In contrast, where
bank filtration is the primary filtration process, meaning that a PWS
uses bank filtration to comply with the Cryptosporidium treatment
requirements of the IESWTR or LT1ESWTR, PWSs must collect samples in
the surface water source (e.g, the river).
c. Summary of Major Comments
    Public comments on the August 11, 2003, LT2ESWTR proposal supported
awarding Cryptosporidium treatment credit for bank filtration. Many
commenters, however, stated that the proposed levels of credit (0.5-
and 1.0-log) were insufficient. To address this issue, commenters
supported allowing PWSs to obtain greater treatment credit by
performing a site-specific study of bank filtration removal efficiency.
Commenters recommended that site-specific bank filtration studies
involve the measurement of surrogates for Cryptosporidium removal using
monitoring wells located along the shortest flow path between the
surface water and the production well.
    EPA agrees that some bank filtration sites may achieve greater than
1.0-log Cryptosporidium removal. Today's rule establishes the proposed
bank filtration Cryptosporidium treatment credits of 0.5- and 1.0-log
and allows PWSs to apply to the State for higher levels of credit
through a site-specific demonstration of performance. In such a study,
PWSs must measure the removal of Cryptosporidium or a State-approved
surrogate using monitoring wells located along the flow path, as
recommended by commenters.
    Some commenters cited research addressing appropriate surrogate
organisms for estimating Cryptosporidium removal in surface water
treatment plants and bank filtration sites. Commenters recommended that
EPA recognize aerobic endospores as a surrogate measure in
Cryptosporidium removal studies, including those for bank filtration.
    EPA agrees that based on available information, aerobic spores are
suitable Cryptosporidium removal surrogates for bank filtration
processes due to their size, resistance to inactivation, and
concentration in surface and ground waters. Data from several bank
filtration sites on the use of aerobic spores as a Cryptosporidium
removal surrogate are available. The Toolbox Guidance Manual identifies
aerobic spores as suitable in conjunction with other hydrogeologic data
for making site-specific determinations for additional Cryptosporidium
removal credit.
    In guidance, EPA suggests that where feasible, PWSs measure diatom
species in conjunction with aerobic spores in bank filtration studies
because Cryptosporidium oocysts are intermediate in size between the
two surrogate groups. Further, EPA recognizes the current uncertainties
and limitations in available information on surrogates for bank
filtration and will update guidance as warranted by new information.
7. Combined Filter Performance
a. Today's Rule
    For water treatment plants that use filtration, the turbidity of
the filtered water is an indicator of how effectively the plant is
removing particulate matter, including microbial pathogens, from the
raw water. PWSs using conventional filtration treatment or direct
filtration receive an additional 0.5-log Cryptosporidium treatment
credit during any month the PWS meets the following standard:
     The turbidity level of representative samples of a PWS's
filtered water (i.e., the combined filter effluent) is less than or
equal to 0.15 NTU in at least 95 percent of the measurements taken each
month. PWSs must continue to measure turbidity as specified in 40 CFR
141.74(a) and (c), which generally require sampling at least every four
hours using approved methods.

PWSs using other types of filtration processes, including slow sand,
diatomaceous earth, membranes, bag, or cartridge filtration, are not
eligible for this treatment credit.
b. Background and Analysis
    Turbidity is a method defined parameter that is based on measuring
the amount of light scattered by suspended particles in a solution.
This measure can detect the presence of a wide variety of particles in
water, including microorganisms, but cannot provide specific
information on particle type, number, or size. In filtered water, the
turbidity level indicates how well the filtration and other upstream
clarification processes have performed in removing particles from the
raw water, with lower turbidity indicating better particle removal.
Thus, lower filtered water turbidity is associated with a decreased
likelihood that microbial pathogens like Cryptosporidium have passed
through the filtration plant and into the water distributed to
consumers.
    Under existing regulations, PWSs that filter must monitor turbidity
in the combined filter effluent (CFE) at least every four hours using
approved methods, although States may reduce this frequency to once per
day for PWSs serving 500 people or fewer (40 CFR 141.74(a) and (c)).
For PWSs using conventional or direct filtration, at least 95 percent
of the CFE turbidity measurements must be less than or equal to 0.3
NTU, and the turbidity must never exceed 1 NTU (40 CFR 141.173(a) and
141.551(a)-(b)).
    The Stage 2 M-DBP Advisory Committee recommended an additional 0.5-
log Cryptosporidium treatment credit for PWSs that achieve a CFE
turbidity less than or equal to 0.15 NTU in at least 95 percent of
measurements per month (USEPA 2000a). This 95th percentile turbidity
standard is one half the level required under existing regulations for
PWSs using conventional or direct filtration, as stated earlier. The
August 11, 2003 proposal included this treatment credit for PWSs using
conventional or direct filtration (USEPA 2003a), and EPA is
establishing it in today's final rule with no changes from the
proposal. The following discussion summarizes the basis for this
treatment credit.
    In the proposal, EPA analyzed the improvement in Cryptosporidium
removal that conventional and direct filtration plants realize when
operating at lower effluent turbidity levels. For this analysis, EPA
estimated that PWSs

complying with the existing 95th percentile CFE turbidity standard of
0.3 NTU will typically operate with filter effluent turbidity between
0.1-0.2 NTU; PWSs complying with a CFE standard of 0.15 NTU were
estimated to operate with filter effluent turbidity less than 0.1 NTU.
Accordingly, EPA compared Cryptosporidium removal efficiencies when
effluent turbidity was below 0.1 NTU with those when effluent turbidity
was in the range of 0.1-0.2 NTU.
    Studies by Patania et al. (1995), Emelko et al. (1999), and Dugan
et al. (2001) observed the average removal of Cryptosporidium to be
0.5-to 1.2-log greater when filter effluent turbidity was less than 0.1
NTU in comparison to removal with effluent turbidity between 0.1-0.2
NTU. These studies, therefore, indicate that PWSs complying with a
filter effluent turbidity standard of 0.15 NTU will achieve at least
0.5-log greater Cryptosporidium removal than PWSs complying with the
existing 0.3 NTU standard. Based on this finding, EPA concluded that an
additional 0.5-log Cryptosporidium treatment credit is appropriate for
PWSs using conventional or direct filtration that meet a 95th
percentile CFE turbidity standard of 0.15 NTU.
    Other types of filtration processes, such as slow sand,
diatomaceous earth, membranes, bag, or cartridge filtration, are not
eligible for this treatment credit. These filtration processes remove
Cryptosporidium through different mechanisms than those operative in
rapid granular media filtration, which is used in conventional and
direct filtration. Available data do not establish a similar
relationship between lower filter effluent turbidity and improved
Cryptosporidium removal efficiency for these other filtration
processes.
    The SAB reviewed the proposed additional Cryptosporidium treatment
credit for PWSs that operate with very low filtered water turbidity. In
their report, the SAB stated that further lowering of turbidity would
result in further reductions in Cryptosporidium in the effluent from
filtration processes, but available data were limited in showing the
exact removal that can be achieved. Based on the data provided, the SAB
recommended that no additional treatment credit be given to plants that
demonstrate a CFE turbidity of 0.15 NTU or less (SAB 2003).
    In addressing this SAB recommendation, EPA recognizes that
precisely quantifying the increase in Cryptosporidium removal that a
particular filtration plant will realize when operating at lower filter
effluent turbidity is not generally feasible. Available data, though,
consistently show that removal of Cryptosporidium is at least 0.5-log
greater when filter effluent turbidity reflects compliance with a 0.15
NTU standard in comparison to a 0.3 NTU standard. Further, treatment
plants operating at lower filter effluent turbidity will achieve
increased removal of other microbial pathogens present in the raw
water. In consideration of these factors, EPA believes that PWSs should
receive an additional 0.5-log Cryptosporidium treatment credit when at
least 95 percent of CFE turbidity measurements are less than or equal
to 0.15 NTU.
    Another key issue in establishing additional treatment credit based
on low filtered water turbidity is the performance of analytical
instruments (turbidimeters) to accurately measure turbidity at low
levels. In the proposal, EPA reviewed studies of low level turbidity
measurements by EPA (1998c), Sadar (1999), and Letterman et al. (2001).
Among the significant findings of these studies are the following:

    (1) On-line turbidimeters typically had a positive bias (i.e., a
higher turbidity reading) in comparison to bench-top turbidimeters.
EPA expects that most PWSs that receive additional treatment credit
for low filter effluent turbidity will use on-line turbidimeters.
This finding suggests that the error in turbidimeter readings may be
generally conservative, so that PWSs will operate at lower than
required turbidity levels.
    (2) Different turbidimeters did not agree well when used to
measure low level turbidity, which may be due to differences in
instrument design. This finding suggests that low level turbidity
measurements may be viewed as a relative indicator of water quality
improvement at a particular PWS but may be less applicable for
making comparisons among different PWSs.

    In addition, the American Society for Testing and Materials (ASTM)
has issued standard test methods for measurement of turbidity below 5
NTU by on-line (ASTM 2001) and static (ASTM 2003) instruments. These
methods specify that the instrument should permit detection of
turbidity differences of 0.01 NTU or less in waters having turbidities
of less than 1.00 NTU (ASTM 2001) and 5.0 NTU (ASTM 2003),
respectively.
    After reviewing these studies and the ASTM methods, EPA concluded
that currently available monitoring equipment can reliably measure
turbidity at levels of 0.15 NTU and lower. Rigorous calibration and
maintenance of turbidity monitoring equipment is necessary, however.
EPA has developed guidance on proper calibration, operation, and
maintenance of turbidimeters (USEPA 1999c).
c. Summary of Major Comments
    Public comment on the August 11, 2003, LT2ESWTR proposal supported
awarding additional Cryptosporidium treatment credit for PWSs that
achieve lower filtered water turbidity. Commenters raised specific
concerns with the criteria for PWSs to receive this credit, the
available data that support this credit, and the performance of
turbidimeters for measuring turbidity at very low levels. A summary of
these comments and EPA's responses follows.
    Most commenters supported awarding 0.5-log additional
Cryptosporidium treatment credit for PWSs that achieve at least 95
percent of CFE turbidity measurements less than or equal to 0.15 NTU. A
few commenters, however, recommended that PWSs only receive additional
treatment credit for demonstrating this level of turbidity performance
in each individual filter effluent (IFE), rather than the CFE. In
addition, one commenter stated that PWSs should be required to monitor
CFE turbidity every 15 minutes, rather than every four hours as
required under current regulations.
    In response, EPA agrees with the recommendation of most commenters
and has established additional Cryptosporidium treatment credit based
on meeting a 95th percentile turbidity level of 0.15 NTU in the CFE.
EPA recognizes, however, that achieving low turbidity in each IFE may
represent a higher level of performance than achieving low turbidity in
the CFE. As described in the next section, EPA has also established
standards for additional Cryptosporidium treatment credit based on low
IFE turbidity in today's rule. EPA does not have data indicating that
PWSs should monitor the CFE turbidity at a higher frequency than every
four hours, as required under existing regulations. Consequently, EPA
is not changing the frequency of required CFE turbidity monitoring as a
condition for PWSs to receive additional treatment credit under today's
rule.
    One commenter summarized additional studies that provide data on
the improvement in Cryptosporidium removal efficiency at lower filter
effluent turbidity levels. According to this commenter, these studies
demonstrate that lowering filter effluent turbidity from 0.3 to 0.15
NTU translates to an improvement in Cryptosporidium removal of more
than 1.5-log, with individual studies showing a range of >0.7-log to
>3-log based on median removal. EPA finds that these studies bolster
the conclusion that PWSs operating to meet 0.15 NTU in the filter
effluent will achieve at least 0.5-

log greater Cryptosporidium removal than PWSs operating to meet 0.3
NTU. Thus, they support the additional 0.5-log Cryptosporidium
treatment credit under today's rule for PWSs meeting 0.15 NTU at the
95th percentile in the CFE.
    In regard to the measurement of low level turbidity, some
commenters raised concerns that turbidimeters used by the U.S. water
supply industry do not agree when used to measure turbidity in the 0.01
to 0.5 NTU range. Further, these differences are independent of the
calibration method used and can be significant when comparing
instruments by different manufacturers. Other commenters stated that
turbidimeters can accurately reflect turbidity values less than 0.15
NTU if properly calibrated, and some commenters cited the ASTM method
development process to support this assessment. In addition, commenters
suggested that available guidance on turbidity measurement provides
quality assurance measure that can reduce analytical uncertainty.
    EPA agrees with commenters that available methods and instruments
are adequate to demonstrate compliance with a 0.15 NTU turbidity level.
In particular, EPA believes that monitoring low level turbidity can be
effective for demonstrating water quality improvements at individual
plants, but also recognizes that the performance of turbidimeters used
at different plants may vary. Further, calibration and maintenance of
turbidity monitoring equipment is critical, and EPA has developed
guidance on these procedures (USEPA 1999c).
8. Individual Filter Performance
a. Today's Rule
    PWSs using conventional filtration treatment or direct filtration
receive an additional 0.5-log Cryptosporidium treatment credit during
any month the PWS meets the following criteria:
     The filtered water turbidity for each individual filter is
less than or equal to 0.15 NTU in at least 95 percent of the
measurements recorded each month; and
     No individual filter has a measured turbidity level
greater than 0.3 NTU in two consecutive measurements taken 15 minutes
apart.
PWSs must continue to monitor turbidity for each individual filter
continuously and record the results every 15 minutes, as required under
40 CFR 141.174 and 141.560.
    PWSs that receive this 0.5-log Cryptosporidium treatment credit for
individual filter performance also receive 0.5-log treatment credit for
combined filter performance, as described in section IV.D.7, for a
total additional treatment credit of 1.0-log. Conversely, PWSs are not
required to pursue individual filter performance credit to remain
eligible for combined filter performance credit.
    If a PWS has received credit for individual filter performance to
comply with its Cryptosporidium treatment requirements and fails to
meet the required criteria for this credit during any month, the PWS
will not incur a treatment technique violation if the State determines
the following:
     The failure to meet the required criteria for individual
filter performance treatment credit was due to unusual and short-term
circumstances that could not reasonably be prevented through optimizing
treatment plant design, operation, and maintenance; and
     The PWS has experienced no more than two such failures in
any calendar year.
    This treatment credit is not applicable to other types of
filtration processes, including slow sand, diatomaceous earth,
membranes, bag, or cartridge filtration.
b. Background and Analysis
    Awarding additional treatment credit for individual filter
performance is based on the expectation that achieving low filtered
water turbidity in each individual filter will provide increased
protection against microbial pathogens. Most treatment plants have
multiple filters. Moderately elevated turbidity in the effluent from a
single filter may not significantly affect the turbidity of the
combined filter effluent, but may indicate a reduction in the overall
pathogen removal efficiency of the filtration process. Consequently, a
primary goal in optimizing water treatment plant performance is
ensuring that each filter always produces very low turbidity water.
    The criteria for PWSs to achieve the additional 1.0-log
Cryptosporidium treatment credit for individual filter performance
reflect goals of Phase IV of the Partnership for Safe Water
(Partnership). The Partnership is a voluntary cooperative program
involving PWSs, professional associations, and Federal and State
regulatory agencies that seeks to increase protection against microbial
contaminants by optimizing water treatment plant performance. The Stage
2 M-DBP Advisory Committee recommended 1.0-log treatment credit for
PWSs that successfully participate in a peer review program and
identified Phase IV of the Partnership as a program where such credit
would be appropriate (USEPA 2000a).
    At the time of the Advisory Committee recommendation, the
performance goals for Phase IV of the Partnership reflected those of
the EPA Composite Correction Program (USEPA 1991a) and involved an on-
site evaluation by a third-party team. Phase IV performance goals for
individual filters included filtered water turbidity less than 0.1 NTU
at least 95 percent of the time based on daily maximum values and a
maximum measurement of 0.3 NTU. The purpose of the on-site evaluation
was to confirm that a PWS had met Phase IV performance goals or had
achieved the highest level of performance given its unique raw water
quality.
    After the Stage 2 M-DBP Agreement in Principle was signed in
September 2000, the Partnership eliminated on-site third-party
evaluation as a component of Phase IV. Instead, Phase IV required
completion of an Optimization Assessment Spreadsheet in which the PWS
entered water treatment data to demonstrate that it had achieved Phase
IV performance levels. The application also required narratives related
to the administrative support and operational capabilities necessary to
sustain performance long-term.
    The August 11, 2003 LT2ESWTR proposal included a 1.0-log
Cryptosporidium treatment credit for PWSs that met the individual
filter performance goals of Phase IV of the Partnership (i.e., 95
percent of daily maximum values below 0.1 and no values above 0.3 NTU)
(USEPA 2003a). Rather than requiring an application package with
historical data and narratives, however, the proposed rule required
PWSs to report filter effluent turbidity data to the State each month
to demonstrate compliance with these filter performance goals.
    The Partnership modified the Phase IV goals for individual filter
performance in 2003. A revised goal is filtered water turbidity less
than 0.10 NTU at least 95 percent of the time based on values recorded
at 15 minute time intervals. Thus, where the earlier goal was based on
daily maximum values for each filter, the revised goal is based on all
values for each filter--a less stringent approach. The Partnership made
this modification after finding that none of the water treatment plants
that had been evaluated could consistently meet the 0.1 NTU goal using
daily maximum values and, further, that this goal was biased against
plants with more filters.
    In today's final rule, EPA has adjusted the criteria from the
proposal for PWSs

to receive additional treatment credit based on individual filter
effluent turbidity. These adjustments are in response to the changes
the Partnership made to Phase IV individual filter performance goals.
Under today's rule, PWSs receive 1.0-log additional Cryptosporidium
treatment credit if effluent turbidity from each filter is less than or
equal to 0.15 NTU at least 95 percent of the time and never exceeds 0.3
NTU in two consecutive measurements taken 15 minutes apart.
    EPA expects that PWSs will operate at less than 0.1 NTU in order to
comply with a regulatory limit of 0.15 NTU. Further, EPA believes that
assessing individual filter compliance with a maximum turbidity level
of 0.3 NTU based on two consecutive measurements taken 15 minutes apart
is appropriate. This approach allows for brief fluctuations in
turbidimeter readings that may not indicate a degradation in filtered
water quality to occur without penalizing a PWS, but it should catch
filters that significantly exceed 0.3 NTU over the course of a month.
EPA applied this approach to individual filter monitoring under the
IESWTR and LT1ESWTR. Consequently, EPA regards these criteria as
comparable to the revised Partnership Phase IV standards for individual
filter performance.
    In addition, today's rule gives States authority to determine
whether to issue a treatment technique violation for PWSs that exceed
individual filter performance limits. This authority applies in the
case where a PWS receives credit for individual filter performance to
meet the treatment requirements of today's rule and fails to achieve
the criteria to receive this credit during a month. If the State
determines that this failure was due to unusual and short-term
circumstances that could not reasonably be prevented through treatment
optimization, the State may choose not to issue a treatment technique
violation, which the PWS otherwise will incur. Because this authority
should be applied only to unusual plant circumstances, a State cannot
make this determination if a PWS has experienced more than two such
failures in any calendar year.
    EPA is granting States this authority because PWSs that
consistently meet the criteria for individual filter performance
treatment credit may occasionally experience short-term deviations from
these criteria due to circumstances largely beyond the PWS's control.
An example of such a circumstance may be malfunctioning equipment that
a PWS quickly removes from service, but that nevertheless prevents the
PWS from fully meeting individual filter performance criteria in a
particular month. EPA believes that States should only apply this
authority in cases where PWSs have consistently achieved the criteria
for individual filter performance treatment credit in previous months.
    The approach in today's final rule for valuing individual filter
performance treatment credit differs from the approach in the proposal.
EPA's intent in both the proposal and today's rule is to award an
additional 1.0-log Cryptosporidium treatment credit to PWSs that meet
the criteria for individual filter performance. In the proposal,
however, PWSs could receive 1.0-log additional treatment credit
specifically for meeting the individual filter performance criteria,
but were then not eligible to receive any treatment credit under the
combined filter performance option. In today's rule, PWSs receive 0.5-
log credit for the individual filter performance option and also
receive an additional 0.5-log treatment credit for the combined filter
performance option (discussed in section IV.D.7), resulting in 1.0-log
total additional credit. EPA has made this modification so that if a
PWS fails in an attempt to achieve individual filter performance
credit, the PWS is clearly still eligible to received combined filter
performance credit.
    In a review of a draft LT2ESWTR proposal, the SAB recommended that
PWSs receive 0.5-log, rather than 1.0-log, additional Cryptosporidium
treatment credit for achieving individual filter effluent turbidity
below 0.15 NTU at the 95th percentile (SAB 2003). In response to this
SAB recommendation, today's rule requires additional individual filter
performance criteria to support 1.0-log total additional treatment
credit. Specifically, today's rule incorporates the Partnership Phase
IV performance goal that individual filter effluent turbidity never
exceed 0.3 NTU (as described earlier, EPA concluded that determining
compliance with this standard based on two consecutive measurements
taken 15 minutes is appropriate and consistent with existing
regulations). Thus, EPA believes that these criteria, in conjunction
with the expectation that controlling effluent turbidity at all filters
individually rather than just the combined filter effluent will
generally result in lower microbial risk, justify 1.0-log additional
treatment credit.
c. Summary of Major Comments
    Public comment on additional treatment credit for individual filter
performance in the August 11, 2003 proposal raised a number of issues:
changes in the Partnership Phase IV criteria and achievability of the
proposed criteria for this credit, credit for participating in peer
review programs, and a review process for data that exceed regulatory
limit. A summary of these comments and EPA's responses follows.
    Several commenters stated that PWSs could not consistently achieve
the proposed individual filter effluent turbidity criterion of 95
percent of daily maximum measurements less than or equal to 0.1 NTU.
Commenters provided data on turbidity levels in PWSs to support this
assertion and indicated that the Partnership modified this criterion in
the Phase IV individual filter performance goals because PWSs could not
meet it. Alternatives recommended by commenters for the final rule
included the use of the revised Partnership Phase IV goals for
individual filter effluent turbidity or a more stringent criterion for
combined filter effluent turbidity.
    In response, EPA agrees that current Partnership Phase IV goals
provide appropriate criteria for awarding 1.0-log total additional
Cryptosporidium treatment credit. Today's rule grants this total credit
to PWSs that meet a 95th percentile individual filter effluent
turbidity limit of 0.15 NTU, and EPA expects that PWSs complying with
this limit will operate under the Partnership goal of 0.10 NTU. EPA
does not support awarding a higher level of additional treatment credit
for a more stringent combined filter effluent turbidity criterion,
beyond the 0.5-log credit available under combined filter performance
(see section IV.D.7). The purpose of the individual filter performance
toolbox option is to recognize the higher pathogen removal PWSs will
likely achieve by maintaining very low effluent turbidity for each
individual filter.
    A few commenters suggested that as an alternative to establishing
numerical criteria for individual filter performance, today's rule
should award additional treatment credit for PWSs that successfully
participate in a peer review program. In addition to the Partnership,
commenters listed the Area Wide Optimization Program and the Texas
Optimization Program as examples of programs that will provide for
comprehensive improvements in treatment performance.
    EPA agrees that participation in peer review programs is beneficial
for PWSs. Further, such programs may assist PWSs in meeting the
filtration performance criteria in today's rule for additional
Cryptosporidium treatment credit. EPA does not believe, however, that
mere participation in a peer review program

is an appropriate basis for awarding additional treatment credit.
Rather, to ensure national consistency in standards for compliance with
treatment requirements, EPA has concluded that additional treatment
credit should be based on PWSs meeting specified criteria for enhanced
treatment performance.
    Another significant issue raised by commenters is the need for a
review process for deviations from the criteria for individual filter
performance due to circumstances that cannot be prevented through plant
optimization. An example given by several commenters is a filter that
malfunctions and is taken out of service, but that may have exceeded
the individual filter performance turbidity criteria for a short period
when the filter was operating.
    EPA agrees that circumstances may occur that are beyond the PWS's
control and that prevent the PWS from fully meeting the criteria for
individual filter performance in a particular month. If a PWS relies on
individual filter performance treatment credit to meet the treatment
requirements of today's rule and the PWS fails to meet all criteria for
this credit in a given month, the State may review the reasons for this
failure. If the State finds that the failure was due to circumstances
that could not be prevented through plant optimization, the State may
choose not to issue a treatment technique violation on up to two such
occasions in a calendar year.
9. Demonstration of Performance
a. Today's Rule
    A demonstration of performance is a site-specific test that
assesses the Cryptosporidium removal efficiency of a water treatment
plant or a treatment process within a plant. Under today's rule, PWSs
may undertake demonstration of performance testing for the following
purposes:

    (1) To establish a Cryptosporidium treatment credit that is
higher than the prescribed treatment credit in today's rule for a
water treatment plant or a treatment process in the microbial
toolbox; or
    (2) To establish a Cryptosporidium treatment credit for a
treatment process that is not included in the microbial toolbox or
that does not meet the design or operational criteria for prescribed
treatment credit in the microbial toolbox.

    The specific requirements that apply to demonstration of
performance testing are as follows:
     PWSs may receive Cryptosporidium treatment credit for a
water treatment plant or a treatment process within a plant that is
based on a site-specific demonstration of Cryptosporidium removal
efficiency. This demonstration of performance treatment credit may be
greater than or less than any prescribed treatment credit in today's
rule.
     The site-specific demonstration of Cryptosporidium removal
efficiency must follow a State-approved protocol and may involve the
use of surrogates rather than Cryptosporidium.
     The State must approve through written notification any
treatment credit based on a demonstration of performance. As a
condition of approval, the State may designate monitoring and treatment
performance criteria the PWS must meet and report on an ongoing basis
to remain eligible for the credit. The State may designate such
criteria to verify that the PWS maintains the operating conditions
under which the State approved the demonstration of performance
treatment credit.
     PWSs are not eligible for prescribed treatment credit for
any treatment process that is included in a demonstration of
performance credit.
b. Background and Analysis
    The prescribed Cryptosporidium treatment credits in today's rule
for water treatment plants and for treatment processes in the microbial
toolbox are based on conservative estimates of mean Cryptosporidium
removal efficiencies. Due to site-specific conditions, however, some
PWSs will achieve greater Cryptosporidium removal than reflected in the
prescribed treatment credits. In addition, some PWSs will have
treatment processes that are not included in the microbial toolbox or
that do not meet microbial toolbox criteria for prescribed treatment
credit. In all these cases, PWSs have the option to undertake
demonstration of performance testing to establish an appropriate level
of Cryptosporidium treatment credit for the treatment plant or
treatment process.
    The option for demonstration of performance testing in today's rule
reflects a recommendation by the Stage 2 M-DBP Advisory Committee.
Specifically, the Committee stated that the LT2ESWTR should allow site-
specific testing both to establish Cryptosporidium treatment credit
above the prescribed credit for microbial toolbox processes and to
demonstrate Cryptosporidium removal for technologies not listed in the
microbial toolbox. The August 11, 2003 LT2ESWTR proposal included the
demonstration of performance option (USEPA 2003a), and EPA is
establishing it in today's final rule.
    Demonstration of performance testing will be specific to a
particular site and will depend on the treatment processes being
tested, water quality, plant infrastructure, PWS resources, and other
factors. Consequently, today's rule does not establish specific
protocols for demonstration of performance testing. Rather, today's
rule gives States the authority to approve testing protocols developed
by PWSs and to determine what level of Cryptosporidium treatment credit
is appropriate. The Toolbox Guidance Manual provides recommendations to
PWSs and States on conducting demonstration of performance testing,
including analytical methods for measuring aerobic and anaerobic
spores.
    In general, demonstration of performance testing should encompass
the full range of expected operating conditions and should
conservatively assess the degree of Cryptosporidium removal that a
treatment process can reliably achieve. Directly quantifying the
removal of Cryptosporidium typically is not feasible in full-scale
testing due to limitations in source water concentrations and
analytical method performance. Consequently, demonstration of
performance testing that is conducted at full-scale may involve the use
of surrogates, such as aerobic spores, that have been shown to
correlate with the removal of Cryptosporidium. PWSs and States may also
consider the use of pilot-scale studies in conjunction with full-scale
studies for demonstration of performance testing.
    As a condition of approving a demonstration of performance credit,
the State may designate treatment performance criteria the PWS must
meet on an ongoing basis to remain eligible for the credit. For
example, if a PWS conducts a demonstration of performance study while
operating with very low filtered water turbidity, the State may
establish as a condition of approving treatment credit based on the
study that the PWS must continue operating at the low filtered water
turbidity. EPA believes this condition is necessary because, in this
example, if the PWS were to begin operating at a higher filtered water
turbidity level, the demonstration of performance study results might
no longer represent the PWSs actual performance.
    PWSs are not eligible for prescribed treatment credit for any
treatment process that is included in a demonstration of performance
credit. For example, if a PWS receives a demonstration of performance
treatment credit of 4-log for Cryptosporidium removal through a
conventional treatment plant (i.e., coagulation/sedimentation/
filtration), the PWS is not

also eligible for additional treatment credit for combined filter
performance. In this case, the demonstration of performance testing
accounts for the removal achieved by filtration.
c. Summary of Major Comments
    Public comment on the August 11, 2003 LT2ESWTR proposed supported
inclusion of the demonstration of performance option to award site-
specific treatment credit to PWSs. Commenters stated that many well-run
surface water treatment plants achieve significantly greater
Cryptosporidium removal than the prescribed treatment credit, and
demonstration of performance testing is needed to award an appropriate
level of credit in such cases. Two aspects of this option that received
significant public comment are the provision for States to award less
than the prescribed treatment credit if indicated by testing results
and the need for guidance on demonstration of performance testing.
These comments and EPA's responses are summarized as follows.
    Several commenters recommended that EPA eliminate the provision
that allows States to award less than the prescribed treatment credit
based on demonstration of performance testing. These commenters stated
that pilot- and full-scale testing is conservative and challenging to
implement and that for past regulations, States generally have not
awarded lower treatment credit based on a site-specific study. If this
provision remains in the regulation, commenters suggested that EPA
provide criteria addressing how it should be applied. Such criteria
should recognize the conservative nature of testing with surrogates for
Cryptosporidium removal and the potential for misleading or flawed
testing results.
    In response, EPA believes that States should have the discretion to
award either more or less treatment credit than the prescribed credit
on a case-by-case basis where a State has site-specific information
that an alternative credit is appropriate. Today's rule allows this.
EPA recognizes, however, that demonstration of performance testing
should be designed to provide a conservative estimate of treatment
efficiency and, as such, is not generally intended to reduce the level
of treatment credit a PWS receives. Further, results from demonstration
of performance testing should be rigorously evaluated for flaws and
bias prior to being used to support either a higher or lower treatment
credit. The Toolbox Guidance Manual identifies approaches States may
wish to consider in awarding higher or lower treatment credit.
    Many commenters stated that EPA should provide thorough guidance on
demonstration of performance testing. Topics for this guidance
suggested by commenters include approaches to demonstrating treatment
credit, minimum duration of testing, the use of safety factors, and
periodic reconfirmation of testing results. Some commenters recommended
that guidance address both full-scale testing with surrogates like
aerobic spores and pilot-scale testing with Cryptosporidium or
surrogates. Other commenters recommended that testing should be limited
to full-scale processes and that testing with pilot-scale
representations of full-scale equipment should be discouraged.
    In the Toolbox Guidance Manual, EPA provides direction on
procedures for demonstration of performance testing that addresses
issues raised by commenters. These issues include surrogates for full-
scale testing, potential roles for pilot-scale testing in conjunction
with full-scale testing, minimum duration of testing to capture the
full range of operating conditions, the analysis of data from testing
to establish treatment credit, and routine monitoring to verify that
the conditions under which demonstration of performance credit is
awarded are maintained during routine operation. EPA believes that this
guidance will assist PWSs and States with implementing demonstration of
performance testing appropriately.
10. Bag and Cartridge Filtration
a. Today's Rule
    Under today's rule, PWSs may receive Cryptosporidium treatment
credit of up to 2.0-log for an individual bag or cartridge filter and
up to 2.5-log for two or more bag or cartridge filters operated in
series. To be eligible for this treatment credit, filters must meet the
definition of a bag or cartridge filter and must undergo challenge
testing to demonstrate removal efficiency with an applied safety
factor, as described in this section.
    Today's rule defines bag and cartridge filters as pressure driven
separation processes that remove particulate matter larger than 1
micrometer using an engineered porous filtration media through either
surface or depth filtration. Bag filters are constructed of a non-
rigid, fabric filtration media housed in a pressure vessel in which the
direction of flow is from the inside of the bag to the outside.
Cartridge filters are typically constructed as rigid or semi-rigid,
self-supporting filter elements housed in a pressure vessel in which
flow is from the outside of the cartridge to the inside.
    Today's rule treats bag and cartridge filters equivalently, with
the following exception: If a cartridge filter meets the definition of
a membrane filtration process and can be direct integrity tested
according to the criteria specified in section IV.D.11, a PWS has the
option to seek greater treatment credit for the filter as a membrane.
Section IV.D.11 describes criteria for awarding treatment credit to
membranes.
    Today's rule requires challenge testing to establish
Cryptosporidium treatment credit for bag and cartridge filters. This
challenge testing is product-specific and not site-specific. Once
challenge testing is performed on a specific bag or cartridge
filtration product, PWSs that install the specific filtration product
are not required to repeat challenge testing at individual sites. For a
PWS to receive Cryptosporidium treatment credit for a bag or cartridge
filter, challenge testing must meet the following criteria:
     Challenge testing must be conducted on full-scale filters
that match the filters the PWS will use in materials, construction, and
associated housing or pressure vessel. If treatment credit will be
based on filters operated in series then challenge testing must be
performed on the filters in series.
     Challenge testing must involve measuring the removal by
the filter of either Cryptosporidium or a surrogate that is removed no
more efficiently than Cryptosporidium (i.e., the ``challenge
particulate'').
     The analytical method used to measure removal in the
challenge test must discretely quantify the specific challenge
particulate. The maximum allowable feed water concentration of the
challenge particulate used during a challenge test is 10,000 times the
analytical method detection limit of the challenge particulate in the
filtrate.
     During challenge testing, filters must be operated at the
maximum design flow rate and for a duration sufficient to reach the
maximum design pressure drop (i.e., ``terminal pressure drop''). PWSs
may not operate bag or cartridge filters outside of these design
parameters during routine use. In order to achieve terminal pressure
drop during challenge testing, adding particulate matter, such as fine
carbon test dust or bentonite clay particles, to the test water is
allowed and may be necessary.
     In each challenge test, the removal of the challenge
particulate must be measured during three periods over the

filtration cycle: (1) Within two hours of start-up of a new filter, (2)
when the pressure drop is between 45 and 55 percent of the terminal
pressure drop, and (3) when the pressure drop has reached 100 percent
of the terminal pressure drop. A log removal value (LRV) must be
calculated for each of these periods as follows: LOG10
(filter influent challenge particulate level) - LOG10
(filter effluent challenge particulate level). For each filter tested,
the LRV for the filter (LRVfilter) is equal to the minimum
of these three LRVs.
     The LRVfilter values for each filter that is
tested are used to determine the removal efficiency that is assigned to
the specific bag or cartridge filter product (i.e., a filter product
line) or combination of filters in series. If fewer than twenty filters
are tested, the removal efficiency of the filter product line is equal
to the lowest LRVfilter among the filters tested (today's
rule does not specify a minimum number of filters to test). If twenty
or more filters are tested, the removal efficiency of the filter
product line is equal to the 10th percentile of the
LRVfilter values among the filters tested.
     The Cryptosporidium treatment credit assigned to an
individual bag or cartridge filter is equal to the removal efficiency
established during challenge testing minus a 1.0-log factor of safety,
up to a maximum treatment credit of 2.0-log (e.g., if challenge testing
demonstrates a removal efficiency of 3.0-log or greater, the filter is
eligible to receive 2.0-log Cryptosporidium treatment credit).
     The Cryptosporidium treatment credit assigned to
configurations of two or more bag or cartridge filters operated in
series is equal to the removal efficiency established during challenge
testing minus a 0.5-log factor of safety, up to a maximum treatment
credit of 2.5-log (e.g., if challenge testing demonstrates a removal
efficiency of 3-log or greater, the filter receives 2.5-log
Cryptosporidium treatment credit).
    If a previously tested bag or cartridge filter is modified in a
manner that could change the removal efficiency of the filter product
line, a new removal efficiency must be established for the modified
filter through challenge testing. If approved by the State, data from
challenge testing conducted prior to promulgation of today's rule may
be considered in lieu of additional testing. However, the prior testing
must have been conducted in a manner that demonstrates a removal
efficiency for Cryptosporidium commensurate with the treatment credit
awarded to the filter.
b. Background and Analysis
    Bag and cartridge filters are widely used by very small PWSs and in
point-of-entry applications to remove particulate material from raw
water, including microbial pathogens like Cryptosporidium. Depending on
water quality and treatment plant infrastructure, these filters may be
used as the sole filtration step or as a polishing filter that follows
primary filtration processes. A critical aspect of bag and cartridge
filters as defined in today's rule is that they cannot undergo direct
integrity testing, which is used to detect leaks that could result in
contamination of the treated water. Cartridge filters that meet the
definition of a membrane process and can be direct integrity tested are
considered membranes under today's rule, and these are described in
section IV.D.11.
    The Stage 2 M-DBP Advisory Committee recommended Cryptosporidium
treatment credits of 1.0- and 2.0-log for bag and cartridge filters,
respectively (USEPA 2000a), and the August 11, 2003 LT2ESWTR proposal
included criteria for PWSs to receive these treatment credits. The
proposed criteria required challenge testing and the application of a
1.0-log factor of safety to establish treatment credit. In today's
final rule, EPA has modified these criteria to allow both bag and
cartridge filters to be eligible for 2.0-log credit and to allow 2.5-
log credit with a 0.5-log factor of safety for bag or cartridge filters
operated in series. The following discussion summarizes the basis for
these criteria and for differences between the proposal and today's
final rule.
    In the proposal, EPA reviewed bag and cartridge filtration studies
by Long (1983), Schaub et al. (1993), Goodrich et al. (1995), Ciardelli
(1996a and 1996b), Li et al. (1997), Roessler (1998), Enriquez et al.
(1999), NSF (2001a and 2001b), and Cornwell and LeChevallier (2002).
Results from these studies indicated that both bag and cartridge
filters exhibit variable removal efficiency, ranging from 0.5- to 3.6-
log. No correlation between the pore size rating established by the
manufacturer and the removal efficiency of the filter was apparent.
Additionally, available data did not indicate a strong relationship
between commonly used process monitoring parameters, such as turbidity
and pressure drop, and Cryptosporidium removal efficiency.
    Due to this lack of correlation between either design criteria or
process monitoring and removal efficiency, today's rule requires
challenge testing of filters to establish Cryptosporidium treatment
credit. Challenge testing must measure the removal across the filter of
Cryptosporidium or a surrogate, like polystyrene microspheres, that is
removed no more efficiently than Cryptosporidium (Long 1983, Li et al.
1997, NSF 2002b). Further, because studies have shown the removal
efficiency of some bag and cartridge filters to decrease over the
course of a filtration cycle (Li et al. 1997, NSF 2001a,b), challenge
testing must assess removal efficiency during three periods: within two
hours of startup of a new filter, between 45-55 percent of terminal
pressure drop, and at the end of the run after terminal pressure drop
is realized.
    Bag and cartridge filter challenge testing is product-specific and
not site-specific since the intent of this testing is to demonstrate
the removal capabilities of the filtration device rather than evaluate
the feasibility of implementing the technology at a specific plant.
Challenge testing must be conducted using full-scale filter elements to
assess the performance of the entire unit, including the filtration
media, seals, filter housing and other components integral to the
filtration system. To be eligible for treatment credit when operated in
series, filters must be tested in series. Multiple filters of the same
type can be tested to provide a better statistical basis for estimating
removal efficiency. The Toolbox Guidance Manual provides information on
bag and cartridge filter challenge testing.
    Today's rule establishes the proposed requirement that a 1.0-log
factor of safety be applied to the removal efficiency established
during challenge testing for individual bag or cartridge filters when
determining treatment credit. Thus, to receive a 2.0-log treatment
credit, a removal efficiency of at least 3.0-log must be demonstrated
during challenge testing. EPA believes that this factor of safety is
necessary because integrity testing with bag and cartridge filters is
not possible (note: under today's rule, cartridge filters that can be
integrity tested are classified as membranes and no safety factor is
required; see section IV.D.11).
    Challenge testing provides an estimate of the removal efficiency of
a bag or cartridge filter product line but does not involve testing
every filter. Further, it does not fully capture the variation in
filter performance that will occur over time during routine use. For
membranes, the use of direct integrity tests, such as a pressure hold
test, that is correlated to removal efficiency addresses this problem.
With bag and cartridge filters, however, EPA is aware

of no equivalent test, and parameters like turbidity and pressure
differential that may be monitored with these filters have not been
shown to correlate with Cryptosporidium removal efficiency.
Consequently, a safety factor is necessary to account for variation in
individual filter performance relative to challenge test results.
    Individual bag and cartridge filters are eligible for a maximum
Cryptosporidium treatment credit of 2.0-log. EPA proposed this level of
credit for cartridge filters but proposed a 1.0-log maximum credit for
bag filters, as recommended by the Advisory Committee. However, after
further reviewing available data, EPA has concluded that treatment
studies do not support establishing different limits on treatment
credit for bag and cartridge filters. Accordingly, today's rule treats
bag and cartridge filters equivalently. EPA continues to believe that
2.0-log is an appropriate maximum treatment credit for a single bag or
cartridge filter, based on available data on the removal of
Cryptosporidium and surrogates by these processes and the absence of a
direct integrity test.
    Today's rule also establishes criteria for awarding treatment
credit to bag or cartridge filters operated in series. EPA believes
that the use of these filters in series provides clear advantages in
comparison to operation of a single filter. Series operation will
achieve both greater removal efficiency and improved reliability by
lessening the impact of variation in the performance of a single
filter. In consideration of these factors, bag or cartridge filters
operated in series are eligible for a higher Cryptosporidium treatment
credit of 2.5-log and require a lower safety factor of 0.5-log applied
to challenge test results when determining treatment credit.
c. Summary of Major Comments
    In response to the August 11, 2003 proposal, EPA received
significant public comment on the following issues related to bag and
cartridge filtration: the allowable treatment credit, the factor of
safety applied to challenge testing results to determine treatment
credit, and the procedure for determining the removal efficiency. A
summary of these comments and EPA's responses follows.
    In regard to the proposed treatment credits, several commenters
recommended that bag and cartridge filters should be eligible for up to
2.0- and 2.5-log credit, respectively, if supported by the challenge
test results. Others commented that filters should be allowed to
qualify for removal credits at or below the 1.0- and 2.0-log credits in
the proposal. EPA agrees that additional flexibility should be provided
with respect to the removal credit awarded to bag and cartridge
filters. After reviewing these comments and reassessing data presented
in the proposal on the removal efficiencies of bag and cartridge
filters, EPA revised the proposal to allow up to 2.0-log treatment
credit for either a single bag or cartridge filter. In addition,
today's rule allows up to 2.5-log credit for bag or cartridge filters
operated in series.
    With respect to the 1.0-log safety factor applied to challenge test
results to determine treatment credit, some commenters supported this
approach, while others recommended a reduced safety factor. In
response, EPA continues to believe that a 1.0-log safety factor is
appropriate to address variability in individual filter performance and
in the absence of a direct integrity test for bag and cartridge
filters. Where filters are operated in series, however, EPA agrees that
the safety factor should be reduced. Series operation provides an
intrinsic process safety and will dampen some of the variability in
removal efficiency observed for individual filters. Thus, EPA is
reducing the factor of safety to 0.5-log for configurations consisting
of two or more filters in series.
    Commenters requested that EPA clarify the procedure used to
determine the removal efficiency of bag and cartridge filters. In
response, expanded and clarified guidance on conducting challenge tests
to determine removal efficiency for bag and cartridge filters has been
included in the Toolbox Guidance Manual.
11. Membrane Filtration
a. Today's Rule
    Today's final rule establishes criteria for awarding
Cryptosporidium treatment credit to membrane filtration processes. To
receive removal credit, filters must meet the definition of a membrane
filtration process and undergo challenge testing to establish removal
efficiency; PWSs must periodically verify system integrity through
direct integrity testing and perform continuous indirect integrity
monitoring during use. The removal credit awarded to a membrane process
is based on the removal efficiency demonstrated during challenge
testing and the sensitivity of the direct integrity test.
    For the purpose of today's rule, membrane filtration is defined as
a pressure or vacuum driven separation process in which particulate
matter larger than 1 micrometer is rejected by an engineered barrier,
primarily through a size-exclusion mechanism, and which has a
measurable removal efficiency of a target organism that can be verified
through the application of a direct integrity test.

Membrane Challenge Testing

    Any membrane filter used to meet the treatment requirements of
today's rule must undergo challenge testing to determine its
Cryptosporidium removal efficiency. Challenge testing establishes the
maximum Cryptosporidium treatment credit a membrane filtration process
is eligible to receive, provided this value is less than or equal to
the sensitivity of the direct integrity test, as described later in
this section. Challenge testing for membranes is product-specific, and
PWSs that install membranes that have successfully undergone challenge
testing are not required to repeat testing at their sites. Membrane
challenge testing must meet the following criteria:
     Challenge testing must be conducted on either an identical
full-scale module or a smaller-scale module identical in material and
similar in construction to the membrane modules the PWS will use. A
module is the smallest component of a membrane unit in which a specific
membrane surface area is housed in a device with a filtrate outlet
structure.
     Either Cryptosporidium or a surrogate that is removed no
more efficiently than Cryptosporidium must be used as the challenge
particulate during challenge testing.
     The analytical method used to measure removal in the
challenge test must discretely quantify the specific challenge
particulate. The maximum allowable feed water concentration used during
a challenge test is 6.5-log (3.16 x 10\6\) times the detection limit of
the challenge particulate in the filtrate.
     Challenge testing must be conducted under representative
hydraulic conditions at the maximum design flux and maximum design
process recovery as specified by the manufacturer for the membrane
filtration process. Flux is defined as the throughput of a pressure
driven membrane process expressed as flow per unit of membrane area.
Recovery is defined as the volumetric percent of feed water that is
converted to filtrate over the course of an operating cycle
uninterrupted by events such as chemical cleaning or a solids removal
process (i.e., backwashing).
     The removal efficiency for the membrane is determined from
the results of the challenge test, expressed as a log removal value
(LRV). A LRV must be calculated for each membrane module evaluated
during the challenge

[[Continued on page 703]]

]

[[pp. 703-752]] National Primary Drinking Water Regulations: Long Term 2 Enhanced
Surface Water Treatment Rule

[[Continued from page 702]]

test based on the feed and filtrate concentrations of the challenge
particulate for that module. The individual LRVs for each module are
used to determine the overall removal efficiency of the membrane
product. If fewer than twenty modules are tested, the overall removal
efficiency is assigned a value equal to the lowest of the
representative LRVs for the various modules tested. If twenty or more
modules are tested, then the overall removal efficiency is assigned a
value equal to the 10th percentile of the representative LRVs for the
various modules tested.
     As part of the challenge test, a quality control release
value (QCRV) must be established for a non-destructive performance test
(e.g., bubble point test, diffusive airflow test, pressure/vacuum decay
test) that demonstrates the Cryptosporidium removal capability of the
membrane module. The non-destructive performance test must be applied
to each membrane module a PWS uses in order to verify Cryptosporidium
removal capability. Membrane modules that do not meet the established
QCRV are not eligible for the Cryptosporidium removal credit
demonstrated during challenge testing.
    If a previously tested membrane product is modified in a manner
that could change the removal efficiency of the membrane or the
applicability of non-destructive performance test and associated QCRV,
the modified membrane filter must be challenge tested to establish the
removal efficiency and QCRV. If approved by the State, data from
challenge testing conducted prior to promulgation of today's rule may
be considered in lieu of additional testing. However, the prior testing
must have been conducted in a manner that demonstrates a removal
efficiency for Cryptosporidium commensurate with the treatment credit
awarded to the filter.

Membrane Direct Integrity Testing

    In order to receive Cryptosporidium treatment credit for a membrane
filtration process, PWSs must conduct direct integrity testing in a
manner that demonstrates a removal efficiency equal to or greater than
the removal credit awarded to the membrane filtration process. A direct
integrity test is defined as a physical test applied to a membrane unit
in order to identify and isolate integrity breaches (i.e., one or more
leaks that could result in contamination of the filtrate).
    Each membrane unit must be independently direct integrity tested,
where a membrane unit is defined as a group of membrane modules that
share common valving which allows the unit to be isolated from the rest
of the system for the purpose of integrity testing or other
maintenance. The direct integrity test must be applied to the physical
elements of the entire membrane unit including membranes, seals,
potting material, associated valving and piping, and all other
components which under compromised conditions could result in
contamination of the filtrate.
    Common direct integrity tests include those that apply pressure or
vacuum (such as the pressure decay test and diffusive airflow test) and
those that measure the rejection of a particulate or molecular marker
(such as spiked particle monitoring). Today's final rule does not
stipulate the use of a particular direct integrity test. Instead, the
direct integrity test must meet performance criteria for resolution,
sensitivity, and frequency.
    ``Resolution'' is defined as the smallest leak that contributes to
the response from a direct integrity test. Any direct integrity test
applied to meet the requirements of this rule must have a resolution of
3 micrometers or less. The manner in which resolution is determined
will depend on the type of direct integrity test used (i.e., pressure-
based versus marker-based tests).
    ``Sensitivity'' is defined as the maximum LRV that can be reliably
verified by the direct integrity test. The sensitivity of the direct
integrity test applied to a membrane filtration process to meet the
Cryptosporidium treatment requirements of this rule must be equal to or
greater than the removal credit awarded to the membrane filtration
process. Furthermore, the increased concentration of suspended solids
that occurs on the high pressure side of the membrane in some module
designs must be considered in the sensitivity determination (i.e., the
scouring action of some membrane designs keeps the accumulated solids
in suspension where they may pass through an integrity breach).
Specifically, the sensitivity of the direct integrity test is reduced
by a factor that quantifies the increased concentration of suspended
solids relative to the feed concentration.
    The ``frequency'' of direct integrity testing specifies how often
the test is performed over an established time interval. Direct
integrity tests available at the time of promulgation are applied
periodically and must be conducted on each membrane unit at a frequency
of not less than once per day that the unit is in operation, unless the
State determines that less frequent testing is acceptable. If
continuous direct integrity test methods become available that also
meet the sensitivity and resolution criteria described earlier, such a
continuous test may be used in lieu of periodic testing.
    PWSs must establish a direct integrity test control limit that is
indicative of an integral membrane unit capable of meeting the
Cryptosporidium removal credit awarded to the membrane. If the control
limit for the direct integrity test is exceeded, the membrane unit must
be taken off-line for diagnostic testing and repair. The membrane unit
may only be returned to service after the repair has been completed and
confirmed through the application of a direct integrity test. A monthly
report must be submitted to the State summarizing all direct integrity
test results above the control limit and the corrective action that was
taken in each case.

Continuous Indirect Integrity Monitoring

    Available direct integrity test methods are applied periodically
since the membrane unit must be taken out of service to conduct the
test. In order to provide some measure of process performance between
direct integrity testing events, PWSs must perform continuous indirect
integrity monitoring on each membrane unit. Continuous indirect
integrity monitoring is defined as monitoring some aspect of filtrate
water quality that is indicative of the removal of particulate matter
at a frequency of at least once every 15 minutes. If a continuous
direct integrity test is implemented that meets the resolution and
sensitivity criteria described previously in this section, continuous
indirect integrity monitoring is not required.
    Unless the State approves an alternative parameter, continuous
indirect integrity monitoring must include continuous filtrate
turbidity monitoring. If the filtrate turbidity readings are above 0.15
NTU for a period greater than 15 minutes, the PWS must perform direct
integrity testing on the associated membrane unit.
    If the State approves an alternate parameter for continuous
indirect integrity monitoring, the State must approve a control limit
for that parameter. If the parameter exceeds the control limit for a
period greater than 15 minutes, the PWS must perform direct integrity
testing on the associated membrane unit.
    PWSs must submit a monthly report to the State summarizing all
continuous indirect integrity monitoring results triggering direct
integrity testing and the corrective action that was taken in each
case.
    EPA has developed the Membrane Filtration Guidance Manual to assist

systems with implementation of these requirements. This guidance may be
requested from EPA's Safe Drinking Water Hotline, which may be
contacted as described under FOR FURTHER INFORMATION CONTACT in the
beginning of this notice.
b. Background and Analysis
    In the August 11, 2003 proposed LT2ESWTR, EPA proposed to establish
criteria for awarding credit to membrane filtration processes for
removal of Cryptosporidium (USEPA 2003g). The Agency based these
criteria on data demonstrating the Cryptosporidium removal efficiency
of membrane filtration processes, a critical evaluation of available
integrity monitoring techniques, and study of State approaches to the
regulation of membrane filtration for pathogen removal. This
information is summarized in the report Low-Pressure Membrane
Filtration for Pathogen Removal: Application, Implementation, and
Regulatory Issues (USEPA 2001g).
    As summarized in this report, a number of studies demonstrate the
ability of membrane filtration processes to remove pathogens, including
Cryptosporidium, to below detection levels (USEPA 2001g). In some
studies that used Cryptosporidium seeding, measured removal
efficiencies were as high as 7-log (Jacangelo, et al., 1997; Hagen,
1998; Kachalsky and Masterson, 1993). In other studies, removal
efficiencies ranged from 4.4- to 6.5-log and were only limited by the
seeded concentration of Cryptosporidium oocysts (Dwyer, et al. 1995,
Jacangelo et al. 1989, Trussel, et al. 1998, NSF 2000a-g, Olivieri
1989). Collectively, these results demonstrate that an integral
membrane module (i.e., a membrane module without any leaks or defects,
with an exclusion characteristic smaller than Cryptosporidium) is
capable of removing this pathogen to below detection in the filtrate,
independent of the influent concentration.
    The 2003 proposal included a provision for challenge testing
membranes to demonstrate the removal efficiency of Cryptosporidium. EPA
believes this requirement is necessary due to the proprietary nature of
these products and the lack of any uniform design criteria for
establishing the exclusion characteristic of a membrane. Guidance on
the design and conduct of a challenge test to meet the requirements of
this rule is presented in the Membrane Filtration Guidance Manual.
    Challenge testing is required on a product-specific basis, rather
than a site-specific basis; thus, modules used in full-scale facilities
will generally not be directly challenge tested. The removal capability
of production membrane modules is verified through the application of a
non-destructive performance test, such as a bubble point test. A
quality control release value (QCRV) for the non-destructive
performance test can be related to the results of the challenge test
and used to demonstrate the ability of production modules to achieve
the Cryptosporidium removal efficiency demonstrated during challenge
testing. Most membrane manufacturers have adapted some form of non-
destructive testing for the purpose of product quality control and have
established a QCRV that is indicative of an acceptable product. It may
be possible to apply these existing practices to meet the requirements
of today's final rule.
    While challenge testing demonstrates the removal efficiency of an
integral membrane module, defects or leaks in the membrane or other
system components can result in contamination of the filtrate unless
they are identified, isolated, and repaired. In order to verify
continued performance of a membrane system, today's final rule requires
direct integrity testing of membrane filtration processes used to meet
the Cryptosporidium treatment requirements of this rule.
    An evaluation of available direct integrity tests indicates that
pressure-based tests are widely applied and sufficiently sensitive to
provide verification of removal efficiencies in excess of 4-log.
Marker-based direct integrity tests are also available, and new direct
integrity tests may be developed that present an improvement over
existing tests. Rather than specify a particular direct integrity test,
today's final rule defines performance criteria for direct integrity
testing. These criteria are resolution, sensitivity, and frequency, as
previously described. EPA believes that this approach will provide
flexibility for the development and implementation of future
innovations in direct integrity testing while ensuring that any test
applied to meet the requirements of this rule will achieve the required
level of performance.
    Since available direct integrity tests require taking the membrane
unit out of service to conduct the test, today's rule establishes a
minimum test frequency for direct integrity testing. Currently, there
is no standard frequency for direct integrity testing that has been
adopted by all States and membrane treatment facilities. In a 2000
survey, the required frequency of integrity testing was found to vary
from once every four hours to once per week; however, the most common
frequency for conducting a direct integrity test was once every 24
hours (USEPA 2001g). Specifically, 10 out of 14 States that require
periodic direct integrity testing specify a frequency of once per day.
Furthermore, many membrane manufacturers of systems with automated
integrity test systems set up the membrane units to automatically
perform a direct integrity test once per day.
    EPA believes that daily direct integrity testing is appropriate for
most membrane filtration installations, but under some circumstances,
less frequent testing may be adequate. Thus, EPA is allowing States to
approve less frequent direct integrity testing on the basis of
demonstrated process reliability, use of multiple barriers effective
for Cryptosporidium, or reliable process safeguards.
    Due to the periodic nature of direct integrity testing, today's
rule includes a provision for continuous indirect integrity monitoring.
While indirect monitoring is not as sensitive as direct testing, it
provides an indication of process performance to ensure that a major
failure has not occurred between application of direct integrity tests.
c. Summary of Major Comments
    In response to the 2003 proposal, the Agency received significant
comments on the following issues related to membrane filtration: the
frequency of direct integrity testing; the procedure necessary to
determine removal credit for membrane filtration; and the requirement
for continuous indirect integrity monitoring.
    The 2003 proposal requested comment on the proposed minimum direct
integrity test frequency of once per day. Some commenters supported the
daily frequency and commented that many states have already adopted
this standard. Others commented that direct integrity testing once per
day is too frequent, citing the lack of data in the proposal
documenting the rate of membrane failure, as well as the loss in
production that occurs when the membrane unit is taken off-line for
testing.
    While EPA recognizes these concerns, a critical factor in
establishing a testing frequency is the amount of time that water from
a compromised membrane unit is supplied to the public before the
integrity breach is detected. EPA believes that this factor is most
important to public health protection and that daily direct integrity
testing is appropriate for the majority of membrane systems. However,
EPA also acknowledges that there may be

circumstances under which less frequent testing may provide adequate
public health protection, and has revised the rule to allow States to
permit less frequent direct integrity testing based on demonstrated
process reliability, use of multiple barriers effective for
Cryptosporidium, or reliable process safeguards.
    Several commenters expressed concern with the process needed to
determine appropriate removal credit for membrane filtration. However,
many commenters also supported the flexibility provided to States in
determining the appropriate removal credit for membrane filtration
based on the criteria defined in the 2003 proposal. EPA believes that
the proposed approach for awarding Cryptosporidium removal credit to
membrane filtration is supported by the available data and analysis,
and will allow higher removal credits to be considered on a
scientifically sound basis. EPA recognizes that the flexibility
provided in the regulation does increase the complexity of determining
removal credits for membrane filtration. To address this issue, EPA has
developed extensive guidance to support the implementation of
requirements for membrane filtration.
    EPA received comment that continuous indirect integrity monitoring
is unnecessary due to the poor sensitivity of currently available
methods. EPA acknowledges that currently available indirect monitoring
methods are less sensitive than available direct integrity tests.
However, EPA believes that continuous indirect integrity monitoring is
necessary to protect public health. Specifically, continuous monitoring
may alert a system of potentially severe integrity breaches that could
result in bypass of unfiltered water around the membrane filtration
process and pose a risk to public health. Furthermore, EPA has provided
States with the flexibility to permit use of more sensitive continuous
indirect monitoring methods and/or to establish lower control limits.
Also, implementation of continuous direct integrity testing would
preclude the need to implement any form of indirect integrity
monitoring.
12. Second Stage Filtration
a. Today's Rule
    PWSs may receive 0.5-log credit towards the Cryptosporidium
treatment requirements of today's rule for a second filtration stage.
To be eligible for this credit, the second-stage filtration must meet
the following criteria:
     The filter must be a separate second stage of granular
media filtration, such as sand, dual media, or granular activated
carbon (GAC), that follows a first stage of granular media filtration
(e.g., follows a conventional treatment or direct filtration plant).
     The first filtration stage must be preceded by a
coagulation process.
     Both filtration stages must treat 100 percent of the
treatment plant flow.
     The State must approve the treatment credit based on an
assessment of the design characteristics of the filtration process.
    This microbial toolbox option does not apply to bag filters,
cartridge filters, membranes, or slow sand filters, which are addressed
separately in the microbial toolbox. Further, this options does not
apply to roughing filters, which are pretreatment processes that
typically consist of coarse media and are not preceded by coagulation.
States may consider awarding treatment credit to roughing filters under
a demonstration of performance.
    PWSs may not receive additional treatment credit for both second-
stage filtration and lower filter effluent turbidity (i.e., combined or
individual filter performance) that is based on turbidity levels
following the second filtration stage. PWSs may receive credit for both
options based on turbidity following the first filtration stage.
b. Background and Analysis
    The Stage 2 M-DBP Advisory Committee recommended a 0.5-log
Cryptosporidium treatment credit for a roughing filter with the
stipulation that EPA identify the design and operational conditions
under which such credit is appropriate. After reviewing available data,
however, EPA was unable to determine conditions under which a roughing
filter is likely to achieve at least 0.5-log removal of
Cryptosporidium. Roughing filters consist of coarse media like gravel
and usually are not preceded by coagulation. They are used to remove
sediment and large particulate matter from raw water prior to the
primary treatment processes. EPA identified no studies indicating that
roughing filters would be effective for removal of Cryptosporidium
(USEPA 2003a).
    In contrast, numerous studies have demonstrated that granular media
filtration can be effective for removing Cryptosporidium when preceded
by coagulation (Patania et al. 1995, Nieminski and Ongerth 1995,
Ongerth and Pecoraro 1995, LeChevallier and Norton 1992, LeChevallier
et al. 1991, Dugan et al. 2001, Nieminski and Bellamy 2000, McTigue et
al. 1998, Patania et al. 1999, Huck et al. 2000, Emelko et al. 2000).
PWSs may implement a second granular media filtration stage to achieve
various water quality objectives, such as increased removal of organic
material in biologically active filters or removal of inorganic
contaminants. Consequently, EPA believes that consideration of
additional Cryptosporidium treatment credit for a second granular media
filtration stage is appropriate.
    The August 11, 2003 LT2ESWTR proposal included an additional 0.5-
log Cryptosporidium treatment credit for PWSs that use a second
separate filtration stage consisting of rapid sand, dual media, GAC, or
other fine grain media. A cap, such as GAC, on a single stage of
filtration did not qualify. In addition, the proposal required the
first stage of filtration to be preceded by a coagulation step and both
stages had to treat 100 percent of the plant flow. Today's final rule
establishes this treatment credit with minimal changes from the
proposal. The basis for this credit and for changes from the proposed
rule are summarized in the following discussion.
    While the studies of Cryptosporidium removal by granular media
filtration cited previously evaluated only a single stage of
filtration, the same removal mechanisms will be operative in a second
stage of granular media filtration. Secondary filters may remove
Cryptosporidium that were destabilized but not trapped in primary
filters or that were trapped but subsequently detached from primary
filters prior to backwash. Thus, EPA believes these studies are
supportive of additional removal credit for a second filtration stage.
    An important finding of these studies is that coagulation is
necessary to achieve significant Cryptosporidium removal by granular
media filtration (does not apply to slow sand filtration, which is
addressed in the next section). Consequently, today's rule requires
that the first filtration stage be preceded by coagulation for a PWS to
receive treatment credit for second-stage filtration. This requirement
is necessary to ensure that both filtration stages are effective for
Cryptosporidium removal. PWSs will already comply with this requirement
where a second filtration stage is applied after conventional treatment
or direct filtration.
    In the proposal, EPA also reviewed data provided by a PWS on the
removal of aerobic spores through GAC filters (i.e., contactors)
following conventional treatment. As discussed earlier, studies have
demonstrated that aerobic spores can serve as an indicator of
Cryptosporidium removal by granular

media filtration (Dugan et al. 2001, Emelko et al. 1999 and 2000, Yates
et al. 1998, Mazounie et al. 2000). Over a two year period, the mean
removal of aerobic spores across the GAC filters exceeded 0.5-log.
These results support the finding that a second stage of granular media
filtration can reduce Cryptosporidium levels by 0.5-log or greater.
    Today's rule does not establish design criteria such as filter
depth or media size for second-stage filters to be eligible for
treatment credit. While filter design will influence Cryptosporidium
removal efficiency, EPA recognizes that appropriate filter designs will
vary depending on the application. States have traditionally provided
oversight for treatment process designs in PWSs. Accordingly, today's
rule requires State review and approval of second-stage filter design
as a condition for PWSs to receive additional treatment credit for this
process. The Microbial Toolbox Guidance Manual addresses second-stage
filtration for Cryptosporidium treatment credit.
c. Summary of Major Comments
    Public comment on the August 11, 2003 LT2ESWTR proposal generally
supported additional treatment credit for second-stage filtration.
Commenters raised specific concerns with EPA establishing design
requirements for filtration, the sufficiency of data to support
prescribed treatment credit, and the expansion of this credit to
include other filtration technologies. These comments and EPA's
responses are summarized as follows.
    In the proposal, EPA requested comment on whether a minimum filter
depth should be required for PWSs to receive treatment credit for a
second filtration stage. All commenters opposed EPA setting regulatory
design standards for filters on the basis that PWSs and States need the
flexibility to determine appropriate treatment designs. In response,
EPA agrees that effective filter designs will vary depending on the
application. Consequently, EPA is not establishing filter design
criteria in today's rule, but is requiring that States approve designs
for PWSs to receive treatment credit for second-stage filtration.
    Many commenters stated that available data support the prescribed
0.5-log Cryptosporidium treatment credit for second-stage filtration.
Some commenters provided additional data on the removal of aerobic
spores through GAC filters following conventional treatment that showed
a mean reduction greater than 1-log. In contrast, other commenters were
concerned about the lack of data to support increased removal through a
second filtration stage. These commenters recommended that treatment
credit for second-stage filtration should be awarded only on a site-
specific basis through a demonstration of performance.
    EPA has concluded that available data are sufficient to support the
prescribed 0.5-log treatment credit for second-stage filtration.
Studies of granular media filtration demonstrate high levels of
Cryptosporidium removal and one study has shown greater than 1.0-log
removal through secondary GAC filters. Secondary filters can remove
Cryptosporidium that pass through or detach from the primary filters.
This added removal will help to stabilize finished water quality by
providing a barrier during periods of the filtration cycle when the
primary filters are not performing optimally. Therefore, EPA is
establishing this credit in today's rule.
    Several commenters recommended that EPA expand the second-stage
filtration option to include membranes, bag filters, and DE filtration.
EPA notes that today's rule establishes prescribed treatment credits
specifically for bag and cartridge filters and membranes as microbial
toolbox options, and prescribed credit for DE filtration is addressed
in section IV.B. PWSs may seek treatment credit for other filtration
technologies through a demonstration of performance under today's rule.
13. Slow Sand Filtration
a. Today's Rule
    PWSs may receive a 2.5-log credit towards the Cryptosporidium
treatment requirements in today's rule for implementing slow sand
filtration as a secondary filtration stage following a primary
filtration process. To be eligible for this credit, the slow sand
filtration must meet the following criteria:
     The slow sand filter must be a separate second stage of
filtration that follows a first stage of filtration like conventional
treatment or direct filtration;
     There must be no disinfectant residual in the influent
water to the slow sand filtration process;
     Both filtration stages must treat 100 percent of the
treatment plant flow from a surface water or GWUDI source; and
     The State must approve the treatment credit based on an
assessment of the design characteristics of the filtration process.
    Slow sand filtration used as a primary filtration process receives
a prescribed 3-log Cryptosporidium treatment credit, as described in
section IV.B.
b. Background and Analysis
    Slow sand filtration is a process involving passage of raw water
through a bed of sand at low velocity (generally less than 0.4 m/h),
resulting in substantial particulate removal. Several studies have
demonstrated that slow sand filtration can achieve significant
Cryptosporidium removal (Schuler and Ghosh, 1991, Timms et al. 1995,
Hall et al. 1994). Slow sand filtration is typically used as a primary
filtration process, usually in small systems, rather than as a
secondary filtration stage following conventional treatment or another
primary filtration process. EPA expects, however, that slow sand
filtration would be effective for Cryptosporidium removal in such an
application, which warrants consideration of treatment credit under
today's rule.
    The Stage 2 M-DBP Advisory Committee recommended that slow sand
filtration receive 2.5-log or greater Cryptosporidium treatment credit
when used in addition to existing treatment that achieves compliance
with the IESWTR or LT1ESWTR. The August 11, 2003 LT2ESWTR proposal
included 2.5-log treatment credit for slow sand as a secondary
filtration process, with the only associated condition being no
disinfectant residual in the water influent to the filter. In today's
rule, EPA is establishing this treatment credit with minimal changes
from the proposal. The following discussion summarizes the basis for
this credit and for changes from the proposal.
    Removal of microbial pathogens in slow sand filters is complex and
is believed to occur through a combination of physical, chemical, and
biological mechanisms, both on the surface and in the interior of the
filter bed. In particular, biological activity in the upper layers of
the filter is believed to promote microbial removal. Based on
previously cited studies demonstrating greater than 4-log removal of
Cryptosporidium through slow sand filtration, today's rule awards a
prescribed 3-log Cryptosporidium removal credit to slow sand filtration
as a primary filtration process.
    The effectiveness of slow sand as a secondary filtration process is
more uncertain. In general, EPA expects that the same microbial removal