UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WSG71
Date Signed: February 1, 1994
MEMORANDUM
SUBJECT: Clarification of Turbidity Requirements for Filtered Systems under the Surface
Water Treatment Rule (SWTR)
FROM: James R. Elder, Director
Office of Ground Water and Drinking Water
TO: Water Management Division Directors, Regions I-X
Environmental Services Division Directors, Regions I-X
This water supply guidance clarifies three issues related to turbidity requirements for
filtered water systems under the SWTR1. Specifically, these are (1) where turbidity samples are
to be collected for filtered water systems, (2) when a system is to notify the State and the public
of high turbidity levels and anticipated turbidity violations, and (3) backwashing practices.
1. Turbidity sampling location for filtered water systems
As specified above, the SWTR requires systems that filter to measure the turbidity level
in representative samples of a system's filtered water. The rule did not specify a more precise
sample location in order to allow for differences in configuration among Public Water Systems
(PWS) . Therefore, the rule clearly places the burden on the PWS to determine where to take
'The SWTR established treatment technique requirements for Giardia lamblia. viruses, heterotrophic plate
count bacteria, Legionella. and turbidity. These requirements consist of installing and properly operating water
treatment processes that reliably achieve at least 99.9% (3-log) removal and/or inactivation of Giardia Lamblia cysts
and at least 99.99% (4 -log) removal and/or inactivation of viruses (40 CFR 141.70). Further the SWTR established
disinfection requirements in § 141.72 and filtration requirements in § 141.73. Failure to meet any of the requirements
in §141.70, §141.72, or §141.73 is a treatment technique violation.
The turbidity requirements with which filtered systems must comply are contained in §141.73:
- For all systems, the turbidity level of representative samples of filtered water must at no time exceed 5
NTU.
- For systems using conventional or direct filtration, the turbidity level of representative samples of a
system's filtered water must be less than or equal to 0.5 NTU in at least 95% of the measurements taken each month,
measured as specified in §141.74 (a) (4) and (c) (1) . The State may substitute a higher level if the State determines
that the system is capable of achieving at least 99.9% removal/inactivation of Giardia lamblia cysts at some higher
level. The State may not approve a turbidity limit that allows more than 1 NTU in more than 5% of the samples
taken each month.
- For systems using slow sand or diatomaceous earth filtration, the turbidity level of representative samples
of a system's filtered water must be less than or equal to 1 NTU in at least 95% of the measurements taken each
month, measured as specified in §141.74(a)(4) and (c)(l).
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samples and to ensure that these samples are representative of the quality of the filtered water.
Moreover, any information which the PWS has on turbidity in filtered water must be evaluated
by the PWS to determine if that is representative. If so, then these measurements should be
reported to the State.
Turbidity is an indicator of (1) filtration efficiency for removal of pathogens and other
particles, and (2) the treatability of the water by disinfection. A high turbidity level indicates the
potential for pathogen breakthrough and interference with disinfection efficiency. Unless other
circumstances in the PWS make another locations more representative, turbidity samples for
filtered water systems should be collected immediately after t the confluence of flow from all the
filters, and if possible, before the clearwell. In addition, EPA guidance (SWTR Guidance
Manual, October 1990, Section 4.3.2) recommends that systems regularly monitor the turbidity
level in the water exiting from each filter to isolate problems with individual filter performance.
Failure by a PWS to Perform this monitoring obligation properly (e.g., choosing sites-
that are not representative of filtered water quality, deliberately reporting data only from those
sampling sites that are known to be under the limits specified in the SWTR) is a violation of
requirements in the SWTR. Moreover, it-could subject the owner/operator of the PWS to
criminal action under Title 18 of the U.S. Code, that is. deliberately making false statements to
the Federal Government or State primacy agent is a federal criminal offense.
2. Notification by the PWS of high turbidity levels
The SWTR requires filtered systems to report a number of
items to the State on a monthly basis. These include turbidity measurements, as required by
§ 141.74(c)(l) and disinfection information specified in § 141.74(c)(2) and § 141.74(c)(3). The
specific items that must be reported are contained in § 141.75(b). These items must be reported
within ten days after the end of each month.
The SWTR also requires filtered systems to report three specific items to the State as
soon as possible, but no later than the end of the next business day (§141.75(W3V). These are: a
waterborne disease outbreak potentially attributable to the water system, any time the turbidity
exceeds 5 Nephelometric Turbidity Units (NTU), and any time the disinfectant residual falls
below 0.2 mg/1 in the water entering the distribution system. (In this third case, the system must
also notify the State by the end of the next business day whether or not the residual was restored
to at least 0.2 mg/1 within 4 hours.)
In addition, anytime a system fails to meet any of the requirements of the SWTR, it
incurs a treatment technique violation. This requires notification to the State in accord with
§141.31 and notification to the public in accord with §141.32. Treatment technique violations are
considered non-acute violations and therefore, public notification must be completed as soon as
possible, but in no case later than 14 days after the violation.
It is important that the standard for reporting to the State and the public be clearly
understood. In both instances, the requirement is to report as soon as possible. This means that
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the reporting must be accomplished not within the maximum timeframe allowed by the
regulations but as soon as the system is physically able to do so.
For example, if a large city exceeds 5 NTU on Tuesday at 7:00 a.m., the city should
report that to the State as soon as the State offices open. The notice to the public should be in
that evening's newspaper or, if there is no evening paper, then in the next day's paper. Note that
the city does not have until the end of the next business day to report to the State nor do they
have 14 days to issue the public notice because it is possible to accomplish both of these items in
less than the maximum time allowed by the regulations. For purposes of reporting to the State,
there are very few instances where that cannot be done almost immediately after the violation
occurs. For purposes of reporting to the public there are very few situations where this cannot
occur within 1-2 days of a violation; an obvious example would be a rural community that is
served only by a weekly newspaper.
Failure by a system to report as soon as possible, even if it is accomplished within the
time allowed by the regulations, is a violation and could subject the system to an enforcement
action.
There are sound public health reasons for requiring notification as soon as possible.
Pathogens are likely to accompany the turbidity particles that exit the filters, especially with poor
quality source waters. High turbidity levels in the filtered water, even for a limited time, may
represent a significant risk to the public. Increasing the disinfection residual in such cases is
essential, but some pathogens (e.g., Giardia and Cryptosporidium) are relatively resistant to
disinfection. States then should require the system to issue an immediate public notice of the
turbidity violation if the violation is considered to be an immediate health concern.
Moreover, if at any point during the month it becomes apparent that a system will exceed
the monthly turbidity performance standards, as specified earlier in this memorandum, or if a
system exceeds the monthly turbidity performance standards in §141.73 for an extended period
of time (e.g., more than 12 hours), the system should advise the State as soon as possible for
reasons of public health.
3_, Backwashing Practices
Systems often recycle backwash water. Unless precautions are taken, this water may
contain high pathogen densities that challenge the filter and result in a breakthrough. Several
recent waterborne disease outbreaks have been associated with questionable backwash
procedures.
In the interest of public health, systems should either run backwash waters to waste or
treat these waters before reuse. Treatment may consist of coagulation and settling, and/or
disinfection. As an additional measure, a system may also wish to monitor the source water for
Cryptosporidium and avoid recycling the backwash water when the Cryptosporidium density in
the source water exceeds a particular value (the Severn-Trent Water Authority in England uses a
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value of five oocysts/liter). For determining the Cryptosporidium density, we recommend that
systems use the analytical method specified in the upcoming monitoring and reporting
regulation, the "Information Collection Requirements", which would be used to gain additional
information for the Disinfection Byproducts Rule and the Enhanced Surface Water Treatment
Rule.
Section 4.3.2 of the SWTR Guidance Manual recommends procedures for preventing
spikes in the turbidity level in the filter effluent when the filter is placed back on line after
backwash. The section also includes filter-to-waste operating guidelines critical to the
implementation of these procedures is the continuous routine turbidity monitoring of each filter
effluent. I am attaching a copy of Section 4.3.2 of the SWTR Guidance Manual for your
information.
Should you have any questions, please contact Brian Maas at (202) 260-5530.
Attachment
cc:
Drinking Water Branch Chiefs, Regions I-X
Drinking Water Section Chiefs, Regions I-X
Enforcement Coordinators, Regions I-X
PWSS Regional Counsel Contacts, Regions I-X
Kathy Summerlee, Office of Enforcement
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ATTACHMENT
GUIDANCE MANUAL
FOR
COMPLIANCE WITH THE
FILTRATION AND DISINFECTION REQUIREMENTS
FOR
PUBLIC WATER SYSTEMS
USING
SURFACE WATER SOURCES
for
Science and Technology Branch
Criteria and Standards Division
Office of Drinking Water
U.S. Environmental Protection Agency
Washington, D.C.
Contract No. 68-01-6989
by
Malcolm Pirnie, Inc. HDR Engineering, Inc.
100 Eisenhower Drive 5175 Hillsdale Circle
Paramus, New Jersey 07653 Eldorado Hills, CA 95630
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October, 1990
4.3.2 General
The following recommendations apply to all filtration plants:
a. All filtration plants should provide continuous turbidity monitoring of the effluent
turbidity from each individual filter.3',4 If continuous monitoring is impractical,
routine monitoring of individual filters is recommended as a minimum.
b. All filtration systems should be concerned with the peak turbidity levels in the
filtered water after backwashing and make every attempt to operate the filters to
minimize the magnitude and duration of these turbidity spikes.5
Individual filters should be monitored as discussed in Section 4.3.2.a and when excessive
turbidity spikes are found, corrective actions taken. During these turbidity peaks, Giardia cysts
and other pathogens may be passed into the finished water. There is evidence that a 0.2 to 0.3
NTU increase in the turbidity during the first period of the filter run can be associated with rises
in Giardia cyst concentrations by factors of twenty to forty (Logsdon, 1985). Special studies
should be conducted to determine the extent of the turbidity spike problems.
There are basically four approaches available for correcting problems with turbidity
spikes after backwashing. These are as follows (Bucklin, et a] 1988):
- Proper chemical conditioning of the influent water to the filter can minimize the
magnitude and duration of these turbidity spikes. This could include proper
control of the primary coagulant chemicals such as alum or iron compounds. In
some cases filter aids using polymers may be needed to control the turbidity
spikes
'Although this is not a requirement of the SWTR, it is recommended because of the possibility that not all
filters in a treatment plant will produce the same effluent turbidity. This may be due to a variety of conditions that
include bed upsets, failure of media support or underdrain systems, etc. Although the combined effluent from all the
filters may meet the turbidity requirements of the SWTR, the turbidity level from an individual filter may
substantially exceed the limits. This may result in the passage of Giardia cysts or other pathogens.
Validation should be performed at least twice a week based on the procedure outlined in Part 214A in the
16th Edition of Standard Methods. It should be noted that improper installation of continuous monitors may allow
for air bubbles to enter the monitor resulting in false turbidity spikes. To avoid air bubbles reaching the turbidimeter
the sample tap should be installed below the center line of the pipe and an air release valve may be included on the
sample line.
5For most high rate granular bed filters, there is a period of conditioning, or break-in immediately following
backwashing, during which turbidity and particle removal is at a minimum, referred to as the break-in period. The
turbidity peaks are thought to be caused by remnants of backwash water within the pores of and above the media
passing through the filter, and/or floe breakup during the filter ripening period before it can adequately remove
influent turbidity.
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- Gradually increasing the filtration rate in increments when placing the filter in
operation. Starting the filter at a low flow rate and then increasing the flow in
small increments over 10 to 15 minutes has been shown to reduce the turbidity
spikes in some cases (Logsdon, 1987).
- Addition of coagulants to the backwash water has also been shown to reduce the
extent of turbidity spikes after backwash. Typically the same primary coagulant
used in the plant is added to the back-wash water. Polymers alone or in
combination with the primary coagulant may also be used.
- Filter-to-waste may be practiced where a portion of the filtered water immediately
after starting the filter is wasted. This is only possible where the filter system has
provided the necessary valves and piping to allow this procedure. There is some
concern whether or not this practice is beneficial. The extra valve operations
needed for filter-to-waste can disrupt the filter flow rate to the extent that they
create their own turbidity spikes. Some knowledge of the time actually needed
for filter-to-waste is also needed before it can be determined that this is an
effective procedure for controlling turbidity spikes. If the length of time the
filter-to-waste is practiced is less than that before the turbidity spike passes, the
disruption caused by the valve operation may actually increase the turbidity spike.
Different plants and the individual filters within the plant may have different turbidity
spike characteristics. The four approaches presented above, therefore, must be evaluated on a
case-by-case basis. Special studies will be required to identify those filters with the turbidity
spike problems and assist in selecting which of the four approaches is best for correcting the
problem. It has been generally found that turbidity spikes can be minimized through one or a
combination of the first three approaches.
In order to establish filter-to-waste operating guidelines, the following procedure is
suggested:
- Review the effluent turbidity data for each filter and determine which filter
historically has the highest effluent turbidity.
- Following backwashing of the filter with the poorest performance, place that filter
into service and collect grab samples every 5 to 10 minutes for a period of at least
60 minutes.6
- Analyze the grab samples for turbidity and determine how long the filter must be
in operation before the effluent turbidity drops
6Continuous turbidity monitoring can be used in place of grab sampling.
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- to less than or equal to 0.5 NTU
- or 1 NTU in cases where a filtered water turbidity of less than or equal to
1 NTU is allowed.
Limited information exists on the typical magnitude and duration of peak turbidity levels
after backwashing and what levels are considered acceptable to assure that these turbidity spikes
are not associated with passage of Giardia cysts. Information from plant scale tests, showing the
typical magnitude and duration of these turbidity spikes is available from two plants (Bucklin et
al.. 1988). Studies conducted at these plants over a year showed that these peaks occurred within
the first few minutes after the filter was placed back in operation, their effects lasted for several
hours, and varied in magnitude from 0.08 to 0.35 NTU on average.
For existing plants without provisions for filter-to-waste, the decision to add the
necessary piping to provide this capability should be made only after carefully evaluating the
other three approaches. If the results of special studies show that the other three options are not
effective in minimizing the turbidity spikes then the expense of adding the filter-to-waste
capabilities may be justified.
For new plants the capability of filter-to-waste may be required by the Primacy Agency
or should be considered. By having this capability, additional flexibility will be available for
turbidity spike control. This flexibility may also be useful for other filter maintenance functions
such as after media replacement or when heavy chlorination of the filter is needed after
maintenance.
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