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Required C × T Value for 5-log Virus Inactivation at Full
Scale
MICHAEL J. ADELMAN,1 MICHAEL PHELPS,2 ROBERT T. HADACEK,1 OLIVER R. SLOSSER,1
SIMON CALVET,1 JOAN OPPENHEIMER,1 AND JAMES H. BORCHARDT1
1MWH
Global, Pasadena, Calif.
Water District, Camarillo, Calif.
2Camrosa
The required free chlorine C × T (concentration times time) value
for 5-log virus inactivation was measured at a full-scale water
recycling plant through a suite of tracer, chlorine decay, and virusseeding tests on a baffled-channel chlorine contactor. This plant
produces fully nitrified, low-turbidity filtered effluent. MS2
bacteriophage virus was seeded into the contactor at 106 pfu/mL,
and inactivation of this virus, along with native coliform, were
measured. A consistent MS2 inactivation rate >0.28 log-L/mg-min
was observed for three different flows, consistent with other benchand pilot-scale studies. At C × T values >22 mg-min/L, the facility
complied with California recycled water disinfection requirements
by achieving >5.6-log inactivation of MS2 and reducing 104 cfu/100
mL influent total coliform to <2.2 cfu/100 mL in the effluent. These
results showed how complete nitrification and effective suspendedsolids removal allowed a full-scale plant to realize favorable
disinfection kinetics with free chlorine.
Keywords: C × T value, disinfection, free chlorine, inactivation kinetics, MS2 bacteriophage, water reuse
There is interest in determining the C × T value (the mathematical product of concentration and time) required for adequate
chlorine disinfection of recycled water at full scale. As the need
for water recycling around the world increases in response to
growing demand, limited supplies, and climatic uncertainty, the
questions of which disinfectant to use and what C × T value to
target are important to both existing facilities seeking to increase
capacity and new facilities for which a rational design basis must
be selected. Beyond the chlorination step itself, the effectiveness
of disinfection also depends on the design and operation of the
upstream processes at these facilities to produce effluent that is
readily disinfected.
Existing guidelines for recycled water disinfection are often quite
stringent. For example, water quality requirements in California
(CDPH 2014) stipulate a chlorine C × T value of 450 mg-min/L to
disinfect tertiary (i.e., filtered) recycled water, which is based on
the kinetics of chloramine disinfection (Dryden et al. 1979). Recognizing that different disinfection processes inactivate pathogens
at different rates, recycled water guidelines often allow for alternative disinfectants. For example, §60301.230.a.2 of the California
regulations (CDPH 2014) also allows an exception from the
C × T requirement of 450 mg-min/L if the disinfection process is
validated by demonstrating 5-log inactivation of a surrogate
virus. For any disinfection process, an appropriate surrogate virus
should be more resistant than the pathogenic human viruses to
the proposed disinfectant so that the measured reduction of the
surrogate will be conservative. MS2 bacteriophage is commonly
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used in disinfection studies, and it has been demonstrated as a
conservative surrogate for the specific cases of chlorination (Tree
et al. 1997) and disinfection of wastewater effluents for reuse
applications (Sigmon et al. 2015).
A variety of constituents in wastewater affect chlorination by
reacting with chlorine (e.g., ammonia) or shielding microorganisms (e.g., suspended solids), and removal of these constituents
can improve disinfection efficiency (Tchobanoglous et al. 2003).
For water recycling plants that produce fully nitrified, low-turbidity effluent with low chlorine demand, it is possible to use free
chlorine and achieve kinetics for pathogen inactivation that are
much better than those realized with chloramination (Hirani et
al. 2014). C × T values lower than the 450 mg-min/L California
regulation would allow higher flows at existing plants and smaller
footprints for the disinfection infrastructure at new plants. Selecting a C × T value appropriate to the wastewater quality also
prevents overchlorination and reduces the formation of disinfection by-products that may affect potential reuse.
In the current study, the required free chlorine C × T value for
5-log virus inactivation was measured at a full-scale water recycling facility. The goal of this research was to re-rate the chlorine
contactor at the plant to operate at lower C × T values, and this
was achieved through a suite of tracer tests, chlorine decay tests,
and virus-seeding tests. The disinfection process at this plant
initially operated at the default minimum C × T of 450 mg-min/L,
even though the plant reliably achieves nitrification and produces
low-turbidity filtered effluent. Other plants in California have
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been approved for operation at lower C × T values. For example,
at the San Jose Creek East Water Reclamation Plant in Los Angeles
County, a lower C × T value was approved on the basis of pilot
testing (Huitric et al. 2013). The novel methodological contribution of the current study was to carry out the testing in support
of re-rating the disinfection process at full scale.
MATERIALS AND METHODS
Treatment plant description. This study took place at the Camrosa
Water Reclamation Facility (CWRF) in Camarillo, Calif., which
is owned and operated by the Camrosa Water District. The liquidphase treatment train of this plant (Figure 1) consists of bar
screening, biological treatment in an oxidation ditch, secondary
clarification, media (sand) filtration, and chlorination. Because
virtually all effluent from this plant is reclaimed for crop irrigation, it is essential that the facility comply with recycled water
treatment standards. Before the current study, the plant could
meet these standards at flow rates only up to 1.5 mgd, at which
point the chlorine contact basins would provide insufficient
C × T under the default regulatory minimum. This limitation
sparked interest in studying the disinfection process for the
purposes of re-rating this portion of the plant.
The CWRF is operated with a mean cell residence time of 20
to 25 days to achieve nitrification, and the ammonia concentration of the effluent typically is below 1.0 mg/L ammonia as
nitrogen (NH3-N). The mixed liquor suspended solids concentration is maintained at between 3,500 and 4,000 mg/L, and the
dissolved oxygen in the oxidation ditch is maintained between
0.2 and 0.5 mg/L. During normal operation, chlorine is dosed
into the clarifiers (to control algal growth and meet part of the
chlorine demand), as well as into the contact basin, directly
downstream of the tertiary filters. The chlorine dose is approximately 10 mg/L in each location. Turbidity and free chlorine in
the disinfected filtered effluent are monitored by the CWRF
FIGURE 1
control system. Continuous monitoring allows plant operators
to verify that a free chlorine residual of at least 1–2 mg/L can be
maintained during the disinfection process. Variable frequency
drives on the oxidation ditch aerators provide operational flexibility to achieve fully nitrified secondary effluent by fine-tuning
the aeration achieved in the oxidation ditch.
Chlorine contactor description. The disinfection process at the
CWRF consists of two parallel, baffled contact basins rated for
a total flow of 1.5 mgd. These contactors, which predate the rest
of the CWRF, limit the overall rating of the plant; the other treatment processes at the CWRF can treat up to 3.25 mgd. Chlorine
is dosed in the form of sodium hypochlorite (12.5%) at the inlet
box, where it is blended into the filtered effluent by an electric
mixer. At the inlet box, flow is split into the two parallel basins.
In the contactor, water flows over and under a series of wooden
baffles and over an outlet weir and into a common outlet box.
The dimensions of the contactor are shown in Figure 2. The
contact basins are 10 ft wide and 110 ft long between the inlet
box and the outlet box. When the plant is operating at the current rated capacity, the water depth in the basins is 7.7 ft, which
gives a volume of 8,470 ft3 (63,360 gal). The theoretical retention time is 122 min for each basin at the current rated capacity
of 0.75 mgd (519 gpm) per basin. The baffles divide the reactor
into 20 baffle spaces, each of which has a volume of 423.5 ft3
(3,168 gal) and a theoretical contact time of 6.1 min at the current rated capacity.
The expectation that the existing contactor would be effective at
higher flow rates was attributable to a high expected baffling factor,
based on the favorable geometry to prevent short-circuiting and a
length-to-width ratio of 11:1. Long, narrow geometry and significant baffling in a reactor typically favor advection over dispersion
and allow the volume to be more fully used (Tchobanoglous et al.
2003). The specific case of a long channel with closely spaced vertical baffles allows kinetic processes to be modeled on the basis of
Process flow schematic of the CWRF, with chlorination points and disinfection analyzers
NaOCI
Influent flow
splitter
Ammonia
turbidity
NaOCI
Free chlorine
turbidity
Clarifiers
Treated
effluent
Oxidation ditches
Tertiary filters
Chlorine contactors
Headworks
Return activated sludge
Waste activated sludge
CWRF—Camrosa Water Reclamation Facility, NaOCl—sodium hypochlorite
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contact time (Weber-Shirk & Lion 2010). Residence times of at least
90% of the theoretical value have been reported in other studies of
well-baffled contactors (Rodgers & Butler 2010).
Test flow rates and flow control. Testing was conducted at four
different flow rates. The low flow rate corresponded to the current rated peak capacity (1.5 mgd), the medium flow rate corresponded to the proposed average daily capacity (2.25 mgd), and
the high flow rate corresponded to the proposed peak hourly
capacity of the chlorine contactor (3.24 mgd). A medium-high
flow provided an additional test point within the range of flow
rates that the basin was likely to experience (2.74 mgd) and was
used only in the tracer study. Table 1 presents the test flows for
the study and the corresponding theoretical contact times at the
end of the contact basins, as well as the sample points used at
each flow rate.
The plant flows listed in Table 1 were the total flows when
both contact basins were in use. For this study, only one contact
FIGURE 2
basin (south) was used at the test flow rates shown in the table.
The south basin was selected for testing on the basis of a preliminary slug test that showed that its residence time was slightly
lower than that of the north basin with both in operation. Running the tests at one-half of the proposed plant flows on the
portion of the contactor that is less effectively baffled was
conservative, because the contact time achieved by the two
basins in parallel was expected to be greater than the contact
time of the south basin alone.
To achieve consistent flow rates, a temporary pumping configuration was used (Figure 3). A temporary gate was inserted at
the entrance to the second channel (north) to isolate this channel
from the inlet box. The flow rate for each test was controlled by
a diesel-powered temporary pump, which transferred water from
the inlet of the north channel to the inlet of the south channel.
The pump discharge hose was pointed toward the channel bottom
to facilitate mixing of the first baffle space. A magnetic flow
Plan-view diagram of the existing contactor
110 ft
High baffle
N
Submerged baffle
Inlet box
10 ft
NaOCl dosing
Outlet weir
NaOCl—sodium hypochlorite
TABLE 1
Test flow rates, theoretical contact time, and sample points
Test Scenario
Plant Flow (Two Basins)
mgd (gpm)
Test Flow (One Basin)
gpm
Theoretical Contact Time
min
Sample Pointsa
2, 3, 5, 7, 12
Low flow
1.5 (1,042)
521
122
Medium flow
2.25 (1,562)
781
81
3, 5, 7, 9, 12
Medium-high flow
2.74 (1,903)
951
66
3, 6, 8, 10, 12
High flow
3.24 (2,260)
1,125
56
4, 7, 9, 11, 12
aThe
location of sample points is shown in Figure 3.
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meter1 on the inlet piping connected to the pump measured the
flow rate for the test. Varying the pump motor speed provided
coarse flow control, and an adjustable-position butterfly valve
was used to fine-tune the flow rate. The total plant flow rate
always remained above the test flow rate for the duration of each
test to ensure an adequate supply of water to the pump.
Sample points are shown in Figure 3. At each flow rate, sample
points were selected (Table 1) at theoretical residence times of
approximately 10, 20, 30, and 40 min. Samples were also taken
from the last baffle space during every test. For each flow rate,
the same sample points were used for each phase of testing.
Tracer study and chlorine decay testing. A set of tracer tests was
conducted to measure the actual modal contact time at each
sample point. The tracer tests consisted of a slug addition of
fluoride at the first baffle space of the reactor along with monitoring conducted at points downstream to measure the reactor
response. The slug of fluoride was added at the discharge point
of the temporary pump for effective mixing. Fluoride ion selective electrodes2 (ISEs) were placed at the end of the contact basin
and four intermediate locations (Table 1). The probes continuously monitored the fluoride concentration, and the data were
electronically logged.
The chlorine decay study was conducted on the filtered effluent
at three different test flows. On the basis of the average range of
chlorine doses at the CWRF, the chlorine decay tests were performed at doses of 6, 8, and 10 mg/L into the filtered effluent at
each test flow rate. Under normal operation, chlorine is also dosed
into the secondary clarifiers at a constant 10 mg/L to prevent
algae growth on the weir. Although the chlorine added at this
location does not produce a free residual, it does satisfy a portion
of the chlorine demand. Therefore, the experiments for the chlorine decay and virus seeding were conducted with this chlorine
addition at the clarifiers. During this testing, 1-L samples were
taken at five points along the contactor (Table 1) and analyzed
FIGURE 3
for free chlorine as well as temperature, pH, nitrite, nitrate,
ammonia, and ultraviolet (UV) absorbance. General weather
conditions were also recorded.
Virus-seeding study. The final phase of testing was a virusseeding study in which MS2 bacteriophage was injected into the
contactor influent as a surrogate virus. This virus is one of the
surrogate viruses identified in section §60301.230.a.2 of the
California recycled water regulations (CDPH 2014). To quantify
the rate and extent of disinfection, both MS2 virus and coliform
bacteria were monitored along the contactor during the virusseeding test.
Seed stock solution with an MS2 bacteriophage concentration
of approximately 1.0 × 1011 pfu/mL was obtained from a commercial laboratory3 for the virus-seeding studies. The MS2 solution was refrigerated on receipt at the CWRF. For all tests, the
seed stock solution was diluted 10 times to obtain an MS2 bacteriophage concentration of approximately 1.0 × 1010 pfu/mL. A
32-gal plastic bin was used to store and mix the diluted seed stock
solution. The stock was metered into the contactor with a peristaltic pump to achieve an initial concentration of 1.0 × 106 pfu/
mL. The virus was injected near the temporary pump outlet in
the first baffle space to facilitate mixing. The stock tank was
sampled at the beginning and end of each test to verify that it
maintained the target virus concentration.
Before each test, 100-mL samples of tertiary effluent were collected to evaluate the background concentration of coliform and
MS2 bacteriophage. During the test, chlorine was injected into
the contactor at a dose selected to give 1–2 mg/L free residual at
the end of the contactor, based on the results of the chlorine decay
study. During the course of testing, four samples were taken at
each of five sample points (Table 1), at least 20 min past the
measured modal contact time for that location. One 1-L sample
was analyzed immediately on site for free chlorine, pH, temperature, and UV absorbance; triplicate 100-mL sample bottles were
Temporary pump configuration and sample point locations
North contact basin
N
12
11
10
9
8
7
6
5
4
3
2
1
Meter
South contact basin (inlet closed, basin used for testing)
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TABLE 2
Summary of analytical methods
Parameter
Analytical Method
Minimum
Sample Volume
mL
Time of Analysis
(for the Current Study)
Preservatives
Fluoride
Method 9214a (USEPA 1996)
100
NA
Continuous (sensor)
BOD
Method 5210B (Standard Methods 2012)
300
NA
Within one day of collection
Alkalinity
Method 3020B (Standard Methods 2012)
200
Refrigeration
Within one day of collection
NH3-N
Method 350.1 (USEPA 1993a)
100
H2SO4
Immediately after collection
NO3-N/NO2-N
Method 353.2 (USEPA 1993b)
200
H2SO4
Immediately after collection
UV254
NA
NA
NA
Immediately after collection
Free/total chlorine
Method 334.0 (USEPA 1978)
NA
NA
Immediately after collection
Turbidity
Method 2130B (Standard Methods 2012)
NA
NA
Continuous (sensor)
pH
Method 4500-H+ (Standard Methods 2012)
NA
NA
Immediately after collection
Temperature
NA
NA
NA
Immediately after collection
TOC
Method 5310C (Standard Methods 2012)
100
H2SO4, refrigeration
Off site, within 28 days
Total coliform bacteria
Method 9222B (Standard Methods 2012)
100
Sodium thiosulfate, refrigeration
Off site, within 24 hours
MS2 bacteriophage
Method 1602 (USEPA 2001)
100
Sodium thiosulfate, refrigeration
Off site, within 24 hours
BOD—biochemical oxygen demand, H2SO4—sulfuric acid, NA—not applicable, NH3-N—ammonia-nitrogen, NO2-N—nitrite-nitrogen, NO3-N—nitrate-nitrogen, TOC—total organic carbon,
USEPA—US Environmental Protection Agency, UV254—ultraviolet absorbance at 254 nm
aModified
method to allow for continuous in situ monitoring
preserved with sodium thiosulfate pellets, cooled in ice, and sent
off site for microbial analysis. At the end of the test, the chlorine
pump was turned off with the MS2 injection still on, and triplicate 100-mL samples were taken from the first baffle space. This
allowed for an accurate measurement of the initial concentration
unaffected by chlorine.
One full virus-seeding test was run at each flow rate, and two
additional tests were started but not completed because of
problems with the setup (loss of chlorine feed or insufficient
MS2 dose). Some data from these latter tests were used before
the test was stopped. Each full virus-seeding test generated a
total of 34 sample bottles for MS2 and coliform analysis. The
34 samples comprised
•• duplicate samples from the MS2 stock tank at the beginning
of the test and duplicate samples at the end of the test, for a
total of four samples;
•• triplicate samples of the tertiary effluent for background
concentrations;
•• triplicate samples from the first baffle space (both with and
without chlorine feed) as well as from the first three intermediate sample points, for a total of 15 samples; and
•• two sets of triplicate samples from the last intermediate
sample point and the last baffle space, for a total of 12
samples (these double sets of triplicate samples were generated to confirm effective disinfection in the water leaving the
contactor).
Analytical methods. Quantification of fluoride during the
tracer testing used a modified version of Method 9214 (USEPA
1996), as previously demonstrated for full-scale tracer testing
(Loux 2011). This required calibration of the ISE with a series
of standard solutions in the expected range of the measurement
(e.g., 0 to 60 mg/L) each day that testing was performed. In the
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case of the current study, fluoride standard solutions were made
up in a matrix of tertiary wastewater from the plant so that a
calibration curve could be developed for each probe, specific to
the conditions of the CWRF and in the absence of an ionic
strength adjustment buffer.
Table 2 summarizes the analytical methods used in this study.
Some analyses were conducted continuously (with sensors). Some
analyses were performed by off-site laboratories, and the remainder were performed at the CWRF laboratory, either immediately
or within one day of collection.
RESULTS AND DISCUSSION
Measured contact time and free chlorine residual. During tracer
testing, the modal contact time (corresponding to the peak tracer
concentration) was observed to be very close to the theoretical
value at the end of the contactor for all flow rates, which indicates
effective baffling (Table 3).
In the chlorine decay testing, free chlorine was monitored along
the contactor at chlorine doses of 6, 8, and 10 mg/L. This helped
to characterize the chlorine demand and decay characteristics
TABLE 3
Summary of tracer test results
Flow
gpm
Theoretical
Contact Time
min
Modal Contact Time
min
Low flow
521
122
119
Medium flow
781
81
81
Medium-high flow
951
67
70
1,125
56
59
Test
High flow
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TABLE 4
Conditions during each virus-seeding test
Filtered Water
Average Conditions
C × T Value
NH3-N—mg/L
Turbidity—ntu
UV254
pH
Temperature—°C
Free Cl2 Residual—mg/L
Final C × T—mg-min/L
Low flow
0.06
0.73
0.13
7.9
24.5
1.1
125
Medium flow
0.04
1.21
0.14
7.7
24.5
1.8
146
High flow
0.15
0.73
0.13
7.6
25.7
2.6
153
Test
C × T—concentration times time, Cl2—chlorine, NH3-N—ammonia-nitrogen, UV254—ultraviolet absorbance at 254 nm
within the contactor under different flow conditions. The measured
chlorine demand/decay ranged from 3.8 to 8.5 mg/L, leaving a final
free residual from 0.7 to 5.3 mg/L. This test showed that an adequate free chlorine residual could be maintained in the contactor
while dosing between 6 and 10 mg/L of chlorine. This was made
possible by effective nitrification and suspended solids removal by
the CWRF. With low total organic carbon (2–4 mg/L), biochemical
oxygen demand (1–3 mg/L), ammonia (<0.04 mg/L), UV absorbance (0.09–0.15), and nitrite (<0.01 mg/L) in the filtered water,
the variability in chlorine decay was largely the result of weather
conditions and residence time. Increased sunshine led to
FIGURE 4
Virus and coliform results for high-flow test
MS2 (dosed into reactor)
Coliform (in situ)
C × T value
MS2—pfu/mL, Total Coliform—cfu/100 mL, and C × T—mg-min/L
1.0 × 107
NT
pC* = –log 
N0  
1.0 × 106
1.0 × 105
1.0 × 104
1.0 × 103
1.0 × 102
1.0 × 10
1.0
0
22
38.5
55
77
Distance Along Contactor—ft
C × T—concentration times time
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increased chlorine loss attributable to UV oxidation; longer
residence time led to increased decay because of an increased
time for kinetically limited reactions between the chlorine and
dissolved organics to progress.
Results for virus-seeding study. During the virus-seeding tests, the
chlorine contactor was monitored to quantify conditions that affect
the extent of disinfection. Table 4 presents the average temperature
and pH along the reactor during each virus-seeding test as well as
the free chlorine residual and C × T value at the end of the contactor. The low ammonia and turbidity values indicated effective
nitrification and solids removal during each test.
For each sampling point, the geometric mean was calculated
from the triplicate sampling data for MS2 bacteriophage (dosed
into the reactor) and the native total coliform. The geometric
mean gives a count of the numerical concentration of MS2 and
coliform, which allows log inactivation to be calculated as
shown in Eq 1:
110
(1)
where pC* is the log inactivation at a given point, NT is the concentration at the sample point (pfu/mL or cfu/100 mL), and N0
is the initial concentration at the first baffle space (pfu/mL or
cfu/100 mL). In addition, the C × T value at each sample point
(in units of mg-min/L) was calculated from the free chlorine
concentration measured during the test and the observed modal
contact time from the tracer study.
Figure 4 shows an example plot of the virus and coliform
results for the high-flow test. The value on the bar graph is the
geometric mean of the triplicate samples at each point, and the
error bars show the standard error. Table 5 summarizes the
observed extent of inactivation during the virus-seeding studies
at each flow rate. Following chlorine addition in each test, all
points at the end of the contactor and most intermediate points
were below the detection limit, including all 17 data points for
MS2 and 16 of 22 data points for coliform. Values in Table 5 and
subsequent tables are reported accordingly.
The contactor demonstrated compliance with the California
requirements for disinfected tertiary recycled water.
•• Log inactivation. The contactor achieved greater than 5.6log inactivation of MS2 bacteriophage virus at each flow
rate, compared with the required 5-log inactivation.
•• Coliform count. The final effluent contained <1.7 cfu/100
mL at each flow rate, which is less than 2.2 MPN/100 mL
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TABLE 5
Extent of MS2 and coliform inactivation at the end
of the contactor
Date
MS2
Inactivation
log
Coliform
Inactivation
log
Final Coliform
cfu/100 mL
Low flow
8/4/2014
>5.66
>2.78
<1.1
Medium flow
7/23/2014
>5.73
>2.89
<1.7
High flow
9/2/2014
>6.14
>3.98
<1.2
Test
TABLE 6
Observed kinetic parameters for inactivation
MS2 Virus
Total Coliform
Log
inactivation
Rate (K)
log-L/mg-min
Log
inactivation
Rate (K)
log-L/mg-min
Low flow
>5.56
>0.29
2.83
0.15
Medium
flow
>5.92
>0.29
2.56
0.13
High flow
>6.14
>0.28
4.06
0.19
Test
K—normalized kinetic parameter
FIGURE 5
MS2 inactivation versus C × T for all tests
Analytical values
Measurements below the detection limit
7
6
MS2 Inactivation—log
5
5-log inactivation
4
3
C × T = 22 mg-min/L
2
1
0
0
50
100
C × T Value—mg-min/L
C × T—concentration times time
150
Each point represents the geometric mean of triplicate samples; points
above C × T = 100 mg-min/L represent double triplicate samples.
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California standard for recycled water to be used for ediblecrop irrigation under §60301.230.b (CDPH 2014).
Disinfection kinetics. For both MS2 and coliform, a normalized kinetic parameter K was calculated between the first and
second sample point for each test. In disinfection processes,
conventional engineering practice approximates inactivation of
microorganisms as a first-order process based on Chick’s law
and assumes that the rate of disinfection is linearly proportional
to free chlorine concentration (Tchobanoglous et al. 2003). Both
of these assumptions are implicit in the C × T concept. Therefore, the kinetic parameter was normalized to chlorine concentration using Eq 2:
pC*1–2
K= 
C2 × T2 (2)
where K is the normalized kinetic parameter (log-L/mg-min),
pC*1–2 is the observed log inactivation from the first to the second
sample point (log), and C2 × T2 is the actual C × T value at the
second sample point (mg-min/L).
Table 6 shows the observed inactivation rate of MS2 and coliform for each test. Coliform levels were detectable at the second
sample point, and a relatively consistent inactivation rate was
observed for coliform in all experiments. For MS2, the fact that
the concentration was below the detection limit by the second
sample point means that only the lower limit of this kinetic
parameter can be reported (as shown in the table).
This kinetic parameter was comparable in its order of magnitude to values calculated from the literature, including pilotscale studies (Hirani et al. 2014, Mansell et al. 2008) and
bench-scale MS2 inactivation data (Coulliette et al. 2010). The
disinfection process used in the current study demonstrated the
favorable kinetics of virus inactivation that are possible with
free residual chlorine in low-turbidity (≈1 ntu) nitrified water,
compared with the much slower kinetics of combined chlorine
disinfection that provide the basis for the California recycled
water regulations. In addition, the observation that coliform
bacteria were inactivated at a slower rate than MS2 was also
consistent with expectations for free chlorine disinfection of
wastewater effluent (Mansell et al. 2008).
Minimum required C × T value. Data presented in Table 5 show
that a 5-log reduction in MS2 concentration was reached for the
three tested flows at the end of the basin. However, it should be
noted that this 5-log reduction occurred very early in the disinfection process and therefore at lower C × T values than the reductions at the end of the contactor.
The minimum required C × T must consider both MS2 inactivation and coliform concentration to meet the California regulatory requirements for disinfected tertiary recycled water. To
determine the minimum required C × T, the data from the virusseeding tests were plotted as MS2 inactivation versus C × T
(Figure 5) and total coliform count versus C × T (Figure 6),
including results from all intermediate sample points along the
basin. Each point on these figures represents the geometric mean
of triplicate samples, and points above C × T = 100 mg-min/L
represent double triplicate samples. Hollow points on these
2016 © American Water Works Association
JANUARY 2016 | 108:1
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Adelman et al. | http://dx.doi.org/10.5942/jawwa.2016.108.0001
Peer-Reviewed
FIGURE 6
Total coliform versus C × T for all tests
Analytical values
Measurements below the detection limit
12
Coliform Count—cfu/100 mL
10
C × T = 22 mg-min/L
8
6
4
2.2 cfu/100mL
2
0
0
50
100
150
C × T Value—mg-min/L
C × T—concentration times time
Each point represents the geometric mean of triplicate samples; points
above C × T = 100 mg-min/L represent double triplicate samples.
TABLE 7
Data points near proposed minimum C × T value
MS2 Virus
C × T Value
mg-min/L
Total Coliform
pC*
log
pC*
log
Count
cfu/100 mL
18.2
ND
>2.3
<1
19.3
>5.6
2.8
1.0
figures indicate analytical values, and filled points indicate
measurements that were below detection limit. Table 7 shows
the data points plotted in Figures 5 and 6 that had C × T values
around 20 mg-min/L.
On the basis of the data in Table 7, a free chlorine C × T value
of at least 22 mg-min/L is required; this C × T value is plotted
in Figures 5 and 6. Above this CT value, full compliance with
California recycled water disinfection requirements, including
the MS2 inactivation (>5 log) and total coliform count (<2.2
MPN/100 mL), was demonstrated at all three flow rates.
This minimum C × T value is consistent with what would be
calculated from the kinetic parameters in Table 6. The C × T value
for 5-log virus inactivation in this study is also similar to values
found by other studies with dispersed virus, as shown in Table 8.
Given the difficulty of measuring very low C × T values at full
scale, it is possible that the minimum required C × T may be even
lower than reported here under the conditions at the CWRF. This
would certainly be consistent with the observation of Huitric and
colleagues (2013), who found that sufficient disinfection could
be achieved by a free chlorine C × T value of only 3 mg-min/L.
It is also useful to compare these results with the familiar C × T
values used for chlorine clearwells in potable water disinfection.
Water treatment plants must meet the Surface Water Treatment
Rule requirements of 4-log inactivation of viruses and 3-log
inactivation of Giardia cysts (USEPA 2014), and they will often
achieve 2-log removal of Giardia in the filtration process. Under
the conditions in the current study (pH ≈8, temperature ≈25°C,
and free chlorine residual ≈1 mg/L), the standard disinfection
tables predict a C × T value of 2.0 mg-min/L for 4-log virus
inactivation and a C × T of 19 mg-min/L for 1-log inactivation
of Giardia cysts (USEPA 1991). The lower C × T value for virus
inactivation is consistent with the very rapid kinetics observed
by Huitric and colleagues (2013), and the governing C × T
around 20 mg-min/L represents a similar minimum operating
point as was found in this study.
19.4
>4.7
3.3
11
CONCLUSIONS
20.4
>5.9
2.6
5.3
21.6
>6.1
4.06
1.0
33.9
>6.6
4.00
2.1
The required free chlorine C × T value for 5-log inactivation of
MS2 bacteriophage was measured in a full-scale water reclamation
facility that produces fully nitrified, low-turbidity filtered effluent.
A consistent MS2 inactivation rate greater than 0.28 log-L/mg-min
was observed for three different flow rates, which is consistent
with rates observed in other bench- and pilot-scale studies. At
free chlorine C × T values of at least 22 mg-min/L, the facility
demonstrated full compliance with California recycled water
disinfection requirements for MS2 virus and total coliform.
These results demonstrated how complete nitrification during
secondary treatment and effective solids removal during tertiary
treatment allow a full-scale treatment plant to take advantage
of favorable disinfection kinetics in clear, ammonia-free water.
In such a facility, operating at lower C × T values allows for a
smaller footprint and lower capital costs to complete the disinfection process. The results of the current study can be used (along
with appropriate factors of safety) to inform the design or operating targets for free chlorine disinfection of recycled water, provided that the tertiary water is of sufficient quality.
C × T—concentration times time, ND—no data at a given point, pC*—log inactivation at a
given point
TABLE 8Reported C × T values for 5-log MS2 inactivation
Matrix
Experiment
C × T for
5-log
Inactivation
Reference
MBR effluent
Bench and pilot tests
30
Hirani et al. 2014
MBR effluent
Pilot test with simulated
fiber breakage
30
Mansell et al. 2008
Well water
Bench-scale jar tests
18
Coulliette et al. 2010
Tertiary
effluent
Pilot tests, including
episodes of high
turbidity
3
Huitric et al. 2013
C × T—concentration times time, MBR—membrane bioreactor
JOURNAL AWWA
2016 © American Water Works Association
JANUARY 2016 | 108:1
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Adelman et al. | http://dx.doi.org/10.5942/jawwa.2016.108.0001
Peer-Reviewed
ACKNOWLEDGMENT
The authors thank the many individuals who collaborated on
this study, including Melanie Holmer, Nathan Griffin, Mauricio
Gonzalez, Patrick Stahl, June Lovel, and Mary Portillo at MWH
Global; and Keith Kohr, Kevin Wahl, Graham Moland, Robert
Barone, and Terry Curson from the Camrosa Water District in
Camarillo, Calif. The authors especially thank Zakir Hirani
(now with ConocoPhillips) for his work on the test protocol,
and Richard Lin (now with Metropolitan Water District of
Southern California) for reviewing the manuscript. The Camrosa
Water District provided financial support for this work.
1WaterMaster, ABB,
Stäfa, Switzerland
Hach, Loveland, Colo.
Laboratories, Benicia, Calif.
Huitric, S.-J.; Munakata, N.; Tang, C.-C.; Kuo, J.; Ackman, P.; Friess, P.; Souza, K.; &
Barnard, R., 2013. Determining Free Chlorine Residual CT Values to Meet
California Title 22 Five-Log Virus Inactivation Requirement. Proc. Water
Environmental Federation Technical Exhibition and Conference (WEFTEC)
2013, Chicago. http://dx.doi.org/10.2175/193864713813692225.
Mansell, B.; Huitric, S.-J.; Munakata, N.; Kuo, J.; Tang, C.-C.; Ackman, P.; Friess, P.;
& Selna, M., 2008. Disinfection of Membrane Bioreactor Permeate Using
Free Chlorine: Virus Inactivation and Disinfection Byproducts Formation.
Proc. WEFTEC 2008, Chicago. http://dx.doi.org/10.2175/193864708788733846.
2IntelliCALTM,
ABOUT THE AUTHORS
Michael J. Adelman (to whom
correspondence may be addressed) is an
environmental engineer with MWH Global,
300 N. Lake Ave., Ste. 400, Pasadena, CA
91101 USA; [email protected]. He
has a BS degree from Lafayette College in
Easton, Pa., and an MS degree from Cornell
University in Ithaca, N.Y. He is interested in
the intersection of theory and practice; his process
background includes filtration, sedimentation, disinfection,
membrane treatment, ion exchange, bioremediation, and
municipal waste composting. Michael Phelps is water quality
manager for the Camrosa Water District, Camarillo, Calif.
Robert T. Hadacek is an environmental engineer, Oliver R.
Slosser is a civil engineer, and Simon Calvet is an
environmental engineer with MWH Global in Pasadena. Joan
Oppenheimer is principal environmental scientist and James
H. Borchardt is water treatment tech director at MWH
Global in Pasadena.
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JANUARY 2016 | 108:1