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Biological control in cooling water systems using
nonchemical treatment devices
Scott Duda,
1
Janet E. Stout,
1,2
and Radisav Vidic
1,∗
1
Department of Civil & Environmental Engineering, University of Pittsburgh, 949 Benedum Hall, 3700 O’Hara St.,
Pittsburgh, PA 15261, USA
2
Special Pathogens Laboratory, 1401 Forbes Ave., Suite 209, Pittsburgh, PA 15219, USA
∗
Corresponding author e-mail: vidic@pitt.edu
The objective of this investigation was to provide a controlled, independent, and scientific evaluation
of several classes of nonchemical water treatment devices (NCDs) for controlling biological activity
in a model cooling tower system. Five NCDs (magnetic, pulsed electric field, electrostatic, ultrasonic,
and hydrodynamic cavitation) were evaluated for efficacy in reducing planktonic and sessile microbial
populations. Two model cooling towers were designed and operated to simulate field conditions. One tower
served as the untreated control (T1), while the NCD was installed on the second tower (T2). Each trial was
conducted over a four-week period. Heterotrophic plate counts (HPC) were used to monitor planktonic and
sessile biological growth. Make-up water for both systems was dechlorinated city tap water. No statistically
significant difference in planktonic or sessile microbial concentrations (HPC) was observed between the
control tower and the tower treated by NCD during any of the five trials. Chemical treatment of the towers
with free chlorine generated appreciable reduction in both planktonic (2–3 log) and sessile (3–4 log)
microbial growth in these towers. The results of this study conducted under well-controlled conditions
indicate that NCDs did not control biological growth under the test conditions in the pilot-scale cooling
tower systems.
Introduction
The presence of microorganisms in cooling water
systems can lead to bacterially induced corrosion,
inefficient heat transfer due to coating of surfaces
with heavy microbial growth (biofilm), and the po-
tential for disease transmission (Legionnaires’ dis-
ease). It is currently a standard practice to apply
chemical biocides to control microbial growth in
cooling water systems. The most popular of these
chemical agents is free chlorine, which is utilized
Received December 15, 2010; accepted April 21, 2011
Scott Duda, MS, is Project Manager. Janet E. Stout, PhD, Member ASHRAE, is Research Associate Professor. Radisav Vidic,
PhD, PE, is William Kepler Whiteford Faculty Professor and Chairman.
as a primary disinfectant in the United States and
throughout the world.
Mechanical or physical water treatement devices
have been marketed as an alternative to chemical
biocides that claim to offer an “environmentally
friendly” and effective method for the treatment of
cooling towers to control microbial growth. Physical
water treatment systems include magnetic, pulsed-
power, electrostatic, ultrasonic, and hydrodynamic
cavitation. Selection of a water treatment program
should be made based on objective evidence that
872
HVAC&R Research, 17(5):872–890, 2011. Copyright
C
2011 American Society of Heating, Refrigerating and Air-Conditioning Engineers,
Inc.
ISSN: 1078-9669 print / 1938-5587 online
DOI: 10.1080/10789669.2011.587588
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HVAC&R RESEARCH 873
demonstrates efficacy. Manufacturers of some non-
chemical water treatment devices (NCDs) claim
that their products are capable of controlling scal-
ing, corrosion, and microbial growth. Unfortunately,
few independent studies have been performed that
demonstrate the efficiency of NCDs (Baker and Judd
1996; Kitzman et al. 2003; Phull et al. 1997; Vega-
Mercado et al. 1997). Studies that report anecdotal
and uncontrolled observations, batch experiments
performed in highly controlled laboratory settings
that do not incorporate field conditions, and studies
conducted or supported by the device manufacturer
are less relevant than controlled prospective stud-
ies conducted under conditions that simulate typical
cooling t ower operation.
The investigation described in this report was
funded by the American Society of Heating, Refrig-
eration and Air-Conditioning Engineers (ASHRAE
Report RP-1361) with the intention of providing an
independent, controlled, and unbiased study to de-
termine if NCDs can control biological growth in
cooling t ower systems.
Materials and methods
Cooling tower system description
Two pilot-scale model cooling tower systems
were used to evaluate the performance of selected
NCDs. Each test tower was designed to include
heat load, evaporative cooling, a blowdown system,
make-up water, and to reject heat transfer r ed into
the cooling water system by a heat load from a
heat exchanger. The two model cooling towers used
in this study were designed to be identical and to
meet the following operational parameters: water
temperature entering the tower between 95
◦
F and
100
◦
F(35
◦
C and 38
◦
C), sump water temperature
between 85
◦
F and 90
◦
F(29
◦
C and 32
◦
C), tempera-
ture differential (temperature entering tower – sump
temperature) of approximately 10
◦
F(5.6
◦
C), four to
five cycles of concentration, and halogen-free make-
up water. A schematic outlining the cooling system
setup is shown in Figure 1.
In each pilot-scale system, water is stored in a
60-gal (227-L) holding tank prior to being pumped
at a rate of 7 gpm (26.5 L/min) by a 2-hp (1.5-kW)
centrifugal pump into a copper coil submerged in
a stainless-steel heating bath. Immediately prior to
entering the heating bath, the flow of water is split,
and each flow path continues into one of two 0.5-
in. (12.7-mm) outer diameter copper coils wrapped
around a 15-kW (14.2-BTU/s) immersion heater,
and the entire heating apparatus is surrounded by
a stainless-steel box containing dechlorinated wa-
ter. The box is sealed by a lid made of 0.5-in.-thick
(12.7-mm) plexiglass in order to minimize evap-
orative losses. The immersion heater is controlled
by a thermostat, which was adjusted throughout the
experimental trials to maintain a water bath temper-
ature of approximately 120
◦
F(49
◦
C). This heating
bath temperature provides enough thermal energy to
elevate the temperature of the system water to 95
◦
F
to 100
◦
F(35
◦
Cto38
◦
C)
Once the system water passes through the two
copper coils, the flow paths are combined. The
flow is then diverted through a sampling rack
containing a series of biofilm sampling coupons.
The sampling coupons were 5.61-cm
2
(0.87-in.
2
)
stainless-steel washers that were scrubbed and
autoclaved at 121
◦
F(49
◦
C) prior to installation
in the experimental towers. These coupons were
installed at the beginning of each device trial, and
they were used to quantify biofilm growth within
each of the cooling tower systems. Coupons were
installed parallel to the flow direction.
Upon exiting the sampling rack, the system flow
passes through a number of sensors for data collec-
tion, including a pH probe, an oxidation-reduction
potential (ORP) probe, a conductivity probe, and
a thermometer that records the water temperature
prior to tower entrance. Each of these probes is
connected to an automated data collection system
(AquaTrac MultiFlex 5), which records data values
at 1-h intervals. The flow then passes through a flow
meter to verify that a s ystem flow rate of 7 gpm (26.5
L/min) is maintained. Immediately before tower en-
trance, the flow travels past an additional conductiv-
ity meter. This conductivity meter is connected to
a blowdown control system that uses conductivity
readings to control when blowdown occurs based
on a user-defined set-point. The set-point is chosen
based on the make-up water conductivity, and it was
selected to maintain four to five cycles of concen-
tration in the cooling tower system.
Flow enters each of the cooling towers by way
of a 110
◦
full cone square spray nozzle. This allows
the flow to be distributed evenly over the surface
of the CF1200 packing installed in each tower. The
height of the packing in each tower is adjusted so
that the spray from the nozzle contacts the packing
at its uppermost edge, diverting the flow through
the interior of the packing rather than down the side
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874 VOLUME 17, NUMBER 5, OCTOBER 2011
Figure 1. Pilot-scale cooling system schematic.
wall of the tower. A total of three units of packing
(1 ft
3
[0.028 m
3
] each) were installed vertically in
each tower system, for a total packing height of 3 ft
(0.91 m).
Once the water has travelled through the packing,
it is deposited into a 20-gal (76-L) sump. In order to
minimize water losses from splashing, screening is
placed around the perimeter of each towers’ support
legs. Upon entering the sump, the water temperature
is 85
◦
Fto90
◦
F(29
◦
Cto32
◦
C), thereby maintain-
ing a temperature differential across the packing of
approximately 10
◦
F(6
◦
C). This cooling is accom-
plished by a variable frequency axial fan placed at
the top of the tower, above the water entrance. The
rate of airflow generated by this fan is controlled by
a potentiometer to produce the desired 10
◦
F(6
◦
C)
temperature differential. The 20-gal (76-L) sump is
connected to the 60-gal (227-L) holding tank via
a 2-in.-diameter (51-mm) PVC pipe, and as water
travels through the system, it is pulled from the sump
back into the holding tank, completing the cooling
water cycle.
Make-up water used for all experiments in this
study was dechlorinated City of Pittsburgh tap wa-
ter. Dechlorination was accomplished by passing
the water through a fixed-bed activated carbon ad-
sorber (Loret et al. 2005). The cylindrical activated
carbon adsorber had a height of 6 f t (1.8 m) and a
diameter of 12 in. (0.3 m). The column contained
8.7 gal (33 L) of activated carbon (coconut shell
based, 8 × 30 mesh size, activity = 1000), and the
flow rate through the column during make-up wa-
ter generation was maintained at or below 3 gpm
(11 L/min) in order to ensure a minimum contact
time of 3 min. Make-up water for both cooling tow-
ers was stored in four 125-gal (473-L) polyethylene
tanks to provide enough water for two days of tower
operation (approximate tank residence time = 48
h). In between device trials, the carbon column was
flushed by passing tap water at twice the flow rate
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HVAC&R RESEARCH 875
used for chlorine removal (>6 gpm [>23 L/min])
for a minimum of 1 h.
Device trial protocol
A total of five NCDs were tested over the course
of this investigation:
r
Magnetic device (MD): RT-750-K Superior Wa-
ter Conditioner
R
(Magnatech Corp., Fort Wayne,
IN, USA),
r
Pulsed electric field device (PEFD): Dolphin
Series 3000 (Clearwater Systems Corp., Essex,
CT, USA),
r
Electrostatic device (ED): FluidTron
R
(Electro-
Static Technolgies, Inc., Kansas City, KS, USA),
and
r
Ultrasonic device (UD): Sonoxide
R
B02 (Ash-
land Water Technologies, Wilmington, DE,
USA).
r
Hydrodynamic cavitation device (HCD):
VRTX-10, VRTX Technologies, Schertz, TX
For each device trial, a control tower and a test
tower were utilized. The control tower (T1) received
no treatment during the testing process, while the
tower equipped with the device (T2) received treat-
ment from the NCD being evaluated. The device
was activated at the beginning of the study, and it
was not turned off until the investigation had been
completed. For the remainder of this report, the con-
trol tower in each device trial will be referred to as
T1 (control), and the device tower will be referred
to as T2 (device). Lights in both the shower room
containing the two test towers and the locker room
containing the make-up water storage tanks were
kept on during each device trial. No algal growth
was observed in either of the towers or in the make-
up water storage tanks during any of the device trials
or chlorination tests.
Placement location of each NCD on T2 was se-
lected and verified by each device manufacturer
prior to initialization of the respective device trial.
Each device manufacturer was given an opportu-
nity to visit the testing site to verify testing con-
ditions and proper NCD placement and operation.
Site visits were made by all device manufacturers
prior to the beginning of their respective device trials
with the exception of the representative from Elec-
troStatic Technologies, Inc. (the ED). The proper
placement and operation of the ED was verified us-
ing photographs in liu of a physical site visit prior
to initialization of t he device trial.
Before each device trial, several measures were
taken to ensure consistent starting conditions. Each
tower received 4 gal (15 L) of dilute acetic acid and
8.5 oz (250 mL) of 5.25% sodium hypochlorite so-
lution, and the towers were allowed to operate for
several hours in order to eliminate residual microor-
ganisms present in the system and to remove scale
formed during the previous trial. Both towers and
their corresponding sumps and holding tanks were
scrubbed with 5% acetic acid to remove as much
scale as possible. Each system was drained com-
pletely using a shop vacuum and refilled with fresh
make-up water. The draining and refilling process
was repeated a minimum of two times for each tower
prior t o the next device trial. Additionally, the plas-
tic packing in each of the towers was replaced for
each new test. The new packing was installed after
the tower had been drained and rinsed to reduce the
amount of residual solid material that it collected.
In order to maximize the cooling potential of
the packing, each tower underwent a “seasoning”
process prior to trial onset. This process was in ac-
cordance with the packing manufacturer’s specifica-
tions. To season the packing, each tower was allowed
to operate with a heat load for approximately 1 h.
Following this period of operation, each tower sys-
tem was shut off, allowing heated water deposited
on the packing surface to evaporate and leaving a
thin layer of deposited minerals on the packing sur-
face. This process was repeated a minimum of two
times prior to the beginning of each device trial,
and the entire process occurred over approximately
three days. Each tower system was drained and re-
plenished following the final seasoning of the pack-
ing. Make-up water storage tanks were also drained
and refilled prior to the beginning of a new device
trial. Device trials began less than 24 h after the final
refilling of both the tower systems and the make-up
water storage tanks.
A number of physical, chemical, and biological
parameters were monitored during the performance
of each device trial. The parameters measured, as
well as their corresponding frequencies of measure-
ment and associated standard methods (where ap-
plicable), are shown in Table 1.
Microbiological monitoring
Bulk water samples for biological analysis were
collected using 250-mL (8.45-oz) sterile sampling
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876 VOLUME 17, NUMBER 5, OCTOBER 2011
Tabl e 1. Physical, chemical, and biological parameters analyzed during investigation.
Frequency of
Parameter Source measurement Standard method
Temperature entering tower Tower Continuous —
Sump t emperature Tower Continuous —
pH MU and tower Daily —
ORP Tower Daily —
Conductivity MU and tower Continuous Method 2510
Alkalinity MU and tower Daily Method 2320 B
TDS Tower Daily Method 2540
TDS MU Monthly Method 2540
Chloride MU and tower Weekly Ion chromatography
Chlorine MU Weekly DPD method
Calcium hardness Tower Daily Method 3111 B and Method 2340 B
Magnesium hardness Tower Daily Method 3111 B and Method 2340 B
Total hardness Tower Daily Method 3111 B and Method 2340 B
Calcium hardness MU Monthly Method 3111 B and Method 2340 B
Magnesium hardness MU Monthly Method 3111 B and Method 2340 B
Total hardness MU Monthly Method 3111 B and Method 2340 B
Planktonic HPC MU and tower Biweekly (2×/wk) Method 9215B
Sessile HPC Tower Weekly Modified Method 9215B
MU = make-up water. TDS = Total dissolved solids.
bottles containing enough sodium thiosulfate to
neutralize up to 20 ppm of chlorine. Sodium thiosul-
fate was necessary to neutralize free chlorine resid-
ual present during the chlorine disinfection tests.
All biological samples were kept chilled during
transport to the laboratory. Samples were processed
within 1 h of collection.
Upon arrival at the laboratory, samples were
shaken thoroughly and subjected to a series of di-
lutions. Dilution schemes were determined using
pre-device trial testing. A series of three dilutions
was plated for each bulk water and biofilm sam-
ple, and all dilutions were plated in duplicate. Ten-
fold dilutions were prepared by transferring 1.0 mL
(0.034 oz) of sample water to a test tube contain-
ing 9.0 mL (0.30 oz) of sterilized deionized water.
Hundred-fold dilutions were accomplished by trans-
ferring 0.1 mL (0.0034 oz) of sample water into a
test tube containing 9.9 mL (0.33 oz) of sterilized
deionized water. The range of dilutions used for
make-up water analysis was 10
−2
to 10
−4
for this
investigation, while the b ulk water tower dilution
range was 10
−3
to 10
−5
and the biofilm sample di-
lution range was 10
−4
to 10
−6
. Samples were plated
on plate count agar (PCA), and quality control spec-
imens were prepared for each set of samples using
sterile deionized water and PCA.
Samples were processed for planktonic and ses-
sile microbiological analysis according to Standard
Method 9125B (pour-plate method). For sessile mi-
crobial analysis, sampling coupons (5.61 cm
2
[0.87
in
2
]) were first swabbed to remove biofilm. The
swab and coupon were then suspended in 10 mL
(0.34 oz) of sterile deionized water. This suspension
was vortexed for 30 s prior to the performance of
Method 9215B. Microbiological results are reported
in CFU/mL (planktonic) or CFU/cm
2
(sessile).
Statistical analyses
For statistical analysis, microbiological counts
from each device trial were transformed to log 10
data. Log 10 transformed data were analyzed using
a Shapiro-Wilk test to verify that data were normally
distributed. The data sets from T1 (control) and T2
(device) were then compared using a paired t-test.
A p-value below 0.05 was considered statistically
significant.
Statistical analyses of make-up water and T1
(control) microbiological counts collected during
this investigation were performed using single-
factor ANOVA. If the ANOVA indicated a statis-
tically significant difference, the counts were then
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HVAC&R RESEARCH 877
analyzed for all possible pair-wise comparisons uti-
lizing the Bonferroni adjustment for multiple tests.
Chemical disinfection test protocols
During the course of the investigation, three
chlorine disinfection tests were performed. These
tests provided evidence that a standard disinfec-
tion method was capable of controlling microbial
growth in the experimental system operated in this
study. The selection of free chlorine as a positive
control was based on common practice in cooling
water treatment. In addition, chlorine was previ-
ously tested in a model system and was shown to
reduce microbial counts [Thomas et al. 1999]. For
each chemical disinfection test, a chlorine stock so-
lution was prepared by adding 60 mL (2.0 oz) of
5.25% sodium hypochlorite solution per gallon of
dechlorinated water, resulting in a free chlorine con-
centration of 832 ppm. Individual test protocols are
discussed later in this article.
Results
Tower operation
The reproducibility of this investigation was es-
tablished by analyzing the T1 (control) tower data
and documenting operational consistency among
different tests. This analysis was performed to verify
that each experimental device operated under sim-
ilar conditions during its trial. The make-up water
entering each of the two tower systems was tested
for similarity during each device trial. Additionally,
data were analyzed to demonstrate that control con-
ditions were essentially constant over the duration
of each NCD test.
Make-up water
Make-up water chemistry and biological activity
were monitored over the course of each trial. Bio-
logical activity in the make-up water for each of the
device trials is shown in Figure 2, and a summary of
the make-up water chemical and biological charac-
teristics is shown in Table 2. Analysis of the make-
up water chemistry and biological activity indicated
that make-up water characteristics remained consis-
tent throughout this investigation. Statistical analy-
sis of the make-up water microbiological counts via
single factor ANOVA generated a p-value of 0.252,
indicating that there was no statistically significant
difference in the make-up water heterotrophic plate
count (HPC) between device trials.
Figure 2. Average make-up water planktonic microbial populations (HPC). Error b ars represent range of observed values.
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878 VOLUME 17, NUMBER 5, OCTOBER 2011
Tabl e 2. Make-up water summary table.
Parameter Average
Standard
deviation
Conductivity, mS/cm 0.307 0.039
pH 7.33 0.22
Alkalinity, mg/L as
CaCO
3
26 5
Calcium hardness, mg/L
as CaCO
3
28 7.3
Magnesium hardness,
mg/L as CaCO
3
24 5.9
Total hardness, mg/L as
CaCO
3
52 11
TDS, mg/L 208 25
Chloride, mg/L 37 4
Free chlorine, mg/L 0.017 0.009
Copper, mg/L ND —
Iron, mg/L ND —
Sulfate, mg/L 40.7 6.3
Phosphate, mg/L 1.15 0.32
Planktonic HPC,
CFU/mL
2.66E+04 6.13E+04
ND = Not detected. TDS = Total dissolved solids.
Control tower
Because T1 (control) operated under similar
conditions during all device trials except for PEFD
trial 2/2, the data collected from T1 (control) during
each individual device trial were consolidated
and analyzed in order to determine whether the
operating conditions were comparable between the
trials. Planktonic microbial activity in T1 (control)
during each of the device trials is shown in Figure 3.
A pair-wise comparison was performed using T1
(control) planktonic HPCs from each device trial,
and the results of these statistical analyses are shown
in Table 3. These results indicate that a statistically
significant difference in the planktonic HPC was
observed only when the MD trial was compared to
the UD and HCD trials and when PEFD trial 1/2 was
compared with the HCD trial. All other pair-wise
comparisons showed no statistically significant
differences in planktonic HPC counts in the T1
(control) tower. A summary of the chemical and
biological characteristics observed in T1 (control)
over the course of the investigation is shown in
Table 4.
Figure 3. T1 (control) average planktonic microbial populations for each device trial. Error bars represent range of observed values.
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HVAC&R RESEARCH 879
Tabl e 3. T1 (control) pair-wise comparison of planktonic HPC for each device trial.
MD PEFD (1/2) PEFD (2/2) ED UD
PEFD (1/2) 1.000
PEFD (2/2) 0.079 0.713
ED 0.137 1.000 1.000
UD 0.012
∗
0.162 1.000 1.000
HCD 0.000
∗
0.002
∗
0.688 0.320 1.000
∗
Values indicate statistically significant difference between device trials.
Device trials
MD trial
Magnetic water conditioners have been applied
to reduce scaling and corrosion in industrial sys-
tems for several decades. There are no claims, how-
ever, that MDs are capable of controlling microbial
growth in cooling tower systems. These devices
function by allowing the water to pass through a
Tabl e 4. T1 (control) parameter averages.
Parameter Units
T1 (control)
average
Temperature
entering t ower
◦
F(
◦
C) 99.3(37.4)
Sump
temperature
◦
F(
◦
C) 88.3(31.3)
Daily make-up
water
consumption
gal (L) 115 (435)
Daily blowdown gal (L) 17 (64)
Temperature
differential
◦
F(
◦
C) 11 (6.1)
Conductivity mS/cm 1.174
pH None 8.64
Alkalinity mg/L as CaCO
3
113
Calcium
hardness
mg/L as CaCO
3
205
Magnesium
hardness
mg/L as CaCO
3
122
Total hardness mg/L as CaCO
3
328
TDS mg/L 853
LSI None 1.23
RSI None 6.19
PSI None 7.3
Planktonic HPC CFU/mL 6.77E+05
Sessile HPC CFU/cm
2
2.57E+06
TDS = Total dissolved solids. LSI = Langelier saturation in-
dex. RSI = Ryznar stability index. PSI = Puckorius scaling
index.
fixed magnetic field, which is purported to alter the
water chemistry to prevent the formation of “hard”
scales on cooling surfaces.
The planktonic HPCs observed in each of the
tower systems throughout the MD trial are shown
in Figure 4. This figure reveals that the HPC in T1
(control) was higher than that of T2 (device) on
seven out of the ten days for which data are avail-
able for both tower systems. However, there was less
than 1 log difference on each of these seven days.
The average planktonic HPC in T1 (control) was
1.60 × 10
6
CFU/mL with a standard deviation of
1.42 × 10
6
CFU/mL, while the average planktonic
HPC in T2 (device) was 1.04 × 10
6
CFU/mL with
a standard deviation of 6.79 × 10
5
CFU/mL. Anal-
ysis of the HPC data indicated that the observed
difference between the plate counts for T1 (con-
trol) and T2 (device) was not statistically significant
(p = 0.15). At no point during the device trial did
T2 (device) maintain a microbial population below
the commonly accepted maximum of 10
4
CFU/mL.
PEFD trial 1/2
Pulsed-power treatment, also referred to as
pulsed electric field (PEF) treatment (Rieder et al.
2008) or electropulse treatment (Danilenko et al.
2005), involves the bombardment of the water with
pulses of electromagnetic energy. Theorized mech-
anisms of microbial inactivation via PEF treatment
include electroporation (“[t]he bi-electrical break-
through of the phosphorus lipid double layer in bi-
ological membranes” caused by pulses of electro-
magnetic energy [Rieder et al. 2008, p. 2036]) and
encapsulation (agglomeration of molecules around
the microbe, leading to physical inactivation).
HPCs for each of the tower systems during the
device trial are shown in Figure 5. Although T2 (de-
vice) demonstrated lower planktonic HPC values on
seven out of nine of the biological sampling days, the
observed differences between the plate counts from
T1 (control) and T2 (device) were less than 1 log.
On each sampling day, the microbial population in
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880 VOLUME 17, NUMBER 5, OCTOBER 2011
Figure 4. Planktonic microbial populations (HPCs) for MD trial.
Figure 5. Planktonic microbial populations (HPC) for PEFD trial 1/2.
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HVAC&R RESEARCH 881
Figure 6. Planktonic microbial population enumerated by HPC for PEFD trial 2/2.
T2 (device) was higher than the commonly accepted
maximum of 10
4
CFU/mL. The average planktonic
HPC in T1 (control) was 1.23 × 10
6
CFU/mL with
a standard deviation of 1.54 × 10
6
CFU/mL, while
the average planktonic HPC in T2 (device) was 5.50
× 10
5
CFU/mL with a standard deviation of 4.63 ×
10
5
CFU/mL. Statistical analysis was not performed
using data from this PEFD trial. A comparative sta-
tistical analysis was performed using the combined
data from PEFD trials 1/2 and 2/2. The results of
this analysis are included with the results of PEFD
trial 2/2.
PEFD trial 2/2
This investigation was performed to determine
if a higher scaling index would improve biological
control in the cooling tower. The primary parame-
ter altered during the second investigation was the
blowdown conductivity set-point. In the previous
investigation of the PEFD, the blowdown conduc-
tivity set-point was chosen to establish a steady state
of four to five cycles of concentration. For the sec-
ond PEFD investigation, the set-point was raised to
establish a steady state of six to seven cycles of con-
centration. The increased cycles of concentration
were verified by total dissolved solids, conductivity,
and chloride measurements, although calcium hard-
ness and alkalinity of the make-up water decreased
during this device trial in comparison to PEFD trial
1/2.
Planktonic HPCs for each tower system over the
course of the device trial are shown in Figure 6. The
microbial population was higher in T2 (device) than
in T1 (control) on seven of the nine sampling days.
On June 20, the HPC in T2 (device) was approxi-
mately 1 log higher than in T1 (control). This was
the largest observed difference between the plank-
tonic microbial populations of the two tower sys-
tems. The average planktonic HPC in T1 (control)
was 4.12 × 10
5
CFU/mL with a standard deviation
of 5.00 × 10
5
CFU/mL, while the average plank-
tonic HPC in T2 (device) was 1.17 × 10
6
CFU/mL
with a standard deviation of 2.24 × 10
6
CFU/mL.
Statistical analysis of the combined planktonic HPC
data from PEFD trials 1/2 and 2/2 indicated that the
observed difference between the planktonic HPCs
for T1 (control) and T2 (device) was not statistically
significant (p = 0.92).
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882 VOLUME 17, NUMBER 5, OCTOBER 2011
Figure 7. Planktonic microbial population (HPC) for ED trial.
ED trial
The technology by which the ED operates is sim-
ilar in principle to that employed by the PEFD.
While the PEFD bombards the water with pulses
of electromagnetic energy, the ED exposes the wa-
ter in the reactor chamber to a fixed electrostatic
field. Theoretically, the energy increases molecu-
lar collisions between suspended particles, causing
them to form precipitates, which may easily be re-
moved from cooling systems, r ather than hard scale
on system surfaces. Hypothesized mechanisms for
the control of biological growth in the system in-
clude electroporation and encapsulation.
HPCs for each tower system over the course
of the ED trial are shown in Figure 7. The mi-
crobial population in T2 (device) was higher than
in T1 (control) on six of the nine sampling days.
Both towers maintained planktonic microbial pop-
ulations in excess of acceptable maximum levels
(10
4
CFU/mL). The average planktonic HPC in T1
(control) was 3.89 × 10
5
CFU/mL with a standard
deviation of 2.80 × 10
5
CFU/mL, while the aver-
age planktonic HPC in T2 (device) was 4.99 × 10
5
CFU/mL with a standard deviation of 3.66 × 10
5
CFU/mL. Statistical analysis of the HPCs measured
during the device trial indicated that the observed
difference between the HPCs in T1 (control) and T2
(device) was not statistically significant (p = 0.31).
UD trial
The UD operates by diverting water from the
cooling system sump or holding tank through a ven-
turi and into an ultrasonic treatment cell. Once the
flow velocity has been increased by passing through
the venturi, air is introduced into the water stream.
The water/air mixture then enters an ultrasonic treat-
ment chamber containing six ceramic transducers,
where the application of ultrasonic waves to the wa-
ter/air mixture induces cavitation. It is theorized that
the explosion of the cavitation bubbles generated
during this process is responsible for microbial de-
struction. Upon exiting the treatment cell, the water
passes through a basket filter prior to discharge back
into the cooling system sump.
The planktonic HPCs for each tower system dur-
ing this device trial are s hown in Figure 8. Each
tower system had HPCs higher than the commonly
accepted maximum of 10
4
CFU/mL. T2 (device)
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HVAC&R RESEARCH 883
Figure 8. Planktonic microbial population (HPC) for UD trial.
maintained lower microbial populations than T1
(control) on six of the nine sampling dates. The
average planktonic HPC in T1 (control) was 3.81 ×
10
5
CFU/mL with a standard deviation of 4.06 ×
10
5
CFU/mL, while the average planktonic HPC in
T2 (device) was 2.04 × 10
5
CFU/mL with a stan-
dard deviation of 1.98 × 10
5
CFU/mL. Statistical
analysis of the HPCs measured during the device
trial indicated that the observed difference between
the HPCs in T1 (control) and T2 (device) was not
statistically significant (p = 0.45).
HCD t rial
Operation of the HCD involves the diversion of
water from the cooling system sump or holding tank
into the device, where treatment is administered and
the water is returned to the sump from which it was
initially withdrawn. Water drawn from the system
sump enters a pressure-equalization chamber. The
flow of water is then split into two separate streams,
and each of these streams enters a vortex nozzle.
This nozzle creates a conical flow path for each of
the streams, and these streams are forced to collide
in a low-pressure stabilizing chamber. The collision
of these two conical streams creates a vacuum re-
gion, which results in the formation of cavitation
bubbles. The collapse of these bubbles generates lo-
cal regions of high shearing forces, temperatures,
and pressures, theoretically leading to microbial in-
activation.
Planktonic HPCs for each tower system over the
course of the HCD trial are shown in Figure 9. Nei-
ther T1 (control) nor T2 (device) maintained plank-
tonic microbial levels below the commonly accepted
maximum of 10
4
CFU/mL. The average planktonic
HPC for T1 (control) was 9.56 × 10
4
CFU/mL with
a standard deviation of 4.50 × 10
4
CFU/mL, while
the average value for T2 (device) was 1.24 × 10
5
CFU/mL with a standard deviation of 9.03 × 10
4
CFU/mL. Statistical analysis of the HPCs measured
during the HCD trial indicated that the observed
difference between the HPCs in T1 (control)
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884 VOLUME 17, NUMBER 5, OCTOBER 2011
Figure 9. Planktonic microbial population enumerated by HPC for HCD trial.
and T2 (device) was not statistically significant
(p = 0.51).
A summary of the planktonic HPC results for
each of the device trials is shown in Table 5.
Tabl e 5. Summary of planktonic HPCs, log(CFU/mL).
Standard
Device trial Tower Average deviation p
a
MD T1 (control) 6.20 6.15 0.15
T2 (device) 6.02 5.83
PEFD (1/2) T1 (control) 6.09 6.19 0.92
T2 (device) 5.74 5.67
PEFD (2/2) T1 (control) 5.62 5.7
T2 (device) 6.07 6.35
ED T1 (control) 5.59 5.45 0.31
T2 (device) 5.74 5.56
UD T1 (control) 5.58 5.61 0.45
T2 (device) 5.31 5.3
HCD T1 ( control) 4.98 4.65 0.51
T2 (device) 5.09 4.96
a
For p, statistically significant difference is demonstrated when
p < 0.05.
Sessile HPCs during device trials
In addition to planktonic HPCs, sessile HPCs
were also monitored during each device trial. Data
reflecting sessile microbial activity in the two tower
systems during each device trial are shown in Table
6. The p-values shown in this table reveal that the
difference between sessile HPCs in T1 (control) and
T2 (device) was not statistically significant during
any of the device trials. Average sessile HPCs were
higher in T2 (device) than in T1 (control) during the
MD, ED, UD, and HCD trials.
Chlorine disinfection tests
Chlorine disinfection test #1
This chlorination test was performed on both T1
(control) and T2 (device) before any of the NCD
tests. During this test, both T1 (control) and T2 (de-
vice) operated untreated from January 15, 2009, to
January 22, 2009. The chlorination process began
with a spike dose of chlorine (80 mL [2.7 oz] of
6.0% sodium hypochlorite), resulting in an initial
chlorine dose of approximately 16 ppm. Following
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HVAC&R RESEARCH 885
Tabl e 6. Sessile HPCs, log(CFU/cm
2
).
Week number
Device trial Tower 1 2 3 4 (1) 4 ( 2) Average p
a
MD T1 (control) 6.48 5.10 6.23 5.95 5.51 6.08 0.09
T2 (device) 6.46 5.48 6.36 6.56 6.63 6.43
PEFD (1/2) T1 (control) 5.48 6.03 6.95 6.40 — 6.50 0.13
T2 (device) 5.20 6.15 6.64 6.26 — 6.29
PEFD (2/2) T1 (control) 6.29 6.25 6.25 6.48 6.25 6.32
T2 (device) 6.16 6.37 6.33 6.40 6.03 6.28
ED T1 (control) 5.85 6.25 6.20 5.95 — 6.10 0.46
T2 (device) 5.85 6.20 6.20 6.33 — 6.18
UD T1 (control) 5.51 7.10 7.20 6.33 — 6.89 0.47
T2 (device) 5.57 6.73 7.68 6.51 — 7.15
HCD T1 ( control) 5.45 5.85 5.68 6.03 — 5.80 0.06
T2 (device) 6.20 6.51 6.40 6.03 — 6.32
a
For p, statistically significant difference is demonstrated when p < 0.05.
this spike dose, chlorine stock solution was pumped
into each tower system to maintain a chlorine con-
centration of 0.5 to 1.5 ppm for three days. Based
on approximate flow rates of chorine stock solu-
tion recorded throughout the disinfection test, the
average chlorine feed for each tower was estimated
at approximately 8 ppm. Chlorine residual in each
tower was maintained at approximately 1 ppm, and
the disinfectant demand for this experimental sys-
tem heavily colonized with microbial growth was
estimated at approximately 7 ppm. Similar disinfec-
tant demand was observed in chlorine disinfection
test #2 (described below).
As can be seen from Figure 10, addition of free
chlorine to a heavily colonized (>10
5
CFU/mL)
tower resulted in a 2 log reduction in the planktonic
heterotrophic bacteria population within three days
of maintaining approximately 1 ppm free chlorine
residual.
Chlorine disinfection test #2
This chlorine disinfection test was performed im-
mediately following the ED trial and before the tow-
ers were cleaned and prepared for the UD trial. Chlo-
rination was performed on T2 (device) only. The
chlorination process for this disinfection test began
on August 21 with a spike dose of chlorine (approx-
imately 16 ppm) and was followed by maintenance
of approximately 0.5 to 1.5 ppm free chlorine resid-
ual for a period of two days. As can be seen from
Figure 11, the planktonic heterotrophic population
in T2 (device) decreased by two orders of magni-
tude within 24 h of the start of the disinfection test
and was maintained below the commonly accepted
maximum of 10
4
CFU/mL. On the other hand, the
planktonic heterotrophic population i n T1 (control)
remained at 10
5
CFU/mL since this tower did not
receive any chlorine.
Chlorine disinfection test #3
This chlorine disinfection test was performed im-
mediately following the UD trial and before the
towers were cleaned and prepared for the HCD
trial. Chlorination was performed on T2 (device)
only. Contrary to the other two chlorination tests,
no shock dose of chlorine was used for this test. In-
stead, the chlorine dose in T2 (device) was gradually
increased until the residual concentration reached
1 ppm. Based on approximate chlorine flow rates
recorded throughout the disinfection test, the aver-
age initial chlorine feed for each tower was esti-
mated at approximately 11.4 ppm. Considering that
the chlorine residual in each tower was maintained
at approximately 1 ppm, the disinfectant demand
for this experimental system heavily colonized with
microbial growth can be estimated at approximately
10.4 ppm. Chlorine demand in this disinfection test
was higher than in the previous two disinfection
tests, which can be attributed to the fact that no ini-
tial shock dose of chlorine was applied during this
test.
As indicated in Figure 12, the planktonic het-
erotrophic population in T2 (device) did not de-
crease as was expected. Instead, the planktonic HPC
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886 VOLUME 17, NUMBER 5, OCTOBER 2011
Figure 10. Chlorine disinfection test #1 planktonic microbial population. The planktonic HPCs in both T1 (control) and T2 (device)
were reduced by approximately 2 log after three days of chlorination.
Figure 11. Chlorine disinfection test #2 planktonic microbial population. The planktonic HPC in T2 (device) was reduced by
approximately 2 log after three days of chlorination.
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HVAC&R RESEARCH 887
Figure 12. Chlorine disinfection test #3 planktonic microbial population. Gradual chlorination did not reduce the planktonic HPC
in T2 (device).
increased from 4.0 × 10
4
CFU/mL to 1.2 × 10
5
CFU/mL. This outcome can be due to entrainment of
biofilm from system surfaces as a result of removal
due to chlorination. This effect is often observed in
fields where an effective biocide treatment causes
release of biomass from surfaces into the recirculat-
ing water, which results in very high but transient
planktonic count (McCoy 2010). In the previous
chlorination tests, the initial shock dose of chlorine
would have effectively oxidized this material, but
the lower chlorine dose used during this particular
test was unable to effectively oxidize the majority
of this material during three days of operation. This
hypothesis is indirectly confirmed by the extremely
low sessile heterotrophic population observed in T2
(device) after three days of chlorination, as is seen
in Table 7.
Summary of chemical disinfection test
microbial results
In addition to planktonic HPCs, sessile HPCs
were also analyzed during each of the three
chemical disinfection tests. A summary of the
microbiological activity observed in each tower
system during the chlorine disinfection tests,
including planktonic and sessile microbial popu-
lations, is shown in Table 7. These results indicate
that the microbial population in the tower systems
receiving chlorine was effectively reduced by the
Tabl e 7. Chemical disinfection test HPCs planktonic, log(CFU/mL); sessile, log(CFU/cm
2
).
Chemical Before Following chlorination Following chlorination Total log
disinfection t est Tower chlorination (immediate) (end of test) reduction
#1 (planktonic) T1 (control) 5.15 3.90 3.15 2.00
T2 (device) 5.34 3.70 2.78 2.56
#1 (sessile) T1 (control) 6.29 3.85 2.30 3.99
T2 (device) 7.20 3.72 2.85 4.36
#2 (planktonic) T2 (device) 5.30 3.56 3.26 2.04
#2 (sessile) T2 (device) 6.33 2.85 3.48
#3 (planktonic) T2 (device) 4.60 5.08 −0.48
#3 (sessile) T2 (device) 6.51 3.79 2.71
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888 VOLUME 17, NUMBER 5, OCTOBER 2011
disinfectant in only a few days. A shock chlorine
dose was necessary in order to prevent entrainment
of biofilm from causing recolonization of the tower
system with planktonic bacteria.
Discussion
Two pilot-scale cooling towers were successfully
constructed and operated under relevant process
conditions (i.e., typical temperature profile, liquid
loading rate, detention time in the system, and cy-
cles of concentration) for the purposes of this study.
Both towers were operated under identical condi-
tions, and five different NCDs were evaluated for
their ability to control biological growth by a direct
comparison of planktonic and sessile populations in
untreated and treated towers.
Under the controlled experimental conditions
used in this study, none of the devices were shown to
control microbial growth. There was no statistically
significant difference in the concentration of plank-
tonic and sessile HPC bacteria observed between the
control tower and a tower treated by any of the five
NCDs evaluated in this study. The p-values calcu-
lated from paired t-tests comparing planktonic HPC
results for the experimental and control towers were
0.15, 0.92, 0.31, 0.45, and 0.51 for the MD, PEF,
ED, UD, and HCD trials, respectively. The p-values
from t-tests comparing sessile HPC results for the
experimental and control towers were 0.09, 0.13,
0.43, 0.47, and 0.06 for the MD, PEF, ED, UD, and
HCD t rials, respectively.
Repeated chemical treatment of the pilot-scale
cooling towers using an industry accepted chemical
biocide of known efficacy (chlorine) achieved sig-
nificant reduction (i.e., ≥2 log) in microbial growth
in these towers. These “positive control” experi-
ments demonstrated that the model system, when
treated with an active biocide, was capable of re-
flecting this antimicrobial effect. Chlorine feed rates
applied during these disinfection tests were typical
to actual field disinfectant application rates.
Chlorine was also selected as a chemical bio-
cide of known effiacy based on a previous investi-
gation involving controlled disinfection of a model
cooling system (Thomas et al. 1999). Thomas et
al. (1999) used a series of cross-flow cooling tower
cells and maintained a heterotrophic bacterial popu-
lation >10
6
CFU/mL after 48 h of operation without
disinfectant. In the first phase of the Thomas et al.
study, the chlorine treatment protocol (0.5–1.5 ppm
as free residual oxidant) reduced planktonic het-
erotrophic bacteria by at least two orders of mag-
nitude (99.9%) and reduced heterotrophic bacteria
in biofilms by three to four orders of magnitude
(+99.9%) compared to controls. This study demon-
strated that chlorination may be used as an effective
means of biological control when applied to a model
cooling tower system.
In addition to not being able to achieve significant
reduction in planktonic or sessile microbial activity
in the experimental device tower system (T2 (de-
vice)) when compared to the control tower system
(T1 (control)), planktonic microbial levels in the
experimental device tower (T2 (device)) were con-
sistently higher than the commonly accepted maxi-
mum of 10
4
CFU/mL. Furthermore, microbial levels
in the device tower (T2) were never lower than those
observed in the incoming make-up water, indicating
that no biological control was demonstrated by these
five NCDs under the experimental conditions used
in this study.
Microbiologic targets for the control tower (T1)
were established prior to the initialization of the
device trials. The desired microbial populations
within T1 (control) during each device trial were
10
5
CFU/mL (planktonic) and 10
7
CFU/cm
2
(ses-
sile). While the original project protocol required
the towers to be artificially seeded with microor-
ganisms should these targets not be achieved, no
additional microbial seeding of either T1 (control)
or T2 (device) was necessary during any of the de-
vice trials. The average sessile microbial population
in T1 (control) was approximately one order of mag-
nitude below the target value established prior to the
beginning of the investigation. However, no corre-
lation was observed between planktonic and sessile
microbial populations in the tower systems (Figure
2 and Table 6). As a result, microbial seeding of
the make-up water was not necessary. The T1 (con-
trol) average planktonic microbial populations for
the individual device trials were within one order
of magnitude of each other over the course of the
investigation. The lower limit of 10
5
CFU/mL was
achieved during all device trials.
Planktonic HPCs in the make-up water supplied
to both towers during each device trial (average =
2.7 × 10
4
CFU/mL) were higher than those gen-
erally observed in potable water supplies (<500
CFU/mL). The results of this investigation may in-
dicate that the evaluated NCDs are unable to provide
microbial control in the presence of high microbial
loading rates, as may be observed in cooling systems
that use nonpotable water as a make-up source or
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HVAC&R RESEARCH 889
those that are exposed to significant environmental
influences (e.g., atmospheric debris).
Data collected during this investigation do not
indicate that sessile or planktonic microbial control
was exhibited by any of the NCDs. These results dif-
fer from those reported in independently performed
in vitro studies. While batch studies have demon-
strated the effectiveness of the treatment mecha-
nisms purported by several of the device manufac-
turers, the applicability of these treatment principles
to larger-scale installations, including model cool-
ing systems and field applications, has not been veri-
fied via independent, well-controlled investigations.
A review of in vitro investigations of NCD biocidal
efficacy discussed below reveals a number of bench-
scale testing conditions that vary significantly from
conditions associated with field application.
Biocidal efficacy of PEF water treatment has
been demonstrated in several bench-scale labora-
tory investigations. Feng et al. (2004) reported high
reductions in aerobic plate counts prepared from
circulating water treated with a pulsed-power de-
vice. However, further analysis revealed that the
methods of microbial inactivation observed during
this investigation were the result of electrochem-
ical, rather than physical, interactions, indicating
that the presence of free chlorine may be neces-
sary for biocidal efficacy. The generation of free
radicals (OH
−
,ClO
−
) in water receiving PEF treat-
ment has been documented in other studies as well
(Vega-Mercado et al. 1997; Oshima et al. 1997;
Feng et al. 2000). Further batch-scale investigations
demonstrated a several log reduction of saprophytic,
sulfate-reducing, and iron bacteria in oil field injec-
tion water (Xin et al. 2008), but these reductions
required over 20 min of continuous treatment of a
very small volume of water (1 L).
UDs and HCDs have also been subject to bench-
scale analysis. In vitro investigations of ultrasonic
and hydrodynamic cavitation biocidal efficacy re-
vealed that the technology has the ability to signif-
icantly reduce microbial activity, primarily in con-
junction with other disinfectants, including hydro-
gen peroxide (Jyoti and Pandit 2003) and ozone
(Jyoti and Pandit 2004). However, these reductions
required substantial treatment times (15–60 min),
which may not be achievable in field applications. In
addition, the volume of water treated during these in-
vestigations was relatively small compared to water
volumes associated with full-scale cooling systems.
Hybrid treatment experiments combining ultrasonic
and hydrodynamic cavitation with hydrogen perox-
ide disinfection were performed using 75 L (19.8
gal) of filtered bore well water (Jyoti and Pandit
2003), while only 10 L (2.64 gal) of filtered bore
well water was used for evaluations of cavitation
treatment coupled with ozonation (Jyoti and Pan-
dit 2004). Each of these investigations revealed that
both ultrasonic and hybrid cavitation are capable
of reducing microbiological activity far more effi-
ciently in combination with oxidizing biocides in
comparison to application as stand-alone treatment
processes.
Numerous case studies and anecdotal reports
from device manufacturers are available that pro-
vide evidence of PEFD biocidal efficacy. A case
study involving an ice skating rink cooling tower
receiving PEF treatment required over two months
of continuous NCD operation before antimicrobial
effects were observed (Opheim 2001). A compara-
tive study performed on a set of cooling towers in
Goose Creek, SC, indicated that PEF and hydrody-
namic cavitation treatment provided more consistent
microbial control than chemical treatment (Kitzman
et al. 2003). One of the major flaws with this case
study, however, was the inclusion of a cyclonic sep-
arator on the NCD cooling towers. This separator
was absent from the chemically treated tower, and
during the investigation, large quantities of airborne
carbon dust from a nearby carbon silo entered the
chemically treated tower. This additional carbon was
removed from the towers treated with NCDs via cy-
clonic separation. It should also be noted that during
this investigation, none of the towers demonstrated
HPCs below the commonly accepted maximum of
10
4
CFU/mL.
A field examination of a hydrodynamic cavita-
tion treatment device installation on a cooling tower
located in an automotive test lab demonstrated the
ability of this treatment technology to maintain mi-
crobial levels below the commonly accepted maxi-
mum of 10
4
CFU/mL (Gaines et al. 2007). However,
several HPCs in excess of 10
4
CFU/mL were also
observed during this case study.
Conclusions
None of the NCDs evaluated in this study demon-
strated significant biological control in a model
cooling tower system operated under realistic pro-
cess conditions that may be encountered in the field.
However, this study still offers several opportunities
for continued investigation of NCDs. The effects of
residual chlorine in the incoming make-up water
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890 VOLUME 17, NUMBER 5, OCTOBER 2011
were not analyzed during this investigation, and the
effect of some NCDs may be augmented by the pres-
ence of chlorine. In addition, the combined effects
of chemical (e.g., oxidizing and nonoxidizing bio-
cides) and physical treatment technologies on the
control of biological growth in cooling towers may
offer significant advantages.
The results from this study show that effective
microbial control in cooling water systems may not
be achieved using a NCD as the sole method of wa-
ter treatment. Consequently, equipment operators,
building owners, and engineers should periodically
perform microbiological analyses, including plank-
tonic and sessile HPC bacteria and testing for hu-
man pathogens (Legionella), on systems that rely
on NCDs for biological control. If the testing shows
an issue, one possible measure is to apply chemical
treatment to their system.
Acknowledgment
The authors of this study thank Marilyn Wagener
for performing statistical analyses of data collected
during t his investigation.
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