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By R. J. Strittmatter, B Yang and D. A. Johnson, Nalco Chemical Company
Reprint
R-567
SM
Application of Ozone in
Cooling Water Systems
Presented at the National Association of Corrosion Engineers
Corrosion ‘92
Meeting
, Nashville, Tennessee,
April 27–May 1, 1992.
applications have been published. In water-short parts of
the world, this carries an obvious appeal.
Balancing these significant benefits are some equally sig-
nificant concerns:
The incompatibility of ozone with other inhibitors — Ozone
is one of the strongest oxidizing agents known. Although
little specific information has been published on the com-
patibility of ozone with industry standard corrosion and
deposit control agents, the general feeling of the industry
is that they are incompatible.
The impact of ozone on system materials of construction —
Because of its oxidizing capacity, ozone has the potential to
attack metals, wood, and elastomers used in cooling tower
construction.
The lack of mechanistic understanding of claimed proper-
ties of ozone — It has been suggested in the literature that
ozone itself can function as the sole treatment of a cooling
tower, in which case it must provide corrosion and scale
control. Case histories which have been presented are
generally inadequately documented and do not have enough
experimental control to conclusively assign these proper-
ties to the application of ozone. Many of the explanations
which have been presented are hypothetical and/or not con-
sistent with the principles of objective science. For example,
it has been asserted that it is not possible to realistically
study the properties of ozone in the laboratory. If this is
true, then it logically follows that ozone behaves in some
extra-natural manner in cooling towers.
The lack of general application guidelines — In general,
the application of ozone has been on a case-by-case, quasi-
experimental basis. To date, no body of information which
allows a potential user to unequivocally evaluate the suit-
ability of ozone to a particular situation has been made avail-
able to the public.
The purpose of this paper is to present some of the findings
of an extensive research project which has been aimed at
answering some of the above questions, with particular
emphasis on understanding the mechanistic properties
of ozone. These studies include intercorrelated labo-
ratory and field investigations. The laboratory
investigations provide the critical aspect of
ABSTRACT
The first comprehensive study — bench-top laboratory in-
vestigations, pilot scale testing, and critical monitoring and
evaluation of field applications — addressing the effects of
ozone as a stand-alone cooling water treatment program is
presented. The study also represents the first critical com-
parison of ozone-treated systems with non-treated systems.
Excellent corrosion control can be attained in ozone-treated
cooling water systems. However, the corrosion rates are com-
pletely dominated by the water chemistry of the system and
have no dependence on the presence of ozone at typical use
levels. Good control of fouling can also be attained. How-
ever, as was the case with corrosion control, deposition on
the heat exchange surfaces is not determined by the pres-
ence of ozone, but by several factors that traditionally influ-
ence fouling in a system. The strong biocidal properties of
ozone resulted in excellent microbiological control in all PCT
investigations, and in both case studies. Excellent agree-
ment was observed among all stages of testing.
INTRODUCTION
The use of ozone in cooling tower treatment has received a
great deal of attention in recent years. There are a number
of factors which give this concept a great deal of appeal to
cooling tower users. They include:
Minimal on-site chemical inventory — With the advent of
SARA Title III and other legislation, the storage and
handling of chemicals in general, and biocidal agents in
particular, is more regulated and difficult. Since ozone is
generated as it is used, these concerns are minimized.
Little or no toxicant discharge — The Clean Water Act and
state and local regulation are placing increased pressure
on cooling tower discharge into receiving streams. The high
toxicity of ozone in water solution makes it an effective
biocide. However, its rapid decomposition minimizes any
downstream toxicity concerns. The by-products of ozonation
of cooling tower water have not yet been subjected to the
same scrutiny as have the by-products of halogen applica-
tion, and are consequently not as stringently regulated.
The potential of water conservation — Numerous case
histories of the use of ozone in “zero discharge” cooling tower
2
PILOT COOLING TOWER TESTS
The pilot cooling tower (PCT) apparatus contains all the
features of a standard industrial cooling tower and related
heat exchanger system, and has been described previously.1,2
It is designed to simulate the processes in an open recircu-
lating cooling tower system as closely as possible. All PCT
tests were grouped in sets of two and run concurrently
under identical conditions with the exception that one was
treated with ozone and the other was either treated with
bromine or had no treatment. All heat exchange tubes were
either stainless steel or titanium. Unheated mild steel sur-
faces were also investigated. The conditions investigated
were typical of HVAC or light industrial cooling water
systems.
The makeup water conditions are given in Table 1. Some
tests were started at one cycle of concentration and main-
tained at 8 cycles of concentration; some tests were started
at one cycle of concentration and run under zero-blowdown
conditions (final cycles = 20 to 30), and some tests were
started at high cycles (20 to 30) and run under zero-
blowdown conditions. The recirculating and makeup
water were analyzed daily for calcium, magnesium, “M”
alkalinity, “P” alkalinity, silica, conductivity, and pH; chlo-
ride, sodium, sulfate, and nitrate were analyzed periodically.
In addition to the water chemistry analyses, total aerobic
bacteria counts were analyzed three times per week. Each
test was equipped with a Bridger Scientific DATS™ fouling
monitor, an on-line mild steel Rohrback Corrater®, and mild
steel corrosion coupons.
The ozone-treated tests were run on a PCT situated in a
hood, along with an OREC model #SP-AR 0.5 lb/day
(0.2 kg/day) ozonator. The feed gas to the ozonator is house
compressed air, which is dried and filtered prior to ozona-
tion. The ozone is injected by means of a Venturi eductor
into a sidestream loop of water drawn from the basin. The
ozonated stream is then returned to the basin. Normal ozone
dosage was based on two separate criterion: tests were run
with a targeted ozone concentration of 0.05 to 0.10 ppm
ozone immediately before the heat exchange tubes, and tests
were run with a targeted total aerobic bacteria count of
103 CFU/ml or less. Ozone concentration was usually
analyzed by the Indigo method, but the DPD method was
used for quick and approximate measurements.
Due to scale-down factors, ozonator-produced NOx artifi-
cially reduced the “M” alkalinity of the recirculating water
in the PCT tests to a much greater extent than found in
good control of important variables, such as the presence
or absence of ozone. In addition to their obvious importance,
the field investigations also provide an important measur-
ing stick for appraising the applicability of the laboratory
results. Some specific areas which are addressed are:
• The relative effects of ozone and water chemistry on cor-
rosion of mild steel. Mechanistic analysis of what is and
is not occurring.
• The effect of ozone on the corrosion of copper and brasses.
• Studies of the effectiveness of ozone in preventing scalant
precipitation and modifying crystal structures.
• Pilot cooling tower studies of the influence of ozone on
scaling and corrosion under both blowdown-limited and
zero-blowdown conditions.
• Field case histories of the application of ozone to cooling
towers under both blowdown-limited and zero-blowdown
conditions.
In order to conclusively define the properties associated
with ozone, whenever possible, performance results
obtained under identical conditions with and without ozone
will be considered.
EXPERIMENTAL PROCEDURES
LABORATORY CORROSION STUDIES
In order to independently study the effect of ozone on cor-
rosion of metals and the interactions of this effect with other
parameters, a laboratory apparatus was constructed. It
consisted of the following elements:
• A standard 0.75-liter laboratory electrochemical corro-
sion cell consisting of 0.5" cylindrical working electrodes
of the appropriate metal alloy, graphite rod or platinum
wire counter electrodes, and a Fisher saturated calomel
reference electrode with a Luggin probe. In some experi-
ments, the working electrode was rotated by a Pine
rotator to simulate fluid dynamic effects.
• A Princeton Applied Research (PAR) model 273
potentiostat, controlled by a personal computer using
the PAR 342C software system.
• A PCI model GL-1 corona discharge ozone generator
rated at 1.0 lb/day (0.4 kg/day) of ozone production. Stan-
dard laboratory compressed air was first passed through
an air preparation system (also provided by PCI) which
removed entrained oil and moisture. The ozone/air mix-
ture output by this unit was bubbled into the test cell.
Ozone residuals were measured using a colorimetric test
kit (Hach Chemical). Ozone residuals were controlled
by modulating the output of the ozone generator and/or
by bleeding off part of the ozone stream into a waste
collector.
Corrosion rates were determined from either the polariza-
tion resistance method or from Tafel extra-polations. In
either case, the PAR software was used for curve fitting
and data analysis. Appropriate Tafel constants for each sys-
tem were used to calculate corrosion rates from the linear
polarization data.
Table 1 — Makeup water for pilot cooling tower tests
Ion ppm Unit
Ca 58 CaCO3
Mg 43 CaCO3
“M” 74 CaCO3
SiO213 ion
SO448 ion
Cl 99 ion
Na 84 ion
3
typical field applications. Therefore, unless otherwise noted,
the ozone-treated tests incorporated a dilute NaOH feed to
neutralize the NOx. The caustic feed was based on the pH
and “M” alkalinity of the concurrent non-treated test.
One test set was equipped with a chemostat which sup-
plied a continuous feed of bacteria and nutrient to the tower
basin. The nutrient was a mixture of distilled water,
tryptic soy broth, and dextrose; and the bacteria culture
consisted of a typical cross-section of bacteria found in open
recirculating cooling tower systems. The basin was also
slugged with the nutrient mixture at the start of the tests
and once during the tests.
CASE HISTORY MONITORING
A complete analysis of tower and makeup water samples
was performed weekly, and the following species were
closely monitored: sodium, calcium, magnesium, potassium,
“M” alkalinity, silica, chloride, sulfate, nitrate, phosphorus,
and conductivity. Total aerobic bacteria levels were also
monitored. A Bridger Scientific DATS fouling monitor and
two Rohrback Corraters continuously collected fouling and
corrosion data, respectively. Corrosion coupons were also
used for corrosion measurements. The fouling monitor,
Corraters, and corrosion coupons were all located in the
return line. Each system was visually inspected, including
inspection of the heat exchangers.
LABORATORY CORROSION STUDIES
In order to gain independent information about the relative
corrosion effects of ozone and other factors such as water
chemistry, as well as to obtain information about operative
corrosion mechanisms, a series of laboratory corrosion tests
were conducted. Three test sequences are reported here:
1. A study of the relative effects of ozone on carbon steel
using authentic ozone-treated water samples
2. A study of the relative effects of ozone and water chem-
istry factors on carbon steel using laboratory-prepared
synthetic water
3. A series of tests on copper alloys comparing the relative
effects of ozone with other oxidizers, using a single water
chemistry
CORROSION OF CARBON STEEL IN
AUTHETIC OZONE-TREATED WATER
In the first comparison, a sample of authentic ozone-treated
water from a medium-sized tower system was obtained.
This tower was using ozone as the sole treatment and was
operating in a “zero blowdown” mode. The composition of
the makeup and tower water is shown in Table 2.
Examination of the water compositions shows that the
“cycles” of nonprecipitating species such as sodium and
potassium are much higher than those of calcium, alkalinity,
and silica, indicating that considerable precipitation had
occurred in the tower and that the water was still super-
saturated with calcium carbonate (RSI <4.5).
Corrosion experiments using carbon steel working elec-
trodes were conducted as described previously, using the
water as received in the absence of ozone, in the presence
of 0.1 ppm of ozone, and in the presence of 1.0 ppm of ozone.
The results of a series of linear polarization measurements
taken over time are shown in Table 3. At the conclusion of
each of these experiments, a Tafel scan was taken; the re-
sults of which are shown in Figure 1.
A number of conclusions may be drawn from this compari-
son. Comparing the linear polarization corrosion rates
(Table 3) obtained at comparable time periods (16.5 hours)
shows that low and comparable corrosion rates were
observed in the absence of ozone and in the presence of
0.1 ppm of ozone (1.1 mpy vs. 0.9 mpy, 0.028 mm/y vs.
0.023 mm/y). These rates were in good agreement with
coupon data obtained from the operating tower. Another
notable factor is the high and increasing carbon steel
corrosion observed in the presence of 1.0 ppm of ozone. How-
ever, the lower pH of the system in this experiment may be
the overriding factor in the higher corrosion rate result.
Table 2 — Makeup water for laboratory corrosion
test #1
Makeup Tower
Ion (typical) (authentic) Unit
Na 40–100 ppm 5100 ppm ion
Ca 60–70 490 CaCO3
Mg 50–70 1900 CaCO3
K 3–5 120 ion
SiO210–14 120 ion
“M” 70–80 40 CaCO3
Figure 1 — Tafel scans of carbon steel in authentic
ozone-treated water. Temperature = 100
°
F, rotation
speed = 500 rpm, pH = 8.7 to 8.9, air saturated.
4
The results are shown in Table 4. Examination of the data
shows that, under the conditions of the field tower discussed
in the previous section (490 ppm Ca, 402 ppm “M”), silica
does not play a significant role in determining the corro-
sion rate. Clearly, the saturation level of calcium carbonate
is the most important factor.
With regard to the effect of ozone, the data can be divided
up into three zones:
1. Under conditions of elevated alkalinity (>300 mg/l CaCO3),
ozone at typical use levels does not play any role in
determining the carbon steel corrosion rate. Under these
conditions, industry-acceptable corrosion rates are
observed (<3.0 mpy, <0.076 mm/y).
2. Under conditions of marginal alkalinity (200 to 300 mg/l
CaCO3), 0.1 ppm of ozone appears to result in improved
carbon steel corrosion rates. However, the corrosion rates
observed under these conditions with or without ozone
are greater than is normally considered acceptable by
the water treatment industry (>4.0mpy, >0.10 mm/y).
3. At low alkalinity (<200 mg/l CaCO3), low levels of ozone
gave a slight acceleration of corrosion. Corrosion rates
under these conditions are significantly greater than
industry-standard corrosion rates (>6.0 mpy, 0.15 mm/y).
COMPARISON OF OZONE WITH OTHER
OXIDIZERS ON CORROSION OF COPPER
ALLOYS
The third sequence was performed to determine the
behavior of common copper alloys in the presence of ozone
and other oxidizers under a specific water chemistry. Static
specimens of copper, admiralty brass, 70/30 cupronickel,
and 90/10 cupronickel were exposed to synthetic Lake
Examination of the open circuit potentials (Table 3) and
the Tafel scans (Figure 1) indicates that the corrosion inhi-
bition is attributable to the formation of a cathodically
induced oxygen barrier film. The low open circuit potentials
(less than –600 mV vs. SCE) and the general shape of the
anodic branch of the Tafel plots are strong evidence that
anodic passivation contributes little to corrosion control in
this system, either in the presence or absence of ozone.
Based on this data, the observed low corrosion rates are
entirely a result of the water chemistry, with ozone playing
no role. This conclusion is in contrast to previously pub-
lished hypotheses3.
EFFECTS OF OZONE vs. WATER CHEMISTRY
ON CARBON STEEL CORROSION
The first test sequence provided indications that the pre-
cipitation of mineral species was playing a dominant role
in corrosion control in ozone-treated systems. A second
series of experiments were performed using the same meth-
odology but with synthetic test solutions. In this series, the
concentrations of scale-producing solutes (calcium, alka-
linity, and silica) were varied and comparisons were made
of carbon steel corrosion rates in the presence and absence
of low levels of ozone.
Table 4 — Corrosion rates of AISI 1010 synthetic water,
100
°
F, 500 rpm
Steady state
Ca HCO3SiO2corr. rate
(ppm) (ppm) (ppm) Condition mpy(mm/y)
490 402 0 Aerated 2.2 (0.056)
490 402 120 Aerated 1.4 (0.036)
490 402 120 0.2 ppm O31.4 (0.036)
400 303 120 Aerated 3.2 (0.081)
400 303 120 0.1 ppm O32.7 (0.069)
303 215 120 Aerated 20 (0.5)
303 215 120 0.1 ppm O34.2 (0.11)
303 215 120 1 atm O24.3 (0.11)
303 215 120 1 ppm O37.2 (0.18)
200 112 120 Aerated 7.0 (0.18)
200 112 120 0.1 ppm O314 (0.36)
Figure 2 — Corrosion rates of copper and its alloys in
synthetic Lake Michigan Water.
Table 3 — Corrosion rates of mild steel in an authentic
ozone-treated tower water
Corrosion
Immersion rate Ecorr
Condition time mpy(mm/y) pH mV/SCE
Aerated 0.5 hr 5.73 (0.146) 8.85 –527
Aerated 16.5 hr 1.07 (0.027) 9.13 –594
0.1 ppm O316.5 hr 0.85 (0.022) 8.80 –604
0.1 ppm O319.0 hr 0.92 (0.023) –606
0.1 ppm O321.3 hr 0.78 (0.020) 8.70 –613
0.1 ppm O323.5 hr 0.71 (0.018) 8.79 –616
1 ppm O316.5 hr 4.51 (0.115) 7.23 –555
Michigan water in the presence of various oxidizing envi-
ronments (deaerated, aerated, 1 atmosphere oxygen,
NaOCl aerated, 0.1 ppm ozone, and 1 ppm ozone). Linear
polarization measurements of corrosion rates vs. time were
done with Tafel scans at the end of each experiment. The
data are summarized in Figures 2 through 6.
A number of conclusions are apparent from the data. In
contrast to previously published reports,4 ozone gave slight
to pronounced increases in the corrosion rates of all the
alloys versus normal levels of oxygenation (air column,
Figure 2). High (1 ppm) levels of ozone were extremely
aggressive to all the metals tested. Typical use levels of
ozone (0.1 ppm) either gave no effect (copper, 90Cu/10 Ni,
5
admiralty) or was slightly aggressive (70 Cu/30 Ni). Sodium
hypochlorite addition also typically gave slight increases in
corrosion to the yellow metals.
Lu and Duquette4 noted the elevation of the open circuit
(corrosion) potential of 70 Cu/30 Ni upon exposure to ozone.
The effect was not noted in our study. The only factor
observed to make significant changes in open circuit
potential was the dissolved oxygen level.
What was observed was an inhibition of the formation of
anodic passive regions upon the addition of ozone, particu-
larly in the case of the cupronickels (Figure 4 and Figure
5). It is this effect, rather than significant changes in open
Figure 3 — Tafel plots for copper in medium hardness
water. Temperature = 100
°
F, scan rate = 0.5 mV/sec,
static electrode, synthetic Lake Michigan water.
Figure 4 — Tafel plots for 90/10 cupronickel in
medium hardness water. Temperature = 100
°
F, scan
rate = 0.5 mV/sec, static electrode, synthetic Lake
Michigan water.
Figure 5 — Tafel plots for 70/30 cupronickel in
medium hardness water. Temperature = 100
°
F, scan
rate = 0.5 mV/sec, static electrode, synthetic Lake
Michigan water.
Figure 6 — Tafel plots for admiralty brass in medium
hardness water. Temperature = 100
°
F, scan rate = 0.5
mV/sec, static electrode, synthetic Lake Michigan
water.
circuit potential, which seems to be the determining factor
in the effect of ozone on yellow metal corrosion. The most
obvious explanation of this phenomenon is that ozone is
attacking or inhibiting the formation of a protective cuprous
oxide layer on the metal surface.
PILOT COOLING TOWER
INVESTIGATIONS
The laboratory corrosion investigations provide an easily
controlled environment, allowing determination of the
unconfounded effects of ozone on corrosion. However, in a
dynamic cooling water environment, several interdepen-
dent factors couple to determine the performance results.
Therefore, it is important to compare the results obtained
at the bench top with results obtained under open recircu-
lating cooling water system conditions. PCT investigations
include all of the important factors, while allowing the criti-
cal ability to systematically control variables.
The PCT results are divided into three major sections:
1. The effects of ozone on corrosion
2. The effects of ozone on scale formation
3. Biocidal effects of ozone
THE EFFECTS OF OZONE ON CORROSION
As was the case in the laboratory studies, the corrosion
rate of mild steel is dominated by the saturation level of
the water, and not by the presence or absence of ozone in
the system. This point is clearly illustrated by comparing
the average coupon corrosion rates of ozone-treated and
non-treated tests based on saturation level (cycles of con-
centration) during the exposure period:
Ozone-treated Non-treated
Cycles mpy (mm/yr) mpy (mm/yr)
1-8 10.9 (0.277) 10.0 (0.254)
>20 3.1 (0.079) 1.8 (0.046)
The low-cycle corrosion rates for the ozone-treated tests
and the non-treated tests are essentially equal, and are
more than three times the acceptable value. A slight differ-
ence exists between the high-cycle corrosion rates, but this
is likely due to lower pH in the ozone-treated tests (run
with no NOx neutralization). More importantly, a dramatic
decrease in corrosion rate occurs in both the ozone-treated
and non-treated tests as the saturation level of the water
is increased.
The mild steel tube corrosion rates corroborate the coupon
data. Ordering the mild steel tubes first by the initial cycles
of concentration to which they were exposed, and second
by the final cycles of concentration to which they were
exposed, clearly illustrates the relationship between satu-
ration level of recirculating water and corrosion rate (Fig-
ure 7). The low-cycle tests have the highest corrosion rates,
while the high-cycle tests have extremely low corrosion
rates. The difference between the corrosion rates in the non-
treated and ozone-treated tests for the first set of tests is
large; however, both values are well above accepted indus-
try standards. As the cycles increase, and the corrosion rates
become more acceptable, the difference between the ozone-
treated and non-treated tests becomes insignificant.
THE EFFECTS OF OZONE ON SCALE
FORMATION
Scale formation in the PCT tests was determined by sev-
eral factors that traditionally influence deposition on heat
exchange surfaces, including water chemistry, nucleation
sites, system dynamics, and skin temperature, but was not
directly influenced by the presence of ozone in the system.
(Scale formation was indirectly influenced due to the bio-
cidal effects of ozone, which will be addressed in the next
section.) The deposit rates on the heat exchange tubes
exhibited clear and dramatic responses to changes in the
test variables, ranging from excellent deposit control to 35
times the acceptable value, but the observed trends in the
deposit rates were identical in the ozone-treated tests and
the non-ozone-treated tests.
Water Chemistry
Conventional chemical treatment programs prevent scal-
ing and deposition by dispersion of mineral scale or by com-
pletely inhibiting precipitation of mineral scale. Therefore,
the most common and most convenient method for moni-
toring and evaluating system performance in terms of scale
formation is water chemistry analyses. If 100% transport
is maintained in the system, i.e., all ions are accounted for
in the water analysis, then the heat transfer surface re-
mains free of mineral scale.
Ozone alone clearly does not prevent nor promote precipi-
tation of mineral scales. No difference in the precipitation
of CaCO3 is observed between ozone-treated systems and
Figure 7 — A comparison of ozone-treated and non-
treated mild steel PCT tube corrosion rates as a
function of tower water saturation level.
6
non-treated systems, as demonstrated by a representative
plot of calcium, “M” alkalinity, and conductivity cycles of
concentration for equivalent non-treated and ozone-treated
tests (Figure 8). The agreement between the concentration
ranges of Ca and “M” for the two tests is remarkable:
Ozone-treated Non-treated
Actual Ca 99–233 ppm 107–210 ppm
Theoretical Ca 450 ppm 450 ppm
% Transport 22–52 24–47
Actual “M” 114–166 ppm 114–170 ppm
Theoretical“M” 570 ppm 570 ppm
% Transport 20–29 20–30
The results presented here are in close agreement with pre-
vious observations reported in the literature.5
Crystal Growth and Formation of “Sandy”
CaCO3
One theory for the prevention of scale in ozone-treated sys-
tems is that ozone changes the crystal morphology of the
precipitated CaCO3, rendering it less adherent to heat ex-
change surfaces and causing it to settle in the basin.6 A
common assertion is the appearance of a CaCO3 “sand” in
the basin of ozone-treated systems.
Off-white “sandy” deposits have been found in the basin of
some of the PCT tests; however, the deposits were found in
both the ozone-treated tests and the non-treated tests. The
chemical composition of a typical deposit was 54% calcium,
45% carbonate, and 1% silicon, in accord with that expected
from the water analyses. The formation of a basin deposit
under certain conditions, both with and without ozone,
indicates that this phenomenon is due to system dynamics
and not related to ozone.
To directly analyze the effects of ozone on the crystal mor-
phology of CaCO3, a scanning electron microscope (SEM)
study was performed on CaCO3 crystals formed in both a
synthetic water (low organic levels) and a natural water
(typical organic levels). No modification of the crystal
morphology occurs in the ozone-treated tests relative to the
non-treated tests, as demonstrated by magnification at
2000X of the CaCO3 crystals from natural water tests
(Figure 9). Similar results were observed in analogous
investigations on actual PCT tube deposits, which were
formed on heat exchange surfaces under recirculating
water conditions.
Figure 8 — Calcium, “M” alkalinity, and conductivity
cycles of concentration for equivalent non-treated and
ozone-treated tests.
7
Figure 9 — SEM photographs of CaCO3 crystals
formed in a natural water, (top) non-treated and
(bottom) ozone-treated.
Nucleation
A key requirement for crystallization from solution of a
material directly on site of scale formation is nucleation.7
Heterogeneous nucleation, the deposition of solute on a pre-
existing substrate, requires less energy than homogeneous
nucleation, deposition which does not require the presence
of a foreign substance. Cooling water systems inherently
contain numerous nucleating sites such as the walls of pipes
and the tower fill. Also, foreign substances such as dust,
gas bubbles, and microorganisms, and factors such as
vibration and agitation cause nucleation.8 Therefore,
heterogeneous nucleation is the predominant scale deposi-
tion mechanism in cooling water systems.
Under severely precipitating conditions, as is the case in
the systems in question, nucleation has a profound influ-
ence on the location of scale formation. At the PCT scale,
the presence, quantity, and location of nucleation sites can
be manipulated to simulate different conditions. For
example, the surface of the heat exchange tubes can be
varied, and/or the presence of nucleation sites on the tower
fill and in the basin can be varied.
The importance of mineral scale nucleation sites away from
the heat exchange tubes was clearly demonstrated by the
PCT tests. Tests run with “clean” (nucleation sites absent)
heat exchange tubes and “dirty” (nucleation sites present)
tower fill and basin, resulted in all tubes being free of depo-
sition after 14 days of operation, despite CaCO3 precipita-
tion commencing on day five. Similar tests run with com-
pletely “clean” systems (nucleation sites absent from heat
exchange tubes, tower fill, and basin), resulted in unac-
ceptable deposit rates for at least four of the eight tubes,
some tubes having deposit rates greater than ten times the
acceptable value. These nucleation site effects caused simi-
lar results in both the ozone-treated and the non-treated
systems, and serve to demonstrate that scale formation can
be highly system dependent.
Ozone vs. Bromine
In order to investigate the direct effects of ozone on scale
information, all difficult-to-control variables which may
influence deposit rates and fouling need to be removed from
the experiment. This entails excluding any easily corroded
metallurgy (e.g., mild steel), and ensuring that both the
ozone-treated system and the control is free of microbio-
logical contamination, i.e., the control is treated with an
effective biocide.
Comparison of a representative bromine-treated test with
an identical ozone-treated test, both of which incorporated
only stainless steel heat exchangers, clearly demonstrates
the similarity in scale control between the two treatments.
Under a variety of heat fluxes and skin temperatures, the
eight-tube averaged, stainless steel deposit rates were nearly
identical, with the bromine-treated test averaging 86.0 mg/
cm2/yr and the ozone-treated test averaging 82.6 mg/cm2/
yr, both being greater than eight times the acceptable value
(criterion for successful control is <10mg/cm2/yr)2. In addi-
tion, the distribution of deposit rates relative to skin tem-
perature and heat flux was extremely similar between the
two tests, in spite of its two order of magnitude range.
Real-time monitoring of fouling in the systems strongly sup-
ports the weight measurement results (Figure 10). Both
the ozone- and bromine-treated tests show a dramatic in-
crease in fouling after a comparable amount of elapsed time,
followed by a leveling of fouling at a nearly equivalent point.
The sharp increase in fouling observed correlates well with
the water chemistry analyses — calcium and “M” alkalin-
ity were no longer in balance with the more soluble ions
after three days in the bromine-treated test, and after four
days in the ozone-treated test. The slight time difference
between initialization of fouling is readily accounted for by
the slightly lower “M” alkalinity in the ozone-treated tests at
any given time, caused by the necessary lag-time for proper
NOx neutralization.
BIOCIDAL EFFECTS OF OZONE
Excellent microbiological control was observed in all ozone-
treated tests, as measured by total aerobic bacteria counts.
As expected, a significant difference in microbiological
activity was observed between the ozone-treated and non-
treated tests (Figure 11). The ozone-treated tests had total
aerobic bacteria counts of 103 CFU/ml or less, while the
non-treated tests had counts of 104 to 106 CFU/ml. The
difference in bacteria counts is clearly due to the presence
of ozone and not to some other system parameter, as exhib-
ited by an increase in aerobic bacteria to the non-treated
test levels upon discontinuation of the ozone feed.
Figure 10 — Comparison of fouling monitor data for
a chlorine/bromine-treated PCT test and an ozone-
treated PCT test run under identical conditions.
8
The effects of ozone on microbiologically-induced fouling
were investigated by a set of tests incorporating an artifi-
cial nutrient and bacteria feed (as described in Experimen-
tal Procedures). The bacteria levels of the non-treated test
increased throughout, approaching 108 CFU/ml by the end
of the 14-day period (Figure 12). The levels during the first
half of the ozone-treated test were high, but decreased to
102 CFU/ml as the ozone output was increased to meet the
demand of the bacteria and nutrient being added to the
system. The fouling of the heat transfer tubes exhibited a
definite and immediate response to changes in the ozone
feed system, as measured by fouling monitor data (Figure
13). Severe fouling immediately commenced upon initia-
tion of the bacteria/nutrient feed mixture, indicating that
the fouling is microbiologically induced. In the non-treated
system the fouling increased throughout the test, increas-
ing most rapidly after the addition of a nutrient slug.
In the ozone-treated system, the fouling was decelerated,
suppressed, or even reversed upon increase of the ozone
output. The responses of the fouling monitor indicate that
ozone may actually clean a fouled system if the foulant is
microbiologically based. However, it should be noted that
the quick response observed in this system was only
achieved after maximizing the output of the ozonator, which
is an usually large amount of ozone for the volume of water
treated (0.2 kg/day for 50 l of water).
CASE HISTORIES
Excellent agreement was observed between the laboratory
corrosion studies and the PCT investigations. However,
these results can be deemed of little practical importance
unless it is clearly shown that they correlate to results
obtained in actual field applications. In this light, the
results from two closely monitored case histories are pre-
sented here. As the variables are harder to control than at
the PCT stage, the results have been divided into three
major categories:
1. Water chemistry
2. Fouling control
3. Corrosion control
Figure 11 — A representative plot of total aerobic
bacteria counts from a non-treated and an ozone-
treated PCT test.
Figure 12 — Total aerobic bacteria levels for a non-
treated and an ozone-treated PCT test incorporating
a nutrient and bacterial feed.
Figure 13 — Heat transfer resistance vs. time as
measured by a Bridger fouling monitor for a non-
treated and an ozone-treated PCT test incorporating
a nutrient and bacterial feed.
9
CASE HISTORY #1
Case history #1 was conducted on a small comfort cooling
system servicing a facility in southern California. The met-
allurgy of the heat exchanger is copper and the transmis-
sion lines are mild steel. Ozone was used as a stand-alone
chemical treatment, and the results represent one-year
operation. Ozone is injected by means of a Venturi eductor
into a sidestream loop of water drawn off of the basin, and
the ozonated stream is returned to the basin.
Water Chemistry
The composition of the makeup water for this facility is
presented in Table 5. During the first four months of the
study, the concentration of the tower water was slowly
raised from 3 to 14 cycles (the tower cycles refer to the
sodium cycles, which is representative of the other highly
soluble ions). For the next eight months, the system was
operated at approximately 5 to 10 cycles of concentration,
with stress conditions as high as 20 cycles. CaCO3 precipi-
tated throughout this time period, as demonstrated by the
Ca and “M” alkalinity cycles of concentration being consis-
tently 3 to 4. Silica and/or magnesium silicate also precipi-
tated, but to a much lesser extent.
Fouling Control
As was demonstrated in the PCT investigations, precipita-
tion does not necessarily mean that scaling of the heat ex-
change surfaces is occurring. However, the online fouling
monitor does provide real-time information regarding the
formation of scale on the heat exchange surfaces. The foul-
ing monitor in question was operated in a “worse-case
scenario” manner, such that any potential for fouling of the
system’s heat exchange surface would be detected by the
fouling monitor, prior to scale forming in the system.
Under normal operation, fouling of the system was not a
problem. However, a direct correlation between an increase
in the cycles of concentration and a general increase in foul-
ing, as measured by the fouling monitor, was realized. In
addition to this correlation, sharp and dramatic increases
in fouling were observed. These increases occurred over a
short period of time, and coincided with an increase in bulk
water temperature. The fouling decreased slowly over a
period of days after the sharp rises (Figure 14). An expla-
nation for this type of behavior is that the scale forms dur-
ing the hottest skin temperature, but is not tenacious, be-
ing sloughed off by the water due to its high fluid velocity.
Support for this explanation is found in the fact that an
off-white-to-brown, flaky, solid material was observed in the
basin. The chemical composition of the material is 43% Ca,
28% carbonate, and 18% Si, in close agreement with that
expected from the water chemistry analyses. Visual inspec-
tion of the system indicated those large amounts of deposi-
tion formed on the tower fill and in the basin. However, no
significant scaling was observed on the surface of the heat
exchange tubes.
Corrosion Control
Good mild steel and copper corrosion control was achieved
throughout the trial. Initially, a dramatic decrease in cor-
rosion rate was observed, going from approximately 5 mpy
(0.1 mm/y) to less than or equal to 1 mpy (0.02 mm/y) in
the mild steel corrosion rate, and 0.2 mpy (0.005 mm/y) to
less than 0.1 mpy (0.002 mm/y) in the copper corrosion rate.
The Corrater-measured corrosion rates remained at
approximately 1 mpy (0.02 mm/y) for mild steel and less
than 0.1 mpy (0.002 mm/y) for copper for the duration of
the trial. Excellent agreement was found between both mild
steel and copper coupon data and the Corrater data, lend-
ing confidence to the results.
The trends in the corrosion rates correlate extremely well
to the water chemistry of the system, but have no correla-
tion with the presence of ozone in the system. Immediately
prior to initialization of ozone treatment, the system was
operating with no conventional chemical corrosion protec-
tion, thus the high mild steel corrosion rates. The initial
dramatic decrease in corrosion rate coincided with both an
Ion ppm Unit
Ca 66 CaCO3
Mg 43 CaCO3
“M” 109 CaCO3
SiO221 ion
SO443 ion
Cl 58 ion
Na 54 ion
Table 5 — Makeup water, Site #1
Figure 14 — Fouling monitor data for Site #1
10
Figure 15 — Mild steel corrosion rates from Site #1.
Note the periods of no ozone feed to the system.
increase in the cycles of concentration of the recirculating
water and the initialization of ozone treatment. The tower
was operating at approximately 2.5 cycles initially, and the
concentration was steadily increased to approximately
5 cycles. For the remainder of the trial, the cycles of con-
centration of the tower water was greater than or equal to
five. The ozone treatment was held constant during the
initial decrease in corrosion, but then was discontinued for
a ten-day period. Also, for the remainder of the trial, the
ozone feed rate was varied several times, including two more
discontinuations of ozone feed. Using the mild steel corro-
sion rate to demonstrate the point, Figure 15 clearly shows
no response to the changes in the ozone treatment.
CASE HISTORY # 2
Case history #2 was conducted on a light industrial system
in the western United States. Ozone was used as a stand-
alone chemical treatment, and injection is similar to that of
case history #1 and the PCT studies.
Water Chemistry
The composition of the makeup water for this facility is
presented in Table 6. The system operated at 10 to 20 cycles
of concentration for the first six weeks of ozone treatment
(again, the tower cycles refer to the sodium cycles, which is
representative of the other highly soluble ions). In contrast
to case history #1, the concentration of the tower water was
then rapidly raised from 10 to 20 cycles to nearly 70 cycles
of concentration in a seven-week period by complete shut-
off of the blowdown. The system operated at a concentra-
tion as high as 90 to 100 cycles. Calcium, “M” alkalinity,
and silica were severely out of balance, with the concentra-
tion cycles of each being between 7 and 12 throughout the
trial.
Fouling Control
An increase in fouling was observed corresponding to the
rapid increase in concentration of the recirculating water.
Removal and inspection of the fouling monitor heat
exchange tube revealed an off-white deposit covering the
entire region of applied heat. Inspection of the system re-
vealed an almost identical, off-white, tenacious scale, which
was uniform throughout the heat transfer equipment. The
chemical composition of the deposit was 60% Si, 13% Mg,
9% Ca, and 8% carbonate, with minor components com-
prising the remaining 10%. Also, the tower fill was covered
with scale. The chemical composition of the tower fill scale
was 49% Ca, 29% carbonate, and 16% Si, in accord with
the water analyses. The zero blowdown operation contin-
ued for over one year during a water-short period. Due to
overdesign of the system, the observed fouling did not
affect operations until increases in ambient temperatures
caused significant increases in bulk water temperatures.
The water analyses indicate that a tremendous amount of
solids have precipitated from solution in this system. From
the commencement of ozone treatment until the inspection
was performed, calculations indicate that approximately
10,000 kg of CaCO3 and 2,000 kg of SiO2 precipitated from
solution, representing approximately 5 m3 of solid mate-
rial. The tower fill for this facility has nearly 500,000 ft2
(50,000m2) of surface area. A uniform coverage of scale would
result in a layer 0.1 mm thick. The observed scale was
approximately 0.2 to 0.6 mm thick, easily accounting for
the precipitated solids.
Corrosion Control
As was the case with site #1, the corrosion rates correlate
extremely well with the saturation level of the water. A mild
steel corrosion coupon, which was exposed while the recir-
culating water was at 10 to 20 cycles, had a corrosion rate
of 1.5 mpy (0.038 mm/y). An admiralty coupon exposed
during the same time period had a surprisingly high corro-
sion rate of 0.7 mpy (0.018 mm/y). Both mild steel and
admiralty coupons, which were exposed while the system
was operating at greater than 20 cycles of concentration,
had corrosion rates of <0.1 mpy (<0.003 mm/y). However,
upon drying, a white film covering the entire surface of the
coupons became apparent. The film consisted primarily of
silica.
SUMMARY AND CONCLUSIONS
The work presented here represents the first comprehen-
sive study — bench-top laboratory investigations, pilot scale
testing, and critical monitoring and evaluation of field ap-
plications — addressing the effects of ozone as a stand-alone
cooling water treatment program. It also represents the
first investigation to critically compare ozone-treated
systems to non-treated systems. Excellent agreement was
observed among all stages of testing, establishing not only
the relevance, but the importance — due to control of vari-
ables not possible in the field — of investigating the prop-
erties of ozone in the laboratory. The use of established
pilot testing provides a means of evaluating the suitability
of ozone to a particular situation.
Several conclusions concerning the mechanistic aspect
of ozone treatment have been clearly demonstrated:
1. Excellent corrosion control can be attained in ozone-
treated cooling water systems. However, the corrosion
rates are completely dominated by the water chemistry
of the system and have no dependence on the presence of
ozone at typical use levels. The laboratory corrosion rates
agree nearly quantitatively with the pilot scale corro-
sion rates, both of which agree nearly quantitatively with
corrosion rates obtained from field applications.
11
Table 6 — Makeup water, Site #2
Ion ppm Unit
Ca 75 CaCO3
Mg 73 CaCO3
“M” 76 CaCO3
SiO214 ion
SO460 ion
Cl 101 ion
Na 93 ion
2. Ozone has no direct effect on the precipitation of min-
eral scale in a recirculating cooling water system, nor
does it affect the crystal morphology of precipitating
species. Again, nearly quantitative agreement is ob-
served between water chemistry at the pilot scale and
water chemistry in field applications. Also, as have been
reported in ozone field applications elsewhere and
observed in the present field studies, “sandy” deposits
formed in the basin of some of the PCT tests, irrespec-
tive of the presence of ozone.
3. Good control of fouling can be attained in ozone-treated
cooling water systems. However, as was the case with
corrosion control, deposition on the heat exchange sur-
faces is not determined by the presence of ozone, but by
several factors that traditionally influence fouling in a
system.
4. The strong biocidal properties of ozone resulted in ex-
cellent microbiological control in all PCT investigations,
and in both case histories studied. The unique combina-
tion of high toxicity during treatment with no toxicant
discharge makes ozone an attractive biocide.
ACKNOWLEDGEMENTS
The authors would like to thank D.P. Pruss, M.V. Buncio,
O.X. Fedyniak, T.A. Krol, A.M. Blondin, P.R. Young, T.L.
Stuebner, and R.W. Cloud for assistance in collecting the
data.
REFERENCES
1. (a) R. Nass and D.T. Reed, “Small-Scale Short-Term
Methods of Evaluating Cooling Water Treatments...
Are They Worthwhile?” International Water Conference,
1975. (b) K.E. Fulks and A.M. Yeoman, “Performance
Evaluation of Non-Metal Cooling Water Treatments,”
CORROSION/83, paper no. 279, (Houston, TX: NACE,
1983).
2. E.B. Smyk, J.E. Hoots, K.P. Fivizzani and K.E. Fulks,
“The Design and Application of Polymers in Cooling
Water Programs,” CORROSION/88, paper no. 14, (Hous-
ton, TX: NACE, 1988).
3. G. Wofford, C. Slezak and M. Bukey, Industrial Water
Treatment 23, 2 (1991): p. 33, and references therein.
4. H.H. Lu and D.J. Duquette, Corrosion 46, (1990): p. 843.
5. D.A. Meier and J.D. Lammering, ASHRAE Transactions
93 (part 2) (1987).
6. J.M. Brooke and P.R. Puckorius, “Ozone for Cooling
Tower Systems: Is It a Panacea?” CORROSION/91,
paper no. 212, (Houston, TX: NACE, 1991). And refer-
ences therein.
7. J.D. Cowan and D.J. Weintritt, Water-Formed Scale
Deposits, Gulf Publishing, Houston, 1976, p. 204.
8. Ibid, p. 209.
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