A COMPARISON OF SCALE POTENTIAL INDICES WITH
PROGRAM RESULTS IN OZONATED SYSTEMS
French Creek Software, Inc.
Kimberton & Hares Hill Roads, Box 684
Kimberton, PA 19442
Arthur Freedman Associates
1415 Crystal Court
Naperville, IL 60563-0142
Phone: (630) 857-3094
Presented at CORROSION '94
Ion association model saturation levels were calculated for ozonated cooling systems. This paper
compares treatment program results, including deposits present, to indices calculated for calcium
carbonate and other scale forming species typically encountered in cooling systems.
Keywords: cooling water, scale, indices, ozone
Water chemistry studies reported for ozonated cooling systems have historically evaluated scale
potential in terms of the simple calcium carbonate scale potential indices: the Langelier Saturation
Index, the Ryznar Stability Index, and the Practical Scaling Index. These indices can provide
reliable predictions of calcium carbonates scale potential in low dissolved solids, low sulfate water.
But the indices do not account for "common ion effects" such as the apparent increase in calcium
carbonate solubility in high sulfate waters. Their usefulness is limited to ionic strengths of systems
operating at a total dissolved solids well below that of the super cycled zero blow down systems.
And perhaps as importantly, these indices are limited to calcium carbonate scale potential. They
predict nothing of other scale forming species such as silica, magnesium silicate as a co-precipitate
of magnesium hydroxide and silica, or calcium sulfate.
This study evaluated the water chemistry of ozonated cooling systems using an ion association
model to predict the scale potential for calcium carbonate, magnesium hydroxide, amorphous silica,
and calcium sulfate.
The ion association model used is applicable to the ionic strength range of the highest concentration
ratio ozonated systems. The model accounts for the common ion effects through the use of ion
pairing and the calculation of indices based upon free, uncombined ions concentrations rather than
analytical values. Alkalinity is corrected for non-carbonate contributions by the model so that
calcium carbonate scale potential indices calculated are based upon the most accurate estimate of
the carbonate species distribution. The results are reliable even in the presence of free hydroxide
alkalinity and high silica levels.1
Scale potentials are reported as saturation level (degree of supersaturation). This index describes
the ratio of the observed water chemistry to the water chemistry at equilibrium, for the reactants
For example for calcium carbonate:
Saturation Level = _______________
- (Ca) is the free ion activity of calcium in the water.
- (CO3) is the free ion activity of carbonate in the water.
- Ksp is the solubility product for calcium carbonate such that
(Ca)(CO3) = Ksp at equilibrium.
Saturation ratios can also be calculated for other scale forming species.
Saturation Level = ________________ ______________________for calcium sulfate
________________ _Ksp CaSO4
Saturation Level = ______________ _______________for magnesium hydroxide
Saturation Level = _____________________ __________for amorphous silica
_________________(H2O)2 . Ksp SiO2
These indices can be loosely interpreted as follows:
- If the saturation level is less than 1.0, a water is undersaturated with this scale forming
specie and will tend to dissolve the scale if present in a solid form.
- If the saturation level is 1.0, the water will not tend to form or dissolve the scale. The
water is at equilibrium with respect to the scale forming specie.
- As the saturation level rises above 1.0, the driving force for this scale to form
Saturation level calculations can be applied to estimating the driving force for scale formation for
common scale forming species.
Saturation levels were calculated in this manner for:
- Calcium carbonate as the calcite polymorph.
- Silica as amorphous silica.
- Magnesium hydroxide as brucite.
- Calcium sulfate as gypsum.
Each system was evaluated by:
- Comparing actual makeup and recirculating water chemistry, and operating parameters to
estimate the actual concentration ratio of the systems.
- The theoretical concentration of the recirculating water chemistry was calculated based upon
the makeup water and apparent, calculated concentration ratio.
- The Theoretical and Actual ion concentrations were compared to determine precipitation of
- The driving force for precipitation for the major scale forming species were calculated for the
Actual and Theoretical recirculating cooling water chemistry.
- Differences in the Theoretical to Actual chemistry were compared to the cleanliness of the
cooling systems with respect to heat transfer surface scale, scale formation in valves and non
heat transfer surfaces, and precipitate buildup in the tower fill and basin.
SYSTEM OPERATING PARAMETERS
The cooling systems included in this evaluation were comfort heating and cooling systems, where
the cooling water serviced freon exchangers, or light industrial systems. The heat exchangers
serviced by the cooling systems can be characterized as low heat load, high flow velocity with tube
side cooling water.
All of the cooling water systems described in this paper were treated solely with ozone. No other
chemical treatment compounds, including sulfuric acid, were used during the course of this work.
Ozone was added to these systems at dosages ranging from 0.05 to 0.2 ppm in the circulating
pump suction pit. The intent, in each case, was to apply ozone continuously to each system.
Because of the various mechanical problems and changes in operation required by the tower
owners, the ozone application varied substantially in most cases. Unfortunately, accurate records of
ozone application over time for each system is not available for inclusion in this paper.
WATER CHEMISTRY RANGE
Three categories of systems were encountered in the evaluation:
- CATEGORY 1: Those where the theoretical recirculating water chemistry is not scale
forming when cycled;
CATEGORY 2: Those where the recirculating water would have a moderate to high
calcium carbonate scale forming tendency when cycled. Recirculating chemistry for these
systems is similar to that encountered in conventional alkaline cooling water programs;
CATEGORY 3: Those with an extraordinarily high scale potential for calcium carbonate
and magnesium hydroxide when cycled. These systems operated with a recirculating water
chemistry more like that of a softener than of a conventional cooling system.
RESULTS VERSUS WATER CHEMISTRY
Table 1 outlines the theoretical versus actual water chemistry for the thirteen (13) systems
evaluated. Table 2 presents saturation levels for the theoretical and actual recirculating water
- Category 1 - Recirculating Water Not Scale Forming:
In the case of category 1 systems, no scale formation was observed, as expected.
Category 2 - Conventional Alkaline Control Range:
Scale formation was observed when category 2 water chemistry was present in eight of nine
systems evaluated. In these cases, the driving force for scale formation was in the range
where a traditional scale control treatment program could prevent scale deposition on heat
transfer surfaces. It was not above the level where inhibitors such as phosphonates lose
In one case, scale potential was low (calcite saturation level less than 40), and holding time
index relatively short (12 hours half life). No scale was observed under these conditions.
Category 3 - The Cooling Tower as a Softener:
Deposit formation on heat transfer surfaces was not observed in most of these systems. In
one case, deposits were observed in a valve.
The driving force for scale formation was so high in these systems that gross precipitation of
calcium carbonate, magnesium hydroxide, and in some cases, silica would be expected. Yet
no deposits were found on heat transfer surfaces.
RATIONALE FOR RESULTS
Category 2 systems fall into the general operating range for conventional alkaline cooling water
systems. Typical calcite saturation ratios for such systems fall into the range of 20 to 150
([Ca][CO3]/Ksp). In the absence of chemical treatment, scale formation in these systems normally
occurs by growth on existing active sites. Because of the inverse solubility of calcite with
temperature, most accumulation is on heat transfer surfaces.
Even though clearly visible scale formation occurred in the Category 2 systems listed in Tables 1
and 2, the saturation ratios are low. That is, since the circulating water is only slightly supersaturated
with calcite, only a small amount of the total reactive material in the solution precipitates.
By comparison, the saturation ratios in Category 3 systems listed in Tables 1 and 2 range well
above 1,000. These levels of supersaturation are typical of calcium carbonate softening processes.
Under these conditions, calcite tends to precipitate as finely divided particles in the bulk solution,
rather than by growth on specific sites as described above.
Once bulk precipitation begins, calcite formation on metal surfaces is greatly reduced because of
the greater surface area of the suspended calcite crystals. The flow velocity of water through the
condenser tubes tends to keep the crystals in suspension, so that calcite does not accumulate in the
condenser. In these cases, calcite accumulates in the basin, and sometimes on the tower deck and
fill. This explains why Category 3 systems, with much higher saturation ratios than Category 2
systems, show less tendency to form calcium carbonate deposits on heat transfer surfaces.
THE ROLE OF OZONE IN CALCIUM CARBONATE SCALE
The data in this paper do not show that ozone played any significant role in calcite precipitation or
scale formation on heat transfer surfaces. Although there were variations in the dosage, frequency
and continuity of ozone feed to these cooling systems, the overall feed rates were in the 0.05 to 0.2
ppm range, as explained above. There does not seem to be any real correlation between ozone
dosage and feeding method with calcium carbonate scale formation in the system. Conventional
water treatment technology recognizes two different approaches to controlling calcium carbonate
scale formation. These are sometimes described as "stabilization chemistry" and "precipitation
chemistry." Stabilization chemistry refers to the use of specific chemicals to "stabilize"
supersaturated solutions of calcium carbonate, with saturation ratios typical of the Category 2
systems in this paper. The chemicals stabilize the calcium carbonate so that it will not precipitate
during the residence time in the system. Precipitation chemistry, on the other hand, refers to the use
of dispersants and biocides to maintain system cleanliness and keep precipitating calcium carbonate
in suspension and prevent deposition on heat transfer surfaces. The performance of ozone as a
biocide in cooling water systems is well known. It may be that the role of ozone in preventing scale
formation is to keep both the internal system surfaces and the bulk water clean and free of
biofouling that can attract and "glue" small crystals to surfaces.
Analysis of the water chemistry data from a number of recirculating cooling systems treated solely
with ozone show that calcium carbonate (calcite) scale forms most readily on heat transfer surfaces
in systems operating in a calcite saturation ratio in the range of about 20 to 150. This range is
typical of conventional alkaline cooling water operations. At much higher saturation ratios, above
1,000, calcite precipitates in the bulk water. Because of the very high surface area of the
precipitating crystals compared to the metal surface in the system, continuing precipitation leads to
crystal growth in the bulk water rather than scale formation on heat transfer surfaces. The presence
of ozone in these systems does not appear to have influenced calcite precipitation and/or scale
formation. Although not conclusively proven, it seems that the main role of ozone may be to keep
the system clean and free of biofouling that can encourage scale formation.
1 R.J. Ferguson, Computerized Ion Association Model Profiles Complete Range of Cooling
System Parameters, International Water Conference, 52nd Annual Meeting, Pittsburgh,
Table 1: Theoretical Versus Actual Recirculating Water Chemistry
|1 (1)||56||43||13||28||36||-8||40||52||-12||No scale
|4 (2)||288||180||108||216||223||-7||66||48||18||Valve scale|
|6 (3)||803||163||640||495||607||-112||162||143||19||No scale
|7 (3)||1,464||200||1,264||549||135||414||112||101||11||No scale
|8 (3)||800||168||632||480||78||402||280||78||202||No scale
|9 (3)||775||95||680||496||78||418||186||60||126||No scale
|10 (3)||3,904||270||3,634||3,172||508||2,664||3,050||95||2,995||Slight valve
|11 (3)||4,170||188||3,982||308||303||5||126||126||0||No scale
|12 (3)||3,660||800||2,860||2,623||2,972||-349||6,100||138||5,962||No scale
|13 (3)||7,930||68||7,862||610||20||590||1,952||85||1,867||No scale
Table 2: Theoretical Versus Actual Recirculating Water Saturation Level
|1 (1)||0.03||0.02||<0.001||<0.001||0.20||0.25||No scale
|2 (2)||49||5.4||0.82||0.02||0.06||0.09||Basin buildup|
|3 (2)||89||611||2.4||0.12||0.10||0.12||Heavy scale|
|4 (2)||106||50||1.3||0.55||0.13||0.16||Valve scale|
|6 (3)||540||51||5.3||0.73||0.35||0.49||No scale
|7 (3)||598||28||10||0.17||0.40||0.52||No scale
|8 (3)||794||26||53||0.06||0.10||0.33||No scale
|9 (3)||809||6.5||10||<0.01||0.22||0.27||No scale
|10 (3)||1,198||62||7.4||0.36||0.31||0.35||Slight valve
|11 (3)||1,670||74||4.6||0.36||0.22||0.44||No scale
|12 (3)||3,420||37||254||0.59||1.31||0.55||No scale
|13 (3)||7,634||65||7.6||0.14||1.74||0.10||No scale