Applications of Calcium Hypochlorite in Cooling Towers

Written by: Ellen Meyer, Product Safety and Government Affairs Manager


Fouling of cooling towers by microbiological growth can lead to operational issues such as increased corrosion and decreased heat transfer efficiency. Since some microorganisms produce noxious odors, they may also be a nuisance. More importantly, some microorganisms such as Legionella are pathogenic and have been known to cause disease outbreaks.1 The primary groups of microorganisms found in cooling towers are algae, fungi and bacteria. The microorganisms may be introduced from the makeup water, process water leaks, and from the air that is “scrubbed” in the cooling tower. These organisms may be free floating in the water (i.e. planktonic), but most likely they will be present in the form of biofilm (i.e. sessile). Biofilms protect microorganisms, reduce heat transfer efficiency, and promote corrosion, so any cooling tower treatment program needs to minimize biofilm formation.

Cooling tower operators have a number of technologies available to them for controlling microorganisms in cooling towers. Cooling tower biocides are typically broken down into two groups: oxidizing and non-oxidizing. Oxidizing biocides including chlorine are a mainstay of cooling tower treatment due to their broad spectrum of activity and fast kill rates. There are a variety of oxidizing biocides available, so an operator can choose which disinfectant best fits their system and treatment program objectives. Table 1 lists the various choices that are available.

Each of these products has its advantages and disadvantages. Operators can choose whichever option has the properties they require, depending on their cooling water system, their storage facilities, and environmental discharge permits.

Table 1

Chemical Name
Chemical Formula
Other Names
% Active
Dissolution Rate
Sodium hypochlorite
Calcium hypochlorite
Cal hypo
Granular Solid
Calcium hypochlorite
Cal hypo
Solid briquette
Calcium hypochlorite
Cal hypo
Solid tablet
Calcium hypochlorite
Cal hypo
Solid tablet
BCDMH stabilized bromine
Solid tablet
Chlorine dioxide

Application 1: Continuous Chlorination

Calcium hypochlorite is a low-cost, solid biocide option for treating cooling towers. It has a number of advantages over the other options listed in Table 1.

Transportation, Storage and Handling

Treatment of cooling towers with chlorine gas is becoming increasingly rare. Solid halogen sources have the advantage of no potential for gas leaks or liquid spills.

Chlorine dioxide is an explosive gas at concentrations greater than 10% in air.2

Calcium hypochlorite is a highly concentrated form of chlorine and so minimizes shipping costs compared to sodium hypochlorite.


In NFPA 400, the National Fire Protection Association recommends secondary containment that is sufficient to contain a spill from the largest vessel plus 20 minutes of fire water.3 Fire water volumes will vary, but the following table shows the storage volume of NaOCl vs cal hypo.

Table 2

Pounds of AvCl
NaOCl, ft3
Ca(OCl)2, ft3

NaOCl (12% AvCl, 10 lb/gal (75 lb/ft3), 9 lb AvCl /ft3)
Ca(OCl)2 (65% AvCl, 50 lb/ft3, 32 lb AvCl /ft3)


Because of its instability, chlorine dioxide cannot
be shipped. It must be produced on-site and used immediately.

Liquid sodium hypochlorite bleach is often used due to its relatively low price. However, liquid bleach will quickly lose its strength during transportation, storage, and when circulated into a cooling water system. The following graph shows the effect of temperature on degradation rates based on an equation from White’s Handbook of Chlorination.4 However, bleach degradation may occur much quicker than shown here and may be accelerated with higher concentrations, sunlight and impurities.

Figure 1. Naocl Stability, Ln C = Ln Co K Co3 Days
Figure 1. NaOCl Stability, Ln C = ln CO K CO3 days

Chlorine feed control is complicated by the fact
that the sodium hypochlorite solution strength is constantly changing. Calcium hypochlorite has much greater stability which produces more consistent solutions strengths.

When sodium hypochlorite decomposes, it produces chlorate and perchlorate, which could be an issue for discharge of cooling tower water, particularly for once through systems. The following table compares the average contaminant concentration contributed when the water is treated with 10 ppm AvCl.

Table 3. Contaminant contributions, ppb (AWWA 2009)5

710 ± 840
1.39 ± 0.94
0.37 ± 0.70
130 ± 10
0.81 ± 0.08
0.0087 ± 0.0004

Because calcium hypochlorite is stored as a solid and solutions are made on demand, decomposition products such as chlorate and perchlorate are minimized.

Decomposition of sodium hypochlorite also leads to the formation of gas which may cause feed pump cavitation.

The shelf life of solid cal hypo is much greater than liquid sodium hypochlorite. Bleach degrades in weeks/months, while solid cal hypo is stable for months.

Dissolution Rate

The dissolution rate of cal hypo is much higher than stabilized bromine, so uncycled makeup water can be used in the feeder. This is not possible with hydantoins such as BCDMH. Dissolution with makeup water has the advantage of not allowing corrosion/deposit control treatment residuals (phosphonates, azoles) to see elevated chlorine levels in the feeder.

Low Cost

When evaluating cost differences, it is important to consider that cal hypo is 65-68% active chlorine content compared to 12.5% active in bleach. Therefore, a pound for pound chemical evaluation should not be used. Rather, the comparison should be based on price per lb of available active chlorine. Pricing will vary by region and supplier. If the price per pound of available chlorine from cal hypo is greater than the price per pound of available chlorine from bleach, savings can be realized by decreased capital and maintenance costs, as no storage tanks are required. Even considering regional differences, the cost of cal hypo is much less per pound of active compared to other solid chlorine/bromine products such as BCDMH.

Easy to Apply

Calcium hypochlorite tablet and briquette feeders are an easy way to provide a consistent chlorine residual feed to the cooling tower. Various feeder designs are available to select from, depending on system parameters and treatment objectives.

Effects on Corrosion and Scale

When chlorine gas is added to water, the pH will decrease. This can cause significant system pH upsets in low alkalinity waters during times of chlorine feed.

Sodium hypochlorite and calcium hypochlorite will raise pH. This increase in pH can impact the carbonate/bicarbonate ratio and cause calcium carbonate scale to form on heat transfer surfaces or precipitate in the bulk water.

Calcium hypochlorite also increases the carbonate and calcium content of the water. The calcium contribution is approximately 1 ppm calcium (as calcium carbonate)/ ppm of available chlorine. Cal hypo feed solutions may contain suspended solids that can settle in the system and cause fouling. These suspended solids may act as nucleation seeds for crystal growth and result in a rapid loss of deposit control.

Due to its effect on water balance, calcium hypochlorite is best suited to low-alkalinity, low-hardness water. Cal hypo is also better suited to once-through systems or systems with low cycles of concentration. As a general rule to minimize the chance of deposit control issues, cal hypo should be used in evaporative cooling water systems with LSI of less than +1.0. For systems running stabilized phosphate and zinc-phosphate programs, calcium phosphate precipitation can occur. Therefore, it is important to monitor filtered (0.45 micron) and unfiltered phosphate and zinc residuals. As a general rule, the difference between filtered and unfiltered levels should not be more than 1 ppm. If higher levels are observed, bulk precipitation of the corrosion inhibitor is probably occurring.

Application 2: Hyperchlorination Disinfection

Best practice recommendations from OSHA6, ASHRAE7 and CTI8 for the operation and maintenance of cooling towers include routine (every 6 months) online or offline cooling tower cleaning and disinfection. In addition, cooling tower hyperchlorination disinfections may need to be performed as remedial actions in response to positive Legionella test results or other system upsets. Bleach is typically used in this application. Stabilized bromine should not be used to shock dose for risk of system over-stabilization.

Cal hypo has several advantages over bleach in this application. Because it is a stable solid with a longer shelf life, it can be stored on-site until hyperchlorination is needed. Because of its short shelf life, bleach may need to be specially ordered for these applications and used immediately before the solution strength declines. The longer shelf life and high chlorine concentration of solid calcium hypochlorite makes it a superior choice for storage and handling concerns as well.

Calcium hypochlorite briquettes can be fed using feeders, or the granular product can be applied directly into the sump/basin.


Calcium hypochlorite is a high concentration, low-cost, solid biocide option for treating cooling towers where liquid bleach presents degradation and/or handling issues. For continuous treatment, the best applications are systems that are once-through or run low cycles of concentration, or where makeup water quality is of low-hardness and low-alkalinity.

For routine or remedial cooling tower hyperchlorination disinfection, calcium hypochlorite presents several advantages over liquid bleach. It is shelf stable and can be stored on-site to be used on-demand, as needed. It is also much easier to transport, handle and apply compared to bleach.


  1. Barsky, A., Lackraj, D., Tripathi, P.S., Cooley, L., Lee, S., Smith, J., Edens, C. Legionnaires’ Disease Surveillance Summary Report, United States 2016-2017, Centers for Disease Control and Prevention, 2020.
  2. Masschelein, W.J., Rice, R. G., Chlorine Dioxide, Chemistry and Environmental Impact of Oxychlorine Compounds, Ann Arbor Science, Ann Arbor MI, 1979, ISBN 0-250-40224-6. 
  3. NFPA 400, Hazardous Materials Code, National Fire Protection Association, 2019, ISBN 978-145592079-2.
  4. White’s Handbook of Chlorination and Alternative Disinfectants, 5th Edition, Black and Veatch Corporation, Wiley, 2010.
  5. Hypochlorite- An assessment of factors that influence the formation of perchlorate and other compounds, American Water Works Association and Water Research Foundation, 2009.
  6. OSHA Technical Manual (OTM), OSHA Instruction TED 01-00-0015, Section III, Chapter 7 Legionnaire’s Disease, Occupational Safety and Health Administration, accessed 11-18-20
  7. ASHRAE Guideline 12 -2000, Managing the Risk of Legionellosis Associated with Building Water Systems, American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2000.
  8. CTI Guidelines WTB-148 (08), Legionellosis Guideline: Best Practices for Control of Legionella, Cooling Technology Institute, July 2008.