A multitude of microscopic organisms such as bacterial colonies, fungi, and algae can easily thrive within the moderately high temperatures present inside a cooling tower… Source: https://commons.wikimedia.org/wiki/File:Cooling_Tower_ Algal_Mat.jpg

Biofouling of cooling towers can lead to operational issues such as increased corrosion and decreased heat transfer efficiencies. Areas of the system with slow water flow and stagnation often encourage the rapid formation of biofilms, which are extremely complex ecosystems that can block free water flow and reduce water quality significantly.

Sodium hypochlorite (liquid bleach/NaOCl) has long been the most widely used biocide for controlling microorganisms and biofilm formation in cooling towers. This popularity is primarily due to its broad spectrum of activity, fast kill rates, and low cost. Other cooling tower operations have adopted alkaline water systems and have switched from bleach to stabilized bromine, such as bromochlorodimethylhydantoin (BCDMH).

Cal Hypo Option Offers Advantages

Stressing on the effectivity, Stacy Willis, Global Key Account Manager for Sigura, said, “It’s important to consider calcium hypochlorite as an important alternative to these other oxidizing biocides. Cal hypo can provide equivalent biocidal efficacy as bleach, stabilized bromine, or chlorine dioxide, and its use can present several key advantages over these other products in many conventional cooling tower applications.”

“Advantages include far greater efficiencies in chemical transportation, storage and handling than bleach, a higher rate of dissolution than stabilized bromine, and significantly better stability and solution consistency than bleach,” he continued.


Although bleach is often used due to its relatively low price, it can quickly lose strength during transportation, storage, and when circulated into a cooling water system. The shelf life of DryTec calcium hypochlorite is greater than sodium hypochlorite.

DryTec calcium hypochlorite offers cooling tower operators a highly effective biocide designed for controlling microorganisms to maintain system cleanliness, achieve associated production efficiency gains, and can be used as part of an overall Legionella control program. DryTec is a highly concentrated form of chlorine that effectively kills bacteria, controls algae and removes organic contaminants that promote biofilm formation.

Cost Effective

The cost of DryTec calcium hypochlorite briquettes is much less per pound of active biocide than other solid chlorine/bromine products such as BCDMH. And, in areas or regions where the price per pound of available active chlorine is lower for bleach than calcium hypochlorite, DryTec biocide can still provide cost advantages.

Focusing on its cost effectiveness, Willis said, “No large storage tanks are required when using calcium hypochlorite, so significant savings can be realized in reduced capital and maintenance costs. Plus, there’s no liquid product sitting in storage tanks losing solution strength, and value.  Sodium hypochlorite loses strength much quicker than calcium hypochlorite and our engineered feed systems make readily available hypochlorite solution on demand.”

High Concentration, Long Shelf Life

There are many cooling tower operations where the long shelf life, high concentration, and compact storage of DryTec calcium hypochlorite chemistry provides a number of key advantages.

Pointing out to many cooling towers that are located on rooftops serving industrial processing facilities or large institutional buildings or entertainment complexes, Willis said, “These facilities typically don’t have elevators serving their rooftop, so getting a heavy chemical liquid to these locations can often be tricky, especially when there is a spill hazard associated with sodium hypochlorite.”

By switching to DryTec, which contains 65% chlorine equivalent, the operator and its chemical supplier are now handling only about one-fifth the weight of liquid bleach. Bleach has a 12.5% concentration of equivalent chlorine; the rest is primarily water.

Drawing attention on the volumetric reduction on carrying the chemical, Willis said, “As far as chlorine concentration is concerned, a five-gallon pail of DryTec calcium hypochlorite briquettes is equivalent to approximately 34 gallons of sodium hypochlorite. This makes transport and handling much easier because 50-pound pails are being moved rather than 55-gallon drums, which weigh about 539 pounds.”

The dry form and high concentration of DryTec biocide also greatly improves the consequence of lengthy transport. A large cooling tower that would typically be treated with bleach on the mainland, for example, finds it difficult and expensive to use bleach where no ready source is available. This is the case for many cooling towers serving island industrial and commercial facilities and power plants. For these facilities, the long shelf life and high concentration of DryTec calcium hypochlorite make storing large quantities of an oxidizing biocide a viable and far less expensive approach than bleach or BCDMH.

Higher Dissolution Rate than Stabilized Bromine

The dissolution rate of Drytec calcium hypochlorite is much higher than stabilized bromine. This allows uncycled makeup water to be used in the cal hypo feed system, which is not a viable approach with hydantoin biocides such as BCDMH. The high dissolution of cal hypo using makeup water keeps corrosion/deposit control treatment residuals away from the elevated chlorine levels in the feeder.

Chlorine Residual Consistency

The solution strength of bleach is constantly degrading, which often makes accurate chlorine feed control difficult. Calcium hypochlorite has much greater stability than bleach, which brings more consistency in solution strength.

Highlighting yet another advantage of the Drytec product, Willis said, “Unlike bleach, decomposition products such as chlorate and perchlorate are minimized with calcium hypochlorite because it’s stored as a solid and solution is made on demand. Plus, bleach generates off-gasses as it decomposes in storage. This off-gassing can often cause metering pumps to become air-bound. Air-bound pumps are not an issue when using Drytec calcium hypochlorite briquettes and Drytec feed systems.”

Meeting Treatment Program Objectives

There are a variety of oxidizing biocides available for cooling water treatment, so operators can choose which one best fits their system and treatment program objectives.

Indicating the most appropriate area for application of Drytec calcium hypochlorite, Willis said, “Calcium hypochlorite is best suited to low-alkalinity, low-hardness water and systems with lower cycles of concentration. Drytec calcium hypochlorite briquettes also have several advantages over bleach for cooling tower hyperchlorination disinfection applications, like those used for remedial actions in response to positive Legionella test results or other system upsets. For these applications, Drytec calcium hypochlorite can be stored on-site until hyperchlorination is needed.”

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