Wet cooling towers often are preferred over dry systems for process cooling applications because water provides more efficient heat transfer and evaporative cooling. However, the warm water in wet systems contains dissolved and suspended solids, making it an excellent medium for microorganism growth. This growth is further encouraged by the recent trends to use reclaimed wastewater as makeup water and to increase cooling tower cycles of concentration. While these efforts help facilities satisfy goals to reduce fresh water use, minimize water and sewer charges, and comply with stricter environmental regulations, the biological “costs” in terms of increased microorganism growth can be significant. The uncontrolled growth of microorganisms in cooling water can cause severe problems such as the risk of Legionnaire's disease, blocked cooling water passages, accelerated corrosion and reduced heat exchanger efficiency.

Enlarge this picture Table 1. Many commonly used oxidizing and nonoxidizing biocides present a worker hazard due to their toxicity.

Many cooling tower biological control methods rely on toxic chemicals such as chlorine, ozone, chlorine dioxide, dithiocarbamate, isothiazolin, hydantoin and glutaraldehyde, which are collectively termed “biocides.” There are more than 300,000 cooling towers in the United States using an estimated 40 million pounds of such chemicals on an annual basis. While these biocides are often effective, their use embodies substantial health and safety and environmental concerns.

Gases from oxidizing biocides such as chlorine, chlorine dioxide and ozone present a serious safety issue. Low water solubility, reagent spills and leakage can expose workers to toxic levels of gases and explosion hazards. Liquid oxidizers such as sodium hypochlorite and n,n,dibromosulfamate are corrosive and reactive, exposing workers to potential chemical burns, toxic gas evolution and explosions. Solid oxidizers such as hydantoin are reactive and can explode when mixed with many organic materials, ranging from sawdust or to even flour. Chlorine gas commonly is used in larger cooling water applications due to its low cost and thus is present on industrial sites in large amounts, often in 1-ton cylinders. This chemical is extremely toxic in its gas form and, if released in large amounts, represents a risk for serious injuries and fatalities within the facility and the surrounding community.

Enlarge this picture Table 2. Both oxidizers and nonoxidizers are extremely toxic to most aquatic life, and even small product spills and leaks can produce catastrophic effects.

The nonoxidizing biocides often used in cooling towers also present a worker hazard due to their toxicity. Several of these products can be readily absorbed through the skin. Table 1 summarizes some relevant toxicity data on five chemicals commonly used as cooling water biocides.

Smaller operations, which include many cooling tower users, represent a special worker safety concern because cooling water treatment and biocide application may fall to workers not trained in handling toxic chemicals.

In addition to health and safety concerns, oxidizing and nonoxidizing biocides present other risks. Both oxidizers and nonoxidizers are extremely toxic to most aquatic life, and even small product spills and leaks can produce catastrophic effects. Table 2 summarizes some aquatic toxicity data for several commercial cooling water biocides along with the typical cooling water dosage range.

Cooling towers are basically evaporative coolers, with about 80 percent of the input heat load removed by evaporation. Cooling water solids content increases rapidly, and routine blowdown is required to prevent scale formation. A typical cooling tower operating at four cycles of concentration will evaporate 2,655 gal/day and blowdown 885 gal/day for every 100 tons of thermal load. This blowdown has been recognized as a substantial source of highly toxic chemicals released to the environment, depending on the biocide(s) and discharge treatment used. In several cases, environmental agencies either have banned the use of or required treatment for various biocides for the direct stream discharge of blowdown. The biocide content in a large blowdown discharge easily could wipe out the mixed liquor biomass of a smaller publicly owned treatment works (POTW).

A Bromine-Based Option

The electrolytic bromine process is based on a containerless electrolytic cell constructed of impregnated electrolytic graphite.

While most nonoxidizing biocides are long-lived and difficult to destroy, oxidizing biocides can be destroyed easily by adding a reducing reagent to the blowdown stream, thereby minimizing the environmental impact of the blowdown. For this reason, oxidizers are preferred over nonoxidizers, although they still present significant hazards during transport, storage and use. Bromine, an oxidizing biocide, has been recognized as a good cooling water biocide for many years. Unfortunately, conventional bromine delivery methods suffer from the same environmental, health, and safety issues as other oxidizers, and they are also more expensive.

The use of on-site electrolysis to make aqueous electrolytic bromine may be appealing for some cooling water applications. Sodium bromide solutions are nonhazardous and relatively low in cost, and the electrolysis process has been used successfully to produce both chlorine and bromine for more than 100 years. However, most electrolysis technology for manufacturing aqueous electrolytic bromine uses platinum-plated titanium in the electrolysis cells, and the systems operate with a typical bromide-to-bromine conversion efficiency of 35 percent.

Given the advantages of bromine for cooling water biological control, a project was started in 2001 to devise a cost-effective electrolysis-based delivery technology. This work resulted in the development of a system that can produce aqueous electrolytic bromine on-site from a nonhazardous precursor bromide salt solution. The process is based on a containerless electrolytic cell constructed of impregnated electrolytic graphite. The technology also uses a mixed solution of sodium bromide and chloride salts to obtain at least 85 percent conversion of bromide ion to bromine. The toxicities of the two salts used in the electrolysis process — sodium bromide at 3,500 mg/kg and sodium chloride (table salt) at 3,000 mg/kg — are lower than other biocides.

The electrolytic bromine solution produced by the process is made on-demand and fed immediately into the cooling tower water. Workers are not exposed to the material, which minimizes health and safety risks. To put the potential toxicity hazard of the produced electrolytic bromine solution into a common perspective, household bleach is a highly alkaline (greater than 13.5 pH), 5 percent sodium hypochlorite solution. The active product produced by the electrolysis process is a mildly alkaline (less than 10.0 pH), 0.8 percent aqueous bromine solution. At the designed 0.8 percent oxidizer content, the output of the electrolysis cell is below the hazardous designation level of 1.0 percent for oxidizers, as established by the U.S. Occupational Safety and Health Administration.

Electrolytic bromine units have proven to be a reliable means of controlling the growth of microorganisms in cooling water.

The recommended dose of electrolytic bromine for typical cooling water is 0.5 to 1.0 mg/l, measured as total bromine. Following a dose, the bromine degrades to harmless bromide ion, often in as little as one to two hours. Many cooling tower controllers can be programmed to “lock out” blowdown during, and for a set time after, a biocide feed event. With proper programming of the cooling tower controller, any discharge of electrolytic bromine in cooling water blowdown often can be avoided. In some cooling systems, due to makeup water characteristics or specific thermal requirements, it might be impossible to lock out blowdown for the required time to degrade the electrolytic bromine. In these cases, an appropriate feed of a reducing agent such as sodium sulfite into the blowdown can be used to destroy the residual biocide.

Considering that typical sanitary wastewater is highly reducing, the discharge of electrolytic bromine-treated cooling water blowdown to sanitary sewers does not present any problems unless the blowdown flow is a significant portion of the total flow to the receiving POTW.

As facilities with wet cooling towers look for ways to decrease their environmental impact and minimize health and safety issues, additional installations of the electrolysis-based bromine delivery technology are possible in the future.