There is no such thing as a microorganism-free industrial cooling water system; conditions are simply too beneficial for their growth. Over time, microorganisms such as algae, bacteria and fungi in cooling water systems can form biofilm (i.e., slime), which is protected by a naturally occurring matrix composed of extracellular polymeric substance (EPS). This enables biofilm to thrive on surfaces ranging from steel and concrete to plastic fill. It is critical to monitor and mitigate biological growth within industrial cooling water systems since biofilm reduces overall efficiency significantly and can harbor pathogenic bacteria. Scale and corrosion substances often stick to the tacky biofilm and combine to create biofouling.

This can lead to many problems, including reduced heat exchanger efficiency, microbially influenced corrosion (MIC) proliferation, blockage of filters and screens, increased downtime and creating a conducive environment for harmful bacteria, such as Legionella pneumophila, to thrive. Various groups of microorganisms are known to cause these problems, including aerobic and anaerobic bacteria, fungi, algae and protozoa.

Remediation of biofouling versus biofouling prevention

Mechanical removal of biofouling using scrapers, brushes and foam balls can be a useful first step in serious remediation situations, but killing the bacteria requires the use of one or more biocides. An array of biocides with different mechanisms of action are available. Plant operators should consult with water treatment service company experts to determine which combination of biocides will work best in their facility for remediation and, ideally, ongoing monitoring and prevention programs that optimize cooling water operations.

Industrial biocides are either oxidizing or nonoxidizing based on their mechanism of action. Nonoxidizing biocides can be either electrophilic or lytic. They all have advantages as well as limitations under particular conditions, which is why many microbiological control programs use combinations of oxidizing and nonoxidizing biocides.

Oxidizing biocides

The most commonly used treatment for biofouling in industrial cooling water systems is oxidizing biocides due to their effectiveness, low cost and rapid biodegradation to nontoxic molecules. They demonstrate broad-spectrum activity against bacteria, fungi and algae and are capable of killing microorganisms within a matter of seconds. The mechanism of action is chemical oxidation of the cellular structure and subsequent cell lysis. Oxidizing agents can readily pass through cell membranes, leading to cell death. Although they are effective at killing microorganisms in water, oxidizing biocides are poor at penetrating biofilms and dispersing anaerobic infestations, and they do not offer extended prevention of microorganism growth.

Nonoxidizing biocides

Nonoxidizing biocides inhibit microbial growth through interference with cell metabolism and structure. For example, electrophilic agents, such as isothiazolones, react covalently with cellular nucleophiles to inactivate enzymes and initiate the formation of intracellular free radicals, which contribute to cell death. Alternatively, cationic membrane active biocides such as quaternary ammonium (also known as quats) and phosphonium compounds (such as THPS) destabilize membranes, leading to rapid cell lysis.

Nonoxidizing biocides are capable of killing microorganisms within minutes to hours after contact. Due to their greater persistence, nonoxidizing biocides are more effective than oxidizing biocides at long-term prevention of biofilm growth. Although nonoxidizing biocides can be more expensive on a per-unit basis, they are less likely to cause corrosion and are more compatible with other water treatment additives.


Table 1. Comparison of oxidizing and nonoxidizing biocides


Biocide optimization

With few exceptions, microbial control experts recommend using oxidizing and nonoxidizing biocides in tandem to optimize immediate and ongoing microorganism control. Typically, a low residual oxidant is used to reduce the planktonic population in a system and assist in destroying the biofilm EPS layer. A nonoxidizing biocide is used to penetrate and destroy biofilm. Nonoxidizing biocides also can be synergistic when combined with other biocides, making the effect of two biocides greater than the sum of activity of both products used individually. This results in enhanced efficacy and improved microbiological control.

“Microbial control experts recommend using oxidizing and nonoxidizing biocides in tandem to optimize immediate and ongoing microorganism control.”

Ongoing optimization of biocides is critical for maintaining effective microbial control in industrial water treatment applications. The optimization of biocide programs enhances biocide performance and overall treatment costs. 

Reducing the risk of Legionella in cooling towers

Industrial cooling tower systems can present an ideal environment for growth of Legionella pneumophila, a bacterium found in untreated freshwater worldwide. When people breathe in water droplets contaminated with Legionella pneumophila, they can contract Legionnaires’ disease — a serious, sometimes lethal pneumonia — or Pontiac fever, an influenza-like, self-limited illness.

Legionella is a critical issue in the global water treatment industry since the waterborne bacterial infection kills about one in 10 patients. In Europe, Australia and the U.S., there are about 10 to 15 cases detected per million population per year, according to the World Health Organization (WHO).1 The 6,238 cases reported nationwide in the U.S. as of mid-December 2017 represented a 13.6 percent increase in cases since 2016, nearly double the increase of 7.8 percent from 2015 to 2016.2

Many health-associated organizations, including WHO, the U.S. Center for Disease Control (CDC) and the European Working Group for Legionella Infections (EWGLI), have published recommendations for reducing the risk of Legionnaires’ disease. The U.S. Environmental Protection Agency (EPA), the National Science Foundation (NSF) and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) are currently developing new protocols or guidelines for mitigating Legionella in industrial and public water systems.

Unfortunately, the ecology of Legionella pneumophila in water systems is not fully understood, and current best practices used to control it have been met with limited success. Laboratory studies have revealed that limiting biofilm formation with nonoxidizing biocides may reduce Legionella growth. In addition, it is known that Legionella prefer to live inside amoeba and, as a result, higher, oftentimes corrosive levels of oxidizing biocides are needed in order to reduce growth. For these reasons, the use of oxidizing and nonoxidizing biocides as part of a robust water treatment program is recommended for reducing the risk of Legionella in cooling towers.

Efficient cooling tower operations

The benefits of using an effective combination of oxidizing and nonoxidizing biocides include:

• The combination of oxidizing and nonoxidizing biocides provides an optimized balance of speed of kill and duration of effectiveness against microorganisms.

• It helps minimize corrosion and extend asset life by including nonoxidizing biocides in a water treatment program.

• Nonoxidizing biocides are more effective at controlling biofilm formation and growth.

• An optimized combination of oxidizing and nonoxidizing biocides helps control biofouling, protect equipment and increase operational efficiency.

Biocide labeling improvements

As the need to address industrial biofouling has increased, so has the need for enhanced biocide labeling that helps users better understand the chemistry and recommended dose rates for remediation and ongoing prevention. For instance, DBNPA (2,2-dibromo-3-nitrilopropionamide) is an effective antimicrobial product with a history of successful use in a variety of industrial water treatment applications for more than 30 years, but DBNPA labels have never offered guidance between dose levels needed to treat planktonic bacteria versus biofilm.

Dow Microbial Control conducted studies to assess the effect of DBNPA treatment on the growth and control of microbial biofilm development on industrial surfaces. Using an in vitro biofilm technique, it was demonstrated that DBNPA can inhibit microorganisms in cooling water systems at levels as low as 1 part per million (ppm) active; however, this level does not offer sufficient eradication of bacteria growing within biofilm communities. 

Due to this new data, Dow Microbial Control recently updated its EPA labels to provide a 5 ppm to 24 ppm dosage recommendation (based on active percentage) for customers to effectively control nonpublic health biofilms to reduce system fouling in industrial or commercial cooling water systems (see Figure 1).


Figure 1. Dow Microbial Control EPA label sample for DBNPA dosages. Graphic courtesy of Dow Microbial Control.


Note: While Dow product approvals address nonpublic health claims, Dow does not have a registered biocide in the U.S. with Legionella control claims on its label.

Microbial control saves energy, water and money

The United Nations estimates that by 2030 the world population is expected to increase from 7 billion to 8.3 billion people. This means the world will require 30 percent more water and 40 percent more energy. Both water and energy scarcity issues worldwide are increasing the pressure on private industry to reduce its reliance on freshwater sources and to conserve water through enhanced efficiency, such as cooling towers.

“Optimized, ongoing use of biocides … is critical for enhancing heat exchange and other process efficiencies, decreasing energy and water use and minimizing downtime for maintenance and repairs.”

Future market reports predict the water treatment sector will be the biggest driver of future biocide growth due to the increasing demand for treated water for municipal and industrial purposes, especially in water-intensive industries, such as manufacturing sectors and power generation.3 Optimized, ongoing use of biocides to control the growth of microorganisms in industrial cooling water systems is critical for enhancing heat exchange and other process efficiencies, decreasing energy and water use and minimizing downtime for maintenance and repairs. Supporting proper water quality in a cooling system not only helps reduce the risk of unwanted, sometimes dangerous microorganisms, but also helps operators comply with existing and pending regulations. 


1. WHO Legionellosis fact sheet, Updated November 2017.

2. Legionnaires’ Disease Is Rising at an Alarming Rate in the U.S., U.S. NEWS 12/14/2017.

3. Biocides Market: Global Industry Analysis and Opportunity Assessment 2017-2027, Future Market Insights.



Christina Pampena is the regional marketing manager for Europe and North America at Dow Microbial Control. She is responsible for regional marketing for the oil and gas and water treatment market segments. Pampena holds a bachelor’s degree in chemistry from the University of Pittsburgh. She can be reached at




Brian Corbin is a customer application scientist at Dow Microbial Control. He is responsible for the industrial water treatment segment and provides technical service and drives growth for the North America region. Corbin holds a Ph.D. in microbiology and molecular genetics from the University of Texas Health Science Center at Houston and completed a post-doctoral fellowship at Vanderbilt University. He can be reached at