Biofilms can become a large problem in cooling water systems and can cause increased corrosion, fouling, and deposition rates. This can result in damages to your equipment, increased energy consumption, and increased costs. Biofilms in cooling systems can also harbor Legionella and other harmful bacteria.
KEY INSIGHT: Legionella is a bacteria that can cause diseases including Legionella pneumonia and Pontiac fever. It appears in both natural (rivers and lakes) and artificial (cooling towers and fountains) water environments.
With heightened awareness around sanitization and respiratory health, it is important to consider your cooling system in your safety plan and educate your teams in current water chemistry hygiene methods.
In many process and cooling water systems, microbiological fouling can present itself as an issue. Microbiological fouling, or “biofouling”, is almost always due to the presence of biofilm in the system.
A biofilm is a configuration or collection of many different types of bacteria, held together and protected by exopolysaccharide (EPS) – a substance produced by the bacteria in the biofilm. Biofilms have both an aerobic and anaerobic layer, which house their respective type of bacteria. Biofilm can form on system surfaces such as piping, sump walls, cooling tower fill, and heat exchangers.
KEY INSIGHT: Aerobic layers contain a significant amount of oxygen which allows microbes to quickly break down organic matter. Anaerobic layers occur in areas that are oxygen-depleted, making decomposition happen at a slower rate.
Through harboring and protecting bacteria, biofilms become excellent environments to foster the growth of Legionella. In addition to sheltering bacteria, EPS collects nutrients and additional bacteria and can increase the risk of scale formation. Furthermore, biofilms can corrode most metals, even stainless steel, through what is known as “microbiologically induced corrosion” (MIC). MIC occurs when biofilm bacteria creates low pH environments which corrodes the metal surface to which the biofilm is attached. Bacteria can also generate a cathode and anode to induce the flow of ions and spur corrosion. In stainless steel, once the oxide layer is penetrated, bacteria will begin to “tunnel” through the rest of the metal.
Biofilms also insulate heat transfer up to three times more than calcium carbonate scale, decreasing efficiency and driving up energy costs.
Biofilm is formed when bacteria present in process water begins to stick to surfaces and indicates for other bacteria to join. The rate of bacterial and biofilm formation increases in warmer condenser water and process cooling water. These factors should be discussed when considering the control of biological growth and biofilm formation.
Biofilm formation occurs more readily in areas of low flow; it is important to eliminate all piping dead-legs and ensure regular circulation of system water during periods of low load, the shoulder season, and the off-season.
KEY INSIGHT: Shoulder season is the period between the heat of summer and the cold of winter. Especially during this transitional time between peak seasons, it’s important to care for equipment and machines to prevent damage.
Both the availability of nutrients and the presence of collection surfaces for bacteria help contribute to bacterial growth and biofilm formation. Filtration is vital to remove the suspended solids associated with these factors. It is recommended that filters be monitored to ensure they do not become breeding grounds for additional bacteria as a result of the accumulation of these suspended solids.
Physical cleanings on tower systems are recommended twice annually to help ensure proper biological control, especially following shoulder seasons or downtime. After the physical cleaning, which breaks up biofilm, it is crucial to eliminate the newly freed bacteria by disinfecting the system.
Copper is naturally resistant to bacterial growth and excretes copper ions when corroded. These ions poison bacteria, thus helping manage biofilm formation.
Biodispersants are chemicals added to process water that are specifically designed to break up biofilm. An industry best practice, the consistent addition of a biodispersant is recommended in all open recirculating cooling systems.
Biocides are employed to kill bacteria in many scenarios and consist of either an oxidizing or non-oxidizing agent. It is important to note that not all biocides penetrate biofilms equally, and many oxidizing biocides are very weak penetrators. The use of a biodispersant is recommended in conjunction with an oxidizing biocide.
Physical treatment devices that are designed to kill bacteria are only able to kill bacteria in the bulk water circulating through them. In these situations, the use of a biodispersant may be needed to ensure biofilm growth is managed.
There are many devices available in the marketplace for monitoring biofilm formation, ranging from simple screen coupons to sophisticated devices that analyze electrical interferences. Unexplained reductions in heat exchange efficiency or visual indications including slimes on sump walls or piping also indicate biofilm formation.
A simple test that is a general indicator of biological activity which may be used is a dipslide or an ATP level test. Although these will indicate bacteria levels present in the bulk water of the system, they don’t necessarily indicate the presence of biofilm.
The best way to ensure you do not have biofilm is to employ a properly designed water treatment program that minimizes or prevents scale and corrosion, incorporating both a biocide and biodispersant.
"*" indicates required fields
