If you are responsible for maintaining your plant’s cooling towers, you know when you have an algae problem. You can see algae in the tower basin, the tower fill and the distribution deck. Other microbiological problems might not be as obvious; however, their effects on your cooling system can be equally destructive, if not more so.

Many people believe that algae are not harmful as long as they do not break loose or get into the bulk circulating water system, but this assumption is not true. Algae can serve as a protective layer for other organisms and provide a source of usable organic carbon that feeds slime-forming bacteria. They also can cause plugging of the distribution deck on cooling towers, which will reduce the system’s efficiency. Massive growths of algae in the distribution deck can break loose from the top deck and cause exchanger fouling.

Fungal growth is another problem found in many cooling systems. Often colonies of fungi develop on the surface of the tower fill to the extent that the evaporative capacity and overall efficiency of the cooling tower is reduced significantly. In addition, fungal colonies provide an optimum growth environment for other problem-causing microorganisms, particularly bacteria.

As unicellular microscopic organisms that are considerably smaller than most fungi and algae, bacteria are not photosynthetic and therefore do not require light to produce an energy source nutrient. They simply require a carbon source and water; in addition, some require oxygen. Many bacteria can attach to surfaces, where they become difficult to remove. Once the bacteria are “glued” to the surface, they begin to produce a material called extra-cellular biopolymer, or “slime.” The slime provides an accommodating environment for the bacteria to multiply. The microbiological growth creates a biofilm that begins the deposit process and eventually becomes the “glue” that holds the deposits together.

Slime-forming bacteria are among the most predominant types of bacteria occurring in open recirculating cooling water systems. These bacteria are aerobes and are naturally present in the environment. Because the cooling tower also acts as an air washer, the biofilm mass that contains the filamentous bacteria (glue) can serve as a nucleus for agglomerating and depositing waterborne suspended solids.

Biological fouling has been estimated to comprise 37 percent of tower fill fouling, second only to mud and silt at 41 percent. In some cases, cooling towers have collapsed under their own weight due to the tremendous weight of the deposits and deterioration of the tower. In other cases, sections of fill weighing more than 40 lb/ft³ have been cut out of towers, when the fill should have weighed closer to 5 lb/ft³. Can you imagine the inefficiency that occurred in these systems, along with the associated costs?

Identifying and Correcting a Biofilm Problem

Enlarge this picture This cooling tower is treated regularly with biocides to prevent biofilm problems. The biocides are added automatically from the small white tank shown in the foreground.

To determine whether you have a biofilm problem, observe the water in the cooling tower. Is it cloudy or murky? Are algae floating in the water or sticking to the surfaces? Go to a corner or other low-flow area of the tower basin and use your hand to feel the inside of the tower basin beneath the water line. If it feels slippery, slick or slimy, you have a biofilm problem. Inspect the tower fill and listen to the flow of water through the fill to see if it sounds restricted. Check the head pressures and temperatures of the condensers and compressors to determine whether they are normal. You can use dip slides to determine the bacterial count or fungal count in the planktonic water (bulk water), but these readings will not tell you how much sessile (surface) fouling might be in the system. Monitor and chart energy use and efficiency, and inspect filters and strainers for slime, algae and fungal contamination.

Biofilm problems can be eliminated by using a biodispersant and a chlorine-based compound. Put the biodispersant into the system and let it circulate at least once through the system before adding the chlorine compound. The results usually are immediate and phenomenal. You can stand by the tower and hear the increased water flow through the tower fill; you can watch the temperature and pressure gauges move to normal operating ranges; and you can expect your automatic blowdown controller to open the solenoid and begin blowing down the excess solids that have been dislodged and are circulating in the open water system. You may need to make several applications before the biofilm is completely eliminated.

Of course, the ultimate goal is to prevent biofilm problems from occurring in the first place. This goal can be achieved through the regular use of biocides as part of a preventive maintenance program. The biocide dosage and number of applications will be determined by the conditions involved. Generally, alternating an oxidizing biocide with a nonoxidizing biocide is an effective approach to controlling biofilm production. This method is successful not because the bacteria build up a resistance or immunity to a particular biocide, but because it is a broad-spectrum control program.