Understanding Biofilms in Heat-Transfer Equipment, Part 2
By Jim Lukanich, Senior Technical Specialist,
Buckman Laboratories International
Part 1 of this article detailed how biofilms originate in cooling towers and other water-handling equipment, and what problems they create for maintaining performance. It can be seen that the growth of bacteria on surfaces in cooling and process water systems can lead to significant deposit and corrosion problems. Once this is understood, then the importance of controlling biofilms becomes quite clear.
Too often microbiological control efforts focus only on planktonic counts, that is to say the number of bacteria in the bulk water. While some useful data may be gathered from monitoring daily bacterial counts, monthly or weekly counts are simply time wasted and have little meaningful use. Planktonic counts do not necessarily correlate to the amount of biofilm present. In addition, planktonic organisms are not generally responsible for deposit and corrosion problems. There are a few exceptions, such as a closed loop system, where planktonic organisms may degrade corrosion inhibitors, produce high levels of H2S, or reduce pH. Systems with low planktonic counts may have a significant biofilm problem and vice versa. Therefore, efforts should focus on general biofilm control.
Biofilms can be controlled through the use of microbicides, biodispersants, and by limiting nutrient. Microbicides, both oxidizing and nonoxidizing, can be effective in overall biofilm control when applied properly. The oxidizing microbicides, such as chlorine, bromine, chlorine dioxide, and ozone, can be extremely effective in destroying both the extracellular polysaccharide and the bacterial cells. When using oxidizing microbicides, one must be sure to obtain a sufficient residual for sufficient duration to effectively oxidize the biofilm.
Unfortunately, there are those who are overly concerned with the corrosive nature of the oxidizing microbicides and fail to apply the needed residual oxidant. Low residual oxidant levels may significantly reduce planktonic counts but may not be sufficient to control biofilm. The level of oxidant and duration required will vary from system to system. It is generally more effective to maintain a higher residual for a duration of several hours than it is to continuously maintain a low residual. Continuous low-level feed may not achieve an oxidant level sufficient to oxidize the polysaccharide and expose the bacteria to the oxidant.
Another misconception is with the use of chlorine at alkaline pH (>8.0). It is often thought that chlorine is ineffective for controlling microorganisms at elevated pH. This is only half true. Certainly, the hypohalous acid form of chlorine (HOCl) is more effective at killing cells than the hypohalite form (OCl-). However, the hypohalite is actually very effective at oxidizing the extracellular polysaccharide and the proteinaceous attachment structures. Therefore, the use of chlorine in alkaline cooling waters can still be extremely effective. This is especially true when combining chlorine with bromine or with a compatible nonoxidizing microbicide such as a polyquat. When this is done, you achieve both oxidation of the extracellular material and sufficient kill of the microorganism.
Nonoxidizing microbicides are also effective in controlling biofilm. Effective control is greatly dependent on frequency of addition, level of feed, and resistance of the incumbent population to the product being fed. Control cannot generally be achieved by once-a-week additions as is common in "full service" applications. Typical application for effective control may include a slug addition of product two to five times a week. As with oxidizing microbicides, frequency and dosage will depend on the system conditions. It is generally most effective to alternate nonoxidizing microbicides at every addition to ensure broad spectrum control.
Most nonoxidizing microbicides will have little effect in destroying the extracellular polysaccharide found in the biofilm. However, many microbicides may be able to penetrate and kill bacteria found within the biofilm. Combining the use of nonoxidizing and oxidizing microbicides is a very effective means of controlling biofilm. When using a nonoxidizing microbicide in conjunction with an oxidizing agent, there should be no residual oxidant present in the system at the time of addition. Sufficient time should be allowed for the nonoxidizing microbicide to work before resuming oxidant feed unless an oxidant compatible microbicide is being used ( i.e., polyquat).
Biofilm control programs can be made more effective through the utilization of a biopenetrant/dispersant product. Products that penetrate and loosen the biopolymer matrix will not only help to slough the biofilm but will also expose the microorganisms to the effects of the microbicide. These products are especially effective when dealing with systems that have a high TOC loading and a tendency to foul. These products are typically fed in slug additions prior to microbicide feed. Low-level continuous feed may not be as effective, since it often takes a certain threshold amount to produce the desired effect. Buckman Laboratories offers a variety of products, including BSC 8000 and 8020, that are effective when used with biocides. The products can be slug-fed into a system at a rate of 60-150 ppm daily (depending on the product used) and then to maintain system cleanliness with loading at a rate of 10-60 ppm, one to three times a week, depending on the severity of the loading.
Recent developments in biodispersant technology is making this approach more effective and popular than ever before. Enzyme technologies that will break down the extracellular polysaccharides and degrade bacterial attachment structures (fimbriae) are currently being developed and patented. These technologies, although expensive, may provide biofilm control where microbicide use is environmentally restricted or provide a means of quickly restoring fouled cooling water systems to a clean, efficient operable state.
The importance of biofilm control must not be taken lightly. It is the fundamental basis for controlling a high percentage of deposition and corrosion problems in process and cooling water systems. Once these fundamentals are understood, effective treatment strategies can be developed.
For more information: Buckman Laboratories International, 1256 N. Mclean Blvd., Memphis TN 38108. Tel: 901-278-0330.