The proliferation of microorganisms (bacteria, fungi and algae) presents a significant challenge in the maintenance and operation of process cooling water systems. Excessive accumulation of biofilm and the activity of the microorganisms within, can lead to fouling, formation of mineral scales, amplification of pathogens (e.g. Legionella spp.) and microbiological corrosion (MIC). If biofilm is not well controlled, the result can be loss of heat transfer and reduction in energy efficiency, downtime for cleaning, illness or death, corrosion-related failure and costly equipment repair or replacement. In order to minimize or prevent these issues, it is important to have a robust cooling water treatment program with a strong emphasis on biofilm control. There are a number of effective strategies for cooling water biofilm control, and this article will primarily discuss the application of chlorine.
Oxidizing microbicides such as chlorine, bromine and chlorine dioxide make up the fundamental backbone of cooling water biofilm control programs. This has always been the case, but more recent concern for the control of pathogens, especially those causing Legionnaire’s disease, have placed increased emphasis on their use. The oxidizing microbicides are important in the control of biofilm and biofilm-related problems as oxidizing microbicides not only kill microorganisms, but also help to destroy the extracellular biofilm components. This is especially important in reducing the risk for Legionnaire’s disease.
Chlorine is most often fed in the form of liquid sodium hypochlorite (bleach) but may also be applied as chlorine gas or solid forms such as calcium hypochlorite. Chlorine can also be generated electrolytically from salt (NaCl) and is an effective way to provide halogen or mixed oxidant without the need to store a large amount of hazardous materials on-site. In any case, it is essential to feed chlorine in a manner that does not allow biofilm to accumulate. In most cases, the use of supplemental non-oxidizing microbicides and dispersants, which work either synergistically or complementary with chlorine, will assist with penetration into biofilm and algal masses as well as provide protection for off-line equipment. The frequency of addition will depend on need, but typically one to three days a week.
Those responsible for the treatment of process cooling water systems, including water treatment professionals, often struggle with effective ways to manage and control chlorine feed. There is a school of thought that chlorine is not effective in cooling water systems operating with a pH of greater than 8.0. The basis for this is that chlorine dissociates into two forms in water; hypochlorous acid (HOCl) and the hypochlorite ion (OCl-). At a higher pH, a larger percentage of the chorine will be present in the OCl- form, which is a weaker oxidant and carries a negative charge. In practice this tends to have less relevance for cooling water where chlorine is applied continuously when compared to sanitizing and disinfection applications where exposure to chlorine may last only a few minutes.
About Chlorine Residuals
Chlorine residuals are measured and expressed in two ways: total and free chlorine. Free chlorine consists of both the HOCl and OCl- forms, which are the more effective oxidants, while the remaining portion of the total chlorine will consist of chlorine that has combined with amino acids, ammonia or other nitrogenous compounds. These will tend to have less microbicidal activity, depending on the form. Monochloramine is one form of combined chlorine that can be generated on-site and is often used in municipal drinking water. It does not have the ability to oxidize biofilm components but can penetrate into biofilm and algal cells to provide antimicrobial activity. Because it is readily stripped over cooling towers and cannot oxidize the extracellular biofilm components, its use in cooling towers has been limited. When chlorine is added to cooling water systems, any free residual will quickly dissipate once the feed is discontinued as the oxidant readily reacts with organic compounds and reducing agents. It is imperative to test free chlorine residual within a minute of obtaining the sample rather than gathering a sample and walking it back to the test station. Free chlorine residuals do not persist once the chemical feed is stopped and why biofilm populations will quickly recover. Just to understand how quickly bacteria can reproduce, one bacterium with a generation time of 30 minutes, will result in a population of 28,147,497,660,000 bacteria in 24 hours! The rapid reproduction rate of bacteria is the primary reason feeding chlorine a few days a week is not an effective biofilm control strategy. Another important point to make is that low level continuous feed of chlorine (≤ 0.2 – 0.5 ppm) can result in a scenario where the rate of bacterial reproduction and biofilm accretion keeps pace with, or exceeds, the rate of kill, and is often the reason for failure of chlorination and other oxidant programs to control biofilm. Chlorine residuals can be monitored with easy-to-use test kits are available in a variety of options, including test strips, color comparators, titration, colorimetric or spectrophotometric.
Controlling Chlorine Feed
Considering low level continuous feed of chlorine may not always be sufficient to maintain control of biofilm, a strategy that has been shown to work well is to apply timed spikes of chlorine with the frequency and duration dependant upon the relative cleanliness of the system. This could be as little as a few hours one day a week in clean systems, or as much as daily in others. An example would be to hold a continuous free residual of 0.2 ppm and then spike to 0.5 – 0.75 ppm two days a week for a period of 3 hours. The most popular and reliable method for the feed and control of chlorine in cooling water systems is ORP (oxidation-reduction potential). While ORP is not a direct measurement of chlorine residual, it can be used to maintain relatively consistent control once dialed in. There are several off-the-shelf cooling water controllers or PLCs capable of managing this type of control. Most controllers with an ORP sensor input will allow for continuous control but not all will have the flexibility to add the controlled timed event. If the current controller being used does not have this capability, an upgrade to one that does is recommended. Free and total chlorine probes are available and can be used. However, they are prone to issues and require a great deal of maintenance and care, especially in cooling water systems with heavier solids loading. When using ORP or a chorine probe, it is important to fine tune the feed to maintain tight control around the setpoint. If the pump is oversized or set too high, overfeed can occur. This will result in a pattern of continuous over and under feed of chlorine, which can negatively impact corrosion rates.
The question often comes up as to how to monitor the effectiveness of a chlorination program outside of residual measurement and control. Unfortunately, this is not as easy as it may seem, and multiple observations must be used to determine performance. First, it is important to understand the organisms growing on system surfaces are what we are trying to control. This means bulk water enumeration may not always be the best method for measuring performance. However, dip slides or other culture techniques and/or ATP measurement should make up part of the monitoring program and are the simplest way to trend overall microbial activity in a cooling water system. With continuous halogen feed, bulk water bacterial counts or ATP should be at very low levels (<1,000 cfu/mL). ATP control limits will depend on the actual instrument being used. Organisms are constantly being shed into the water from the surfaces, so if high numbers are found in the bulk water, then chances are, there is either a significant surface population or contamination through an outside source. Heat exchanger performance monitoring and visual observation of cooling tower cleanliness are simple practices and provide hard evidence of cleanliness. Commercially available test heat exchangers or biofilm monitors can be used to measure fouling, though they are costly and sometimes difficult to use. There are also a number of other useful tools that can be used to measure performance, including fouling coupons, activity specific media and surface swab techniques.
Alone, continuous low-level chlorine feed may not be enough to control cooling water biofilm and its related problems. For improved protection, chlorine should be continuously fed, monitored and controlled. The effectiveness of chlorination programs can be increased with the addition of controlled residual spike(s) and supplemented with non-oxidizing microbiocides and disperants, when possible. To ensure accurate measurements when reading free chlorine residuals, measure samples within one minute of gathering. There are numerous tools available for monitoring results, including microbiological tests, visual observation, heat exchanger efficiency monitoring and test heat exchangers. Work with your water treatment provider to determine the best protection method for your unique system.