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 can lead to system fouling and corrosion, loss of heat transfer and reduction in energy efficiency, downtime for cleaning, illness or death, 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 biofilm management, and oxidizing microbicides such as chlorine, typically make up the fundamental backbone of cooling water biofilm control programs. 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. It is essential to feed chlorine in a manner which does not allow the biofilm to accumulate, and those responsible for the treatment of process cooling water systems often struggle with effective ways to manager and control chlorine feed.
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, allowing biofilm populations to quickly recover. The rapid reproduction rate of bacteria is the primary reason feeding chlorine a few days a week is not an effective biofilm control strategy. A low level continuous feed of chlorine 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.
A chlorination strategy shown to be effective is to add timed spikes to the continuous feed. The frequency and duration vary and are dependent on the relative cleanliness of the system. (i.e. 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 controlling chlorine feed 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 PLC's 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. 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 very important to fine tune the feed to maintain tight control around the set point. If the pump is over-sized 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. First, it is important to understand the organisms growing on system surfaces are what need to be controlled, so bulk water enumeration may not always be the best method for measuring program performance. Dip slides and ATP measurement are the simple ways to trend overall microbial activity in a cooling water system and should be included in the monitoring program. However, these may not always indicate the level of surface biofilm activity. Heat exchanger performance monitoring and visual observation of cooling tower cleanliness are simple practices and may provide hard evidence of cleanliness. Commercially available test heat exchangers or biofilm monitors can be used to measure fouling, though costly and sometimes difficult to use. Other useful tools which can be used to measure program performance include fouling coupons, activity specific media, and surface swab techniques.
Work with your water treatment provider to determine the best protection method for your cooling water system.