Chlorine and bromine based biocides are widely used for microbiological control in cooling tower systems. These halogen compounds form strong oxidizing agents in water, which is how they kill bacteria and other microorganisms. Routine testing is important to confirm effective halogen residuals are maintained. Low residuals can result in poor microbiological control. High residuals can cause corrosion and destroy inhibitors.
The DPD test commonly used for measuring chlorine and bromine residuals in cooling towers is simple, quick, and relatively accurate. However, there are some important things you need to understand about halogen testing in cooling towers that will help you use the results for maximum benefit.
1. Standard DPD test method does not distinguish between chlorine and bromine
DPD free and total chlorine test methods are not specific for chlorine or bromine. To avoid confusion, halogen test residuals are typically reported as ppm of chlorine (Cl2) whether the biocide contains chlorine, bromine, or produces a mixture of both.
2. Three types of oxidizer residuals
Free and Total Chlorine test results can be used to monitor three types of oxidizer residuals:
Free halogen residual measures the concentration of unreacted chlorine and/or bromine available to kill microorganisms. It is typically the best measure of biocidal efficacy.
In chlorine treated systems, the free residual reflects the amount of hypochlorous acid and hypochlorite ion in solution. Where bromine is used, the free residual measures the amount of hypobromous acid and hypobromite ion. Unlike chlorine, bromine is an effective oxidizer across the pH range in which cooling towers operate. Some halogens generate both chlorine and bromine.
The free residual does not vary with changes in pH. However, the Oxidation Reduction Potential (ORP) of the water does vary with pH, with the ORP decreasing as the pH increases reflecting its reduced strength as an oxidizer.
Combined halogen residual measures the concentration of oxidant that has been consumed in reactions with any ammonia, nitrite, or organic nitrogen to form a range of chloramine and/or bromamine compounds. Unlike chloramines, bromamines can be effective biocides. Where a stabilized chlorine or bromine biocide is used, it also measures the oxidant bound to a nitrogen based stabilizer.
Total halogen residual is the sum of the free and combined chlorine and/or bromine residuals. Because bromamines can be good biocides, the total residual is sometimes used to monitor bromine treated systems.
Free and total halogen residuals can be directly measured using DPD test methods. Combined residual is determined by calculation:
3. Only a fraction of the oxidizer added is reflected in a test residual
Most of the halogen added during an oxidizing biocide addition is consumed by demand reactions and off-gassing.
Oxidizer is directly consumed by reactions with any naturally occurring organic matter including decaying vegetation, bacteria, and biofilm, and to a lesser extent with organic inhibitors such as azoles and phosphonates that are added. They also combine with any nitrogen compounds present to form chloramines or bromamines which are measured by the combined residual. The dirtier the system and the more organic matter present, the higher the oxidizer demand.
To maintain a sustained residual, sufficient oxidizer must also be added to compensate for losses due to off gassing as the water circulates over the cooling tower fill, which can be significant. These losses are a function of the oxidizing biocide, pH, and cooling tower design/operation. Stabilized biocides release free chlorine and/or bromine on a controlled basis to reduce losses due to demand reactions and off-gassing. Their lower reactivity and volatility translates into a more persistent residual at lower feed rates.
4. pH impacts biocidal efficacy when using chlorine, but not as much as you think
In chlorine treated systems, the pH dictates how much of the free residual is hypochlorous acid and how much is hypochlorite, which is only one-tenth as potent as an oxidizer.
Hypochlorite is still a relatively strong oxidizer that reacts with microorganisms and biofilm. Because the ratio of hypochlorous acid to hypochlorite ion is fixed for a given pH, as hypochlorous acid is consumed, the available hypochlorite shifts to become hypochlorous acid. And since hypochlorite is not volatile, chlorine losses due to off-gassing are greatly reduced in high pH systems. Although higher residuals may be required, chlorine can be effective in high pH systems. Key concerns using sodium hypochlorite (bleach) revolve around its stability.
5. Beware of phantom free residuals
High levels of combined chlorine or bromine (> 0.5 ppm) can react with the DPD free chlorine test to cause a phantom free residual reading that gradually increases. If a true free residual is present, you should get an immediate color change. If the reading only gradually appears, it is probably not free chlorine, but combined chlorine instead. This interference can be minimized by taking the free reading immediately after adding the DPD free reagent. It is particularly notable in makeup samples treated with monochloramine.
6. Disappearing residuals can mean very high levels
High levels can cause the DPD indicator to fade or bleach out completely making you think there is little or no chlorine or bromine in the water. This starts to be a problem when the residual exceeds 20 ppm. This can often be verified by watching for a pink when the DPD reagent is first added. The simplest solution is to dilute a fresh sample with chlorine-free water and retest. Alternatively, a higher range test method can be used.
7. Routinely monitor both free and total residuals
It’s important to understand the oxidizer demand when treating a cooling tower system. A high demand translates into higher oxidizer feed rates, and can make it difficult to achieve an effective free residual without a potentially corrosive high total residual also being developed. The relationship between the free and total residuals provides valuable information that can be used to help identify and troubleshoot systems with a high oxidizer demand.
A low free residual in comparison with the total residual typically indicates a high demand. This demand can be reduced by a more aggressive cleaning and disinfection regimen. Filtration and routine hyper halogenation should be considered.
8. Sample when it means something
Routine monitoring is important to confirm the target oxidizer residuals are achieved, and reduce the potential for problems related to over and under feed. Unless oxidizer is being fed to maintain a continuous residual, this requires sampling following the conclusion of a biocide feed cycle.
Oxidizing biocide feed schedules and control ranges vary widely. A typical intermittent feed program involves adding an oxidizer 3 to 7 times per week with a target residual maintained for a minimum of one hour. Better results are achieved if the oxidizer is slowly added over a 1-4 hour period to achieve a sustained residual.
The preferred sample point for testing is off the water treatment control loop before oxidizer injection. The sample bottle should be rinsed several times with the water to be tested and analyzed immediately. The readings can change significantly in 30 minutes. If the water is to be transported any distance, make sure the sample bottle is filled completely and capped.
When setting up an oxidizing biocide program, it is important that the residual be monitored throughout the feed cycle to verify the target range is achieved. For routine monitoring, residuals should ideally be measured following the conclusion of an oxidizing biocide feed cycle. Where this is not possible, it may be practical to manually initiate a biocide feed cycle prior to testing. A web enabled SMART controller with ORP monitoring capabilities can also be used to verify residuals are achieved. However, ORP readings should be regularly correlated with halogen test results.
Conclusions
Chem-Aqua’s aquaDART controller can help monitor oxidizing biocide residuals and keep you in range. Contact Chem-Aqua today for more information about the aquaDART. For a more in-depth view about chlorine testing refer to our tech bulletin.
Written by: Allan Browning