Which Chlorine Monitoring Method is Most Effective?

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2 Which Chlorine Monitoring Method is Most Effective? It's no secret that contaminated water can threaten the health of your laboratory animals and the validity of your research. That is why a residual disinfectant such as chlorine is commonly used in automated watering systems to kill harmful bacteria found in the drinking water. Chlorine also helps minimize the growth of biofilm in the watering system. But how much chlorine is effective? Maintaining the proper amount in your system is very important. If there is too much chlorine present it can damage the watering system by causing corrosion of stainless steel drinking valves, manifolds, and room distribution piping. But if there is not enough chlorine available, bacteria in the water will not be destroyed. As a general guideline, drinking water should contain 2 to 3 parts per million (ppm) chlorine throughout an automated watering system. This level is effective at destroying upwards of 99% of all bacteria present, but a level of 0.5 to 10 ppm is considered acceptable. Edstrom Industries recommends free chlorine levels in the animal watering system be tested on a daily basis. This will ensure the system is maintaining the appropriate amount of residual disinfectant. Free chlorine is the sum of the concentration of dissolved chlorine gas (Cl2), hypochlorous acid (HOCl) and hypochlorite ion (OCl-). This form of chlorine is the strongest disinfectant, and the measure of its concentration is the number that is of concern when determining the chlorine residual in water. Monitoring Methods The methods for measuring chlorine range from very basic to quite sophisticated, with the following methods being most commonly used: color comparator method using a handheld test kit, colorimetric method and amperometric method. When taking a chlorine measurement, it is critical to test the sample immediately. Chlorine in water is not stable, and the concentration will decrease rapidly. Exposure to sunlight or other strong light or agitation will accelerate the reduction of chlorine. Samples to be analyzed for chlorine should not be stored. At a minimum, chlorine concentration in an animal watering system should be measured at points farthest from the incoming supply water right before flushing. Chlorine concentration will be the lowest in these areas. Samples should be taken from the last interconnect quick disconnect or rack manifold drain valve in the room farthest from the source of chlorine injection. At the same time, measure chlorine at the chlorine injecting outlet and compare the concentrations. A higher concentration may be needed to compensate for chlorine dissipation in the automated watering system. (see diagram on last page) Color Comparator Method - Handheld Test Kit The color comparator method using a handheld test kit is the most basic method for detecting chlorine levels in an automated watering system. It involves taking a water sample and mixing it with a chemical indicator, N,N-Diethyl-p-Phenylenediamine (DPD). This chemical reacts with the chlorine by dyeing the sample water a shade of pink that is indicative of the chlorine content in the sample. To determine the chlorine concentration in the water, the tester must compare the sample color to a standard on the test kit's

3 color wheel. The wheel contains a gradient of colors, and the darker the color pink, the higher the chlorine concentration. The handheld test kit is effective at detecting low chlorine levels from 0 to 3.5 ppm. Colorimetric Method The colorimeter is an automated machine installed in the watering system to continuously measure chlorine levels. The colorimeter is similar to the handheld test kit in that both instruments use the chemical indicator DPD and both measure the same color, but how they measure the color is what differs. While the handheld kit relies on the human eye to read the sample, a colorimeter uses a photodiode or electric eye to measure the intensity of the sample color. A light beam is passed through the sample, and the amount of light transmitted depends on the amount of color present in the sample. If the sample is very dark in color, limited light will pass through, which indicates a high chlorine concentration. Colorimeters are capable of generating an output signal for remote monitoring. These machines commonly have 4 to 20 milliamp outputs to send the measurement readings to a remote system such as a monitor or controller. Colorimeters typically detect chlorine levels in the range of 0 to 5 ppm, but vary by manufacturer. Amperometric Method The third and most sophisticated technique for measuring chlorine levels is the amperometric method. With this method, chlorine levels in the water are continuously monitored by a sensor probe that contains the electrolyte Potassium Chloride (KCl). The probe is located in the distribution piping, and as the water travels by, the chlorine moves across the membrane located on the bottom of the probe and reacts with the KCl to generate an electric current. The probe then measures this current to determine the chlorine concentration in the water. The concentration will determine the strength of the current, delivering a stronger current when the concentration is high. Like the colorimetric method, the amperometric method typically utilizes 4 to 20 milliamps to send readings to a remote system to provide alarm notification and/or data archiving. This system often gauges chlorine levels from 0 to 20 ppm. Strengths/Weaknesses of Each Method The color comparator, colorimetric, and amperometric methods all have benefits and drawbacks when it comes to monitoring chlorine. While one method may be easier to conduct than another, it may not be as accurate. And while another method may be more accurate, it may require more expense and maintenance to ensure that accuracy. The following paragraphs discuss the strengths and weaknesses of each technique. Handheld Test Kit Using a conveniently packaged handheld test kit is the simplest and least expensive means for measuring chlorine concentration. This method involves few materials and no maintenance of a complex system. It represents a fairly accurate picture of the chlorine level in the water if the person reading the test does so correctly. However, testing chlorine levels with this method is not continuous. Rather, the measurement is simply a snapshot in time of the chlorine present when the sample was taken. Colorimetric Method Of the three methods discussed in this paper, the colorimetric method is middle of the road as far as expense and complexity. The colorimetric method delivers continuous chlorine testing with instantaneous results. Readings can even be forwarded to a remote

4 system for alarm notification. The colorimetric method eliminates the need for visual analysis by an operator and the concern for background lighting, but its accuracy is dependent upon several criteria: a constant supply of clean water for taking the samples, a large quantity of chemicals, and perfect operation of components such as pumps and tubing. Hundred of gallons of water are potentially required each day because the system continuously takes water samples. It also consumes a large quantity of the chemical indicator DPD, as well as ph buffer. This system is ultimately more accurate than a handheld test kit, but a great deal of maintenance is required to ensure the colorimeter operates correctly. The amount of maintenance needed depends upon the quality of the inlet water. Tubing needs to be replaced periodically, and the chemical solution bottles need to be replenished. Amperometric Method As stated earlier, the amperometric method is the most sophisticated method for monitoring chlorine, as well as the most costly. It provides continuous, automatic testing of chlorine levels and like the colorimeter, the amperometric method allows remote alarm notification and removes the need for human interpretation. Of the three methods, it is the most accurate when used with water that is not reverse osmosis (RO)-purified. This method does not add chemicals to the water and is a truly continuous system since it does not rely on water sampling. Furthermore, it does not result in wasted water like the colorimetric method, which demands large volumes of water for regularly taking samples. Constant flow is required for this system to perform as specified. Limitations of the Amperometric Method with Reverse Osmosis Water While the amperometric method may sound like the ideal way to monitor chlorine levels, it is not suitable for all applications. The major disadvantage with this method is its inability to effectively monitor chlorine levels in RO water. This is due to the purity of RO water and its need to seek out nutrients. The result is that the electrolyte diffuses out of the probe as a consequence of the mutual attraction between the RO water and the electrolyte. This leaves no indicator to generate an electric current with the chlorine in the probe. Because of this, the chlorine cannot be accurately measured. Many facilities that are interested in a sophisticated chlorine measurement tool are using RO water, thereby making their chlorine monitoring choice quite complicated. To make the amperometric method work with RO water, it is necessary to inject a salt solution into the water prior to the chlorine monitor to raise the conductivity of the water. By doing this, the osmotic pressure is reduced, conserving the electrolyte in the probe. Increasing the conductivity would require an additional tank and injection system, would involve the mixing of chemicals, and would result in wasted water, making this already expensive system even less economical, not to mention impractical. Maintaining such a system would be extremely time-consuming and labor-intensive. Raising the conductivity basically turns a clean water source into a dirty one. Cleaning up the water once again is a large undertaking that needs to be considered. Conclusion A perfect method for monitoring chlorine levels does not exist. Each technique has its flaws. The color comparator method using a handheld test kit will provide fairly accurate snapshots of chlorine levels in the watering system when performed on a daily basis, but does not provide continuous testing and is subjected to human error when reading the color table. The colorimetric method provides continuous testing with online monitoring,

5 but at the expense of large quantities of water, chemicals, and maintenance. Finally, the amperometric method, while the most accurate of the three, offers no way for effective chlorine monitoring of RO water. The challenges associated with chlorine monitoring are not easily overcome. When determining the appropriate solution for your facility, it is important to consider your goals in testing, as well as the expense and maintenance you are willing to endure. Edstrom Industries does not provide an online chlorine monitoring solution because of the reasons mentioned above. However, if you are considering purchasing this type of system from another vendor, we are happy to make accommodations for it in our watering system. References White, G.C. Handbook of Chlorination, 2nd ed. Van Nostrand Reinhold, New York, NY, pp , Greater Vancouver Regional District (GVRD). Chlorine Monitoring and Dechlorinating Techniques Handbook, pp McFeters, G.A. Drinking Water Microbiology, Springer-Verlag New York Inc., New York, NY, pp , Color Comparator Method Colorimetric Method Amperometric Method Advantages Simple to perform Inexpensive Conveniently packaged Provides fairly accurate picture of chlorine concentration at a specific point in time Constant flow is not required Provides continuous testing with instantaneous results Remote alarm notification Provides very good accuracy when maintained properly Provides continuous testing with instantaneous results Remote alarm notification Most accurate method when used with water that is not RO-purified Does not use any chemicals Truly continuous since it does not take water samples Disadvantages Subjective analysis that relies on human eye to read results Does not provide continuous testing Uses large volumes of water Uses large volumes of chemicals Requires constant flow Potentially requires great deal of maintenance Very expensive Requires constant flow Does not work with ROpurified water Table 1: Advantages/disadvantages of chlorine monitoring methods

6 Samples should be taken from the following locations: 1. Chlorine injecting outlet. 2. Last interconnect quick disconnect or rack manifold drain valve in room farthest from the source of chlorine injection.