Rosemount TCL system from Emerson Process Management offers continuous total chlorine measurement with low reagent requirements.

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By Ryo Hashimoto, Senior Manager, Emerson Process Management,

Rosemount Asia Pacific

astewater is produced everywhere from homes, s c h o o l s , h o s p i t a l s ,

businesses, factories, refineries, power plants, and more. Regardless of the source, whether municipal or industrial, all of that wastewater eventually has to b e t r e a t e d a n d disinfected before it can be released back into the environment safely. Wastewater is over 99% water and about 0.3% dissolved and suspended solid material. Wastewater treatment involves several steps to make s u r e t h a t t h e wastewater streams m e e t l o c a l a n d national regulations that safeguard the env i ronment and water quality.

At a minimum, treatment removes suspended solids and reduces the concentration of organic matter. Other steps may remove nutrients, such as nitrogen and phosphorus. Treatment must also kill disease-causing microorganisms. Chlorine gas and sodium hypochlorite are common disinfectants.

The accurate measurement of total chlorine is the wastewater treatment plant’s first line of defense in assuring that the water has been sufficiently treated and the chlorine is at appropriate levels to meet regulatory requirements for safe discharge into the environment. Yet, selecting the right chlorine analyzer in water treatment applications can be a challenge for instrument technicians. Sometimes it is difficult for technicians to select the right analyzer. For example, in an application that requires measurement of free residual chlorine, a technician may incorrectly select a free chlorine analyzer, but a total chlorine analyzer is actually more suitable in this case.

This article explores and discusses the typical chlorination process in wastewater. The selection of a chlorine analyzer in seawater chlorination is also examined with the use of an actual case study performed at a global chemical customer site.

Wastewater TreatmentTypically, wastewater treatment includes four basic treatment stages: primary, secondary, sludge, and final treatment. In the primary treatment stage, larger solids are removed from wastewater by screening and settling. Secondary treatment is a large biological process for further removal of the remaining suspended and dissolved solids. Sludge is generated during the first two stages and is treated to convert it into a stable organic solid. In the final treatment, wastewater is disinfected to kill any remaining harmful microorganisms prior to being released. The various stages of wastewater treatment include physical, chemical, and biological treatment processes.

Chlorination and dechlorination are involved in the final stage of wastewater treatment. In this stage, disinfectants are added to the water to kill disease-causing organisms and

microorganisms that were used in the previous treatment stages. Disinfection inactivates or destroys pathogenic organisms and prevents the spread of waterborne diseases to downstream users and the environment. Chlorine gas is often used, but some plants

use their own chlorine so lut ion , such as sodium hypochlorite.

Chlorination Process in Wastewater TreatmentWhen ch lor ine i s added to wastewater, a p o r t i o n o f i t i s c o n s u m e d b y r e a c t i o n s w i t h chemicals in the water, making it unavailable f o r d i s i n f e c t i o n . Moreover, the chlorine that remains has often been converted to

forms that are significantly poorer

Effective Chlorine Measurement in Wastewater

to Meet Environmental Requirements

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dis infectants than the sodium hypochlorite or chlorine gas originally added.

Only two forms of chlorine have disinfecting properties: free chlorine and combined chlorine. Free chlorine, which is by far the better disinfectant, is produced when sodium hypochlorite or chlorine gas is added to water. Combined chlorine is monochloramine a n d d i c h l o ra m i n e , w h i c h a re compounds formed by the reaction between free chlorine and ammonia. Because wastewater treatment seldom completely removes ammonia and organic amines, chlorinated wastewater typically contains combined chlorine, primarily monochloramine. Free chlorine will appear only if enough chlorine has been added to completely oxidise the chloramines, so-called

breakpoint chlorination. Breakpoint chlorination is rarely practiced in municipal treatment plants.

The sum of free and combined chlorine is often called total chlorine. The term is misleading because standard analytical methods define total chlorine by the way it is measured, not as the aggregate of certain chemicals. Accordingly, total chlorine is all the oxidants, some of which may not contain chlorine, in the sample capable of oxidising potassium iodide to iodine when the pH is between 3.5 and 4.5. The amount of iodine produced under these conditions is defined as total chlorine.

For chlorine, either free or combined, to be effective, the concentration and contact time must be controlled. Most treatment plants have a contact chamber where chlorine is injected,

mixed, and allowed to remain in contact with the water for the required time. Flow rate into the chamber and the chlorine concentration at the outlet are used to control the chlorine injection rate.

Dechlorination ProcessIn many areas, local and national government agencies regulate the amount of chlorine allowed in the final plant effluent before being discharged into lakes, rivers, or the ocean. This requires dechlorination, w h i c h re m o v e s t h e f re e a n d combined chlorine residuals to reduce the toxicity after chlorination and before discharge. The chlorine concentration in reused wastewater can be as high as several ppm. In discharged wastewater, the permitted

Figure 1

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concentrat ion is low, typical ly between 0.01 to 0.30 ppm of chlorine. Chlorine is closely regulated because even small amounts are harmful to aquatic organisms. Typically, plants are required to monitor their waste streams and report chlorine levels to a regulatory agency. Agencies can require either continuous or grab sample testing.

Chlorine is added to the effluent from the final clarifier as it enters the chlorine contact chamber. Typically, the chemical injection rate is controlled using the flow rate and the chlorine concentration at the outlet of the contact chamber (see Figure 1). A second contact chamber for dechlorination is not needed because dechlorination reactions are almost instantaneous. Most wastewater applications require measuring total chlorine, although free chlorine might also be required. Excess chlorine is removed in a dechlorination stage by adding sulfur dioxide, sodium bisulfite, sodium sulfite, or sodium metabisulfite. The chlorine concentration is measured in both the

chlorination and dechlorination stages. Direct measurement of trace

chlorine can be difficult. If the discharge limit is close to the detection limit, the relative measurement error will be high. If the discharge limit is below the detection limit, the analyzer cannot be used at all. Many chlorine sensors, particularly amperometric ones, are calibrated against the results of a grab sample test. Because chlorine sensors cannot be accurately calibrated if the sample contains only a trace of chlorine, there must be a provision to interrupt the normal sample flow and replace it with chlorinated water. Chlorinated water is also needed for periodically checking the response of the analyzer to confirm that low chlorine readings are valid and not the result of a failed sensor.

Seawater ChlorinationIn a reaction of chlorination in normal water, sodium hypochlorite or chlorine gas forms hypochlorous acid (HOCl) and hypochlorite ions (OCl-). A free

chlorine amperometic sensor reacts with HOCl and converts it into a free residual chlorine measurement. When it comes to seawater chlorination, free chlorine (HOCl and OCl-) is not an issue as it does not exist in chlorinated seawater because of its bromide (Br-) content. Seawater contains as much as 65 ppm bromide. When chlorine is added to seawater, it oxidises with bromide to create a mixture of hypobromous acid (HOBr) and hypobromite ion (OBr-). In the process, free chlorine becomes chloride (Cl-), so free chlorine no longer exists. Free chlorine will also oxidise trace iodide (I-) in seawater to hypoiodous acid (HOI) and hypoiodite ions (OI-). Finally, if ammonia and o r ga n i c a m i n e s a r e p r e s e n t , inorganic and organic chloramines and bromamines might form. For this reason, free chlorine won’t be present in chlorinated seawater, so when measuring it, it is best to monitor total chlorine. Typical chlorine applications are listed in Table 1 on the following page.

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A Real-World ExampleA major global chemical processing company uses seawater for process cooling. Because that cooling water is later returned to the nearby estuary, the plant must use only the minimum concentration of chlorine to be sure it meets environmental discharge regulations. The company had previously used an analyzer based on diethyl-p-phenylenediamine (DPD) to monitor chlorine, but this technology had become outdated and was expensive to run. The DPD analyzer was a colorimetric method and wasn’t well-suited for seawater. The water is salty, contains a lgae , seaweed, musse ls , and sand, and these contents impact the accuracy and reliability of the measurement. As a result, the sensor had to be cleaned nearly every day. The plant shifted to amperometric chlorine analyzers for total chlorine measurement. As chlorine diffuses into the sensor through a membrane, an electrochemical reaction generates current proportional to the diffusion rate, which itself is proportional to the chlorine concentration. The company

Ryo Hashimoto is senior manager, Emerson Process Management, Rosemount l iquid analytical in Asia-Pacific. Further information can be found at www.EmersonProcess.com/Rosemount-Water Quality Panels.

found this method to be effective in dirty seawater and to provide a consistent and accurate measurement as the water content and pollution and temperature changes do not affect its reliability. It also provides a faster measurement, so problems with chlorine levels can be identified a n d a d d re s s e d m o re q u i c k l y, improving environmental compliance and avoiding regulatory penalties and fines. Faster, more accurate measurement of total chlorine allows the plant to optimise both the chlorine content and the microbiological activity.

When considering the chlorine analyzers and sensors the plant chose, one important selection factor was the reagent used in the systems. The analyzer they opted for had very low reagent requirements so the reagent lasted for more than three months, which reduced costs and maintenance. Because air is dosed in the analyzer the plant selected, it offers a self-cleaning aspect, so the measuring cell rarely if ever needs cleaning, a huge maintenance improvement over the previous system.

About the Author

Responsible for Regulatory ComplianceRegardless of whether treated water was used in industrial processes, such as for cooling water, or used by municipalities in homes, schools, businesses, and more, the resulting wastewater must be treated so it can be returned to the environment. Chlorine is critical to disinfecting water to prevent disease, but it also poses its own risk as a dangerous poison to sea life and the environment, so water treatment plants, both industrial and municipal, must ensure its careful removal before water is discharged. Accurate, reliable chlorine monitoring is vital to ensure safe discharge water, compliance with environmental regulations, and an optimised treatment process. WWA

FRC: Free Residual Chlorines • CAC: Combined Available Chlorines •TRC: Total Residual Chlorines

Table 1. Typical Chlorine Applications