Online Analyzers Improve Control of Chloramination Processes in Milwaukee

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Courtesy of ASA Analytics

The Milwaukee Water Works is a national leader in providing high quality drinking water and monitoring water quality. Since 1998, it has invested $227 million in its infrastructure, from treatment plants to distribution systems, to ensure high quality drinking water and reliable supply. The Milwaukee Water Works treats Lake Michigan water with Ozone gas as a primary disinfectant because it destroys microorganisms, reduces chlorinated disinfection byproducts, and removes taste and odor. It then further treats its water with coagulation, settling, and filtration to remove additional particles, and adds Chloramine as a secondary disinfectant.

Chloramine is a more stable disinfectant than Chlorine and better extends the disinfectant residual throughout the distribution system. It also reduces the formation of trihalomethanes and Halogenic Acetic Acids and the taste and odor problems associated with Chlorine. However, producing Chloramine can be a challenge because it requires precise control of a complex process.

In 2004 the City of Milwaukee invested in a ChemScan UV-2150S online Chloramine analyzer in its Howard Avenue Water Treatment Plant to improve control of its Chloramination process. Chloramine has been used as a secondary disinfectant since the 1990s when the plant's primary disinfection process was converted to Ozone in what was then the world's largest retrofit project of its kind. The original design of the Howard Avenue plant adds an additional degree of complexity to controlling the Chloramination process because the sample point is so close to where the ammonia is added. As a result the ammonia may not be mixed as thoroughly as it would have been had it been located further downstream, making it even more critical for plant operators to have timely access to accurate information about the process.

'Our challenge had been mixing the ammonia and maintaining our ratios,' said John Gavre, Howard Avenue Plant manager. 'We wanted a more accurate instrument than our current one and an instrument that works online so we can get the results faster. That would allow us to more closely monitor the process and adjust it.'

Chloramine is formed by mixing Ammonia with Chlorine. It's a complex process often depicted by the breakpoint curve that separates the Chloramination process into a series of steps. In the early stages of the Chloramination process, Ammonia added to free Chlorine produces Monochloramine. All of the Ammonia and available Chlorine are combined to form Monochloramine when the process is in control. However, when the process begins running out of control, an imbalance occurs between the amounts of Ammonia and Chlorine. When excess Chlorine is present it continues to combine with Monochloramine, converting it to Dichloramine. Additional Chlorine then combines with Dichloramine to form Trichloramine. Unfortunately, this can result in drinking water tainted with unpleasant tastes and smells.

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