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Ozone replacing chlorine as disinfectant for wastewater


July, 2017
G. Wilkins
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Town of Coubourg, Ontario, Canada

City of Cobourg, Ontario, Canada

Source: Coolty via Wikipedia Commons

Chlorine jugs

Chlorine plays an important, but dangerous part of wastewater treatment.

Source: Maksym Kozlenko via Wikipedia Commons

Some wastewater treatment facilities in North America are dropping chlorine as a disinfecting agent and switching to ozone.

Disinfection, of course, is important for wastewater treatment. It rids wastewater of pathogens that could spread disease, and since a treatment facility handles water usually released for use by a city full of people, disinfection is a crucial step in the treatment process.

Chlorine, however, is a very dangerous substance. On its own it is deadly, but we use it to clean swimming pools and our cities’ water supplies because its effectiveness and can be neutralized.

“So what we had to do is add [chlorine] to kill off the pathogens, and then at the end of the contact chamber we had to add sulphur dioxide to take the chlorine out,” Bill Peeples told Northumberland News. Peeples is the manager of environmental services for the town of Cobourg, Ontario. “It’s a real fine balancing act and if one messes up you’re in trouble with either the federal government or the province.”

Peeples expects that by 2020 new regulations will prohibit the release of chlorine residuals.

Cobourg recently had its wastewater system retrofitted to remove the chlorine-related components and replace them with those needed for an ozone-based system. This was a much cheaper alternative than building an entirely new infrastructure. There are even more savings: ozone can be produced onsite, and now the plant does not have to worry about cost and safety risks associated with shipping and handling chlorine. In addition, workers no longer have to worry about putting chlorine into wastewater and removing it before discharge. Better, no one has to worry about the effects of a chlorine leak–a very deadly situation.

According to the EPA, ozone is more effective than chlorine at disinfecting wastewater. In a short contact time of 10-30 minutes, it kills viruses and bacteria and quickly decomposes, leaving behind only oxygen and water without the “harmful residuals” that a chlorine systems create. Furthermore, when the ozone does its job, microorganisms do not regrow into the water “except those protected by particulates in the wastewater stream.”

There are some drawbacks to ozone systems. Wastewater treatment plants need a high dosage of ozone to kill the highest amount of bacteria and viruses possible, and a fair amount of power to generate that high dosage of ozone (an issue that can be largely remedied by solar power). In addition, ozone is not completely safe either; it’s corrosive and possibly even toxic. Steps should still be taken to make sure workers are not directly exposed.

Peeples believes this to be a worthwhile trade, as does the vice president of marketing and sales for Aclarus, Inc., Adam Doran. Aclarus, Inc. developed the new ozone system in Cobourg, who called the system “a ‘win-win’ regarding operational cost savings and environmental footprint.” Aclarus has other types of ozone systems for various purposes throughout Canada, but it worked for two years to develop this particular system, and another one is scheduled for installation in Montreal in 2018.

  1. https://www.northumberlandnews.com/news-story/7394325-cobourg-gets-greener-by-replacing-chlorine-with-ozone-at-waste-water-plant/
  2. https://www3.epa.gov/npdes/pubs/ozon.pdf


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