Water Industry News

New Massachusetts water treatment depends on ozone

By Jon Brodkin / Daily News Staff
August 19, 2005

After six years of construction and $340 million spent, the John J. Carroll Water Treatment Plant began operating July 27 and now disinfects drinking water for 2.3 million people in 41 Boston-area communities.

The plant uses ozone gas, which MWRA officials said makes water cleaner and better-tasting without leaving the by-products of chlorine. Ozone has been used as a disinfectant for more than 100 years, but many water providers have been reluctant to adopt the method because of its cost.

"It was very costly before and it still is, but it's become much more manageable and much more cost-effective than it ever was before," plant operations manager Ken Perry said after the ceremony, as he toured the plant with a reporter and photographer.

The facility has four ozone generators that use electricity to convert oxygen gas into ozone. The process is like a lightning storm, which forms electrical charges that make ozone.

"Did you ever smell the nice clean air after a storm? There's a lot of ozone in there," Perry said.

The 2.5-acre building, located on a 30-acre property on D'Angelo Drive, treats an average of 220 million gallons of water per day and is capable of treating up to 405 million gallons each day.

Water travels into the plant from the Wachusett and Quabbin reservoirs. After treatment, it enters the underground MetroWest Water Supply Tunnel, which stretches from Marlborough to Weston.

Before the plant opened, the MWRA's primary disinfectant was chlorine, which was injected into the Wachusett Reservoir. One of the benefits of ozone is that it kills cryptosporidium, a parasite that infected the Milwaukee, Wis., water supply in 1993, reportedly sickening 400,000 people and killing 70. 

"The ozone attacks the cell wall of the organisms and destroys it, basically blows them away," Perry said. "It's a very unstable molecule, but it's a very reactive molecule. That's what makes it such a good disinfectant."

Chlorine is still used by the plant in smaller amounts, so the overall amount of chlorine by-products in the water is a fraction of what it used to be, Perry said.
The plant also uses carbon dioxide and sodium carbonate to prevent corrosion of water pipes, limiting the amount of copper and lead in drinking water. Fluoride is added to improve dental health.