Mixing Systems to be Installed at Crosby Drive and Pine Hill Water Tanks

By Kim Siebert MacPhail

While presenting information about the winning contract bid from Utility Service Co, Inc., of Georgia for installation of mixing systems at the Crosby Drive and Pine Hill Road water towers, Town Engineer Adrienne St. John reminded those in attendance at the Selectmen’s meeting on Monday night about last summer’s repeated problems with total coliform bacteria in the Bedford’s water system.

“You may recall last summer we spent several months chasing water quality issues, mostly related to [the] age of water, temperature of water, and the inability to keep a good level of chlorine residuals on the outskirts of the distribution system,” St. John explained.

“This contract is for a tank mixing system that will be installed in the two tall tanks, Crosby Drive and Pine Hill Road,” St. John continued. “What this tank mixing system does [is that] it keeps the water in the tank from stratifying. Since the water at the top of tank doesn’t move, that water can sometimes in the summer approach 90 degrees, whereas at the bottom of the tank the water is much cooler.

“The tank mixing system keeps the tank at constant, uniform water quality, and that’s going to help us in that we can keep the chlorine residual in the tank all the way through the whole [water] column,” St. John added. “It will prevent ’nitrification,’ which is a chemical reaction of [accumulated] organisms and bio-growth, Bedford being the last community in the MWRA distribution system.”

St. John said that installing the mixing system is one of three strategies that will be used to combat the water quality problem. The other two strategies are scraping/relining of the water main that runs under Great Road from the Lexington town line, and changing the chemical additive injection system at the Shawsheen treatment plant to offset the loss of chlorine residuals experienced last summer.

“We’re hoping that with these steps taken throughout the system, we’ll have a much [better] summer than we did last year,” St. John said.

While the tanks are empty to prepare for the mixing system installation, they will be chemically cleaned to assure that no bio-films are present. The mixer to be installed is very small—about four feet high—and sits at the bottom of the tank. A propeller runs continuously, moving water around in the tank. St. John anticipates that no maintenance will be needed for the mixers; the life cycle for the units is about 20 years.

The total cost of the contract is $113,200, with $96,600 for the tank mixers and $16,600 for cleaning three tanks: Crosby Drive, Pine Hill and Reeves Road. The water main cleaning project and Shawsheen treatment plant upgrades fall under separate contracts.

At the Special Town Meeting last November, $95,000 was appropriated for the mixing systems. St. John said that the difference between the appropriation and the total project cost will be covered by the Water Department’s operating budget, made possible by savings due to inactivity at the Shawsheen pumping station during the total coliform bacteria incidents.