To combat a long-standing water-quality problem, the Town of Shrewsbury Water and Sewer Division in Massachusetts recently completed a comprehensive unidirectional flushing project aimed at removing manganese from its water mains.

The roughly $800,000 project, funded by water revenues, started in December 2019 and was completed in November 2021. It’s the latest in a series of steps the utility has taken over the years to remove manganese from the town’s drinking water, says Dan Rowley, water and sewer superintendent for the town, located about 35 miles west of downtown Boston.

The pandemic caused some project delays, Rowley notes. But the flushing program, which uses large quantities of water, also stopped during summers for fear the town’s wells wouldn’t be able to keep up with the extra demand during the peak-use months.

A local contractor, Hydra Tech, performed the flushing, using a plan developed by Tata & Howard, a firm that’s been the utility’s consultant since the 1990s. The high-velocity flushing scoured away manganese lining the pipes, as well as tuberculation.

“We used an outside contractor because we’d never done a flushing program before,” Rowley says. “We also had no idea about the condition of the water main valves, so we needed a contractor with flushing experience and that could repair any damaged valves.”

The water system features about 207 miles of mains, mostly made of asbestos cement, ductile iron and PVC pipes. It includes more than 11,000 service connections and serves about 38,000 people. Water is supplied by eight gravel-packed wells that pump groundwater. The wells vary in depth from 40 to 70 feet; they’re located in the northwestern section of the town, near Lake Quinsigamond.

High levels of manganese

Manganese is a naturally occurring mineral that’s prevalent in New England. It’s found in rocks, soil and groundwater. While it’s an essential mineral for proper nutrition, it can be harmful in elevated concentrations.

The U.S. Environmental Protection Agency and the Massachusetts Department of Environmental Protection established an “aesthetics‐based” secondary maximum contaminant level for manganese of 50 parts per billion. When manganese concentrations exceed the 50 ppb level, it can produce discolored water, which was the case in Shrewsbury, Rowley says.

The town has been dealing with manganese since around 1990. It typically turns drinking water brown or black and can stain clothes and kitchenware. It can also carry an odor and make the water taste bad.

“On the technical side, manganese may build up in pipelines, pressure tanks, water heaters and the like, reducing the effective size of pipes, requiring more frequent replacement and increasing the operation and maintenance costs due to the extra power needed to pump water through smaller pipes,” according to a town report.

Chemical solution

To deal with the manganese, the utility started injecting a chemical blend of half polyphosphates and half orthophosphates into the water system in the 1990s, using a chemical pump at the town’s water treatment plant. The phosphates prevent the manganese from oxidizing and keeps it in soluble form.

“That worked for many years, so we didn’t have to take action,” Rowley says. “But the levels slowly crept up and eventually, it would break off in the pipes, which created significant water quality issues.

“Around 2014 and 2015, we started to receive complaints about discolored water,” he adds. “In 2016, we had one water quality report with manganese levels at 500 ppb. So we had to do something to remove the manganese.”

Rowley believes the increased manganese levels might have stemmed from using different wells as the town’s primary well. A severe drought in 2016 and 2017 also likely raised levels in the groundwater.

“Droughts can significantly change the pumping level or height of water in a well,” Rowley notes. “When a drastic change in the well level occurs, mineral concentrations can change significantly.”

New treatment plant

After considering different options, the utility decided to build a $12 million, 8,580-square-foot water treatment plant equipped with a Mangazur manganese-filtration system. The project was funded by a low-interest loan from a state revolving fund.

“Our existing treatment center (built in 1990) couldn’t accommodate the large filters in the Mangazur system, which are 14 feet in diameter,” Rowley says. “The old treatment plant was primarily designed to remove volatile organic compounds using air scourers and it would’ve been difficult to retrofit it for a Mangazur system. So it was more economically feasible to start with a brand-new design.”

Developed in Europe by SUEZ Water Technologies and Solutions, the system uses a natural biological filtration process instead of chemicals to remove manganese from water.

A key component is naturally occurring bacteria found in raw water. In simplest terms, these microorganisms grow and create a biofilm that coats the giant filters — one in each of four large tanks — and oxidizes (solidifies) the manganese as it passes through. The manganese particles then get trapped in the filters.

Operators backwash the filters periodically to remove the solidified manganese, which flows into decant tanks and then into the town’s sanitary sewer system. The biofilm remains in the filters even after backwashing, allowing for continual operation and no filter replacements needed for long periods of time.

“I’m extremely impressed with the quality of water that the Home Farm Treatment Plant has produced,” says Rich Fox, the assistant superintendent of water treatment pumping stations and a 36-year employee of utility. “And the 700 hours in between filter backwashes is unheard of, compared to conventional filtration.” 

Construction of the Home Farm Water Treatment Plant began in July 2017. The facility started operating in early October 2018. The project also included refurbishing the eight wells with new pumps, motors and motor-control centers.

The plant can treat up to 7 million gallons of water per day but treats an average of 3.3 to 3.5 mgd. Rowley says operators have to closely watch the pH levels in the water and periodically add oxygen to keep the microorganisms alive.

The utility increased its water rates by 40% in 2018 to cover rising operational costs and to generate revenue to pay off the loan, Rowley says.

More work to do

The project was a success; within several weeks, water samples tested at the plant showed no presence of manganese.

But there was one issue the new treatment plant didn’t solve; the manganese that still coated the waterlines in the town’s water distribution system. That’s where the unidirectional flushing program came into play.

UDF differs from conventional waterline flushing because water flows only in one direction — downstream from a water tank or treatment plant ­— and the flushing occurs only in one isolated pipe segment at a time. By cutting off other water flows, the velocity of the flushed water, created by opening fire hydrants, increases to 5 to 10 feet per second, compared to 1 to 3 feet per second in conventional flushing, Rowley says.

The Shrewsbury water system has three pressure zones; each one functions independently with its own water tanks.

“The town has such elevational differences and we have to have the water system function in line with those different elevations. One area of town is very high, so it has two water tanks.”

Block-by-block flushing

Under the game plan developed by Tata & Howard, the town was divided into 12 sections. A flushing sequence was laid out for each section, block by block, starting at either a water tank or at the treatment plant.

“You’re essentially working away from the source of the water,” Rowley says. “Step by step, the contractor would close off the appropriate valves at an intersection and then open a hydrant, which creates a high-velocity flush in one direction, created by the hydraulic pressure of the system.

“Opening the hydrants moves an incredible amount of water. It can remove not only the manganese, but tuberculation, too,” he adds. “Then you systematically flush the side streets.”

Mains larger than 12 inches in diameter sometimes required opening a couple hydrants at the same time in order to generate the required velocity. The system has about 1,538 hydrants.

The flushing continues until the water runs clear. That could take as little as five minutes or as long as a couple of hours, Rowley says.

“It’s a very visual process. The water might be pretty rusty initially, but then it clears up.”

The water from the hydrants then drains into the town’s stormwater system.

Residual benefits

There are some side benefits to unidirectional flushing. For starters, the process uses an average of about 40% less water than conventional flushing, experts note.

In addition, removing tuberculation in pipes extends the life of water mains, Rowley says.

“We also found sections of water mains where the gates were closed and we didn’t know about it. And we found that some gates in new subdivisions were buried under pavement, so we dug them up to expose them.

“It’s also good to open the hydrants and be sure they’re functioning correctly.”

As he looks back on the project, Rowley says he’s appreciative of the support received by the town’s leadership and the patience of the residents and businesses as the work took place. He also praised the entire Department of Public Works staff and Hydra Tech for their tireless efforts in making the project a success, noting it was truly a team effort.

“With close to 20 years of experience in water distribution and treatment, I’ve never been a part of a project of this scale,” adds Joe Kenney, assistant superintendent of water and sewer operations. “The ability to map out and execute a UDF program of this size is an incredible achievement.” 

UDF will continue

Looking ahead, the UDF program will continue indefinitely each spring and fall, taking two to three years to complete a full pass through the town. And because the pipes have already been scoured, the process should go faster and use less water going forward.

Rowley thinks there’s a lesson here for other utilities, starting with proof for UDF doubters.

“If someone doesn’t believe that unidirectional flushing works, our experience proves how effective it really is when done right,” he says. “Water utilities shouldn’t let their flushing programs lag. It’s an easy thing to push off and push off, but it’s really critical for optimizing your system.”

John Samia, chair of the town’s governing oard of selectman, which also serves as the town’s water commissioners, says completing the flushing project was an important step for the town of about 38,000 residents.

“Improving water quality has been one of the most important goals for the town over the past several years,” he says. “The completion of the first full pass of all 12 flushing zones represents a significant step toward improved town water quality.

“This important work will continue and we look forward to even more water-quality improvements upon further rounds of unidirectional flushing zone work and future water quality improvement projects.”

Would Rowley call the program a success? Absolutely, he says.

“When we started the UDF program in 2019, we had a lot of complaints about water quality. And now we have hardly any. So we’re in a much better place than we were a few years ago.”