The timing was right for a solar energy project at the Home Farm Water Plant in Shrewsbury, Massachusetts.

For one thing, the plant had been rebuilt so that it could treat for manganese. The rebuilding left an old foundation that became the stage for a solar array. That saved the cost of removing the old foundation.

On top of that, the state had set aside $4 million in grant money for energy efficiency or renewable power projects at water and wastewater facilities. Shrewsbury secured a $200,000 grant, about 88% of the cost for a 59.2 kW solar array. The town share was $27,525. The 160-module array, completed in June, is expected to produce 75,582 kWh per year, less than 5% of demand, but a savings of almost $11,000 a year in electricity costs.

“The timing was perfect,” says Dan Rowley, superintendent of the Water and Sewer Division. “Now we have a brand-new plant and a new solar array, and everything is working out well. They added another 4 or 5 inches of concrete to the old foundation slab and trimmed it out to build the solar array.”

Town owned

Many water and wastewater treatment plants have solar arrays built under power purchase agreements with solar development companies. Under those arrangements, the plants can buy the power at a fixed rate for a number of years without having to invest to build the solar panels. That wasn’t an option for Shrewsbury, which has its own power utility.

Although the town had to pay for the solar array, the system is simpler than under a power purchase agreement, since there is no need for infrastructure to connect to the power grid or for batteries to store power.

“We have five variable-frequency drive motor starters. There’s a lot of power usage,” Rowley says. “There is no storage needed. We use the power immediately. There’s no grid connection, no separate meter. It gets used up right in the building.”

Shrewsbury, population 37,000, is about 40 miles west of Boston. The 7 mgd (design) water plant pumps about 4 mgd in summer and about 3.1 mgd for the rest of the year, drawing from eight wells. There are six water storage tanks, three pressure zones and 207 miles of mains.

Manganese removal

Before the rebuilding, completed in the fall of 2018, the plant’s product water was approaching the limit for manganese. “Over the years, it just kept building up and getting worse as the town grew,” Rowley says. “The more we pumped the wells, the more manganese we pulled out. It got to the point where we were right at the limits.”

The town chose biofiltration for manganese removal. “Biological treatment is kind of new in this part of the country,” Rowley says. “I believe this is the largest biological filter plant this side of the Mississippi. It’s working great. Our manganese now is almost nondetectable.”

The rebuilding brought other changes. The previous water treatment process used chlorine gas for disinfection; the new process uses liquid sodium hypochlorite, which is safer to store and safer for the workers.

Rich Fox, assistant superintendent of the Water and Sewer Division and chief plant operator, is pleased with the change, calling chlorine gas “scary stuff. Now that it’s gone, we’ll never have it back.”

The plant used to keep fluoride as a liquid but now uses powdered fluoride. The liquid was easier to use, Fox says, but the powdered form is safer. “I think they’ve geared everything toward operator safety, which is good,” he says.

Quick payback

The solar power presents no problems for the operators. It works when the sun shines, and the power produced can be monitored on a website that the public can view. It shows daily and yearly power output and pounds of carbon dioxide emissions prevented.

If the solar array continues to perform as in the early months, the payback on the town’s investment will be less than three years. Without the $200,000 state grant, the payback would be about 23 years.

But the financial impact was not the only consideration, Rowley says: “The big part is doing what’s right for the environment. Every little bit helps.”