"We're in the business of cleaning the environment," says Mark Young, executive director of the Lowell (Mass.) Regional Wastewater Utility. "We want to be an environmentally progressive city where people want to live and work."

With an investment of $22 million, including $4.66 million in federal stimulus funds, the city's wastewater plant has improved its aeration system, installed active solar heating and photovoltaic systems, and installed green roofs and other stormwater mitigation strategies.

The project aligns with the vision of city manager Bernie Lynch: "The reason for the wastewater plant is to produce clean water for a safer and better environment. Sooner or later, we have to replace roofs, windows and heating systems. This gets it all done, and it's paid for by the savings."

The 32 mgd (design) activated sludge plant, now with a staff of 48, went online in May 1980 to serve 180,000 people in Lowell and four neighboring communities. While the average flow is about 26 mgd, wet-weather flow from combined sewers can reach 110 mgd. The utility has just begun work to separate the sewers and eliminate overflows to the Merrimack River watershed.

Starting with aeration

The upgraded plant began operating with a new fine-bubble aeration system in November 2011. One 500 hp and three 350 hp centrifugal blowers were replaced by four 300 hp energy-efficient turbo blowers from HSI Blowers. "We were very vulnerable," notes Young. "We were not in a good place. The old blowers were unreliable, expensive to maintain, very loud, and energy inefficient."

Besides a huge improvement in reliability, the plant will see electrical savings of about $75,000 a year at current rates. "We had visited some facilities where these turbo blowers were installed and realized that it was a good investment," says Young. Also included in the upgrade was an overhaul of the entire electrical system.

Photovoltaic systems were installed to reduce power demand from the local utility. Half of the administration building roof houses a 1,080-square-foot, 33 kW photovoltaic array, while another 15 kW system covers 2,400 square feet of the maintenance building roof. The renewable energy is expected to save $3,330 a year.

Stormwater management

To reduce runoff from the property, the upgrade included 14,600 square feet of green roofs on all five plant buildings. Green roofs use soil and filtering media to grow vegetation, which also provides insulation, reducing heating and cooling needs.

Lowell uses two kinds of green roofs. There are four "extensive" roofs with soil 1 to 5 inches thick for low-growing grasses, and one "intensive" roof with about a foot of soil to grow flowering plants and shrubs. "We have blueberry bushes growing on ours," notes Young.

The 6,300-square-foot intensive roof and photovoltaic system on the administration building are accessed by an exterior stairway to accommodate public tours. A 1,950-gallon cistern collects water that isn't absorbed by the green roof, and it is used for landscape irrigation.

Pervious asphalt and concrete were added to three parking areas to further reduce runoff. Rainwater drains through the pavement and is naturally filtered by the soil before returning to the groundwater. "I was skeptical, but I poured a five-gallon pail of water on the concrete, and it went through it like a sponge," says Young.

The property also includes two rain gardens. A 375-square-foot garden outside Young's office is mainly for appearance and demonstrations. A 2,500-square-foot rain garden handles runoff from pavement in front of the influent pump building. "We wanted to be able to showcase the wastewater department as a municipal entity that initiated all these green projects," he says.

Reducing odors

Green innovation also extends to the plant's biosolids operation. Biosolids are composted in Maine or landfilled in Vermont. Until the recent upgrade, primary and secondary sludges were co-thickened in three gravity thickeners. "In the summertime, it would come alive because we, in essence, were mixing food and bugs," says Young. "It would spill over into the wet well and make its way to the aeration system and load it up with BOD, causing a lot of process upsets."

There had been many odor complaints over the years, so the project converted one of the gravity thickeners to a thickened waste activated sludge tank with a roof and a rotary drum thickener (RDT) for secondary sludge, which was enclosed in a building.

"We combine 20 percent secondary sludge from the RDT, 60 percent primary sludge from the gravity thickeners, and 20 percent septage and pump it to the belt filter presses for dewatering," Young explains. "It helps with odors and our secondary treatment process."

The chemical system for the belt filter presses was also improved to enhance conditioning of the biosolids and maximize dewatering with lower chemical dosing. Since the upgrade, the plant has had only one odor complaint.

The septage receiving station was also relocated and modernized. "We wanted to be able to get trucks in and out of here quicker, accommodate haulers' needs, and make it more attractive to them so that we could increase our revenue," says Young.

Going citywide

Along with the investment at the wastewater plant, Lowell has invested in other city operations. "We have a performance energy contract with Ameresco who did an energy audit of all our municipal facilities," says Lynch.

The improvements have included lighting retrofits, solar panels and new heating systems. "They are making improvements to the buildings to make them less costly and more environmentally sustainable and it's paying for itself," Lynch says.