Seeking to meet mandated cuts for the discharge of nutrients into the nation’s largest estuary, the Chesapeake Bay, a Maryland wastewater treatment plant and its design, construction and technology team are using what they say is the world’s largest application of a pollutant-reduction technology as part of a $138.7-million facility upgrade.

Team officials at the Patapsco wastewater treatment plant in Baltimore say use of the “fixed-film denitrification system” for tertiary treatment at the facility will reduce nitrogen and phosphorous discharges into the Patapsco River by 83% and 85%, respectively. The waterway is a key tributary of the bay, now on the U.S. Environmental Protection Agency’s list of “impaired waters.” High discharge levels feed the rapid growth of algae, which depletes oxygen and kills fish.

Construction gets under way just after the release last month by Maryland officials of a new 234-page plan for bay cleanup over the next seven years. In addition to plant upgrades, it addresses non-point-source pollution such as farm and street runoff and fertilizer use. The plan could cost more than $10 billion, according to published reports.

The $22.7-million denitrification system, manufactured by Severn Trent Services, Fort Washington, Pa., will be deployed in the upgrade of the 1920s-era plant, which will boost its capacity, on average, to 81 million gallons per day from 73 mgd and, at peak, to 150 mgd. A Severn Trent executive says it is the largest implementation of the system to date. The largest one that Severn has installed is at the Howard F. Curren advanced wastewater treatment plant in Tampa, which has an average flow rate of 55 mgd, says the company.

Fixed-film denitrification uses bacteria and layers of granular filter media to reduce the amount of nitrogen and phosphorous to acceptable levels. The system is the last step in a treatment train that enables wastewater to meet state standards before it flows into the river.

The system’s 34 beds will use specially sized and shaped granular media and denitrifying bacteria to lower the Patapsco plant’s nitrogen discharge to 0.5 mg/l and total phosphorous discharge to 0.3 mg/l. Suspended solids discharges also will be reduced to mandated levels.

Each bed is 100 ft long, 11 ft, 8 in. wide and 17 ft, 9 in. deep.

Plant upgrade engineer Rummel, Klepper, Kahl LLP, Baltimore, specified the technology. Fru-Con Construction Corp., Woodbridge, Va., is project contractor for the overall upgrade and denitrification installation. “The amount of water that needs to be processed, space limitations, cost and ease of integration with other plant processes led us to select Severn Trent’s fixed-film denitrification system,” says Robert Andryszak, RKK project manager.

The Patapsco plant processes about 25% of the area’s wastewater. While Maryland has more than 65 wastewater treatment plants, the facility and another in Baltimore together provide 45% of the state’s total wastewater-treatment capacity, says Andryszak.

Work began in January; completion is scheduled for March 2013.

Because the plant sits on a soft site created by fill dumped in the river during the late 1800s, Fru-Con is driving about 1,100 hollow steel friction piles, each up to 24 inches in dia. and 105 ft long, to support foundations for the current upgrade and future plant improvements. The contractor says construction will include placement of more than 30,000 cu yd of reinforced concrete. Before work could begin, Fru-Con had to remediate soil contaminated with chromium.

“To keep the plant functioning, we will reroute potable water, sanitary, storm-drain, electrical and plant water lines to the perimeter of the property in a by-pass configuration until the newly installed permanent lines take over,” says Fru-Con Vice President Michael R. Fischer.