Chesapeake Bay and its surrounding waters provide many functions—recreation, fishing, travel. But these functions can only serve their purpose if the water maintains a certain level of quality, a task that local wastewater treatment plants (WWTPs) take seriously—because with well-functioning WWTPs come enjoyable bay waters.
Located just off the Back River, the 180-million-gal-per-day Back River WWTP’s treated water eventually flows into Chesapeake Bay. To adequately meet the Chesapeake Bay total maximum daily loads (TMDLs) and improve the quality of water entering the bay, the WWTP needed an upgrade. In October 2013, the city of Baltimore began constructing a fourth activated sludge plant to provide the additional level of treatment necessary to achieve enhanced nutrient removal (ENR) of nitrogen and phosphorus.
“This is one of two ongoing ENR projects at the plant, valued at $550 million, that will reduce the annual loading of nitrogen and phosphorus to the Chesapeake Bay by 2.7 million lb per year and 400,000 lb per year, respectively,” said Timothy W. Wolfe, P.E., BCEE, vice president of KCI Technologies Inc. and principal-in-charge. “These projects will make significant improvements to the water quality of the bay.”
The plant upgrade includes six 40-million-gal activated sludge reactors in an MLE configuration. The reactors incorporate swing zones to maintain complete nitrification during varying flow and weather conditions, and the anoxic and swing zones are equipped with Invent hyperboloid mixers for increased mixing coverage and energy efficiency.
It also has a blower building housing 14 350-hp turbo blowers; 12 120-ft-diameter secondary clarifiers; and three sludge pump stations for pumping return-activated sludge (RAS) and waste-activated sludge. A flushing water pumping station, consisting of two 5,250-gal-per-minute vertical turbine pumps and a 30-in. pipeline network, also was installed. The large-diameter pipeline network includes 48- and 60-in. clarifier effluent lines, 48-in. mixed liquor lines and 24-in. RAS lines. All of the equipment was chosen under the premise of incorporating “significant energy-saving technology for oxygen transfer,” Wolfe said.
While major design elements of the project were somewhat complex, the Back River WWTP upgrade faced additional unique challenges. For example, crews needed to construct the facility within the footprint of the existing plant without disturbing operations—no small feat for the nearly 400 craftsmen on site during peak periods. Despite the site conditions, this large number of craftsmen was required to maintain the expected velocity of the project; the facility, spanning an area of 5.5 football fields, has a timeline of two years and seven months.
While the Back River WWTP upgrade necessitated rigorous construction and dedicated crews, the city of Baltimore did not lose sight of the goal: ensure the water flowing into Chesapeake Bay meets its TMDLs and maintains high water quality.