The Chesapeake Bay, the largest estuary in the United States, is a vital economic, cultural and ecological resource for the region and the nation. The Chesapeake Bay watershed stretches across more than 64,000 square miles, encompassing parts of six states — Delaware, Maryland, New York, Pennsylvania, Virginia and West Virginia — and the entire District of Columbia. Chemical contaminants, air pollution, landscape changes, erosion and over-harvesting of fish and shellfish stress the Bay and its wildlife. However, the watershed's worst problem is nutrient pollution, which fuels the growth of algae blooms that impact water quality and aquatic life. Excess runoff and discharges of nutrients – particularly nitrogen and phosphorous – from farms, pavement, wastewater treatment plants and other sources have placed the Bay on the USEPA's List of Impaired Waters.
Over the past 25 years, billions of dollars have been spent on the Bay's restoration. Yet many governmental entities, nonprofit groups and universities agree little progress is being made. In 2009, the University of Maryland Center for Environmental Science graded the Bay with a C-, or a score of 43 percent, a one percentage-point improvement from 2007. In its 2009 State of the Bay report, the nonprofit Chesapeake Bay Foundation graded the Bay with an "unacceptable" D. And the USEPA's Chesapeake Bay Program reported in March 2009 that the Bay was at only 38 percent of its desired health in 2008, the same rating it received in 2007.
But there are positive developments in the effort to reduce nutrient pollution in the Chesapeake Bay. Many wastewater treatment plants and industries in the six-state Chesapeake Bay watershed, for example, are looking to install new equipment to reduce the amount of nutrients that are discharged into the Bay's tributaries. Concurrently, states are evaluating their regulatory parameters for total nitrogen (TN) and total phosphorous (TP). Of the six states in the watershed, Maryland has the most stringent regulations, limiting TN to 3 mg/l and TP limit to 0.3 mg/l, and other states are likely to bring their nutrient limits in line with Maryland's.
Upgrading sewage treatment plants for nutrient removal is one of the top environmental priorities in the watershed. In fact, the Baltimore City Department of Public Works is in the process of upgrading and expanding its Patapsco Wastewater Treatment Plant using a technology proven to reduce nitrate-nitrogen (NO3-N) and total suspended solids (TSS) to meet stringent government standards. The system, a TETRA® Denite® denitrification system from Severn Trent Services, will be the largest fixed-film denitrification system in the world.
Denitrification: A proven nutrient-reduction technology
The Denite system is an economical solution for the removal of NO3-N and TSS in a single treatment step. The fixed-film biological denitrification process also serves as a deep bed filtration system capable of removing suspended solids to virtually any final effluent requirement. The system integrates well with other plant treatment processes to provide superior TN and phosphorous removal. Denite is used as the final treatment step in the TN removal process to help facilities meet discharge limits of 3 mg/l.
The specially sized and shaped granular media used in the Denite process is an excellent support medium for denitrifying bacteria and the deep bed environment is conducive to efficient NO3-N and solids removal. The contact between wastewater and biomass is excellent and hydraulic short-circuiting is negligible even during plant upsets. The media allows for heavy capture of solids of at least 1.0 pound of solids per square foot of filter surface area (4.88 kg/m²) before backwashing is required.
Installation of the denitrification system at the Patapsco Wastewater Treatment Plant is expected by 2013. Until then, the world's largest denitrification plant is at the Howard F. Curren Advanced Wastewater Treatment Plant operated by the City of Tampa, Fla. The Curren plant installed a Denite denitrification system in 1979 and expanded it in 1986, and the improved effluent quality of the large flow from the plant has been a documented factor in the revitalization of Tampa Bay over the last few decades. Sea grasses and sport fish have returned to once barren areas of that waterway.
The Curren plant's incoming wastewater contains approximately 200 ppm biological oxygen demand (BOD), 200 ppm suspended solids (SS) and 40-45 ppm total Kjeldahl nitrogen (TKN). Effluent standards are 5 mg/l (or ppm) CBOD, 5 ppm TSS and 3 ppm TN. The plant has received a variance on TP, as eastern Tampa Bay is the largest U.S. depository of bone rock, which is used for fertilizer production and is the cause of background TP in the Bay.
Ten years of effluent data at the Curren plant from 1997 to 2006 revealed an average flow of 55.43 mgd, BOD of 2.52 mg/l, TN of 2.46 mg/l and TSS of 0.93 mg/l. Over the years, the Denite system has proven to be the only fixed-film denitrification system that can reduce NO3-N to 0.5 mg/l or less at average daily flow rates.
Patapsco plant will realize 80% reduction in total nitrogen, 90% reduction in total phosphorous
The Denite system to be installed at the Patapsco plant will help reduce its discharge of TN by more than 80 percent and TP by more than 90 percent, significantly reducing nutrients discharged to the Patapsco River and, ultimately, the Chesapeake Bay. When completed, the plant will treat an average flow rate of 81 mgd (307,000 m³/day) and a maximum of 150 mgd (568,000 m³/day). The TETRA Denite denitrification system will enable the Patapsco facility to reduce NO3-N to 0.5 mg/l and TSS to 5 mg/l at the plant's average flow rate. And Severn Trent has provided a process performance guarantee for the effluent parameters of the Denite system.
As wastewater treatment facility operators across the Chesapeake Bay watershed upgrade their plants with proven nutrient removal technologies such as denitrification, assessments of Bay restoration efforts are likely to brighten considerably.
For more information, e-mail info@severntrentservices.com.
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