Denitrification system cleans up largest U.S. estuary
Chesapeake Bay is one of the most important bodies of water in the world and is the largest estuary in the U.S. Comprising 4,700 sq miles and accounting for nearly 12,000 miles of waterfront, the bay is home to myriad freshwater plant and animal species, provides recreational pleasure to millions of people, and is a critical waterway for commercial and national defense purposes.
The Chesapeake Bay watershed covers 64,000 sq miles—including portions of Maryland, Virginia, West Virginia, Pennsylvania and New York—and includes hundreds of thousands of creeks, streams and rivers. More than 300 species of fish, shellfish and crab also call the bay home.
While the bay has unparalleled beauty, pollution from agriculture, wastewater treatment plants and urban runoff is a huge problem for it and its watershed. Three of every 10 acres in the watershed are used for agriculture, with more than 83,000 farms contributing $10 billion in annual agricultural output. While thousands of wastewater treatment plants (WWTPs), providing service to nearly 18 million people, dump their effluent into rivers and lakes that flow to the bay, 470 WWTPs are designated by the U.S. Environmental Protection Agency (EPA) as significant sources of nutrients and total suspended solids (TSS). Additionally, factories, power plants, and millions of gallons of storm water runoff contribute to pollution.
Setting the Bar
The situation was so acute that on Dec. 29, 2010, the EPA established the Chesapeake Bay Total Maximum Daily Load (TMDL), a daily pollution diet that puts rigorous accountability and measurements on the total amount of nitrogen and phosphorus allowed in the bay. The TMDL is the largest pollution diet ever developed by the EPA and is designed to ensure that all pollution control measures needed to restore the bay and its tidal rivers are in place by 2025.
The effort to turn the tide within the watershed encompasses water filtration and nutrient removal efforts for over a dozen waterways, including the Susquehanna River, Back River, Patapsco River, Patuxent River, Potomac River, Rappanhannock River, York River, James River, Shenandoah River, Antietam Creek, Seneca Creek, Beabsco Creek, George’s Creek, Town Run and many more. The rivers and streams that feed into Chesapeake Bay read like a who’s who of American waterways.
The major nutrient culprits in the pollution of the bay are nitrogen and phosphorus. Both nutrients cause algal blooms in the receiving waters. The algal blooms reduce the dissolved oxygen (DO) levels in the water, which kills plant and animal life—everything from marsh grasses to blue crabs to rockfish. While there are many sources of these pollutants, the major contributors are agricultural runoff, wastewater treatment plant and combined sewer overflows (CSOs), forests, urban runoff and septic system leaching. In 1985, agriculture and wastewater and CSO contributed more than 70% of the nitrogen and more than 80% of the phosphorus.
With the WWTP upgrades driven by the Clean Water Act National Pollution Discharge Elimination System (NPDES), the percentage of nutrients attributed to WWTPs has fallen dramatically, from 28% to 16% of nitrogen and 39% to 16% of phosphorus.
Knocking Down WWTP Pollutants
Within each wastewater treatment plant, nitrogen and phosphorus can be reduced with several different processes or technologies. De Nora Tetra Denite technology is one such technology for a facility that requires additional treatment to meet the NPDES permit regulations of 3 mg/L total nitrogen (TN) and 0.3 mg/L total phosphorus (TP) for the Chesapeake watershed. The system converts nitrates into nitrogen gas using a biologically active deep bed sand filter. A supplemental carbon source is added to the filter’s influent water to grow the denitrifying biomass. It simultaneously reduces TSS.
While much of the wastewater’s phosphorus content is removed upstream from the filter, residual solids containing phosphorus are captured in the filter media. Additionally, phosphorus is consumed via the denitrification mechanism, where it is incorporated into the biomass. The nitrogen gas is periodically purged or ‘bumped’ from the filter bed via brief upflow backwash water—typically multiple times per day. The filtered solids and biomass are removed from the filter bed via air and water backwash—typically once every two to five days.
The system employs three components that make the system effective. First, the underdrain supports the media, and during backwash, it acts to start the distribution of the air and water. The underdrain is a plastic, jacketed, concrete underdrain block that has been effective in German and U.S. steel mills dating back to the 1930s.
The second component is the mono-media sand. The sand is a support for the denitrifying bacteria and a filter media to capture the influent TSS and the biomass generated from the denitrification reaction. The 2- to 3-mm sand has critical physical properties, including a low uniformity coefficient, a high sphericity and a high Moh hardness. These properties combine to provide a system that can operate for decades with little or no media loss or degradation.
Finally, De Nora uses a proprietary algorithm called TetraPace to control the flow of supplemental carbon to the filter. The algorithm uses a feedforward and feedback loop to optimize the carbon usage and minimize the residual total organic carbon in the treated effluent water.
Developing the Solution
When a municipality is required to comply with new TN and TP effluent limitations, the process engineering group works with a municipality’s consultant or engineer to develop a system. Critical variables to evaluate are the influent flowrate (average daily flow and peak flow), influent nitrate concentration to the filter, required effluent nitrate concentration and the space available for the filter system. Once the process design for the filters has been finalized with the engineer, process and proposal groups collaborate with the engineer to size the auxiliary equipment in a Denite system, including the backwash pumps, backwash air blowers, mudwell pumps, level and flow instrumentation, and nutrient analyzer.
More than 20 WWTPs in the Chesapeake Bay watershed use the Tetra Denite technology to solve nutrient removal needs. The Back River WWTP in Baltimore has the largest denitrification filter in the U.S., with an average daily design flow of 188 million gal per day (mgd) and peak of 300 mgd. The Back River WWTP is comprised of 52 11-ft-8-in.-wide-by-100-ft-long filters arranged in four quadrants. The 81-mgd average flow Patapsco WWTP in Baltimore is currently under construction and consists of 34 11-ft-8-in.-wide-by-100-ft-long Denite filters.
Additional WWTPs in operation using this technology are the Arlington County Water Pollution Control Plant, the Washinton Suburban Sanitary Commission Seneca Creek WWTP, York River WWTP, Cumberland WWTP, H.L. Mooney Advanced Water Reclamation Facility, Lebanon WWTP, Parkway WWTP in Laurel, Md., and the City of Richmond WWTP. Richmond’s WWTP was a pioneer in the early days of nutrient removal for the bay. The facility retrofitted its nozzle-bottom filters with an underdrain in 1986 to allow for fixed-film denitrification in tertiary filters.
The system has a 30-year history of success in Tampa Bay, and its continuing success in Chesapeake Bay with more than 450 mgd under treatment have caught the attention of China, which has a dire need for treatment of its waterways. De Nora has dozens of plants in the People’s Republic of China, with more plants going online every month. China is a major market for denitrification technology as it attempts to revitalize rivers and lakes across the country.
For the U.S., algal blooms are becoming commonplace on Lake Erie. In the summer of 2017, the shallowest of the Great Lakes had the third-worst algal bloom in 15 years, covering 280 sq miles with a thick, paint-like scum. The Gulf of Mexico had an 8,776-sq-mile “dead zone” in the summer of 2017—the largest ever recorded. And the U.S. continues to struggle to address algal blooms off the shorelines of both the east and west coasts.
Upgrades to wastewater treatment plants can play a major role in reducing nitrogen and phosphorus levels in streams, lakes and rivers. Technology such as Tetra Denite is one solution to this growing issue.