About the author: Mark Drake, P.E., LEED AP, is biological product manager for Veolia Water Technologies Inc. Drake can be reached at [email protected] or 919.653.4564.
Wastewater enters at the bottom of a series of treatment cells and flows up through the bed of beads, where both biological treatment and filtration occur.
Protecting the sensitive estuaries of the eastern seaboard of the U.S. from excessive nutrient pollution has long been a driver for treatment system upgrades. While some initiatives have been aimed at the protection of large water bodies (e.g., Chesapeake Bay), many others are more locally driven. This includes an area of southeastern New Hampshire that drains to the Atlantic via the Piscataqua River at Portsmouth.
The city of Portsmouth, N.H., is a historic seaport and summer tourist destination, with approximately 21,200 full-time residents. The Peirce Island Wastewater Treatment Facility (WWTF) is one of the city’s primary treatment facilities. Originally constructed in 1974, the facility has provided primary treatment with disinfection since its inception. Upgrades in 1990 and 2002 provided for advanced and chemically enhanced primary treatment, but no biological treatment or removal of any nutrients. With new permit issuance in 2007, it was indicated that secondary treatment would be required in the future, and subsequent regulatory action indicated a total nitrogen limit would also need to be achieved.
Technology Selection
In the late 2000s, the city embarked on a mission to identify the right technology for its unique situation. Not only did the technology need to address nitrogen removal, but also it had to work within a limited site footprint. A desktop evaluation of eight treatment technologies narrowed the selection to three, and those three technologies were evaluated via onsite, side-by-side pilot studies. The technologies piloted included moving bed biofilm beactors (MBBRs) with dissolved air flotation for solids removal, magnetite-ballasted conventional activated sludge (CASB) with secondary clarification, and biologically active filters (BAFs).
Conducted in 2012, the pilot study explored hydraulic and pollutant loading rate capabilities of the technologies, effluent performance, and operational characteristics. A full-scale cost analysis was completed with the benefit of the pilot study and the technologies were further ranked on many factors to determine a total weighted criteria score. The BAF system offered the highest score of 353, followed by the MBBR system at 333, and finally the CASB at 255; the design of the BAF system was pursued. This included an additional pilot study in 2014 to further explore and learn the system. This allowed for further verification of design parameters along with additional time for operations staff to become more familiar with the operation of the technology.
BAF Process
The BAF system that was pilot tested was the Biostyr system from Veolia. The system is a compact up-flow BAF, combining biological treatment and filtration in a single process. A fixed bed of submerged Biostyrene beads inside the system provides surface area for fixed film growth and filtration. The beads are offered in sizes ranging from 3.3 to 5 millimeters in diameter, which makes them fine enough for good filtration, yet with sufficient void space for optimized biological attachment and growth. The system is provided in both aerobic and anoxic configurations to meet different treatment objectives.
In the process, wastewater enters at the bottom of a series of treatment cells and flows up through the bed of beads where both biological treatment and filtration occur. The fixed bed typically will be designed at a depth of 8 to 12 ft and is held in place within the cell by a concrete nozzle deck containing plastic strainers situated at the top of each cell. The nozzle decks allow the treated effluent to exit while maintaining the buoyant media within the cell. Below the media is an aeration grid. In aerobic cells, the aeration meets treatment needs and provides a scouring function. In anoxic cells, the aeration is only used for scouring during backwash.
As needed based on system head loss or time, one cell at a time is backwashed. This process occurs by gravity flow of clean effluent back through the filter, removing excess biomass and other total suspended solids from the system. A series of valves allows the treated effluent to flow back through the cell and out through the influent pipe. This flow is then routed to a backwash holding tank where it is later sent to the headworks. A single cell is backwashed at a time, one time per day or less, for approximately 20 minutes. While one cell is backwashed, all remaining cells are available for treatment.
Design Characteristics
The design for the Peirce Island WWTF features two treatment stages. Stage 1 includes a set of six treatment cells. These cells are aerobic for biochemical oxygen demand removal and nitrification. Stage 2 includes another set of six cells that operate as an anoxic environment for denitrification. Primary clarifier effluent flows to the Stage 1 cells, and effluent from Stage 1 flows to the Stage 2 cells. The Stage 1 cells each have a cross-sectional area of 1,268 sq ft, and the Stage 2 cells each have an area of 340 sq ft, providing the ability to remove total nitrogen (TN) at a design maximum monthly flow of 9.06 million gal per day. The compact system treats an average of 940 gal per sq ft of reactor footprint per day.
Both stages of cells send backwash wastewater to a single backwash holding tank, where it is pumped back to the head of the existing primary clarifiers. The backwash return includes a provision for chemical addition to add flexibility to the operations and minimize total chemical used for primary clarification.
The process offers a solution that will allow the Peirce Island WWTF to meet its anticipated effluent TN limits in both the near and far term. Designed with an effluent TN level of 3.0 mg/L in mind, the BAF system also fits within an existing fence line. This minimizes the land-use-impact to the small island compared to alternative technologies considered. The facility upgrades began construction in 2016 and is expected to be operational by the end of 2019.