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For more than three decades, wastewater treatment plants have faced increasingly stringent regulations regarding total maximum loads for nutrient discharges. During that period, deep bed denitrification filters have proven to be an effective treatment technology used by wastewater plants to meet low total nitrogen (TN) limits. But cold water temperatures in more northern locales have proven to be a challenge to the effectiveness of the denitrification process. Recent wastewater treatment plant operating results, however, have shown that deep bed down flow filters are as reliable as tertiary treatment to remove suspended solids and achieve lower effluent nitrogen levels in cold weather conditions.

The combination of denitrification and solids removal in a deep bed filter process was first patented in 1979, and since then the technology has been responsible for helping improve wastewater quality at treatment plants across the country. When full nitrogen removal is required, one of the available treatment methods is biological denitrification. Nitrate-nitrogen (NO3-N) is biologically converted into nitrogen gas, thus playing an integral role in maintaining the integrity of the wastewater treatment plant's receiving waters.

Deep Bed Filtration
Filtering waters through deep beds of porous granular media to improve their clarity is widespread in municipal and industrial practice. This treatment process is now often used in tertiary wastewater filtration for reuse. Additionally, the removal of nutrients provides advanced wastewater treatment quality effluent.

The TETRA Denite® process from Severn Trent Services is a proven DeepBed™ filtration technology used for denitrification process. As both a bioreactor and effluent filter, the TETRA Denite system combines deep bed filtration and fixed-film biological denitrification to achieve a high level of process synergy. Simultaneous removal of total suspended solids (TSS) and effluent NO3-N of less than 1 ppm allows facilities to achieve 3 ppm TN or less. An added benefit to the Denite process is the removal of phosphorus, which is consumed in the cell wall biology of the biomass and removed via solids capture.

During the fixed-film biological denitrification process, wastewater is forced to flow around nitrogen gas bubbles that accumulate in media voids in the filtration vessel, improving biomass contact and filtration efficiency. Effective removal of nitrate-nitrogen is undertaken by introducing a supplemental carbon using the TETRAPace™ automatic dosing control. This dosing control scheme is based on an influent flow signal combined with an influent and effluent concentration analyzer.

The advantages of tighter carbon control can be significant if the plant has a stringent biochemical oxygen demand (BOD) limit in combination with a low total nitrogen limit. Under these conditions, the tighter control and reduced risk can be a critical component in ensuring the plant meets limits reliably. The accuracy of the proprietary algorithm used to feed methanol during the denitrification process enables TETRAPace to yield savings of up to 30 percent in chemical consumption costs while guaranteeing effluent quality with a "no net TOC pickup across the filter."

Severn Trent Services employs a "bump" operation to purge accumulated gas - mostly nitrogen that builds up in the filter media. Backwash water is used to bump the gas out. If desired, this "bumping" can be accomplished without stopping the flow of backwash water using SpeedBump™. Backwash water is applied to the bottom of the filter, bumping multiple filters in sequential order, instead of one filter at a time.

An added benefit to the Denite process is the removal of phosphorus, which is consumed in the cell wall biology of the biomass. The trapped solids are backwashed out of the filter by a simultaneous injection of air and water, and returned to the upstream biological treatment units.

More than 400 Denite filter cells are currently in operation at wastewater treatment plants throughout the United States — including the Howard F. Curren Advanced Wastewater Treatment Plant in Tampa, Fla., the world's largest denitrification system. In 2013, a Denite system will be installed at the Patapsco Wastewater Treatment Plant in Baltimore, Md. And, started up in 2015, it will supplant the Curren facility as the world's largest system.

Challenges of cold water temperatures
Cold water temperatures have always posed a challenge to the effectiveness for the biological treatment process. Lower water temperatures affect reaction time, or empty bed detention time. Since the denitrification process is time-dependent, filtration rates are typically designed for 1-3 gpm/ft2 for water temperatures down to 8 degrees C and 2-5 gpm/ft2 in warmer waters. The bacteria needed to convert NO3-N in the Denite process adapt well to temperatures less than 10 degrees C.

Denitrification systems in operation in Scituate, Mass., and Cumberland, Md., are showing the effectiveness of the Denite denitrification process in cold water temperatures.

Scituate, Massachusetts
The Scituate Advanced Wastewater Treatment facility is owned by the Town of Scituate, which is about 25 miles south of Boston. The plant was designed for an average flow of 1.60 mgd, a maximum daily flow of 2.36 mgd and a peak hourly flow of 4.34 mgd. The plant uses four 9.5 ft. wide x 32 ft. long deep bed filters and employs the TETRAPace automatic dose control system. This system uses plant flow, influent and effluent nitrate-nitrogen to pace methanol dosage.

Commissioned in 2000, daily data from 2003 to 2004 showed very consistent denitrification to below 0.5 mg/L nitrate-nitrogen during cold weather operation. The wastewater temperature averaged between 6.7 and 12 degrees C. Later results in 2006 and 2007 showed continued consistent denitrification at average monthly temperatures as low as 7.6 degrees C. Averaging cold weather data revealed the following results from January 2007 and April 2007: Average flow of 1.47 mgd; water temperature of 8.7 degrees C; influent NO3-N of 9.8 mg/L; and effluent NO3-N of 0.34 mg/L.

But the most recent evidence of the efficacy of the denitrification process even in cold water temperatures is in the Appalachian Mountains in western Maryland.

Cumberland, Maryland
The Celanese Wastewater Treatment Plant, owned and operated by Allegany County, Md., is located in Cumberland, Md. The Celanese plant is one of the coldest operating sewage nutrient removal facilities in the state. It removes nutrients that contribute to the algae blooms suffocating the Chesapeake Bay from wastewater before sending the water back into the ecosystem.

Commissioned in 2005, the plant was designed for an average flow of 1.66 mgd, a maximum monthly flow of 2.86 mgd and a peak hourly flow of 6.6 mgd.

From January 2009 to April 2012, cold weather operating data showed the Celanese plant to have an average flow of 1.82 mgd; water temperature of 11.2 degrees C; carbonaceous biochemical oxygen demand of 3.1 mg/L; TSS of 2.8 mg/L; nitrate-nitrogen of 1.0 mg/L; and total nitrogen of 2.7 mg/L.

Cold water temperatures have long posed a challenge for denitrification systems. But the TETRAPace system with its feed-forward and feed-back controls based on flow and influent and effluent NO3-N enhances the operation and reliability of the denitrification process without risk of methanol overdose. By more closely matching the methanol feed to demand, and reducing the variations in effluent NO3-N to more closely match the setpoint at temperatures below 10 degrees C, the efficacy of the denitrification process even in cold temperatures can be ensured.

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