2011 Honor Award - Research

Comparison of Parallel IFAS and ASP Reactors: Oxygen Transfer and Uptake, Nutrient Removal, Carbon and Energy Footprint

Location: McLeansville, North Carolina
Entrant: Hazen and Sawyer, P.C.
Engineer in Charge: Alan Stone, P.E.
Media Contact: Nicole Pennington; 919-833-7142; NPennington@hazenandsawyer.com

The T.Z. Osborne Water Reclamation Facility, a 40 mgd facility located in Greensboro, NC., is the City's largest wastewater treatment facility.	Aeration Tank No. 11 (right) and Aeration Tank No. 12 (left) are shown. Each tank contains three anoxic cells followed by six aerobic cells. The anoxic cells are covered by concrete and visible in the lower portion of the photograph. The IFAS process is piloted in Aeration Tank 12; Aeration Tank 11 is a conventional activated sludge process.	IFAS media in the full-scale pilot, white in color when placed in the basin, now has a biofilm layer. The biofilm growth on the media increases the biomass inventory in the basin.
A closer view of Aeration Basin No. 12 shows screens placed on top of baffle walls to separate the three IFAS cells and keep media in place.	This figure shows the off-gas testing layout. This non-invasive testing method allows the concurrent measurement of the actual oxygen transferred to the water (oxygen transfer efficiency [OTE, %], oxygen transfer rate [OTE, kgO₂/h], and oxygen uptake rate [OUR, mgO₂/L/h].	The compact off-gas analyzer used in testing was assembled in the field and contains components that yield results identical to the components used in the original Ewing off-gas instrument developed for this test.

Entrant Profile

Hazen and Sawyer has been a leader in the water and wastewater industry for over 60 years. Since first working with the City of Greensboro in the 1950s, Hazen and Sawyer has earned the position of trusted technical advisor. Hazen and Sawyer was the original designer for the City's largest wastewater treatment facility, the 40 mgd T.Z. Osborne Water Reclamation Facility. It is at this facility that innovative research to compare oxygen transfer efficiency, air use, and oxygen uptake of parallel IFAS and ASP reactors was conducted. This research was conducted through collaborated efforts between Hazen and Sawyer, the City of Greensboro's Water Reclamation Department, and Dr. Diego Rosso, an Assistant Professor in the Civil and Environmental Engineering Department at the University of California, Irvine.

Hazen and Sawyer, through focused applied research such as the Parallel IFAS and ASP Reactors Research Study, serves to provide leadership and guidance, pushing the cutting edge of science while remaining at the very center of practicality. It also provides a platform for dialogue with regulators and utility managers, while fostering education and outreach to stakeholders and collaborators.

The City of Greensboro has reputation for environmental stewardship and strongly supports hands-on approaches and applied research efforts to evaluate wastewater process technologies. Dr. Rosso is an expert in the industry focusing on aeration. He has a Ph.D. in Environmental Engineering from the University of California at Los Angeles, and is a Chemical Engineering Laureate from the University of Padua, Italy.

Project Description

The T.Z. Osborne Water Reclamation Facility (WRF), located in Greensboro, NC, is owned and operated by the City of Greensboro. This 40 million gallon per day (MGD) wastewater treatment plant discharges to the Haw River, a tributary of Jordan Lake, and has a history of producing a high-quality effluent. As a result of North Carolina Department of Environment and Natural Resources (NCDENR) basin-wide planning and the development of a nutrient management strategy for the Jordan Lake Watershed, the City must address point and non-point sources entering the waters of the State.

The Jordan Lake Nutrient Reduction Rules were formally adopted in August 2009 and required the City to evaluate process alternatives to meet annual total nitrogen (TN) and total phosphorus (TP) mass allocations of 901,032 lb/year and 113,258 lb/year, respectively. This corresponds to TN and TP effluent concentrations 5.29 mg/L and 0.66 mg/L, respectively, at the City's combined permitted flow of 56 mgd.

The City of Greensboro, with the technical assistance of Hazen and Sawyer, constructed a full-scale Integrated Fixed-Film Activated Sludge (IFAS) pilot in 2007 in order to verify the viability of IFAS as a nutrient reduction strategy. IFAS involves the addition of fixed or free-floating media, either plastic or fabric, to an activated sludge basin. The IFAS process improves upon the activated sludge process by providing a larger biomass inventory in the aeration tank by facilitating biofilm growth on the media in addition to suspended solids which are present in the activated sludge process. The higher biomass inventory increases treatment capacity. The enhanced removal of chemical oxygen demand (COD) and nutrients has been demonstrated in IFAS applications (Randall et al., 1996).

Objective and Originality of the Study

The objective of this comparative study was to be the first independent investigation to identify oxygen transfer efficiency, air use, and oxygen uptake of an IFAS basin relative to an adjoining activated sludge basin. Identifying these parameters was critical since they are necessary to accurately quantify process operating costs and energy footprint, and ultimately help the City identify the most appropriate, costeffective, and efficient technology to meet forthcoming limits. This study was a collaborative effort between the City of Greensboro, Hazen and Sawyer, and Dr. Diego Rosso, an Assistant Professor at UCIrvine specialized in aeration testing. Previously, one off-gas test was performed by an IFAS manufacturer, Infilco Degremont (Viswanathan, 2008). To our knowledge, this is the first independent test published to date.

Demonstration of an Innovative, Integrated Approach

The off-gas testing method, a gas-phase mass balance technique for measuring oxygen transfer efficiency of diffused air devices, is described in detail in the Standard Guidelines for In-Process Oxygen CalibriTransfer Testing (ASCE, 1997). This method requires the capture of a representative sample of the gas which exits the aeration basin surface, typically using a floating "hood" that provides a seal between the liquid surface and the headspace. Two tests were performed, one in the winter season (January 2010) and one in the summer season (June 2010). No unusual plant conditions existed at the time of either testing.

Results and Discussion

The results show that in both tests the activated sludge process had lower dissolved oxygen, air flux, and air use than the IFAS process, which in turn was characterized by lower OTE. Due to the elevated DO requirements for the IFAS process, the normalized efficiency (aSOTE) was comparable between the two processes, after data normalization to standard conditions (i.e., zero DO). Nevertheless, the air used (as mass of oxygen per mass of load removed) by the IFAS process is between 1.6 and 2.0 times higher than the activated sludge process, corresponding to the analogous ratio for energy footprint. Nitrous oxide in the off-gas was measured in the winter test, and at this time is still insufficient to support any conclusion on the carbon footprint of two processes.

Social and Economic Advancement

The study team recognized that social, economic and environmental issues are interrelated and that a sound decision must consider how a decision will impact people, cost and the environment. In this particular case, the City faced implementing one of two alternatives: the more energy intensive IFAS process not requiring construction of additional tank volume or a conventional BNR strategy that required construction of addition concrete tankage on the treatment site. Without a doubt, this project considers all environmental media; the project team evaluated off-gas from two wastewater treatment processes in order to help the City decide the best upgrade process option. The final decision resulted in the adoption of an alternative that will save the City of Greensboro millions of dollars over the lifespan of the process.

The City of Greensboro constructed its own instrument to be used for testing. This photograph shows Mike Adcock (City of Greensboro) and Sarah Lothman (Hazen and Sawyer) observing the instrument at work.	T.Z. Osborne staff constructed two off-gas collection hoods. These hoods, one for each basin, were each 8ft. by 4ft. in area and constructed from marine plywood with Styrofoam® for additional flotation.	The testing team used a backhoe to lift off-gas hoods over divider screens; the screens were put into place by City of Greensboro staff in order to prevent media from traveling over baffle walls into downstream cells.
This photograph shows the June rainstorm that moved in quickly to interrupt testing.	A team member adjusts an off-gas collection hose. Off-gas is collected under the hood and travels through the hose to the analyzer.	This schematic shows both testing basins, aeration tank No. 11 and aeration tank No. 12. IFAS media is located in cells D, E, and F of aeration tank No. 12. The numbered yellow boxes represent locations of gas capture hoods where readings were taken.

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May 22, 2012

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