Excellence in Environmental Engineering Award for IFAS Study
(RALEIGH, NC – May 2, 2011) – The American Academy of Environmental Engineers has bestowed the Excellence in Environmental Engineering Honor Award for Research on the comparative study of parallel IFAS and ASP reactors at the T.Z. Osborne Water Reclamation Facility. This 40-mgd wastewater treatment plant, located in Greensboro, NC, 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 technical assistance from 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, cost-effective, 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 UC-Irvine 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 Transfer 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.