Real-Time Efficiency Monitoring for Wastewater Aeration Systems Shao-Yuan Leu a , Diego Rosso b , Pan Jiang a , Lory E. Larson c , Michael K. Stenstrom a a Civil and Environmental Engineering Department, 5714 Boelter Hall, University of California, Los Angeles, Los Angeles, CA 90095-1593, U.S.A. E-mail: syleu@ucla.edu (S.-Y. Leu); stenstro@seas.ucla.edu (M. K. Stenstrom). b Civil and Environmental Engineering Department, University of California, Irvine, Irvine, CA 92697, U.S.A. c Southern California Edison, Rosemead, CA 91770, U.S.A. Abstract: Aeration is the most energy intensive unit operation in municipal wastewater treatment. To improve oxygen transfer rate, fine-pore diffusers have been wildly applied in aeration practice. However, during operation, this type of diffuser suffers from fouling and scaling problems, which cause a rapid decline in aeration performance and significant increase in energy consumption. Diffusers must be cleaned periodically to reduce energy costs. The cleaning frequency of diffusers is site-specific, and can be evaluated by oxygen transfer efficiency (OTE) measurements over time in operation. Off-gas testing is the only technique that directly measures oxygen transfer efficiency in real-time. This paper presents a time-series of off-gas measurements which demonstrate the value of implementing energy- conservation practices. Our results include the real-time prediction of plant load and alpha factors from off-gas testing, as well as the quantification of the increased energy costs caused by fouling. Our off-gas analyzer can be used to develop an aeration efficiency monitoring protocols, and an aeration feedback control system for blowers. Keywords: Aeration; off-gas technique; oxygen-transfer efficiency. INTRODUCTION Municipal wastewater treatment plants have been converted to fine pore diffusers which have resulted in significant energy savings. Fine pore diffusers work well but have the pitfall of fouling and scaling, which causes a decrease in oxygen transfer efficiency (OTE, %) resulting in an increase in the energy cost per unit oxygen transferred (Rosso and Stenstrom, 2006a). A common way of quantifying OTE is the off-gas technique, which measures mass transfer by analyzing the oxygen content in the air leaving the surface of the aeration tank. The mass difference between oxygen fed (21% mole fraction) and off-gas, which can be measured independently of off-gas flow rate, is used to calculate the OTE. If the air flow rate is also measured, the oxygen uptake rate (OUR) can be calculated as well as the overall plant loading, quantifying the oxygen requirements and the bacterial activity of the activated sludge process (Yuan, et al., 1993, Koch, 2000). This technique is applicable for measuring instantaneous wastewater treatment plant loadings, and to optimize blower systems. The off- gas technique is the process water aeration efficiency measurement with the highest accuracy and precision (ASCE, 1997). The modern off-gas analysis and its instrument were developed by Redmon et al. (1983) under the sponsorship of US EPA and ASCE. It uses a vacuum pump to collect the off-gas stream from the aeration tank thorough a floating hood. The original off-gas procedure works very well, but requires an analyzer, the capture hood, a vacuum pump, and a crew of at least two to three experts to perform the analysis, which may take one to three days to perform for a typical activated sludge treatment plant. For this reason, off-gas testing is not routinely performed, but is used for periodic assessment of aeration efficiency or to collect data for plant design or upgrading. This paper presents a simplified, automated off-gas monitoring instrument (design data are in the public domain), which operates in real-time and is self-calibrating. In addition, we present the results of real-time off-gas testing and their Water Practice & Technology Vol 3 No 3 © IWA Publishing 2008 doi: 10.2166/WPT.2008064