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White Paper constitute important parameters, as they influence aeration loss. The need to appreciate an oxidation ditch as a system with dynamic behavior — whose individual components operate in unison — must be emphasized. Figure 3. Depiction of the correlation between the Froude number and the aeration loss factor in horizontal flow-aeration dynamics Lessons Learned In recent years, significant improvements have been observed at wastewater treatment plants (WWTPs) that have operated oxidation ditch biological reactors with the use of mechanical surface aerators, illustrated at Big Gulch, WA. This plant replaced mechanical aeration with diffused aeration in combination with submersible lowspeed mixers, experiencing improved performance and reduced maintenance needs. At Big Gulch WWTP, until 2009, two oxidation ditches were in operation using mechanical brush aerators. The plant experienced maintenance issues concerning the mechanics of the surface aerators in addition to aerosols around the bioprocess basins. To cope with increased influent total suspended solids (TSS) and biochemical oxygen demand (BOD) loadings, the two ditches were upgraded to operate with a diffused aeration system with automatically dissolved oxygen controlled turbo blowers. Low-speed mixers were installed to generate the required horizontal flow (U.S. Environmental Protection Agency, 2010). Although the intent with the upgrade was primarily to improve operation and reduce maintenance and noise levels, the plant also enjoyed lower power consumption following the upgrades. The diffused aeration upgrade provided an annual energy savings of $10,000 compared to the four years before the upgrade when mechanical aeration was used. From a process perspective, the diffused aeration system improved performance by reducing the impact of filamentous organisms on sludge settling. Chlorine usage, previously used to control filamentous outbreaks, has been reduced with the aeration system upgrade. Fine-bubble diffused aeration has provided a much wider control range, enabling the operators to control dissolved oxygen (DO) levels during the aerobic phases of the process, along with oxidation reduction potential (ORP) monitoring during the anoxic phases. 28 wateronline.com ■ The facility has also benefited from the elimination of aerosol previously produced by the brush aerators, providing a safer area around the basins. Clearly, Big Gulch showcases operational advantages apart from energy savings. Summary And Conclusions Two viable alternatives for aerating biological reactors in municipal water resource recovery facilities often stand side by side: mechanical surface aerators and fine-bubble diffused aeration. Mechanical surface aerators are often found in oxidation ditches but lack many of the features required to provide low operating cost with optimized aeration efficiency. Such installations face problems in maximizing oxygen transfer capacity without compromising mixing. Diffused aeration is an engineered solution that requires insight into the factors affecting the degree of oxygen transfer for a given reactor geometry and diffuser configuration. However, with experience, such installations offer tangible advantages to mechanical surface aeration, including high aeration efficiency, independent mixing with increased process control flexibility, low maintenance, and a safer working environment. Oxidation ditches present additional challenges in achieving an adequate horizontal flow throughout the reactor. Highlighted in this study is just one example of successfully incorporating diffused aeration systems with low-speed horizontal flow mixers to lower operational costs, while at the same time improving treatment performance and reducing equipment maintenance. It has been demonstrated that, with the proper engineering experience, diffused aeration and submersible low-speed mixers can provide increased treatment capacity, improved energy efficiency, and operational flexibility when compared to mechanical surface aerators. REFERENCES -American Society of Civil Engineers (2007). Measurement of Oxygen Transfer in Clean Water; ASCE/EWRI 2-06. Reston, VA. -European Committee for Standardization (2003) Wastewater Treatment Plants – Part 15: Measurement of the Oxygen Transfer in Clean Water in Aeration Tanks of Activated Sludge; EN 12255-15:2003 E. Brussels, Belgium. -Tchobanoglous, G.; Burton, F. L.; Stensel, H. D. (2003) Wastewater Engineering: Treatment and Reuse. McGraw-Hill Education. London, UK. -ISO (2007) Pumps – Testing – Submersible Mixers for Wastewater and Similar Applications; ISO 21630:2007. -Uby, L. (2012) Handbook of Mixing for Wastewater and Similar Applications. Xylem Water Solutions AB. Sundbyberg, Sweden. -Reardon, R. D.; Karmasin, B. M.; Prieto, L. M.; Hurley, W. J.; VonMutius, J. (2003) A Side-by-Side Comparison of Step-Feed BNR and Simultaneous NDN. CDM: Maitland, FL. U.S. Environmental Protection Agency (2010). Evaluation of Energy Conservation Methods for Wastewater Treatment Facilities; EPA 832-R-10-005, 1-14. Mark Gehring is business development and market manager, biological treatment, for Xylem Water Solutions USA. With 20 years of experience in wastewater treatment process, Mark is currently responsible for development of the Sanitaire treatment and Flygt mixing portfolio. Gehring has a degree in Water and Wastewater Treatment from the University of Wisconsin — Stevens Point, and carries a Grade 4 Wisconsin Wastewater Treatment Plant Operators Certificate. John Lindam is a biological treatment process engineer for Xylem, and holds a masters degree in Environmental Engineering. John has worked on biological process and applications within biological treatment, with a special focus on energy optimization. He joined Xylem in 2008 after having worked in the biotechnology industry and graduating from The University of Queensland, Australia in 2007. Water Online The Magazine