Extending the Modeling of High Purity Oxygen Wastewater Treatment Processes: Transition from Closed to Open Basin Operations – A Full Scale Case Study

About 15% of municipal wastewater treatment in the United States is carried out using high purity oxygen (HPO) wastewater treatment processes (Praxair, 2011). HPO systems enable compact process footprint, produce good settling sludge, good DO control, process flexibility and low VOC emissions (Jiang et al, 2010). First generation HPO systems have completely covered basins, and the oxygen feed is charged directly to the headspace. A surface aerator sprays liquid droplets into the headspace, and the oxygen diffuses into the water droplets and is incorporated into the bulk liquid. However, closed basin high purity oxygen systems can have a limited capacity for Nitrification (Metcalf & Eddy, 2003; Sears et al, 2003; Randall & Cokgor, 2001) mainly due to the accumulation of CO2 in the headspace which could lead to depressed pH in the process and Nitrifier growth inhibition. Second generation HPO systems are operated in uncovered activated sludge basins, hence avoiding the issue of headspace CO2 accumulation and enabling effective Biological Nutrient Removal (BNR).A number of first generation HPO wastewater systems have been upgraded to enhance their BNR capabilities (Randall & Cokgor, 2001). The application of biological process modeling tools can enable the cost effective evaluation of upgrade and retrofit options (Henze, et al, 2000). While robust and reliable process modeling tools exist for conventional aeration systems, there are few options available for modeling pure oxygen based processes (Tzeng et al, 2003). Because of the differences in the oxygen transfer efficiency, vent gas volumes, gas exchange characteristics, CO2 stripping conditions, and off gas volume controlled evaporative cooling effect between conventional aeration and pure oxygen based systems, extensions to the traditional ASM models are necessary to adequately model HPO operations. Even within HPO systems, closed vs. open basin HPO operations also vary, especially with regards to CO2 accumulation in the headspace in closed basin systems, with implications for biological process modeling.In this study, we describe the application of Hydromantis' GPS-X™ platform to enable the effective modeling of open basin pure oxygen systems using data from a full scale pure oxygen activated sludge plant that was retrofit from closed to open basin pure oxygen operations.