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

摘要

About 15% of municipal wastewater treatment in the United States is carried out usinghigh purity oxygen (HPO) wastewater treatment processes (Praxair, 2011). HPO systemsenable compact process footprint, produce good settling sludge, good DO control,process flexibility and low VOC emissions (Jiang et al, 2010). First generation HPOsystems have completely covered basins, and the oxygen feed is charged directly to theheadspace. A surface aerator sprays liquid droplets into the headspace, and the oxygendiffuses into the water droplets and is incorporated into the bulk liquid. However, closedbasin 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 accumulationof CO_2 in the headspace which could lead to depressed pH in the process and Nitrifiergrowth inhibition. Second generation HPO systems are operated in uncovered activatedsludge basins, hence avoiding the issue of headspace CO_2 accumulation and enablingeffective Biological Nutrient Removal (BNR).A number of first generation HPO wastewater systems have been upgraded to enhancetheir BNR capabilities (Randall & Cokgor, 2001). The application of biological processmodeling tools can enable the cost effective evaluation of upgrade and retrofit options(Henze, et al, 2000). While robust and reliable process modeling tools exist forconventional aeration systems, there are few options available for modeling pure oxygenbased processes (Tzeng et al, 2003). Because of the differences in the oxygen transferefficiency, vent gas volumes, gas exchange characteristics, CO_2 stripping conditions, andoff gas volume controlled evaporative cooling effect between conventional aeration andpure oxygen based systems, extensions to the traditional ASM models are necessary toadequately model HPO operations. Even within HPO systems, closed vs. open basinHPO operations also vary, especially with regards to CO_2 accumulation in the headspacein closed basin systems, with implications for biological process modeling.In this study, we describe the application of Hydromantis’ GPS-X? platform to enablethe effective modeling of open basin pure oxygen systems using data from a full scalepure oxygen activated sludge plant that was retrofit from closed to open basin pureoxygen operations.