Modeling Fixed Film Processes: Practical Considerations and Current Limitations from a Consulting Viewpoint


Fixed film technologies such as integrated fixed film activated sludge (IFAS), moving bed bioreactors (MBBR), biological aerated filters (BAF) and denitrification filters are gaining popularity in the United States for a number of reasons. As a result, there is a renewed desire to model these processes, particularly for consulting engineers interested in designing these processes. Models are powerful tools for applying pilot data to full scale designs, as examples in
this paper show. Westchester County (NY) piloted both the IFAS and MBBR technologies at the Mamaroneck Wastewater Treatment Plant (WWTP) as part of a nitrogen removal study, but not all operating conditions could be evaluated during the six month study. A dynamic model calibrated with pilot data provided insight to how the processes would handle wet weather events. At the Henry N. Wochholz WWTP in Yucaipa (CA), models were used to simulate both a plastic media IFAS system and a sponge media system, the latter being modeled as an activated sludge process in which a portion of the biosolids are retained in the aeration tanks. Three years of full-scale operational data at the Broomfield WWTP proved to be invaluable for calibrating a new model for the Phase 2 expansion. Commercially available software packages such as BioWin (Envirosim) and GPS-X (Hydromantis) are excellent tools for modeling fixed film processes, though the modeler must take care to not just use the simulators “out of the box” as the models are complex and require expert input to calibrate and use them correctly. Pilot testing and specialized sampling is highly recommended to enhance modeling efforts, especially for designs with low effluent limits. Despite the advances in fixed film modeling in recent years, there are still many questions that remain, and this paper touches on some of the more urgent research needs to improve fixed film models for design and other practical applications.

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