Attached and suspended biomass kinetics in an IFAS-MBR system operated under intermittent aeration: Long-term monitoring under SRT variation

This study thoroughly investigates a Membrane BioReactor - Integrated Fixed Film Activated Sludge - Intermittent Aeration (MBR-IFAS-IA) pilot plant operated from a biokinetic point of view. Specifically, respirometric techniques were applied on suspended and attached biomass to evaluate kinetic and stoichiometric parameters. The main aim was to investigate how the simultaneous presence of biofilm and activated sludge could affect the kinetic behaviour and the role of the Sludge Retention Time (SRT) variation in the kinetic behaviour of the system. The results highlighted a mutual interaction between suspended biomass and biofilm in the IFAS-MBR configuration. In Period I both the heterotrophic yield and growth rate of suspended biomass were higher compared to that of biofilm, thus highlighting higher affinity with organic matter; in contrast, the biofilm showed high affinity with nitrification, with increased nitrification rates with decreasing SRT and sustaining nitrification in the activated sludge due to "seeding" effect. Therefore, the suggestion is that it is possible to operate IFAS-MBR systems at low SRT without hampering the nitrification ability due to the growth of nitrifiers in the biofilm. Respirometry has been confirmed to be an effective tool for evaluating biomass kinetic and stoichiometric parameters. The results of this study highlighted the effect of IFAS configuration and can help apply mathematical models in the design phase and monitor biomass viability during plant operations.