Suspended and attached biomass behaviour in high-rate moving bed biofilm reactors for A-stage carbon redirection
di Biase, Alessandro
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The main goal of the thesis was to develop a high-rate moving bed biofilm (HR-MBBR) for carbon redirection allowing for potential carbon recovery through bioproducts and/or energy recovery. To achieve the goal, understanding the interaction between biofilm and suspended solids as well as developing a method to impose a biofilm solids retention time (SRT) was necessary. Particular attention was paid to biomass fractionation, biofilm versus suspended solids, through kinetic studies in HR-MBBRs operated at similar surface area loading rates (SALRs). A model was developed on data generated from HR-MBBRs in which the HRT at 11, 5.5, and 2.8 hours was the only variable maintaining the SALR. Calibration and validation of influential parameters was conducted through steady-state simulations on the highest and lowest HRTs. Calibrated and validated parameters were verified using steady-state simulations with the remaining HRTs. Dynamic simulations were run to evaluate specific biofilm and suspended solids removal rates at all HRTs. It was shown that activated sludge and digestion models do not accurately predict performance, kinetic and stoichiometric parameters for biofilm, and suspended biomass in HR-MBBRs. Therefore, biofilm and suspended biomass must be considered separately in biofilm models. A physicochemical technique to partially detach biofilm was developed. pH between 2 and 12, mixing intensities 30 to 120 rpm, and 20-100 % filling fractions were evaluated. Subjecting biofilm to pH 12 yielded 65 – 75 % biofilm detachment, while no significant difference was found when varying mixing intensities and filling fractions. Biofilm activity was preserved at 40 % compared to untreated biofilm. The developed method (i.e., pH 12 and 30 rpm) imposed biofilm SRTs in continuously operated HR MBBRs with 60 % biofilm removal to compare similar nominal SRTs ranging from 2 to 8 days with 100 % biofilm removal by applying different biocarrier replacement rates. This study showed that applying a nominal SRT of 2 days resulted in an actual biofilm SRT of 12 hours using a 60 % biofilm removal strategy and under 3 hours SRT using the 100 % removal strategy with a yield of 1.1 ± 0.3 g-TSS g-COD-1. These strategies significantly reduced suspended solids COD removal rates, favoring sequestration of organic carbon into the biofilm enabling potential energy recovery.