Modelling the long-term dynamics and inhibitory effects of crude glycerol impurities in a methanogenic and sulfidogenic UASB bioreactor

The performance of Upflow Anaerobic Sludge Blanket (UASB) bioreactors treating sulfate (SO42-) -rich effluents depends on multiple factors, including microbial interactions and operational conditions. The high complexity of these systems necessitates the use of mathematical modelling tools to better understand the process and predict the long-term impacts of various operational variables. In this work, a mathematical model describing the long-term operation of a sulfate-fed 2.5 L UASB reactor was developed, calibrated and validated. Crude glycerol was used as electron donor to achieve sulfate reduction. The hydraulic model of the UASB was described as a set of CSTRs in series to represent its plug flow-like behavior. The kinetic model included 8 fermentation processes using glycerol as the primary electron source, 5 sulfate-reduction processes using organic and inorganic electron sources, and 2 methanogenic processes. The model tackled the long-term accumulation of the impurities coming from the crude glycerol solution, namely slime -like-substances (SLS), and their inhibitory effects over the three different trophic groups: fermenters, sulfate-reducers and methanogens. A sensitivity analysis and calibration of the most relevant parameters was performed using the experimental data from 280 days of continuous operation of a lab-scale UASB. Volatile suspended solids (VSS), carbon (C) and sulfur (S) species profiles as well as microbial dynamics from initial methanogenic conditions to non-methanogenic conditions due to SLS impact were properly predicted by the model under steady-state feeding conditions. Furthermore, the model was validated using another independent set of data under dynamic-feeding conditions, containing 6 different phases with varying HRT, inlet sulfate and organic carbon concentrations. After successfully validating the model, a scenario analysis was conducted to evaluate two case studies, with different inlet sources: crude glycerol with varying SLS concentrations and pure glycerine (SLS-free). The results of the simulations suggest that heterotrophic SR have greater long-term resistance to the inhibitory effects of SLS, compared to methanogens. Methane production increased with higher C and S loading rates, and the balance between sulfate reduction efficiency and COD removal was optimal at a C/S ratio of 1.6 g C g S-1.