How does CO2 supply shape the biofilm microenvironment in denitrifying H2-based membrane biofilm reactor: A modeling study

Abstract

CO2 supply to a hydrogen-based membrane biofilm reactor (H2-based MBfR) is an imperative approach for maintaining its performance. The supply modes (via bulk liquid or membrane) and dosages of CO2 can largely determine the efficiency of H2-based MBfR, but their effects on the biofilm microenvironment remain unclear. To bridge this knowledge gap, a denitrifying H2-based MBfR equipped with an in-situ pH microsensor was constructed, and an expanded model was developed by incorporating the modified denitrification kinetic algorithm, based on the measured pH gradients in the biofilm. Results showed that the greatest NO3− removal was achieved at an influent CO2 concentration of 50 mg/L and a CO2 fraction of 20 %. The carbon source constituted a decisive factor, limiting the denitrification activity in the inner and outer layers of the biofilm when CO2 was supplied below the optimum dosage (50 mg/L or 20 %) via bulk liquid and membrane, respectively. Increasing the CO2 dosage to 100 mg/L or 30 % enabled the sufficient carbon source, but led to low-pH inhibition in the entire biofilm. The outcomes of this research provided mechanistic insights into the denitrification behaviors of H2-based MBfR, and the developed model could help devise CO2 supply strategies for the management of the reactor.