Model-based evaluation on simultaneous nitrate and arsenite removal in a membrane biofilm reactor

Nitrate (NO3−) and arsenite (As(III)) are two major contaminants in groundwater, which could cause significant risks to human wellbeing and ecological system. In this work, a single-stage membrane biofilm reactor (MBfR) coupling denitrifying anaerobic methane (CH4) oxidation (DAMO) and autotrophic As(III) oxidation processes was proposed for the first time to achieve the in-situ or ex-situ simultaneous removal of NO3− and As(III) from groundwater. CH4 is supplied to the MBfR through gas-permeable membranes while NO3− and As(III) are provided in the bulk liquid. A mathematical model was developed by integrating the well-established biokinetics of DAMO microorganisms with the kinetics of As(III)-oxidizing bacteria (AsOB). The key parameter values of AsOB were specifically estimated using the batch experimental data of an enriched pure AsOB culture in conjunction with thermodynamic state calculations. The maximum specific growth rate of AsOB (μAsOB) and the yield coefficient for AsOB (YAsOB) were determined to be 0.00161 h−1 and 0.016 g COD g−1 As, respectively. The modeling results demonstrated that both influent surface loading (or hydraulic retention time (HRT)) and CH4 surface loading played important roles in controlling the steady-state microbial community structure and thus significantly affected the system performance. The As(III)/NO3− ratio between 0.1 and 2 g As g−1 NO3−-N in the influent would have no significant impact on the overall system performance despite the varying microbial composition in the biofilm. Through properly adjusting the influent surface loading (or HRT) and CH4 surface loading whilst maintaining a sufficient biofilm thickness at a suitable influent As(III)/NO3− ratio, the maximum removal efficiencies of total nitrogen and As(III) could both reach above 95.0%, accompanied by a high CH4 utilization efficiency of up to 99.0%.