Incorporating membrane gas diffusion into a membrane bioreactor for hydrogenotrophic denitrification of groundwater

A hydrogenotrophic denitrification system, comprising a suspended growth membrane bioreactor (MBR) with membrane hydrogen gas diffusion, was developed to remove nitrate from groundwater. A hollow fiber gas permeable membrane module was designed for hydrogen delivery and a commercially available hollow fiber membrane module was used for solid/liquid separation. The MBR was operated at an SRT of 20 days and at room temperature. Four nitrate loading rates of 24, 48, 96 and 192 NO3−-N mg l−1 d−1 were applied to the system. As the nitrate loading was raised, pH increased due to increased denitrification and release of OH− ions. The oxidation reduction potential (ORP) remained fairly stable when full denitrification was achieved, but increased when nitrate loading rates reached 192 NO3−-N mg l−1 d−1 and residual nitrate was present in the reactor. Nitrate removal was complete (100%) in the first three nitrate loadings and 72% in the system with 192 NO3−-N mg l−1 d−1. Nitrate utilization rates of 30.6, 23.4, and 37.7 g NO3−-N m−3 d−1 were achieved in the first three loadings. Average effluent dissolved organic carbon (DOC) concentration of approximately 8 mg l−1 was observed in all four nitrate loading regimes, possibly owing to the generation and release of soluble microbial bi-products (SMP).