HYDROGENOTROPHIC DENITRIFICATION OF GROUNDWATER IN A MEMBRANE BIOREACTOR ASSISTED BY MEMBRANE GAS DIFFUSION

A hydrogenotrophic denitrification system, comprising of a suspended growth membrane bioreactor (MBR) and a membrane hydrogen gas diffuser, 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 for 224 days. Four nitrate loading rates of 24, 48, 96 and 192 NO3−-N mg/L·d were applied to the system. As the nitrate loading was increased, pH increased due to increased denitrification and release of OH- ions. The oxidation reduction potential (ORP) remained fairly stable when full denitrification was achieved; however, it increased in the initial period of the fourth stage as residual nitrate was present in the reactor. It later decreased when 240 mg/L of NaHCO3 was provided. Nitrate removal was complete (100%) in the first three nitrate loadings and average of 91% of nitrate removal was achieved in the system with 192 NO3−-N mg/L·d when sufficient carbon source was supplied to the system. Nitrate utilization rates (NUR) of 30.6, 23.4, 37.7 and 184.2 mg NO3−-N/L·d were achieved in the four nitrate loadings, respectively. Average effluent dissolved organic carbon (DOC) concentration of approximately 8 mg/L observed in all four nitrate loading regimes. Average effluent soluble chemical oxygen demand (COD) of approximately of 14.1 mg/L was also observed in the last two stages, possibly due to release of soluble microbial bi-products (SMP).