Sand Media Type and Charge Effects on Tce Cometabolism in a Fluidized- Bed Bioreactor

Fluidized-bed biological reactors (FBBRs) may be used to remove chloroethenes such as trichloroethene (TCE) from groundwater. Proper selection of FBBR aqueous growth medium, biofilm solid support media type and size, and bed charge (bed depth to column length fraction) are critical for establishing a biomass of sufficient quantity and activity to transform TCE via aerobic cometabolism, which requires a supplemental growth substrate. Thus, proper media selection involves facilitating efficient conversion of growth substrate to cometabolic activity for TCE degradation. In this study, five types of sand media (quartz, garnet, ilmenite, hematite, and magnetite) were evaluated in small-scale FBBRs under different conditions of media charge, size, and type. Also, biofilm growth and TCE degradation rates were measured for three aqueous growth media with phenol as the growth substrate: mineral salt waters (MSW), groundwater with excess nutrients (NGW), and amended groundwater with 10 times excess nutrients (AGW). MSW produced the highest level of biomass and highest TCE degradation rates, but the other media were selected for use in the FBBRs due to practical considerations. Differences in biofilm cometabolic properties were related to carbonate versus phosphate buffering of the growth media. Biofilms grown in the small-scale FBBRs on the different sand media were harvested and assayed for their maximum specific TCE degradation rate (k c) and transformation capacity (T c). Quartz sand with 30/35 mesh (500 to 594 mm) particle size and 20% column sand charge was identified as a near-optimal media with a k c of 181 mg-TCE/g-VSS/d and a T c of 0.23 mg-TCE/g-VSS. Hematite sand was also identified as a media that enhanced cometabolism. When implemented in a continuously operated laboratory pilot-scale FBBR at a flow of 1 L/min and a detention time of 2.4 minutes, TCE removal up to 81% was obtained for an inlet TCE concentration of 123 mg/L using the quartz sand. INTRODUCTION Trichloroethene (TCE) has been widely used in the past and its uncontrolled disposal has led