Biological activated carbon in fluidized bed reactors for the treatment of groundwater contaminated with volatile aromatic hydrocarbons

A comparison of fluidized bed reactor systems with (1) adsorptive removal capacity only using granular activated carbon (GAC) without microbial growth, (2) combined biological and adsorptive removal mechanisms using GAC with microbial growth and (3) biological removal only using nonactivated carbon with microbial growth was performed. These three systems were fed groundwater contaminated with benzene, toluene and xylene (BTX). The breakthrough profiles, steady-state removal of BTX and system responses to step increases in applied organic loading rates were investigated. During start-up, even though the same amount of inoculum was added to the two biological systems, the time required until effective biodegradation commenced in the system employing GAC as a biomass carrier was less than that observed for the system using non-activated carbon (200 vs 500 h). Significantly less BTX was released during this period by the system with combined removal mechanisms and the development of a contiguous biofilm was more rapid. Under constant, steady state, organic loading conditions (3 and 6 kg-COD/m 3-day), BTX removals were similar for the two biological systems, although the system employing the GAC carrier had lower effluent concentrations at the lower loading rate. More than 90% of the BTX were removed in both systems. During step increases in organic loading, however, the combination of biological and adsorptive removal capacity resulted in enhanced BTX removal and more stable operation. Scanning electron microscopy was used to examine the extent of surface coverage of the GAC and non-activated carbon by the biofilm. Particles from both systems were observed to be completely covered by a contiguous, thick biofilm.