Accumulation of Metabolic Intermediates during Shock Loads in Biological Fluidized Bed Reactors

Abstract

Groundwater containing benzene, toluene, and p-xylene (BTX) was treated in fluidized bed reactors using nonactivated carbon (FBR) and activated carbon (BAC-FBR) as the biofilm carriers. BTX removal and accumulation of metabolic intermediates were studied at steady state and during single-substrate step-loading increases. At steady state (organic loading rate of 2.2 kg-COD/m3-d), no intermediates were detected in the effluents. However, intermediates were found in the effluents of both systems during a sevenfold p-xylene step increase under different supply levels of dissolved oxygen (DO) and nutrients. Intermediate concentrations increased with increasing dissolved oxygen in the influent but were not affected by the level of nutrients. The accumulation of intermediates was also evaluated quantitatively by subjecting both reactor systems to 20-, 12-, and 7-fold step increases in the organic loading rates of benzene, toluene, and p-xylene, respectively. In all of the step increase experiments, the effluent intermediate concentrations were less for the BAC-FBR than for the FBR system. We conclude that the combination of biological and adsorptive removal mechanisms in the BAC-FBR resulted in enhanced removal of both BTX and metabolic intermediates under shock-load conditions.