A design methodology for activated carbon load equalization systems applied to biofilters treating intermittent toluene loading

Biofiltration has been used successfully as an air pollution control technology in many applications during the past several decades, however, several design and operational challenges remain. Among these is the fact that contaminant concentrations in most waste gas streams vary with time, and an appropriate basis of design for treatment of dynamically-varying waste gas streams has not yet been well established. Use of an integrated system consisting of a column packed with granular activated carbon (GAC) as a passively controlled load equalization mechanism in series before a biofilter has the potential to mitigate many of the adverse effects of unsteady loading. This paper describes results from a series of experiments conducted to characterize the capacity of Calgon BPL 4 × 6 mesh GAC for load equalization of toluene contaminated air under intermittent loading conditions consisting of contaminant supply during only 8 h/day. Experimental testing was conducted for a variety of loading conditions with GAC column empty bed contact times (EBCTs) as low as 1.5 s and influent toluene concentrations as high as 1000 ppmv. Results demonstrate that appreciable load equalization can be accomplished at low EBCTs (on the order of a few seconds) for the range of contaminant concentrations typically treated by biofiltration. A pore and surface diffusion model (PSDM), calibrated and validated using experimental data, was successful in predicting dynamic adsorption and desorption behavior. The calibrated PSDM was used to develop a series of design curves that can provide guidance in reactor sizing and operating parameter estimation. Examples are presented to demonstrate how implementation of this load equalization approach may facilitate use of smaller, less expensive biofilters, and how it can minimize much of the uncertainty that accompanies biofilter design and operation.