Feasibility of Adsorption/Desorption Cycles in a Two-Bed Adsorption Unit for Dampening Biofilter Performance Fluctuation  
 
Paper #: 13 
 

Daekeun Kim, Zhangli Cai, George A. Sorial* 
 

Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0071 
 

* Author to whom all correspondence should be made 
 
 

ABSTRACT

      Biofilters are best operated at steady loads of contaminants. However, variations in contaminant load are common in real application. Recent research has indicated the possibility of using a carbon filter for buffering the biofilter. The buffer capacity of the adsorbents depends on the desired concentration range of the contaminants and on the time available for desorption. Regeneration of adsorbents is the most important consideration for practical application of a carbon filter.

      In order to provide long-term stable operation of a biofilter, a 2-bed adsorption system involving adsorption/desorption cycles was applied in this study. The 2-bed adsorption unit was designed for operating in a 2-step cycle, i.e., feeding (adsorption) and purging (desorption), in a fixed bed of adsorbent using gas pressure variation as the principal operating parameter. Hence, regeneration of adsorbents was effectively applied with the 2-bed adsorption system.

      The 2-bed adsorption unit was packed with activated carbon (bituminous base BPL). Toluene was tested to investigate the feasibility of adsorption/desorption cycle for dampening biofilter performance fluctuation during square wave change in VOC concentrations. The obtained results revealed that the net effect of the 2-bed adsorption was a reduction of the toluene concentration that makes it amenable for biofiltration. It was concluded in this study that the 2-step cycle in 2-bed adsorption can serve successfully as a polishing unit to abate the initial acclimation for the biofilter and a buffering unit to damp the biofilter performance during the fluctuating toluene loading 
 
 

INTRODUCTION

      Biofiltration has been recognized as a cost-effective technology for the purification of air contaminated with low concentrations of biologically degradable organic compounds.^1-4 The performance is strongly dependent on a stable, constant influent concentration of the contaminant.⁵ In practice most off gases contaminated with VOCs have variable flow rates, a wide range of concentration, unsteady loading, and shock loading, which limit the handling efficiency of biofiltration.^6-11 This limitation provides a challenge to biofiltration for the VOC abatement. An attenuation of input fluctuation is significant for regulatory compliance, especially for sources where a fluctuating loading is common.

      It has been reported that the use of “treatment trains” or “coupled systems” may be advantageous compared with any single biological technology.¹² Specifically, the biofilter preceded by a carbon filter can provide an effective solution for VOC control of emissions with unstable pollutant load.^13-17 Weber and Hartmans¹³ observed that application of an adsorbent to minimize fluctuation of toluene loading improved the overall removal efficiency. Li and Moe¹⁷ installed a single GAC load-dampening unit followed by a biofilter under discontinuous loading basis (e.g. 8 hour work per day), demonstrating that the GAC can dampen effectively the effects of the intermittent loading for multi-component waste gas. They recently developed design methodologies for activated carbon load dampening system for biofilters.¹⁸ Although these efforts have verified the effectiveness of adsorption unit in dampening biofilter performance fluctuation during transient VOC loading, this application has been restricted to the desired contaminant loading duration and its concentration range. Since the buffer capacity of the adsorbents depends on the desired concentration range of the contaminants and on the time available for desorption,¹³ long-term maintenance of the adsorption unit is not practical without regeneration of adsorbent. Therefore, an adsorption unit as a buffer column for a biofilter should carefully be designed and operated.

        In this study, a 2-bed adsorption unit was proposed to provide long-term stable buffer capacity of adsorption unit. The 2-bed adsorption system involves a short term cycle adsorption/desorption process occurring in a fixed bed of adsorbent using gas pressure variation as the principal operating parameter. Regeneration of adsorbents was conducted within the 2-bed adsorption system. The net effect of the 2-bed adsorption unit was to flatten and reduce the VOC concentration in the treated air for long-t