Transient Response of Flow-Direction-Switching Vapor-Phase Biofilters

Abstract

Transient loading of vapor-phase biofilters may result in exceedence of the local reaction or mass transfer capacity of the inlet region. In such cases, higher concentrations of contaminants are carried deeper into the bed where there is less active biomass and, in some cases, breakthrough of contaminants may occur. Previous studies have demonstrated that periodic reversal of the flow direction results in improved transient-loading response. However, quantitative information on the extent of the benefit is lacking.Step function increases in toluene concentration were applied to unidirectional-flow and flow-direction-switching laboratory reactors operated in parallel. Contaminant concentration was monitored at several points along the packed beds. Relative to unidirectional mode of operation, periodic flow reversal produced a more uniform distribution of microbial reaction capacity along the length of the packed bed. Directional switching at a 12-h interval did not result in a loss of activity or removal capacity. Mass-removal rates under transient-loading conditions were similar in the first-half of both biofilters but, in the second-half of the units, significant removals were observed only in the flow-direction-switching biofilter. As a result, maximum mass-removal rates under transient-loading conditions were approximately twice as great for the flow-direction-switching biofilter relative to the conventional unidirectional-flow biofilter receiving similar mass loading.

Regular Transient Loading Response in a Vapor-Phase Flow-Direction-Switching Biofilter

“…2-4͒ and this favorably distinguishes FDS biofilters from conventional unidirectional biofilters which typically have minimal levels of reaction capacity in the second half of their beds ͑Ergas et Kinney 1996;Song and Kinney 2001;Wright 2004. The results of this study support conclusions reached by Song and Kinney ͑2001͒ that relative to unidirectional mode of operation, periodic flow reversal produced a more uniform distribution of reaction capacity along the length of the packed bed.…”

“…In such cases higher concentrations of contaminants are carried deeper into the bed and, in some cases, pass through the unit untreated. Using a directionally switching mode of operation results in greater removal rates in the downstream bed sections and greater capacity to respond to transient loadings ͑Kinney 1996; Song and Kinney 2000;Wright 2004. The principal objective of this study was determination of the response of a vapor-phase flow-direction-switching biofilter to regular transient loadings.…”

“…An air-stream containing toluene vapor and nutrient aerosol was conveyed to the inlet of the FDS biofilter column ͑top or bottom depending on the cycle phase͒ using a double solenoid, four-way, five-port valve and electronic controller. A more complete description of the experimental apparatus, nutrient solution, and air-stream sampling and analysis have been described previously ͑Wright 2004, except, in this study, only one FDS biofilter was used, new media was used at startup, and the feed apparatus did not include a humidifier or aerosol reduction device.…”

“…Correspondingly, off-gas characteristics from manufacturing and commercial operations will typically vary with time due to changes in processes, shut downs, shift changes, and spills. The performance of biofilters subject to transient loadings have been studied by a number of researchers ͑Al- Rayes et al 2001;Campbell and Connor 1997;Damborsky et al 1999;Deshusses 1995;Deshusses et al 1995Deshusses et al , 1996Deshusses et al , 2001DirkFaitakis and Allen 1998;Ergas et al 1995a, b;Hodge and Divinney 1995;Irvine and Moe 2001;Kinney 1996;Kirchner et al 1994; Leson and Winer 1991;Marek et al 2000;Martin and Loehr 1996;Métris et al 2001;Moe and Irvine 2001;Mysliwiec et al 2001;Park and Kinney 2001;Parks and Loehr 1994;Schroeder et al 2000;Schroeder 2002;Seed and Corsi 1994;Shi et al 1995;Kinney 2000, 2001;South Coast Air Quality Management District 1994;Tang et al 1995;Tang and Hwang 1997;Tonga and Frisch 1993;Weber and Hartmans 1995;Woertz et al 2001;Wright 2004;Wright et al 1997. However, more information is needed because the relationship between transient loadings and biofilter response remains poorly understood ͑Deshusses 1995; Deshusses et al 1996;Irvine and Moe 2001;…”

“…The focus of this research was to investigate the response of the most common type of vapor-phase biological treatment, biofilters, to transients resulting from regular flow-direction reversal, and cyclical feed-on/off operation. Regular flow reversal has been shown to increase reaction capacity in biofilter response to transient loadings ͑Kinney 1996; Song and Kinney 2000;Wright 2004;.͒. Cyclical on/off operations result from temporary shut downs of manufacturing and commercial processes for periods typically ranging from seconds to days.…”