Evaluating design options for biofilters

Biofilters are now increasingly offered and accepted in North America for air pollution control. However, prospective buyers often lack the technical and economic criteria to select a suitable biofilter design for their applications. This paper summarizes past experiences with full-scale biofilters and emerging design trends. Since control of the filter media's moisture content is essential for effective pollutant removal, the processes affecting moisture content are reviewed and ranges for evaporation rates and dry-out periods are estimated. High VOC concentrations and elimination rates cause high moisture removal and require additional irrigation; excessive irrigation or condensation of moisture from hot off-gases routinely cause over-watering and poor performance. Consequently, an application's moisture balance should be evaluated carefully prior to selecting a biofilter design. High VOC loadings and the need for low maintenance and verifiable pollutant removal favor fully enclosed vessels with automatic moisture control. Open biofilters with a low level of moisture control are suited for dilute off-gases, e.g., in non-critical odor control applications. Typical cost ranges for biofilter installations in North America are presented. Other design selection criteria, including choice of media, space demands, and monitoring requirements are reviewed. Three case examples of full-scale biofilters for VOC control illustrate our findings.