Nonlinear finite element analysis of coupled heat and mass transport applied to the drying of a compost biofilter

Biofilters are unsaturated, packed bed reactors used to remove metabolizable compounds from air. As waste gases are blown through the packing, contaminants partition into the liquid phase and are degraded by attached microorganisms. Successful long term operation of a biofilter system depends upon maintaining a suitable environment for microbial growth. Evaporation, induced by heating from microbial respiration, and water seepage, deplete the system moisture content, resulting in biofilm deactivation. As nutrients are consumed for cell growth, the biofilm shifts from a high activity growth state, to a low activity maintenance state, resulting in reduced contaminant elimination capacity. To date, there has been no modeling effort to capture the internal dynamics of gas and liquid flow, nutrient cycling, and biofilm activity.; In an effort to model the highly interconnected processes effecting long term biofilter performance, a mathematical study of biofilter heat and mass transport was conducted. The method of volume averaging was used to spatially smooth the 3-phase (solid, liquid, and gas) conservation equations over the biofilter domain. A finite element model of the resulting nonlinear, coupled heat and mass transport equations was developed, tracking eight filed variables. A multiphase flow model was adapted from previous studies of unsaturated groundwater flow. The functional description, and maximal values of the nonlinear rates used within the model were perturbed as part of a sensitivity analysis. The code was written entirely in Visual C++ maintaining an object oriented (OO) paradigm.; Typical biofilter operating schemes were investigated to illustrate the usefulness of the model as a design tool. Evaporation, induced by biological heating, was found to be the major removal mechanism for water. Because the region of maximum evaporation corresponded to the region of maximum biological degradation, water flow through the packing, required to replenish moisture content and nutrient supply, greatly affected the biofilter performance. Model results support the investigation of the packing water flow parameters as a means of increasing the predictability of long term biofilter performance. Given the flow characteristics of a medium, it would be possible to optimize the rewetting and fertilization schedules to insure long term high performance biofiltration.