Biofiltration is an effective approach for the remediation of carbon-based air pollutants in which the degradation is achieved through the bioactivity of certain microorganisms present in the bioreactor. The growth of the microorganisms within the biofilter not only depends on the carbon source, but also on the other nutrients. Nitrogen, for example, is considered to be essential for microbial growth and survival. Most biofilter models do not consider nitrogen limitation, and also assume a constant microbial concentration within the biofilter, which is not always true. Hence, a growth model was developed in this study which can describe the relationship between microbial growth and nitrogen utilization within a toluene biofilter. From the simulation results, it was estimated that at 1 g m-3 inlet toluene concentration and an empty bed residence time of 72 s, 87.6% removal efficiency was achieved on day 100 without any additional nitrogen supplement when the initial nitrogen content was 1% (w/w). A model describing toluene mass transfer by diffusion in the biofilm was also developed. The simulation results suggest that the effect of diffusion limitation on toluene biodegradation is significant in a thick biofilm (i.e., 100 μm) as opposed to a thin biofilm (i.e., 1 μm).
An experimental evaluation of toluene biofiltration was carried out using compost. Preliminary experiments led to a low and unstable activity. It was hypothesized that isolation of the toluene degrading strain and inoculation on sterile compost will lead to higher and more stable activity. Bench-scale experiments were conducted to isolate and characterize toluene degrading microorganisms from compost. DNA analysis and sequencing data suggested that the strain was Nocardia sp. The Nocardia strain was inoculated on a sterile biofilter. The biofilter was operated for 50 days with 0.4, 1.1 and 1.4 g m-3 inlet concentrations resulting in 96.1%, 82% and 69.8% average removal efficiencies respectively, confirming the hypothesis. This is the first report that shows Nocardia sp. as the sole toluene degrader in a compost biofilter. The experimental data were compared with the growth model and the results suggest that the growth model can successfully predict the degradation efficiency of the compost biofilter.