Although many experimental works on biofiltration, both at laboratory and full-scale size, have been published during the last two decades, a critical comparison of results is difficult due to various reasons, among which are the use of German and Dutch as the most common languages in this research field, the use of different criteria to express the performances, the utilization of different volatile organic and inorganic compounds, the different types of biofilters used, and so on. Nevertheless, the work of Ottengraf (20) can be taken as a useful reference basis to provide a complete overview on biofilter performance, as well as on the various effects influencing the process macrokinetics.
For most organic pollutants, the steady-state continuous elimination follows zero-order kinetics, so that pollutant concentration decreases with the column height. In the presence of mixtures of pollutants characterized by different biodegradability, a nearly complete removal of the most easily degradable compounds takes place at the bottom of the filter, whereas the most recalcitrant are mainly removed at the top. This suggests that different microbial populations could be responsible in the filter bed for the degradation of different organic components.
The path of Figure 4 explains another typical behavior of biofilter performances besides the one illustrated earlier. At low organic loads (corresponding to low concentrations for a given residence time), the elimination capacity linearly increases with this parameter, which means that nearly complete pollutant conversion is achieved. Over a critical load, the elimination capacity reaches a maximum value (corresponding to the apparent zero-order kinetic rate constant, Ko) and stays nearly constant. Maximum thresholds of 21, 27, 32, and 32 g m—3 h—1, calculated for a mixture of toluene, ethyl acetate, butyl acetate, and bu-tanol in a five-stage system simulating a biofiltration column with variable height (20), seem to reflect a direct dependence of this parameter on the pollutant degradability.
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