Abstract
Biotrickling filtration (BTF) of malodours from Sewage Treatment Works, is investigated in detail, as the most economical and environmentally friendly air treatment option. Experimental data were produced on the removal of H2S and VOCs by a Biotrickling filter (BTF) demonstration plant, namely a SULPHUSTM, which was installed by Thames Water in late 2015. The two widely used models of Ottengraf and van den Over (1983) for BTFs were found to be inadequate with Root Mean Squared Errors (RMSE) of 22 and 4.8 mg m-3 of H2S respectively. These models are based on zero-order kinetics in the biofilm. Neither of these two existing models accounts for the possibility of diffusion limitation emerging at a point within the bed height; therefore, a novel hybrid model was developed for this possibility, which failed to improve the fit provided by the existing zero-order models. This confirms that the zero-order kinetics assumption is main source of error. The Michaelis Menten (M-M) kinetic model, predicts that zero-order is likely to be inaccurate at low pollutant concentrations when the kinetics should asymptote to fist order kinetics. However, first-order kinetics, which is found in the literature on BTFs, also fails to follow the trend of the data with RMSE of 0.36 mg m-3. A novel derivation based on the M-M kinetics is found to fit the data better the rest of the models with RMSE of 0.26 mg m-3. All models were also compared to the total VOC removal for the SULPHUSTM trial, and the M-M equation was also found to provide the best fit. In the SULPHUSTM unit used in the trials, the product of the retention time and the specific surface area was higher than typically practiced. Thus, whilst the inlet concentration reached values too high for the first-order model, the concentration in most of the bed had been reduced low enough to render the zero-order model inaccurate. The zero-order models provide a good fit to some of the laboratory H2S data by others at lower Empty Bed Retention Times (EBRT) and higher inlet concentrations than our case study. The zero order model fitting results of these data sets and the model fitting of the M-M model to our data all produced reaction rate constants of about 0.3 g/m3/s. This advance in the mathematical modelling of bio-tricking filtration has made it possible to demonstrate consistency in a seemingly disparate sets of experimental data within the biological air pollutant removal literature.