Effects of Biomass Growth on Gas Pressure Drop in Biofilters

The effects of biomass accumulation and distribution on air pressure losses in biofilters were experimentally studied. Two bench-scale biofilters, one packed with inert porous pellets (Nova Inert) and the other with wood chips, were operated under similar conditions with excess nutrients to treat an airstream containing methanol, at loading rates of 100-150 g methanol/m3 bed/h. Localized biomass accumulation in the biofilter beds was the key factor increasing the pressure drop, which was caused by local bed clogging due to biomass growth. The highest pressure drops in the beds (wood chips: 2,600 Pa/m; Nova Inert: 550 Pa/m) occurred in sections where there were high biomass levels with high water content. The pressure drop varied nonlinearly with the amount of accumulated biomass and the amount of methanol consumed. Sixfold higher pressure drops were measured in the wood chip biofilter than in the Nova Inert biofilter because of more biomass growth and bed compaction. A model, based on the Ergun equation, was developed to predict biomass-affected porosity and pressure drop as a function of the biomass concentration in a bed packed with spherical pellets. A comparison of the experimental and the predicted pressure drops showed that the model provided good estimates of biomass-affected porosity and pressure drop in the biofilter packed with spherical porous pellets with even biomass distribution.