In order to eliminate organic pollutants in waste gases, a biological filter bed technique has been developed, with a high self‐regenerating capacity and a low pressure drop. The bed consists of an appropriate filling material (mainly peat compost), in order to let the microorganisms grow on the solid surface and to supply them with inorganic nutrients. Most organic compounds are oxidized to carbon dioxide and water. The compositions of the solid phase and the viable organisms present are such that aging is prevented, as a result of which a relatively high activity can be maintained during a long period of time (years). Experiments have been carried out in laboratory‐scale columns with composite gas mixtures at varied concentrations and superficial gas velocities. The (macro) kinetics of the elimination processes have been studied, which enables the prediction of the elimination capacity of the filter bed.

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Microelectrodes for ammonium, oxygen, nitrate, and pH were used to study nitrifying aggregates grown in a fluidized-bed reactor. Local reactant fluxes and distribution of microbial activity could be determined from the microprofiles. The interfacial fluxes of the reactants closely reflected the stoichiometry of bacterial nitrification. Both ammonium consumption and nitrate production were localized in the outer shells, with a thickness of approximately 100 to 120 ,m, of the aggregates. Under conditions in which ammonium and oxygen

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