A new type of waste gas treatment was previously proposed in the packed column containing the immobilized activated sludge gel beads of 5 mm in diameter together with the hollow plastic balls of 10 mm in the optimal diameter with their optimal volume ratio of 1 to 2 to avoid compaction of the gel beads. The same type of biofilter is employed and analyzed for removal of acetaldehyde or propionaldehyde as a model of toxic and malodorant pollutants under a wide range of the inlet aldehyde concentrations. The same spherical gel beads as prepared previously by the PVA-boric acid method are stable and durable during storage as well as use in the removal operation. The biofilter gives a very low pressure drop to the waste gas stream through the packed bed. The acetaldehyde or propionaldehyde removal in the column decreases with an increase in the inlet aldehyde concentration. The kinetics data on the gel beads suspended in an air-tight batch reactor reveal that the Michaelis-Menten type rate equation is applicable to both respiration and pollutant biodegradation by the gel beads with negligible mass transfer effect. A model for removal of a single pollutant is developed assuming that the Henry's law constant determines the dissolved pollutant concentration in the gel bead, the pollutant is biodegraded according to the above rate equation and the contact efficiency of gel beads with waste gas has the same value of 0.24 as determined previously. The observed removals agree well with those calculated from the design equation developed. As a result the observed decrease in removal of aldehyde with its inlet concentration is due to the fact that the biodegradation rate itself approaches asymptotically the maximum one with the aldehyde concentration.