Method for removal of malodorous and/or toxic components from waste gases by biological treatment of the same. Such removal is accomplished by passing the gas stream through a humidifying temperature conditioning section and then through a filter bed. The filter bed consists of a mixture of an organic biological active material, like compost, an inert fraction and additives for neutralization and organic buffening. Also disclosed is the inoculation of the filter bed with specific micro-organisms adapted to remove constituents of the gas stream which are difficultly biodegradable, e.g. methylenechloride.