Bio filtration is where vapour-phase organic contaminants are passed through a bed of material and adsorb to the substrate surface, where they are degraded by micro-organisms. The bed material may be soil, bark, compost or any mixture of these components.

Bioreactors are a development of the bio filter and operate in a similar way, but use an inert support medium such as plastic rings, scoria or pumice. The micro-organisms are cultured as a bio film on the surface of the support medium, which is supported by recirculating water.