Abstract: Microorganisms which may be in the form of homogenated anaerobic sludge are immobilized in a polymeric gel matrix containing chitosan and lignosulphonate. A mixture of chitosan solution and a microorganism is added dropwise to a solution of lignosulphonate to crosslink chitosan droplets and form beads of chitosan/lignosulphonate matrix membrane encapsulating the microorganism. Alternatively, the mixture is added to a water-insoluble organic phase to form chitosan beads in an emulsion, and to the emulsion is added a lignosulphonate solution to crosslink the beads and form a chitosan/lignosulphonate matrix membrane encapsulating the microorganism. In another embodiment, anaerobic bacteria in the form of granules are encapsulated in the chitosan/lignosulphonate matrix membrane and the membrane contains another bacterial specie(s).