NOVELTY - Anaerobic bioreactor comprises conversion layer (2) for anaerobic conversion from lipid rich biomass (5) to biogas comprising micro-organisms, a lipid rich layer (3) having pH which is lower than pH of conversion layer, a biogas layer (4) arranged above lipid rich layer, where reactor further comprises spraying unit (7) arranged above lipid rich layer, which spraying unit spray continuously or discontinuously fluid comprising micro-organisms taken from conversion layer onto top of lipid rich layer, where lipid rich layer remains intact.    USE - As anaerobic bioreactor for anaerobic conversion from a lipid rich biomass to biogas (claimed), where the lipid rich biomass is oils, or fats.    ADVANTAGE - The anaerobic bioreactor removes the non-fermentable subject matter, or removes a part or the entire lipid rich layer, is that the anaerobic bioreactor can be cleaned efficiently.    DETAILED DESCRIPTION - Anaerobic bioreactor for anaerobic conversion from a lipid rich biomass (5) to biogas, comprises during the anaerobic conversion from the lipid rich biomass to biogas the following layers: a conversion layer (2) for anaerobic conversion from the lipid rich biomass to biogas comprising micro-organisms; a lipid rich layer (3) having a pH which is lower than the pH of the conversion layer, where the lipid rich layer is arranged above the conversion layer where the lipid rich layer is in contact with the conversion layer, a biogas layer (4) arranged above the lipid rich layer, which anaerobic bioreactor further comprises an inlet (10) for the lipid rich biomass; an outlet (6) on top of the anaerobic reactor for the biogas formed by the anaerobic conversion from the lipid rich layer to biogas, where the reactor further comprises spraying unit (7) arranged above the lipid rich layer, which spraying unit spray continuously or discontinuously a fluid comprising micro-organisms taken from the conversion layer onto the top of the lipid rich layer, where the lipid rich layer remains intact. An INDEPENDENT CLAIM is included for method for anaerobic conversion of lipid rich biomass to biogas in an anaerobic bioreactor involving adding the lipid rich biomass to the anaerobic bioreactor thus forming a lipid rich layer situated above and in contact with a conversion layer for the conversion of the lipid rich biomass to biogas which conversion layer comprises micro-organisms, tapping a fluid comprising micro-organisms from the conversion layer; spraying the fluid tapped from the conversion layer on top of the lipid rich layer where the lipid rich layer remains intact, and recovery of the produced biogas, where the lipid rich layer has a pH which is lower than the pH of the conversion layer.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of anaerobic bioreactor.    Conversion layer (2)    Lipid rich layer (3)    Biogas layer (4)    Lipid rich biomass (5)    Outlet (6)    Spraying unit (7)    Conduit (9)    Inlet (10)