Anaerobic bioreactor suitable for anaerobic conversion from a lipid rich biomass to biogas, which anaerobic reactor is arranged to comprise during the anaerobic conversion from the lipid rich biomass to biogas the following layers: a conversion layer for anaerobic conversion from the lipid rich biomass to biogas comprising micro-organisms; a lipid rich layer having a pH which is lower than the pH of the conversion layer, wherein the lipid rich layer is arranged above the conversion layer and wherein the lipid rich layer is in contact with the conversion layer; a biogas layer arranged above the lipid rich layer; which anaerobic bioreactor comprises: (i) an inlet suitable for the lipid rich biomass; (ii) an outlet on top of the anaerobic reactor suitable for the biogas formed by the anaerobic conversion from the lipid rich layer to biogas; (iii) an inlet for a fluid suitably arranged for forming a conversion layer; (iv) an outlet for a fluid suitably arranged for tapping fluid from the conversion layer; (v) an outlet arranged on the bottom of the anaerobic bioreactor suitable for removing precipitated residue; and (vi) an outlet suitably arranged for removing non fermentable subject matter from the lipid rich layer and/or conversion layer; wherein the reactor further comprises spraying means arranged above the lipid rich layer, which spraying means are configured to spray continuously or discontinuously a fluid comprising micro-organisms taken from the conversion layer onto the top of the lipid rich layer, such that the lipid rich layer substantially remains intact, wherein the inlet for lipid rich biomass is arranged above the lipid rich layer such that during the anaerobic conversion from the lipid rich biomass to biogas the added lipid rich biomass is poured continuously or discontinuously on the lipid rich layer. a conversion layer for anaerobic conversion from the lipid rich biomass to biogas comprising micro-organisms; a lipid rich layer having a pH which is lower than the pH of the conversion layer, wherein the lipid rich layer is arranged above the conversion layer and wherein the lipid rich layer is in contact with the conversion layer; a biogas layer arranged above the lipid rich layer; which anaerobic bioreactor comprises: (i) an inlet suitable for the lipid rich biomass; (ii) an outlet on top of the anaerobic reactor suitable for the biogas formed by the anaerobic conversion from the lipid rich layer to biogas; (iii) an inlet for a fluid suitably arranged for forming a conversion layer; (iv) an outlet for a fluid suitably arranged for tapping fluid from the conversion layer; (v) an outlet arranged on the bottom of the anaerobic bioreactor suitable for removing precipitated residue; and (vi) an outlet suitably arranged for removing non fermentable subject matter from the lipid rich layer and/or conversion layer; wherein the reactor further comprises spraying means arranged above the lipid rich layer, which spraying means are configured to spray continuously or discontinuously a fluid comprising micro-organisms taken from the conversion layer onto the top of the lipid rich layer, such that the lipid rich layer substantially remains intact, wherein the inlet for lipid rich biomass is arranged above the lipid rich layer such that during the anaerobic conversion from the lipid rich biomass to biogas the added lipid rich biomass is poured continuously or discontinuously on the lipid rich layer. (i) an inlet suitable for the lipid rich biomass; (ii) an outlet on top of the anaerobic reactor suitable for the biogas formed by the anaerobic conversion from the lipid rich layer to biogas; (iii) an inlet for a fluid suitably arranged for forming a conversion layer; (iv) an outlet for a fluid suitably arranged for tapping fluid from the conversion layer; (v) an outlet arranged on the bottom of the anaerobic bioreactor suitable for removing precipitated residue; and (vi) an outlet suitably arranged for removing non fermentable subject matter from the lipid rich layer and/or conversion layer; wherein the reactor further comprises spraying means arranged above the lipid rich layer, which spraying means are configured to spray continuously or discontinuously a fluid comprising micro-organisms taken from the conversion layer onto the top of the lipid rich layer, such that the lipid rich layer substantially remains intact, wherein the inlet for lipid rich biomass is arranged above the lipid rich layer such that during the anaerobic conversion from the lipid rich biomass to biogas the added lipid rich biomass is poured continuously or discontinuously on the lipid rich layer.