NOVELTY - Operating anaerobic digestion system (10) involves feeding, under gravity, fresh organic material feedstock to an anaerobic digestion stage (14) along organic material feedstock feed line; subjecting the feedstock to anaerobic digestion in anaerobic digestion stage; withdrawing biogas along a digestion stage biogas transfer line from the digestion stage; passing withdrawn biogas to and collecting withdrawn biogas in a biogas collection stage (18); withdrawing, under gravity, digestate generated in anaerobic digestion stage from anaerobic digestion stage.    USE - As operating an anaerobic digestion system (claimed), used for treating organic waste materials, such as sewage, anaerobic digestion treatment processes, for biogas production.    ADVANTAGE - The anaerobic digester system and the control of the operating conditions allows the residence time to be minimized, due to automated control of operating conditions and separate digestate handling, thus allowing for the construction of a smaller anaerobic digestion stage to deal, in semi-batch or even in continuous fashion, with an equivalent volume of organic waste feedstock which is treated by standard non-automated anaerobic digester in simple batch fashion. Anaerobic digestion stage residence time is, importantly, further reduced by provision of the digestate phase separation stage, which allows for discrete digestate product recovery on a semi-batch or quasi-continuous, or even fully continuous basis separately from the anaerobic digestion stage. The manner in which the system is arranged thus provides for a dedicated anaerobic digestion stage and a dedicated digestate product recovery stage. It is also an advantage of the system as described that anaerobic digestion stage residence time need not be extended to allow for virtually complete digestion to occur, considering that digestion of undigested organic material can continue in the digestate phase separation stage, also under anaerobic conditions.    DETAILED DESCRIPTION - Operating an anaerobic digestion system (10) involves feeding, under gravity, fresh organic material feedstock to an anaerobic digestion stage (14) along an organic material feedstock feed line; subjecting the feedstock to anaerobic digestion in the anaerobic digestion stage; withdrawing biogas along a digestion stage biogas transfer line from the digestion stage; passing withdrawn biogas to and collecting withdrawn biogas in a biogas collection stage (18); withdrawing, under gravity, digestate generated in the anaerobic digestion stage from the anaerobic digestion stage; transferring the withdrawn digestate to a digestate phase separation stage (16) along a digestate transfer line (24); and allowing the digestate to settle and separate into its phases in the digestate phase separation stage. An INDEPENDENT CLAIM is included for an anaerobic digester system, comprising fresh organic material feedstock feeder (12); an anaerobic digestion stage which is located operatively lower than the organic material feedstock feeder; an organic material feedstock feed line (20) which leads from the fresh organic material feedstock feeder to the anaerobic digestion stage and along which fresh organic material feedstock can be passed, under gravity, from the fresh organic material feedstock feeder to the anaerobic digestion stage and which is provided with a fresh organic material feedstock feed regulator; a digestate phase separation stage, which is located operatively lower than the anaerobic digestion stage; a digestate transfer line leading from the anaerobic digestion stage to the phase separation stage, along which digestate transfer line anaerobic digestate generated in the anaerobic digestion stage can be passed, under gravity, from the anaerobic digestion stage to the digestate phase separation stage, where the digestate transfer line being provided with a digestate withdrawal feed regulator; a biogas collection stage; and an anaerobic digestion stage biogas transfer line (22) which leads into the biogas collection stage from the anaerobic digestion stage and along which biogas generated in the anaerobic digestion stage can be passed from the anaerobic digestion stage to the gas collection stage.    DESCRIPTION OF DRAWING(S) - The figure shows a side view of Operating an anaerobic digestion system.    Anaerobic digestion system (10)    Feedstock feeder (12)    Anaerobic digestion stage (14)    Digestate phase separation stage (16)    Biogas collection stage (18)    Organic material feedstock feed line (20)    Anaerobic digestion stage biogas transfer line (22)    Digestate transfer line (24)