NOVELTY - Anaerobic digester system comprises a feedstock slurry feeder, conduits connecting components of the system, a pressurizable anaerobic digester, pressurizer, and gas processor(s) (in)directly connected to the anaerobic digester (1). The headspace of the anaerobic digester is pressurized to at least 10 psi to form enriched effluent and useful gas(es).    USE - Used for converting cellulose-containing feedstock into useful materials, i.e. for bioconverting waste material (in a fermentative process) into at least methane to operate gas-powered machine(s). It is installed in a swine ranch, chicken farm, cattle ranch, feedlot, or dairy cattle milking facility (claimed).    ADVANTAGE - The system recovers energy, reduces pollution potential, and adds value to organic waste. It uses existing handling and storage equipment at the farm and requires minimal supervision and skill by the operator. It is not affected by high concentrations of volatile acids and ammonia or nitrogen. Its productivity in terms of methane gas production and quality is high. It requires only a single reaction vessel. It results to a process that is low cost and does not interfere with regular farm operations.    DETAILED DESCRIPTION - Anaerobic digester system includes a feedstock slurry feeder, conduits connecting components of the system, pressurizable anaerobic digester, pressurizer, and gas processor(s) (in)directly connected to the anaerobic digester. The anaerobic digester includes an agitator, feed port(s), discharge port(s) and a reaction vessel for holding reaction solution. The reaction solution comprises anaerobic microbes which convert aqueous slurry of cellulose containing feedstock into useful gas(es) and an enriched effluent. The headspace of the anaerobic digester is pressurized to at least 10 psi to form the enriched effluent and the useful gas(es).    INDEPENDENT CLAIMS are also included for the following:    (A) a useful material prepared from the anaerobic system, comprising methane, ammonia, hydrogen, hydrogen sulfide, carbon dioxide, nitrogen rich fertilizer, feed additive, insecticidal mixture, and/or organic acid; and    (B) a method of converting an aqueous slurry of cellulose-containing feedstock into useful gas(es) and enriched effluent using the anaerobic digester system.    DESCRIPTION OF DRAWING(S) - The figure shows a process flow diagram of the anaerobic digester system.    Anaerobic digester (1)