NOVELTY - An anaerobic process (P1) for producing biogas, involves: injecting feed material into a reactor having anaerobic microorganisms to form volume of bulk liquid in the reactor; monitoring oxidation-reduction potential, pH and temperature of bulk liquid to determine whether oxidation-reduction potential, pH and temperature are each within a predetermined range; and adjusting amount of feed material fed to reactor in response to a determination, to adjust oxidation-reduction potential, pH or temperature of bulk liquid within the predetermined range; and producing biogas having methane.    USE - For producing biogas having methane (claimed).    ADVANTAGE - The process or apparatus is improved process or apparatus for enhanced and optimized anaerobic mesophilic and thermophilic biological digestion of wastes or energy feedstocks (e.g. biomass). The biomass solids returned to the reactor provides the following significant advantages to the digester operation/performance: biomass inventory management and control; food to microorganism ratio (F/M) process control through biomass recycle; improved digestion process stability and performance enhancement; alkalinity recycle and pH control; enhanced and optimized biomethane production; and increased nutrient (nitrogen and phosphorus) value of the residual biomass solids.    DETAILED DESCRIPTION - An anaerobic process (P1) for producing biogas, involves: injecting feed material into a reactor having anaerobic microorganisms to form a volume of bulk liquid in the reactor; monitoring the oxidation-reduction potential, the pH, and temperature of the reactor bulk liquid to determine whether oxidation-reduction potential, pH, and temperature are each within a predetermined range; and adjusting the amount of feed material fed to the reactor in response to a determination that one of the oxidation-reduction potential, pH, and temperature of the reactor bulk liquid are outside the corresponding predetermined ranges to adjust the oxidation-reduction potential, pH, or temperature of the reactor bulk liquid to within the predetermined range; and producing biogas having methane. INDEPENDENT CLAIMS are included for the following:    (1) an anaerobic process (P2) for producing biogas, involving: injecting an influent mixture into a reactor having anaerobic microorganisms to form a volume of bulk liquid in the reactor, where the influent mixture comprises a feed material and recycled effluent; monitoring oxidation-reduction potential, pH, and temperature of the reactor bulk liquid; adjusting at least a selected one of the rate of injection of the recycled effluent and the amount of the feed material fed to the reactor in response to the oxidation-reduction potential, pH, and temperature of the reactor bulk liquid to maintain the temperature of the reactor bulk liquid within a predetermined range; and producing biogas having methane; and    (2) an apparatus for producing biogas, comprising: a reactor containing anaerobic microorganisms and bulk liquid; a feed conduit operably coupled to the reactor which facilitates introduction of feed material into the reactor; a biogas conduit which facilitates removal of biogas from the reactor; an effluent conduit which facilitates removal of treated effluent from the reactor; a temperature sensor for measuring the temperature of the bulk liquid; a pH sensor for measuring the pH of the bulk liquid; and an oxidation-reduction potential sensor for measuring the oxidation-reduction potential of the bulk liquid.    DESCRIPTION OF DRAWING(S) - The figure shows a flowchart for the anaerobic control process for producing biogas.