NOVELTY - The process comprises introducing a feedstock into a pretreatment tank to form a bulk liquid, performing pretreatment of the feedstock in the pretreatment tank, monitoring oxidation-reduction potential, pH and temperature of the bulk liquid during pretreatment to determine the oxidation-reduction potential, pH and temperature within a predetermined range, adjusting the volume of the feedstock in the pretreatment tank in response to a determination that the oxidation reduction potential, pH and/or temperature of the bulk liquid is outside the corresponding predetermined ranges.    USE - The process is useful for a pretreatment of feedstock e.g. biomass such as sugarcane, sugarcane extract, sugar beets, corn kernels, corn starch, paper, paper products, paper waste, wood, particle board, sawdust, agricultural waste, sewage, silage, grasses, rice hulls, bagasse, cotton, jute, hemp, flax, bamboo, sisal, abaca, straw, corn cobs, corn stover, switchgrass, alfalfa, hay, rice hulls, coconut hair, cotton, syntheticcelluloses, seaweed and algae for processing the waste materials and energy feedstocks to produce biogas.    ADVANTAGE - The process is capable of effectively pretreating the feedstock with reduced particle size thus enhancing the biogas (biomethane) production in an economical manner.    DETAILED DESCRIPTION - The process comprises introducing a feedstock into a pretreatment tank to form a bulk liquid, performing pretreatment of the feedstock in the pretreatment tank, monitoring oxidation-reduction potential, pH and temperature of the bulk liquid during pretreatment to determine the oxidation-reduction potential, pH and temperature within a predetermined range, adjusting the volume of the feedstock in the pretreatment tank in response to a determination that the oxidation reduction potential, pH and/or temperature of the bulk liquid is outside the corresponding predetermined ranges to maintain the oxidation-reduction potential, pH, and temperature of the bulk liquid within the predetermined ranges, blending the feedstock or combinations of various feedstocks to form a blended feed, maintaining the oxidation-reduction potential of the treated material -150 to -300 mV, maintaining the pH of the treated material of 4-9.5, monitoring parameters consisting of volatile fatty acids (500-5,000), chemical oxygen demand, total Kjeldahl nitrogen, total phosphorus, total solids, total volatile solids, total suspended solids and volatile suspended solids, performing anaerobic digestion on the treated material, producing a biogas having methane, and monitoring the carbon dioxide content of the biogas (15-40%). The pretreatment of the feedstock or blended feed is preacidified. The process is performed mesophilic at the temperatures 80-110 degrees F and thermophilic at temperatures 120-160 degrees F. The pH in anaerobic digestion is 6.5-8. The total alkalinity is 1000-10000. The bicarbonate alkalinity is 500-8000. An INDEPENDENT CLAIM is included for an apparatus for pretreatment of a feedstock.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a vessel for pretreating the feedstock.    Vessel (12)    Top of the vessel (16)    Motor (29)    Vent (35)    Liquid line level. (40)