NOVELTY - Production of ethanol from cellulosic substrate, comprises providing within a reaction vessel, a reaction mixture in the form of a slurry comprising cellulosic substrate, an enzyme and a fermentation agent; determining an optimum feeding frequency; and providing additional quantities of cellulosic substrate and enzyme into reaction vessel at an interval(s) according to optimized feeding frequency.    USE - For producing ethanol from a cellulosic substrate e.g. woody biomass, forage grasses, herbaceous energy crops, agricultural residue, waste paper sludge or municipal solid waste (claimed).    ADVANTAGE - The method provides for higher substrate conversion at a given enzyme loading (or alternatively a lower enzyme loading used to achieve the same conversion) in a reactor operated according to a semi-continuous feeding protocol. The method reduces costs by using lower enzyme loading concentrations to achieve a given conversion efficiency relative to a fully continuous process.    DETAILED DESCRIPTION - Production of ethanol from cellulosic substrate, comprises:    (1) providing within a reaction vessel, a reaction mixture in the form of a slurry comprising cellulosic substrate, an enzyme and a fermentation agent, where the reaction mixture is reacted under conditions to initiate and maintain a fermentation reaction;    (2) determining, by experiment or theoretical calculations, an optimum feeding frequency; and    (3) providing additional quantities of the cellulosic substrate and the enzyme into the reaction vessel at an interval(s) according to the optimized feeding frequency.    INDEPENDENT CLAIMS are also included for:    (A) a method of operating a reactor system to provide semi-continuous solids-fed fermentation of paper sludge, comprising advancing a paper sludge under a semi-continuous controlled flow rate; chopping the paper sludge as it advances to subdivide the paper sludge into comparatively smaller particles; digesting the comparatively smaller particles in an agitator (145) by the action of a hydrolyzing enzyme; and controlling the flow rate and amount of hydrolyzing enzyme added to the agitator for use in the step of digesting to establish a semi-continuous state of flow through the reactor system over an extended period of time while also maintaining a predetermined conversion efficiency of the paper sludge by action of the hydrolyzing enzyme at an optimized loading concentration; and    (B) a semi-continuous solids-fed bioreactor (100) comprising a feed tube (110) containing a plug of solid waste (115); a screw driven by a timer-controlled motor, the screw acting to advance the plug of solid waste into a chopper (125); and an agitator that receives solid waste particles from the chopper and enzymes, fermentation agents and liquid reagents from an inlet valve, the solid waste particles, enzymes, fermentation agents and liquid reagents forming a slurry in the agitator, where the agitator includes a mixer driven by a second motor and at least one conduit from which products may be removed.    DESCRIPTION OF DRAWING(S) - The figure is a schematic diagram of a semi-continuous, solids-fed reactor system.    Solids-fed bioreactor (100)    Feed tube (110)    Solid waste (115)    Isolation valve (120)    Chopper (125)    Agitator (145)    Bearing (160)