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A steady state model for anaerobic digestion of sewage sludge is developed that comprises three sequential parts - a kinetic part from which the % COD removal and methane production are determined for a given retention time; a stoichiometry part from which the gas composition (or partial pressure of CO₂), ammonia released and alkalinity generated are calculated from the %COD removal; and a carbonate system weak acid / base chemistry part from which the digester pH is calculated from the partial pressure of CO₂ and alkalinity generated. From the stoichiometry and weak acid base chemistry parts of the model, for a given % COD removal, the digester gas composition, ammonia released, alkalinity generated and digester pH are completely defined by the influent sludge composition, i.e. X, Y, Z and A in C_XH_YO_ZN_A of the hydrolysable organics; volatile fatty acid (VFA) concentration; and …