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Anaerobic sludge digestion may be divided into two stages: hydrolysis -acidogenesis and methanogenesis. These phases may be separated to improve the efficiency of the digestion. The hydrolysis of sludge was studied in laboratory fermenters with the aim of improving volatile fatty acids production during the first phase. The influence of pH, temperature and agitation speed was studied. A mathematical approach to this problem was used by means of a statistical tool : methodology experimental research (M.E.R.). The best running conditions for hydrolysis were found to be : pH = 6.8, temperature = 50°C and agitation speed = 545 rpm.