Studies on the microbiology of anaerobic sludge disclosed that nonmethanogenic strictly anaerobic bacteria comprised one of the most numerous groups of organisms present. Euryoxic bacteria constitute only a minor fraction of the normal bacterial flora but can become predominant in very short time if fermentable carbohydrates are present. This finding emphasizes the necessity of using unenriched fermentations for studying the sludge digestion process. Lipids are the main substrates responsible for generation of acetate in the unaltered normal sludge fermentation. Organisms responsible for lipid decomposition are apparently slow-growing and are washed out of the digestor at short retention times. Recycling of the bacteria would enable the operator to shorten retention time without a loss in efficiency of decomposition. (Author)