Laboratory experiments supported by full-scale operational observations have been utilized to assess the possible yield of methane-rich biogas via decomposition of COD and volatile fatty acids (VFAs) from organic matter trapped and settled in a municipal wastewater treatment plant. The impact of operational conditions (mainly solids' retention time) on VFAs and methane-rich gas generation was estimated by fractionation of COD in sludge and supernatant. Assessment of efficiency of this process was done by steady state measurements of COD solubilization. Full-scale investigations supported by laboratory tests enlightened some specific operational problems that may occur in wastewater treatment plants with an intensive acidic hydrolysis. Relatively high consumption of biodegradable carbon is usually one of adverse and unfavorable effects.