In this study, a bench-scale fermenter was operated at room temperature to investigate COD solubilization and fermentation of primary sludge at Atakoy treatment plant in Istanbul, Turkey. The objective was to evaluate and compare the potential of short-chain volatile fatty acid (VFA) generation by simple uncontrolled fermentation of primary sludge under mixed and unmixed conditions. The results clearly show that the predominant fermentation products are always acetic and propionic acids, independent from with/without mixing. The COD equivalent of VFA for mixed and unmixed conditions was obtained to be 1.25 and 1.28 g COD/g VFA, respectively. Under mixing conditions more than 40% improvement in VFA yields was achieved.