Organic carbon is the main source of energy and matter necessary for the development of micro-organisms. Removing the suspension from the wastewater in the primary settling tank causes a reduction in the concentration of organic compounds and their hydrolysis products including volatile fatty acids (VFA). As a result, the ratio of organic matter to nitrogen and phosphorus in wastewater after sedimentation is lower than in raw wastewater. Volatile fatty acids, especially acetic acid, are the most desirable and assimilable form of organic carbon. In biological wastewater treatment processes, the content of easily biodegradable organic carbon significantly influences the rate and efficiency of denitrification and dephosphorylation. There are a many of solutions that increase the content of organic matter, including VFA, in wastewater. The basic solutions include sedimentation time reduction, and in particular the use of the concepts of active primary settling tank with acid fermentation process. Other technological solutions are based on the use of acid fermentation in fermentors or special thickeners. The rate of acid fermentation process, leading to the formation of VFA is dependent on many factors. The basic limiting factors include content of biodegradable organic substance and temperature. The aim of the study was to evaluate the possibility of using sonication to intensify the production of volatile fatty acids from primary sludge in acid fermentation. The aim of the study was also to determine the combined effect of sonication time (sonication energy) and temperature of acid fermentation on the yield of volatile fatty acids. During the study, VFA generation was inhibited by the increase in sonication energy. The most effective acid fermentation, due to the production of LKT, was carried out under conditions of psychrophilic temperature i.e. 20°C.