Prof. L. Wolny's field of research is intensification of sewage sludge preparation before mechanical dewatering with ultrasonic field energy application. The trend of investigations is based on low-frequency ultrasounds (22–30 kHz) application to sewage sludge or polyelectrolytes, which are subsequently used in the sludge conditioning process. One of the aims of the investigation is the possibility of reducing the dose of chemical reagents by combining both methods of sludge preparation (physical and chemical). Recently, the impact of ultrasonic hydrolysis on the VFA generation in the acid fermentation of excess sludge has been investigated.