Wastewater treatment plants are the largest municipal energy consumers. Although wastewater theoretically contains enough energy to cover the entire treatment process, often only a fraction of the wastewater’s chemical energy is recovered. Energy recovery is generally carried out during the anaerobic treatment of the sewage sludge produced during the purification process. However, typical degrees of degradation are only about 50% to 60% without a sludge (pre)treatment. A relatively large amount of energy therefore remains in the digestate, which is incinerated after dewatering and drying with high energy effort. Hence, an enhanced anaerobic degradability at the same residence time in the digester could not only increase the methane yield but also reduce the amount of sewage sludge to be disposed. In addition to a variety of other methods, sludge treatment with ultrasound is an interesting option to enhance digestibility. Through ultrasound-induced cavitation, sludge flocs are disintegrated, microorganism cells are disrupted and intracellular enzymes are released. This results in an improved microbiological degradation and, consequently, in an increased biogas yield. In addition, it was reported that sonication can improve sludge dewaterability, which would further reduce the amount of sewage sludge. Current commercially available ultrasonic units for wastewater treatment plants are mainly based on systems in which the ultrasound is induced into the medium by rod-shaped sonotrodes. However, practical experience has shown that these sonotrode-based systems are relatively susceptible to interference. As an alternative, a tube reactor with plate transducers can be used, where the ultrasound is transmitted via the tube wall. The central aim of the project is to maximize the methane yield while minimizing the power consumption due to an optimization of process parameters (e.g., acoustic intensity, duration and amplitude) and system configuration (parallel vs. series treatment and raw sludge vs. digested sludge treatment). In addition to tests on a laboratory scale, selected wastewater treatment plants will be equipped with the ultrasound system to obtain reliable data from real plants. At the end of the project, the main results will be summarized in a guideline.
