Energy Properties of Sewage Sludge in Biogas Production—Technical and Economic Aspects

The biogas sector is undergoing development as a result of the growing demand for renewable energy. Methane fermentation allows for the acquisition of energy that is universally usable, while also facilitating the neutralization of problematic waste. Sewage sludge generated as a result of a number of technological processes occurring during wastewater treatment requires appropriate management, and its volume increases every year. In this work, the task was to determine the suitability of sewage sludge for co-digestion with agricultural biomass. The research allowed for the determination of the positive impact of using sewage sludge for fermentation with agricultural biomass. The amount of biogas produced and the methane content were higher compared to the single-component fermentation of agricultural biomass. Mixed sludge had a particularly beneficial effect on fermentation. The largest amount of biogas was obtained from maize silage input and mixed sludge, i.e., 309 Ndm3·k−1 d.m. The methane content in this mixture reached a maximum level of 63%. The least productive was mixture no. 4, consisting exclusively of apple pomace. It produced the smallest amount of biogas (96 Ndm3/kg d.m.) and the process occurred with the greatest delay. The rate of the process was similar for mixtures 3 and 4 for an extended period. In the case of mixture no. 2, there was initially a slightly higher inhibition of the process, but by day 17, it had reached the biogas yield level of mixture 3. The amount of biogas produced for mixtures 2 and 3 was 119 and 133 Ndm3/kg d.m., respectively. From day 22 onwards, the process for all mixtures was coming to an end, with no significant biogas yields observed until the end of the study period. Such a high methane content increases the energy value of biogas, which in practice means a higher yield of electricity and heat from the same amount of feedstock, and thus lower unit costs of energy production. Co-digestion of maize silage, apple pomace, and beet pulp with sewage sludge can be a successful practice in biogas plants.