Co-Digestion of Sewage Sludge and Cattle Slaughterhouse Waste for Wastewater Treatment Plant Energy Self-Sufficiency—A Case Study

The need to increase the energy self-sufficiency of wastewater treatment plants (WWTPs), and ultimately to achieve energy neutrality of these facilities, encourages the search for solutions that intensify biogas production. Co-digestion of sewage sludge (SS) with high-energy potential organic waste is a solution that enables an increase in biogas yield in WWTPs. Such an approach allows not only for more efficient use of existing infrastructure but also for increased energy recovery from waste. This article evaluates the use of cattle slaughterhouse waste (CSW) as a co-substrate in the co-digestion process with sewage sludge. The analysis covered biogas production efficiency, its quality, and the significance of this approach for the operation of WWTPs. The study was conducted in laboratory reactors operating under mesophilic conditions (36–37 °C), with different percentage shares of CSW (2, 8, 12, and 25% of TS). The obtained biogas yield increased from 0.487 ± 0.024 m3/kg VS for SS to 0.527 ± 0.026 m3/kg VS at 25% CSW, while for CSW alone this parameter reached 0.625 ± 0.031 m3/kg VS. Methane yield increased from 0.322 ± 0.016 Nm3 CH4/kg VS (SS) to 0.344 ± 0.017 Nm3 CH4/kg VS (25% CSW), while for CSW it reached 0.452 ± 0.023 Nm3 CH4/kg VS. In the simulation of the co-digestion process with this waste, an increase in energy production from 3.1% to 51.2% was achieved depending on the percentage share of CSW. The conducted research clearly demonstrated that the addition of CSW improves the energy balance of WWTP, which represents a significant technological benefit.