Assessment of Energy Self-Sufficiency of Wastewater Treatment Plants—A Case Study from Poland

Currently, one of the main goals is to make municipal wastewater treatment plants (WWTPs) energy-neutral. However, advanced wastewater treatments and sewage sludge processing are still classified as highly energy-intensive. In this study, the energy self-sufficiency potential assessment of the WWTP located in Krosno (Poland) was evaluated. Moreover, the possible paths for improving the energy balance of the analyzed facility are presented in this paper. The performed evaluation indicated that in 2016–2019, the energy consumption at WWTP Krosno varied from 0.25 to 0.71 kWh/m3 of wastewater (average 0.51 kWh/m3), and the highest energy utilization values in each year were recorded around the summer season. An analysis of the data showed that as the pollutant load flowing into the WWTP increased, its energy utilization decreased. Such results indicate that the treatment cost per cubic meter decreases as the load increases due to the capital cost being the same. The estimated self-sufficiency of the facility in the years analyzed was 50.5%. The average energy recovery from 1 m3 of wastewater was 0.27 kWh/m3, and the average energy recovery from 1 m3 of biogas was 1.54 kWh/m3. Since the energy balance of this wastewater treatment plant, determined primarily by the continuously increasing cost of energy purchases, has to be improved, two courses of action were identified that will allow for increasing self-sufficiency. The co-digestion strategy was indicated as the easiest solution to implement, given the on-going anaerobic stabilization of sewage sludge at this WWTP. Moreover, the possible co-substrates that can be obtained from local suppliers were indicated. The second course of action, which requires a thorough economic analysis, is sludge pre-treatment, which might improve sewage sludge properties, resulting in a more favorable biogas yield.