This study investigates the cumulative energy demand (CED) of different systems for the management of urban wastewater, following the methodology of Life Cycle Assessment. In a hypothetical case study for an urban area (5,000 inhabitants), all relevant processes for wastewater collection and treatment and the construction of infrastructure are described in a substance flow model. The conventional system requires 1,250 MJ/(pe*a), with the operation contributing 45%, the infrastructure 7%, and the system expansion (production of mineral fertilizer and electricity) 48% to the total CED. The separation systems have a CED of 930–1,182 MJ/(pe*a) depending on their configuration. Results of the impact assessment show that recovering energy from the organic matter of toilet wastewater and household biowaste in a digestion process can decrease the cumulative energy demand by 13–26%. Energetic benefits of mineral fertilizer substitution are relatively small compared to the energy recovered from organic matter. Decisive parameters for the energy analysis are the amount of biowaste which is co-digested with toilet wastewater and the energy demand of the vacuum plant.
Keywords: cumulative energy demand, energy analysis, Life Cycle Assessment, source-separation system, sustainable wastewater treatment