Life cycle assessment of conventional and source separation systems for urban wastewater management

Abstract

Source-separation systems for urban wastewater management collect the different wastewater flows separately to facilitate the recovery of valuable resources from wastewater (energy, nutrients). Thus, they are claimed to be more sustainable than the conventional concept of combined drainage and treatment. This hypothesis is verified in this study by comparing the environmental impacts of conventional and sourceseparation systems with the methodology of Life Cycle Assessment (ISO 14040/44). In a hypothetical case study for an urban area (5000 inhabitants), twelve different scenarios for the integrated management of household wastewater and biowaste are set up in a substance flow model. Required inventory data for all relevant core processes of wastewater collection and treatment is compiled from pilot projects and literature and is complemented by qualified assumptions. Secondary functions of separation systems (supply of energy and nutrients) are considered by expanding the conventional system with the respective production processes for grid energy and mineral fertilizer. Resource demand and emissions are aggregated for each scenario and evaluated in Life Cycle Impact Assessment with a set of eight indicators for energy and resource demand, global warming, eutrophication, acidification, and human and ecotoxicity. Results of the impact assessment show that separation systems offer significant potentials for an increase in sustainability. Recovering energy from the organic matter of toilet wastewater and especially biowaste in a digestion process can decrease the cumulative energy demand by up to 40% and related emissions of greenhouse gases by up to 46%. Energetic benefits of mineral fertilizer substitution are relatively low, but the quality of organic fertilizers from urine and faeces is superior to mineral fertilizer or sewage sludge in terms of lower heavy metal content. The remaining greywater can be treated in an activated sludge process with less energy demand and better effluent quality than in the conventional system. Natural treatment in soil filters can further reduce the energy demand considerably, but the insufficient retention of phosphorus in soil filters can seriously increase the eutrophication potential by up to 140%. Greywater can also be adequately treated for non-potable reuse with membrane bioreactors, although the energetic benefits of wastewater reuse are marginal. During the application of liquid organic fertilizers from urine and faeces, increased emissions of ammonia lead to a higher potential for acidification (+ 60-110%) and should be minimized by adequate application techniques. Overall, grouping and weighting of the indicators reveal significant benefits in ecological sustainability for separation systems. However, the choice of an appropriate combination of process technology for separation systems is essential for a realization of these potential benefits, because the conventional system has already been optimized in terms of energy demand and effluent quality. In sensitivity analysis, decisive key parameters of the inventory are identified. Functional definitions and the choice of both indicators for impact assessment and valuation methods can have a considerable impact on the results of this LCA.