Comparative assessment of a decentralized blackwater and organic household waste treatment system using LCA : Campus Ås showcase

This study evaluates the environmental performance of blackwater and organic household waste treatment in Campus Ås Showcase, located at the Norwegian University of Life Sciences, in Ås municipality, Norway. The Campus Ås Showcase is part of the SiEUGreen Project (www.sieugreen.eu) that aims to demonstrate that a combination of known and emerging technologies contributes to a more resilient and environmentally friendly urban development, with near zero emission, low water footprint and adoption of the circular economy concept. In the Campus Ås Showcase, black water and organic household waste are collected from a dormitory, which is equipped with a vacuum toilet and a grinder system for organic waste. The treatment system comprises an anaerobic digestion reactor (AD), with recovery of biogas, and post-treatment methods that aim at plant nutrient recovery. The post-treatments are liquid fertilizer production (LF), struvite precipitation (SP) and microalgae cultivation (PBR). The systems demonstrated high efficiency of treatment and recovery of nutrients. The analysis was carried out using the Life Cycle Assessment tool, described by ISO14040-44. The impact categories selected for analysis were the global warming potential (GWP), based on the 100-year potential, eutrophication potential (EP), water and energy consumption. The analysis was done for the construction and operational phase, and the fertilizer produced in the system was considered as an avoided burden of commercial fertilizer production. The results were compared to the conventional centralized treatment. The results for the construction phase assessed the environmental impacts associated with the production of the materials used in the system under study. Only the main materials for each system were accounted for in this analysis. The production process of the stainless steel is responsible for the main impacts of the construction phase. Two scenarios were evaluated. Scenario 1 comprises the AD, LF and PBR systems, and Scenario 2 includes struvite precipitation to the process. Scenario 1 presented better results in the GWP category than Scenario 2. The EP and the water consumption were similar for both scenarios. Scenario 2 consumed more energy due the addition of the struvite precipitation process. With the results, it is possible to infer that the addition of the struvite precipitation to the treatment process does not enhance the environmental performance of the system. However, it can be an interesting alternative in case the fertilizer should be transported from the production place. The results of Scenario 1 were compared with the conventional treatment. The Nordre Follo Wastewater Treatment Plant was chosen for comparison and the operational data from the treatment plant was obtained. The anaerobic digestion treatment for organic waste is a theoretical set up. The comparative study has limitations as the technological representation of Campus Ås Showcase and the conventional system are not identical. Campus Ås Showcase demonstrated to have a better environmental performance with regards to GWP, EP and water consumption. On the other hand, the centralized system presented better results on energy consumption. In the sensitivity analysis, the impact of the volume of flushing water used in the vacuum toilet on the performance of the system was investigated for Scenario 1. Volumes of 1 l, 0.75 l, 0.5 l and 0.25 were chosen. The results of the sensitivity analysis showed that the amount of biogas recovered can be increased significantly, as a result of a longer hydraulic retention time in the AD reactor. The liquid fertilizer produced had higher concentrations and lower volumes. There was not much difference from the results achieved in Scenario 1 in the categories GWP, EP and energy consumption. The water consumption decreased from 5.13 m^3 in Scenario 1 to 3.49 m^3, in the case of 0.25 l flushing volume. Campus Ås Showcase demonstrated to be an environmentally friendly treatment process with low water consumption and of great potential for plant nutrient recovery and CO2 biofixation. To improve the system further, the biogas recovered can be used to supply heat to the AD reactor and it is also possible to complement the electricity source with solar power and decrease the tap water consumption with lower flush volume. To strengthen and validate the results of this study, it is recommended to conduct a LCA with the full-scale system once it is in full operation