Life Cycle Assessment of Microalgal Biomass Valorization from a Wastewater Treatment Process

Purpose: The life cycle assessment (LCA) performed in this work evaluates the potential environmental impacts of an activated-sludge-based wastewater treatment plant (WWTP) coupled with microalgal cultivation, including the algal biomass recovery. The system is compared with the WWTP without algal integration to evaluate the potential benefits derived by the coupling. In addition to more conventional valorization strategies for the algal biomass, a special focus is given to the production of biostimulants, that give the chance to replace some chemical fertilizers. Method: Four scenarios are compared. They differ in the algal biomass valorization route: direct use in agriculture (S1), incineration with energy recovery (S2), use in cement plant as an auxiliary fuel (S3), and biostimulants production and use on crops (S4). The environmental impacts of the system are assessed including 16 categories, to comprehensively cover its potential impacts. Results: S4 allows for significant impact reductions (compared to the WWTP without algal integration) only when the increase of the nutrient-uptake efficiency of crops is taken into account. Assuming a 5% reduction of fertilizers application, S4 shows an improvement in 14 out of the 16 impact categories. Conversely, when the amount of substituted fertilizers is calculated comparing just the macronutrient content of biostimulants with that of algae, S4 is comparable with S1, S2, and S3, where just four to five impact categories show lower impacts than the WWTP without algal integration. Conclusions: The LCA confirms the environmental benefits of biostimulants application on crops, although the modeling approach requires further research as it strongly influences the results.