Life cycle assessment of sewage sludge valorisation for bio-based crotonic acid and sustainable waste management

The case study presented in this work aligns to the context of sustainable development by proposing an alternative route for the production of crotonic acid, as an example of drop-in chemical currently obtained from fossil sources. The biological alternative route investigated in this work applies to sewage sludge inflows from the agrifood sector a novel process that combines hydrothermal carbonization, anaerobic fermentation, biochemical fermentation and a thermolytic distillation stage to the production of crotonic acid. The process is funded through the EU funding scheme LIFE and it is named “CROtonic acid from Sewage Sludge (CROSS-LIFE)”. The main implications of CROSS-LIFE include i) valorization of wastewater treatment sludge and reduction of its volume to dispose of, and ii) replacement of fossil-based products with bio-based alternatives. To fully characterize the environmental implications of CROSS-LIFE, life cycle assessment (LCA) was applied with a cradle to grave approach. The LCA model has enabled a first carbon footprint evaluation of fossil-based crotonic acid to understand the actual environmental load and served as a basis for the comparison with the carbon footprint of crotonic acid produced through CROSS-LIFE. The study highlighted a reduction in CO2 emissions of about 43% for bio-based crotonic acid (CA) compared to fossil-based crotonic acid, with a carbon footprint estimation for bio-based crotonic acid of 7.87 kgCO2eq/kgCA produced. Process-contribution analysis pointed out that the most impactful steps in the process correspond to the more energy-demanding, but it also showed potential environmental benefits achievable through the anaerobic digestion stage from energy recovery. In addition, this work was used to conduct a preliminary quantification of the potential carbon emission savings achievable during the disposal of wastewater sludge by applying the CROSS-LIFE process in the current management scenarios at end-of-life.