(Re)visiting the Sustainability Thresholds: Are Product Titers of 1 g L-1 Enough for (Bio)chemical Processes?

In a previous work, equations were reported to estimate global warming potential (GWP, kg CO2 kg product-1) of (bio)catalytic reactions, using substrate loadings and conversions. The approach is useful for reactions with similar mass-to-mass proportion (substrate to product). For processes with mass-to-mass change, or for fermentations, where the relationship substrate to product is elusive, GWP equations using product titers are more straightforward. This article introduces such product-titers-based GWP equations estimating the CO2 released from energy (up- and downstream), as well as from the waste treatment, namely wastewater and solvents. Equations are applied to discuss whether aqueous (bio)transformations with product titers of 1 g L-1 and extractive downstream are sustainable. When effluents are not recycled, >6000 kg CO2 kg product-1 is generated, because large aqueous media (1000 L) and solvents (1000-3000 L) are needed for one product kilogram. Even when solvent and water effluents are almost completely recycled (95%), >100 kg CO2 kg product-1 is still generated. For better environmental figures, process intensification with less diluted systems is mandatory. Setting product titers of ≈20 g L-1 with large recycling loops (95%) and mild wastewater treatment decrease the overall GWP to ≈10 kg CO2 kg product-1. GWP estimations at early development are useful in improving processes with low TRLs.