Biocatalysis in Water or in Non-Conventional Media? Adding the CO₂ Production for the Debate

Biocatalysis can be applied in aqueous media and in different non-aqueous solutions (non-conventional media). Water is a safe solvent, yet many synthesis-wise interesting substrates cannot be dissolved in aqueous solutions, and thus low concentrations are often applied. Conversely, non-conventional media may enable higher substrate loadings but at the cost of using (fossil-based) organic solvents. This paper determines the CO₂ production-expressed as kg CO₂·kg product^-1-of generic biotransformations in water and non-conventional media, assessing both the upstream and the downstream. The key to reaching a diminished environmental footprint is the type of wastewater treatment to be implemented. If the used chemicals enable a conventional (mild) wastewater treatment, the production of CO₂ is limited. If other (pre)treatments for the wastewater are needed to eliminate hazardous chemicals and solvents, higher environmental impacts can be expected (based on CO₂ production). Water media for biocatalysis are more sustainable during the upstream unit-the biocatalytic step-than non-conventional systems. However, processes with aqueous media often need to incorporate extractive solvents during the downstream processing. Both strategies result in comparable CO₂ production if extractive solvents are recycled at least 1-2 times. Under these conditions, a generic industrial biotransformation at 100 g L^-1 loading would produce 15-25 kg CO₂·kg product^-1 regardless of the applied media.