Primary and secondary waste in (bio)catalysis: What matters is not what is produced but what permanently remains!

(Bio)catalysis creates waste, composed of wastewater and an organic fraction. Different metrics are used for the environmental impact of processes, being E-Factor and process mass intensity (PMI) the prominent ones. These metrics are typically applied to wastewater and spent organic fraction, which is the process “Primary Waste.” However, “Primary Waste” is industrially treated before it is released to the environment. When treating “Primary Waste,” CO 2 is generated, from the wastewater treatment plant and from the organic fraction incineration. That CO 2 , coined as “Secondary Waste,” remains in the planet and thus should be the target for environmental assessments. When E-Factor and PMI calculated from “Primary Waste” are compared to the values of produced CO 2 , results mismatch because the CO 2 depends on the proportion wastewater/organic fraction since incineration leads to higher CO 2 production than wastewater treatment. Sustainable options for (bio)catalysis would be process intensification, incorporating renewable energy and biogenic solvents (neutral carbon), and developing (new) wastewater treatment strategies to deliver pure(r) water effluents to the environment. • (Bio)catalytic reactions generate waste—solvents and wastewater—which are “Primary Waste.” • Traditional environmental mass-metrics focus on those “Primary Waste.” • “Primary Waste” is treated in industry before it is released, generating CO 2 as “Secondary Waste.” • Environmental metrics based on “Primary Waste” show discrepancy with the CO 2 formed. • An assessment of the CO 2 produced may become useful to compare processes.