Integrating biomass-derived oils into existing petroleum refineries is one of the fastest routes toward large-scale deployment of renewable liquid fuels. Yet co-processing bio-oils with refractory fossil streams, such as vacuum gas oil, cycle oils or coker gas oils, poses persistent hurdles: their high oxygen content, thermal instability and heteroatom-rich matrices accelerate catalyst fouling and drive hydrogen consumption. This Perspective argues that a modular hydrotreating strategy, in which tailored pretreatment, grading and active catalyst beds are arranged as interchangeable cartridges, offers a pragmatic path to bridge refinery and biorefinery operations. Drawing from recent continuo... More >