This thesis investigated hydrothermal carbonisation (HTC) and pyrolysis for upcycling textile microfibre waste into high-value carbonaceous products, while contributing to strategies to reduce microfibre release from washing machines. The work was motivated by legislation mandating washing-machine filters to capture textile microfibres before wastewater discharge. Although these measures reduce emissions at source, they have not been accompanied by clear strategies for managing the retained waste. This thesis therefore explored whether captured microfibres could be treated as a recoverable resource through thermochemical upcycling.

The collected washing-machine microfibres were characterised as PET fibres of 1–2 mm length and ~18 µm width, and cotton fibres of 4–7 mm length and ~20 µm width, range within the definition of microfibre. The results showed that HTC and pyrolysis are technically feasible, while highlighting the importance of catalyst design, operating conditions and feedstock characteristics. In catalytic pyrolysis, solid carbon yields of 28% for PET and 43% for cotton were obtained.

Overall, pyrolysis and HTC should be regarded as complementary rather than competing routes. Pyrolysis is particularly suitable when valuable gas streams and structured carbonaceous materials are targeted, whereas HTC offers advantages for wetter or more heterogeneous feedstocks. In HTC of real textile waste, carbon retention in the solid phase increased from 12% at 250 °C for 4 h to 25% at 300 °C for 4 h, showing the effect of process conditions on carbon partitioning.

The environmental performance of the selected routes was evaluated through life cycle assessment. Per kg of microfibre waste treated, HTC achieved climate-change impacts of −0.0395 kg CO₂-eq for activated carbon and −0.573 kg CO₂-eq for graphite, while pyrolysis achieved −0.723 kg CO₂-eq for PET and −0.803 kg CO₂-eq for cotton. Overall, captured textile microfibres can be treated as a secondary resource for producing useful carbonaceous materials.