Next-gen pre-treatment strategies for lignocellulose in biohydrogen production via dark fermentation: A review

Hydrogen is a clean fuel that emits only water during combustion. Biohydrogen production from biomass waste via dark fermentation represents an attractive approach that can be integrated into existing waste treatment facilities for simultaneous waste treatment and biofuel production. Lignocellulosic biomass (LCB) is a rich source of sugars for dark fermentation, but it faces challenges such as its recalcitrant structure and inherent variability in physiochemical properties, which make it difficult to pre-treat and can limit biohydrogen yield. Wet torrefaction (WT) is a promising pre-treatment strategy that can co-process LCB and organic wastewater, releasing sugars from LCB while recovering organic matter from wastewater. The addition of organic wastewater also supplements nutrients (i.e. nitrogen and phosphorus) and helps balance the C/N ratio of the substrate. Palm oil mill effluent (POME) has been identified as a suitable organic waste stream and is used as a model for discussion in this review. The WT process produces two products: hydrochar and hydrolysate. The hydrolysate contains readily fermentable sugars that can be used in dark fermentation without the enzymatic saccharification typically required in conventional pre-treatment methods. Moreover, the hydrochar is also important for offsetting energy requirements and providing environmental benefits to the process. The utilisation of POME further contributes to greenhouse gas mitigation by reducing emissions associated with conventional POME treatment, which annually releases substantial amounts of greenhouse gases to the atmosphere. These factors may translate into carbon credits, providing additional financial benefits and improving the overall economic viability. The feasibility and status of WT are also assessed in this review compared with other conventional pre-treatment methods. Nevertheless, several operational challenges of WT are discussed, and potential solutions to address them are evaluated.