Blackwater as pretreatment agent to enhance straw saccharification for biohydrogen production

Blackwater (toilet flushing water) resource recovery is recognized as an effective approach to addressing ammonia nitrogen in municipal wastewater. In this study, blackwater was innovatively combined with alkali to pretreat straw, followed by co-fermentation for hydrogen production. Alkali-blackwater pretreatment conditions were optimized with mechanisms characterized. The co-fermentation of pretreated straw and blackwater was explored to enhance hydrogen production and substrate conversion efficiencies. Under conditions of 4 °C, a 1:50 solid-liquid ratio, 2-h duration, and 15 mg/mL NaOH concentration, a maximum reducing sugar yield of 219.56 mg/g was achieved by the alkali-blackwater pretreatment, representing a 2.55-fold increase over alkali pretreatment alone. Characterization analysis revealed that significant microstructural disruption of straw was induced by alkali-blackwater pretreatment, resulting in twisted/bent fibers with rough, layered surfaces. Lignin characteristic spectral bands weakened significantly, with substantial lignin and xylan removal observed. When co-fermented with blackwater at pH = 7, a maximum hydrogen yield of 29 mL/g was achieved, representing a 52.6% increase in hydrogen production rate over alkali pretreatment alone. Blackwater served dual functions: nitrogen source for fermentation and alkali synergist during pretreatment, enhancing sugar release and straw bioavailability. This study provided theoretical guidance for waste resource utilization, biomass energy research, and the application of straw and blackwater.