Performance of a Continuous Dark–Photo Fermentation System to Produce Hydrogen from Simulated Sugar–Rich Processing Wastewater Under Suboptimal pH and Temperature Conditions

The integration of coupled dark fermentation (DF) and photofermentation (PF) systems represents a promising approach for concurrent H2 production and organic load reduction in carbohydrate–rich wastewater. However, their behavior under non–optimized, continuous industrial operation remains inadequately characterized, hindering practical implementation. This case study evaluated an integrated DF–PF system treating a synthetic sugar mixture mimicking acidic fruit and dairy processing wastewater. The system presented herein (5-L reactors) serves as an initial prototype to facilitate scaling to both pilot (40-L reactors) and, ultimately, industrial (600-L) scales using real effluents within the framework of a research project. The bioreactors were operated continuously with a 10 h hydraulic retention time and an organic loading rate of 2.5 g COD L−1 d−1. The pH and temperature were monitored but intentionally left uncontrolled. The DF stage facilitated by hydrogen–producing bacteria achieved H2 concentrations of up to 57% (v/v) and a maximum production rate of 177 mL H2 L−1 d−1; however, it demonstrated notable process instability due to the absence of controls. In contrast, the PF stage exhibited negligible H2 production (1.8% v/v) attributable to the displacement of Rhodopseudomonas species by microbial competition, thereby functioning predominantly as a polishing step. Overall, the coupled system achieved an average COD removal of 34%, highlighting the functional differentiation between stages and identifying microbial competition as the primary constraint under suboptimal conditions.