Integrated dark-photo fermentative hydrogen production from synthetic gelatinaceous wastewater via cost-effective hybrid reactor at ambient temperature

Abstract

The industrial application of anaerobic digestion to treat protein-rich wastewater (e.g., gelatin) is promising; however, inhibitory effects such as released excess ammonia on methanogens causes limitations. This study investigated the potential of sequential dark and photo-fermentation for wastewater treatment and simultaneous bio-hydrogen production (as energy source). To this end, a new configuration, namely dark-photo circular baffled reactor (DP-CBR) was introduced and operated at ambient temperature (21 ± 10 °C). The reactor was composed of four identical compartments, where fluorescent tubes were installed to the last two compartments, i.e., C1-C2 (dark) and C3-C4 (photo). The long-term impact of main operational parameters (i.e., hydraulic retention time [HRT] of 6, 12 and 24 h at initial pH of 5.5 and 6.5) was assessed. Maximum hydrogen yield (HY) of 0.4 L/gCOD, COD removal of 82%, and organic-N removal of 95% were obtained at HRT of 24 h and initial pH of 6.5. Increasing HRT was found to maintain the reactor efficiency at ambient temperature. Lowering initial pH to 5.5 deteriorated the dark-treatment at C1 and C2, resulting in lower local HY and ammonification efficiency. Further, the results confirmed that higher HY was achieved in the photo-fermentation, as the protein hydrolysis was mainly achieved in the dark-fermentation. The residual free ammonia (<0.36 mg/L), at all examined conditions, was below the inhibition limit of photosynthetic bacteria. The microbial community analysis revealed the development of purple non-sulfur bacterial family Rhodospirillaceae at C3. The economic efficiency of DP-CBR was also evaluated by considering the capital cost, annual costs (i.e., lightning, pumping, nutrients, and gas purification), and revenues (i.e., bio-hydrogen energy and removal add-value). Overall, the techno-economic assessment of DP-CBR performance emphasizes its feasibility in affordable removal of organics and bioenergy recovery when dealing with gelatin-rich wastewater.