Biological hydrogen promotion via integrated fermentation of complex agro-industrial wastes

The presence of phenolic compounds and/or metals in mixed fruit peels (MFPs) and paper mill sludge (PMS) yielded hydrogen production values of 121.6 ± 9.2 and 163.5 ± 13.4 mL, respectively. MFPs contained total phenolic compound concentrations of 2.43 ± 0.11 mg_GAU/g, which reduced anaerobic activity. However, the integration of MFPs (30%) into PMS (70%) mitigated the inhibition of hydrogen production and resulted in a 3.01 and 2.24 increase in hydrogen generation compared to the separate fermentation of MFPs and PMS, respectively. Moreover, integrated fermentation of 30% MFPs and 70% PMS synergistically balanced the concentrations of trace metals in terms of calcium, iron, magnesium, manganese, nickel, zinc, sodium, potassium and cobalt at 2016.1 ± 119.3, 473.0 ± 42.6, 243.3 ± 21.7, 23.0 ± 2.7, 14.2 ± 1.3, 109.6 ± 7.5, 113.9 ± 9.1, 888.2 ± 102.4 and 1.2 ± 0.1 mg/kg, respectively. Total ammonia nitrogen and alkalinity values were augmented up to 539.20 ± 28.02 mg/L and 5.23 ± 0.73 gCaCO₃/L, respectively, at 70% PMS, which resulted in a limited pH drop of 1.31 ± 0.41. Furthermore, the 30/70 MFP/PMS substance yielded a net energy profit of 32.2 ± 4.2 kJ/kg_feedstock. This value corresponded to a payback period of 2.6 years compared to 11.5 and 5.4 years for the separate fermentation of MFPs and PMS, respectively.