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 gCaCO(3)/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. (C) 2016 Elsevier Ltd. All rights reserved.