Continuous H-2 production from alkali-hydrolyzed rice straw in two mesophillic anaerobic baffled reactors (ABRs) was investigated in parallel. ABR(1) was inoculated with thermally pretreated sludge while the sludge in ABR(2) was amended with magnetite nanoparticles (MNPs). Both reactors were operated at organic loading rates (OLRs) ranged from 0.8 to 4.8 g(COD)/L/day. The addition of MNPs significantly improved the H-2 production and yield, ie, maximum H-2 yield was 21.4 +/- 3.2 and 60.6 +/- 5.7 mL/g CODremoved for both ABR(1) and ABR(2), respectively, at OLR of 1.6 g(COD)/L/day. Likewise, substrate degradation efficiency (SDE) was augmented from 48.4 +/- 3.9% (ABR(1)) to 55.8 +/- 4.2% (ABR(2)). The enhancement effect of MNPs can be emphasized by hydrogenase enzyme activity increase from 0.097 +/- 0.013 to 0.205 +/- 0.019 mg MBreduced/min at ABR(1) and ABR(2), respectively. Besides, ABR(2) witnessed higher concentrations of acetate (HAc) and butyrate (HBu) coupled with lower values of propionate (HPr). Additionally, HAc/HBu ratio for ABR(2) (3.03 +/- 0.31) was higher than that for ABR(1) (1.95 +/- 0.29). Microbial analysis indicated that Bacillus and Clostridium species became dominant in mixed culture bacteria supplemented with MNPs. Furthermore, economic analysis revealed that the minimum payback period of 10.5 years at OLR of 3.2 g(COD)/L/day was recorded for ABR(2) compared with 14.6 years at OLR of 4.8 g(COD)/L/day for ABR(1).