This study examined the effects of Fe2+ and its bioavailability for controlling VFAs during organic shock loads in batch reactors and a submerged anaerobic membrane bioreactor (SAMBR). When seed grown under Fe-sufficient conditions (7.95 +/- 0.05 mg Fe/g-TSS), an organic shock resulted in leaching of Fe from the residual to organically bound and soluble forms. Under Fe-deficient seed conditions (0.1 +/- 0.002 mg Fe/g TSS), Fe2+ supplementation (3.34 mg Fe2+/g-TSS) with acetate resulted in a 2.1-3.9 fold increase in the rate of methane production, while with propionate it increased by 1.2-1.5 fold compared to non-Fe2+ supplemented reactors. Precipitation of Fe2+ as sulphides and organically bound Fe were bioavailable to methanogens for acetate assimilation. The results confirmed that the transitory/long term limitations of Fe play a significant role in controlling the degradation of VFAs during organic shock loads due to their varying physical/chemical states, and bioavailability. (C) 2016 Elsevier Ltd. All rights reserved.