Supplementation of cobalt to UASB reactors by pulse dosing: CoCl2 versus CoEDTA2− pulses

The effect of chelation on the dosing strategy of cobalt to restore the performance of a cobalt limited methanol-fed bioreactor was investigated. Three upflow anaerobic sludge bed (UASB) reactors (30 °C, pH 7.0) were operated with methanol as the substrate at an organic loading rate of 8.5 g COD L−1 d−1. One UASB reactor was supplied with several pulses of cobalt bound to EDTA, and its operation was compared to that of another UASB reactor to which several pulses of CoCl2 were given. The addition of cobalt (5 μmoles cobalt per litre of reactor volume) in the form of CoCl2 creates a pool of cobalt in the granular sludge matrix due to the high cobalt retention (around 90%). The methanogens present in the granular sludge are able to use that cobalt pool for stable methane formation from methanol during the subsequent 15 days. When added as Co-EDTA2−, only around 8% of the cobalt added is retained. The small amount of retained cobalt in case of Co-EDTA2− addition supports methylotrophic methanogenesis only a few operational days. Furthermore, the side-effects EDTA has on the granule matrix or microbial cells make EDTA an unsuitable ligand for cobalt dosage in full-scale applications.