Iron speciation and iron species transformation in activated sludge membrane bioreactors

Iron speciation and iron species transformation were investigated in three membrane bioreactors (MBRs) differing in feed iron concentration (and oxidation state) and the presence or absence of an anoxic chamber to simulate various feed stream conditions and operational configurations. The concentration of dissolved Fe(II) was below detection limit (i.e., <0.1 μM) in all chambers while the concentration of dissolved Fe(III) was found to be around 0.25 μM. H2O2 was detected as a quasi-stable reactive oxygen species with concentrations in the μM range in all MBR chambers. H2O2 acted as the primary potential oxidant of Fe(II) in the anoxic chamber. Batch experiments showed that the rate constant for oxygenation of dissolved Fe(II) in the liquid phase of the activated sludge compartment was as high as 78 M−1 s−1. The half-life time of dissolved Fe(II) in all chambers was found to be no longer than 1 min. The stability constants of the Fe(III)SMP complexes were far from uniform. A large quantity of Fe(II) (over 0.036% of the sludge dry mass) was found to be adsorbed by the bacterial flocs suggesting the active reduction of adsorbed Fe(III). The content of adsorbed Fe(II) was found to increase if the MBR was supplied with iron in the Fe(II) form. Over 60% of iron fed to the reactors was converted to highly insoluble ferric oxyhydroxide in all MBRs. A model has been developed which satisfactorily describes the oxidation of Fe(II) in the activated sludge liquid phase and which provides valuable insight into the relative importance of redox processes occurring which mediate the speciation of iron in the system.