Unravelling key factors controlling vivianite formation during anaerobic digestion of waste activated sludge.

As a product of phosphorous recovery from anaerobic digestion (AD) of waste activated sludge (WAS), vivianite has received increasing attention. However, key factors controlling vivianite formation have not yet been fully addressed. Thus, this study was initiated to ascertain key factors controlling vivianite formation. A simulation of chemical equilibriums indicates that interfering ions such as metallic ions and inorganic compounds may affect vivianite formation, especially at a PO ₄ ^3- concentration lower than 3 mM. The experiments demonstrated that the rate of ferric bio-reduction conducted by dissimilatory metal-reducing bacteria (DMRB) and the competition of methane-producing bacteria (MPB) with DMRB for VFAs (acetate) were not the key factors controlling vivianite formation, and that ferric bio-reduction of DMRB can proceed when a sufficient amount of Fe ³⁺ exists in WAS. The determined affinity constants (K _s ) of both DMRB and MPB on acetate revealed that the K _HAc constant (4.2 mmol/g VSS) of DMRB was almost 4 times lower than that of MPB (15.67 mmol/g VSS) and thus MPB could not seriously compete for VFAs (acetate) with DMRB. As a result, vivianite formation was controlled mainly by the amount of Fe ³⁺ in WAS. In practice, a Fe/P molar ratio of 2:1 should be enough for vivianite formation in AD of WAS. Otherwise, exogenously dosing Fe ³⁺ or Fe ²⁺ into AD must be applied in AD.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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