Improvement of Anaerobically Digested Sludge Dewatering by Tannin-Mediated Fenton Conditioning: Significance of Fe(III) Chelation and Reduction

To address the technical bottlenecks of the classic Fenton oxidation for anaerobically digested sludge (ADS) dewatering (e.g., high doses of Fe2+/H2O2/acid usage and slow reduction of Fe3+), we employed biodegradable tannin compounds as iron chelating and reducing agents to improve the Fenton reaction. Compared with the classic Fenton (pH 3.0), tannic acid (TA)-assisted Fenton enhanced the degree and rate of ADS dewatering (pH 5.2) with reductions of 99% in specific resistance filtration, 96% in capillary suction time, and 97% in filtration–compression time. Besides, this modification reduced the consumption of Fe2+/H2O2 by 50%. Mechanistically, TA could chelate Fe3+ to generate iron complexes while facilitating the reduction of Fe3+ to Fe2+. This phenomenon avoided Fe3+ precipitation and accelerated Fe3+/Fe2+ cycling to produce more hydroxyl radical (•OH).•OH oxidation disintegrated CHOP-, CHONS-, and CHONSP-containing protein-/lipid-like substances within tightly bound extracellular polymeric substances. Bioflocs were dispersed into an irregular block-shaped structure with smaller particle sizes. Employing low-cost and readily accessible tannin-rich plant crude extracts derived fromAcacia mangiumas an alternative to purified TA in Fenton conditioning of ADS exhibited comparable enhancements in dewaterability characteristics. These findings demonstrated that the synergy between tannin compounds and Fenton oxidation could facilitate the cost-effective dewatering of ADS.