Mechanistic insights into a novel nitrilotriacetic acid-Fe0 and CaO2 process for efficient anaerobic digestion sludge dewatering at near-neutral pH

Abstract

Traditional Fenton or Fenton-like oxidation has been widely studied for waste activated sludge dewaterability. However, the narrow pH range (2.0–4.0) and the instabilities of Fe2+ and H2O2 have hindered its commercial application. Owing to the high alkalinity of anaerobic digestion (AD) sludge, traditional Fenton or Fenton-like oxidation is economically unfeasible for its dewatering. In this study, we successfully demonstrated a novel and feasible method that used nitrilotriacetic acid (NTA)-Fe0 combined with CaO2 (NTA-Fe0/CaO2) at near-neutral pH (∼6.0) (a slight pH adjustment) in which capillary suction time ratio (CST0/CST) and centrifuged weight reduction (CWR) improved by 6 folds and 42.98 ± 0.37%, respectively, under the optimal conditions. The presence of NTA accelerated the Fe0 corrosion, Fe2+ stability and turnover between Fe2+ and Fe3+. As such, Fe0 could effectively catalyze CaO2 to produce hydroxyl radicals (•OH) under near-neutral conditions. Accordingly, various molecular weight hydrophilic compounds in different extracellular polymeric substances fractions were significantly reduced after treatment. The hydrophilic functional groups especially protein molecules were largely reduced. Consequently, the viscosity of sludge and particle size effectively decreased, while the release of bound water, surface charge, flocculation, and flowability of sludge were improved. The cost-benefit analysis further demonstrated the NTA-Fe0/CaO2 treatment has high reusability and stability and is also more economical over the FeCl3/CaO and Fenton’s reagent/CaO treatments. In summary, the NTA-Fe0/CaO2 process is a cost-effective and practically feasible technology for improving AD sludge dewaterability.