Characterization of changes in extracellular polymeric substances and heavy metal speciation of waste activated...

Biopolymer solubilization is considered to be the rate-limiting stage of anaerobic digestion of waste activated sludge (WAS). Oxidation processes have been proven to be effective in disrupting sludge flocs and causing solubilization of the solid biopolymers. In this study, WAS was treated by NaNO₂ or H₂O₂ oxidation at pH of 2. The changes in extracellular polymeric substances properties and the speciation of heavy metals were investigated. The results revealed that both NaNO₂ and H₂O₂ treatments were effective in solubilizing organics in WAS, while the conversion of biopolymers in the two treatment processes was different. Free nitrous acid destroyed the gel network structure of EPS, and organic materials were released from the solid phase to the supernatant. Indigenous peroxidase catalyzed H₂O₂ to produce hydroxyl radicals which caused significant solubilization of biopolymers, and the protein-like substances were further degraded into micro-molecule polypeptides or amino acids at high dosages of H₂O₂. During the oxidation processes, Zn, Cd and Cu, with excellent mobility, tended to migrate to the supernatant, and thus were easy to remove through the liquid-solid separation process. Ni and As showed moderate migration ability, of which the residual fraction tended to transform into reducible and soluble fractions. With poor mobility, Cr and Pb mainly existed in the forms of residual and oxidizable fractions, which were difficult to dissolve and remove from WAS. Both NaNO₂ and H₂O₂ treatment resulted in the enhancement of sludge solubilization efficiency and heavy metal mobility in WAS, but different heavy metals showed distinct migration and transformation behaviors.