Study of sludge conditioning using organic acids chelated ferrous ion catalyzed NaClO oxidation: Evolution of extracellular polymeric substances and floc structure

In previous studies, we interestingly found that several ligands (e.g., nitrilotriacetate and humic acid) could significantly accelerate the oxidation of NaClO by ferrous ion, via complexing with Fe(II). This inspired us to hypothesis that organic acids (OA) chelated Fe(II) might enhance the NaClO oxidation for improving sludge dewaterability. In this work, the OA chelated ferrous ions activated NaClO (OA-Fe(II)–NaClO) process was utilized for sludge conditioning, which exhibited significantly improved dewatering performance, as evidenced by capillary suction time (CST), specific resistance to filtration (SRF) and cake moisture of sludge. The normalized CST (CSTn) and SRF decreased to 1.22 s × L/g and 3.24 × 108 m/kg, when ratio dosage of [ClO−]/[Fe2+] (NaClO: 0.75%(v/v)) was 1 at pH of 2. During the NaClO activation by Fe(II), tightly bound extracellular polymeric substances (TB-EPS) was cracked into soluble EPS (S-EPS), followed by oxidizing into smaller molecular organic matters due to the production of hydroxyl radicals. Meanwhile, OA complexed to ferrous ion inhibited hydrolysis of ferric ions, leading to better catalytic performance of NaClO under neutral pH conditions. In addition, the molecular structure of OA determined the low complexing ability and steric hindrance for OA-Fe(II). When molar ratio of [Oxalic acid]/[Fe2+] was 1, oxalic acid-Fe(II)–NaClO improved sludge dewaterability significantly due to the coagulation and skeleton builder properties of formed iron oxalate precipitates. This work provides an eco-friendly and cost-effective method for improving sludge dewaterability. [Display omitted] •Acidic environment is beneficial for sludge conditioning with Fe2+-NaClO process.•Fe2+-NaClO treatment exerted dual functions of oxidation and coagulation.•Organic acids improved sludge conditioning efficiency of Fe2+-NaClO at neutral pH.•Oxalic acid performed better in sludge conditioning due to formation of iron oxalate.