A three-stage process of Mn(VII)-Fe(III)/PDS system for enhancing sludge dewaterability: Effective driving of Fe(II)/Fe(III) cycle and adequate assurance of ROS

Abstract

Sludge dewatering helps to reduce the cost of sewage treatment plants and environmental impact. Hence, we used a type of permanganate (Mn(IV)) activated ferric chloride (Fe(III))/ peroxydisulfate (PDS) system to enhance sludge dewaterability. At optimal dose, the simultaneous addition of Mn(IV), Fe(III) and PDS (MFP treatment) reduced the value of specific resistance to filtration (SRF) from 6.76 × 108 s2/g to 2.01 × 108 s2/g. There is a three-stage process during MFP conditioning: oxidation/flocculation, adsorption/oxidation, and reflocculation. X-ray photoelectron spectroscopy (XPS) and Electron spin resonance (ESR) analyses were employed to investigate the oxidation mechanism. Meanwhile, the analysis of Mn valence was improved by considering metal leaching rates. The cycles of Mn(II)/Mn(III)/Mn(IV) were initiated under the reaction between Mn(VII) and extracellular polymeric substances (EPS)/PDS. This process sustained the cycle of Fe(II)/Fe(III) to promote activation of PDS, which continuously promoted to generate OH•, SO4-•, •O2–, 1O2 and Fe(IV). Additionally, sludge EPS was proven to be the primary electron donor of Fe(II)/Fe(III) cycle, resulting in the increasing of Fe(II) proportion. Notably, the hydrophilicity and water-holding capacity of EPS declined significantly following excellent oxidation, which promoted the conversion of bound water. The effective destruction of hydrogen bonds and protein secondary structures explained the outstanding effects. Meanwhile, XDLVO calculation revealed that sludge coagulation performance enhanced through Fe(III) flocculation, MnO2 adsorption and hydrophobic flocculation. Raw and converted free water would be effectively removed through the more hydrophobic surface of large and tough drain pores formed after the reflocculation process. The efficient drainage of sludge water created strong sludge dewaterability. This study verified the practicality of MFP treatment by exploring the oxidation and coagulation mechanisms to clarify the dewatering mechanism.