Unraveling the water states of waste-activated sludge through transverse spin-spin relaxation time of low-field NMR

The physical states of water are crucial for the dewatering efficiency of waste-activated sludge (WAS). However, to date, there is still lack of promising methods for the distinct differentiation of water states in colloidal microbial aggregates. This study proposed that the transverse spin-spin relaxation time (T-2) distribution of low-field nuclear magnetic resonance (NMR) could be a useful tool to unravel the occurrence state of water in WAS. Due to the different interaction strengths of protons with the surrounding environment, the three water states with different T-2 ranges were identified. The water strongly trapped on the surface of solid phase through hydrogen bound could be classified as vicinal water; interstitial water refers to the water physically trapped in bio-floc by steric hindrance or adsorption; and the water that is least affected by solid compositions is categorized as moderately mobile water. The potential ways of typical conditioning approaches for shifting water states were also investigated. The removal of hydrophilic compounds in extracellular polymeric substances (EPS) and surface charge neutralization were both found to be possible ways to influence the percentage of vicinal water (Pearson correlation coefficient R-p > 0.950, p-value <= 0.05). The disintegration or compaction of colloidal microbial aggregates could induce the transformation of interstitial water into moderately mobile water. All the above results are believed to deepen the mechanism insights into the differentiation and interactive transformation of water states in bio-floc of WAS. (C) 2019 Elsevier Ltd. All rights reserved.