Low-field nuclear magnetic resonance detection of changes in the water distribution in citric acid biosludge during dewatering

Activated sludge treatment of citric acid wastewater creates large quantities of residual biological sludge that is difficult to dewater and dispose of. In order to deepen the understanding of the sludge dewatering process, low-field nuclear magnetic resonance (NMR) spectroscopy was used to study changes in internal, vicinal, interstitial, and free water contents and binding degree during citric acid sludge dewatering. The effects of polyaluminum chloride (PAC), PAC and calcium oxide (PAC + CaO), and cationic polyacrylamide (CPAM) treatment and altering the doses of these coagulants were evaluated. Obvious changes were detected in the distribution of water between interstitial and free forms during sludge dewatering. The transverse relaxation time (T-2) and water content obtained by low-field NMR spectroscopy indicated that CPAM played an important role in the sludge cake formation stage because of its flocculation. In this process, the vicinal, interstitial, and free water contents of the sludge decreased by 0.581 g/g-dry matter (DM), 6.695 g/g-DM, and 51.867 g/g-DM, respectively, under vacuum filtration (t = 2 min), that are much higher than other conditioning agents. In the sludge cake shrinkage stage with vacuum filtration for 8 min, PAC + CaO pretreatment gave a higher dewatering rate (10.783 g/g-DM) than CPAM (7.098 g/g-DM) because of the role of the skeleton structure. Overall, the treatment effects of these three conditioners were ranked in the order CPAM > PAC + CaO > PAC. Low-field NMR spectroscopy proved to be a convenient and reliable method to investigate changes in the water distribution in sludge during the treatment process.