Reassessing LF-NMR applications: Unraveling water state transformations in sludge and their implications for dewatering performance

The distribution and transformation of water content critically influence sludge dewatering performance and remain central to sludge treatment research. Low-field nuclear magnetic resonance (LF-NMR), known for its non-destructive and rapid analysis, has emerged as a vital tool for characterizing water distribution in sludge, though its limitations require further investigation. This study employed LF-NMR to analyze T₂ relaxation spectra of sludge under various pretreatments and with differing water contents (97 %–62 %). Results indicated that, despite similar overall water content, sludge subjected to different pretreatments exhibited significant variations in T₂ peak counts and times. Notably, T₂₁, T₂₂, T₂₃, and T₂₄ showed significant exponential relationships with water content (p < 0.0001). The transformation and migration of adsorbed, interstitial, and free water demonstrated complex bidirectional continuity. Removal of adsorbed and interstitial water significantly altered sludge structure, diminishing the reversibility of water reabsorption, whereas removal of free water had minimal impact on microstructure. Overlapping T₂ relaxation peaks suggested continuous transitions among water states, challenging the traditional discrete categorization. Neglecting changes in T₂ peak times may lead to misinterpretation of water states and affect assessments of dewatering performance. While LF-NMR shows great potential, its limitations in resolving overlapping peaks and quantitatively assessing water states must be acknowledged. Consequently, LF-NMR should be regarded as a qualitative or semi-quantitative method rather than a purely quantitative tool as often assumed. Future research should integrate higher-resolution T₁–T₂ two-dimensional NMR techniques with advanced data processing to more accurately elucidate the mechanisms of water migration and transformation within sludge.