Characterizing the sludge moisture distribution during anaerobic digestion process through various approaches

Exploring moisture distribution plays an important role in improving sludge dewatering. However, the moisture content usually varies based on the measurements, and a comparison of various measurement methods for the same sample during the anaerobic digestion (AD) process is lacking. In this study, the moisture distributions of two types of sludge samples with different digestion times during the AD process are characterized by four methods (i.e., thermal drying, thermal gravimetric analyzer (TGA), thermogravimetry differential scanning calorimetry (TG/DSC), low field ¹H nuclear magnetic resonance (LF-NMR)). The advantages and disadvantages of each test method are evaluated and discussed. It is found that, compared with the other three methods, LF-NMR has the advantages of high recovery rate and coincidence degree between total moisture content (TMC) and moisture content measurements. In addition, the moisture content in various occurrence states is compared. Experimental results show that the content of free moisture increases, while that of bound moisture decreases during the AD process, suggesting that bound moisture is converted into free moisture, which improves the sludge dewaterability. However, these findings can provide an important reference for characterizing the sludge moisture distribution and identifying the effect of the AD process on sludge dewatering.