## 3 Results and Discussion
### Characteristics of Sludge  
The appearances of raw sludge and thermally solidified sludge were also greatly different under SEM. As shown in Figure 3b, the sludge particles adhered to each other to form a uniform coralline-like compact structure after thermal solidification, and a much larger number of pores in the interior of sludge were observed in comparison with the raw sludge (the SEM image of raw sludge is shown in Figure 3a). The cooling sludge had greater hardness and could not be broken easily. Definitely, solidification and shrinkage of sludge, as well as bigger pores between sludge particles, will all be propitious to the release of water from sludge. The DSC thermograms of sludge are shown in Figure 4a,b. Large amounts of heat were released during the range of \(-18\) to \(-27\)\({}^{\circ}\)C. When the temperature was below \(-30\)\({}^{\circ}\)C, no obvious exothermic peak was observed. The heat released by the sample during cooling  
Figure 2: Effect of temperature (**a**) and time (**b**) on the dewatering ratio of sludge and the final sludge volume. The oxygen-containing functional groups exert a great influence on the surface properties of sludge particles. Oxygen functionalities on the sludge surface have been categorized as hydroxyl (-OH), carboxyl (-COOH), carbonyl (C=O), and ether (-O-) [17,18]. The first three groups (hydroxyl, carboxyl, and carbonyl) have strong hydrophilicity and enhance the water-absorbing capacity of the sludge. In this study, XPS was used to obtain information about the surface characteristics of sludge particles. The O 1s/C 1s peak area ratio (see Table 1) decreased when the sludge was treated by the thermal solidification method at 75 \({}^{\circ}\)C for 30 min. This may be explained by the fact that parts of carboxyl groups of acrylic resins were enclosed within the sludge after thermal solidification treatment. For the sake of a better understanding of the surface chemistry change during the thermal solidification treatment, XPS Peak 4.1 was employed to deconvolve the high-resolution C 1s spectra in this study. As shown in Figure 5, the high-resolution C 1s spectra of sludge samples were resolved into three individual component peaks: C-C/C-H (peak 1, 284.7-284.8 eV), C-O/C-OH (peak 2, 286.3 eV), and O=C-O (peak 3, 288.9-289.1 eV). Among these groups, C-C and C-H (peak 1) are hydrophobic groups, while hydroxyl (peak 2) and carboxyl groups (peaks 3) are hydrophilic groups. Table 1 summarizes the results of the C1 s peaks. C-C and C-H were the predominant carbon chemical bonds for raw sludge and accounted for the 84.0% peak area ratio at the C 1s peak, reflecting that its relative amount was significantly higher than that of the carbon bonded to oxygen-containing functions (16.0%). Therefore, it could be identified that the primary carbonic compounds in the sludge sample were aliphatic compounds. For the sample after thermal solidification treatment, the relative amount of C-C and C-H (peak 1) was increased to 88.5%, and the content of hydrophilic groups was decreased to 11.5%. The fitting data in the C 1s peak can only give quantitative information on the relative contents of oxygen-containing functional groups, but we can conclude that the hydrophobicity of the sludge after thermal solidification treatment was decreased. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**C 1s (\%Amun)**} \\ \cline{3-6}
**Sludge Sample** & **O/C** & \begin{tabular}{c} **Peak 1** \\ **284.7–284.8 eV** \\ \end{tabular} & \begin{tabular}{c} **Peak 2** \\ **286.3 eV** \\ \end{tabular} & \
\begin{tabular}{c} **Peak 3** \\ **288.9–289.1 eV** \\ \end{tabular} &
\begin{tabular}{c} **Peak 2** \\ **286.3 eV** \\ \end{tabular} \\ \cline{3-6}  & & **C–C and C–H** & **C–O and C–OH** & **O=C–O** & **Hydrophilic Groups** \\ \hline Raw sludge & 0.244 & 84.0 & 11.2 & 4.8 & 16.0 \\ \hline Thermal solidified sludge & 0.231 & 88.5 & 7.8 & 3.7 & 11.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Relative contents of functional group in C 1s from XPS spectra.