## 3 Results and Discussions  
### Mass Balance and Chemical Composition of the Untreated and Pretreated Substrate and Effluent  
The mass balance of _L. grandiflora_ after SE pretreatment at 180 \({}^{\circ}\)C for 30 min was evaluated (Table 1). Approximately 126 kg-wwt of untreated _L. grandiflora_ was used for the SE pretreatment. Around 70.34 kg-wwt of solid fraction and 175.14 kg-wwt of liquid fraction were obtained after the SE pretreatment. The solid and liquid fractions were collected separately due to the mechanism of the SE equipment. A conventional type of SE equipment was used by [7] in which the sample is collected in the form of slurry. The heating method used in this study was by supplying high-temperature steam instead of boiling the substrate at a specific temperature. The pH of the liquid fraction was 4.64, which was low owing to the release of the organic acids, such as acetic acid or levulinic acid, during the substrate degradation [11; 35]. A low pH can cause an imbalance in the bacterial population during the AD process. The TS and VS contents in the untreated _L. grandiflora_ were 194.5 g L\({}^{-1}\) and 166.7 g L\({}^{-1}\), respectively (Table 2). They further decreased to 187.1 g L\({}^{-1}\) and 162.1 g L\({}^{-1}\) for the solid fraction and 8.6 g L\({}^{-1}\) and 5.4 g L\({}^{-1}\) for the liquid fraction. A similar decrease in the TS and VS values after the SE pretreatment was observed [36]. The TS in the effluent on day 0 and day 98 was 28.50 g L\({}^{-1}\) and 20.50 g L\({}^{-1}\), respectively. This proves that approximately 70% of TS were still present in the reactor even after the 98-day operation, showing an increase in the insoluble solid content inside the reactor [37]. This increase in the insoluble solid content can further affect the biogas yield. However, the VS content on day 0 (15.80 g L\({}^{-1}\)) and day 98 (15.70 g L\({}^{-1}\)) did not differ much, indicating that a higher amount of organic compounds was still present in the reactor leading to a minor VS reduction. The cellulose and lignin values in the untreated _L. grandiflora_ were 32.83 \(\pm\) 0.5 and 25.22 \(\pm\) 4.6%TS, respectively, whereas hemicellulose was not detected. Higher lignin content was obtained in a study conducted on _L. grandiflora_[7]. After the pretreatment, the cellulose value decreased to 28.02 \(\pm\) 0.2%TS, and the lignin value increased to 29.15 \(\pm\) 0.5%TS. Similar increments in the lignin content were also obtained in previous studies, indicating the condensation and depolymerisation reactions during SE pretreatment. These reactions result in the increment of pseudo lignin and dissolved lignin, affecting AD, as mentioned by [22; 38]. On day 98, the lignocellulosic values in the effluent were calculated to observe substrate degradation during the AD process (Table 2). The cellulose value on day 98 was 5.03 \(\pm\) 0.8%TS, leading to a higher cellulose degradation in the effluent. The cellulose value \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Values** \\ \hline Solid fraction (kg-wwt) & 70.34 \\ Liquid fraction (kg-wwt) & 175.14 \\ Solid/Liquid & 0.40 \\ pH of liquid fraction & 4.64 \\ \hline Total untreated _Ludwigia grandiflora_ before steam explosion pretreatment = 126 kg-wwt. Total solid and liquid fraction after pretreatment = 245.48 kg-wwt. \\ \hline \end{tabular}
\end{table} 
Table 1: Mass balance of the steam-exploded _Ludwigia grandiflora_ at 180 \({}^{\circ}\)C for 30 min at a severity factor of 3.8.