## 3 Results and Discussions
### Biogas Production Yield and Methane Content in the CSTR Reactor  
The AD process showed a fluctuating biogas production trend (Figure 4). The average biogas production during the operation was 265 mL g-VS\({}^{-1}\). The highest biogas production was 400 mL g-VS\({}^{-1}\) on day 49, and the lowest biogas production (71.4 mL g-VS\({}^{-1}\)) was recorded on day 67. The initial OLR and HRT were 0.9 g-VS L\({}^{-1}\) day\({}^{-1}\) and 30 days, as shown in Figure 4. NaOH addition from day 67 until the last operational day 98 did not show an increasing trend in the pH but showed a significant change in biogas production. The gas production tended to be less fluctuating and more stable towards the end of the operation. In previous studies, the NaOH addition effect was advantageous in incrementing biogas production and preventing the destabilisation of AD processes [54,58]. In a study performed by [54], due to the addition of NaOH to poultry waste and the digestate, the methane yield was increased by 3.03% from the untreated. However, in this study, the relationship between the NaOH addition and the methanogenesis bacteria needs to be studied to clarify the increment in the biogas yield.  
Figure 4 also shows the methane content in the produced biogas. The methane content in the biogas was stable, but its value was relatively lower than the other CSTR reactors [25,59,60]. For example, a methane content of 63% was observed when grape pomace was used as a substrate [25]. The average value of the methane content in this  
Figure 4: Relationship of the biogas yield and methane content in the produced biogas.  
study was 49.04%. The low methane content in the biogas could be due to the high carbon present in the SE pretreated \(L\). _grandiflora_, as discussed before in Table 2.  
When comparing the methane content produced in this study with another pretreatment like the pyrolysis process, a different trend was seen [61, 62]. The studies performed high-temperature pyrolysis on sewage sludge and plastics as substrates. These studies observed that the methane content observed during the process was different to the methane content in conventional biogas produced during no pretreatment from garbage or sludge was used as substrates. The value of C1 (vol%), mainly methane, ranged from 0.24 to 4.94% [61]. However, when sludge was used for pyrolysis, the value was 33.41% higher than the methane produced by conventional biogas (29.12%) using garbage [62].  
The reason for the low biogas production, its fluctuation and low methane content in this study can be the high lignin content of 29.15 \(\pm\) 0.5%TS in the pretreated \(L\). _grandiflora_[39]. As shown in Table 1, the lignin amount in the effluent on the last operational day 98 was 17.0 \(\pm\) 1.0%TS, which is higher than that of other untreated plant biomasses, such as _Potamogeton maackianus_ (20.7%TS) and _Eichhornia crassipes_ stem (17.5%TS) [63, 64]. Furthermore, \(L\). _grandiflora_ has a woody structure because of which it does not fully degrade even after SE pretreatment at 180 \({}^{\circ}\)C. Lignin contains 60-65% of carbon inside the plant biomass [64]. This high amount of carbon can lead to an imbalance of nutrients. Therefore, another evidence can be the high residual carbon present inside the reactor, owing to which the biogas production fluctuated and had a lower value.