## 1 Introduction
For instance, when biomethane potential tests of _L. grandiflora_ were performed [7], methane production was found to have increased 3.2-fold compared with that of untreated biomass. In a study by [25], grape pomace AD was performed using batch and continuous digesters. The cumulative methane production by batch tests was 0.125 Nm\({}^{3}\) kg COD\({}^{-1}\), whereas in a pilot-scale reactor with an organic loading rate (OLR) of 2.5 kg COD\({}^{3}\)d\({}^{-1}\) and a hydraulic retention time (HRT) of 30 days, the biogas production was 6.43 \(\times\) 10\({}^{-3}\) Nm\({}^{3}\)d\({}^{-1}\). The pilot reactor efficiency, when compared with the batch process, was 81.2%. Therefore, to fully understand substrate properties and maximise the methane potential, semi-continuous/continuous operations should be implemented to help predict substrate digestibility and inhibitory effects in full-scale industrial AD. Some studies reported the effects of SE pretreated lignocellulosic biomass in semi-continuous/continuous modes [26; 27]. Although the studies mentioned above were limited in discussing the biomasses from industries or mills, no study discussed the semi-continuous AD of the invasive aquatic weeds especially _L. grandiflora_. Also, SE pretreatment can be easily applied to a practical industrial scale, as mentioned by previous studies [28; 29]. For example, Canada Sunopta Company manufactured an industrial steam explosion plant. This plant uses around 500 kg/h substrate continuously. Similarly, other industrial-scale SE plants are operated by Masonite batch process and IUogen Corporation in Canada. In a study shared by [30], the SE process was simulated to study the economic feasibility of biogas production from wheat straw. When wheat straw (200,000 tons/year) was used as a substrate for a biogas production plant, the total project investment was around 61.8 million euros. On the other hand, the total project costs for a plant without a SE pretreatment unit was lower: 56.4 million euros. However, when calculating the average annual production of biogas from the wheat straw, it observed an increase from 36.8 to 70.5 million Nm\({}^{3}\)/year if the pretreatment unit is used [31]. In the coming times, the primary overall energy consumption (-40-45%) mainly depends on various biomass resources. In this study, performing SE pretreatment before AD is the first step for the degradation of biomass (_L. grandiflora_) which does not require any pre-processing like adding chemicals or washing the substrate, making it profitable from other pretreatments. Hence, the economic viability of the SE pretreatment is directly related to its global energy consumption and must be integrated with an electricity production system through biogas combustion. Hence, the problem caused by the excessive growth of _L. grandiflora_ as mentioned before, this study decided to use the harvested _L. grandiflora_ as a substrate for a long-term AD operation. The study aimed to run a long-term (98 days) AD operation to treat lignin-rich _L. grandiflora_ using a continuously stirred tank reactor (CSTR). SE was performed before the AD for the overall substrate solubilisation and evaluated the effect of steam-exploded _L. grandiflora_ on AD. The temperature of 180 \({}^{\circ}\)C can help evaluate the effect of inhibitory compounds produced during the SE pretreatment. Owing to the high lignin content of _L. grandiflora_, during the SE pretreatment, lignin can be distributed into both solid and liquid fractions, making it an inhibitory compound affecting the AD process. Also, due to the long retention time of 30 minutes, the lignin can be condensate in the solid fraction during the SE pretreatment. Therefore, this study will evaluate substrate solubilisation and the effect of inhibitors with _L. grandiflora_ used as a substrate. This study is the first which performs a semi-continuous AD operation with _L. grandiflora_ for further industrial application.