## 1 Introduction  
Anaerobic digestion (AD) is a biological process that converts organic matter into biogas (70% CH\({}_{4}\) and 30% CO\({}_{2}\)) under oxygen-free conditions. AD is a cost-effective technology that produces renewable energy and reduces the volume of organic waste [1] such as food waste, lignocellulosic waste, microalgae and wastewater treatment plant sludge. The easy availability and low moisture content of lignocellulosic substrates [2] make them suitable for producing biogas using AD. Furthermore, due to the availability of higher nutrients, the digested effluent can be used as a liquid fertilizer [34]. Lignocellulosic biomasses are mainly comprised of cellulose (30-40%), hemicellulose (20-30%) and lignin (15-30%), the contents of which vary according to the plant origin [5]. Cellulose and hemicellulose are available in bundles covered by hardly degradable lignin in the plant cell wall. Thus, initial treatment must be performed to remove this covering of lignin [6]which help to increase the biodegradability of cellulose and hemicellulose, leading to higher solubilisation. _Ludwigia grandiflora_ is an emergent invasive lignocellulosic aquatic weed that has caused several social, economic and ecological problems worldwide owing to its overgrowth [7]. _L. grandiflora_ is commonly referred to as water primrose. _L. grandiflora_ has a rapid growth rate, efficient reproductive capacity and broad ecological tolerance; therefore, it's harvesting is necessary. According to some climatic models, global warming will further accelerate the spread of these species [8]. For instance, in Turc pond, Landes, France, _L. grandiflora_ covered more than 3 hectares in 10 years [9]. Although the treatment of this harvested biomass is a major concern, AD is expected to efficiently treat harvested _L. grandiflora_ due to its cost-effectiveness and recovery of energy from wet organic waste. However, the recalcitrant nature of lignin present in _L. grandiflora_ can affect AD efficiency because it is hardly degradable [10; 11]. Consequently, hydrolysis often becomes a rate-limiting step during AD of lignocellulosic biomasses [12; 13]. Therefore, to increase the degradability of _L. grandiflora_, a pretreatment before AD should be performed. Various pretreatments, including physical, chemical, thermal and biological pretreatments, help in the delignification of rigid lignocellulosic structures [14]. Several factors must be considered while selecting an efficient pretreatment, such as the applicability of the biomass, maximum carbohydrate recovery, production of minimum inhibitors and minimum use of chemicals [7]. Moreover, it should be readily applicable to industrial purposes, making it economically stable [6]. Steam explosion (SE) is a thermal pretreatment process and is among the most efficient and cost-effective treatment methods used in industrial applications because of its environment-friendly properties [15]. SE helps in solubilising the biomass, increasing cellulose digestibility and further preventing the formation of inhibitory compounds [16]. During SE, the lignocellulosic biomass is subjected to elevated temperatures starting from a lower temperature of 120 to a higher temperature of 260 \({}^{\circ}\)C for a retention time of 20 s to 60 min. The steam entering the biomass leads to the expansion of the cell wall making the surface more accessible. Temperatures higher than 150 \({}^{\circ}\)C help in improving the hydrolysis step by solubilising the cellulose, whereas temperatures >180 \({}^{\circ}\)C simultaneously cause substrate solubilisation and the formation of inhibitory compounds [17]. The combination of treatment temperature and retention time is an essential factor for deciding conditions for SE pretreatment. The severity factor (SF) is used to evaluate the intensity of the SE pretreatment [18; 19]. It helps to characterise the combined effects of temperature and time. The effect of SE pretreatment on the AD of different biomasses has been primarily investigated in the batch mode [20; 21; 22; 23; 24].