# Thermophilic Methane Production from Hydrothermally Pretreated Norway Spruce (_Picea abies_)  
[PERSON]  
1 Department of Process, Energy and Environmental Technology, University of South-Eastern Norway, Kjolnes Ring 56, NO 3918 Forsgrunn, Norway; [EMAIL_ADDRESS] (R.B. ); [EMAIL_ADDRESS] (W.H.B. )1 Department of Process, Energy and Environmental Technology, University of South-Eastern Norway, Kjolnes Ring 56, NO 3918 Forsgrunn, Norway; [EMAIL_ADDRESS] (R.B. ); [EMAIL_ADDRESS] (W.H.B. )1 Department of Process, Energy and Environmental Technology, University of South-Eastern Norway, Kjolnes Ring 56, NO 3918 Forsgrunn, Norway; [EMAIL_ADDRESS] (R.B. )1 Department of Process, Energy and Environmental Technology, University of South-Eastern Norway, Kjolnes Ring 56, NO 3918 Forsgrunn, Norway; rune. advantages and is widely accepted as a green technology as it involves only lignocellulosic biomass and water and avoids corrosion problems, acid recycling, and formation of neutralization sludge [3; 4; 5]. HWE, usually carried out at temperatures between 120 \({}^{\circ}\)C and 230 \({}^{\circ}\)C and various pressure conditions for different retention times, effectively dissolves hemicellulose sugars which may be beneficial to the anaerobic digestion in theory [6]. This liquid hydrolysate is rich in oligomeric and monomeric products like xylose, glucose, mannose, arabin, and galactose [7] and is a suitable substrate for AD [8]. AD on hydrolysate, instead of AD on the original lignocellulosic, overcomes problems related to traditional solid-state (SS) or semi SS-AD such as higher retention time, poor biodegradation, low methane yield, and acidification [9]. In addition to the hemicellulose sugars in the liquid hydrolysate, inhibiting compounds are also formed due to degradation of sugar molecules during HWE [10] which usually inhibit bacteria and archea and operating parameters of the pretreatment need to be optimized to reduce their formation [11]. The nature and concentration of such inhibiting compounds depend upon the condition of pretreatment of the raw material types [12]. The main inhibitors are furfural and 5-hydroxymethylfurfural (HMF) formed by the dehydration of pentose and hexose sugars respectively in addition to degradation products of lignin polymer such as phenol, cresol, syringaldehyde, and vanillin [12]. Temperature plays a crucial role in AD as increased temperature leads to increased reaction rate in biochemical systems [13]. Thermophilic AD (55-60 \({}^{\circ}\)C) is considered a highly-efficient system due to a better pathogen inactivation and enhanced biogas production rate compared to mesophilic AD (35-40 \({}^{\circ}\)C) [13]. However, several studies have reported that thermophilic AD is susceptible to VFAs accumulation (especially propionic acid), inhibiting the methanogenic activity and decreasing the pH-buffer system [14; 15; 16]. Besides the operating condition, a higher proportion of feeding can also influence the rate of anaerobic digestion. The organic loading (OL) of the reactor with a reasonable amount of inoculum, also called substrate-to-inoculum ratio (S/I), is an important parameter when estimating methane potential [17]. High OL leads to VFA accumulation inhibiting the methanogens, thus lowering the amount of methane produced. On the contrary, lower OL cannot provide enough nutrition for microorganism growth, thus hampering the AD process [18]. Although hot water extracted hydrolysates of several agricultural residues and energy crops have been anaerobically digested to produce methane [11; 19; 20], AD of woody biomass hydrolysate (especially spruce) is still lacking. Among the available experiments, very few [21] are conducted in a thermophilic condition although it would save energy cost cooling the hydrolysates prepared at high temperatures. It is also imperative to test AD of hydrolysate prepared at different pretreatment conditions to have an overview of the effects of possible inhibitors in methane production due to the severity of the pretreatment.