Anaerobic digestion is a complex process involving various chemical reactions such as hydrolysis, acidogenesis and methanogenesis by means of several bacterial strains industrially applied for bio-gas production. This process can be applied to a wide range of biomass types, especially those with high moisture content. There is a growing interest in the application of this technology to transformation of lignocellulosic biomass. Pure lignocellulosic biomass represents an under-utilised source for biogas and — ethanol production (Hendriks and Zeeman 2008), primarily due to the recalcitrance of lignocellulose to biological degradation. Hydrolysis of lignocellulosic materials is the first step for either digestion to biogas or fermentation to ethanol, but it is considered to be the most rate-limiting step (Lissens et al. 2004; Sanders et al. 2000). Pretreatments developed for alcoholic fermentation (see Sect. 7.3.1) can improve the efficiency of lignocellulose hydrolysis during anaerobic digestion, which leads to increased yields and productivity while decreas­ing residence times for lignocellulose digestion (Taherzadeh and Karimi 2008). Among physicochemical processes, steam pretreatment, lime pretreatment, liquid hot water pretreatment and ammonia based pretreatments have high potential for application to biogas production (Hendriks and Zeeman 2008). Steam explosion and thermal pretreatments are widely investigated for improving biogas production from different materials such as forest residues (Hooper and Li 1996). Liu et al. (2002) investigated and developed steam pressure disruption as a treatment step to render lignocellulose-rich, solid municipal waste more digestible, which would result in increased biogas yields. During most steam pretreatment processes small amounts of sugar degradation products are formed, with varying amounts of furfural, acetic acid, HMF and soluble phenolic compounds present in the pretreated lignocellulose. These by-products may be inhibitory to fermentation by methanogenic bacteria and thus methane production. The consortium of microorganisms involved in anaerobic digestion is however capable of adapting to overcome the effects of such com­pounds, even though there are limits to the adaptive potential. Bacterial adaptation to fermentation in the presence of inhibitory compounds was demonstrated by Benjamin et al. (1984), Fox et al. (2003), over several bacterial generations.

Dilute acid

CO2 hydrothermal treatment Autohydrolysis at Severity = 4.67 Dilute acid (160 °С. 0.75 % acid concentration for 10 min)

Acid hydrolysis (1.2 % H2SO4, 121 °С for 45 min) Delignification (60 % ethanol) at H = 125,001 Autohydrolysis at So = 4.67 Autohydrolysis and organosolv So = 3.64,

TD = 198, C = 60 kg ethanol/100 kg liquor

Hot water (160 °С for 30 min) and ball milling (BM) (20 min)

Two step liquid hot water (180 °С for 20 min followed by 200 °С for 20 min)

Steam explosion (25 atm for 3 min) and anaerobic fermentation

Steam explosion (210 °С for 4 min)

Dilute acid 180 °С. 0.8 % H2S04 for 15 min Hydrothermal followed by alkaline treatment

(NaOH concentration = 4.5 %, 130 °С for 3 h). Autohydrolysis

CSF = 1-1.6 93 % xylose recovery CSF = 2.48 73 % glucan conversion and 18 g/1 ethanol

80 % glucose yield

15.1 g/1 ethanol

82 % of total sugars

28.7 g/1 ethanol 35 g/1 ethanol 67.4 g/1 ethanol

18.1 kg of saccharides (as monomers and sugars) from first step. 17.9 kg of soluble lignin and 41.9 kg glucose from second step

70 % yield of total sugar with a cellulase loading of 4 FPU/g substrate (10 times reduction) total sugar recovery of 96.63 %

80 % conversion of cellulose into biogas

17 g/1 ethanol (62.5 % yield)

10.3 g/1 ethanol

47.3 g glucose/100 g solids from autohydrolysis 77-99 % xylose solubilization

SHF overall ethanol yield of 72 %, SSF 63 % of theoretical total (22 g/1)

25.1 % of monomeric sugars

Total sugar recovery of 66.1 %

41 % recovery of total theoretical glucose 50 % of the theoretical glucose 270 1/t wood

55-60 % digestibility providing 70 % of total theoretical yield