Pretreatment of forest residues using N - methylmorpholine - N - oxide (NMMO or NMO) prior to anaer obic digestion was investigated , where the effects of particle size, NMMO concentration , and pretreatment time were the primary focus. The pretreatments were carried out on forest residues; with different particle size s of 2, 4 and 8 mm , at 120 °C for 3, 7 , and 15 h in two different modes of NMMO - treatment : dissolution by 85% NMMO and swelling without dissolution using 75% NMMO solution in water . The pretreatment process led to minor changes in the composition of the forest residues . The best improvement in methane yield of the forest residues was achieved by pretreatment using 85% NMMO for 15 h at 120 °C. This treatment resulted in 0.1 7 Nm 3 /kg VS methane yield , which corresponds to 83 % of the expected theoretical yield of carbohydrates present in the material. Additionally, the accumulated methane yield and the rate of the methane production were highly affected by the amounts of remaining NMMO when it was not well separated during the washing and filtration step s after the treatment. The p resence o f concentrations even as low as 0.008 % NMMO resulted in a decrease in the final methan e yield by 45% , while the presence of 1% of this solvent in the digester completely terminated the anaerobic digestion process.

Methane production from two types of wool textile wastes (TW1 and TW2) was investigated. To improve the digestibility of these textiles, different pretreatments were applied, and comprised thermal treatment (at 120 ◦C for 10 min), enzymatic hydrolysis (using an alkaline endopeptidase at different levels of enzymatic loading, at 55 ◦C for 0, 2, and 8 h), and a combination of these two treatments. Soluble protein concentration and sCOD (soluble chemical oxygen demand) were measured to evaluate the effectivity of the different pretreatment conditions to degrade wool keratin. The sCOD as well as the soluble protein content had increased in both textile samples in comparison to untreated samples, as a response to the different pretreatments indicating breakdown of the wool keratin structure. The combined treatments and the thermal treatments were further evaluated by anaerobic batch digestion assays at 55 ◦C. Combined thermal and enzymatic treatment of TW1 and TW2 resulted in methane productions of 0.43 N m3/kg VS and 0.27 N m3/kg VS, i.e., 20 and 10 times higher yields, respectively, than that gained from untreated samples. The application of thermal treatment by itself was less effective and resulted in increasing the methane production by 10-fold for TW1 and showing no significant improvement for TW2.

Utilisation of wheat straw and wool textile waste in dry anaerobic digestion (AD) process was investigated. Dry-AD of the individual substrates as well as co-digestion of those were evaluated using different total solid (TS) contents ranging between 6 to 30%. Additionally, the effects of the addition of nutrients and cellulose- or protein-degrading enzymes on the performance of the AD process were also investigated. Dry-AD of the wheat straw resulted in methane yields of 0.081 – 0.200 Nm3CH4/kgVS with the lowest and highest values obtained at 30 and 21% TS, respectively. The addition of the cellulolytic enzymes could significantly increase the yield in the reactor containing 13% TS (0.231 Nm3CH4/kg VS). Likewise, degradation of wool textile waste was enhanced significantly at TS of 13% with the addition of the protein-degrading enzyme (0.131 Nm3CH4/kg VS). Furthermore, the co-digestion of these two substrates showed higher methane yields compared with the methane potentials calculated for the individual fractions at all the investigated TS contents due to synergetic effects and better nutritional balance.

The effects of N-methylmorpholine-N-oxide (NMMO) pretreatment on barley straw and forest residues were investigated for biogas production. The pretreatments were performed at 90 °C with 85% NMMO for 3–30 h. The best pretreatment conditions resulted in 100% improvement in methane yield during the subsequent digestion compared to that of the untreated lignocelluloses. Methane yields of 0.23 and 0.15 Nm3 CH4/kg VS were obtained from barley straw and forest residues, respectively, corresponding to 88% and 83% of the theoretical yields. In addition, the effects of the pretreatment with recovered and reused NMMO was also studied over the course of five cycles. Pretreatment with recycled NMMO showed the same performance as the fresh NMMO on barley straw. However, pretreatment of forest residues with recycled NMMO resulted in 55% reduction in methane yield.

The methane potential of forest residues was compared after applying organic solvent, i.e., acetic acid, ethanol, and methanol pretreatments using batch anaerobic digestion (AD). The pretreatments were performed at 190 °C with 50% (V/V) organic solvent for 60 min. The accumulated methane yields after 40 days of AD from pretreated forest residues were between 0.23 and 0.34 m3 CH4/kg VS, which shows a significant improvement compared to 0.05 m3 CH4/kg VS, from untreated forest residues. These improvements count up to 50% increase in the methane yields from the pretreated substrates based on expected theoretical yield from carbohydrates. Among the organic solvents, pretreatments with acetic acid and ethanol led to highest methane yields, i.e., over 0.30 m3 CH4/kg VS. However, techno-economical evaluation showed, pretreatment with methanol was more viable financially. The capital investments of the plant operating 20,000 tons of forest residues varied between 56 and 60 million USD, which could be recovered in less than 8 years of operation.