The present study focused on conversion of cellulosic part of waste textiles into biogas and its challenges. The annual global fiber consumption exceeded 70 Mt with a cellulosic fraction of around 40%. This huge amount of fiber is further processed into apparel, home textiles and industrial products and after a certain time delay end up in waste streams. This amount of cellulose has the potential of production of approximately 20 billion liters of ethanol. Assuming a good collection and waste management system, however, there are still challenges facing the process of conversion. For instance, high crystallinity of cotton cellulose makes it hard to achieve enzymatic or bacterial hydrolysis. In addition, waste textiles are composed of different materials including natural and synthetic fibers, and the cellulosic fibers should be separated from the other materials. Furthermore, presence of dyes and reagents in the fibers can also be challenging in the bioprocessing of textile waste. In the present work, we examined the process of dilute acid hydrolysis of viscose and cotton (i.e. jeans) textiles. Hydrolyses were performed at different lengths of time (8 and 15 min), temperatures (180 and 200 °C), and acid concentrations (0.5, 1.5, and 3% w/w). Hydrolysis of viscose and jeans under identical conditions resulted in significantly different yields of glucose. This may be due to differences in the structure, i.e. high crystalline cellulose in jeans and low crystalline cellulose in viscose.

The changes of the reactivity of cellulose during enzymatic hydrolysis and the adsorption capacity of cellulase into insoluble cellulose are two central features of several mechanistic models presented for enzymatic hydrolysis of cellulose. In the present study, these parameters were investigated for regenerated cellulose, i.e. amorphous cellulose. The regenerated pure cellulose was obtained by dissolution cotton linter in N-Methylmorpholine-N-oxide (NMMO) at 120 °C, and then regeneration by adding water. The data of adsorption of cellulase into cellulose was examined with the Langmuir isothermal model. The maximum adsorption capacity was obtained as 212, 35, and 5 mg protein/g substrate for regenerated cellulose, Avicel, and cotton, respectively. The substrate reactivity (SR) was measured while the effect of end product inhibition and also enzyme inactivation during enzymatic hydrolysis was eliminated. The SR of regenerated cellulose was declined from 1 to 0.43 after conversion of 30% of substrate in 15 min. Then, there was no significant change in the SR until conversion of 92% of cellulose which was achieved by 6 h hydrolysis. This study shows that by regeneration of cellulose its capacity of cellulase adsorption is dramatically increased. However, the effect of reactivity of cellulose on the rate of hydrolysis is eliminated and SR may be deleted from its hydrolysis kinetic model.

This work deals with conversion of four construction and demolition (C&D) lignocellulosic wastes including OSB, chipboard, plywood, and wallpaper to ethanol by separate enzymatic hydrolysis and fermentation (SHF). Similar to other lignocelluloses, the wastes were resistant to the enzymatic hydrolysis, in which only up to 7% of their cellulose was hydrolyzed. Therefore, the lignocellulosic wastes were treated with phosphoric acid, sodium hydroxide, or N-methylmorpholine-N-oxide (NMMO), which resulted in improving the subsequent enzymatic hydrolysis to 38.2–94.6% of the theoretical yield. The best performance was obtained after pretreatment by concentrated phosphoric acid, followed by NMMO. The pretreated and hydrolyzed C&D wastes were then successfully fermented by baker’s yeast to ethanol with 70.5–84.2% of the theoretical yields. The results indicate the possibility of producing 160 ml ethanol from each kg of the C&D wastes at the best conditions.

Pretreatment of lignocelluloses with cellulose dissolution reagents is efficient and can be applied under relatively mild conditions. NMMO is an industrial cellulose solvent that can dissolve cellulose by breaking intramolecular bonds. The solvent can be recycled with over 99% recovery and does not produce toxic waste pollutants. After dissolution, the cellulose can be regenerated by fast precipitation with an anti-solvent that is usually water. The dissolution can severely modify the structure of cellulose and reduce its crystallinity, which is very important in hydrolysis of softwoods. Native species of spruce was debarked, cut, milled, and screened to achieve a size of less than 1 mm. The treatment was performed using 85%w/w NMMO solution at 120ºC for 1, 3, and 15 h. The pretreated wood species were then regenerated by addition of boiling distilled water, followed by vacuum filtration and washing. The pretreated and untreated wood species were enzymatically hydrolyzed by commercial cellulase (15 FPU/g) and β-glucosidase (30 IU/g) at 45°C for 96 h. Then, the hydrolyzates were fermented by a flocculation strain of Saccharomyces cerevisiae (CCUG 53310) at 30°C for 24 h. The results showed that the pretreatment, in general, did not significantly affect the composition of the wood, while increased the yield of hydrolysis and fermentation. The cellulose hydrolysis was increased from 11% for native spruce to more than 98% for the wood treated with NMMO for 15 h, and, correspondingly, the yield of ethanol production was increased from 8.1% to over 86.1%.

The work of the present thesis focused on conversion of the cellulosic part of waste textiles into biogas and ethanol, and its challenges. In 2009, the global annual fiber consumption exceeded 70 Mt, of which around 40% consisted of cellulosic material. This huge amount of fibers is processed into apparel, home textiles, and industrial products, ending up as waste after a certain time delay. Regretfully, current management of waste textiles mainly comprises incineration and landfilling, in spite of the potential of cellulosic material being used in the production of ethanol or methane. The volume of cellulose mentioned above would be sufficient for producing around 20 billion liters of ethanol or 11.6 billion Nm3 of methane per year. Nevertheless, waste textiles are not yet accepted as a suitable substrate for biofuel production, since their processing to biofuel presents certain challenges, e.g. high crystallinity of cotton cellulose, presence of dyes, reagents and other materials, and being textiles as a mixture of natural and synthetic fibers. High crystallinity of cotton cellulose curbs high efficient conversion by enzymatic or bacterial hydrolysis, and the presence of non-cellulosic fibers may create several processing problems. The work of the present thesis centered on these challenges. Cotton linter and blue jeans waste textiles, all practically pure cellulose, were converted to ethanol by SSSF, using S. cerevisiae, with a yield of about 0.14 g ethanol/g textile, only 25% of the theoretical yield. To improve the yield, a pretreatment process was required and thus, several methods were examined. Alkaline pretreatments significantly improved the yield of hydrolysis and subsequent ethanol production, the most effective condition being treatment with a 12% NaOH-solution at 0 °C, increasing the yield to 0.48 g ethanol/g textile (85% of the theoretical yield). Waste textile streams, however, are mixtures of different fibers, and a separation of the cellulosic fibers from synthetic fibers is thus necessary. The separation was not achieved using an alkaline pretreatment, and hence another approach was investigated, viz. pretreatment with N-methyl-morpholine-N-oxide (NMMO), an industrially available and environment friendly cellulose solvent. The dissolution process was performed under different conditions in terms of solvent concentration, temperature, and duration. Pretreatment with 85% NMMO at 120 °C under atmospheric pressure for 2.5 hours, improved the ethanol yield by 150%, compared to the yield of untreated cellulose. This pretreatment proved to be of major advantage, as it provided a method for dissolving and then recovering the cellulose. Using this method as a foundation, a novel process was developed, refined and verified, by testing polyester/cellulose-blended textiles, which predominate waste textiles. The polyesters were purified as fibers after the NMMO treatments, and up to 95% of the cellulose content was regenerated. The solvent was then recovered, recycled, and reused. Furthermore, investigating the effect of this treatment on anaerobic digestion of cellulose disclosed a remarkable enhancement of the microbial solubilization; the rate in pretreated textiles was twice the rate in untreated material. The overall yield of methane was, however, not significantly affected. The process developed in the present thesis appears promising for transformation of waste textiles into a suitable raw material, to subsequently be used for biological conversion to ethanol and biogas.

A novel process has been developed for separation of the cellulose, i.e. cotton and viscose, from blended-fibers waste textiles. An environmentally friendly cellulose solvent, N-methylmorpholine-N-oxide (NMMO) was used in this process for separation and pretreatment of the cellulose. This solvent was mixed with blended-fibers textiles at 120 °C and atmospheric pressure to dissolve the cellulose and separate it from the undissolved non-cellulosic fibers. Water was then added to the solution in order to precipitate the cellulose, while both water and NMMO were reused after separation by evaporation. The cellulose was then either hydrolyzed by cellulase enzymes followed by fermentation to ethanol, or digested directly to produce biogas. The process was verified by testing 50/50 polyester/cotton and 40/60 polyester/viscose-blended textiles. The polyesters were purified as fibers after the NMMO treatments, and up to 95% of the cellulose fibers were regenerated and collected on a filter. A 2-day enzymatic hydrolysis and 1-day fermentation of the regenerated cotton and viscose resulted in 48 and 50 g ethanol/g regenerated cellulose, which were 85% and 89% of the theoretical yields, respectively. This process also resulted in a significant increase of the biogas production rate. While untreated cotton and viscose fibers were converted to methane by respectively, 0.02% and 1.91% of their theoretical yields in 3 days of digestion, the identical NMMO-treated fibers resulted into about 30% of yield at the same period of time.

Alkaline pretreatment with NaOH under mild operating conditions was used to improve ethanol and biogas production from softwood spruce and hardwood birch. The pretreatments were carried out at different temperatures between minus 15 and 100ºC with 7.0% w/w NaOH solution for 2 h. The pretreated materials were then enzymatically hydrolyzed and subsequently fermented to ethanol or anaerobically digested to biogas. In general, the pretreatment was more successful for both ethanol and biogas production from the hardwood birch than the softwood spruce. The pretreatment resulted in significant reduction of hemicellulose and the crystallinity of cellulose, which might be responsible for improved enzymatic hydrolyses of birch from 6.9% to 82.3% and spruce from 14.1% to 35.7%. These results were obtained with pretreatment at 100°C for birch and 5°C for spruce. Subsequently, the best ethanol yield obtained was 0.08 g/g of the spruce while pretreated at 100°C, and 0.17 g/g of the birch treated at 100°C. On the other hand, digestion of untreated birch and spruce resulted in methane yields of 250 and 30 l/kg VS of the wood species, respectively. The pretreatment of the wood species at the best conditions for enzymatic hydrolysis resulted in 83% and 74% improvement in methane production from birch and spruce.