Circular Economy Approaches for Sustainable Agri-Residue Management: Integrating Pyrolysis and Biochemical Conversion for Developing Zero-Waste Fiber Production Processes
Abstract
An ever-increasing population has directed an enormous growth in agricultural activities to ensure food security. This has led to higher agricultural residue production worldwide. The effective and resourceful management of these residues remains a challenge to this day. With the increasing population, there has also been a higher demand for textiles which are mostly fulfilled by fossil fuel derived synthetic fibers. This further results in the generation of toxic effluents, hence becoming an environmental liability. This project deals with the development of zero-waste methods for the management of agri-residues, by integrating the thermochemical treatment of the agri-residues with the biochemical process of fiber production.
In the first stage of the project, various agri-residues, such as wheat straw, hemp bast fibers, banana short fibers and agave straw, were pyrolyzed in batch and continuous processes. The products derived from the pyrolysis process - bio-oil, biochar and gas, were analyzed and compared. Wheat straw bio-oil had the maximum acetic acid concentration and was chosen for further application. In the second stage, the retting (biological degumming) process of decorticated pineapple leaf fibers (PALF) was investigated. The activities of cellulase, xylanase and pectinase enzymes were monitored, and process conditions were optimized to reduce retting time and improve efficiency. It was found that the addition of bio-oil, to adjust the initial pH of the system, along with bubbling (aeration) improved the efficiency of the process by reducing the time from 18 days to 4 days. In addition, the pectin and wax content of the fibers reduced by 55% and 36.5%, respectively, in comparison to the traditional retting where the wax content reduced only by 13.6%. The fibers were further processed to increase the alpha cellulose content, by steam explosion in the presence of an alkali. Biochar extracted water, with a pH of 12.8, was used to replace NaOH, which is traditionally used in the fiber treatment process. The fibers had a crude cellulose content of 87.21%, which was comparable to the NaOH treated fibers. Additionally, the crystallinity index and SEM images show that the biochar extracted water was a suitable alternative. Additionally, the effluent from the retting process was pH corrected using biochar, to be used for hydroponic growth of leafy greens. The results suggested that the treatment of the effluent did not have any negative effects on arugula growth when diluted 30:70 (v/v) with fertilizer solution. The biochar remaining from the effluent treatment was further utilized as a germination medium for the hydroponic system to replace rockwool. Finally, a qualitative life cycle assessment showed the benefits of the proposed scheme in comparison to the conventional treatment of agri-residues.
Overall, this project demonstrates the valorization and resourceful application of all waste streams generated during the process hence successfully demonstrating the implementation of a circular economy approach through a zero-waste process.