Nanocellulose from wastewater solids

Abstract

Cellulose is a biopolymer commonly used as a renewable constituent in the production of green materials. Wastewater solids can be an attractive source of cellulose, mainly originating from toilet paper. Cellulosic solids can be recovered (i) through sieving before the sewage treatment - resulting in a Fine Sieve Fraction (FSF), or (ii) after biological treatment in the waste sludge – such as in Nereda® Excess Sludge (NES). Its recovery and valorisation into high-end bio-based products, such as crystalline nanocellulose, could improve the techno-economic feasibility of cellulose recovery from wastewater. A cellulose extraction method from wastewater solids was developed during this project. Reference materials (microcrystalline cellulose and toilet paper) and wastewater-extracted cellulosic pulps were studied for the production of cellulose nanocrystals (CNCs). CNCs were prepared by controlled acid hydrolysis followed by centrifugation, filtration, dialysis, neutralisation and ultrasonication. A detailed protocol for CNCs isolation from toilet paper and wastewater-recovered pulps can be found in the appendix section. CNCs could be extracted at around 30 % yield (g g pulp-1). FSF showed to be an attractive source of nanocellulose since the yield per g FSF was 22 %, while for NES it was only 4 %. The wastewater-CNCs presented rod-like morphology with high aspect ratio (10 – 14), crystallinity (62 – 68 %), and chemical structure very similar to commercially available CNCs. However, there is still great extent for improvements since impurities might be still present and the isolation process needs reproducibility and optimization. Bionanocomposites consisting of CNCs (commercial and lab-made) and alginate materials were investigated by measuring the stiffness of freestanding films. A 50 % loading of commercial-CNCs reinforced alginate films by a factor of 1.6 (19 GPa). For the same loading, CNCs isolated from toilet paper and FSF reinforced alginate by a factor of 1.8 and 1.5 (22 – 18 GPa) respectively. Both characterisation results and reinforcement efficiency of CNCs isolated from toilet paper and wastewater-recovered pulps confirm the good quality of this potential product. Furthermore, we have observed changes in the nematic-isotropic phase diagram of CNCs in alginate suspensions. We developed a model hypothesizing that an alginate-shell decorates the CNCs rods increasing its volume fraction and allowing the nematic phase to appear at lower concentrations. The model was highly representative to the experimental data. Lastly, the market relevance of wastewater-CNCs is discussed together with a research outlook.