Bio-adhesives derived from sewage sludge via hydrothermal carbonization: Influence of aqueous phase recycling

Abstract

This paper describes the preparation of formaldehyde-free plywood bio-adhesive using hydrochar from sewage sludge in the presence (and absence) of aqueous phase containing a high content of inorganic matter. Aqueous phase recycling was observed to enhance the removal of low molecular weight proteins and the retention of large molecular weight proteins in hydrochar. The hydrochar-based bio-adhesive, incorporating the aqueous phase, exhibits excellent wet shear strength, meeting the Chinese national standard GB/T 9846-2015 (≥0.7 MPa). The wet shear strength of the bio-adhesive with tap water surpasses that of the bio-adhesive with the aqueous phase. During hot-pressing, both low molecular weight proteins in the aqueous phase and large molecular weight proteins in hydrochar participate in existing reactions including those between inter-components in bio-adhesives and bio-adhesives/plywood. The presence of low molecular weight proteins in the aqueous phase negatively affects wet shear strength and water resistance. The formation of covalent bonds, such as C–N and C–O–C, emerges as the key factor to enhance the wet shear strength of bio-adhesives. This study presents a novel approach for the valorization of sewage sludge, particularly the aqueous phase.