Investigations of Alkaline and Enzymatic Membrane Cleaning of Ultrafiltration Membranes Fouled by Thermomechanical Pulping Process Water

The pulp and paper industry is one of the most important industrial sectors worldwide, and has considerable potential for the sustainable fractionation of lignocellulosic biomass to provide valuable compounds. Ultrafiltration (UF) is a suitable separation technique for the profitable production of hemicelluloses from process water from thermomechanical pulping (ThMP), but is limited by membrane fouling. Improvements in cleaning protocols and new alternative cleaning agents are required to ensure a long membrane lifetime, and thus a sustainable process. This study, therefore, focuses on the cleaning of polymeric UF membranes after the filtration of ThMP process water, comparing alkaline with enzymatic cleaning agents. The aim was to develop a cleaning procedure that is efficient under mild conditions, resulting in a lower environmental impact. It was not possible to restore the initial permeability of the membrane when cleaning the membrane with enzymes alone, but the permeability was restored when using a two-step cleaning process with enzymes in the first step and an alkaline cleaning agent in the second step. Scanning electron microscopy gave a deeper inside into the cleaning efficiency. Attenuated total reflectance Fourier-transform infrared spectroscopy analysis confirmed that not only polysaccharides, but also extractives are adsorbed onto the membrane surface.

The effect of delignification on the properties of cellulosic fiber material

The behavior of pressed poplar chemi-thermomechanical pulping (CTMP) without additive the focus of our study. Four CTMPs with decreasing lignin contents were prepared by the sodium chlorite/acetic acid method and the holocellulose, α-cellulose, pentosan and Klason lignin contents of the delignified CTMPs were determined. The surface composition, aggregation structure and microstructure of the delignified fibers were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscope (SEM), and the mechanical properties of the fiber material were investigated by means of tensile and bending tests. As shown by XPS, the lignin content of the Pr-CTMP surface layer firstly increased and then decreased as the lignin content of CTMP decreased. With the delignification time increased from 0 to 240 min, the crystallinity index (CrI) of CTMP increased from 60.1% to 65.7%. TheCrIof all CTMPs at different delignification degrees showed significant elevated values after hot-pressing. The fiber cell wall became thinner and the cells were flattened and thus elevated the contact area among fibers and, as a consequence, the density of material gradually increased at higher delignification degrees. The tensile strength increased by ca. 10%, when the lignin content decreased from 24.9% to 13.1%, and by ca. 53%, when the lignin content decreased from 13.1% to nearly zero. The bending strength increased with increasing delignification. When the removal rate of lignin increased from 47% to 54%, the bending strength increased from 101 to 122 MPa.