Membrane fouling and cleaning in lignocellulosic biorefineries

Introduction
The pulp and paper sector is a key global industry. Presently, pulp mills prioritize the production of pulp, fibers, and electricity. By transforming traditional pulp mills into lignocellulosic biorefineries, they can become a key element of a future, circular bioeconomy. To accomplish this, pulp mills need to close their loops and optimize the use of the lignocellulosic feedstock, not just for pulp and fiber production but also for co-production of biochemicals, biofuels, and other high-value materials. Membrane processes can play an important role in this transition as electrifiable, low energy and high-selective separation processes. A key challenge and important cost factor when using membrane processes is membrane fouling and subsequent membrane cleaning. In this lecture two studies on membrane fouling and cleaning with reference to two process streams in the pulping industry will be presented.
Bleaching plant effluent from sulphite pulping process
In this study tubular polymeric UF membrane used for several months for concentration of bleach plant effluent in a magnesium sulphite pulp mill were investigated. The initial analysis of the fouling layer on the membrane was conducted in combination of scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and attenuated total reflection Fourier transformed infrared spectroscopy (ATR-FTIR). Additionally, high-performance anion-exchange chromatography coupled with pulsed amperometric detection (HPAEC-PAD) and heteronuclear single quantum coherence 2D nuclear magnetic resonance spectroscopy (HSQC-2D-NMR) were used. The comprehensive analysis showed that a 4.5 µm thick fouling layer consisting mainly of magnesium had been formed on the membrane during its lifespan. Based on this, alkaline membrane cleaning was tested on samples of the fouled membrane and SEM-EDS analysis was conducted before and after cleaning. Before the analysis the membrane samples were placed for 1h and 20 h in the alkaline cleaning solution. The subsequent analysis showed that a 1 h exposure to the cleaning agent had hardly any impact on the fouling layer, while after 20 h the fouling layer nearly completely disappeared. Hence, this study underlined the importance of contact time in membrane cleaning.
Process water from a thermo-mechanical pulp mill
The second study is related to the development of a process for the concentration and purification of hemicellulose in a thermomechanical pulp mill process water. Flat-sheet polymeric UF membranes fouled and cleaned during lab-scale conditions were analyzed using various techniques such as SEM, ATR-FTIR, Brunauer-Emmett-Teller nitrogen adsorption desorption analysis (BET) and Quartz crystal microbalance with dissipation monitoring (QCM-D). The studies revealed among others that membrane fouling resulted in gel layer formation, blocking and pore narrowing. It was further shown that the hemicelluloses i.e. galactoglucomannan were key components of the membrane fouling layer. A cleaning study using alkaline and enzymatic cleaning showed that alkaline cleaning was very efficient to remove the following layer but in the longer term a regular additional enzymatic cleaning could boost the membrane capacity over the membrane lifetime.
Conclusions and outlook
Both fouling and cleaning are important for the sustainable implementation of membrane processes in lignocellulosic biorefineries. These two studies revealed the importance of cleaning solution, contact time and cleaning chemical sequency. Overall, this work will support the development of new cleaning protocols for membrane processes in lignocellulosic biorefineries and thus the establishment of membrane processes in the industry.
Acknowledgements
This work is related to Task XVII: “Membrane Processes in Biorefineries” of the IEA Technology Collaboration Programme: “Industrial Energy-Related Technologies and Systems (IETS-TCP)”.