The current fossil-based economy must be transformed into a bio-based circular economy. A cornerstone of this transformation is the conversion of pulp and paper mills into lignocellulosic biorefineries that allow not only the production of pulp, paper, and electricity, but also novel biochemicals. Highly selective, electrifiable, and low-energy-consuming separation technologies will be key in the complete and economically feasible utilization of valuable compounds from lignocellulosic biomass such as wood. These requirements are met by the pressure-driven membrane processes microfiltration, ultrafiltration (UF), nanofiltration and reverse osmosis. The greatest challenge to the broad implementation of membrane processes in lignocellulosic biorefineries is membrane fouling. Membrane fouling not only alters the capacity and selectivity of the membrane, but also increases the investment and operational costs of the separation process. Membrane fouling can often only be overcome by chemical cleaning. Improvements in chemical cleaning processes would result in a reduction in operational costs and lessen the environmental impact of the process.
In the work presented here, the fouling and cleaning of membranes used for the UF of process streams from lignocellulosic biorefineries were investigated. This was done with the help of ex situ and in situ analytical methods.
Suitable in situ real-time monitoring techniques for this task were identified and evaluated through a literature review and a survey carried out among industrial membrane users and analytical equipment suppliers. Membrane fouling was studied ex situ using techniques such as scanning electron microscopy, atomic force microscopy, attenuated total reflectance-Fourier transform infrared spectroscopy, and Brunauer-Emmett-Teller surface analysis. Quartz crystal microbalance with dissipation monitoring was used for in situ real-time adsorption studies. UF of thermomechanical pulping process water with the commercially available UFX5-pHt membrane from Alfa Laval was used as a reference system. This process water contained hemicelluloses, lignin, wood extractives, and inorganic residues. UF of other solutions associated with lignocellulosic biorefineries, such as black liquor from Kraft pulping, hot water extract from sawdust, and bleach plant effluent from sulfite pulping, were also investigated. For the reference system, it was found that especially hemicelluloses were immediately adsorbed on the membrane, forming a thin rigid fouling layer. This layer became thicker and softer over time, due to the incorporation of droplets of colloidal extractive stabilized by hemicelluloses, until equilibrium was reached. After reaching equilibrium, water was lost from the fouling layer, and it became thinner and more rigid again. It was also found that foulants entered the inner structure of the membranes, blocking the pores or being adsorbed on the pore walls. Rinsing with water revealed that a very thin fouling layer consisting not only of hemicelluloses, but also of wood extractive residues such as fatty and resin acids, remained adsorbed on the membrane polymer. Alkaline cleaning with a commercial cleaning agent removed irreversible fouling, and the initial membrane permeability was recovered. However, this was only possible after one filtration cycle. When the same membrane was used for several filtration cycles and subjected to several cleaning cycles, alkaline cleaning was no longer effective. A home-made enzyme cocktail was investigated as an alternative and more environmentally sound cleaning agent. The membrane permeability was not completely recovered when this cocktail was used alone, but promising results were obtained when it was used in combination with the alkaline cleaning agent.
Overall, the findings presented in this thesis will help improve membrane filtration processes in lignocellulosic biorefineries. It was shown that detailed analysis of membrane fouling allowed the processes leading to reductions in capacity and selectivity during UF of process streams to be identified. Comprehensive knowledge on the causes of membrane fouling will make it easier to tailor membrane cleaning processes. This will prolong the membrane lifetime, and reduce plant downtime and the usage of chemicals and water, resulting in a reduction in process costs and a more environmentally sound process. The findings of this work will also contribute to the wider implementation of membrane processes in the transition from a fossil-based economy to a fossil-free bio-based circular economy.