Fouling and cleaning of membranes in biorefineries

Wood components such as cellulose, lignin and hemicelluloses all have potential to be used in biobased value-added products. Once extracted from the wood these components have to be separated and purified from co-extracted compounds. Membrane filtration can be used to achieve this separation and is believed to play an important role in biorefineries.

Fouling is the single greatest obstacle to the introduction of membrane processes in biorefineries. Fouling affects membrane performance in several ways: it reduces flux, alters separation properties and necessitates frequent cleaning which shortens membrane life-time. Cleaning, used to recover flux, needs to be optimized in order to reduce downtime costs. As a rule of thumb, alkaline cleaning agents are used to remove organic matter and acid cleaning agents are used to remove inorganic scalants. The complex nature of process streams in biorefineries complicates the choice of cleaning agents in these applications. Lignin, extractives, polysaccharides and inorganics are all potential foulants.

This work illustrates the importance of choosing the right cleaning agent, and the right sequence of cleaning agents when multistep cleaning is used. A combination of microfiltration (MF) and ultrafiltration (UF) was used to isolate hemicelluloses in thermomechanical pulp (TMP) mill process water. Suspended and colloidal matter was removed in the MF stage and hemicelluloses were concentrated and purified in the UF stage. Organic compounds are the main constituents of the process water and, hence, alkaline cleaning is assumed to be the most efficient cleaning method in this case. However, in this work it is shown that alkaline cleaning is not necessarily the most effective cleaning method.

The influence of different cleaning agents on flux recovery when cleaning MF and UF membranes after treatment of TMP process water was investigated. Methods to measure fouling and determine efficiency of different cleaning agents on flux recovery are presented.