Separation of dispersed substances and galactoglucomannan in thermomechanical pulp process water by microfiltration

Several valuable substances are released from wood during the production of thermomechanical pulp. These substances can be isolated and purified for use as value-added chemicals. The hemicellulose galactoglucomannan (GGM) has attracted a great deal of interest because of its possible applications as an oxygen barrier film in packaging materials, as a hydrogel in biomedical products and as an emulsion stabilizer in food and feed. In order to exploit GGM in pulp mill process water, it must be recovered at a high concentration and high purity. The first step in the purification process is the separation of GGM and dispersed substances. In this work, the influence of membrane pore size, cross-flow velocity and backpulsing on membrane performance during the microfiltration of thermomechanical pulp process water was investigated. The flux of 0.4 and 0.8 μm membranes was lower than the flux of a 0.2 μm membrane, and it was not possible to recover the pure water flux of the 0.4 and 0.8 μm membranes. Increased cross-flow velocity and backpulsing had a positive effect on the flux, but only a minor influence on the retention. The mean molecular mass of GGM in the process water was 9 kg/mol. The retention was therefore expected to be <10%. However, the retention of GGM was >50%, irrespective of the operating conditions.

► Dispersed and dissolved substances in thermomechanical pulp waste water were separated by microfiltration.
► Significant difference in mean molecular mass of galactoglucomannan (9 kg/mol) and lignin (0.5 kg/mol).
► High retention of galactoglucomannan (>40%).
► The high retention is not due to cake filtration.