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Fungal and enzyme treatment of mechanical pulp and paper mill white water : impact on white water, fiber, and paper properties

University of British Columbia

Growing economic and environmental concerns have resulted in a move to reduce fresh water utilization at mechanical pulp and paper mills. This involves retaining and reusing process waters, which consequently provides benefits such as lower effluent treatment costs, energy savings, and improved environmental performance. However, mills experience major problems when attempting water system closure, such as the accumulation of dissolved and colloidal substances (DCS) within the water system. The buildup of DCS can lower paper quality, increase rates of corrosion, and reduce paper machine runnability. Consequently, efficient and cost effective treatment strategies are required for the removal of these contaminants in order for mills to achieve effective water system closure. Unfortunately, existing treatment technologies are far from ideal, and as a result, fresh water usage at mills remains high. Our group has previously demonstrated the potential of a fungal and enzyme treatment strategy for the removal of DCS from mill process waters. To further the development of this treatment strategy, the work conducted in this thesis initially focused on fungal growth and enzyme production, followed by an evaluation of enzyme thermostability. Fungal growth was then carried out in a larger scale (18 L) bioreactor to identify potential changes to extracellular enzyme production as a result of scale-up. The enzymes produced in both cases were used to treat fresh white water and pulp from a mechanical pulp and paper mill, and changes to white water, fiber, and paper properties were determined. The white-rot fungus, Trametes versicolor, utilized in this treatment strategy did not grow well at 45 or 60°C, and the production of cellulolytic, hemicellulolytic, lipolytic, and oxidative enzymes were significantly reduced when compared to the values detected after growth at 30°C. However, these same enzymes were found to maintain substantial percentages of their original activity when incubated at 65°C for extended periods of time. The scale-up of fungal growth in a bioreactor produced very comparable enzyme activities to those determined previously in shake flask cultures. Enzymatic treatment of fresh white water and pulp resulted in changes to both white water and fiber properties. The average colloidal particle size within the treated white water was reduced when compared to untreated white water, while the average molecular weight of the phenolic compounds present in the white water increased. Additionally, the average zeta potential of the colloidal particles was decreased in the treated water, indicating reduced colloidal particle stability. The changes made to the white water contaminants as a result of the enzymatic treatments significantly enhanced DCS removal by precipitation when an alum post treatment step was employed. Mechanical pulp added to the enzyme treated white water showed increased surface charge when compared to pulp blended with the corresponding control water. Handsheets were prepared from enzyme treated or control pulps and were formed in enzyme treated or control white waters, with or without alum post treatment. Paper consolidation and dry strengths were unaffected by any of the treatments, as was illustrated by the very similar densities, scattering coefficients, and tensile, tear, burst, and zero-span indices measured. Enzyme treatment of white water and pulp, followed by alum post treatment significantly enhanced paper surface properties by lowering handsheet roughness and porosity. However, enzyme addition hindered paper optical properties, resulting in lower brightness values. The loss in pulp brightness was largely overcome after a twostage hydrogen peroxide brightening sequence was applied. Brightening resulted in the enzyme treated pulp reaching a comparable final brightness to that of bleached control pulp.

Thesis/Dissertation

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2009-08-14

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http://hdl.handle.net/2429/12246

Forestry

University of British Columbia

UBCV

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