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UBC Theses and Dissertations

The potential of using a combined fungal and enzyme treatment system to remove detrimental dissolved and colloidal substances from TMP/newsprint mill process waters Zhang, Xiao

Abstract

Pulp and paper making is one of the most important industries in the world and contributes significantly to the world economy. However, this industry is known to be a major consumer of increasing scarce water resources, and it is also accused of being one of the largest sources of water pollution. Ongoing environmental concerns and the advent of more stringent Environmental Protection Agency (EPA) regulations have meant that the pulp and paper industries have to further clean up their effluents or attempt to close the processing water system to achieve zero discharge. Despite many obvious benefits, several potential problems are expected to be encountered with a closed water system. One of these problems is the build up of dissolved and colloidal substances (DCS) within the recycling white water system. These substances have been shown to cause detrimental effects on the papermaking machinery and greatly reduce product quality. Therefore, it would be desirable to develop a process water treatment system that could remove these detrimental substances and enhance the closure of the mill's "water loop". The main objective of this thesis was to assess the potential of using a combined fungal and enzyme system as an internal treatment "kidney" to control the build up of detrimental dissolved and colloidal substances present in a TMP/newsprint mill process. water. Some specific objectives were achieved as a result of this study. A clear knowledge of the composition of a "typical" TMP process white water gave us a better understanding of the influence that the DCS components had on paper properties and it also helped in the design of an appropriate treatment technology. However, the huge variations which exist among the different white water/process water streams make it almost impossible to obtain a "representative water" sample. With the intention of obtaining key information on the range of concentrations of the various components of the DCS contained within the white water, a series of white water samples were obtained. These included mill white waters collected from a typical TMP/newsprint mill and "model" recycled white waters (one to five times recycled white waters) made in the PAPRICAN pilot plant. A comprehensive protocol was established to analyze each of DCS components present in the different white water samples and to determine the various physical properties of each of the white water samples. By examining the physical properties of the handsheets prepared by using these different white waters, it was recognized that the properties of the papersheets formed in the white water were not only influenced by the different chemical natures of the white water DCS components, but also by their physical form (i.e. dissolved or colloidal). The major detrimental components present in the white water were shown to be lignin and lipophilic extractives. Thus, the next objective of the study was to screen various fungi for their activities against these substances and to find a suitable strain that could carry out effective fungal and enzyme treatment of the white waters. As anticipated, the fungi utilized a small portion of the white water from the main stream which was cooled to 30°C before feeding into a bioreactor to produce enzymes present within the fungal culture filtrate (FCF). More importantly, many of the enzymes required to break down the DCS substrates could then be released into the white water. It was expected that, in this way, a highly active fungal culture filtrate (FCF) could be continuously decanted into the process water so that the enzymes could react with the DCS components while the whole paper making process continues. Since the substances present in the white water constitute the only nutritional source for fungal growth, the enzymes produced in the culture filtrate should be specific for these dissolved and colloidal components and also be much cheaper to use than commercial sources of enzymes. Over twenty fungal strains were screened through a two step-process. The first screening was based on the growth of these fungi on agar plates containing white water without any additional nutrients. The secondary screening was used to determine the ability of the selected strains to degrade extractives, the major detrimental substances present within the white water. From the two screening steps, the white-rot fungus Trametes versicolor showed both the highest growth on white water and highest activity against the DCS components present in the TMP white water. Therefore, this fungus was selected for further study. When the efficiency of the fungal removal of the white water DCS components was tested, a significant decrease in the total dissolved and colloidal substances, carbohydrates and extractives was detected after 2 days growth at 30°C. Of the carbohydrate fraction, the mannans and glucans were extensively degraded (over 70%) before a significant reduction in galactan was detected. Trametes versicolor was able to degrade almost all of the fatty acids, lignans and ester bonded extractives (over 90%). There was also a good removal of the resin acids by Trametes versicolor. An increase in the acid insoluble lignin content was observed after 2-3 days of treatment, probably due to the polymerization of low molecular weight phenolic materials (e.g. lignans). Several different white water streams, including mill white water, model recycled white water and membrane filtered white water were used to grow Trametes versicolor for 7 days. Although the levels of each enzyme produced in the FCF by Trametes versicolor grown on the different streams were slightly different, the overall enzyme production profiles were similar. The mill white water collected from the cloudy white water tank was found to be enriched in extractives. It is also abundantly available within the mill and it usually has a long settling time. Besides, It resulted in the highest level of enzyme activities detected in the FCF. These facts made this type of stream favourable for fungal growth after cooling down to ambient temperature. Thus the fungal culture filtrate produced from mill cloudy white water was selected for further enzyme treatments. The FCF treatments of both the mill white water and the model recycled white water were carried out at 65°C for 3 hours and the ratio between the FCF and white water was 1 to 2. The fungal culture filtrate (FCF) treatment of white water resulted in the degradation of most of the DCS components. Carbohydrates were extensively hydrolyzed into monomers while the lignin content increased as a result of lignan polymerization. The FCF was also active in degrading most of the extractives present in the white water, resulting in a reduction of more than 90% of the lignans, steryl esters and triglycerides and a significant decrease (20-30%) in the resin and fatty acids. In order to achieve a better understanding of the interactions between the various components of the fungal enzyme system and the DCS components, the model recycled white water and colloidal-free white water were treated with commercial sources of the three main types of enzymes (cellulases, laccases and lipases) found in the Trametes versicolor culture filtrate. The active hydrolysis of carbohydrates by cellulase/hemicellulase and degradation of lipids by lipase/esterase are well documented in the literature and correlated well with our results. Although laccase-mediated polymerization of low molecular weight phenolics to higher molecular weight lignin like materials was anticipated, surprisingly, the laccase treatment also resulted in significant degradation on lipophilic extractives, such as steryl esters and triglycerides. The laccase-catalyzed reactions of lipophilic extractives were further investigated using three extractive model compounds, methyl linoleate, cholestryl linoleate and trilinolein, and one laccase from VTT Food and Biotechnology Lab and two laccases from Novo Nordisk Ltd. All three laccases degraded trilinolein well, however, degradation of methyl linoleate was only detected after VTT laccase treatment. In summary, the white rot fungi, Trametes versicolor was shown to be able to grow on unsupplemented process waters, while effectively removing various white water organics. The fungus also produced a large spectrum of enzymes during its growth on the different white water streams and the fungal enzyme treatments resulted in a significant degradation or modification of various DCS components. The results presented in this thesis show that a combined fungal and enzyme system could possibly be used as an internal treatment "kidney" to remove detrimental organic substances present in TMP/newsprint mill with a closed water system.

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