VARIATIONS IN EXTRACTIVE COMPOUNDS DURING HYDROTHERMAL TREATMENT OF LIGNOCELLULOSIC SLUDGE

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1 VARIATIONS IN EXTRACTIVE COMPOUNDS DURING HYDROTHERMAL TREATMENT OF LIGNOCELLULOSIC SLUDGE Saeid Baroutian, John Andrews, Murray Robinson, Anne-Marie Smit, Ben McDonald, Suren Wijeyekoon, Daniel Gapes Scion Te Papa Tipu Innovation Park, 49 Sala Street, Private Bag 3020, Rotorua 3046, New Zealand ABSTRACT Wood extractives including resin acids, fatty acids and phenolics can be a high percentage of the total organics that exist in pulp and paper mill solid organic waste. These substances are difficult to break down under anaerobic digestion conditions, and have inhibitory effects on methane production. In this study, two different hydrothermal treatment techniques (wet oxidation and thermal hydrolysis) were used to treat the extractive compounds from a lignocellulosic solid waste. The treatment processes were carried out at 220 C with initial pressure of 20 bar, under oxidative and non-oxidative conditions. Time-related changes in compound concentrations were investigated, with oxidative conditions demonstrating significant destruction within 20 minutes. INTRODUCTION Increased costs of landfill waste management, as well as increased public and government pressure to limit the waste to landfill disposal methods, has compelled many industries to increasingly consider alternative waste management solutions. Lignocellulosic sludge is a significant waste product of the pulp and paper manufacturing industry. Anaerobic digestion of this material is an attractive disposal route as it both reduces the amount of final sludge solids for disposal and enables the process to generate a product in the form of biogas, which can contain % (by volume) methane (Strong et al., 2011). The solid waste from softwood processing is commonly characterised by a high levels of wood extractives (Verta et al., 1996, Leach and Thakore, 1978). Most of these extractives are not easy to break down under anaerobic digestion conditions. Further, and particularly for the resin acid fraction, these extractives may inhibit biological growth and methane production (Sekido et al., 1990). Thus, technologies which achieve effective removal of these extractives components are needed to maximise methanogenic energy production from such feedstocks. Hydrothermal processing, including wet oxidation and thermal hydrolysis, is a waste management option that could potentially achieve simultaneous waste degradation and formation of valuable byproducts. Hydrothermal processing involves liquid phase deconstruction of organic and inorganic components at elevated temperatures and pressures. As the reactions are completed in the water phase, the technology eliminates the need for water removal prior to treatment. Several recent studies on hydrothermal processing (Strong and Gapes, 2012, Molina, 2006, Shende and Levec, 1999, Abelleira et al., 2012) indicated there is renewed interest in these thermal aqueous processes due to their inherent advantages in handling a wet waste and potential for resource recovery from waste. The mechanism of

2 hydrothermal treatment of organic compounds is very complicated, even for singlecomponent solutions. Typically, oxidation proceeds via a very complex reaction scheme, altering compounds to form distinctive intermediates, such as short-chain organic acids. This study investigated the transformation of wood extractives during oxidative and non-oxidative hydrothermal treatment of lignocellulosic waste biomass. METHODOLOGY Material The substrate utilised was an organic solid waste fraction derived from a mechanical pulping process which utilised Pinus radiata species feedstock. The solids were diluted with tap water to obtain a dry solids feed of 3 % as Total Suspended Solids (TSS). The diluted material was kept at 4 C and homogenized prior to starting the experiments. Hydrothermal treatment Batchwise oxidative and non-oxidative hydrothermal processing (wet oxidation and thermal hydrolysis, respectively) was carried out in a high temperature-high pressure Parr reactor. The reactor (Figure 1) was equipped with a pre-heating system to heat the substrate to 90 C to minimize the temperature gradient inside the reactor. The reactor was also equipped with a non-automotive sampling system to facilitate sampling during the reaction process. On sampling, the samples were immediately cooled down to stop further reaction. The sampling line was washed and drained after each sampling. The reactor was initially charged with 150 ml water and pressurized using 20 bar pure oxygen or nitrogen. The reactor was heated to 220 C and then the pre-heated material was transferred to the reactor by means of a pressure difference when it reached set temperature. The raw material was diluted inside the reactor to achieve the desired solid concentration of approximately 1.5 wt%. Sub-samples were taken from the reactor throughout the experiment. The sampling tube was flushed with water followed by nitrogen before each 10 ml sub-sample was collected. Fig. 1: Experimental set-up for high pressure hydrothermal processing 2

3 Analysis Wood extractives were obtained via solvent extraction from the aqueous samples. This method is suitable for samples from mill treatment system effluents and mill process waters. The liquid samples were ph adjusted then extracted using Dichloromethane (DCM). The extracts were concentrated, silylated, and analysed by GC/MS. RESULTS AND DISCUSSION Overall Transformation Lignocellulosic sludge was subjected to oxidative and non-oxidative processes for 120 min. Degradation was observed based on the concentrations of total extractives. As is evident in Figure 2, greater degradation was observed for oxidative treatment, with >99 % degradation after 120 min processing time, and 98 % of the extracted organic compounds were degraded after only 20 min. During thermal hydrolysis, the concentrations of the organic extractives initially increased and then decreased at a slower rate than that for the oxidative process. The initial increase in the concentration of organic compounds can be attributed to improvement in solvent extractability of materials or transformation of other compounds into the organic extractives. In this study, the organic extractives are divided into three major groups: resin acids, fatty acids and phenolics. Resin and fatty acids are the predominant compounds in lignocellulosic sludge with inhibitory effect to methane production. The results of the extractions show that the concentrations of certain components were considerably reduced, particularly when the reaction was carried out under the oxidative condition. At the end of the oxidative reaction, the removal of these compounds was almost complete (Figures 3-5). Resin acid transformations As shown in Figure 3, abietic acid was the major resin acid present in the extracted compounds, and was quickly degraded under oxidative conditions. It was also found that the non-oxidative process was not successful in degrading resin acids, with only 36 % of abietic acid removed from the raw material after 2 h. Fig. 2: Degradation of the total organic extractives during oxidative and non-oxidative processes 3

4 It is well known that resin acids have inhibitory effects on biological growth and enzyme activities (Sekido et al., 1990) and require removal prior to further biological treatment. Figure 3 also shows a significant increase in abietic acid concentration early on in the non-oxidative process. As mentioned before, this can be due to the improved extractability or transformation of other compounds into resin acids. Fatty acid transformations During pulping, fatty acids (in particular long chain carboxylic acids) are formed as hydrolysis products of esters. Variations in fatty acids are shown in Figure 4. Notably, oleic and linoleic acids are the major fatty acids in the lignocellulosic sludge. It was established that a 20 min reaction time is sufficient for complete removal of fatty acids through the oxidative process. In contrast, thermal hydrolysis did not successfully remove fatty acids from the raw sludge. Fig. 3: Degradation of resin acids during oxidative and non-oxidative hydrothermal processing Fig. 4: Degradation of fatty acids during oxidative and non-oxidative hydrothermal processing 4

5 Phenolic transformations A comparison between oxidative and non-oxidative treatments revealed that wet oxidation can remove phenolic compounds that exist in pulp and paper sludge. Vanillin, pinosylvin mono-methyl-esters, acetovanillone and homovanillic acid which are the main phenolic compounds in lignocellulosic sludge were reasonably degraded after 120 min oxidative treatment (Figure 5). During the oxidative process, vanillin was initially produced through degradation and oxidation of lignin and further oxidation led to complete degradation of vanillin. Vanillin is a high-added value compound and it can be produced through a controlled oxidation process. The transformation of lignin to vanillin during high temperature oxidative reaction has been demonstrated (Fargues et al., 1996, Araújo et al., 2010). In contrast, more phenolic compounds were produced/extracted during non-oxidative conditions and this process was not able to treat phenolic compounds successfully CONCLUSION Extractives from softwood pulp and paper mill solid waste consist of long-chain acids, resin acids and phenolic compounds. These compounds may have inhibitory activities against methane production and energy value extraction from the lignocellulosic sludge. This study examined two hydrothermal treatment methods- namely wet oxidation and thermal hydrolysis-to treat the extractive compounds from a lignocellulosic solid waste. The oxidative process (wet oxidation) showed a rapid and near complete destruction of the extractives. Thermal hydrolysis at 220 C does not seem to be a suitable method for the destruction of extractive compounds. For certain compounds, the major reduction in concentrations occurred within 20 min of the oxidative reaction. These results demonstrate the efficacy of oxidative hydrothermal processing (wet oxidation) in the treatment of inhibitory substances found in biomass. Fig. 5: Degradation of phenolics during oxidative and non-oxidative hydrothermal processing 5

6 ACKNOWLEDGEMENT The authors gratefully acknowledge Winstone Pulp International (WPI) Ltd for support of this research. REFERENCES ABELLEIRA, J., PÉREZ-ELVIRA, S. I., SÁNCHEZ-ONETO, J., PORTELA, J. R. & NEBOT, E Advanced Thermal Hydrolysis of secondary sewage sludge: A novel process combining thermal hydrolysis and hydrogen peroxide addition. Resources, Conservation and Recycling, 59, ARAÚJO, J. D. P., GRANDE, C. A. & RODRIGUES, A. E Vanillin production from lignin oxidation in a batch reactor. Chemical Engineering Research and Design, 88, FARGUES, C., MATHIAS, Á. & RODRIGUES, A Kinetics of Vanillin Production from Kraft Lignin Oxidation. Industrial and Engineering Chemistry Research, 35, LEACH, J. M. & THAKORE, A. N compounds toxic to fish in pulp mill waste streams. Progress in Water Technology, 9, MOLINA, V. G Wet Oxidation Processes for Water Pollution Remediation. PhD, University of Barcelona. SEKIDO, H., TAKEZAWA, J.-I., MOTOKI, G. & AKATSUKA, T Inhibitory Effect of Abietic Acid against (Na + +K + )- and (H + +K + )-ATPases. Agricultural and Biological Chemistry, 54, SHENDE, R. V. & LEVEC, J Wet Oxidation Kinetics of Refractory Low Molecular Mass Carboxylic Acids. Industrial & Engineering Chemistry Research, 38, STRONG, P. J. & GAPES, D. J Thermal and thermo-chemical pre-treatment of four waste residues and the effect on acetic acid production and methane synthesis. Waste Management, 32, STRONG, P. J., MCDONALD, B. & GAPES, D. J Combined thermochemical and fermentative destruction of municipal biosolids: A comparison between thermal hydrolysis and wet oxidative pre-treatment. Bioresource Technology, 102, VERTA, M., AHTIAINEN, J., NAKARI, T., LANGI, A. & TALKA, E The effect of waste constituents on the toxicity of TCF and ECF pulp bleaching effluents. In: SERVOS, M. R. (ed.) Environmental Fate and Effects of Pulp and Paper: Mill Effluents. Taylor & Francis. 6