Variations in Extractive Compounds during Hydrothermal Treatment of Lignocellulosic Sludge

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 60-70 % (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