Industrial Crops and Products 54 (2014) 248–256 Contents lists available at ScienceDirect Industrial Crops and Products jo u r n al homep age: www.elsevier.com/locate/indcrop The potential of paper mill sludge for wood–plastic composites Joël Soucy a , Ahmed Koubaa a,∗ , Sébastien Migneault a , Bernard Riedl b a Canada Research Chair on Wood Development, Characterization and Processing, Université du Québec en Abitibi-Témiscamingue, 445 Boulevard de l’Université, Rouyn-Noranda, Québec, J9X 5E4, Canada b Centre de recherche sur les matériaux renouvelables, Université Laval, Pavillon Eugène Kruger, Ste-Foy, Québec, G1K 7P4, Canada a r t i c l e i n f o Article history: Received 7 October 2013 Received in revised form 6 January 2014 Accepted 8 January 2014 Keywords: Primary and secondary sludge Sludge characterization Pulping process Wood–plastic composites Mechanical properties Water immersion a b s t r a c t Recent studies have demonstrated the potential of primary sludge (PS) as reinforcing fibers and secondary sludge (SS) as binder or co-binder in wood–plastic composites (WPC). A comparative study was conducted using paper mill sludge produced by three different pulping processes at two SS to PS ratios. The objec- tives were to determine the impact of PS and SS on the development of high density polyethylene (HDPE) WPC properties. Sludge produced by thermomechanical pulping (TMP), chemithermomechanical pulp- ing (CTMP), and Kraft pulping were used at three different proportions (20%, 30%, and 40%) for composite manufacturing. The use of mixed sludge containing 30% SS resulted in lower tensile, flexural, and impact performance of the WPC compared to mixed sludge containing only 10% SS for the three pulping pro- cesses. Sludge type had a significant impact on the WPC physical and mechanical properties. Kraft sludge produced the best WPC properties, followed by CTMP and TMP sludge. Increasing the sludge proportion produced increasingly negative impacts on water absorption and thickness swelling, but improved the flexural and tensile properties. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The pulp and paper industry requires large quantities of water to operate. The wastewater must then be treated to reduce the total suspended solid content and the oxygen demand prior to disposal (Gilbride and Fulthorpe, 2004). The sludge is the solid residue generated by the water treatment plant. Each ton of paper produces about 45 kg of dry sludge (Son et al., 2004). Sludge dis- posal can account for up to 60% of the total water treatment cost (Mahmood and Elliott, 2006). Certain fine particles that are mechanically removed from the wastewater are called primary sludge (PS). These suspended solids contain mostly cellulose, hemi- celluloses, and lignin, as well as other possible residues such as bark and additives used as filler in paper production (Mahmood and Elliott, 2006; Chen et al., 2002). The remaining suspended solids are sent to a secondary treatment plant for bacterial digestion. The mixture of biosolids and the remaining suspended solids is called the secondary sludge (SS). The large numbers of bacteria in the SS make it difficult to dewater and dry, because a large portion of the water is trapped inside the living cells, and therefore tends to jellify (Mahmood and Elliott, 2006; Tchobanoglous et al., 2003; Mabee, 2001). Bacteria are made up of approximately 80% water and 20% dry material, of which 90% is organic and 10% inorganic. An ∗ Corresponding author. Tel.: +1 819 762 0971x2579. E-mail address: ahmed.koubaa@uqat.ca (A. Koubaa). approximate formula for the organic portion is C 5 H 7 O 2 N (Cheremisinoff, 1996). The nitrogen content in the simplified molecular structure is a particular feature of SS. When the SS is digested in the presence of oxygen, the aerobic process generates mainly heat, water (H 2 O), and carbon dioxide (CO 2 ). When a secondary treatment plant operates in the absence of oxygen, the digestion produces methane (CH 4 ) and carbon diox- ide (CO 2 ). This anaerobic process considerably reduces the biomass produced by the digestion of suspended solids (Baudez, 2001). The mixture of PS and SS is called mixed sludge. Paper mills usually dis- pose of mixed sludge by burning and landfilling (Beauchamp et al., 2002). However, burning nitrate-rich paper mill sludge in industrial boilers has been criticized because it generates powerful green- house gases such as nitrate oxide (NOx) and provides low calorific value. In Canada, the public has begun complaining about land fill- ing because of the unpleasant smell of the active bacterial culture in the sludge and the potential to produce elements that are toxic to fauna and flora (Pearson, 2005). Studies have recommended com- posting, ethanol production, and other thermal treatments such as pyrolysis, vitrification, and gasification for sludge disposal, but these alternatives are either economically unviable or liable to generate other sub-products that could be protested by environ- mentalists (Beauchamp et al., 2006; Mahmood and Elliott, 2006). Due to the very high volume of sludge worldwide, there is growing interest in either increasing agricultural use of sludge or processing sludge for use in producing other products (Chen et al., 2002). For example, Ou-Yang and Wu (2002) observed a 50% higher plant 0926-6690/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2014.01.013