Enzymatic saccharification of pretreated wheat straw: Comparison of solids-recycling, sequential hydrolysis and batch hydrolysis Ville Pihlajaniemi ⇑ , Satu Sipponen 1 , Mika H. Sipponen, Ossi Pastinen, Simo Laakso Aalto University, School of Chemical Technology, Department of Biotechnology and Chemical Technology, Espoo, Finland highlights  Solids-recycling at steady state was compared to sequential and batch hydrolysis.  Volumetric productivity was increased by solids-recycling and sequential hydrolysis.  Solids-recycling is a method for product removal rather than enzyme recycling.  Solids-recycling requires less unit operations than sequential hydrolysis.  A solids-recycle process was modeled, based on a geometrical series. article info Article history: Received 14 October 2013 Received in revised form 18 November 2013 Accepted 21 November 2013 Available online 1 December 2013 Keywords: Enzymatic hydrolysis Cellulase Solids-recycling Sequential hydrolysis Wheat straw abstract In the enzymatic hydrolysis of lignocellulose materials, the recycling of the solid residue has previously been considered within the context of enzyme recycling. In this study, a steady state investigation of a solids-recycling process was made with pretreated wheat straw and compared to sequential and batch hydrolysis at constant reaction times, substrate feed and liquid and enzyme consumption. Compared to batch hydrolysis, the recycling and sequential processes showed roughly equal hydrolysis yields, while the volumetric productivity was significantly increased. In the 72 h process the improvement was 90% due to an increased reaction consistency, while the solids feed was 16% of the total process constituents. The improvement resulted primarily from product removal, which was equally efficient in solids- recycling and sequential hydrolysis processes. No evidence of accumulation of enzymes beyond the accumulation of the substrate was found in recycling. A mathematical model of solids-recycling was constructed, based on a geometrical series. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The production of biofuels from renewable lignocellulose mate- rials is currently the subject of substantial efforts of research and development. Growing industrial interest in the area and emerging demonstration plants (Larsen et al., 2012; Pescarolo, 2013) indicate that the technology for the conversion of these recalcitrant materials to fuels is reaching economic viability. Wheat straw is one of the major agricultural residues in Northern Europe, and it is frequently studied as a lignocellulosic raw material across the world. A prominent conversion route consists of pretreatment, enzymatic saccharification and fermentation to biofuels, but the cost of enzymes remains a major obstacle for this approach. Typical pretreatments include autohydrolysis or thermochemical delignification methods using alkali, acid or organic solvents (Petersen et al., 2009; Sánchez et al., 2011). Autohydrolysis is performed at high temperature and pressure in presence of water, leading to the cleavage of the acetyl groups in hemicellulose in the form of acetic acid, which then acts as a catalyst for hemicellulose hydrolysis. NaOH-delignification is an alkaline pretreatment closely related to soda pulping, where the major effects are the cleavage of ester bonds between lignin and hemicellulose and the swelling of the cellulose fibers, resulting in better enzymatic hydrolyzability (Chen et al., 2013). The enzyme costs have been addressed in different ways, by using reactor configurations facilitating product removal and enzyme recycling and by using additives to enhance the hydrolysis. Different possibilities for cellulase recycling exist due to the partition of cellulases into an insoluble and a soluble fraction dur- ing saccharification of lignocellulosics (Vallander and Eriksson, 1987; Lee et al., 1994). A majority of the cellulases is adsorbed onto the solid substrate by binding to cellulose and lignin. Enzymes adsorbed on cellulose are mostly released after hydrolysis, whereas 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.11.060 ⇑ Corresponding author. Address: Aalto University, School of Chemical Technol- ogy, Department of Biotechnology and Chemical Technology, P.O. Box 16100, FI-00076 Aalto, Espoo, Finland. Tel.: +358 50 3717 697; fax: +358 9462373. E-mail address: ville.pihlajaniemi@aalto.fi (V. Pihlajaniemi). 1 Current address: Tikkurila Oyj, Kuninkaalantie 1, 01301 Vantaa, Finland. Bioresource Technology 153 (2014) 15–22 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech