Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech Insights into the economic viability of cellulases recycling on bioethanol production from recycled paper sludge Daniel G. Gomes a , Sebastián Serna-Loaiza b , Carlos A. Cardona b , Miguel Gama a , Lucília Domingues a, ⁎ a Centre of Biological Engineering, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal b Instituto de Biotecnología y Agroindustria, Universidad Nacional de Colombia Sede Manizales, Manizales-Caldas, Colombia GRAPHICAL ABSTRACT ARTICLE INFO Keywords: Economic analysis Cellulase recycling Lignocellulosic ethanol Recycled paper sludge ABSTRACT The economics of Recycled Paper Sludge conversion into ethanol was here assessed with emphasis on integrating a cellulase recycling system. Without cellulases recycling this process presented positive economic outputs (payback period of 7.85 years; 10.90 Million US$ of accumulated NPV) despite the modest ethanol titers. Recycling both free and solid-bound enzymes allowed considerable savings of enzyme but also an increase on annual costs (0.88%), resulting on a superior economic output: payback period decreased to 7.25 years; accumulated NPV increased to 14.44 Million US$. Recycling exclusively the liquid fraction enabled a clear costs reduction, however, also total ethanol de- creased, attenuating the abovementioned benefits. Targeting higher ethanol concentrations, superior solids consistencies were also evaluated. Despite a costs reduction, total ethanol decreased due to a higher ethanol retention on the solid. A sensitivity analysis further revealed that the cost of enzymes and ultrafiltration membrane may be critical on enzyme recycling economic feasibility. 1. Introduction Over the last years, cellulosic ethanol is increasingly establishing itself as a feasible alternative to fossil fuels. This can have an important boost on small and local economies, while at the same time eliminating the potential competition with food crops. The cost of both feedstock and the enzymes is critical regarding the economic performance of this process (Johnson, 2016; Liu et al., 2016; Pirzadah et al., 2017). Thus, more attention has been given to residues (from forest, agriculture, industry, etc.) rather than energy crops, since they may have a negative https://doi.org/10.1016/j.biortech.2018.07.056 Received 20 May 2018; Received in revised form 8 July 2018; Accepted 9 July 2018 ⁎ Corresponding author. E-mail address: luciliad@deb.uminho.pt (L. Domingues). Bioresource Technology 267 (2018) 347–355 Available online 12 July 2018 0960-8524/ © 2018 Elsevier Ltd. All rights reserved. T