Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman Review Biomethane production through anaerobic co-digestion with Maize Cob Waste based on a biorefinery concept: A review Elena Surra a , Maria Bernardo b , Nuno Lapa a,* , Isabel A.A.C. Esteves b,** , Isabel Fonseca b , José P.B. Mota b a LAQV-REQUIMTE, Departamento de Ciências e Tecnologia da Biomassa, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516, Caparica, Portugal b LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516, Caparica, Portugal ARTICLE INFO Keywords: Activated carbon Anaerobic co-Digestion Bio-methane Biorefinery Maize cob waste Pre-treatments ABSTRACT Maize Cob Waste (MCW) is available worldwide in high amounts, as maize is the most produced cereal in the world. MCW is generally left in the crop fields, but due to its low biodegradability it has a negligible impact in soil fertility. Moreover, MCW can be used as substrate to balance the C/N ratio during the Anaerobic co- Digestion (AcoD) with other biodegradable substrates, and is an excellent precursor for the production of Activated Carbons (ACs). In this context, a biorefinery is theoretically discussed in the present review, based on the idea that MCW, after proper pre-treatment is valorised as precursor of ACs and as co-substrate in AcoD for biomethane generation. This paper provides an overview on different scientific and technological aspects that can be involved in the development of the proposed biorefinery; the major topics considered in this work are the following ones: (i) the most suitable pre-treatments of MCW prior to AcoD; (ii) AcoD process with regard to the critical parameters resulting from MCW pre-treatments; (iii) production of ACs using MCW as precursor, with the aim to use these ACs in biogas conditioning (H 2 S removal) and upgrading (biomethane production), and (iv) an overview on biogas upgrading technologies. 1. Introduction Anaerobic Digestion (AD) offers the opportunity to produce biogas from organic wastes (Liu et al., 2015). Biogas can be upgraded to a higher methane content (> 97% v/v), resembling the quality of natural gas (NG). Bio-methane (bioCH 4 ) is suitable for grid injection, or to be used as transportation fuel, and is economically more profitable than biogas combustion. Also, it can contribute to satisfy the increasing demand on renewable fuels (EBA, 2015). Maize, the most produced cereal in the world (URL1, 2016) gen- erates residual stover (leaves, stalks and maize cob) that is commonly left in the field as natural fertilizer (J. Zheng et al., 2014). Unlike stover, Maize Cob Waste (MCW) has a negligible impact on soil fertility due to its low biodegradability, and its removal from crop fields may increase maize yield (Jeschke and Heggenstaller, 2012). MCW is sometimes harvested as feedstock for full-scale production of ethanol and xylitol (L. Wang et al., 2012), furfural (H. Li et al., 2014) and activated carbons (ACs) (Hou et al., 2013). The potentially avail- able MCW worldwide exceeds the industry and market demands (Gu et al., 2014). AD of standalone organic substrates characterized by higher Nitrogen (N) content than Carbon (C) content (low C/N ratio), such as animal manure, sewage sludge, abattoir wastes and organic fraction of municipal solid wastes, is not a well-balanced process due to the lack of carbohydrates, leading to ammonia accumulation. The use of MCW as co-substrate can balance the C/N ratio towards the optimal range, stabilizing the AD process and enhancing biogas yield. Currently, most of the European biogas plants uses maize silage as co-substrate. The silage is produced as an energy crop that affects ne- gatively the availability of arable lands for food production. MCW when harvested separately from grains can be used as carbon source in AcoD process, without any significant impact on feed or food chains. MCW is mainly composed of cellulose, hemicellulose, and lignin that are embedded together in a complex matrix (Menon and Rao, 2012). Cellulose and hemicellulose are fermentable polysaccharides after hydrolysis, while lignin, a complex aromatic and hydrophobic amorphous heteropolymer having a cement role in the cross-linking between cellulose and hemicellulose, cannot be degraded during https://doi.org/10.1016/j.jenvman.2019.109351 Received 6 June 2018; Received in revised form 10 June 2019; Accepted 1 August 2019 * Corresponding author. ** Co-corresponding author. E-mail addresses: ncsn@fct.unl.pt (N. Lapa), iaesteves@fct.unl.pt (I.A.A.C. Esteves). Journal of Environmental Management 249 (2019) 109351 0301-4797/ © 2019 Elsevier Ltd. All rights reserved. T