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applied sciences Editorial Editorial of the Special Issue “Anaerobic Co-Digestion of Lignocellulosic Wastes” 1 , 1 Luis Isidoro Romero-García * , Carlos José Álvarez-Gallego and Luis Alberto Fernández-Güelfo Department of Chemical Engineering and Food Technology, University of Cádiz, 11510 Puerto Real, Cádiz, Spain; email@example.com Department of Environmental Technologies, University of Cádiz, 11510 Puerto Real, Cádiz, Spain; firstname.lastname@example.org * Correspondence: email@example.com Received: 4 October 2020; Accepted: 14 October 2020; Published: 22 October 2020 1. Introduction Carbohydrates from vegetal biomass (wood and agricultural biomass) are the focus of bioreﬁnery strategies . In the search for a cheap, widespread and available carbon source not related to the food market (second generation biomass), all eorts have been focused on lignocellulosic biomass as the main substrate. However, lignocellulosic structures are dicult to depolymerize and pretreatments are needed in order to access the readily degradable carbon. On the other hand, anaerobic digestion is a technologically mature microbial process which has been considered as a theoretically carbon-neutral process when vegetal biomass is used as a substrate. Even a substantial reduction of greenhouse gases impact is possible if natural and decontrolled anaerobic decomposition is considered as the usual fate of some types of waste (e.g., animal manures or forestry residuals) . In this issue, Muhayodin et al.  present a global study about the applicability of anaerobic digestion as a process for reducing the greenhouse gases from rice straw as an example of lignocellulosic material including a mass balance for nutrients. The anaerobic digestion of rice straw is a clear case of reducing the impact of greenhouse gases. In data extracted from Scival , the anaerobic digestion/digester/methane production topic (T155) is ranked 7th of 1621 topics in the Environmental engineering subject by prominence index based on Citation count, Scopus view count and average CiteScore, with a percentile of 99.937 . In the present Special Issue, a collection of research and review papers has been presented for the discussion of the possibilities of anaerobic co-digestion of lignocellulosic biomass for bioenergy production with a special emphasis on the pretreatment of biomass to enhance the biodegradability and the subsequent biogas production. Five reviews have been included to provide a full dimension of the topic. 2. Pretreatments in the Hotspot As a reﬂection of the clear interest in pretreatment procedures for the improvement of the anaerobic digestion of lignocellulosic biomass, the ﬁrst half of the papers in the Special Issue are related to pretreatments. The reviews by Sayara et al.  and Hernández-Beltrán et al.  contribute with an overview on the possible types of pretreatment and the subsequent conditions of the anaerobic digestion process for biogas production. In addition, the latter paper includes a techno-economic and environmental analysis. Acid pretreatment is a highly eective chemical technique used to disrupt the lignocellulosic matrix by cleavage of glucosidic bonds, where mild acid pretreatments are the most promising technology . Sorghum stalks from sixteen representative pedigreed sorghum mutant lines were tested by Xu et al.  with a classical diluted acid pretreatment as a Appl. Sci. 2020, 10, 7399; doi:10.3390/app10217399 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7399 2 of 3 previous step for enzymatic sacchariﬁcation. The authors obtained a glucose recovery of 80–85% from cellulose and hemicellulose with minimal degradation products, proposing a pre-washing step where water-extractive content exceeded 35% (w/w), before being subjected to the acid pretreatment process in order to avoid sugar losses or degradation. Furthermore, an eective option is the organosolv pretreatment, which Sulbarán-Rangel et al.  veriﬁed using corncob wastes. The organosolv process is a thermochemical deligniﬁcation process that consists of breaking the internal lignin and hemicellulose bonds of the lignocellulosic material with solvents. In this case, ethanol/acetic acid (1:10) was applied as organosolv reagent to obtain the hydrolysates. Improvement in biogas production was four times greater in the batch test. As a common co-substrate, animal manures are particulate organic-rich materials with negative eects on the kinetics of the anaerobic process. The eect of enzymatic (cellulase) pretreatment on particulate disintegration of this waste was studied by Tassew et al. . Moreover, the review by Ahmed et al.  oers an in-depth insight into hydrothermal pretreatment with a section dedicated to inhibitors formed by the deligniﬁcation process (mainly furfural and 5-HMF) and strategies to overcome it. 3. Anaerobic Co-Digestion Process Aboudi et al.  presented a study of the mesophilic anaerobic co-digestion of lignocellulosic material (exhausted sugar beet pulp) with animal manures (pig and cow) in reactors fed in semi-continuous mode. This mixture shows the huge potential of co-substrates to improve the methane yield (70 and 31% increase in methane production, respectively) by mitigating the inhibitory eect of volatile fatty acids at high loading rate conditions. The tests were analyzed from a novel perspective, evaluating the uncoupling of anaerobic phases (hydrolysis, acidogenesis/acetogenesis and methanogenesis) through the calculation of indirect carbon-related parameters such as acidogenic substrate carbon (ASC). In the same way, Gómez-Quiroga et al.  refer to a similar study with the same type of lignocellulosic material (exhausted sugar beet pulp) under thermophilic operating conditions, where batch tests were assayed, showing that the activity of acetoclastic methanogens was especially aected at thermophilic conditions while organic matter solubilization was more ecient. As a ﬁnal colophon, the review of Sarker et al.  focuses on the dierent types of bioreactors and conﬁgurations for the anaerobic process. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Ajao, O.; Marinova, M.; Savadogo, O.; Paris, J. Hemicellulose based integrated forest bioreﬁneries: Implementation strategies. Ind. Crop. Prod. 2018, 126, 250–260. [CrossRef] 2. Pabón-Pereira, C.P.; Hamelers, H.V.M.; Matilla, I.; Van Lier, J.B. New insights on the estimation of the anaerobic biodegradability of plant material: Identifying valuable plants for sustainable energy production. Processes 2020, 8, 806. [CrossRef] 3. Muhayodin, F.; Fritze, A.; Rotter, V.S. A Review on the fate of nutrients and enhancement of energy recovery from rice straw through anaerobic digestion. Appl. Sci. 2020, 10, 2047. [CrossRef] 4. Scival ®. Report from Scival in the period 2015–2019 Using Topic (T155) and Topic Clusters (TC65 and TC119). Available online: www.scival.com (accessed on 10 September 2020). 5. Sayara, T.; Sánchez, A. A review on anaerobic digestion of lignocellulosic wastes: Pretreatments and operational conditions. Appl. Sci. 2019, 9, 4655. [CrossRef] 6. Hernández-Beltrán, J.U.; Hernández-De Lira, I.O.; Cruz-Santos, M.M.; Saucedo-Luevanos, A.; Hernández-Terán, F.; Balagurusamy, N. Insight into pretreatment methods of lignocellulosic biomass to increase biogas yield: Current state, challenges, and opportunities. Appl. Sci. 2019, 9, 3721. [CrossRef] Appl. Sci. 2020, 10, 7399 3 of 3 7. Woiciechowski, A.L.; Dalmas-Neto, C.J.; Porto de Souza-Vandenberghe, L.; Pedro de Carvalho-Neto, D.; Novak-Sydney, A.C.; Junior-Letti, L.A.; Grace-Karp, S.; Zevallos-Torres, L.A.; Ricardo-Soccol, C. Lignocellulosic biomass: Acid and alkaline pretreatments and their eects on biomass recalcitrance— Conventional processing and recent advances. Bioresour. Technol. 2020, 304, 122848. [CrossRef] [PubMed] 8. Xu, Y.; Li, J.; Xin, Z.; Bean, S.R.; Tilley, M.; Wang, D. Water-soluble sugars of pedigreed sorghum mutant stalks and their recovery after pretreatment. Appl. Sci. 2020, 10, 5472. [CrossRef] 9. Sulbarán-Rangel, B.; Alarcón Aguirre, J.S.; Breton-Deval, L.; del Real-Olvera, J.; Gurubel Tun, K.J. Improvement of anaerobic digestion of hydrolysed corncob waste by organosolv pretreatment for biogas production. Appl. Sci. 2020, 10, 2785. [CrossRef] 10. Tassew, F.A.; Bergland, W.H.; Dinamarca, C.; Bakke, R. Eect of Particulate Disintegration on biomethane potential of particle-rich substrates in batch anaerobic reactor. Appl. Sci. 2019, 9, 2880. [CrossRef] 11. Ahmed, B.; Aboudi, K.; Tyagi, V.K.; Álvarez-Gallego, C.J.; Fernández-Güelfo, L.A.; Romero-García, L.I.; Kazmi, A.A. Improvement of anaerobic digestion of lignocellulosic biomass by hydrothermal pretreatment. Appl. Sci. 2019, 9, 3853. [CrossRef] 12. Aboudi, K.; Gómez-Quiroga, X.; Álvarez-Gallego, C.J.; Romero-García, L.I. Insights into anaerobic co-digestion of lignocellulosic biomass (sugar beet by-products) and animal manure in long-term semi-continuous assays. Appl. Sci. 2020, 10, 5126. [CrossRef] 13. Gómez-Quiroga, X.; Aboudi, K.; Álvarez-Gallego, C.J.; Romero-García, L.I. Enhancement of methane production in thermophilic anaerobic co-digestion of exhausted sugar beet pulp and pig manure. Appl. Sci. 2019, 9, 1791. [CrossRef] 14. Sarker, S.; Lamb, J.J.; Hjelme, D.R.; Lien, K.M. A review of the role of critical parameters in the design and operation of biogas production plants. Appl. Sci. 2019, 9, 1915. [CrossRef] Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aliations. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Applied Sciences – Multidisciplinary Digital Publishing Institute
Published: Oct 22, 2020
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