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      Effect of pH and temperature on microbial community structure and carboxylic acid yield during the acidogenic digestion of duckweed

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          Abstract

          Background

          Duckweeds (Lemnaceae) are efficient aquatic plants for wastewater treatment due to their high nutrient-uptake capabilities and resilience to severe environmental conditions. Combined with their rapid growth rates, high starch, and low lignin contents, duckweeds have also gained popularity as a biofuel feedstock for thermochemical conversion and alcohol fermentation. However, studies on the acidogenic anaerobic digestion of duckweed into carboxylic acids, another group of chemicals which are precursors of higher-value chemicals and biofuels, are lacking. In this study, a series of laboratory batch experiments were performed to determine the favorable operating conditions (i.e., temperature and pH) to maximize carboxylic acid production from wastewater-derived duckweed during acidogenic digestion. Batch reactors with 25 g/l solid loading were operated anaerobically for 21 days under mesophilic (35 °C) or thermophilic (55 °C) conditions at an acidic (5.3) or basic (9.2) pH. At the conclusion of the experiment, the dominant microbial communities under various operating conditions were assessed using high-throughput sequencing.

          Results

          The highest duckweed–carboxylic acid conversion of 388 ± 28 mg acetic acid equivalent per gram volatile solids was observed under mesophilic and basic conditions, with an average production rate of 0.59 g/l/day. This result is comparable to those reported for acidogenic digestion of other organics such as food waste. The superior performance observed under these conditions was attributed to both chemical treatment and microbial bioconversion. Hydrogen recovery was only observed under acidic thermophilic conditions, as 23.5 ± 0.5 ml/g of duckweed volatile solids added. More than temperature, pH controlled the overall structure of the microbial communities. For instance, differentially abundant enrichments of Veillonellaceae acidaminococcus were observed in acidic samples, whereas enrichments of Clostridiaceae alkaliphilus were found in the basic samples. Acidic mesophilic conditions were found to enrich acetoclastic methanogenic populations over processing times longer than 10 days.

          Conclusions

          Operating conditions have a significant effect on the yield and composition of the end products resulting from acidogenic digestion of duckweed. Wastewater-derived duckweed is a technically feasible alternative feedstock for the production of advanced biofuel precursors; however, techno-economic analysis is needed to determine integrated full-scale system feasibility and economic viability.

          Electronic supplementary material

          The online version of this article (10.1186/s13068-018-1278-6) contains supplementary material, which is available to authorized users.

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          Most cited references45

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          Methods for Pretreatment of Lignocellulosic Biomass for Efficient Hydrolysis and Biofuel Production

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            Inhibition of anaerobic digestion process: a review.

            Anaerobic digestion is an attractive waste treatment practice in which both pollution control and energy recovery can be achieved. Many agricultural and industrial wastes are ideal candidates for anaerobic digestion because they contain high levels of easily biodegradable materials. Problems such as low methane yield and process instability are often encountered in anaerobic digestion, preventing this technique from being widely applied. A wide variety of inhibitory substances are the primary cause of anaerobic digester upset or failure since they are present in substantial concentrations in wastes. Considerable research efforts have been made to identify the mechanism and the controlling factors of inhibition. This review provides a detailed summary of the research conducted on the inhibition of anaerobic processes. The inhibitors commonly present in anaerobic digesters include ammonia, sulfide, light metal ions, heavy metals, and organics. Due to the difference in anaerobic inocula, waste composition, and experimental methods and conditions, literature results on inhibition caused by specific toxicants vary widely. Co-digestion with other waste, adaptation of microorganisms to inhibitory substances, and incorporation of methods to remove or counteract toxicants before anaerobic digestion can significantly improve the waste treatment efficiency.
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              Pretreatments to enhance the digestibility of lignocellulosic biomass.

              Lignocellulosic biomass represents a rather unused source for biogas and ethanol production. Many factors, like lignin content, crystallinity of cellulose, and particle size, limit the digestibility of the hemicellulose and cellulose present in the lignocellulosic biomass. Pretreatments have as a goal to improve the digestibility of the lignocellulosic biomass. Each pretreatment has its own effect(s) on the cellulose, hemicellulose and lignin; the three main components of lignocellulosic biomass. This paper reviews the different effect(s) of several pretreatments on the three main parts of the lignocellulosic biomass to improve its digestibility. Steam pretreatment, lime pretreatment, liquid hot water pretreatments and ammonia based pretreatments are concluded to be pretreatments with high potentials. The main effects are dissolving hemicellulose and alteration of lignin structure, providing an improved accessibility of the cellulose for hydrolytic enzymes.
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                Author and article information

                Contributors
                auc261@psu.edu
                mjs697@psu.edu
                trichard@psu.edu
                rab44@psu.edu
                Journal
                Biotechnol Biofuels
                Biotechnol Biofuels
                Biotechnology for Biofuels
                BioMed Central (London )
                1754-6834
                8 October 2018
                8 October 2018
                2018
                : 11
                : 275
                Affiliations
                [1 ]ISNI 0000 0001 2097 4281, GRID grid.29857.31, Department of Civil and Environmental Engineering, , The Pennsylvania State University, ; 212 Sackett Building, University Park, 16802 USA
                [2 ]ISNI 0000 0001 2097 4281, GRID grid.29857.31, Department of Agricultural and Biological Engineering, , The Pennsylvania State University, ; 132 Land and Water Research Building, University Park, PA 16802 USA
                Author information
                http://orcid.org/0000-0001-6780-7552
                Article
                1278
                10.1186/s13068-018-1278-6
                6174553
                2c34ac9f-3015-45b9-b1a7-835dbb373b0e
                © The Author(s) 2018

                Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

                History
                : 30 May 2018
                : 29 September 2018
                Funding
                Funded by: FundRef http://dx.doi.org/10.13039/100005825, National Institute of Food and Agriculture;
                Award ID: 2012-68005-19703
                Award Recipient :
                Funded by: FundRef http://dx.doi.org/10.13039/100011610, Institutes of Energy and the Environment, Pennsylvania State University;
                Categories
                Research
                Custom metadata
                © The Author(s) 2018

                Biotechnology
                volatile fatty acids,acidogenic digestion,carboxylate platform,biohydrogen,duckweed,lemna obscura,marker-gene survey,microbial community analysis

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