Microbial Community Shifts during Biogas ... - Semantic Scholar

Bioengineering 2015, 2, 35-53; doi:10.3390/bioengineering2010035 OPEN ACCESS

bioengineering ISSN 2306-5354 www.mdpi.com/journal/bioengineering Article

Microbial Community Shifts during Biogas Production from Biowaste and/or Propionate Chaoran Li 1,†, Christoph Moertelmaier 1,†, Josef Winter 1 and Claudia Gallert 1,2,* 1

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Institute of Biology for Engineers and Biotechnology of Wastewater, Karlsruhe Institute of Technology (KIT), Am Fasanengarten, Karlsruhe D-76128, Germany; E-Mails: [email protected] (C.L.); [email protected] (C.M.); [email protected] (J.W.) Division Microbiology-Biotechnology, Faculty of Technology, University of Applied Science, Hochschule Emden-Leer, Constantiaplatz 4, Emden D-26723, Germany These authors contributed equally to this work.

* Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +49-4921-807-1586; Fax: +49-4921-807-1593. Academic Editor: Sabine Kleinsteuber Received: 6 November 2014 / Accepted: 30 January 2015 / Published: 9 February 2015

Abstract: Propionate is the most delicate intermediate during anaerobic digestion as its degradation is thermodynamically unfavorable. To determine its maximum possible degradation rates during anaerobic digestion, a reactor was fed Monday to Friday with an organic loading rate (OLR) of 12/14 kg CODbiowaste·m−3·d−1 plus propionate up to a final OLR of 18 kg COD·m−3·d−1. No feed was supplied on weekends as it was the case in full-scale. To maintain permanently high propionate oxidizing activity (POA), a basic OLR of 3 kg CODpropionate·m−3·d−1 all week + 11 kg CODbiowaste·m−3·d−1 from Monday to Friday was supplied. Finally a reactor was operated with an OLR of 12 kg CODbiowaste·m−3·d−1 from Monday to Friday and 5 kg CODpropionate·m−3·d−1 from Friday night to Monday morning to maintain a constant gas production for permanent operation of a gas engine. The propionate degradation rates (PDRs) were determined for biowaste + propionate feeding. Decreasing PDRs during starvation were analyzed. The POA was higher after propionate supply than after biowaste feeding and decreased faster during starvation of a propionate-fed rather than a biowaste-fed inoculum. Shifts of the propionate-oxidizing and methanogenic community were determined.

Bioengineering 2015, 2

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Keywords: anaerobic co-digestion; biowaste; propionate; propionate-oxidizing bacteria (POB); methanogenic community; community shifts

Bullet Points Anaerobic digestion of biowaste + propionate Periodic biowaste replacement by propionate Propionate-oxidizing bacteria and methanogenic archaea Constant biogas production with different substrates Community changes during different operational conditions 1. Introduction Propionate is a key intermediate of anaerobic digestion in general and of biowaste in particular [1–7]. Main sources of propionate in bioreactors are odd numbered fatty acids from lipolysis of fat and oil as well as from carbohydrate and amino acid degradation [3]. Propionate accumulates during disturbances of the anaerobic digestion process, caused by e.g., toxic substances, high dry matter content, or high OLR close to overload, when hydrolysis, acidogenesis, acetogenesis, and methanogenesis, leading to biogas formation from complex organic matter, are no longer balanced [4,7,8]. Degradation of accumulated fatty acids such as propionate, however, is thermodynamically unfavorable [9]. Only a few slow-growing and obligately syntrophic propionate-oxidizing bacteria (POB) can degrade propionate to acetate, CO2, and hydrogen in the absence of electron acceptors such as sulfate. Degradation of propionate under strictly anaerobic conditions requires a hydrogen partial pressure of degradation). Cell numbers of the three genera of POB increased at an OLR of 17–18 kg COD·m−3·d−1 (Figure 6c). The maximum propionate degradation rate was around 2.6 g·L−1·d−1 (calculated from data of Table 2) at a


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daily addition of 2.5 g·L−1 propionic acid into the reactor (contributing 4 kg CSB·m−3·d−1 to the OLR). Thus, only little propionate could have been generated from biowaste. When the reactor was fed with biowaste alone, up to 1.7 g·L−1 propionate was excreted. Propionate production was apparently suppressed during biowaste plus propionate feeding. Co-digestion of propionate seems to be a way to stabilize and increase the number of POB. Compared with the POB numbers in investigations with biowaste alone, the numbers of POB in digesters that were fed with biowaste + propionate were slightly higher: After the start of the full-scale biowaste digester of Karlsruhe 2–3 × 108 POB per mL were found in reactor effluent [6] and during co-digestion of biowaste with wheat and rye bread at high concentrations of propionate 2–5 × 108 POB per mL were detected [26], whereas 5–6 × 108 POB per mL were found after biowaste/propionate co-digestion (Figure 6c). When propionate was fed the most numerous POB were Pelotomaculum sp. ((5.1 ± 1.6) × 108 mL−1), followed by significantly less Syntrophobacter sp. ((8.4 ± 0.3) × 10 7 mL−1) and Smithella sp. ((1.1 ± 0.9) × 107 mL−1, Figure 6c). Pelotomaculum sp. and Syntrophobacter sp. began to grow when co-fermentation of 1 kg CSB·m−3·d−1 of propionate in the presence of 12 kg·m−3·d−1 biowaste was started, whereas Smithella sp. began to grow only when 1.5 kg propionate·m−3·d−1 were present and then approached numbers of Syntrophobacter sp. However, at higher OLR Smithella sp. decreased again to much lower final cell numbers (108 mL−1; Figure 6c, days 90–120). These results indicate that Smithella sp. are not able to compete with the dominant Pelotomaculum sp. and with Syntrophobacter sp. at very low or very high propionate levels. 5. Conclusions Propionic acid as a model substrate for highly acidified organic liquids could immediately substitute for biowaste suspensions in a biowaste reactor. Addition of little propionate to biowaste suspensions improved the propionate oxidation activity in a biowaste reactor at increasing OLR. An almost constant daily biogas production could be maintained during manual feeding of a biowaste suspension twice a day from Monday to Friday (regular biowaste collection period) and continuous pump feeding of propionic acid as a concentrated model substrate for acidified liquid wastes on Saturday and Sunday, when no biowaste is collected. Propionate degradation rates (PDRs) were much higher after propionate feeding than after biowaste feeding. During 5 days’ starvation the PDR decreased by 50%, but remained higher in biowaste suspensions after propionate feeding than after biowaste feeding. The total number of bacteria including POB increased with little propionate feeding to its maximum within a few days, while the archaeal community increased slowly for about 60 days. In the biowaste-fed reactors, members of the genus Pelotomaculum were the abundant POB. Members of the genera Syntrophobacter and Smithella were also present, but at much lower numbers. POB reach their maximum cell numbers 20–40 days after the start. Very low numbers of Smithella sp. were detected under starvation conditions or at high concentrations of propionate. Numbers of acetate-utilizing methanogens increased only slowly but not proportionally to maximal cell numbers of Bacteria, whereas cell numbers of Methanomicrobiales increased faster and proportionally to numbers of Bacteria with increasing propionate load and reached a much higher cell density after about 70 days.


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Acknowledgments We thank Amt für Abfallwirtschaft Karlsruhe for providing biowaste suspension from separately collected biowaste and for inocula from the full-scale digester, and Deutsche Forschungsgemeinschaft (DFG) for financial support (Grant Ga 546/4-2). Author Contributions All authors participated in conceiving and designing the experiments, Chaoran Li and Christoph Moertelmaier performed the experiments and analyses. Data evaluation was done by all authors, who also contributed to writing the paper. Conflicts of Interest The authors declare no conflict of interest. References 1.

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26. Li, C.; Moertelmaier, C.; Winter, J.; Gallert, C. Co-digestion of wheat and rye bread suspensions with the source-sorted municipal biowaste fraction. Waste Manag. 2015, revised. © 2015 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 license (http://creativecommons.org/licenses/by/4.0/).