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Biotechnology for Biofuels

, 10:186

First Online: 17 July 2017Received: 19 April 2017Accepted: 10 July 2017

Abstract

BackgroundThe fact that microalgae perform very efficiently photosynthetic conversion of sunlight into chemical energy has moved them into the focus of regenerative fuel research. Especially, biogas generation via anaerobic digestion is economically attractive due to the comparably simple apparative process technology and the theoretical possibility of converting the entire algal biomass to biogas-methane. In the last 60 years, intensive research on biogas production from microalgae biomass has revealed the microalgae as a rather challenging substrate for anaerobic digestion due to its high cell wall recalcitrance and unfavorable protein content, which requires additional pretreatment and co-fermentation strategies for sufficient fermentation. However, sustainable fuel generation requires the avoidance of cost-energy intensive biomass pretreatments to achieve positive net-energy process balance.

ResultsCultivation of microalgae in replete and limited nitrogen culture media conditions has led to the formation of protein-rich and low protein biomass, respectively, with the last being especially optimal for continuous fermentation. Anaerobic digestion of nitrogen limited biomass low-N BM was characterized by a stable process with low levels of inhibitory substances and resulted in extraordinary high biogas, and subsequently methane productivity 750 ± 15 and 462 ± 9 mLN g volatile solids VS day, respectively, thus corresponding to biomass-to-methane energy conversion efficiency of up to 84%. The microbial community structure within this highly efficient digester revealed a clear predominance of the phyla Bacteroidetes and the family Methanosaetaceae among the Bacteria and Archaea, respectively. The fermentation of replete nitrogen biomass replete-N BM, on the contrary, was demonstrated to be less productive 131 ± 33 mLN CH4 gVS day and failed completely due to acidosis, caused through high ammonia-ammonium concentrations. The organization of the microbial community of the failed replete-N digester differed greatly compared to the stable low-N digester, presenting a clear shift to the phyla Firmicutes and Thermotogae, and the archaeal population shifted from acetoclastic to hydrogenotrophic methanogenesis.

ConclusionsThe present study underlines the importance of cultivation conditions and shows the practicability of microalgae biomass usage as mono-substrate for highly efficient continuous fermentation to methane without any pretreatment with almost maximum practically achievable energy conversion efficiency biomass to methane.Open image in new windowGraphical abstractGrowth condition dependence of anaerobic conversion efficiency of microalgae biomass to methane

KeywordsBiofuel Biogas Methane Microalgae mono-substrate Nitrogen limitation Continuous anaerobic fermentation-digestion Maximal energy conversion efficiency Microbial community Ammonia-ammonium inhibition AbbreviationsADanaerobic digestion

C-Ncarbon-to-nitrogen ratio

DWdry weight

BMbiomass

SEstandard error

SDstandard deviation

FANfree ammonia nitrogen

TANtotal ammonium nitrogen

OLRorganic loading rate

HRThydraulic retention time

VFAvolatile fatty acids

Ccarbon

Nnitrogen

TMPtheoretical methane potential

VSvolatile solids

Electronic supplementary materialThe online version of this article doi:10.1186-s13068-017-0871-4 contains supplementary material, which is available to authorized users.

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Author: Viktor Klassen - Olga Blifernez-Klassen - Daniel Wibberg - Anika Winkler - Jörn Kalinowski - Clemens Posten - Olaf Kruse

Source: https://link.springer.com/







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