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Dynamics of a Perturbed Microbial Community during Thermophilic Anaerobic Digestion of Chemically Defined Soluble Organic Compounds
Linköping University, Department of Thematic Studies, Tema Environmental Change. Linköping University, Faculty of Arts and Sciences.
Linköping University, Department of Thematic Studies, Tema Environmental Change. Linköping University, Faculty of Arts and Sciences.
Swedish Univ Agr Sci, Sweden.
Linköping University, Department of Thematic Studies, Tema Environmental Change. Linköping University, Faculty of Arts and Sciences.
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2018 (English)In: MICROORGANISMS, ISSN 2076-2607, Vol. 6, no 4, article id 105Article in journal (Refereed) Published
Abstract [en]

Knowledge of microbial community dynamics in relation to process perturbations is fundamental to understand and deal with the instability of anaerobic digestion (AD) processes. This study aims to investigate the microbial community structure and function of a thermophilic AD process, fed with a chemically defined substrate, and its association with process performance stability. Next generation amplicon sequencing of 16S ribosomal RNA (rRNA) genes revealed that variations in relative abundances of the predominant bacterial species, Defluviitoga tunisiensis and Anaerobaculum hydrogeniformans, were not linked to the process performance stability, while dynamics of bacterial genera of low abundance, Coprothermobacter and Defluviitoga (other than D. tunisiensis), were associated with microbial community function and process stability. A decrease in the diversity of the archaeal community was observed in conjunction with process recovery and stable performance, implying that the high abundance of specific archaeal group(s) contributed to the stable AD. Dominance of hydrogenotrophic Methanoculleus particularly corresponded to an enhanced microbial acetate and propionate turnover capacity, whereas the prevalence of hydrogenotrophic Methanothermobacter and acetoclastic Methanosaeta was associated with instable AD. Acetate oxidation via syntrophic interactions between Coprothermobacter and Methanoculleus was potentially the main methane-formation pathway during the stable process. We observed that supplementation of Se and W to the medium improved the propionate turnover by the thermophilic consortium. The outcomes of our study provided insights into the community dynamics and trace element requirements in relation to the process performance stability of thermophilic AD.

Place, publisher, year, edition, pages
MDPI , 2018. Vol. 6, no 4, article id 105
Keywords [en]
Thermophilic Anaerobic Digestion; process perturbation; process stability; microbial community dynamics; trace elements
National Category
Microbiology
Identifiers
URN: urn:nbn:se:liu:diva-154125DOI: 10.3390/microorganisms6040105ISI: 000455073000010PubMedID: 30314333OAI: oai:DiVA.org:liu-154125DiVA, id: diva2:1283583
Note

Funding Agencies|European Unions Seventh Framework ATBEST Marie-Curie ITN program [316838]; Linkoping University; Swedish Energy Agency [35624-2]; Biogas Research Center

Available from: 2019-01-29 Created: 2019-01-29 Last updated: 2019-04-02
In thesis
1. Anaerobic Digester Fluid Rheology and Process Efficiency: Interactions of Substrate Composition, Trace Element Availability, and Microbial Activity
Open this publication in new window or tab >>Anaerobic Digester Fluid Rheology and Process Efficiency: Interactions of Substrate Composition, Trace Element Availability, and Microbial Activity
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Reologi och processeffektivitet i biogasreaktorer : Interaktioner mellan substrat, tillgänglighet av spårämnen och mikrobiell aktivitet
Abstract [en]

As the anthropogenic greenhouse gas emissions continue imposing stress on our environment, it is becoming increasingly important to identify and implement new renewable technologies. Biogas production through anaerobic digestion has a great potential, since it links waste treatment with extraction of renewable energy, enabling circular bio-economies that are vital for a sustainable future.

For biogas to have an important role as a renewable energy carrier in society, the scale of its production will need to be increased substantially. New substrates need to be introduced along with raising organic loading rates of the reactors to increase the rate of biogas production. This contributes to challenges in maintaining process stability, thus increasing the risk for process disturbances, including problems that were not commonly encountered before. These difficulties may be particularly pronounced when a broad range of new, largely untested substrates are introduced, leading to an increased heterogeneity of organic material entering the reactors. In the case of currently the most common reactor type; the continuous stirred-tank biogas reactor (CSTBR); such problems may include shifts in rheology (i.e. fluid behaviour) of the anaerobic digester sludge. This may lead to increased energy consumption and decreased digester mixing efficiencies, which in turn may lead to inefficient biogas processes, ultimately decreasing the economic and environmental viability of biogas production. Much is still unknown regarding how rheology shifts happen in biogas reactors, particularly when it comes to what role the substrate plays in rheological dynamics, as compared to the microbial community during varying levels of biogas process stability.

This thesis elucidates the interactions between substrate type, microbial community and its metabolic activity, and anaerobic sludge rheology. A number of sludge samples from mesophilic and thermophilic CSTBRs digesting a broad range of substrates was analysed for their rheology. The specific effects of individual substrate types on CSTBR sludge rheology and the resulting implications for stirring power requirements and mixing efficiency were investigated. In order to also asses to which extent the microbial metabolism affects rheology at different levels of process disturbance, an experiment with a trace-element-induced inhibition of specific metabolic pathways under mesophilic reactor conditions was performed. This was used to identify the sequence of different interactions that occur in the reactor after the process begins to fail, and to evaluate how these interactions link to changes in digester sludge rheology. Finally, a case study of a disturbed thermophilic anaerobic digestion process was performed, including the monitoring of the response of rheology in relation to process stability, which was modified by changing trace element concentrations. The use of artificial substrate without polymeric compounds in both cases allowed for an evaluation of effects of the microbial community and its metabolic products on rheology without including the effects of complex substrates.

The results showed that substrate type has a large effect on how different process parameters correlate with fluid behaviour. This was particularly apparent in the case of total solids and total volatile solids, which correlated well with rheological parameters for samples from reactors digesting agricultural waste, sewage sludge, paper mill waste, or food waste, but not for mesophilic co-digesters. Among the different substrates investigated, food waste was generally observed to lead to the highest limit viscosities (i.e. apparent viscosities at high shear rates, where it becomes linear and constant) of the anaerobic sludge, while digestion of paper mill waste and thermophilic co-digestion led to some of the lowest. No fluid type could be clearly coupled to a specific substrate, but it could be observed that increased solids content could generally be associated with more complex, non-Newtonian rheological behaviour. The differences in fluid characteristics between reactors corresponded to large differences in modelled stirring power requirements and mixing efficiency. The results indicated that fluids with high values of rheological parameters, such as the consistency index (K) or yield stress (τ0), would likely require more power or an adapted stirring system to achieve complete mixing. The substrates generally contributed more to the rheology characteristics of the anaerobic sludge than microbial cells on their own. Trace element-induced process disturbance initially led to the inhibition of specific microbial groups among methanogenic archaea or their syntrophic partners, which later escalated to broader inhibition of many microbial groups due to the accumulation of fermentation products. This resulted in microbial cell washout with a corresponding decrease of the contribution of the cells to anaerobic sludge rheology. A recovery of the thermophilic anaerobic digestion process was possible after the supplementation of selenium and tungsten was increased, resulting in increased propionate turnover rates, growing cell densities, and higher viscosity. Major shifts in the methanogenic community were observed, corresponding to the level of process stability. It could be concluded based on these experiments that the specific effect of microbial cells and their activity on sludge rheology were linked to cell density, which corresponded to process stability.

A conceptual scheme was developed based on the studies in this thesis, defining complex interactions between substrate, microbial metabolism, and anaerobic sludge rheology in biogas processes. The possible causes of rheology shifts are visualised and discussed.

Abstract [sv]

Med anledning av att antropogena utsläpp av växthusgaser fortsätter att påverka vår miljö negativt, blir det allt viktigare att identifiera och implementera förnybara teknologier. Biogasproduktion, genom anaerob rötning, bidrar till att sammanlänka avfallshantering med förnybar energiomvandling samt möjliggör för cirkulära bioekonomier, avgörande för en hållbar framtid.

För att biogas ska utgöra en central roll som förnybar energibärare i samhället måste omfattningen av dess produktion ökas avsevärt. Nya substrat behöver introduceras, samtidigt som den organiska belastningen i existerande biogasreaktorer ökas, med syfte att öka produktionshastigheten. Detta bidrar emellertid till utmaningar avseende att upprätthålla processtabilitet, med ökad risk för processtörningar, inklusive nya typer av problem. Dessa svårigheter kan vara särskilt uttalade när nya, i stort sett otestade substrat, introduceras, vilket leder till ökad heterogenitet av organiskt material till reaktorerna. I tankreaktorer med omrörning (CSTBR), som i nuläget är den vanligast förekommande reaktortypen, kan sådana processförändringar innebära förändringar i reologiska egenskaper hos rötvätskor. Detta kan i sin tur leda till ökad energiförbrukning och lägre omrörningseffektivitet, vilket kan försämra biogasprocessens effektivitet samt bidra till minskad ekonomisk avkastning och minskade miljömässiga vinster. Det råder fortfarande oklarheter gällande reologiska förändringar av rötkammarmaterial i biogasreaktorer, särskilt med avseende på val av substrat i jämförelse med mikroorganismernas roll under varierade betingelser av processtabilitet.

Denna avhandling belyser interaktioner mellan typ av substrat, mikrobiella samhällen och deras metaboliska aktivitet och reologiska egenskaper av rötslam. Reologisk karaktärisering av rötvätskor från mesofila och termofila CSTBRs, som rötade ett brett spektrum av substrat, genomfördes. De specifika effekterna av individuella substrattyper på korresponderande rötvätskors reologiska egenskaper och dess implikationer för omrörning undersöktes. För att även kunna bedöma i vilken utsträckning den mikrobiella metabolismen inverkar på rötvätskors reologi, vid olika nivåer av processtörning, genomfördes en fallstudie med spårelement-inducerad inhibering av specifika metaboliska vägar, under mesofila reaktorbetingelser. Denna studie användes för att identifiera sekvensen av olika interaktioner som uppträder i reaktorn när processtörningar uppstår och för att utvärdera hur dessa interaktioner kopplar till förändringar i slamreologi. Slutligen genomfördes en fallstudie av en termofil biogasprocess innefattande karaktärisering av reaktormaterialets reologi i respons till förändringar i processtabilitet orsakad av förändrade spårelementkoncentrationer. I båda dessa fallstudier, möjliggjorde användningen av ett artificiellt substrat utan komplexa polymerer, att mikrobiella effekter på förändringar i rötvätskans reologi kunde studeras frånkopplat effekter av komplexa substrat.

Resultaten visade att substrattyp har stor inverkan på hur olika processparametrar korrelerar med rötvätskors reologiska egenskaper. Detta var särskilt tydligt med avseende på andelen totala respektive organiska fasta ämnen, vilka korrelerade väl med reologiska parametrar för rötvätskor från reaktorer som rötade jordbruksrester, avloppsslam, avfall från pappersbruk, respektive matavfall, men ej för rötvätskor från mesofila samrötningsanläggningar. Av de undersökta substrattyperna bidrog rötning av matavfall generellt till anaerobt slam med högst uppmätta värdena för gränsviskositet (dvs när viskositeten blir linjär och konstant vid ökade skjuvhastigheter), medan rötning av avfall från pappersbruk respektive termofil samrötning av olika substrat bidrog till de lägsta värdena för gränsviskoistet. Ingen reologisk vätsketyp kunde tydligt kopplas till en specifik substrattyp, men ökad mängd torrsubstans i rötvästkan kunde generellt associeras med komplexa, icke-newtonska, flödesegenskaper. Skillnaderna i flödesegenskaper motsvarade stora skillnader i behov av omrörningskraft och omrörningseffektivitet. Resultaten indikerade att rötvätskor med höga reologiska värden, exempelvis för konsistensindex (K) eller flytgräns (τ0), sannolikt kräver mer energi eller ett anpassat system för effektivare omrörning. Generellt bidrog substratet mer till rötvätskans reologiska egenskaper än de mikrobiella cellerna på egen hand. Inducerad spårämnesbrist i genomfört reaktorförök ledde i början till en hämning av specifika mikrobiella grupper inom metanogena arkéer och deras syntrofa partners, vilket i sin tur bidrog till en bredare hämning av flera mikrobiella grupper orsakad av ackumulering av olika fermentationsprodukter. Detta resulterade i ursköljning av cellbiomassa vilket i sin tur minskade deras effekt på slammets reologiska egenskaper. En återhämtning av den termofila biogasprocessen var möjlig efter ökade tillsatser av spårelementen selen och volfram, vilket resulterade i snabbare omsättning av propionsyra, förhöjd celldensitet och viskositet. Stora förändringar observerades samtidigt inom det metanogena samhället, vilka var kopplade till olika nivåer av processtabilitet. Den specifika effekten av mikroorganismerna och deras aktivitet med avseende på slammens reologiska egenskaper var kopplad till celldensitet, vilket motsvarade processtabiliteten.

Ett konceptuellt schema utvecklades baserat på resultat av beskrivna studier för att visualisera komplexa interaktioner mellan substrat, mikrobiell metabolism, och reologi av anaerobt slam i biogasprocesser. De möjliga orsakerna till reologiska förändringar visualiseras och diskuteras.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2019. p. 41
Series
Linköping Studies in Arts and Sciences, ISSN 0282-9800 ; 768
Keywords
Anaerobic digestion; biogas; rheology; trace elements; microbiology, Anaerob rötning, biogas, reologi, spårämnen, mikrobiologi
National Category
Renewable Bioenergy Research
Identifiers
urn:nbn:se:liu:diva-156030 (URN)10.3384/diss.diva-156030 (DOI)9789176850862 (ISBN)
Public defence
2019-04-26, TEMCAS, Hus T, Campus Valla, Linköping, 10:15 (English)
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Supervisors
Available from: 2019-04-02 Created: 2019-04-02 Last updated: 2019-04-08Bibliographically approved

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