Influence of transitional states on the microbial ecology of anaerobic digesters treating solid wastes

Co-fermentation of sewage sludge and lignocellulosic biomass for production of medium-chain fatty acids

Medium-chain fatty acids (MCFAs) production from sewage sludge and lignocellulosic biomass (fallen leaves and grass) was explored. Co-fermentation of sludge and lignocellulosic biomass significantly accelerated the caproate production and promoted the longer-chain MCFAs formation. Co-fermentation of sludge and grass achieved the highest caproate production of 89.50 mmol C/L, which was 18.04 % and 41.73 % higher than the mono-fermentation of grass and sludge, respectively. Co-fermentation of sludge and leaves produced 63.80 mmol C/L caproate, which was 11.09 % and 1.03 % higher than the mono-fermentation of leaves and sludge, respectively. Microbial analysis showed that co-fermentation enriched CE microbes like genus Clostridium_sensu_stricto_13, Caprocipiproducens, Terrisporpbacter and Praraclostridium, and suppressed the competitive microbes like genus norank_f_Caldilineaceae and Desulfomicrobium. Functional enzymes analysis revealed that co-fermentation of sludge and leaves promoted MCFAs production through strengthening reverse β oxidation (RBO) pathway, while co-fermentation of sludge and grass stimulated MCFAs production by strengthening fatty acid biosynthesis (FAB) pathway.

Acclimatization of anaerobic sludge with cow manure and realization of high-rate food waste digestion for biogas production

Long-term acclimatization of anaerobic sludge was conducted by operating a mesophilic continuously stirred anaerobic reactor (CSTR) with continuous feeding of food wastes (FW) and cow manure (CM). During the long-term acclimatization, continued increase of enzyme activity was revealed, while the microbial structure tended stable as shown by the Shannon index and microbial community. By biomethane potential analysis, the acclimated sludge had a methane yield about 13 times higher than the initial anaerobic sludge. The acclimated sludge was subsequently used for FW digestion with stepwise organic loading rate increase without CM addition. The functional phyla of Bacteroidetes and Proteobacteria, which originated from CM but not very abundant, were significantly enriched not only during sludge acclimatization with CM addition but also in the process of FW digestion without CM addition. A microbe coexistence network was proposed to support an explanation of the metabolic pathways of FW digestion using the acclimated sludge.

Trichococcus shcherbakoviae sp. nov., isolated from a laboratory-scale anaerobic EGSB bioreactor operated at low temperature

A new species of the genus Trichococcus, strain Art1^T, was isolated from a psychrotolerant syntrophic propionate-oxidizing consortium, obtained before from a low-temperature EGSB reactor fed with a mixture of VFAs (acetate, propionate and butyrate). The 16S rRNA gene sequence of strain Art1^T was highly similar to those of other Trichococcus species (99.7-99.9 %) but digital DNA-DNA hybridization values were lower than those recommended for the delineation of a novel species, indicating that strain Art1^T is a novel species of the genus Trichococcus. Cells of strain Art1^T are non-motile cocci with a diameter of 0.5-2.0 µm and were observed singularly, in pairs, short chains and irregular conglomerates. Cells of Art1^T stained Gram-positive and produced extracellular polymeric substances . Growth was optimal at pH 6-7.5 and cells could grow in a temperature range of from -2 to 30 °C (optimum 25-30 °C). Strain Art1^T can degrade several carbohydrates, and the main products from glucose fermentation are lactate, acetate, formate and ethanol. The genomic DNA G+C content of strain Art1^T is 46.7 %. The major components of the cellular fatty acids are C16 : 1 ω9c, C16 : 0 and C18 : 1 ω9c. Based on genomic and physiological characteristics of strain Art1^T, a new species of the genus Trichococcus, Trichococcusshcherbakoviae, is proposed. The type strain of Trichococcusshcherbakoviae is Art1^T (=DSM 107162^T = VKM B-3260^T).

Adaptation of granular sludge microbial communities to nitrate, sulfide, and/or p-cresol removal

Effluents from petroleum refineries contain a toxic mixture of sulfide, nitrogen, and phenolic compounds that require adequate treatment for their removal. Biological denitrification processes are a cost-effective option for the treatment of these effluents, but the knowledge on the microbial interactions in simultaneous sulfide and phenol oxidation in denitrifying reactors is still very limited. In this work, microbial community structure and macrostructure of granular biomass were studied in three denitrifying reactors treating a mixture of inorganic (sulfide) and organic (p-cresol) electron donors for their simultaneous removal. The differences in the available substrates resulted in different community assemblies that supported high removal efficiencies, indicating the community adaptation capacity to the fluctuating compositions of industrial effluents. The three reactors were dominated by nitrate reducing and denitrifying bacteria where Thiobacillus spp. were the prevalent denitrifying organisms. The toxicity and lack of adequate substrates caused the endogenous decay of the biomass, leading to release of organic matter that maintained a diverse although not very abundant group of heterotrophs. The endogenous digestion of the granules caused the degradation of its macrostructure, which should be considered to further develop the denitrification process in sulfur-based granular reactors for treatment of industrial wastewater with toxic compounds.

Terminal restriction fragment length polymorphism is an "old school" reliable technique for swift microbial community screening in anaerobic digestion

The microbial community in anaerobic digestion has been analysed through microbial fingerprinting techniques, such as terminal restriction fragment length polymorphism (TRFLP), for decades. In the last decade, high-throughput 16S rRNA gene amplicon sequencing has replaced these techniques, but the time-consuming and complex nature of high-throughput techniques is a potential bottleneck for full-scale anaerobic digestion application, when monitoring community dynamics. Here, the bacterial and archaeal TRFLP profiles were compared with 16S rRNA gene amplicon profiles (Illumina platform) of 25 full-scale anaerobic digestion plants. The α-diversity analysis revealed a higher richness based on Illumina data, compared with the TRFLP data. This coincided with a clear difference in community organisation, Pareto distribution, and co-occurrence network statistics, i.e., betweenness centrality and normalised degree. The β-diversity analysis showed a similar clustering profile for the Illumina, bacterial TRFLP and archaeal TRFLP data, based on different distance measures and independent of phylogenetic identification, with pH and temperature as the two key operational parameters determining microbial community composition. The combined knowledge of temporal dynamics and projected clustering in the β-diversity profile, based on the TRFLP data, distinctly showed that TRFLP is a reliable technique for swift microbial community dynamics screening in full-scale anaerobic digestion plants.

Intermittent fasting for microbes: how discontinuous feeding increases functional stability in anaerobic digestion

BACKGROUND

Demand-driven biogas production could play an important role for future sustainable energy supply. However, feeding a biogas reactor according to energy demand may lead to organic overloading and, thus, to process failures. To minimize this risk, digesters need to be actively steered towards containing more robust microbial communities. This study focuses on acetogenesis and methanogenesis as crucial process steps for avoiding acidification. We fed lab-scale anaerobic digesters with volatile fatty acids under various feeding regimes and disturbances. The resulting microbial communities were analyzed on DNA and RNA level by terminal restriction fragment length polymorphism of the mcrA gene, 16S rRNA gene amplicon sequencing, and a [2-13C]-acetate assay. A modified Anaerobic Digestion Model 1 (ADM1) that distinguishes between the acetoclastic methanogens Methanosaeta and Methanosarcina was developed and fitted using experimental abiotic and biotic process parameters.

RESULTS

Discontinuous feeding led to more functional resilience than continuous feeding, without loss in process efficiency. This was attributed to a different microbial community composition. Methanosaeta dominated the continuously fed reactors, while its competitor Methanosarcina was washed out. With discontinuous feeding, however, the fluctuating acetic acid concentrations provided niches to grow and co-exist for both organisms as shown by transcription analysis of the mcrA gene. Our model confirmed the higher functional resilience due to the higher abundance of Methanosarcina based on its higher substrate uptake rate and higher resistance to low pH values. Finally, we applied our model to maize silage as a more complex and practically relevant substrate and showed that our model is likely transferable to the complete AD process.

CONCLUSIONS

The composition of the microbial community determined the AD functional resilience against organic overloading in our experiments. In particular, communities with higher share of Methanosarcina showed higher process stability. The share of these microorganisms can be purposefully increased by discontinuous feeding. A model was developed that enables derivation of the necessary feeding regime for a more robust community with higher share of Methanosarcina.

Anaerobic digestion of food waste: A review focusing on process stability

Food waste (FW) is rich in biomass energy, and increasing numbers of national programs are being established to recover energy from FW using anaerobic digestion (AD). However process instability is a common operational issue for AD of FW. Process monitoring and control as well as microbial management can be used to control instability and increase the energy conversion efficiency of anaerobic digesters. Here, we review research progress related to these methods and identify existing limitations to efficient AD; recommendations for future research are also discussed. Process monitoring and control are suitable for evaluating the current operational status of digesters, whereas microbial management can facilitate early diagnosis and process optimization. Optimizing and combining these two methods are necessary to improve AD efficiency.

Blending based optimisation and pretreatment strategies to enhance anaerobic digestion of poultry manure

Anaerobic digestion of poultry manure is limited by the excessive levels of nitrogen and the high concentration of dry matter. These limitations are usually overcome either by applying procedures to remove nitrogen or by employing pretreatments that allows to solubilise organic matter. In this work, the treatment of poultry manure was enhanced by co-digestion with pig manure through the methodological determination of optimal mixtures combined together with a thermochemical pretreatment coupled to ammonia stripping. The optimum poultry-pig mixture, resulting in a 24%:76% (volume basis) poultry-pig manure, was determined by applying a methodology based on linear programming which calculates the proportions of the blend which returns the maximum methane production while keeping a stable process. Pretreatment batch experiments, consisting of increasing both temperature and pH simultaneously with ammonia stripping process was optimised for a temperature of 90 °C and a pH of 10 resulting in a nitrogen removal efficiency of 72% and a 1.2-fold higher methane production in comparison to the unpretreated mixture. Continuous anaerobic co-digestion of pretreated optimum mixture enhanced the COD removal efficiency by 37% when compared with the treatment of unpretreated feedstock (37% vs 27%, respectively). This study indicates that combining blending optimisation of substrates, thermochemical pretreatments and ammonia stripping procedures prior to anaerobic co-digestion becomes a good strategy to overtake the limitations offered by solid- and nitrogen-rich substrates, such as poultry manure.

Methane production in an anaerobic osmotic membrane bioreactor using forward osmosis: Effect of reverse salt flux

This study investigated the impact of reverse salt flux (RSF) on microbe community and bio-methane production in a simulated fertilizer driven FO-AnMBR system using KCl, KNO₃ and KH₂PO₄ as draw solutes. Results showed that KH₂PO₄ exhibited the lowest RSF in terms of molar concentration 19.1mM/(m².h), while for KCl and KNO₃ it was 32.2 and 120.8mM/(m².h), respectively. Interestingly, bio-methane production displayed an opposite order with KH₂PO₄, followed by KCl and KNO₃. Pyrosequencing results revealed the presence of different bacterial communities among the tested fertilizers. Bacterial community of sludge exposed to KH₂PO₄ was very similar to that of DI-water and KCl. However, results with KNO₃ were different since the denitrifying bacteria were found to have a higher percentage than the sludge with other fertilizers. This study demonstrated that RSF has a negative effect on bio-methane production, probably by influencing the sludge bacterial community via environment modification.

Novel insight into the relationship between organic substrate composition and volatile fatty acids distribution in acidogenic co-fermentation

BACKGROUND

Co-fermentation is an attractive technology for improving volatile fatty acids (VFAs) production by treatment of solid organic wastes. However, it remains unclear how the composition of different organic matters in solid waste influences the VFAs distribution, microbial community structure, and metabolic pathway during acidogenic co-fermentation. In this study, different organic wastes were added into waste activated sludge (WAS) as co-fermentation substrates to explore the impact of organic matter composition on VFAs pattern and the microbiological mechanism .

RESULTS

Acetate was the most dominant VFA produced in all fermentation groups, making up 41.3-57.6% of the total VFAs produced during acidogenic co-fermentation under alkaline condition. With the increased addition of potato peel waste, the concentrations of propionate and valerate decreased dramatically, while ethanol and butyrate concentrations increased. The addition of food waste caused gradual decreases of valerate and propionate, but ethanol increased and butyrate was relatively stable. Some inconsistency was observed between hydrolysis efficiency and acidification efficiency. Our results revealed that starch was mainly responsible for butyrate and ethanol formation, while lipids and protein favored the synthesis of valerate and propionate. Microbial community analysis by high-throughput sequencing showed that Firmicutes had the highest relative abundance at phylum level in all fermentation groups. With 75% potato peel waste or 75% food waste addition to WAS, Bacilli (72.2%) and Clostridia (56.2%) were the dominant respective classes. In fermentation using only potato peel waste, the Bacilli content was 64.1%, while the Clostridia content was 53.6% in the food-only waste fermentation.

CONCLUSIONS

Acetate was always the dominant product in acidogenic co-fermentation, regardless of the substrate composition. The addition of carbon-rich substrates significantly enhanced butyrate and ethanol accumulation, while protein-rich substrate substantially benefited propionate and valerate generation. Potato peel waste substantially favored the enrichment of Bacilli, while food waste dramatically increased Clostridia content in the sludge.

Microbial community redundancy in anaerobic digestion drives process recovery after salinity exposure

Anaerobic digestion of high-salinity wastewaters often results in process inhibition due to the susceptibility of the methanogenic archaea. The ability of the microbial community to deal with increased salinity levels is of high importance to ensure process perseverance or recovery after failure. The exact strategy of the microbial community to ensure process endurance is, however, often unknown. In this study, we investigated how the microbial community is able to recover process performance following a disturbance through the application of high-salinity molasses wastewater. After a stable start-up, methane production quickly decreased from 625 ± 17 to 232 ± 35 mL CH₄ L^-1 d^-1 with a simultaneous accumulation in volatile fatty acids up to 20.5 ± 1.4 g COD L^-1, indicating severe process disturbance. A shift in feedstock from molasses wastewater to waste activated sludge resulted in complete process recovery. However, the bacterial and archaeal communities did not return to their original composition as before the disturbance, despite similar process conditions. Microbial community diversity was recovered to similar levels as before disturbance, which indicates that the metabolic potential of the community was maintained. A mild increase in ammonia concentration after process recovery did not influence methane production, indicating a well-balanced microbial community. Hence, given the change in community composition following recovery after salinity disturbance, it can be assumed that microbial community redundancy was the major strategy to ensure the continuation of methane production, without loss of functionality or metabolic flexibility.

Archaea and Bacteria Acclimate to High Total Ammonia in a Methanogenic Reactor Treating Swine Waste

Inhibition by ammonium at concentrations above 1000 mgN/L is known to harm the methanogenesis phase of anaerobic digestion. We anaerobically digested swine waste and achieved steady state COD-removal efficiency of around 52% with no fatty-acid or H₂ accumulation. As the anaerobic microbial community adapted to the gradual increase of total ammonia-N (NH₃-N) from 890 ± 295 to 2040 ± 30 mg/L, the Bacterial and Archaeal communities became less diverse. Phylotypes most closely related to hydrogenotrophic Methanoculleus (36.4%) and Methanobrevibacter (11.6%), along with acetoclastic Methanosaeta (29.3%), became the most abundant Archaeal sequences during acclimation. This was accompanied by a sharp increase in the relative abundances of phylotypes most closely related to acetogens and fatty-acid producers (Clostridium, Coprococcus, and Sphaerochaeta) and syntrophic fatty-acid Bacteria (Syntrophomonas, Clostridium, Clostridiaceae species, and Cloacamonaceae species) that have metabolic capabilities for butyrate and propionate fermentation, as well as for reverse acetogenesis. Our results provide evidence countering a prevailing theory that acetoclastic methanogens are selectively inhibited when the total ammonia-N concentration is greater than ~1000 mgN/L. Instead, acetoclastic and hydrogenotrophic methanogens coexisted in the presence of total ammonia-N of ~2000 mgN/L by establishing syntrophic relationships with fatty-acid fermenters, as well as homoacetogens able to carry out forward and reverse acetogenesis.

Characterization and Adaptation of Anaerobic Sludge Microbial Communities Exposed to Tetrabromobisphenol A

The increasing occurrence of tetrabromobisphenol A (TBBPA) in the environment is raising questions about its potential ecological and human health impacts. TBBPA is microbially transformed under anaerobic conditions to bisphenol A (BPA). However, little is known about which taxa degrade TBBPA and the adaptation of microbial communities exposed to TBBPA. The objectives of this study were to characterize the effect of TBBPA on microbial community structure during the start-up phase of a bench-scale anaerobic sludge reactor, and identify taxa that may be associated with TBBPA degradation. TBBPA degradation was monitored using LC/MS-MS, and the microbial community was characterized using Ion Torrent sequencing and qPCR. TBBPA was nearly completely transformed to BPA via reductive debromination in 55 days. Anaerobic reactor performance was not negatively affected by the presence of TBBPA and the bulk of the microbial community did not experience significant shifts. Several taxa showed a positive response to TBBPA, suggesting they may be associated with TBBPA degradation. Some of these taxa had been previously identified as dehalogenating bacteria including Dehalococcoides, Desulfovibrio, Propionibacterium, and Methylosinus species, but most had not previously been identified as having dehalogenating capacities. This study is the first to provide in-depth information on the microbial dynamics of anaerobic microbial communities exposed to TBBPA.

A mesophilic anaerobic digester for treating food waste: process stability and microbial community analysis using pyrosequencing

Background

Anaerobic digesters become unstable when operated at a high organi c loading rate (OLR). Investigating the microbial community response to OLR disturbance is helpful for achieving efficient and stable process operation. However, previous studies have only focused on community succession during different process stages. How does community succession influence process stability? Is this kind of succession resilient? Are any key microbial indicator closely related to process stability? Such relationships between microbial communities and process stability are poorly understood.

Results

In this study, a mesophilic anaerobic digester for treating food waste (FW) was operated to study the microbial diversity and dynamicity due to OLR disturbance. Overloading resulted in proliferation of acidogenic bacteria, and the resulting high volatile fatty acid (VFA) yield triggered an abundance of acetogenic bacteria. However, the abundance and metabolic efficiency of hydrogenotrophic methanogens decreased after disturbance, and as a consequence, methanogens and acetogenic bacteria could not efficiently complete the syntrophy. This stress induced the proliferation of homoacetogens as alternative hydrogenotrophs for converting excessive H₂ to acetate. However, the susceptible Methanothrix species also failed to degrade the excessive acetate. This metabolic imbalance finally led to process deterioration. After process recovery, the digester gradually returned to its original operational conditions, reached close to its original performance, and the microbial community profile achieved a new steady-state. Interestingly, the abundance of Syntrophomonas and Treponema increased during the deteriorative stage and rebounded after disturbance, suggesting they were resilient groups.

Conclusions

Acidogenic bacteria showed high functional redundancy, rapidly adapted to the increased OLR, and shaped new microbial community profiles. The genera Syntrophomonas and Treponema were resilient groups. This observation provides insight into the key microbial indicator that are closely related to process stability. Moreover, the succession of methanogens during the disturbance phase was unsuitable for the metabolic function needed at high OLR. This contradiction resulted in process deterioration. Thus, methanogenesis is the main step that interferes with the stable operation of digesters at high OLR. Further studies on identifying and breeding high-efficiency methanogens may be helpful for breaking the technical bottleneck of process instability and achieving stable operation under high OLR.

Comparative analysis of taxonomic, functional, and metabolic patterns of microbiomes from 14 full-scale biogas reactors by metagenomic sequencing and radioisotopic analysis

BACKGROUND

Biogas production is a very complex process due to the high complexity in diversity and interactions of the microorganisms mediating it, and only limited and diffuse knowledge exists about the variation of taxonomic and functional patterns of microbiomes across different biogas reactors, and their relationships with the metabolic patterns. The present study used metagenomic sequencing and radioisotopic analysis to assess the taxonomic, functional, and metabolic patterns of microbiomes from 14 full-scale biogas reactors operated under various conditions treating either sludge or manure.

RESULTS

The results from metagenomic analysis showed that the dominant methanogenic pathway revealed by radioisotopic analysis was not always correlated with the taxonomic and functional compositions. It was found by radioisotopic experiments that the aceticlastic methanogenic pathway was dominant, while metagenomics analysis showed higher relative abundance of hydrogenotrophic methanogens. Principal coordinates analysis showed the sludge-based samples were clearly distinct from the manure-based samples for both taxonomic and functional patterns, and canonical correspondence analysis showed that the both temperature and free ammonia were crucial environmental variables shaping the taxonomic and functional patterns. The study further the overall patterns of functional genes were strongly correlated with overall patterns of taxonomic composition across different biogas reactors.

CONCLUSIONS

The discrepancy between the metabolic patterns determined by metagenomic analysis and metabolic pathways determined by radioisotopic analysis was found. Besides, a clear correlation between taxonomic and functional patterns was demonstrated for biogas reactors, and also the environmental factors that shaping both taxonomic and functional genes patterns were identified.

Comparing the inhibitory thresholds of dairy manure co-digesters after prolonged acclimation periods: Part 1--Performance and operating limits

Co-digestion has been used to improve biogas yields and the long-term stability of anaerobic digesters compared to mono-digestion; however, less is known about the ultimate inhibition from co-substrates at their maximum loading rates and mixing ratios because these limits cannot be practically tested by existing facilities. Here, we performed a controlled experiment with long operating periods to ensure sufficient acclimation with the goal to observe ultimate inhibition and the full benefit that can be gained from co-digestion. The three substrates: 1) food waste (FW); 2) alkaline hydrolysate (AH); and 3) crude glycerol (GY) were individually co-digested with dairy manure (MN) for more than 900 days using continuously stirred anaerobic reactors at mesophilic temperatures. Food waste caused no reduction in performance or stability when co-digested with manure up to a total organic loading rate (OLR) of 3.9 g volatile solids (VS)·L(-1)·Day(-1) (MN:FW = 51:49; VS basis), resulting in a specific methane yield (SMY) of 297 ± 3 mL CH4·g VS(-1) for the combined wastes. Alkaline hydrolysate was co-digested with manure up to a total OLR of 2.7 g VS·L(-1)·Day(-1) (MN:AH = 75:25) with a corresponding SMY of 299 ± 6 mL CH4·g VS(-1). However, the free ammonia concentration reached levels previously reported as inhibitory, and may have led to the observed accumulation of volatile fatty acids at higher loading rates. Crude glycerol co-digestion resulted in an optimum SMY of 549 ± 25 mL CH4·g VS(-1) at a total OLR of 3.2 g VS·L(-1)·Day(-1) (MN:GY = 62:38). Stable digestion beyond this level was prohibited by an accumulation of long-chain fatty acids and foaming. These results can be used to implement effective co-digestion strategies. Co-substrates that possess similar inhibiting characteristics should be monitored to prevent severe instability at high loading rates and mixing ratios.

Comparing the inhibitory thresholds of dairy manure co-digesters after prolonged acclimation periods: Part 2--correlations between microbiomes and environment

Here, we studied the microbiome succession and time-scale variability of four mesophilic anaerobic reactors in a co-digestion study with the objective to find links between changing environmental conditions and the microbiome composition. The changing environmental conditions were ensured by gradual increases in loading rates and mixing ratios of three co-substrates with a constant manure-feeding scheme during an operating period longer than 900 days. Each co-substrate (i.e., alkaline hydrolysate, food waste, and glycerol) was co-digested separately. High throughput 16S rRNA gene sequencing was used to examine the microbiome succession. The alkaline hydrolysate reactor microbiome shifted and adapted to high concentrations of free ammonia, total volatile fatty acids, and potassium to maintain its function. The addition of food waste and glycerol as co-substrates also led to microbiome changes, but to a lesser extent, especially in the case of the glycerol reactor microbiome. The divergence of the food waste reactor microbiome was primarily linked to increasing free ammonia levels in the reactor; though, these levels remained below previously reported inhibitory levels for acclimated biomass. The glycerol reactor microbiome succession included an increase in Syntrophomonadaceae family members, which have previously been linked to long-chain fatty acid degradation. The glycerol reactor exhibited rapid failure and limited adaptation at the end of the study.

Key microbial communities steering the functioning of anaerobic digesters during hydraulic and organic overloading shocks

Overloading is one of the most typical process disturbance in anaerobic digesters, resulting in volatile fatty acids (VFAs) accumulation. This work aimed to study the microbial community dynamics during hydraulic (decreasing the hydraulic retention time (HRT)) and organic (increasing the organic loading rate maintaining the HRT constant) overload shocks in anaerobic reactors treating agro-industrial wastes, as well as during the recovery period. In both cases, the organic loading rate increased from 2 to 10gCODL(-1)d(-1), resulting in VFAs accumulation up to 9gL(-1). Both overloads were correlated to an increase in Bacteroidetes and Actinobacteria phyla and with a drop in Syntrophomonadaceae and Pseudomonadaceae families. In contrast, Tissierellaceae family only increased during the organic shock. Active Archaea decreased in both overloads, going from Methanosaeta dominance to Methanosarcina prevalence. During the recovery period, Porphyromonadaceae family increased its presence and Clostridium genus recovered values prior to perturbation.

Effects of sludge inoculum and organic feedstock on active microbial communities and methane yield during anaerobic digestion

Anaerobic digestion (AD) is a widespread microbial technology used to treat organic waste and recover energy in the form of methane ("biogas"). While most AD systems have been designed to treat a single input, mixtures of digester sludge and solid organic waste are emerging as a means to improve efficiency and methane yield. We examined laboratory anaerobic cultures of AD sludge from two sources amended with food waste, xylose, and xylan at mesophilic temperatures, and with cellulose at meso- and thermophilic temperatures, to determine whether and how the inoculum and substrate affect biogas yield and community composition. All substrate and inoculum combinations yielded methane, with food waste most productive by mass. Pyrosequencing of transcribed bacterial and archaeal 16S rRNA showed that community composition varied across substrates and inocula, with differing ratios of hydrogenotrophic/acetoclastic methanogenic archaea associated with syntrophic partners. While communities did not cluster by either inoculum or substrate, additional sequencing of the bacterial 16S rRNA gene in the source sludge revealed that the bacterial communities were influenced by their inoculum. These results suggest that complete and efficient AD systems could potentially be assembled from different microbial inocula and consist of taxonomically diverse communities that nevertheless perform similar functions.

Outlining microbial community dynamics during temperature drop and subsequent recovery period in anaerobic co-digestion systems

To improve the stability of anaerobic reactors, more knowledge is required about how the different communities react against operating perturbations and which specific ones respond better. The objective of this work was to monitor the changes in microbial community structure of an anaerobic digester during a temperature drop by applying different complementary molecular techniques. Temperature decrease led to an increase of Bacteroidales order, Porphyromonadaceae family and Bacteroides genus and a decrease in Syntrophomonas and Clostridium genera. Once the temperature was restored, the reactor recovered the steady state performance without requiring any modification in operational conditions or in the microbiome. During the recovery period, Sedimentibacter genus and Porphyromonadaceae family played an important role in the degradation of the accumulated volatile fatty acids. The hydrogenotrophic methanogens appeared to be the keystone archaeal population at low temperatures as well as in the recovery period. This study stands out that the understanding of microbial community dynamics during temperature drop could be utilized to develop strategies for the mitigation of temperature change consequences and speed up the recovery of stable reactor performance.

Inoculum selection influences the biochemical methane potential of agro-industrial substrates

Obtaining a reliable estimation of the methane potential of organic waste streams in anaerobic digestion, for which a biochemical methane potential (BMP) test is often used, is of high importance. Standardization of this BMP test is required to ensure inter-laboratory repeatability and accuracy of the BMP results. Therefore, guidelines were set out; yet, these do not provide sufficient information concerning origin of and the microbial community in the test inoculum. Here, the specific contribution of the methanogenic community on the BMP test results was evaluated. The biomethane potential of four different substrates (molasses, bio-refinery waste, liquid manure and high-rate activated sludge) was determined by means of four different inocula from full-scale anaerobic digestion plants. A significant effect of the selected inoculum on the BMP result was observed for two out of four substrates. This inoculum effect could be attributed to the abundance of methanogens and a potential inhibiting effect in the inoculum itself, demonstrating the importance of inoculum selection for BMP testing. We recommend the application of granular sludge as an inoculum, because of its higher methanogenic abundance and activity, and protection from bulk solutions, compared with other inocula.

Assessing anaerobic co-digestion of pig manure with agroindustrial wastes: the link between environmental impacts and operational parameters

Anaerobic co-digestion (AcoD) is established as a techno-economic profitable process by incrementing biogas yield (increased cost-efficiency) and improving the nutrient balance (better quality digestate) in comparison to mono-digestion of livestock wastes. However, few data are available on the environmental consequences of AcoD and most of them are mainly related to the use of energy crops as co-substrates. This work analysed the environmental impact of the AcoD of pig manure (PM) with several agroindustrial wastes (molasses, fish, biodiesel and vinasses residues) using life cycle assessment (LCA) methodology. For comparative purposes, mono digestion of PM has also been evaluated. Four out of six selected categories (acidification, eutrophication, global warming and photochemical oxidation potentials) showed environmental impacts in all the scenarios assessed, whereas the other two (abiotic depletion and ozone layer depletion potentials) showed environmental credits, remarking the benefit of replacing fossil fuels by biogas. This was also confirmed by the sensitivity analysis applied to the PM quality (i.e. organic matter content) and the avoided energy source demonstrating the importance of the energy recovery step. The influence of the type of co-substrate could not be discerned; however, a link between the environmental performance and the hydraulic retention time, the organic loading rate and the nutrient content in the digestate could be established. Therefore, LCA results were successfully correlated to process variables involved in AcoD, going a step further in the combination of techno-economic and environmental feasibilities.