Thermophilic anaerobic conversion of raw microalgae: Microbial community diversity in high solids retention systems

Food waste impact on dry anaerobic digestion of straw in a novel reactor: Biogas yield, stability, and hydrolysis-methanogenesis processes

Gradient anaerobic digestion reactor (GADR) can improve substrate utilization efficiency by solving the problem of the "short circuit" of materials. However, the substrate's composition significantly affects the reactor's performance. This study investigated the impact of food waste (FW) levels on corn straw's dry anaerobic digestion (AD) in a novel GADR. The results show that biomethane production can be improved by coupling urban and agricultural solid waste recycling. The mechanism is to increase the hydrolysis and acid production efficiency, and the abundance of enzymes related to methanogenesis. The maximum methane yield (494.2 mL CH4/g VS) and the highest anaerobic biodegradability (85.7 %) were obtained when the FW was added at 60 %. The co-digestion of FW and straw can improve the hydrolysis and acid production efficiency and methane yield, which improves the buffering capacity and stability of the system compared with the single digestion of FW.

Metagenomic insights into improving mechanisms of Fe0 nanoparticles on volatile fatty acids production from potato peel waste anaerobic fermentation

The management of potato peel waste (PPW) has been a challenge faced by the potato industry. This investigation assessed the feasibility of PPW for volatile fatty acids (VFAs) production via anaerobic fermentation, and investigated the impact of Fe⁰ nanoparticles (Fe⁰ NPs) supplementation on the VFAs production. It is found that PPW is a potential feedstock for producing VFAs, achieving a yield of 480.4 mg COD/g-vS Meanwhile, the supplementation of Fe⁰ NPs significantly promoted the VFAs productivity and quality. The higher enrichment of VFAs-producing bacteria, including Clostridium, Proteiniphilum, Fonticella and Pygmaiobacter, contributed to the promotion of the VFAs yield. Furthermore, metagenomic analysis revealed that the encoding genes responsible for carbohydrate metabolism (especially starch), membrane transport, glycolysis and the formation of acetic and butyric acids were remarkably up-regulated,which could be the essential reason for the enhanced metabolic activity and VFAs productivity. This work provides a promising strategy for recycling PPW.

The synergistic effect of rumen cellulolytic bacteria and activated carbon on thermophilic digestion of cornstalk

To explore the bioaugmentation of rumen cellulolytic bacteria (RCB) and activated carbon (AC) on thermophilic digestion of cornstalk, biochemical methane potential tests were carried out. Adding RCB or AC can improve methane production, while simultaneous existence of AC (10 g/L) and RCB (5%) obtained the best performance. The maximum cellulose degradation rate, methane production rate and methane yield were 66.92%, 32.2 L/(kgVS·d), and 144.9 L/kgVS, which increased by 30.23%, 51.17%, and 20.35% compared with control group. The cellulolytic and fermentative bacteria (Hydrogenispora), syntrophic acetate-oxidizing bacteria (norank_o_MBA03), and hydrogenotrophic Methanothermobacter were crucial for thermophilic digestion of cornstalk. The enhancement of AC was due to the enrichment of Hydrogenispora and Methanothermobacter, while RCB can increase the abundance of cellulolytic bacteria (Halocella and norank_o_M55-D21) and mixotrophic Methanosarcina. The synergetic effect of AC and RCB owing to the enriched cellulolytic bacteria, the enhanced syntrophic acetate oxidation and the concentrated carbon metabolic flow to methane.

Thermophilic biogas production from microalgae-bacteria aggregates: biogas yield, community variation and energy balance

Biogas production through anaerobic mesophilic digestion is the most straightforward biofuel production route integrated into microalgae-bacteria wastewater treatment plants. Improvement of this biofuel route without adding pretreatment units is possible through the temperature increase. This paper presents a comprehensive evaluation of the transitory effect of different temperatures (35 °C and 55 °C) and hydraulic retention times (HRT) of 15 and 30 d on the long-term methane production using non-pretreated microalgae-bacteria aggregates as a feedstock. The thermophilic transition from mesophilic inoculum adapted to microalgae-bacteria aggregate increased 1.7-fold the methane production (0.41 m³CH₄ kgVS^-1) at HRT of 30 d. A substantial decrease in the microbial community's diversity present in the anaerobic reactor was observed when thermophilic conditions were applied, explaining the long adaptation period needed. The increase of the operative temperature condition promotes changes in the dominance pathway of methanogenesis from hydrogenotrophic to acetolactic. The energy balance assessment showed a positive net energy ratio when the digester was operated at an HRT of 30 d. A maximum net energy ratio of 1.5 was achieved at mesophilic temperature. This study demonstrated, based on experimental data, that microalgal digestion with an HRT of 30 d favors energy self-sustainability in microalgal wastewater treatment plants.

Effects of biological pretreatments of microalgae on hydrolysis, biomethane potential and microbial community

Parameters of temperature-phased anaerobic digestion (TPAD) were varied to study their effects on hydrolysis, biomethane potential (BMP), and microbial diversity of microalgae biodegradation. Anaerobic pretreatments at 85 °C demonstrated the release of soluble carbohydrate and protein molecules under low microbial metabolic activity. However, at 55 °C, anaerobic pretreatments showed superior performance in methane yield, nutrient release, and volatile fatty acids (VFAs) production due to dominant Clostridium. Furthermore, the highest destruction of volatile solids (VS) was observed during aerobic pretreatments at 55 °C under the influence of various quantities of these genera - Luteimonas, Symbiobacterium, Soehngenia, Thermobacillus, and Ureibacillus. Statistical analysis revealed that hydrolysis and BMP were not correlated. However, soluble nitrogen and phosphorous showed strong correlation with methane (r = 0.623 and 0.948, respectively) under thermo-anaerobic pretreatment, while VS removal and concentrations of acetic and butyric acids and lipids were positively correlated with each other under thermo-aerobic pretreatment.

Comparison of microbial community structures between mesophilic and thermophilic anaerobic digestion of vegetable waste

The anaerobic digestion performance correlates with the functional microbial community. Mesophilic and thermophilic digestions of vegetable waste were conducted, and dynamics of the microbial community were investigated. The mesophilic and thermophilic collapsed stages occurred at organic loading rates of 1.5 and 2.0 g VS/(L d) due to the accumulation of volatile fatty acids with final concentrations of 2276 and 6476 mg/L, respectively. A high concentration of volatile fatty acids caused the severe inhibition of methanogens, which finally led to the imbalance between acetogenesis and methanogenesis. The mesophilic digestion exhibited a higher microbial diversity and richness than the thermophilic digestion. Syntrophic acetate-oxidizing coupled with hydrogenotrophic methanogenesis was the dominant pathway in the thermophilic stable system, and acetoclastic methanogenesis in the mesophilic stable system. The dominant acidogens, syntrophus, and methanogens were unclassified_f__Anaerolineaceae (8.68%), Candidatus_Cloacamonas (19.70%), Methanosaeta (6.10%), and Methanosarcina (4.08%) in the mesophilic stable stage, and Anaerobaculum (12.59%), Syntrophaceticus (4.84%), Methanosarcina (30.58%), and Methanothermobacter (3.17%) in thermophilic stable stage. Spirochaetae and Thermotogae phyla were the characteristic microorganisms in the mesophilic and thermophilic collapsed stages, respectively. These findings provided valuable information for the deep understanding of the difference of the microbial community and methane-producing mechanism between mesophilic and thermophilic digestion of vegetable waste.

Co-digestion of harvested microalgae and primary sludge in a mesophilic anaerobic membrane bioreactor (AnMBR): Methane potential and microbial diversity

Anaerobic co-digestion of primary sludge and raw microalgae (Scenedesmus and Chlorella) was performed in a lab-scale semi-continuous anaerobic membrane bioreactor to assess the biological performance and identify the microbial community involved in the co-digestion process. The reactor was operated at 35 °C for 440 days, working at a solids retention time of 100 days. The system achieved 73% biodegradability and showed high stability in terms of pH and volatile fatty acids. An enriched microbial community was observed. Of the several phyla, Chloroflexi and Proteobacteria were the most abundant. Cellulose-degraders phyla (Bacteroidetes, Chloroflexi and Thermotogae) were detected. Syntrophic microorganisms played an important role in intermediate degradation, enhancing methane production, mainly carried out by Methanosaeta. A nutrient-rich effluent (400 mg NH₄-N·L^-1 and 29 mg PO₄-P·L^-1) and digestate (860 mg N·L^-1 and 151 mg P·L^-1) were obtained. The bio-nutrients released from anaerobic co-digestion could be reused for microalgae cultivation or agricultural applications.

Unveiling microbial structures during raw microalgae digestion and co-digestion with primary sludge to produce biogas using semi-continuous AnMBR systems

Methane production from microalgae can be enhanced through anaerobic co-digestion with carbon-rich substrates and thus mitigate the inhibition risk associated with its low C:N ratio. Acclimated microbial communities for microalgae disruption can be used as a source of natural enzymes in bioenergy production. However, co-substrates with a certain microbial diversity such as primary sludge might shift the microbial structure. Substrates were generated in a Water Resource Recovery Facility (WRRF) and combined as follows: Scenedesmus or Chlorella digestion and microalgae co-digestion with primary sludge. The study was performed using two lab-scale Anaerobic Membrane Bioreactors (AnMBR). During three years, different feedstocks scenarios for methane production were evaluated with a special focus on the microbial diversity of the AnMBR. 57% of the population was shared between the different feedstock scenarios, revealing the importance of Anaerolineaceae members besides Smithella and Methanosaeta genera. The addition of primary sludge enhanced the microbial diversity of the system during both Chlorella and Scenedesmus co-digestion and promoted different microbial structures. Aceticlastic methanogen Methanosaeta was dominant in all the feedstock scenarios. A more remarkable role of syntrophic fatty acid degraders (Smithella, Syntrophobacteraceae) was observed during co-digestion when only microalgae were digested. However, no significant changes were observed in the microbial composition during anaerobic microalgae digestion when feeding only Chlorella or Scenedesmus. This is the first work revealing the composition of complex communities for semi-continuous bioenergy production from WRRF streams. The stability and maintenance of a microbial core over-time in semi-continuous AnMBRs is here shown supporting their future application in full-scale systems for raw microalgae digestion or co-digestion.

Acclimatised rumen culture for raw microalgae conversion into biogas: Linking microbial community structure and operational parameters in anaerobic membrane bioreactors (AnMBR)

Ruminal fluid was inoculated in an Anaerobic Membrane Reactor (AnMBR) to produce biogas from raw Scenedesmus. This work explores the microbial ecology of the system during stable operation at different solids retention times (SRT). The 16S rRNA amplicon analysis revealed that the acclimatised community was mainly composed of Anaerolineaceae, Spirochaetaceae, Lentimicrobiaceae and Cloacimonetes fermentative and hydrolytic members. During the highest biodegradability achieved in the AnMBR (62%) the dominant microorganisms were Fervidobacterium and Methanosaeta. Different microbial community clusters were observed at different SRT conditions. Interestingly, syntrophic bacteria Gelria and Smithella were enhanced after increasing 2-fold the organic loading rate, suggesting their importance in continuous systems producing biogas from raw microalgae.