Microbial Community Drivers in Anaerobic Granulation at High Salinity

De novo anaerobic granulation with varying organic substrates: granule growth and microbial community responses

Anaerobic granulation from dispersed inoculum is recognized as a slow process. However, studies under saline conditions have shown that adding complex proteinaceous substrates can accelerate this process. To explore whether this holds true also under non-saline conditions, we conducted a 262-days experiment with four lab-scale upflow anaerobic sludge blanket reactors inoculated with digested sewage sludge. Each reactor received a synthetic feed containing varying amount of carbohydrate/protein substrate: glucose (RGlu), acetate/tryptone (RAc+Try), glucose/tryptone (RGlu+Try), and glucose/starch (RGlu+Sta). Development of granules with different influent composition was monitored with macroscopy, analysis of the extracellular polymeric substances, and microbial diversity. Granulation was faster in reactors RGlu+Try and RGlu+Sta. Increasing granule diameters positively correlated with the occurrence of bacteria from Muribaculaceae and Lachnospiraceae families, suggesting their involvement in de novo granulation. Granules of RGlu+Try also had high relative abundances of both fermenting bacteria (e.g. Lactococcus, Streptococcus, Trichococcus) and bacteria involved in the oxidation of volatile fatty acids (Smithella, Acetobacteroides). The results of this study provide a basis for strategies to enhance the sludge granulation rate in practice when granular inoculum is not available. Specifically, supplementing small amounts of waste protein during reactor start-up can be effective.

Variation of viscoelastic properties of extracellular polymeric substances and their relation to anaerobic granule's mechanical strength in full-scale treatment plants

Extracellular polymeric substances (EPS) are considered to play a pivotal role in shaping granules' physical properties. In this contribution, we characterized the viscoelastic properties of EPS from granules of 9 full-scale industrial anaerobic reactors; and quantitatively investigated whether these properties correlate with granules' resistance to compression (Egranule) and shear strength (Sgranule). Most granules with a higher shear strength, also exhibited a stronger resistance to compression (r = 0.96, p = 0.002), except those granules that contained relatively more proteins in their EPS. Interestingly, these granules were also the most resistant to shear stress (Sgranule ≥ 110 ± 40 h). Furthermore, the EPS hydrogels of these granules had slower softening rates (κ < 0.9) compared to the others (κ ranged between 0.95 and 1.20), indicating stronger gels were formed. These findings suggest that the EPS hydrogel softening rate could be a key parameter to explain granule's shear strength.

Full-scale study on high-rate low-temperature anaerobic digestion of agro-food wastewater: process performances and microbial community

The fast-growing global population has led to a substantial increase in food production, which generates large volumes of wastewater during the process. Despite most industrial wastewater being discharged at lower ambient temperatures (<20 °C), majority of the high-rate anaerobic reactors are operated at mesophilic temperatures (>30 °C). High-rate low-temperature anaerobic digestion (LtAD) has proven successful in treating industrial wastewater both at laboratory and pilot scales, boasting efficient organic removal and biogas production. In this study, we demonstrated the feasibility of two full-scale high-rate LtAD bioreactors treating meat processing and dairy wastewater, and the microbial communities in both reactors were examined. Both reactors exhibited rapid start-up, achieving considerable chemical oxygen demand (COD) removal efficiencies (total COD removal >80%) and generating high-quality biogas (CH4% in biogas >75%). Long-term operations (6-12 months) underscored the robustness of LtAD bioreactors even during winter periods (average temperature <12 °C), as evidenced by sustained high COD removal rates (total COD removal >80%). The stable performance was underpinned by a resilient microbial community comprising active acetoclastic methanogens, hydrolytic, and fermentative bacteria. These findings underscore the feasibility of high-rate low-temperature anaerobic wastewater treatment, offering promising solutions to the zero-emission wastewater treatment challenge.

Etiology and epidemiology of digital dermatitis in Australian dairy herds

Bovine digital dermatitis (BDD) is an important cause of lameness in dairy cows worldwide. However, very little is known about this disease in Australian herds, which are predominantly managed on pasture. The primary objectives of this cross-sectional study were to describe the presence and prevalence of BDD in Australian dairy herds and to characterize the microbiota of healthy skin and M4 lesions of BDD-affected, pasture-managed cows. Cows from 71 dairy herds were examined at milking time to identify the presence of BDD lesions. True prevalence was estimated using Bayesian methods with informative priors for sensitivity and specificity. Biopsy samples (n = 60) were collected from cows with and without BDD lesions in 7 pasture-based herds. The microbiota in the superficial and deep strata of each tissue biopsy were characterized via sequencing of the V3-V4 region of the bacterial ribosomal RNA gene. Lesions were detected in 1,817 (11.5%) of 15,813 cows and in 68 of 71 (95.8%) herds. The median herd-level apparent and true prevalences of BDD were 8.5% and 18.1%, respectively, but prevalences varied considerably between farms. On farms with BDD, M4 lesions accounted for 70% to 100% of all lesions (interquartile range = 95.1%-100%, median = 100%); M2 lesions (i.e., large ulcerative lesions) were observed at low prevalence (<2.2%) in the few herds (7/71, 9.9%) where they were found. There was a significant difference in the composition of the microbiota between healthy skin and M4 lesions but not between superficial and deep tissue layers. Several gut- and effluent-associated bacterial taxa, including Lentimicrobium and Porphyromonas, which have previously been associated with BDD, were abundant in BDD lesions but not in control biopsies. Our study supports the idea that such taxa are involved in, although possibly not essential to, lesion development and persistence in pasture-managed cows in Australia. Our results also suggest that Dichelobacter may contribute to the disease process. We conclude that BDD is likely to occur in most Australian dairy farms, but that further studies are needed to identify its effect on cow welfare and productivity. Further investigation of the etiology of BDD in Australian dairy herds is also necessary to inform prevention and control strategies.

Investigating functional mechanisms in the Co-biodegradation of lignite and guar gum under the influence of salinity

The biodegradation of guar gum by microorganisms sourced from coalbeds can result in low-temperature gel breaking, thereby reducing reservoir damage. However, limited attention has been given to the influence of salinity on the synergistic biodegradation of coal and guar gum. In this study, biodegradation experiments of guar gum and lignite were conducted under varying salinity conditions. The primary objective was to investigate the controlling effects and mechanisms of salinity on the synergistic biodegradation of lignite and guar gum. The findings revealed that salinity had an inhibitory effect on the biomethane production from the co-degradation of lignite and guar gum. The biomethane production declined with increasing salinity levels, decreasing from 120.9 mL to 47.3 mL. Even under 20 g/L salt stress conditions, bacteria in coalbeds could effectively break the gel and the viscosity decreased to levels below 5 mPa s. As salinity increased, the removal rate of soluble chemical oxygen demand (SCOD) decreased from 55.63% to 31.17%, and volatile fatty acids (VFAs) accumulated in the digestion system. High salt environment reduces the intensity of each fluorescence peak. Alterations in salinity led to changes in microbial community structure and diversity. Under salt stress, there was an increased relative abundance of Proteiniphilum and Methanobacterium, ensuring the continuity of anaerobic digestion. Hydrogentrophic methanogens exhibited higher salt tolerance compared to acetoclastic methanogens. These findings provide experimental evidence supporting the use of guar gum fracturing fluid in coalbeds with varying salinity levels.

Alterations of Glycan Composition in Aerobic Granular Sludge during the Adaptation to Seawater Conditions

Bacteria can synthesize a diverse array of glycans, being found attached to proteins and lipids or as loosely associated polysaccharides to the cells. The major challenge in glycan analysis in environmental samples lies in developing high-throughput and comprehensive characterization methodologies to elucidate the structure and monitor the change of the glycan profile, especially in protein glycosylation. To this end, in the current research, the dynamic change of the glycan profile of a few extracellular polymeric substance (EPS) samples was investigated by high-throughput lectin microarray and mass spectrometry, as well as sialylation and sulfation analysis. Those EPS were extracted from aerobic granular sludge collected at different stages during its adaptation to the seawater condition. It was found that there were glycoproteins in all of the EPS samples. In response to the exposure to seawater, the amount of glycoproteins and their glycan diversity displayed an increase during adaptation, followed by a decrease once the granules reached a stable state of adaptation. Information generated sheds light on the approaches to identify and monitor the diversity and dynamic alteration of the glycan profile of the EPS in response to environmental stimuli.

Acclimation of microbial communities to low and moderate salinities in anaerobic digestion

In recent years anaerobic digestion (AD) has been investigated as suitable biotechnology to treat wastewater at elevated salinities. However, when starting up AD reactors with inocula that are not adapted to salinity, low concentrations of sodium (Na+) in the influent can already cause disintegration of microbial aggregates and wash-out. This study investigated biomass acclimation to 5 g Na+/L of two different non-adapted inocula in two lab-scale hybrid expanded granular sludge bed (EGSB)-anaerobic filter (AF) reactors fed with synthetic wastewater. After an initial biomass disintegration, new aggregates were formed relatively fast (i.e., after 95 days of operation), indicating microbial community adaptation. The newly formed microbial aggregates accumulated Na+ at the expense of calcium (Ca2+), but this did not hamper biomass retention or process performance. The hybrid reactor configuration, including a pumice stone filter in the upper section, and the low up-flow velocities applied, were key features for retaining the biomass within the system. This reactor configuration can be easily applied and represents a low-cost alternative for acclimating biomass to saline effluents, even in existing digesters. When the acclimated biomass was transferred from EGSB to an up-flow anaerobic sludge blanket (UASB) reactor configuration also fed with saline synthetic wastewater, more dense aggregates in the form of granules were obtained. The performances of the UASB inoculated with the acclimated biomass were comparable to another reactor seeded with saline-adapted granular sludge from a full-scale plant. Regardless of the inoculum origin, a defined core microbiome of Bacteria (Thermovirga, Bacteroidetes vadinHA17, Blvii28 wastewater-sludge group, Mesotoga, and Synergistaceae) and Archaea (Methanosaeta and Methanobacterium) was detected, highlighting the importance of these microbial groups in developing halotolerance and maintaining AD process stability.

Unifying concepts in methanogenic, aerobic, and anammox sludge granulation

The retention of dense and well-functioning microbial biomass is crucial for effective pollutant removal in several biological wastewater treatment technologies. High solids retention is often achieved through aggregation of microbial communities into dense, spherical aggregates known as granules, which were initially discovered in the 1980s. These granules have since been widely applied in upflow anaerobic digesters for waste-to-energy conversions. Furthermore, granular biomass has been applied in aerobic wastewater treatment and anaerobic ammonium oxidation (anammox) technologies. The mechanisms underpinning the formation of methanogenic, aerobic, and anammox granules are the subject of ongoing research. Although each granule type has been extensively studied in isolation, there has been a lack of comparative studies among these granulation processes. It is likely that there are some unifying concepts that are shared by all three sludge types. Identifying these unifying concepts could allow a unified theory of granulation to be formed. Here, we review the granulation mechanisms of methanogenic, aerobic, and anammox granular sludge, highlighting several common concepts, such as the role of extracellular polymeric substances, cations, and operational parameters like upflow velocity and shear force. We have then identified some unique features of each granule type, such as different internal structures, microbial compositions, and quorum sensing systems. Finally, we propose that future research should prioritize aspects of microbial ecology, such as community assembly or interspecies interactions in individual granules during their formation and growth.

Facilitating direct interspecies electron transfer in anaerobic digestion via speeding up transmembrane transport of electrons and CO2 reduction in methanogens by Na+ adjustment

The possibility of facilitating direct interspecies electron transfer (DIET) in anaerobic digestion with different concentrations of NaCl was explored. Additional NaCl at 2 or 4 g/L strengthened anaerobic digestion to resist the high-organic loading rate impacts, whereas the higher concentrations of NaCl (6 or 8 g/L) suppressed methanogenesis. Additional MgCl2 with the same ion strength as NaCl at 2 g/L had no effect on performances. Additional NaCl at 2 or 4 g/L dramatically increased the abundance of Methanosarcina species (20.7%/23.4% vs 8.6%) and stimulated the growth of Sphaerochaeta and Petrimonas species that could transfer electrons to the soluble Fe(III) or elemental sulfur. Electrochemical evidences showed that, additional NaCl at 2 or 4 g/L increased capacitances and decreased charge transfer resistances of Methanosarcina-dominant communities. Metagenomic evidences showed that, additional NaCl at 2 or 4 g/L increased the abundance of genes that encoded the type IV pilus assembly proteins (1.98E-04/1.87E-04 vs 1.85E-04) and cytochrome c-like proteins (5.51E-04/5.60E-04 vs 5.31E-04). In addition, additional NaCl at 2 or 4 g/L increased the abundance of genes for methanophenazine (MP)/MPH2 transformation (1.04E-05/1.24E-05 vs 8.06E-06) and CO2 reduction (1.64E-03/1.86E-03 vs 1.06E-03), suggesting a rapid transmembrane transport of electrons and CO2 reduction in methanogens. Both processes were closely associated with F420/F420H2 transformation that required ATP. Additional NaCl at 2 or 4 g/L increased the yield of ATP (256.0/249.3 vs 231.8 nmol/L) that might promote F420/F420H2 transformation in methanogens, which overcame the thermodynamic limitations of combining electrons with protons for the reduction of CO2 to methane and facilitated DIET.

New insights into multi-strategies of sludge granulation in up-flow anaerobic sludge blanket reactors from community succession and interaction

This study aimed to elucidate the multiple strategies employed by anaerobes during granulation in a laboratory upflow anaerobic sludge blanket reactor, based on microbial succession and interactions. The anaerobic granulation process featured staged dominance of microbial genera, corresponding well with the environmental traits. Across the stages (selection, seeding, expansion, and maturation), chemotaxis attraction of nitrogen and/or carbon sources and flagellar motion were the primary strategy of microbial assembly. The second messengers - cyclic adenosine and guanosine monophosphates - partially regulated the agglomeration of filamentous Euryachaeota and Chloroflexi as the inner cores, while quorum sensing mediated the expansion of granules prior to maturation. Antagonism or competition governed the interactions within the phylogenetic molecular ecological network during sludge granulation, which were largely driven by the low-abundance (<1%) taxa. These new insights suggest that better engineering solutions to enhance chemotaxis attraction and species selection could achieve more efficient anaerobic granular sludge processes.

Psychrophilic and mesophilic anaerobic treatment of synthetic dairy wastewater with long chain fatty acids: Process performances and microbial community dynamics

Facilitating the anaerobic degradation of long chain fatty acids (LCFA) is the key to unlock the energy potential of lipids-rich wastewater. In this study, the feasibility of psychrophilic anaerobic treatment of LCFA-containing dairy wastewater was assessed and compared to mesophilic anaerobic treatment. The results showed that psychrophilic treatment at 15 ℃ was feasible for LCFA-containing dairy wastewater, with high removal rates of soluble COD (>90%) and LCFA (∼100%). However, efficient long-term treatment required prior acclimation of the biomass to psychrophilic temperatures. The microbial community analysis revealed that putative syntrophic fatty acid bacteria and Methanocorpusculum played a crucial role in LCFA degradation during both mesophilic and psychrophilic treatments. Additionally, a fungal-bacterial biofilm was found to be important during the psychrophilic treatment. Overall, these findings demonstrate the potential of psychrophilic anaerobic treatment for industrial wastewaters and highlight the importance of understanding the microbial communities involved in the process.

Enhanced anaerobic reduction of nitrobenzene at high salinity by betaine acting as osmoprotectant and regulator of metabolism

Anaerobic technology is extensively applied in the treatment of industrial organic wastewater, but high salinity always triggers microbial cell dehydration, causing the failure of the anaerobic process. In this work, betaine, one kind of compatible solutes which could balance the osmotic pressure of anaerobic biomass, was exogenously added for enhancing the anaerobic reduction of nitrobenzene (NB) at high salinity. Only 100 mg L^-1 betaine dosing could significantly promote the removal efficiency of NB within 35 h at 9% salinity (36.92 ± 4.02% without betaine and 72.94 ± 6.57% with betaine). The relieving effects on salt stress could be observed in the promotion of more extracellular polymeric substances (EPS) secretion with betaine addition. Additionally, the oxidation-reduction potential (ORP), as well as the electron transfer system (ETS) value, was increased with betaine addition, which was reflected in the improvement of system removal efficiency and enzyme activity. Microbial community analysis demonstrated that Bacillus and Clostridiisalibacter which were positively correlated with the stability of the anaerobic process were enriched with betaine addition at high salinity. Metagenomic analysis speculated that the encoding genes for salt tolerance (kdpB/oadA/betA/opuD/epsP/epsH) and NB degradation (nfsA/wrbA/ccdA/menC) obtained higher relative abundance with betaine addition under high salt environment, which might be the key to improving salt tolerance of anaerobic biomass. The long-term assessment demonstrated that exogenous addition betaine played an important role in maintaining the stability of the anaerobic system, which would be a potential strategy to achieve a high-efficiency anaerobic process under high salinity conditions.

Psychrophilic treatment of municipal wastewater with a combined UASB/ASD system, and perspectives for improving urban WWTP sustainability

According to new paradigms of urban wastewater management, energy savings and resources and energy recovery from sewage will assume an ever-increasing importance. Anaerobic processes, aside from being more energy efficient than conventional aerobic ones, are particularly suited to recover embedded organic energy, improving the overall energy balance of treatment processes, however, their performance is limited by low temperatures and slower kinetics. In this study, a pilot Upflow Anaerobic Sludge Blanket (UASB) reactor was operated to treat municipal wastewater at low temperature regime (16.5-18.5 °C) for 22 weeks, both as standalone process and combined with a sidestream anaerobic sludge digester. Process performance highlighted good system robustness, as proved by stable pH and volatile fatty acid/total alkaline buffer capacity ratio, even though observed methane yield was low. Observed COD and TSS removal efficiencies were in the ranges of 60-69% and 63-73%, respectively. Methane production ranged between 0.106 and 0.132 Nm³_CH4/kg_CODrem. An economic assessment was carried out to evaluate the feasibility and benefits of implementing UASB pre-treatment of municipal wastewater in existing conventional facilities (activated sludge and anaerobic sludge digestion), showing that significant energy demand reduction could be achieved for both biological secondary treatment and sludge management, leading to considerable operational economies, and possible positive economic returns within a short pay-back period (3-4 yrs).

Functional Insights of Salinity Stress-Related Pathways in Metagenome-Resolved Methanothrix Genomes

Recently, methanogenic archaea belonging to the genus Methanothrix were reported to have a fundamental role in maintaining stable ecosystem functioning in anaerobic bioreactors under different configurations/conditions. In this study, we reconstructed three Methanothrix metagenome-assembled genomes (MAGs) from granular sludge collected from saline upflow anaerobic sludge blanket (UASB) reactors, where Methanothrix harundinacea was previously implicated with the formation of compact and stable granules under elevated salinity levels (up to 20 g/L Na+). Genome annotation and pathway analysis of the Methanothrix MAGs revealed a genetic repertoire supporting their growth under high salinity. Specifically, the most dominant Methanothrix (MAG_279), classified as a subspecies of Methanothrix_A harundinacea_D, had the potential to augment its salinity resistance through the production of different glycoconjugates via the N-glycosylation process, and via the production of compatible solutes as Nε-acetyl-β-lysine and ectoine. The stabilization and reinforcement of the cell membrane via the production of isoprenoids was identified as an additional stress-related pathway in this microorganism. The improved understanding of the salinity stress-related mechanisms of M. harundinacea highlights its ecological niche in extreme conditions, opening new perspectives for high-efficiency methanisation of organic waste at high salinities, as well as the possible persistence of this methanogen in highly-saline natural anaerobic environments. IMPORTANCE Using genome-centric metagenomics, we discovered a new Methanothrix harundinacea subspecies that appears to be a halotolerant acetoclastic methanogen with the flexibility for adaptation in the anaerobic digestion process both at low (5 g/L Na+) and high salinity conditions (20 g/L Na+). Annotation of the recovered M. harundinacea genome revealed salinity stress-related functions, including the modification of EPS glycoconjugates and the production of compatible solutes. This is the first study reporting these genomic features within a Methanothrix sp., a milestone further supporting previous studies that identified M. harundinacea as a key-driver in anaerobic granulation under high salinity stress.

Impacts of granular activated carbon addition on anaerobic granulation in blackwater treatment

Upflow anaerobic sludge blanket (UASB) reactors, with or without granular activated carbon (GAC) amendment, were applied for blackwater treatment. The impact of GAC on the formation of granules and biomethane recovery was assessed. High organic loading rates (OLRs) up to 15.7 ± 2.1 kg COD/(m³d) were achieved with both reactors. Similar chemical oxygen demand (COD) removal and methane production rate were observed with OLRs ranging from 5.1 ± 1.0 to 9.3 ± 1.5 kg COD/(m³d). Under higher OLR conditions (13.6 ± 1.1 to 15.7 ± 2.1 kg COD/(m³d)), the GAC-amended UASB achieved a higher COD reduction than the UASB without GAC addition. Interestingly, volatile suspended solids (VSS) concentrations, granule size, and extracellular polymeric substance concentrations were lower in the GAC-amended UASB reactor as compared to the UASB without GAC. The methanogenesis activity of the granules in the GAC-amended UASB reactor was significantly higher than the methanogenesis activity of the UASB granules. The microbes o_Bacteroidales and Syntrophus were predominant in both reactors. The acetoclastic methanogens dominated in the UASB reactor without GAC addition; while hydrogenotrophic methanogens dominated in the GAC-UASB reactor. A phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) indicated that syntrophic acetate oxidation improved with GAC addition. The co-occurrence network indicated that interactions between dominant bacteria and archaea were higher in the GAC-amended UASB reactor than in the UASB reactor without GAC addition. This study demonstrated the improved blackwater treatment performance as a result of granulation in UASB with the addition of GAC.

Biofilm formation as a method of improved treatment during anaerobic digestion of organic matter for biogas recovery

The efficiency of anaerobic digestion could be increased by promoting microbial retention through biofilm development. The inclusion of certain types of biofilm carriers has differentiated existing AD biofilm reactors through their respective mode of biofilm growth. Bacteria and archaea engaged in methanogenesis during anaerobic processes potentially build biofilms by adhering or attaching to biofilm carriers. Meta-analyzed results depicted varying degrees of biogas enhancement within AD biofilm reactors. Furthermore, different carrier materials highly induced the dynamicity of the dominant microbial population in each system. It is suggested that the promotion of surface contact and improvement of interspecies electron transport have greatly impacted the treatment results. Modern spectroscopy techniques have been and will continue to give essential information regarding biofilm's composition and structural organization which can be useful in elucidating the added function of this special layer of microbial cells.

The challenges of monitoring and manipulating anaerobic microbial communities

Mixed anaerobic microbial communities are a key component in valorization of waste biomass via anaerobic digestion. Similar microbial communities are important as soil and animal microbiomes and have played a critical role in shaping the planet as it is today. Understanding how individual species within communities interact with others and their environment is important for improving performance and potential applications of an inherently green technology. Here, the challenges associated with making measurements critical to assessing the status of anaerobic microbial communities are considered. How these measurements could be incorporated into control philosophies and augment the potential of anaerobic microbial communities to produce different and higher value products from waste materials are discussed. The benefits and pitfalls of current genetic and molecular approaches to measuring and manipulating anaerobic microbial communities and the challenges which should be addressed to realise the potential of this exciting technology are explored.

Effect of magnetite on anaerobic digestion treating saline wastewater: Methane production, biomass aggregation and microbial community dynamics

In this study, the effect of magnetite amendment on anaerobic digestion was investigated at three increasing salinity levels (0.5%, 1% and 2% NaCl). The amendment of magnetite enhanced the methane yield by 36.3%, 33.3% and 16.5% at low salinity (0.5% NaCl) and high salinity (1% and 2% NaCl), respectively. Meanwhile, a larger proportion of granules was obtained in the magnetite amended reactor (48.05% vs 33.16% at the end of operation). Microbial analysis suggested magnetite could induce more methanogenesis partnerships between hydrogenotrophic methanogens and syntrophic bacteria. Methanosaeta and Methanocorpusculum were the alternating dominant methanogens at low salinity and high salinity. While Streptococcus and Mesotoga were two prevalent bacteria that showed totally different transition tendency in two reactors. Additionally, the supplement of magnetite could relieve the suppression of methanogenesis-related gene expression caused by salinity, thus facilitated the higher methane production.

Assessing effects of Ca2+ addition on membrane bioreactor performance and macro-floc sludge characteristics

Calcium ions (Ca²⁺) can trigger coagulation-flocculation process to form macro-flocculated sludge (MFS). Thus, dosing Ca²⁺-containing reagents into membrane bioreactors (MBRs) is considered as a promising approach to mitigate membrane biofouling. However, a mechanistic understanding of Ca²⁺ addition to MBR performance remains elucidated, such as physicochemical characteristics of MFS and their functionality variations. Consequently, this study was sought to understand the interplays of Ca²⁺ addition and MBR performance with a focus on characterizing MFS in detail. Three parallel MBRs were amended with 82, 208 and 410 mg-Ca²⁺/L final concentrations. Particle size analyses revealed that MFS formation was overall enhanced by the Ca²⁺ addition and granular sludge with diameters of up to 900 μm was formed in the 410 mg-Ca²⁺/L scenario. We believed that cationic bridges facilitated by elevated Ca²⁺ concentrations in conjunction with coagulation-flocculation were primary mechanisms of the formation of large flocs. Moreover, significant portions of soluble proteins and polysaccharides were flocculated and precipitated by Ca²⁺, which demonstrated a negative correlation between extracellular polymeric substances (EPS) concentrations and the formation of MFS. Furthermore, the population abundancies of Thiotrichaceae, Sphingomonadales and Hyphomicrobiaceae decreased in the sludge with Ca²⁺ addition resulted in profound changes of the microbial communities in the MBRs. But MBR performance, such as chemical oxygen demand removal (over 90%), showed no variation during the MBR operation. On the contrary, total nitrogen removal was inhibited in the MBRs. It was because the enlarging MFS formed diffusion barriers to prevent organic component from entering into the sludge flocs to be consumed.

A Distinct, Flocculent, Acidogenic Microbial Community Accompanies Methanogenic Granules in Anaerobic Digesters

The formation of dense, well-settling methanogenic granules is essential for the operation of high-rate, up-flow anaerobic bioreactors used for wastewater treatment. Granule formation (granulation) mechanisms have been previously proposed, but an ecological understanding of granule formation is still lacking. Additionally, much of the current research on granulation only examines the start-up phase of bioreactor operation, rather than monitoring the fate of established granules and how new granules emerge over time. This paper, therefore, attempts to provide an insight into the microbial ecology of granule formation outside the start-up phase of bioreactor operation and develop an ecological granulation model. The microbial communities of granules actively undergoing growth, breakage, and reformation were examined, and an ecological granulation model was proposed. A distinct pregranular microbial community, with a high proportion of acidogenic organisms, such as the Streptococcaceae, was identified and suggested to have a role in initiating granulation by providing simpler substrates for the methanogenic and syntrophic communities which developed during granule growth. After initial granule formation, deterministic influences on microbial community assembly increased with granule size and indicated that microbial community succession was influenced by granule growth, leading to the formation of a stepwise ecological model for granulation. IMPORTANCE Complex microbial communities in engineered environments can aggregate to form surface-attached biofilms. Others form suspended biofilms, such as methanogenic granules. The formation of dense, methanogenic granules underpins the performance of high-rate, anaerobic bioreactors in industrial wastewater treatment. Granule formation (granulation) has been well studied from a physico-chemical perspective, but the ecological basis is poorly understood. We identified a distinct, flocculent, microbial community, which was present alongside granules, comprising primary consumers likely key in providing simpler substrates to granules. This flocculent community is understudied in anaerobic digestion and may initiate, or perpetuate, granule formation. We propose that it may be possible to influence bioreactor performance (e.g., to regulate volatile fatty acid concentrations) by manipulating this community. The patterns of microbial community diversity and assembly revealed by the study indicate that cycles of granule growth and breakage lead to overall diversification of the bioreactor meta-community, with implications for bioreactor process stability.

Calcium phosphate granules formation: Key to high rate of mesophilic UASB treatment of toilet wastewater

Toilet wastewater, a rich source of organic matter and nutrients, can be treated anaerobically to recover energy and resources at mesophilic conditions (35 °C) using an upflow anaerobic sludge blanket (UASB) digester. However, low organic loading rates (OLR) have often been reported, which may be attributed to the flocs biomass applied in previous studies. In the present study, CaP granules were developed in the UASB reactor during the reactor operation of 250 days, which accounted for 89.2% of the UASB sludge, and had high VSS (25.9 ± 0.3 g/L) and high methanogenesis rates (0.34 ± 0.04 g CH₄-COD/(gVSS·d)). An OLR of 16.0 g/(L·d) and a hydraulic retention time (HRT) of 0.25 days, were achieved, with a total chemical oxygen demand (COD) removal rate of 75.6 ± 6.0%, and a methane production rate of 8.4 ± 0.9 g CH₄-COD/(L·d). The efficiency of the hydrolysis of organic substrates ranged from 32.6 ± 2.8% to 43.4 ± 1.4%. Microbial community analysis revealed that syntrophic bacteria Syntrophus, together with diverse H₂-utilizing methanogens, proliferated; and eventually resulted in a hydrogenotrophic dominant pathway in the UASB reactor. The performance, mechanism of CaP granule formation, and the application of the process were discussed in detail. The present paper provided insight of high rate biomethane production from anaerobic toilet wastewater treatment.

Lactate Metabolism and Microbiome Composition Are Affected by Nitrogen Gas Supply in Continuous Lactate-Based Chain Elongation

Chain elongation reactor microbiomes produce valuable medium-chain carboxylates (MCC) from non-sterile residual substrates where lactate is a relevant intermediate. Gas supply has been shown to impact chain elongation performance. In the present study, the effect of nitrogen gas (N2) supply on lactate metabolism, conversion rates, biomass growth, and microbiome composition was evaluated in a lactate-fed upflow anaerobic reactor with continuous or intermittent N2 gas supply. Successful MCC production was achieved with continuous N2 gas supply at low superficial gas velocities (SGV) of 0.22 m∙h−1. Supplying N2 at high SGV (>2 m∙h−1) either continuously (2.2 m∙h−1) or intermittently (3.6 m∙h−1) disrupted chain elongation, resulting in production of short-chain carboxylates (SCC), i.e., acetate, propionate, and n-butyrate. Caproiciproducens-dominated chain-elongating microbiomes enriched at low SGV were washed out at high SGV where Clostridium tyrobutyricum-dominated microbiomes thrived, by displaying higher lactate consumption rates. Suspended growth seemed to be dominant regardless of SGV and gas supply regime applied with no measurable sludge bed formed. The highest MCC production from lactate of 10 g COD∙L−1∙d−1 with electron selectivities of 72 ± 5%was obtained without N2 gas supply at a hydraulic retention time (HRT) of 1 day. The addition of 5 g∙L−1 of propionate did not inhibit chain elongation, but rather boosted lactate conversion rates towards MCC with n-heptylate reaching 1.8 g COD∙L−1∙d−1. N2 gas supply can be used for mixing purposes and to steer lactate metabolism to MCC or SCC production.

New Insights into the Microbial Diversity of Cake Layer in Yttria Composite Ceramic Tubular Membrane in an Anaerobic Membrane Bioreactor (AnMBR)

Cake layer formation is an inevitable challenge in membrane bioreactor (MBR) operation. The investigations on the cake layer microbial community are essential to control biofouling. This work studied the bacterial and archaeal communities in the cake layer, the anaerobic sludge, and the membrane cleaning solutions of anaerobic membrane bioreactor (AnMBR) with yttria-based ceramic tubular membrane by polymerase chain reaction (PCR) amplification of 16S rRNA genes. The cake layer resistance was 69% of the total membrane resistance. Proteins and soluble microbial by-products (SMPs) were the dominant foulants in the cake layer. The pioneering archaeal and bacteria in the cake layer were mostly similar to those in the anaerobic bulk sludge. The dominant biofouling bacteria were Proteobacteria, Bacteroidetes, Firmicutes, and Chloroflexi and the dominant archaeal were Methanosaetacea and Methanobacteriacea at family level. This finding may help to develop antifouling membranes for AnMBR treating domestic wastewater.

Extracellular biopolymers recovered as raw biomaterials from waste granular sludge and potential applications: A critical review

Granular sludge (GS) is a special self-aggregation biofilm. Extracellular polymeric substances (EPS) are mainly associated with the architectural structure, rheological behaviour and functional stability of fine granules, given that their significance to the physicochemical features of the biomass catalysing the biological purification process. This review targets the EPS excretion from GS and introduces newly identified EPS components, EPS distribution in different granules, how to effectively extract and recover EPS from granules, key parameters affecting EPS production, and the potential applications of EPS-based biomaterials. GS-based EPS components are highly diverse and a series of new contents are highlighted. Due to high diversity, emerging extraction standards are proposed and recovery process is capturing particular attention. The major components of EPS are found to be polysaccharides and proteins, which manifest a larger diversity of relative abundance, structures, physical and chemical characteristics, leading to the possibility to sustainably recover raw materials. EPS-based biomaterials not only act as alternatives to synthetic polymers in several applications but also figure in innovative industrial/environmental applications, including gel-forming materials for paper industry, biosorbents, cement curing materials, and flame retardant materials. In the upcoming years, it is foreseen that productions of EPS-based biomaterials from renewable origins would make a significant contribution to the advancement of the circular economy.

Growth and Break-Up of Methanogenic Granules Suggests Mechanisms for Biofilm and Community Development

Methanogenic sludge granules are densely packed, small, spherical biofilms found in anaerobic digesters used to treat industrial wastewaters, where they underpin efficient organic waste conversion and biogas production. Each granule theoretically houses representative microorganisms from all of the trophic groups implicated in the successive and interdependent reactions of the anaerobic digestion (AD) process. Information on exactly how methanogenic granules develop, and their eventual fate will be important for precision management of environmental biotechnologies. Granules from a full-scale bioreactor were size-separated into small (0.6-1 mm), medium (1-1.4 mm), and large (1.4-1.8 mm) size fractions. Twelve laboratory-scale bioreactors were operated using either small, medium, or large granules, or unfractionated sludge. After >50 days of operation, the granule size distribution in each of the small, medium, and large bioreactor sets had diversified beyond-to both bigger and smaller than-the size fraction used for inoculation. Interestingly, extra-small (XS; <0.6 mm) granules were observed, and retained in all of the bioreactors, suggesting the continuous nature of granulation, and/or the breakage of larger granules into XS bits. Moreover, evidence suggested that even granules with small diameters could break. "New" granules from each emerging size were analyzed by studying community structure based on high-throughput 16S rRNA gene sequencing. Methanobacterium, Aminobacterium, Propionibacteriaceae, and Desulfovibrio represented the majority of the community in new granules. H2-using, and not acetoclastic, methanogens appeared more important, and were associated with abundant syntrophic bacteria. Multivariate integration (MINT) analyses identified distinct discriminant taxa responsible for shaping the microbial communities in different-sized granules.