Insight into the Role of Facultative Bacteria Stimulated by Microaeration in Continuous Bioreactors Converting LCFA to Methane

In-situ intermittent micro-aeration in food waste and sewage sludge anaerobic co-digestion: Performance, stability, and microbial dynamics

Anaerobic co-digestion (AcoD) of sewage sludge (SS) with food waste (FW) is a practical approach in urban areas due to spatio-temporal availability of these co-substrates. While micro-aeration could enhance hydrolysis and control acidification during mono-digestion, the effects of micro-aeration on AcoD remains unclear. This study explored the influence of oxidation-reduction potential (ORP)-based in-situ micro-aeration on AcoD performance of SS and FW. Although mono-digestion of FW failed due to acidification and micro-aeration of SS was unstable due to high solids content and viscosity, micro-aeration improved specific biogas and methane yields by 21.1 % and 13.1 %, respectively, at the FW:SS ratio of 60:40 ((volatile solids (VS) basis). This improvement was observed at an organic loading rate of 2.25 g VS/L·d and alkalinity below 5000 mg CaCO3/L. The observed positive effect of micro-aeration was accompanied by changes in relative abundance of Aminicenantes. Functional analysis suggested that AcoD was driven by homoacetogenesis and acetoclastic methanogenesis.

Enhancing methanogenesis from long-chain fatty acids (LCFA) and enrichment of novel bacteria with resuscitation-promoting factors

Long-chain fatty acids (LCFA) are important intermediate metabolites in lipid hydrolysis during anaerobic digestion for biogas production. High LCFA loads inhibit microbial activity by toxicity, impairing the coupling of β-oxidation and methanogenesis, thus reducing LCFA degradation efficiency. This study employed and tested seven stimulants, including the resuscitation-promoting factors (Rpf and YeaZ), the quorum-sensing molecules (cAMP, and AHLs), the chemical stimulants (pyruvate), the growth promoter (fumarate), and yeast extract + peptone (YP) for enhancement of methanogenic degradation of LCFA. The results indicate that the chemical stimulants and resuscitation-promoting factors enhanced maximum methane-production rate 1.58 to 2.20 fold versus the NS, reducing the lag phase by 1.46-9.76 days. Analysis of the microbial community composition revealed that the quorum sensing factors only increased species richness, while Rpf, YeaZ fumarate, and YP stimulated the growth of core members of the communities. Metagenomic analysis detected three previously unreported LCFA-degrading bacterial taxa, Marinisomatota, Thermoanaerobaculaceae and Pelomonas. Particularly, Rpf and YeaZ significantly enriched LCFA-degrading bacteria such as Syntrophomonadaceae, Leptospiraceae, and Marine Group B within the core species, while YeaZ also stimulated methanogenic bacteria, possibly due to resuscitating dormant microbes from unfavorable conditions. Syntrophic interactions between LCFA degraders and non-degraders, rather than methanogen abundance, govern methanogenic LCFA degradation. These results demonstrate that the use of stimulants is an effective approach to enhance LCFA degradation and provide a new pathway for energy recovery.

Enhancing mesophilic methanogenesis in oleate-rich environments through optimized micro-aeration pretreatment

Micro-aeration pretreatment has emerged as a promising technology for improving the performance of anaerobic bioreactors in the treatment of lipid-rich organic waste, particularly in mitigating the accumulation of long-chain fatty acids (LCFAs). Micro-aeration intensity is a critical factor in optimizing substrate hydrolysis and methanogenesis efficiency. In this study, optimal micro-aeration intensities for acetoclastic (30 mL-air/g-COD) and overall methanogenesis (7.5 mL-air/g-COD) were initially determined using acetate and glucose as substrates, respectively. Subsequently, the addition of 0.5 mM oleate (a typical LCFA) increased cumulative methane production by 22.1 % when acetate was used as the substrate after 30 mL-air/g-COD micro-aeration pretreatment. Conversely, it decreased cumulative methane production by 17.3 % when glucose was used as the substrate after 7.5 mL-air/g-COD micro-aeration pretreatment. Additionally, the population of facultative hydrolysis microorganisms, such as the genus Pseudomonas, increased by 25.7 % and 27.8 % when acetate and glucose were used as substrates, respectively. Furthermore, the predominant methane-producing archaea, including the genus Methanosarcina, increased by 27.3 % when acetate was used as the substrate, while the genus Methanosaeta decreased by 65.3 % when glucose was used as the substrate. Collectively, these findings provide insights into the methanogenesis pathway under optimal micro-aeration pretreatment conditions, guiding future research in this field.

Facultative anaerobic bacteria enable syntrophic fatty acids degradation under micro-aerobic conditions

Trace amounts of oxygen stimulate facultative anaerobic bacteria (FAB) within anaerobic bioreactors, which was shown to correlate with enhanced methane production from long-chain fatty acids. The relationship between FAB and fatty acid-degrading syntrophic communities under micro-aerobic conditions is still unclear. In this work, two syntrophic co-cultures, Syntrophomonas wolfei + Methanospirillum hungatei and Syntrophomonas zehnderi + Methanobacterium formicicum, were assembled and incubated with short, medium and long-chain fatty acids, with 0-10 % O2, in the presence and absence of FAB, here represented by Pseudomonas spp. Without Pseudomonas, the syntrophic activity was inhibited by 79 % at 0.5 % O2, but with Pseudomonas, the syntrophic co-cultures successfully converted the fatty acids to methane with up to 2 % O2. These findings underscore the pivotal role of FAB in the protection of syntrophic fatty acid-degrading communities under micro-aerobic conditions and emphasizes its significance in real-scale anaerobic digesters where strictly anaerobic conditions may not consistently be maintained.

Long-chain fatty acids facilitate acidogenic fermentation of food waste: Attention to the microbial response and the change of core metabolic pathway under saturated and unsaturated fatty acids loading

Long-chain fatty acids (LCFAs) are recognized as a significant inhibitory factor in anaerobic digestion of food waste (FW), yet they are inevitably present in FW due to lipid hydrolysis. Given their distinct synthesis mechanism from traditional anaerobic digestion, little is known about the effect of LCFAs on FW acidogenic fermentation. This study reveals that total volatile fatty acids (VFAs) production increased by 9.98 % and 4.03 % under stearic acid and oleic acid loading, respectively. Acetic acid production increased by 20.66 % under stearic acid loading compared to the control group (CK). However, the LCFA stress restricted the degradation of solid organic matter, particularly under oleic acid stress. Analysis of microbial community structure and quorum sensing (QS) indicates that LCFA stress enhanced the relative abundance of Lactobacillus and Klebsiella. In QS system, the relative abundance of luxS declined from 0.157 % to 0.116 % and 0.125 % under oleic acid and stearic acid stress, respectively. LCFA stress limited the Autoinducer-2 (AI-2) biosynthesis, suggesting that microorganisms cannot use QS to resist the LCFA stress. Metagenomic sequencing showed that LCFA stress promoted acetic acid production via the conversion of pyruvate and acetyl-CoA to acetate. Direct conversion of pyruvate to acetic acid increased by 47.23 % compared to the CK group, accounting for the enhanced acetic acid production under stearic acid loading. The abundance of β-oxidation pathway under stearic acid loading was lower than under oleic acid loading. Overall, the stimulating direct conversion of pyruvate plays a pivotal role in enhancing acetic acid biosynthesis under stearic acid loading, providing insights into the effect of LCFA on mechanism of FW acidogenic fermentation.

Metatranscriptomics-guided genome-scale metabolic reconstruction reveals the carbon flux and trophic interaction in methanogenic communities

BACKGROUND

Despite rapid advances in genomic-resolved metagenomics and remarkable explosion of metagenome-assembled genomes (MAGs), the function of uncultivated anaerobic lineages and their interactions in carbon mineralization remain largely uncertain, which has profound implications in biotechnology and biogeochemistry.

RESULTS

In this study, we combined long-read sequencing and metatranscriptomics-guided metabolic reconstruction to provide a genome-wide perspective of carbon mineralization flow from polymers to methane in an anaerobic bioreactor. Our results showed that incorporating long reads resulted in a substantial improvement in the quality of metagenomic assemblies, enabling the effective recovery of 132 high-quality genomes meeting stringent criteria of minimum information about a metagenome-assembled genome (MIMAG). In addition, hybrid assembly obtained 51% more prokaryotic genes in comparison to the short-read-only assembly. Metatranscriptomics-guided metabolic reconstruction unveiled the remarkable metabolic flexibility of several novel Bacteroidales-affiliated bacteria and populations from Mesotoga sp. in scavenging amino acids and sugars. In addition to recovering two circular genomes of previously known but fragmented syntrophic bacteria, two newly identified bacteria within Syntrophales were found to be highly engaged in fatty acid oxidation through syntrophic relationships with dominant methanogens Methanoregulaceae bin.74 and Methanothrix sp. bin.206. The activity of bin.206 preferring acetate as substrate exceeded that of bin.74 with increasing loading, reinforcing the substrate determinantal role.

CONCLUSION

Overall, our study uncovered some key active anaerobic lineages and their metabolic functions in this complex anaerobic ecosystem, offering a framework for understanding carbon transformations in anaerobic digestion. These findings advance the understanding of metabolic activities and trophic interactions between anaerobic guilds, providing foundational insights into carbon flux within both engineered and natural ecosystems. Video Abstract.

Microbial community changes and metabolic pathways analysis during waste activated sludge and meat processing waste anaerobic co-digestion

Waste activated sludge (WAS) and meat processing waste (MPW) were acted as co-substrates in anaerobic co-digestion (AcD), and biochemical methane potential (BMP) test was carried out to investigate the methane production performances. Microbial community structure and metabolic pathways analyses were conducted by 16S rRNA high-throughput sequencing and functional prediction analysis. BMP test results indicated that AcD of 70% WAS+30% MPW and 50% WAS+50% MPW (VS/VS) could significantly improve methane yield to 371.05 mL/g VS and 599.61 mL/g VS, respectively, compared with WAS acting as sole substrate (191.87 mL/g VS). The results of microbial community analysis showed that Syntrophomonas and Petrimonas became the dominant bacteria genera, and Methanomassiliicoccus and Methanobacterium became the dominant archaea genera after MPW addition. 16S functional prediction analysis results indicated that genes expression of key enzymes involved in syntrophic acetate oxidation (SAO), hydrogenotrophic and methylotrophic methanogenesis were up-regulated, and acetoclastic methanogenesis was inhibited after MPW addition. Based on these analyses, it could be inferred that SAO combined with hydrogenotrophic and methylotrophic methanogenesis was the dominant pathway for organics degradation and methane production during AcD. These findings provided systematic insights into the microbial community changes and metabolic pathways during AcD of WAS and MPW.

Microaeration promotes volatile siloxanes conversion to methane and simpler monomeric products

The ubiquitous use of volatile siloxanes in a myriad of product formulations has led to a widespread distribution of these persistent contaminants in both natural ecosystems and wastewater treatment plants. Microbial degradation under microaerobic conditions is a promising approach to mitigate D4 and D5 siloxanes while recovering energy in wastewater treatment plants. This study examined D4/D5 siloxanes biodegradation under both anaerobic and microaerobic conditions ( [Formula: see text]  = 0, 1, 3 %) using wastewater sludge. Results show that the use of microaeration in an otherwise strictly anaerobic environment significantly enhances siloxane conversion to methane. 16S rRNA gene sequencing identified potential degraders, including Clostridium lituseburense, Clostridium bifermentans and Synergistales species. Furthermore, chemical analysis suggested a stepwise siloxane conversion preceding methanogenesis under microaerobic conditions. This study demonstrates the feasibility of microaerobic siloxane biodegradation, laying groundwork for scalable removal technologies in wastewater treatment plants, ultimately highlighting the importance of using bio-based approaches in tackling persistent pollutants.

Microaerophilic Activated Sludge System for Ammonia Retention toward Recovery from High-Strength Nitrogenous Wastewater: Performance and Microbial Communities

A transition to ammonia recovery from wastewater has started; however, a technology for sustainable nitrogen retention in the form of ammonia and organic carbon removal is still in development. This study validated a microaerophilic activated sludge (MAS) system to efficiently retain ammonia from high-strength nitrogenous wastewater. The MAS is based on conventional activated sludge (CAS) with aerobic and settling compartments. Low dissolved oxygen (DO) concentrations (<0.2 mg/L) and short solids retention times (SRTs) (<5 days) eliminated nitrifying bacteria. The two parallel MASs were successfully operated for 300 days and had ammonia retention of 101.7 ± 24.9% and organic carbon removal of 85.5 ± 8.9%. The MASs mitigated N2O emissions with an emission factor of <0.23%, much lower than the default value of CAS (1.6%). A short-term step-change test demonstrated that N2O indicated the initiation of nitrification and the completion of denitrification in the MAS. The parallel MASs had comparable microbial diversity, promoting organic carbon oxidation while inhibiting ammonia-oxidizing microorganisms (AOMs), as revealed by 16S rRNA gene amplicon sequencing, the quantitative polymerase chain reaction of functional genes, and fluorescence in situ hybridization of β-proteobacteria AOB. The microbial analyses also uncovered that filamentous bacteria were positively correlated with effluent turbidity. Together, controlling DO and SRT achieved organic carbon removal and successful ammonia retention, mainly by suppressing AOM activity. This process represents a new nitrogen management paradigm.

Excess methane production and operation stability for anaerobic digestion of oily food waste controlled by mixing intensity: Focusing on heterogeneity of long chain fatty acids

Long chain fatty acids (LCFAs) are the key intermediate of anaerobic digestion of oily food waste, not completely soluble in a water-dominant anaerobic system due to their long hydrocarbon chains with hydrophobic property. Their effective concentration affects release of high methanogenic potential and system stability. A long-term continuous anaerobic digestion of oily food waste demonstrated excess methane production of even more than feedstock in an anaerobic continuous stirred tank reactor (CSTR). Assuming feedstock COD at 100%, approximately 120% of COD as methane could be achieved. Oil floating and crystallization with Ca salt resulting from the distribution heterogeneity of LCFAs in the CSTR were found responsible for the excess methane production. Moreover, slow conversion and accumulation of saturated LCFAs with relatively lower solubility played an important role as well. Compared with unsaturated oleic (C18:1) and linoleic acids (C18:2), around twice slower methane production rate and longer lag time could be observed for those saturated LCFAs. Mixing intensity was proved to be a critical controlling factor for methanogenesis and stability possibly by affecting interaction between oil/LCFAs and anaerobes to change effective lipid loading.

Microaerobic condition as pretreatment for improving anaerobic digestion: A review

Pretreatment of waste before anaerobic digestion (AD) has been extensively studied during the last decades. One of the biological pretreatments studied is the microaeration. This review examines this process, including parameters and applications to different substrates at the lab, pilot and industrial scales, to guide further improvement in large-scale applications. The underlying mechanisms of accelerating hydrolysis and its effects on microbial diversity and enzymatic production were reviewed. In addition, modelling of the process and energetic and financial analysis is presented, showing that microaerobic pretreatment is commercially attractive under certain conditions. Finally, challenges and future perspectives were also highlighted to promote the development of microaeration as a pretreatment before AD.

Effect of ferrate pretreatment on anaerobic digestibility of primary sludge spiked with resin acids

Resin acids are mixtures of high molecular weight carboxylic acids found in tree resins. Due to higher hydrophobicity and low solubility, they tend to adsorb on the suspended solids in pulp and paper (P&P) mill wastewater and accumulate in primary sludge through settling. Anaerobic digestion (AD) is a common practice stabilizing sludge; however, high concentration of resin acids affects the AD process. The aim of this research was mainly to determine the impact of ferrate (Fe (VI)) oxidation on selected resin acids and anaerobic digestibility of ferrate-treated primary sludge (PS) spiked with the resin acids. First, batch control oxidation of model resin acids with Fe (VI) was conducted to identify an optimum dosage, pH and contact time using a Box-Behnken design approach. Thereafter, anaerobic treatability studies of primary sludge spiked with resin acids both under control condition and optimum ferrate pretreatment were conducted. Up to 97% oxidation of resin acids occurred in pure water, while only 44%-62% oxidation of resin acids occurred in PS with an increasing Fe (VI) dosage from 0.034 to 0.137 mg Fe (VI)/mg tCOD_fed. The pretreatment did not affect the anaerobic biodegradability of resin acids; however, it lowered their negative influences on the PS digestibility. About 0.076 mg Fe (VI) dosage/mg tCOD_fed solubilized the sludge increasing the methane production by 40% compared to the untreated digester. The potential benefits of ferrate pretreatment of P&P primary sludge include resin acids oxidation and subsequent toxicity reduction, higher sludge solubilization enhancing methane production and enabling anaerobic digestion at higher COD loading.

Micro-aeration: an attractive strategy to facilitate anaerobic digestion

Micro-aeration can facilitate anaerobic digestion (AD) by regulating microbial communities and promoting the growth of facultative taxa, thereby increasing methane yield and stabilizing the AD process. Additionally, micro-aeration contributes to hydrogen sulfide stripping by oxidization to produce molecular sulfur or sulfuric acid. Although micro-aeration can positively affect AD, it must be strictly regulated to maintain an overall anaerobic environment that permits anaerobic microorganisms to thrive. Even so, obligate anaerobes, especially methanogens, could suffer from oxidative stress during micro-aeration. This review describes the applications of micro-aeration in AD and examines the cutting-edge advances in how methanogens survive under oxygen stress. Moreover, barriers and corresponding solutions are proposed to move micro-aeration technology closer to application at scale.

Strategy to enhance semi-continuous anaerobic digestion of food waste by combined use of calcium peroxide and magnetite

Optimizing methane production from food waste (FW) efficiently is always a hot topic in the field of anaerobic digestion (AD). In this study we aimed to improve the conversion of organics to methane by using CaO₂ and magnetite to enhance the semi-continuous AD of food waste. Under the organic load of 2.5 g VS/L·d^-1, the specific methane yield was increased from 333.9 mL CH₄/g·VS to 423.4 mL CH₄/g·VS by adding 0.01 g/L CaO₂ with 0.4 g/L magnetite, improving the production of methane from FW. We assessed reactor performance, ORP changes, mass balance, enzyme activities and characterized the metagenomic profile of microorganisms involved in digestion. These microorganisms showed rapid conversion of volatile fatty acids and increased expression of genes related to hydrolysis and acid production. Thus, the addition of CaO₂ and magnetite optimized the relationship between fermentation bacteria and methanogenic archaea to enhance the overall production of methane. Microorganisms evolved unique adaptive mechanisms in the co-operative environment of CaO₂ and magnetite, as their energy metabolism patterns combined those controlled by individual CaO₂ and magnetite addition. This method of combining a micro-aeration environment with conductive materials provides a new perspective for optimizing the AD of FW.

Principles, Advances, and Perspectives of Anaerobic Digestion of Lipids

Several problems associated with the presence of lipids in wastewater treatment plants are usually overcome by removing them ahead of the biological treatment. However, because of their high energy content, waste lipids are interesting yet challenging pollutants in anaerobic wastewater treatment and codigestion processes. The maximal amount of waste lipids that can be sustainably accommodated, and effectively converted to methane in anaerobic reactors, is limited by several problems including adsorption, sludge flotation, washout, and inhibition. These difficulties can be circumvented by appropriate feeding, mixing, and solids separation strategies, provided by suitable reactor technology and operation. In recent years, membrane bioreactors and flotation-based bioreactors have been developed to treat lipid-rich wastewater. In parallel, the increasing knowledge on the diversity of complex microbial communities in anaerobic sludge, and on interspecies microbial interactions, contributed to extend the knowledge and to understand more precisely the limits and constraints influencing the anaerobic biodegradation of lipids in anaerobic reactors. This critical review discusses the most important principles underpinning the degradation process and recent key discoveries and outlines the current knowledge coupling fundamental and applied aspects. A critical assessment of knowledge gaps in the field is also presented by integrating sectorial perspectives of academic researchers and of prominent developers of anaerobic technology.

Influence of cysteine, serine, sulfate, and sulfide on anaerobic conversion of unsaturated long-chain fatty acid, oleate, to methane

This study aims to elucidate the role of sulfide and its precursors in anaerobic digestion (i.e., cysteine, representing sulfur-containing amino acids, and sulfate) on microbial oleate conversion to methane. Serine, with a similar structure to cysteine but with a hydroxyl group instead of a thiol, was included as a control to assess potential effects on methane formation that were not related to sulfur functionalities. The results showed that copresence of sulfide and oleate in anaerobic batch assays accelerated the methane formation compared to assays with only oleate and mitigated negative effect on methane formation caused by increased sulfide level. Nuclear magnetic resonance spectroscopy of sulfide-exposed oleate suggested that sulfide reaction with oleate double bonds likely contributed to negation of the negative effect on the methanogenic activity. Methane formation from oleate was also accelerated in the presence of cysteine or serine, while sulfate decreased the cumulative methane formation from oleate. Neither cysteine nor serine was converted to methane, and their accelerating effects was associated to different mechanisms due to establishment of microbial communities with different structures, as evidenced by high-throughput sequencing of 16S rRNA gene. These outcomes contribute with new knowledge to develop strategies for optimum use of sulfur- and lipid-rich wastes in anaerobic digestion processes.

Microaerobic Digestion of Low-Biodegradable Sewage Sludge: Effect of Air Dosing in Batch Reactors

The adoption of prolonged solid retention times during the biological treatment of urban wastewaters is a well-known strategy to reduce sewage sludge production. However, it also results in the production of a biological sludge with low percentages of biodegradable organic matter, also characterized by high humification degrees, which may hamper the anaerobic digestion treatment aimed at sludge stabilization. To accelerate the hydrolytic stage, the application of microaerobic conditions during the anaerobic digestion of low-biodegradable sewage sludge was investigated in this study. In particular, six bio-methanation tests of a real sewage sludge were carried out, introducing air in the bioreactors with doses ranging between 0 and 16.83 L air/kg VSin d, in order to evaluate the air dosage that optimizes the biomethane production and organic matter degradation. Notably, the lower air loading rates investigated in this study, such as 0.68 and 1.37 L air/kg VSin d, led to an increase in methane production of up to 19%, due to a higher degradation of total lipids and proteins. In addition, these microaerobic conditions also resulted in a decrease in the sludge humification degree and in lower volatile fatty acid accumulation.

Microaerobic conditions in anaerobic sludge promote changes in bacterial composition favouring biodegradation of polymeric siloxanes

Volatile organic silicon compounds (VOSiC) are harmful pollutants to the biota and ecological dynamics as well as biogas-based energy conversion systems. However, there is a lack of understanding regarding the source of VOSiCs in biogas, especially arising from the biochemical conversion of siloxane polymers such as polydimethylsiloxanes (PDMS). The biodegradation of PDMS was evaluated under anaerobic/microaerobic conditions (P_O2 = 0, 1, 3, 5%), using wastewater treatment plant (WWTP) sludge as an inoculum and PDMS as a co-substrate (0, 50, 100, 500 ppm). On average, strictly anaerobic treatments produced significantly less methane than the 3 and 5% microaerated ones, which show the highest PMDS biodegradation at 50 ppm. Thauera sp. and Rhodococcus sp. related phylotypes were identified as the most abundant bacterial groups in microaerated treatments, and siloxane-related molecules were identified as remnants of PDMS catabolism. Our study demonstrates that microaeration promotes changes to the native bacterial community which favour the biological degradation of PDMS. This confirms that the presence of VOSiC (e.g., D4-D6) in biogas is not only due to its direct input in wastewaters, but also to the PDMS microbial catabolism. Microaerobic conditions enhance both PDMS and (subsequent) VOSiC degradation in the liquid phase, increasing the concentrations of D4 and D5 in biogas, and the production of less toxic siloxane-based derivatives in the liquid phase. This study suggests that microaeration of the anaerobic sludge can significantly decrease the concentration of PDMSs in the WWTP effluent. However, for WWTPs to become effective barriers for the emission of these ecotoxic contaminants to the environment, such a strategy needs to be coupled with an efficient biodegradation of VOSiCs from the biogas.

Effects of micro-aeration on microbial niches and antimicrobial resistances in blackwater anaerobic digesters

Anaerobic digestion (AD) of source-diverted blackwater (toilet flush) at ambient room temperature presents challenges for fast hydrolysis of particulate matters. This study investigated the effect of different micro-aeration dosages for blackwater AD. Sequencing batch reactors were operated at ambient room temperature (22 ± 1°C) with micro-aeration (0, 5, 10, 50, and 150 mg O₂ g^-1 COD_feed per cycle) and gradually reduced hydraulic retention times from 5 d to 2 d. The methanogenesis efficiencies were greater at low oxygen dosages (i.e., 0, 5, 10) while the volatile fatty acids (VFAs) accumulated more at high oxygen dosages (i.e., 50, 150). Microbial communities were significantly different under different oxygen dosages (p<0.05), with segregation of microbial ecological niches in low and high oxygen dosage communities. The low-oxygen-dosage niche (0, 5, and 10 mg g^-1 COD_feed) was inhabited by fermenting and syntrophic bacteria (e.g., Cytophaga, Syntrophomonas) and methanogens (e.g., Methanobacterium, Methanolinea, Methanosaeta). The high-oxygen-dosage niche (50 and 150 mg g^-1 COD_feed) had significantly (p<0.05) more facultative anaerobic bacteria (Ignavibacteriales and Cloacamonales), and aerobic bacteria (Rhodocyclales). Moreover, blackwater can be a source of antimicrobial resistance genes (ARGs), which are affected by different oxygen dosages. The ARG variation correlated with the microbial community composition (p<0.05). Low-oxygen-dosage communities contained a higher prevalence of mobile gene elements (intI1 and korB) and tetM, ermB, sul1, sul2, and blaCTX-M than the high-oxygen-dosage communities, indicating that oxygen dosage influenced the prevalence of populations carrying ARGs. These findings suggest that application of micro-aeration to AD can be used to control ARG profiles.

Advances towards understanding long chain fatty acids-induced inhibition and overcoming strategies for efficient anaerobic digestion process

The inhibition of the anaerobic digestion (AD) process, caused by long chain fatty acids (LCFAs), has been considered as an important issue in the wastewater treatment sector. Proper understanding of mechanisms behind the inhibition is a must for further improvements of the AD process in the presence of LCFAs. Through analyzing recent literature, this review extensively describes the mechanism of LCFAs degradation, during AD. Further, a particular focus was directed to the key parameters which could affect such process. Besides, this review highlights the recent research efforts in mitigating LCFAs-caused inhibition, through the addition of commonly used additives such as cations and natural adsorbents. Specifically, additives such as bentonite, cation-based adsorbents, as well as zeolite and other natural adsorbents for alleviating the LCFAs-induced inhibition are discussed in detail. Further, panoramic evaluations for characteristics, various mechanisms of reaction, merits, limits, recommended doses, and preferred conditions for each of the different additives are provided. Moreover, the potential for increasing the methane production via pretreatment using those additives are discussed. Finally, we provide future horizons for the alternative materials that can be utilized, more efficiently, for both mitigating LCFAs-based inhibition and boosting methane potential in the subsequent digestion of LCFA-related wastes.

Microbiome taxonomic and functional profiles of two domestic sewage treatment systems

Anaerobic systems for domestic sewage treatment, like septic tanks and anaerobic filters, are used in developing countries due to favorable economic and functional features. The anaerobic filter is used for the treatment of the septic tank effluent, to improve the COD removal efficiency of the system. The microbial composition and diversity of the microbiome from two wastewater treatment systems (factory and rural school) were compared through 16S rRNA gene sequencing using MiSeq 2 × 250 bp Illumina sequencing platform. Additionally, 16S rRNA data were used to predict the functional profile of the microbial communities using PICRUSt2. Results indicated that hydrogenotrophic methanogens, like Methanobacterium, were found in higher abundance in both systems compared to acetotrophic methanogens belonging to Methanosaeta genus. Also, important syntrophic microorganisms (Smithella, Syntrophus, Syntrophobacter) were found in the factory and rural school wastewater treatment systems. Microbial communities were also compared between stages (septic tank and anaerobic filter) of each wastewater treatment stage, revealing that, in the case of the rural school, both microbial communities were quite similar most likely due to hydraulic short-circuit issues. Meanwhile, in the factory, microbial communities from the septic tank and anaerobic filter were different. The school system showed lower COD removal rates (2-30%), which were probably related to a higher abundance of Firmicutes members in addition to the hydraulic short-circuit and low abundance of Chloroflexi members. On the other hand, the fiberglass factory presented higher COD removal rates (60-83%), harboring phyla reported as the core microbiome of anaerobic digesters (Bacteroidetes, Chloroflexi, and Proteobacteria phyla). The knowledge of the structure and composition of wastewater treatment systems may provide support for the improvement of the pollutant removal in anaerobic process.

Synergy of anoxic microenvironment and facultative anaerobes on acidogenic metabolism in a self-induced electrofermentation system

Metabolic potential of two different cultures, facultative (FB) and strict anaerobes (AB) under two microenvironments [anoxic (ANOX) and anaerobic (ANA)] was evaluated to understand acidogenic fermentation in a self-induced electrofermentation (EF) system for the production of short-chain fatty acids (SCFA: C2-C4) and biogas. ANA condition positively influenced FB and AB metabolism towards higher acetic (C2:2390 mg/L) and propionic acid (C3: 717 mg/L) production, while butyric acid (C4:1481 mg/L) favored ANOX microenvironment (AB). ANOX microenvironment showed a better self-induced potential compared to ANA metabolism (0.46 V (FB_ANOX); 0.45 V (AB_ANOX)). An improved H₂ (>30%) fraction was noticed with FB while CH₄ production was found favourable with AB. The study illustrated the role of system microenvironment in association with metabolic function towards regulating electrofermentation towards specific products synthesis.

Effect of Sub-Stoichiometric Fe(III) Amounts on LCFA Degradation by Methanogenic Communities

Long-chain fatty acids (LCFA) are common contaminants in municipal and industrial wastewater that can be converted anaerobically to methane. A low hydrogen partial pressure is required for LCFA degradation by anaerobic bacteria, requiring the establishment of syntrophic relationships with hydrogenotrophic methanogens. However, high LCFA loads can inhibit methanogens, hindering biodegradation. Because it has been suggested that anaerobic degradation of these compounds may be enhanced by the presence of alternative electron acceptors, such as iron, we investigated the effect of sub-stoichiometric amounts of Fe(III) on oleate (C18:1 LCFA) degradation by suspended and granular methanogenic sludge. Fe(III) accelerated oleate biodegradation and hydrogenotrophic methanogenesis in the assays with suspended sludge, with H₂-consuming methanogens coexisting with iron-reducing bacteria. On the other hand, acetoclastic methanogenesis was delayed by Fe(III). These effects were less evident with granular sludge, possibly due to its higher initial methanogenic activity relative to suspended sludge. Enrichments with close-to-stoichiometric amounts of Fe(III) resulted in a microbial community mainly composed of Geobacter, Syntrophomonas, and Methanobacterium genera, with relative abundances of 83-89%, 3-6%, and 0.2-10%, respectively. In these enrichments, oleate was biodegraded to acetate and coupled to iron-reduction and methane production, revealing novel microbial interactions between syntrophic LCFA-degrading bacteria, iron-reducing bacteria, and methanogens.

Different micro-aeration rates facilitate production of different end-products from source-diverted blackwater

The effects of micro-aeration on the performance of anaerobic sequencing batch reactors (ASBR) for blackwater treatment were investigated in this study. Different micro-aeration rates, 0, 5, 10, 50, and 150 mg O₂/L-reactor/cycle, and their effect on the hydrolysis, acidogenesis, and methanogenesis of blackwater were evaluated and compared at ambient temperature. Source-diverted blackwater (toilet water) contains high organic contents which can be recovered as biogas. Previous studies have found that anaerobic digestion of blackwater without micro-aeration can only recover upwards of less than 40% of chemical oxygen demand (COD) to methane at room temperature due to the low hydrolysis rate of biomass content in blackwater. This study achieved increases in blackwater hydrolysis (from 34.7% to 48.7%) and methane production (from 39.6% to 50.7%) with controlled micro-aeration (5 mg O₂/L-reactor/cycle). The microbial analysis results showed that hydrolytic/fermentative bacteria and acetoclastic methanogens (e.g. Methanosaeta) were in higher abundances in low-dose micro-aeration reactors (5 and 10 mg O₂/L-reactor/cycle), which facilitated syntrophic interactions between microorganisms. The relative abundance of oxygen-tolerant methanogen such as Methanosarcina greatly increased (from 1.5% to 11.4%) after oxygen injection. High oxygen dosages (50 and 150 mg O₂/L-reactor/cycle) led to reduced methane production and higher accumulation of volatile fatty acids, largely due to the oxygen inhibition on methanogens and degradation of organic matters by aerobic growth and respiration, as indicated by the predicted metagenome functions. By combining reactor performance results and microbial community analyses, this study demonstrated that low-dose micro-aeration improves blackwater biomethane recovery by enhancing hydrolysis efficiency and promoting the development of a functional microbial population, while medium to high-dose micro-aeration reduced the activities of certain anaerobes. It was also observed that medium-dose micro-aeration maximizes VFA accumulation, which may be used in two-stage anaerobic digesters.

An integrated anaerobic system for on-site treatment of wastewater from food waste disposer

In this study, an integrated system of siphon-driven self-agitated anaerobic reactor (SDSAR) and anaerobic fixed bed reactor (AFBR) was conducted for the treatment of wastewater from food waste disposer (FWD), and the effect of influent total solids (TS) concentration on the process performance was evaluated. When the influent TS concentration increased from 7.04 to 15.5 g/L, the methane gas production rate increased from 0.45 to 0.92 L-CH₄/L/day. However, with the influent TS concentration of food waste (FW) further increased to 23.5 g/L, a large amount of scum formed and accumulated in the SDSAR. According to the result of chemical oxygen demand (COD) recovery, the proportion of COD remained in the effluent at different TS concentrations was only around 2%. On the other hand, with an increase in TS concentration, the proportion of COD remained in the reactors increased significantly. Our results demonstrated that effluent from the integrated system can meet the water quality requirements recommended by Japan Sewage Works Association (JSWA) for wastewater from FWD. In addition, to enhance the process stability, the influent TS concentration should be maintained below 15.5 g/L.

Enhanced Anaerobic Digestion of Long Chain Fatty Acid by Adding Magnetite and Carbon Nanotubes

This study investigated the impact of stimulating direct interspecies electron transfer (DIET), by supplementing nano-sized magnetite (nFe3O4, 0.5 g Fe/g VSS) and carbon nanotubes (CNT, 1 g/L), in anaerobic digestion of oleic acid (OA) at various concentrations (0.10 - 4.00 g chemical oxygen demand(COD)/L). Both supplementations could enhance CH4 production, and its beneficial impact increased with increased OA concentration. The biggest improvements of 114% and 165% compared to the control were achieved by nFe3O4 and CNT, respectively, at OA of 4 g COD/L. The enhancement can be attributed to the increased sludge conductivity: 7.1 ± 0.5 (control), 12.5 ± 0.8 (nFe3O4-added), and 15.7 ± 1.1 µS/cm (CNT-supplemented). Dissolved iron concentration, released from nFe3O4, seemed to have a negligible role in improving CH4 production. The excretion of electron shuttles, i.e., humic-like substances and protein-like substances, were found to be stimulated by supplementing nFe3O4 and CNT. Microbial diversity was found to be simplified under DIET-stimulating conditions, whereby five genera accounted for 88% of the total sequences in the control, while more than 82% were represented by only two genera (Methanotrix concilli and Methanosarcina flavescens) by supplementing nFe3O4 and CNT. In addition, the abudance of electro-active bacteria such as Syntrophomonas zehnderi was significantly increased from 17% to around 45%.

Anaerobic treatment of LCFA-containing synthetic dairy wastewater at 20 °C: Process performance and microbial community dynamics

Facilitating anaerobic degradation of long-chain fatty acids (LCFA) is key for tapping the high methane production potential of the fats, oil and grease (FOG) content of dairy wastewaters. In this study, the feasibility of using high-rate granular sludge reactors for the treatment of mixed LCFA-containing synthetic dairy wastewater (SDW) was assessed at 20 °C. The effects of the LCFA concentration (33-45% of COD) and organic loading rates (2-3 gCOD/L·d) were determined using three parallel expanded granular sludge bed reactors. For the first time, long term anaerobic treatment of LCFA-containing feed at 20 °C was shown to be feasible and was linked to the microbial community dynamics in high-rate reactors. During a two-month operation, a soluble COD removal of 84-91% and COD to methane conversion of 44-51% was obtained. However, granular sludge flotation and washout occurred after two months in all reactors without volatile fatty acids (VFA) accumulation, emphasizing the need for sludge retention for long-term granular sludge reactor operation with LCFA-containing feed at low ambient temperatures. The temporal shifts in microbial community structure were studied in the high-rate treatment of SDW, and the process disturbances (elevated LCFA loading, LCFA accumulation, and batch operation) were found to decrease the microbial community diversity. The relative abundance of Methanosaeta increased with higher LCFA accumulation in the settled and flotation layer granules in the three reactors, therefore, acetoclastic methanogenesis was found to be crucial for the high-rate treatment of SDW at 20 °C. This study provides an initial understanding of the continuous anaerobic treatment of LCFA-containing industrial wastewaters at low ambient temperatures.