Quantitative assessment of exposure to fecal contamination in urban environment across nine cities in low-income and lower-middle-income countries and a city in the United States

Valorization of food waste: A comprehensive review of individual technologies for producing bio-based products

BACKGROUND

The escalating global concerns about food waste and the imperative need for sustainable practices have fuelled a burgeoning interest in the valorization of food waste. This comprehensive review delves into various technologies employed for converting food waste into valuable bio-based products. The article surveys individual technologies, ranging from traditional to cutting-edge methods, highlighting their respective mechanisms, advantages, and challenges.

SCOPE AND APPROACH

The exploration encompasses enzymatic processes, microbial fermentation, anaerobic digestion, and emerging technologies such as pyrolysis and hydrothermal processing. Each technology's efficacy in transforming food waste into bio-based products such as biofuels, enzymes, organic acids, prebiotics, and biopolymers is critically assessed. The review also considers the environmental and economic implications of these technologies, shedding light on their sustainability and scalability. The article discusses the role of technological integration and synergies in creating holistic approaches for maximizing the valorization potential of food waste. Key finding and conclusion: This review consolidates current knowledge on the valorization of food waste, offering a comprehensive understanding of individual technologies and their contributions to the sustainable production of bio-based products. The synthesis of information presented here aims to guide researchers, policymakers, and industry stakeholders in making informed decisions to address the global challenge of food waste while fostering a circular and eco-friendly economy.

Fulvic acid-mediated efficient anaerobic digestion for kitchen wastewater: Electrochemical and biochemical mechanisms

Fulvic acid, prevalent in humus derived from the anaerobic digestion of kitchen wastewater, is crucial in organic matter transformation. However, its effects and underlying mechanisms remain unclear. In this study, the fate of anaerobic digestion of artificial and kitchen wastewater with different fulvic acid contents was investigated. The results showed that 125 mg/L fulvic acid resulted in a 64.02 and 51.72 % increase in methane production in synthetic and kitchen wastewater, respectively. Fulvic acid acted as an electron mediator and increased substrate oxidation by boosting NAD and ATP levels, thereby increasing microbial metabolic rates and ensuring an adequate substrate for methane generation. Isotope analysis suggested that fulvic acid boosts the conversion of volatile fatty acids to methane via the interspecies electron transfer pathway. Gene expression analysis revealed that cytochrome c, FAD, and other electron transport coenzymes were upregulated by fulvic acid, thereby enhancing substrate utilisation and biogas quality. Fulvic acid presented a dual stimulatory and inhibitory effect on anaerobic digestion, with concentrations over 125 mg/L diminishing its positive impact. This dual effect may stem from the properties and concentrations of fulvic acid. This study revealed the effect mechanism of fulvic acid and provided insights into the humus performance in anaerobic digestion.

Impact of using glucose as a sole carbon source to analyze the effect of biochar on the kinetics of biomethane production

The adaptation of biochar in anaerobic digestion (AD) positively influences the conversion of substrate to biomethane and promotes system stability. This study investigated the influence of biochar (BC) doses (0 to 8 g/L) on the Biochemical Methane Potential (BMP) of glucose during a 60-day AD in a mesophilic batch-type reactor. The first 6.5 weeks of the experimentation were dedicated to the microorganism's adaptation to the biochar and degradation of organics from the used inoculum (3 phases of the glucose feeding). The last 2 weeks (4th phase of glucose feeding) represented the assumption, that glucose is the sole carbon source in the system. A machine learning model based on the autoregressive integrated moving average (ARIMA) method was used to model the cumulative BMP. The results showed that the BMP increased with the amount of BC added. The highest BMP was obtained at a dose of 8 g/L, with a maximum cumulative BMP of 390.33 mL CH4/g-VS added. Likewise, the system showed stability in the pH (7.17 to 8.17). In contrast, non-amended reactors produced only 135.06 mL CH4/g-VS and became acidic at the end of the operation. Reducing the influence of carbon from inoculum, sharpened the positive effect of BC on the kinetics of biomethane production from glucose.

Polyethylene microplastic-induced microbial shifts affected greenhouse gas emissions during litter decomposition in coastal wetland sediments

Microplastic contamination has become increasingly aggravated in coastal environments, further affecting biogeochemical processes involved with microbial community shifts. As a key biogeochemical process mainly driven by microbiota in coastal wetland sediments, litter decomposition contributes greatly to the global greenhouse gas (GHG) budget. However, under microplastic pollution, the relationship between microbial alterations and GHG emissions during litter decomposition in coastal wetlands remains largely unknown. Here, we explored the microbial mechanism by which polyethylene microplastic (PE-MP) influenced greenhouse gas (i.e., CH4, CO2 and N2O) emissions during litter decomposition in coastal sediments through a 75-day microcosm experiment. During litter decomposition, PE-MP exposure significantly decreased cumulative CH4 and CO2 emissions by 41.07% and 25.79%, respectively. However, there was no significant change in cumulative N2O emissions under PE-MP exposure. The bacterial, archaeal, and fungal communities in sediments exhibited varied responses to PE-MP exposure over time, as reflected by the altered structure and changed functional groups of the microbiota. The altered microbial functional groups ascribed to PE-MP exposure and sediment property changes might contribute to suppressing CH4 and CO2 emissions during litter decomposition. This study yielded valuable information regarding the effects of PE-MP on GHG emissions during litter decomposition in coastal wetland sediments.

Effects of Fe-modified digestate hydrochar at different hydrothermal temperatures on anaerobic digestion of swine manure

Hydrothermal carbonization temperature is a key factor in controlling the physico-chemical properties of hydrochar and affecting its function. In this study, effects of hydrochar and Fe-modified hydrochar (Fe-HC) prepared at 180 °C (180C-Fe), 220 °C (220C-Fe) and 260 °C (260C-Fe) on anaerobic digestion (AD) performance of swine manure was investigated. Among the three Fe-HCs, 220C-Fe had the highest amount of Fe and Fe2+ on the surface. The relative methane production of control reached 174 %-189 % in the 180C-Fe and 220C-Fe treatments between days 11 and 12. The degradation efficiency of swine manure was highest in the 220C-Fe treatment (61.3 %), which was 14.8 % higher than in the control. Fe-HC could act as an electron shuttle, stimulate the coenzyme F420 formation, increase the relative abundance of Methanosarcina and promote electron transport for acetotrophic methanogenesis in the AD. These findings are helpful for designing an efficient process for treating swine manure and utilizing digestate.

Performance and mechanism of various microaerobic pretreatments on anaerobic digestion of tobacco straw

Tobacco straw is an abundant biomass in China's agricultural ecosystems, and has high potential for methane production. However, the anaerobic digestion (AD) efficiency is limited by the recalcitrant lignocellulose structure of the tobacco straw. In this study, three microaerobic pretreatments were performed for the AD of tobacco straw to increase methane production. Among them, microbial pretreatment with biogas slurry at an oxygen concentration of 4 mL/g VS resulted in the highest methane production of 349.1 mL/g VS, increasing by 19.8 % than that of untreated. During this pretreatment, the relative abundances of Enterococcus and Clostridium sensu stricto 12, which are closely related to acetic acid production and cellulose degradation, were high, and these bacteria might have an important contribution to substrate hydrolysis and the methanogenesis efficiency of the AD process. This study advances the understanding of microaerobic pretreatment processes and provides technological guidance for the efficient utilization of tobacco straw.

Microbiome-functionality in anaerobic digesters: A critical review

Microbially driven anaerobic digestion (AD) processes are of immense interest due to their role in the biovalorization of biowastes into renewable energy resources. The function-versatile microbiome, interspecies syntrophic interactions, and trophic-level metabolic pathways are important microbial components of AD. However, the lack of a comprehensive understanding of the process hampers efforts to improve AD efficiency. This study presents a holistic review of research on the microbial and metabolic "black box" of AD processes. Recent research on microbiology, functional traits, and metabolic pathways in AD, as well as the responses of functional microbiota and metabolic capabilities to optimization strategies are reviewed. The diverse ecophysiological traits and cooperation/competition interactions of the functional guilds and the biomanipulation of microbial ecology to generate valuable products other than methane during AD are outlined. The results show that AD communities prioritize cooperation to improve functional redundancy, and the dominance of specific microbes can be explained by thermodynamics, resource allocation models, and metabolic division of labor during cross-feeding. In addition, the multi-omics approaches used to decipher the ecological principles of AD consortia are summarized in detail. Lastly, future microbial research and engineering applications of AD are proposed. This review presents an in-depth understanding of microbiome-functionality mechanisms of AD and provides critical guidance for the directional and efficient bioconversion of biowastes into methane and other valuable products.

Enhancing acidification efficiency of vegetable wastes through heat shock pretreatment and initial pH regulation

Anaerobic digestion (AD) technology is a practical approach to alleviate severe environmental issues caused by vegetable wastes (VWs). However, its primary product is methane-rich biogas converted from the precursors (mainly volatile fatty acids, VFAs) after long fermentation periods, making traditional AD projects of low economic profits. Intervening in the methanogenesis stage artificially to produce high value-added VFAs can shorten the reaction time of the AD process and significantly improve profits, posing a promising alternative for treating VWs. Given this, this study applied heat shock (HS) pretreatment to inoculum to prevent methane production during AD and systemically investigated the effects of HS pretreatment and initial pH regulation on VFA production from VWs. The results showed that appropriate HS pretreatment effectively inhibited methane generation but promoted VFA accumulation, and VFA production was further enhanced by adjusting the initial pH to 8.0 and 9.0. The highest total VFA concentration of 14,883 mg/L with a VFA yield of 496.1 mg/gVS, 26.98% higher than that of the untreated group, was achieved at an initial pH 8.0 with HS pretreatment of 80 °C for 1 h. Moreover, pH regulation influenced the metabolic pathway of VFA production from VWs during AD, as butyrate was the dominant product at an initial pH of 6.0, while the increased initial pH improved the acetate proportion.

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.

Dosage effect of micron zero-valent iron during thermophilic anaerobic digestion of waste activated sludge: Performance and functional community

This work examined the performance and microbial traits in a thermophilic anaerobic digestion (TAD) of waste activated sludge that was impacted by micron zero valent iron (mZVI). Results showed that methane production was promoted by 0.8, 11.9, and 12.0 times, respectively, when mZVI was at dosages of 25, 100, and 250 mg/g total solid (TS). Also, the consumption of volatile fatty acids was increased by mZVI at higher dosages (100 and 250 mg/g TS). Furthermore, 16S rRNA sequencing demonstrated that microbial community stabilized after day 18 regardless of the dosage of mZVI, and that different dosages of mZVI induced different shifts in the functional community of the archaea rather than the bacteria involved in TAD. As a result, mZVI at 100 mg/g TS could increase the relative abundance of archaeal genera Methanothermobacter the most, increasing by 22.8% at the end of TAD compared to CK. Besides, redundancy analysis revealed that the physicochemical properties explained 79.65% and 89.10% of the variations of bacterial and archaeal abundance, respectively. Also, the findings of the correlation analysis revealed that total dissolved iron, ferrous iron, pH, and ammonium nitrogen, may be the key divers of altering functional communities, particularly archaea. Moreover, mZVI at 100 and 250 mg/g TS boosted the metabolic pathways of environmental information processing (ABC transporters) in bacteria and carbon metabolism and methane metabolism for archaea, as well as relative abundances of enzymes and their activities involved in various methanogenic pathways. This study provides new perspectives on the application of mZVI in solid wastes treatments.

Linking the shifts in the metabolically active microbiota in a UASB and hybrid anaerobic-aerobic bioreactor for swine wastewater treatment

Due to the high concentration of pollutants, swine wastewater needs to be treated prior to disposal. The combination of anaerobic and aerobic technologies in one hybrid system allows to obtain higher removal efficiencies compared to those achieved via conventional biological treatment, and the performance of a hybrid system depends on the microbial community in the bioreactor. Here, we evaluated the community assembly of an anaerobic-aerobic hybrid reactor for swine wastewater treatment. Sequencing of partial 16S rRNA coding genes was performed using Illumina from DNA and retrotranscribed RNA templates (cDNA) extracted from samples from both sections of the hybrid system and from a UASB bioreactor fed with the same swine wastewater influent. Proteobacteria and Firmicutes were the dominant phyla and play a key role in anaerobic fermentation, followed by Methanosaeta and Methanobacterium. Several differences were found in the relative abundances of some genera between the DNA and cDNA samples, indicating an increase in the diversity of the metabolically active community, highlighting Chlorobaculum, Cladimonas, Turicibacter and Clostridium senso stricto. Nitrifying bacteria were more abundant in the hybrid bioreactor. Beta diversity analysis revealed that the microbial community structure significantly differed among the samples (p < 0.05) and between both anaerobic treatments. The main predicted metabolic pathways were the biosynthesis of amino acids and the formation of antibiotics. Also, the metabolism of C5-branched dibasic acid, Vit B5 and CoA, exhibited an important relationship with the main nitrogen-removing microorganisms. The anaerobic-aerobic hybrid bioreactor showed a higher ammonia removal rate compared to the conventional UASB system. However, further research and adjustments are needed to completely remove nitrogen from wastewater.

Mechanistic insights into Fe3O4-modified biochar relieving inhibition from erythromycin on anaerobic digestion

Anaerobic digestion (AD) of antibiotic manufacturing wastewater to degrade residual antibiotics and produce mixture of combustible gases has been investigated actively in the past decades. However, detrimental effect of residual antibiotic to microbial activities is commonly faced in AD process, leading to the reduction of treatment efficiency and energy recovery. Herein, the present study systematically evaluated the detoxification effect and mechanism of Fe3O4-modified biochar in AD of erythromycin manufacturing wastewater. Results showed that Fe3O4-modified biochar had stimulatory effect on AD at 0.5 g/L erythromycin existence. A maximum methane yield of 327.7 ± 8.0 mL/g COD was achieved at 3.0 g/L Fe3O4-modified biochar, leading to the increase of 55.7% compared to control group. Mechanistic investigation demonstrated that different levels of Fe3O4-modified biochar could improve methane yield via different metabolic pathways involved in specific bacteria and archaea. Low levels of Fe3O4-modified biochar (i.e., 0.5-1.0 g/L) led to the enrichment of Methanothermobacter sp., strengthening the hydrogenotrophic pathway. On the contrary, high levels of Fe3O4-modified biochar (2.0-3.0 g/L) favored the proliferation of acetogens (e.g., Lentimicrobium sp.) and methanogen (Methanosarcina sp.) and their syntrophic relations played vital role on the simulated AD performance at erythromycin stress. Additionally, the addition of Fe3O4-modified biochar significantly decreased the abundance of representative antibiotic resistant genes (ARGs), benefiting the reduction of environmental risk. The results of this study verified that the application of Fe3O4-modified biochar could be an efficient approach to detoxify erythromycin on AD system, which brings high impacts and positive implications for biological antibiotic wastewater treatment.

Direct interspecies electron transfer mechanisms of a biochar-amended anaerobic digestion: a review

This paper explores the mechanisms of biochar that facilitate direct interspecies electron transfer (DIET) among syntrophic microorganisms leading to improved anaerobic digestion. Properties such as specific surface area (SSA), cation exchange capacity (CEC), presence of functional groups (FG), and electrical conductivity (EC) were found favorable for increased methane production, reduction of lag phase, and adsorption of inhibitors. It is revealed that these properties can be modified and are greatly affected by the synthesizing temperature, biomass types, and residence time. Additionally, suitable biochar concentration has to be observed since dosage beyond the optimal range can create inhibitions. High organic loading rate (OLR), pH shocks, quick accumulation and relatively low degradation of VFAs, and the presence of heavy metals and toxins are the major inhibitors identified. Summaries of microbial community analysis show fermentative bacteria and methanogens that are known to participate in DIET. These are Methanosaeta, Methanobacterium, Methanospirillum, and Methanosarcina for the archaeal community; whereas, Firmicutes, Proteobacteria, Synergistetes, Spirochetes, and Bacteroidetes are relatively for bacterial analyses. However, the number of defined cocultures promoting DIET is very limited, and there is still a large percentage of unknown bacteria that are believed to support DIET. Moreover, the instantaneous growth of participating microorganisms has to be validated throughout the process.

Role of Polysaccharides from Marine Seaweed as Feed Additives for Methane Mitigation in Ruminants: A Critical Review

Given the increasing concerns regarding greenhouse gas emissions associated with livestock production, the need to discover effective strategies to mitigate methane production in ruminants is clear. Marine algal polysaccharides have emerged as a promising research avenue because of their abundance and sustainability. Polysaccharides, such as alginate, laminaran, and fucoidan, which are extracted from marine seaweeds, have demonstrated the potential to reduce methane emissions by influencing the microbial populations in the rumen. This comprehensive review extensively examines the available literature and considers the effectiveness, challenges, and prospects of using marine seaweed polysaccharides as feed additives. The findings emphasise that marine algal polysaccharides can modulate rumen fermentation, promote the growth of beneficial microorganisms, and inhibit methanogenic archaea, ultimately leading to decreases in methane emissions. However, we must understand the long-term effects and address the obstacles to practical implementation. Further research is warranted to optimise dosage levels, evaluate potential effects on animal health, and assess economic feasibility. This critical review provides insights for researchers, policymakers, and industry stakeholders dedicated to advancing sustainable livestock production and methane mitigation.

Long-term in-situ starvation and reactivation of co-digestion with food waste and corn straw in a continuous AnDMBR: Performance, sludge characteristics, and microorganism community

To explore the effects of in-situ starvation and reactivation in a continuous anaerobic dynamic membrane reactor (AnDMBR), the anaerobic co-digestion system of food waste and corn straw was firstly start-up and stability operated, and then stopped feeding substrate approximately 70 days. After long-term in-situ starvation, the continuous AnDMBR was reactivated using the same operation conditions and organic loading rate as the continuous AnDMBR used before in-situ starvation. Results shown that the anaerobic co-digestion of corn straw and food waste in the continuous AnDMBR can resume stable operation within 5 days, and the corresponding methane production of 1.38 ± 0.26 L/L/d was completely returned to the methane production before in-situ starvation (1.32 ± 0.10 L/L/d). Through analysis of the specific methanogenic activity and key enzyme activity of the digestate sludge, only the acetic acid degradation activity of methanogenic archaea can be partially recovered, however, the activities of lignocellulose enzyme (lignin peroxidase, laccase, and endoglucanase), hydrolase (α-glucosidase) and acidogenic enzyme (acetate kinas, butyrate kinase, and CoA-transferase) can be fully recovered. Analysis of microorganism community structure using metagenomic sequencing technology showed that starvation decreased the abundance of hydrolytic bacteria (Bacteroidetes and Firmicutes) and increased the abundance of small molecule-utilizing bacteria (Proteobacteria and Chloroflexi) due to lack of substrate during the long-term in-situ starvation stage. Furthermore, the microbial community structure and key functional microorganism still maintained and similar with that of starvation final stage even after long-term continuous reactivation. The reactor performance and sludge enzymes activity in the continuous AnDMBR co-digestion of food waste and corn straw can be well reactivated after long-term in-situ starvation, even though the microbial community structure can not be recovered to the initiating stage.

Metataxonomic characterization of the microbial community involved in the production of biogas with microcrystalline cellulose in pilot and laboratory scale

Biogas, produced in anaerobic digestion, is a sustainable alternative for generating energy from agro-industrial and municipal waste. Information from the microbiota active in the process expands the possibilities for technological innovation. In this study, taxonomic annotations, and functional prediction of the microbial community of the inoculum of two processes were carried out: an industrial unit (pilot-scale urban solid waste plant-IU) and a laboratory-scale reactor fed with swine and cattle waste (LS). The biochemical potential of biogas was obtained using tested inoculum with microcrystalline cellulose, obtaining 682 LN/kgVS (LSC-laboratory scale inoculum and microcrystalline cellulose), and 583 LN/kgVS (IUC-industrial unit inoculum and microcrystalline cellulose), which is equivalent to a recovery of 91.5% of total biogas to LSC. The phyla Synergistota and Firmicutes were more abundant in LS/LSC. In the IU/IUC (treatment of restaurant waste and customs seizures), there was a greater microbiological variety and a predominance of the Bacteroidota, Cloacimonadota, Firmicutes and Caldatribacteriota. The genus Methanosaeta predominated in the process, and it was possible to infer the genes (K01895, K00193 and K00625) related to acetoclastic pathway, as well as endoglucanases that are involved in the metabolism of cellulose (LSC). Terpenoids, polyketides, cofactors, and vitamin metabolism were higher in reactors that received different substrates (IU; IUC). The taxonomic and functional differences revealed the importance of determining the microbiota in the analysis of the potential of an inoculum, combined with the use of microcrystalline cellulose, which can provide optimization information in the production of clean energy.

Pilot-scale treatment of municipal garbage mechanical dewatering wastewater by an integrated system involving partial nitrification and denitrification

The municipal solid waste (MSW) with high water content can be pre-treated by the mechanical dewatering technology to significantly decrease the leachate generation in sequential landfill treatment or to improve the efficiency for solid waste incineration, which has attracted great concerns recently. However, the generated mechanical dewatering wastewater (MDW) containing high organics and nitrogenous content has been one of the big challenges for the sustainable treatment of MSW. In this study, a pilot-scale integrated system composed of physiochemical pretreatment, anaerobic sequencing batch reactor (ASBR), partial nitrification SBR (PN-SBR), denitrification SBR (DN-SBR), and UV/O₃ advanced oxidation process, with a capacity of 1.0 m³/d to treat MDW containing over 34000 mg-chemical oxygen demand (COD)/L organics pollutant and 850 mg/L NH₄⁺-N, was successfully developed. By explorations on the start-up of this integrated system and the process conditions optimization, after a long-term system operation, the findings demonstrated that this integrated system could reach the removal efficiency in the COD, NH₄⁺-N and total nitrogen (TN) in the MDW of 99.7%, 98.2% and 96.9%, respectively. Partial nitrification and denitrification were successfully obtained for the TN removal with the nitrite accumulation rate of over 80%. The treatment condition parameters were optimized to be 800 mg/L polyaluminum chloride (PAC) and 2 mg/L polyacrylamide (PAM) under a pH of 9 for pretreatment, 36 h hydraulic retention time (HRT) for ASBR, 24 h for PN-SBR, and 2 h for UV/O₃ unit. The organic sources in the MDW were also found to be feasible for the DN-SBR. Consequently, the resulting final effluent was stably in compliance with the discharge standard with high stability and reliability.

Cross-Feedings, Competition, and Positive and Negative Synergies in a Four-Species Synthetic Community for Anaerobic Degradation of Cellulose to Methane

Complex interactions exist among microorganisms in a community to carry out ecological processes and adapt to changing environments. Here, we constructed a quad-culture consisting of a cellulolytic bacterium (Ruminiclostridium cellulolyticum), a hydrogenotrophic methanogen (Methanospirillum hungatei), an acetoclastic methanogen (Methanosaeta concilii), and a sulfate-reducing bacterium (Desulfovibrio vulgaris). The four microorganisms in the quad-culture cooperated via cross-feeding to produce methane using cellulose as the only carbon source and electron donor. The community metabolism of the quad-culture was compared with those of the R. cellulolyticum-containing tri-cultures, bi-cultures, and mono-culture. Methane production was higher in the quad-culture than the sum of the increases in the tri-cultures, which was attributed to a positive synergy of four species. In contrast, cellulose degradation by the quad-culture was lower than the additive effects of the tri-cultures which represented a negative synergy. The community metabolism of the quad-culture was compared between a control condition and a treatment condition with sulfate addition using metaproteomics and metabolic profiling. Sulfate addition enhanced sulfate reduction and decreased methane and CO₂ productions. The cross-feeding fluxes in the quad-culture in the two conditions were modeled using a community stoichiometric model. Sulfate addition strengthened metabolic handoffs from R. cellulolyticum to M. concilii and D. vulgaris and intensified substrate competition between M. hungatei and D. vulgaris. Overall, this study uncovered emergent properties of higher-order microbial interactions using a four-species synthetic community. IMPORTANCE A synthetic community was designed using four microbial species that together performed distinct key metabolic processes in the anaerobic degradation of cellulose to methane and CO₂. The microorganisms exhibited expected interactions, such as cross-feeding of acetate from a cellulolytic bacterium to an acetoclastic methanogen and competition of H₂ between a sulfate reducing bacterium and a hydrogenotrophic methanogen. This validated our rational design of the interactions between microorganisms based on their metabolic roles. More interestingly, we also found positive and negative synergies as emergent properties of high-order microbial interactions among three or more microorganisms in cocultures. These microbial interactions can be quantitatively measured by adding and removing specific members. A community stoichiometric model was constructed to represent the fluxes in the community metabolic network. This study paved the way toward a more predictive understanding of the impact of environmental perturbations on microbial interactions sustaining geochemically significant processes in natural systems.

Taxonomic and enzymatic basis of the cellulolytic microbial consortium KKU-MC1 and its application in enhancing biomethane production

Lignocellulosic biomass is a promising substrate for biogas production. However, its recalcitrant structure limits conversion efficiency. This study aims to design a microbial consortium (MC) capable of producing the cellulolytic enzyme and exploring the taxonomic and genetic aspects of lignocellulose degradation. A diverse range of lignocellulolytic bacteria and degrading enzymes from various habitats were enriched for a known KKU-MC1. The KKU-MC1 was found to be abundant in Bacteroidetes (51%), Proteobacteria (29%), Firmicutes (10%), and other phyla (8% unknown, 0.4% unclassified, 0.6% archaea, and the remaining 1% other bacteria with low predominance). Carbohydrate-active enzyme (CAZyme) annotation revealed that the genera Bacteroides, Ruminiclostridium, Enterococcus, and Parabacteroides encoded a diverse set of cellulose and hemicellulose degradation enzymes. Furthermore, the gene families associated with lignin deconstruction were more abundant in the Pseudomonas genera. Subsequently, the effects of MC on methane production from various biomasses were studied in two ways: bioaugmentation and pre-hydrolysis. Methane yield (MY) of pre-hydrolysis cassava bagasse (CB), Napier grass (NG), and sugarcane bagasse (SB) with KKU-MC1 for 5 days improved by 38-56% compared to non-prehydrolysis substrates, while MY of prehydrolysed filter cake (FC) for 15 days improved by 56% compared to raw FC. The MY of CB, NG, and SB (at 4% initial volatile solid concentration (IVC)) with KKU-MC1 augmentation improved by 29-42% compared to the non-augmentation treatment. FC (1% IVC) had 17% higher MY than the non-augmentation treatment. These findings demonstrated that KKU-MC1 released the cellulolytic enzyme capable of decomposing various lignocellulosic biomasses, resulting in increased biogas production.

Active microbial communities during biodegradation of biodegradable plastics by mesophilic and thermophilic anaerobic digestion

Biodegradable plastics, if they are not properly managed at their end-of-life, can have the same hazardous environmental consequences as conventional plastics. This study investigates the treatment of the main biodegradable plastics under mesophilic and thermophilic anaerobic digestion using biochemical methane potential test and the microorganisms involved in the process using amplicon sequencing of the 16 S rRNA. Here we showed that, only PHB and TPS undergone important and rapid biodegradation under mesophilic condition (38 °C). By contrast, PCL and PLA exhibited very low biodegradation rate as 500 days were required to reach the ultimate methane yield. Little or no degradation occurred for PBAT and PBS at 38 °C. Under thermophilic conditions (58 °C), TPS, PHB, and PLA reached high levels of biodegradation in a relatively short period (&lt; 100 d). While PBS, PBAT, and PCL could not be converted into methane at 58 °C. PHB degraders (Enterobacter and Cupriavidus) and lactate-utilizing bacteria (Moorella and Tepidimicrobium) appeared to play an important role in the PHB and PLA degradation, respectively. This work not only provides crucial data on the anaerobic digestion of the main biodegradable plastics but also enriches the understanding of the microorganisms involved in this process, which are of great importance for future development of the treatment of biodegradable plastics in anaerobic digestion systems.

Metatranscriptional characterization of metabolic dynamics in anaerobic membrane bioreactor producing methane from low-strength wastewater

An anaerobic membrane bioreactor (AnMBR) with media is an emerging carbon-neutral biotechnology for low-strength wastewater (LSWW) treatment and methane recovery. Understanding metabolic dynamics among methanogens and syntrophic bacteria is important in optimizing the design and operation of AnMBR. However, little is known about it, especially in media-attached microbial communities. This study explored metabolic dynamics to compare media-attached and suspended conditions. Accordingly, metagenomes and metatranscriptomes from AnMBRs with polymeric media and fed with different influent concentrations (350 and 700 mg-COD/L) were analyzed. Metabolic dynamics were profoundly influenced by the different growth habitats and influent conditions, although the applied influent concentrations are within the range of typical LSWW. Metabolic dynamics prediction results suggest that media-attached-growth habitats may have provided a more favorable microenvironment for methanogens to grow and produce methane, especially under low influent conditions. These findings provide significant implications for optimizing floating media design and operation of AnMBR-producing methane from LSWW.

Seasonal characterization of the prokaryotic microbiota of full-scale anaerobic UASB reactors treating domestic sewage in southern Brazil

Upflow Anaerobic Sludge Blanket (UASB) reactors are alternatives in the anaerobic treatment of sanitary sewage in different parts of the world; however, in temperate environments, they are subject to strong seasonal influence. Understanding the dynamics of the microbial community in these systems is essential to propose operational alternatives, improve projects and increase the quality of treated effluents. In this study, for one year, high-performance sequencing, associated with bioinformatics tools for taxonomic annotation and functional prediction was used to characterize the microbial community present in the sludge of biodigesters on full-scale, treating domestic sewage at ambient temperature. Among the most representative phyla stood out Desulfobacterota (20.21-28.64%), Proteobacteria (7.48-24.90%), Bacteroidota (10.05-18.37%), Caldisericota (9.49-17.20%), and Halobacterota (3.23-6.55%). By performing a Canonical Correspondence Analysis (CCA), Methanolinea was correlated to the efficiency in removing Chemical Oxygen Demand (COD), Bacteroidetes_VadinHA17 to the production of volatile fatty acids (VFAs), and CI75cm.2.12 at temperature. On the other hand, Desulfovibrio, Spirochaetaceae_uncultured, Methanosaeta, Lentimicrobiaceae_unclassified, and ADurb.Bin063-1 were relevant in shaping the microbial community in a co-occurrence network. Diversity analyses showed greater richness and evenness for the colder seasons, possibly, due to the lesser influence of dominant taxa. Among the principal metabolic functions associated with the community, the metabolism of proteins and amino acids stood out (7.74-8.00%), and the genes related to the synthesis of VFAs presented higher relative abundance for the autumn and winter. Despite the differences in diversity and taxonomic composition, no significant changes were observed in the efficiency of the biodigesters.

Elucidating interactive effects of sulfidated nanoscale zero-valent iron and ammonia on anaerobic digestion of food waste

In our previous study, anaerobic digestion of food waste could be effectively enhanced by adding sulfidated nanoscale zero-valent iron (S-nZVI) under high-strength ammonia concentrations. In this study, in order to further elucidate the specific interactive effects of S-nZVI and ammonia on anaerobic digestion of nitrogen-rich food waste, the methanogenic performance of anaerobic digestion systems respectively added with nanoscale zero-valent iron (nZVI) and S-nZVI were compared and monitored under different ammonia stress conditions. Both nZVI and S-nZVI could effectively stimulate the methanogenesis process among ammonia concentrations ranging from 0 to 3500 mg/L. However, the enhancing effects of S-nZVI and nZVI on anaerobic digestion of food waste were different, in which anaerobic digestion systems added with S-nZVI and nZVI performed best under 2500 mg/L of ammonia and 1500 mg/L of ammonia, respectively. Furthermore, the analysis of microbial communities suggested that ammonia stress enriched acetoclastic methanogens, while adding nZVI and S-nZVI into anaerobic digestions stimulated the process of hydrogenotrophic methanogenesis. Moreover, S-nZVI performed better in promoting the evolution of DIET-related microorganisms than nZVI, resulting in enhanced methane production under high ammonia-stressed conditions. This work provided fundamental knowledge about the interactive effects of S-nZVI and ammonia on the anaerobic digestion of food waste.

Biochemical methane potential and active microbial communities during anaerobic digestion of biodegradable plastics at different inoculum-substrate ratios

The influence of the inoculum-substrate ratio (ISR) on the mesophilic and thermophilic biochemical methane potential test of two biodegradable plastics was evaluated. Poly(lactic acid) (PLA) and polyhydroxybutyrate (PHB) were selected for this study, the first for being recalcitrant to mesophilic anaerobic digestion (AD) and the second, by contrast, for being readily biodegradable. Several ISRs, calculated on the basis of volatile solids (VS), were tested: 1, 2, 2.85, 4, and 10 g(VS of inoculum).g(VS of substrate)^-1. A high ISR was associated with an enhanced methane production rate (i.e., biodegradation kinetics). However, the ultimate methane production did not change, except when inhibition was observed. Indeed, applying the lowest ISR to readily biodegradable plastics such as PHB resulted in inhibition of methane production. Based on these experiments, in order to have reproducible degradation kinetics and optimal methane production, an ISR between 2.85 and 4 is recommended for biodegradable plastics. The active microbial communities were analyzed, and the active bacteria differed depending on the plastic digested (PLA versus PHB) and the temperature of the process (mesophilic versus thermophilic). Previously identified PHB degraders (Ilyobacter delafieldii and Enterobacter) were detected in PHB-fed reactors. Thermogutta and Tepidanaerobacter were detected during the thermophilic AD of PLA, and they are probably involved in PLA hydrolysis and lactate conversion, respectively.

Impact of different pretreatments on the anaerobic digestion performance of cucumber vine

Copious amounts of cucumber vine (CV) derived from crop growing and harvesting are casually discarded in the field, posing severely negative impacts on public health and the ecological environment. Treating CV via anaerobic digestion (AD) could represent a promising approach while the recalcitrant lignocellulosic structure restricts its conversion efficiency, thus underscoring the importance of valid pretreatments. This study systematically investigated the effects of nine types of commonly applied chemical pretreatments involved H₂SO₄, HCl, H₃PO₄, NaOH, KOH, Ca(OH)₂, CaO, H₂O₂, and alkaline hydrogen peroxide (AHP) pretreatments on methane production of CV. Results showed that alkaline and AHP pretreatments were beneficial to the methane production of CV and obtained the considerable cumulative methane yield and biodegradability of 194.3-241.5 mL·g_VS^-1 and 47.59-59.15%, respectively, 36.83-70.07% higher than untreated. Analyses of lignocellulosic compositions and structural characterizations revealed that alkaline and AHP pretreatments well destroyed both hemicellulose and lignin, which commendably increased the accessibility of cellulose, facilitating the methane production. The findings of this study provide not only efficient pretreatment methods for the disposal and utilization of CV during AD process but also promising alternatives for enhancing methane production performance of similar vine residues, which would be greatly valuable for industrial applications in the future.

River damming enhances ecological functional stability of planktonic microorganisms

Planktonic microorganisms play an important role in maintaining the ecological functions in aquatic ecosystems, but how their structure and function interrelate and respond to environmental changes is still not very clear. Damming interrupts the river continuum and alters river nutrient biogeochemical cycling and biological succession. Considering that river damming decreases the irregular hydrological fluctuation, we hypothesized that it can enhance the ecological functional stability (EFS) of planktonic microorganisms. Therefore, the community composition of planktonic bacteria and archaea, functional genes related to carbon, nitrogen, sulfur, and phosphorus cycling, and relevant environmental factors of four cascade reservoirs in the Pearl River, Southern China, were investigated to understand the impact of damming on microbial community structure and function and verify the above hypothesis. Here, the ratio of function to taxa (F:T) based on Euclidean distance matrix analysis was first proposed to characterize the microbial EFS; the smaller the ratio, the more stable the ecological functions. The results showed that the reservoirs created by river damming had seasonal thermal and chemical stratifications with an increasing hydraulic retention time, which significantly changed the microbial structure and function. The river microbial F:T was significantly higher than that of the reservoirs, indicating that river damming enhances the EFS of the planktonic microorganisms. Structural equation modeling demonstrated that water temperature was an important factor influencing the relationship between the microbial structure and function and thus affected their EFS. In addition, reservoir hydraulic load was found a main factor regulating the seasonal difference in microbial EFS among the reservoirs. This study will help to deepen the understanding of the relationship between microbial structure and function and provide a theoretical basis of assessing the ecological function change after the construction of river damming.

Production of biochar from crop residues and its application for anaerobic digestion

Anaerobic digestion (AD) is a viable and cost-effective method for converting organic waste into usable renewable energy. The efficiency of organic waste digestion, nonetheless, is limited due to inhibition and instability. Accordingly, biochar is an effective method for improving the efficiency of AD by adsorbing inhibitors, promoting biogas generation and methane concentration, maintaining process stability, colonizing microorganisms selectively, and mitigating the inhibition of volatile fatty acids and ammonia. This paper reviews the features of crop waste-derived biochar and its application in AD systems. Four critical roles of biochar in AD systems were identified: maintaining pH stability, promoting hydrolysis, enhancing the direct interspecies electron transfer pathway, and supporting microbial development. This work also highlights that the interaction between biochar dose, amount of organic component in the substrate, and inoculum-to-substrate ratio should be the focus of future research before deploying commercial applications.

Metabolic minimap of anaerobic digestion for undergraduate biochemistry courses

As a representative catabolic reaction that widely exists in nature, anaerobic digestion (AD) exhibits great value regarding the global carbon cycle, renewable energy development, and environmental protection. Such an important biochemical reaction was ignored before and should be introduced into the teaching and textbooks of undergraduate biochemistry courses. However, students may face obstructions when learning AD theories since the metabolic pathways in AD are very complex. To solve these problems, an instructive metabolic minimap of the AD reaction was designed, including its reaction stages, reaction pathways, substrates, and enzymes. Furthermore, the interrelationships between aerobic catabolism (AEC) and anaerobic catabolism (ANC) were also summarized by combining the catabolic pathways of typical biological macromolecules. In this paper, AD theories were first replenished into undergraduate biochemistry courses by metabolic minimap, which not only provided valuable supports for the practical teaching of AD in undergraduate biochemistry courses, but also acted as an important reference for students in biology-related majors and biochemistry teachers.

Combined modifications of CaO and liquid fraction of digestate for augmenting volatile fatty acids production from rice straw: Microbial and proteomics insights

The modification sequence of chemical (CaO) and biological (liquid fraction of digestate, LFD) for augmenting volatile fatty acids (VFAs) production from rice straw was investigated in this study. The coupling order of the modifiers influenced acidification performance, and simultaneous modification (CaO-LFD) was superior to other modes. The highest VFAs production was obtained in CaO-LFD, 51% higher than that in the LFD-first additional modification. The CaO-LFD demonstrated the highest selectivity of acetate production, accounting for 79% of the total VFAs. In addition, CaO-LFD modification changed the direction of the domestication of fermentative bacteria and increased populations of the key anaerobes (Atopostipes sp.) responsible for acidification. The synergistic effect of CaO and LFD was revealed, namely, the effective function of CaO in degrading recalcitrant rice straw, the promotion of transport/metabolism of carbohydrates and acetogenesis by LFD.

Improving production of lactic acid and volatile fatty acids from dairy cattle manure and corn straw silage: Effects of mixing ratios and temperature

Anaerobic co-fermentation (AcoF) of dairy cattle manure (DCM) and corn straw silage (CSS) for producing lactic acid (LA) and volatile fatty acids (VFAs) was investigated. Batch experiments were conducted at seven different DCM/CSS ratios and at mesophilic and thermophilic temperatures. Results indicated that the highest concentration of LA was 17.50 ± 0.70 g/L at DCM:CSS ratio of 1:3 and thermophilic temperature, while VFAs was 18.23 ± 2.45 g/L at mono-CSS fermentation and mesophilic temperature. High solubilization of thermophilic conditions contributed to LA accumulation in AcoF process. Presence of the CSS increased the relative abundance of Lactobacillus for LA production at thermophilic. Meanwhile, the abundance of Bifidobacterium was increased when CSS was added at mesophilic, which could conduce to VFAs production. This study provides a new route for enhancing the biotransformation of DCM and CSS into short-chain fatty acids, potentially bringing economic benefits to agricultural waste treatment.

The effects of ammonia acclimation on biogas recovery and the microbial population in continuous anaerobic digestion of swine manure

This study investigated the ammonia toxicity and the acclimation of anaerobic microbiome in continuous anaerobic digestion of swine manure using unacclimated inoculum. When the total ammonia nitrogen concentration (TAN) reached 2.5 g N/L, the methane yield decreased from 254.1 ± 9.6 to 154.6 ± 9.9 mL/g COD. The free ammonia nitrogen concentration of the inhibited condition was 190 mg N/L. The methane yield was eventually recovered as 269.6 ± 3.6 mL/g COD with a further operation. Anaerobic toxicity assay (ATA) showed that mixed liquor from the recovered phase possessed enhanced tolerance to ammonia, not only within the exposed level in continuous operation (<2.5 g NH₃/L) but also over the range (>2.5 g NH₃/L). Microbial analysis revealed that continuous operation under ammonia stress resulted in the change of both bacterial and archaeal populations. The ammonia adaptation was concurrent with the archaeal population shift from Methanosaeta to Methanosarcina and Methanobacterium. The dominancy of Clostridia in bacterial population was found in the recovered phase. It is highly recommended to use an inoculum acclimated to a target ammonia level which can be pre-checked by ATA and to secure a start-up period for ammonia adaptation in the field application of anaerobic digestion for swine manure.

Degradation of biodegradable plastics by anaerobic digestion: Morphological, micro-structural changes and microbial community dynamics

The serious environmental problem caused by traditional plastics has stimulated the popularization of biodegradable plastics (BPs). However, the rigorous prerequisite for the efficient degradation of BPs has not eliminated its potential hazard to nature. In most biosystems exists the anaerobic environment, but it is still controversial whether BPs can be degraded under such condition. Therefore, this study systematically assessed the anaerobic degradation performance of ten common BPs under mesophilic and thermophilic conditions. Results showed that four BPs were degraded evidently under mesophilic condition with the biodegradability of 57.9%-84.6%, while during thermophilic condition, five BPs showed remarkable degradation performance with the biodegradability of 53.0% to 95.7%. According to morphological and micro-structural analysis, the biodegradation of the BPs probably proceeded via bulk and/or surface erosion. Under mesophilic condition, Anaerolineales, Bacteroidales, Clostridiales, SBR1031, and Synergistales appeared to play an important role. During thermophilic condition, the hydrolysis, acidogenesis, and methanogenesis of most BPs were mainly conducted by Coprothermobacter and the archaea Methanothermobacter. This work not only provides crucial data on the anaerobic biodigestibility of BPs but also enriches the understanding of the BPs degradation mechanisms, which are of great importance for future popularization of BP products and simultaneously relieving the environmental pollution.

Investigating microbial dynamics and potential advantages of anaerobic co-digestion of cheese whey and poultry slaughterhouse wastewaters

Resource recovery and prevention of environmental pollution are key goals for sustainable development. It is widely reported that agro-industrial activities are responsible for the discharge of billions of liters of wastewater to the environment. Anaerobic digestion of these energy rich agro-industrial wastewaters can simultaneously mitigate environmental pollution and recover embedded energy as methane gas. In this study, an assessment of mono- and co-digestion of cheese whey wastewater (CWW) and poultry slaughterhouse wastewater (PSW) was conducted in 2.25-L lab-scale anaerobic digesters. Treatment combinations evaluated included CWW (R1), PSW (R2), 75:25 CWW:PSW (R3), 25:75 CWW:PSW (R4), and 50:50 CWW:PSW (R5). The digestion efficiencies of the mixed wastewaters were compared to the weighted efficiencies of the corresponding combined mono-digested samples. R4, with a mixture of 25% CWW and 75% PSW, achieved the greatest treatment efficiency. This corresponded with an average biodegradability of 84%, which was greater than for R1 and R2 at 68.5 and 71.9%, respectively. Similarly, R4 produced the highest average cumulative methane value compared to R1 and R2 at 1.22× and 1.39× for similar COD loading, respectively. The modified Gompertz model provided the best fit for the obtained methane production data, with lag time decreasing over progressive treatment cycles. PCoA and heatmap analysis of relative microbial abundances indicated a divergence of microbial communities based on feed type over the treatment cycles. Microbial community analysis showed that genus Petrimonas attained the highest relative abundance (RA) at up to 38.9% in the first two cycles, then subsequently decreased to near 0% for all reactors. Syntrophomonas was highly abundant in PSW reactors, reaching up to 36% RA. Acinetobacter was present mostly in CWW reactors with a RA reaching 56.5%. The methanogenic community was dominated by Methanothrix (84.3–99.9% of archaea). The presence of phosphate and Acinetobacter in CWW feed appeared to reduce the treatment efficiency of associated reactors. Despite Acinetobacter being strictly aerobic, previous and current results indicate its survival under anaerobic conditions, with the storage of phosphate likely playing a key role in its ability to scavenge acetate during the digestion process.

Deeper insights into the effects of substrate to inoculum ratio selection on the relationship of kinetic parameters, microbial communities, and key metabolic pathways during the anaerobic digestion of food waste

The substrate to inoculum ratio (S/I) is a crucial factor that affects not only the stability of the anaerobic digestion (AD) of food waste (FW) but also the methanogenic capacity of the substrate. This is of great significance for the start-up of small-scale batch reactors and the directional regulation of methanogenesi and organic acid production. Most studies have merely clarified the optimal S/I ratio for methane production and revealed the basic composition of microbial communities. However, the mechanism of microbial interactions and the metabolic pathways behind the optimal S/I ratio still remain unclear. Herein, the effects of different S/I ratios (VS basis) on the relationship of kinetic parameters, microbial communities, and metabolic pathways during the AD process of FW were holistically explored. The results revealed that high S/I ratios (4:1, 3:1, 2:1, and 1:1) were prone to irreversible acidification, while low S/I ratios (1:2, 1:3, and 1:4) were favorable for methanogenesis. Moreover, a kinetic analysis demonstrated that the methane yield of S/I = 1:3 were the highest. A bioinformatics analysis found that the diversity of bacteria and archaea of S/I = 1:3 were the most abundant, and the enrichment of Bacteroides and Synergistetes could help to establish a syntrophic relationship with hydrogenotrophic methanogens, which could aid in the fulfillment of a unique niche in the system. In contrast to the findings with the other S/I ratios, the cooperation among microbes in S/I = 1:3 was more apparent. Notably, the abundances of genes encoding key enzymes involved in the methanogenesis pathway under S/I = 1:3 were all the highest. This knowledge will be helpful for revealing the influence mechanism of the ratio relationship between microorganisms and substrates on the biochemical metabolic process of anaerobic digestion, thereby providing effective guidance for the directional regulation of FW batch anaerobic reactors.

Biochar and hydrochar in the context of anaerobic digestion for a circular approach: An overview

Biochar and hydrochar are carbonaceous materials with valuable applications. They can be synthesized from a wide range of organic wastes, including digestate. Digestate is the byproduct of anaerobic digestion (AD), which is performed for bioenergy (biogas) production from organic residues. Through a thermochemical process, such as pyrolysis, gasification, and hydrothermal carbonization - HTC, digestate can be converted into biochar or hydrochar. The addition of either biochar or hydrochar in AD has been reported to improve biochemical reactions and microbial growth, increasing the buffer capacity, and facilitating direct interspecies electrons transfer (DIET), resulting in higher methane (CH₄) yields. Both biochar and hydrochar can adsorb undesired compounds present in biogas, such as carbon dioxide (CO₂), hydrogen sulfide (H₂S), ammonia (NH₃), and even siloxanes. However, an integrated understanding of biochar and hydrochar produced from digestate through their return to the AD process, as additives or as adsorbents for biogas purification, is yet to be attained to close the material flow loop in a circular economy model. Therefore, this overview aimed at addressing the integration of biochar and hydrochar production from digestate, their utilization as additives and effects on AD, and their potential to adsorb biogas contaminants. This integration is supported by life cycle assessment (LCA) studies, showing positive results when combining AD and the aforementioned thermochemical processes, although more LCA is still necessary. Techno-economic assessment (TEA) studies of the processes considered are also presented, and despite an expanding market of biochar and hydrochar, further TEA is required to verify the profitability of the proposed integration, given the specificities of each process design. Overall, the synthesis of biochar and hydrochar from digestate can contribute to improving the AD process, establishing a cyclic process that is in agreement with the circular economy concept.

Zero-valent iron mediated alleviation of methanogenesis inhibition induced by organoarsenic roxarsone

Zero-valent iron (ZVI) can enhance anaerobic digestion, and has great potential to alleviate/eliminate methanogenesis inhibition. Little is known about the feasibility of utilizing ZVI to alleviate methanogenesis inhibition that is caused by typical animal feed additive roxarsone in livestock wastewater. In this study, the role of ZVI on alleviating roxarsone-induced methanogenic inhibition and its mechanisms were investigated. With the increase of roxarsone concentration from 5 to 50 mg/L, the inhibition of methanogenesis increased from 3.0% to 65.7%. This inhibition was alleviated by 80.7% and 57.2% when 1.0 and 10.0 g/L ZVI were added, respectively. Due to ZVI addition, an efficient arsenic immobilization onto ZVI (45.4-85.8%) was achieved mainly through the formation of FeAsO₄ precipitate and adsorption by ZVI. Under the function of ZVI, hydrogenotrophic methanogenic activity was obviously restored. The microbial community analysis indicates that the ZVI-regulated alleviation on the methanogenesis inhibition was attributed to the enrichment of Methanobacterium and Methanosarcina. The findings from this study demonstrate that ZVI addition is an effective way for treatment of organoarsenic-contaminated wastewater.

Microbial Pretreatment of Chicken Feather and Its Co-digestion With Rice Husk and Green Grocery Waste for Enhanced Biogas Production

To utilize wastes and residues sustainably and excellently, there is a need to fend for efficient methods and resources for biogas production. Use of poultry waste for biogas production represents one of the most important routes toward reaching global renewable energy targets. The current study involves microbial pretreatment of chicken feather waste, followed by its co-digestion with rice husk and green grocery waste in batch and continuous reactors, respectively. Microbial pretreatment of chicken feathers by keratinase secreting Pseudomonas aeruginosa was an effective and eco-friendly approach to make its recalcitrant structure available as a raw substrate for biogas production. The current study also addressed the enhancement and stability of anaerobic digestion by co-digestion. Results demonstrated that biogas production was increased by microbial pretreatment of chicken feathers and that the percentage increase in biogas yield was 1.1% in microbialy pretreated feathers compared to mono-digestion (non-pretreated feathers) in batch fermentation. The highest yield of biogas was obtained in a batch reactor having co-digestion of pretreated rice husk and microbial pretreated chicken feathers. The co-digestion of chicken feathers hydrolysate with green grocery waste in continuous fermentation mode has also enhanced the biogas yield as compared to average of mono-digestion (chicken feather hydrolysate and green grocery waste) and, therefore, improve the efficiency of the overall process.

Effect of antibiotic mixtures on the characteristics of soluble microbial products and microbial communities in upflow anaerobic sludge blanket

Two upflow anaerobic sludge blanket reactors (UASBs) were used to investigate the effects of three antibiotic mixtures (erythromycin, sulfamethoxazole, and tetracycline) on reactor performance, soluble microbial products (SMPs) composition and microbial community. One reactor (UASB_antibiotics) was fed with antibiotic mixtures, whereas another reactor (UASB_control) was used as a control without the addition of antibiotic mixtures. Compared with those in UASB_control, UASB_antibiotics show lower chemical oxygen demand removal efficiency and biogas content. A higher removal efficiency of antibiotic mixtures was obtained in first few stages in UASB_antibiotics. The SMPs composition of effluent from the two reactors did not differ significantly, and the main components were protein-like substances, which produced higher fluorescence intensity in UASB_antibiotics. Gas chromatography-mass spectrometry analysis revealed that the main compounds identified as SMPs (<580 Da) were alkanes, aromatics and esters, with only 20% similarity of SMPs between UASB_antibiotics and UASB_control. Antibiotics had a significant effect on the microbial community structure. Notably, in UASB_control, hydrogenotrophic methanogens, key microorganisms in anaerobic digestion, had an obvious advantage at all stages compared with UASB_antibiotics, whereas acetoclastic methanogen exhibited the opposite pattern. The above results demonstrated that antibiotic mixtures influenced the effluent quality during anaerobic treatment of synthetic wastewater, resulting in changes in the microbial community structure. This study clarified the effect of antibiotic mixtures on the operation of UASBs. It could contribute to identifying potential strategies for improving effluent quality in anaerobic treatment.

Sulfate-reducing and methanogenic microbial community responses during anaerobic digestion of tannery effluent

Microbial communities were monitored in terms of structure, function and response to physicochemical variables during anaerobic digestion of tannery and associated slaughterhouse effluent in: (i) 2 L biochemical methane potential batch reactors at different inoculum to substrate ratios (2-5) and initial sulfate concentrations (665-2000 mg/L), and (ii) 20 L anaerobic sequencing batch reactors with different mixing regimes (continuous vs. intermittent). Methanogenic and sulfidogenic community compositions in the 2 L reactors evolved initially, but stabilised after the start of biogas generation, although significant (ANOSIM p < 0.05) changes in the physicochemical parameters indicated continued metabolic activity. Both hydrogenotrophic and acetoclastic archaeal genera were present in high relative abundances. Continuous stirring preferentially selected the metabolically versatile genus Methanosarcina, suggesting that higher specific methane generation in the continuously stirred system (168 vs. 19.5 mL methane per gram volatile solids per week) was related to the metabolic activities of members of this genus.

Initial pH-driven production of volatile fatty acid from hybrid Pennisetum

In this work, the impact of initial pH on the production of volatile fatty acids (VFAs) of hybrid Pennisetum was investigated. The batch experiments were conducted under six distinct beginning pH at a mesophilic temperature. Initial pH had an obvious effect on VFA yield and composition, and severe alkaline circumstances (pH_in = 11.0) could boost VFA production and acetic acid selectivity. The highest VFAs yield and acetate proportion were obtained when the initial pH was 11.0, with 518 ± 29 mg g^-1VS and 92%. Furthermore, microbial community analysis showed that alkaliphilic acetogenic anaerobe such as Amphibacillus, Tissierella, and Natronincola were the dominant species when the initial pH was 11.0. The Amphibacillus is the main hydrolysis bacterium under these conditions because of its high ability for xylan degradation at pH 9.0-10.0. Because of the increased VFA yield and superior acetic acid selectivity, the results suggest that adjusting the initial pH to 11.0 in batch mode would be possible for scaling-up purposes.

Carbon-based conductive materials enhance biomethane recovery from organic wastes: A review of the impacts on anaerobic treatment

Amongst the most important sustainable waste management strategies, anaerobic biotechnology has had a central role over the past century in the management of high-pollution load sources, such as food, agricultural and municipal wastes. During anaerobic digestion (AD), valuable by-products such as digestate and biogas are produced. Biogas (mainly composed of methane) is generated through a series of reactions between bacteria and archaea. Enhancement of AD process with higher methane yield, accelerated methane production rate, and shorter start-up time is possible via tapping into a novel methanogenic pathway discovered a decade ago. This fundamentally new concept that is a substitute to interspecies hydrogen transfer is called direct interspecies electron transfer (DIET). DIET, a thermodynamically more feasible way of electron transfer, has been proven to occur between bacteria and methanogens. It is well-documented that amendment of carbon-based conductive materials (CCMs) can stimulate DIET via serving as an electrical conduit between microorganisms. Therefore, different types of CCMs such as biochar and activated carbon have been amended to a variety of AD reactors and enhancement of process performance was reported. In this review, a comparative analysis is presented for enhancement of AD performance in relation to major CCM related factors; electrical conductivity, redox properties, particle size and dosage. Additionally, the impacts of AD operational conditions such as organic loading rate and temperature on CCM amended reactors were discussed. Further, the changes in microbial communities of CCM amended reactors were reviewed and future perspectives along with challenges for CCM application in AD have been provided.

Enrichment of psychrophilic and acidophilic sulfate-reducing bacterial consortia - a solution toward acid mine drainage treatment in cold regions

Failure of sulfate-reducing bacteria (SRB)-mediated treatment of acid mine drainage (AMD) in cold regions due to inhibition of bacteria by acidic pH and low temperature can be overcome by enriching psychrophilic and acidophilic microbial consortia from local metal-rich sediments. In this study, we enriched microbial consortia from Arctic mine sediments at varying pH (3-7) and temperatures (15-37 °C) under anaerobic conditions with repeated sub-culturing in three successive stages, and analyzed the microbial community using 16S rRNA gene sequencing. The enriched SRB genera resulted in high sulfate reduction (85-88%), and significant metal removal (49-99.9%) during the initial stages (stage 1 and 2). Subsequently, sub-culturing the inoculum at pH 3-4.5 resulted in lower sulfate reduction (9-34%) due to the inhibition of SRB by accumulated acetic acid (0.3-9 mM). The microbial metabolic interactions for successful sulfate and metal removal involved initial glycerol co-fermentation to acetic acid at acidic pH (by Desulfosporosinus, Desulfotomaculum, Desulfurospora, and fermentative bacteria including Cellulomonas and Anaerovorax), followed by acetic acid oxidation to CO₂ and H₂ (by Desulfitobacterium) at neutral pH, and subsequent H₂ utilization (by Desulfosporosinus). The results, including the structural and functional properties of enriched microbial consortia, can inform the development of effective biological treatment strategies for AMD in cold regions.

Distribution patterns of functional microbial community in anaerobic digesters under different operational circumstances: A review

Anaerobic digestion (AD) processes are promising to effectively recover resources from organic wastes or wastewater. As a microbial-driven process, the functional anaerobic species played critical roles in AD. However, the lack of effective understanding of the correlations of varying microbial communities with different operational factors hinders the microbial regulation to improve the AD performance. In this paper, the main anaerobic functional microorganisms involved in different stages of AD processes were first demonstrated. Then, the response of anaerobic microbial community to different operating parameters, exogenous interfering substances and digestion substrates, as well as the digestion efficiency, were discussed. Finally, the research gaps and future directions on the understanding of functional microorganisms in AD were proposed. This review provides insightful knowledge of distribution patterns of functional microbial community in anaerobic digesters, and gives critical guidance to regulate and enrich specific functional microorganisms to accumulate certain AD products.

Response of anaerobic granular sludge to long-term loading of roxarsone: From macro- to micro-scale perspective

Extensive use of organoarsenic feed additives such as roxarsone has caused organoarsenicals to occur in livestock wastewater and further within anaerobic wastewater treatment systems. Currently, information on the long-term impacts of roxarsone on anaerobic granular sludge (AGS) activity and the underlying mechanisms is very limited. In this study, the response of AGS to long-term loading of roxarsone was investigated using a laboratory up-flow anaerobic sludge blanket reactor spiked with 5.0 mg L^-1 of roxarsone. Under the effect of roxarsone, methane production decreased by ∼40% due to the complete inhibition on acetoclastic methanogenic activity on day 260, before being restored eventually. Over 30% of the influent arsenic was accumulated in the AGS and the capability of AGS to prevent intracellular As(III) accumulation increased with time. The AGS size was reduced by ∼30% to 1.20‒1.26 mm. Based on morphology and confocal laser scanning microscopy analysis, roxarsone exposure stimulated the excretion of extracellular polymeric substances and the surface spalling of AGS. High-throughput sequencing analysis further indicated roxarsone initially altered the acidogenic pathway and severely inhibited the acetoclastic methanogen Methanothrix. Acetogenic bacteria and Methanothrix were finally enriched and became the main contributor for a full restoration of the initial methane production. These findings provide a deeper understanding on the effect of organoarsenicals on AGS, which is highly beneficial for the effective anaerobic treatment of organoarsenic-bearing wastewater.

Effects of different microbial pretreatments on the anaerobic digestion of giant grass under anaerobic and microaerobic conditions

Microbial pretreatment to lignocellulosic biomass for anaerobic digestion (AD) has achieved increased attention; however, the low efficiency and unclear mechanism of oxygen parameter affecting this process performance limit its practical application. In this study, five readily available microbial consortia were developed to analyze the influences of various oxygen concentrations during pretreatment process upon methane conversion efficiency and microbiota within AD of giant grass. Results found that anaerobic pretreatment by liquid or straw composting inoculant, along with microaerobic pretreatment by cow manure at 10 mL/g VS oxygen concentration, obtained 23.1%, 24.4%, and 16.0% higher methane yields (275.3, 279.8, and 265.3 mL/g VS) than corresponding untreated group, respectively. Microbial community analyses showed that microbial responses to oxygen varied significantly with microbial consortium, which consequently caused different AD performances. The findings will enrich theoretical knowledge of microbial pretreatment and provide a technological guidance for efficient utilization of giant grass and other lignocellulosic biomasses.

Microbiota profile in mesophilic biodigestion of sugarcane vinasse in batch reactors

The vinasse is a residue of ethanol production with the potential for methane production, requiring an allochthonous inoculum. Several microorganisms act in the different phases of anaerobic digestion, and the identification of these microbial communities is essential to optimize the process. The characterization of the microbiota involved in the biodigestion of vinasse was observed in the initial stage (IS), at the peak of methane production (MS) and the end of the process (FS) of the best performance assay by high-throughput sequencing. The highest methane production was 0.78 mmolCH₄.gVS.h^-1 at 243.7 h in the substrate/inoculum ratio of 1.7, with consumption partial of acetic, propionic and isobutyric acids and an 82% reduction of chemical oxygen demand. High microbial diversity was found. The genera Clostridium, Acinetobacter, Candidatus Cloacamonas, Bacteroides, Syntrophomonas, Kosmotoga, the family Porphyromonadaceae and the class Bacteroidia were the most abundant in the maximum methane production. Methane production was driven by Methanobacterium and Methanosaeta, suggesting the metabolic pathways used were hydrogenotrophic and acetoclastic.

Neonicotinoids stimulate H2-limited methane emission in Periplaneta americana through the regulation of gut bacterium community

Methane emitted by insects is considered to be an important source of atmospheric methane. Here we report the stimulation of methane emission in the cockroach Periplaneta americana and termite Coptotermes chaohuensis, insects with abundant methanogens, by neonicotinoids, insecticides widely used to control insect pests. Cycloxaprid (CYC) and imidacloprid (IMI) caused foregut expansion in P. americana, and increased the methane emission. Antibiotics mostly eliminated the effects. In P. americana guts, hydrogen levels increased and pH values decreased, which could be significantly explained by the gut bacterium community change. The proportion of several bacterium genera increased in guts following CYC treatment, and two genera from four could generate hydrogen. Hydrogen is a central intermediate in methanogenesis. All increased methanogens in both foregut and hindgut used hydrogen as electron donor to produce methane. Besides, the up-regulation of mcrA, encoding the enzyme for the final step of methanogenesis suggested the enhanced methane production ability in present methanogens. In the termite, hydrogen levels in gut and methane emission also significantly increased after neonicotinoid treatment, which was similar to the results in P. americana. In summary, neonicotinoids changed bacterium community in P. americana gut to generate more hydrogen, which then stimulated gut methanogens to produce and emit more methane. The finding raised a new concern over neonicotinoid applications, and might be a potential environmental risk associated with atmospheric methane.

Anaerobic and Microaerobic Pretreatment for Improving Methane Production From Paper Waste in Anaerobic Digestion

Having been generated with a tremendous amount annually, paper waste (PW) represents a large proportion in municipal solid waste (MSW) and also a potential source of renewable energy production through the application of anaerobic digestion (AD). However, the recalcitrant lignocellulosic structure poses obstacles to efficient utilization in this way. Recently, anaerobic and microaerobic pretreatment have attracted attention as approaches to overcome the obstacles of biogas production. This study was set out to present a systematic comparison and assessment of anaerobic and microaerobic pretreatment of PW with different oxygen loadings by five microbial agents: composting inoculum (CI), straw-decomposing inoculum (SI), cow manure (CM), sheep manure (SM), and digestate effluent (DE). The hints of microbial community evolution during the pretreatment and AD were tracked by 16S rRNA high-throughput sequencing. The results demonstrated that PW pretreated by DE with an oxygen loading of 15 ml/gVS showed the highest cumulative methane yield (CMY) of 343.2 ml/gVS, with a BD of 79.3%. In addition to DE, SI and SM were also regarded as outstanding microbial agents for pretreatment because of the acceleration of methane production at the early stage of AD. The microbial community analysis showed that Clostridium sensu stricto 1 and Clostridium sensu stricto 10 possessed high relative abundance after anaerobic pretreatment by SI, while Bacteroides and Macellibacteroides were enriched after microaerobic pretreatment by SM, which were all contributable to the cellulose degradation. Besides, aerobic Bacillus in SI and Acinetobacter in SM and DE probably promoted lignin degradation only under microaerobic conditions. During AD, VadinBC27, Ruminococcaceae Incertae Sedis, Clostridium sensu stricto 1, Fastidiosipila, and Caldicoprobacter were the crucial bacteria that facilitated the biodegradation of PW. By comparing the groups with same microbial agent, it could be found that changing the oxygen loading might result in the alternation between hydrogenotrophic and acetoclastic methanogens, which possibly affected the methanogenesis stage. This study not only devised a promising tactic for making full use of PW but also provided a greater understanding of the evolution of microbial community in the pretreatment and AD processes, targeting the efficient utilization of lignocellulosic biomass in full-scale applications.