Effects of digestate recirculation ratios on biogas production and methane yield of continuous dry anaerobic digestion

Enhanced anaerobic digestion of lignocellulosic paunch waste using potassium hydroxide pre-treatment

Alkaline pre-treatment enhances the biological accessibility and methane recovery of lignocellulosic feedstocks during anaerobic digestion. This study investigates the impact of potassium hydroxide (KOH) pre-treatment on paunch (2.5 and 12 g per 100 g paunch TS for 24 hr at 22 °C) using biochemical methane potential tests and 5 L bench-scale CSTR bioreactors. BMP tests showed significant improvements in hydrolysis kinetics and methane yield. Continuous experiments validated these findings, with volatile solids destruction increasing from 33 % for raw paunch to 61 % for 12 % KOH pre-treated paunch. Methane yield was consistently enhanced with an increase of up to 150 % compared to raw paunch. Microbial analysis revealed a shift in dominant methanogens from Methanosaeta to Methanobacterium, in response to KOH pre-treatment. While the results highlight the potential of KOH pre-treatment to improve both degradability and methane recovery, challenges related to process stability were identified, emphasizing the need for further investigation to optimise operational conditions for full-scale applications.

Quantity of inoculum modulates organic methanation and humification in solid-state anaerobic digestion and aerobic composting hybrid processes

In this study, we conducted a comprehensive analysis of the regulatory effects of varying feedstock-to-inoculum (F/I) ratios on the organic matter conversion process within a hybrid solid-state anaerobic digestion (SSAD) - aerobic composting (AC) system. Notably, at an F/I ratio of 1.5, we observed a significant enhancement in the anaerobic fermentation phase, particularly in the microbial community distribution. This adjustment led to a marked enrichment of the acetic acid-utilizing Methanosaeta family, which in turn drove a remarkable increase of over 29.9 % in the volumetric methane production based on reactor volume. Concurrently, the optimized F/I ratio facilitated a more substantial degradation of lignin and cellulose, fostering a conducive environment for the growth of humic acid (HA)-producing microorganisms such as Bacteroidota and Actinobacteria during the aerobic composting phase. The heightened activity of these functional microbes, in conjunction with the upregulation of humification-related genes, culminated in a substantial increase in HA content, ranging from 33.6 % (0.028 g·kg-1 DM) to 60.5 % (1.036 g·kg-1 DM). Thus, our findings demonstrate that by strategically modulating the F/I ratio to 1.5, it is possible to effectively optimize the microbial community structure in the SSAD-AC system, leading to a significant improvement in the efficiency of both organic matter methanation and humification processes.

Techno-economic assessment of digestate management for full-scale continuous dry anaerobic digestion plant

This study investigated techno-economic analysis of digestate management systems for a full-scale continuous dry anaerobic digestion plant treating organic fraction of municipal solid waste (OFMSW). A three-stage dewatering system was considered as the baseline scenario (Sc-1) whereas nitrogen removal unit (Sc-2), the incorporation of membrane bioreactor with nitrification downstream (Sc-3) and direct thermal drying of the digestate (Sc-4) were evaluated. The capital cost for the dewatering system with a cake dryer was $4.2 million. For Sc-2, NH4-N load could be reduced by 60% and Sc-3 would allow for further reduction in COD and NH4-N loads with capital cost of $5.2 and $9.1 million, respectively. To achieve an additional reduction in TN load, an investment of $9.8 million was required in Sc-4. Sc-1 had the lowest operation cost, consequently resulting in the highest income of $22.6/ton, whereas Sc-4 had the highest operation cost, resulting in the lowest income of $16.6/ton OFMSW.

Changes in bacterial diversity of full-scale anaerobic digesters treating secondary sludge

Anaerobic digestion (AD) of secondary sludge (2S) presents challenges because of its high microbial content and complex cell wall structures. The purpose of this study was to investigate the effects of spatiotemporally-variable factors such as water temperature and dietary habits on the 2S bacterial community and its migration into digesters. Bacterial communities and functions were analyzed using high-throughput pyrosequencing. Spatiotemporal variations in bacterial populations were identified, with genera such as Zoogloea and Dechloromonas migrating into digesters and influencing organic degradation. Notably, Zoogloea was negatively correlated with VS removal, potentially due to cell floc formation, whereas Dechloromonas was positively correlated, suggesting its role in acetate metabolism anaerobically. This study emphasizes the importance of microbial migration from 2S to digesters, highlighting the need to monitor microbial communities along with conventional parameters to increase AD performance. These findings provide practical insights into optimizing sludge management and improving biogas production in AD plants.

Effect of differences in residual feed intake on gastrointestinal microbiota of Dexin fine-wool meat sheep

In this study, we examined the effects of different residual feed intakes (RFIs) on nutrient digestibility and the microbiota of the digestive tract of Dexin fine-wool sheep. Fifty 70-day-old Dexin fine-wool meat lambs were selected as the experimental group and fed in a single pen for 100 days. Based on their mid-term metabolic weight, 100-day average daily weight gain and daily feed intake, the male Dexin lambs were divided into a low-RFI group (13), a mid-RFI group (18), and a high-RFI group (11). Six male Dexin lambs were selected from each group to collect feces, rumen digesta and solid digesta. Rectal feces were collected from three lambs in each group. The results showed that the digestibility of dry matter and crude protein by sheep in the L-RFI group was than that in the H-RFI group (P < 0.05). Within the microbial population, f_Anaerovoracaceae, g_Christensenellaceae_R_7_group, p_Proteobacteria, and g_Roseburia were significantly correlated with RFI. Energy metabolism, metabolism of amino acids and carbohydrates, transport and catabolism, and cell migration pathways were upregulated in the L-RFI group. The differences in the microbiota of the digestive tract of sheep with different RFIs were reflected in the presence of some key bacterial genera rather than changes in the overall microbial diversity.

High-solid digestion - A comparison of completely stirred and plug-flow reactor systems

High-solid digestion (HSD) for biogas production is a resource-efficient and sustainable method to treat organic wastes with high total solids content and obtain renewable energy and an organic fertiliser, using a lower dilution rate than in the more common wet digestion process. This study examined the effect of reactor type on the performance of an HSD process, comparing plug-flow (PFR) type reactors developed for continuous HSD processes, and completely stirred-tank reactors (CSTRs) commonly used for wet digestion. The HSD process was operated in thermophilic conditions (52 °C), with a mixture of household waste, garden waste and agricultural residues (total solids content 27-28 %). The PFRs showed slightly better performance, with higher specific methane production and nitrogen mineralisation than the CSTRs, while the reduction of volatile solids was the same in both reactor types. Results from 16S rRNA gene sequencing showed a significant difference in the microbial population, potentially related to large differences in stirring speed between the reactor types (1 rpm in PFRs and 70-150 rpm in CSTRs, respectively). The bacterial community was dominated by the genus Defluviitoga in the PFRs and order MBA03 in the CSTRs. For the archaeal community, there was a predominance of the genus Methanoculleus in the PFRs, and of the genera Methanosarcina and Methanothermobacter in the CSTRs. Despite these shifts in microbiology, the results showed that stable digestion of substrates with high total solids content can be achieved in both reactor types, indicating flexibility in the choice of technique for HSD processes.

Mechanisms of how exogenous CO2 affects methane production in an optimized high-solid anaerobic digester treating co-substrates of sewage sludge and food waste

The CO2 addition could promote anaerobic digestion, but the exploration on bioconversion mechanisms of exogenous CO2 in high-solid anaerobic digestion (HSAD) system is still insufficient. This study investigated the performance of a CO2-added HSAD treating co-substrates of sewage sludge and food waste (FW). The maximum methane yield of 623.4 mL CH4/g-VSremoved was obtained with FW proportion of 75 %, organic loading of 3.7 g-VS/L/d and intermittent stirring. The CO2 addition could improve the methane yield by 11.8 % under the optimized conditions. Thermodynamic analysis showed that the most energetically favorable reaction for CH4 production was acetoclastic methanogenesis (AM), and the main bioconversion pathway of exogenous CO2 was homoacetogenesis (HA). Significantly higher methanogenic activity was achieved with CO2 addition during acetate decomposition testing, suggesting enhanced AM pathway. The AM methanogens Methanosaeta were also enriched. Therefore, the main mechanism of the enhanced methane production by CO2 addition was the facilitation of coupled HA-AM pathway.

Survey of the fecal microbiota of indigenous small ruminants living in different areas of Guizhou

Introduction

Gut microbiota are associated with the health and performance of ruminant species, and they are affected by altitude, host genetics, and sex. However, there has been little research on comparing the fecal microbiota of indigenous small ruminants such as sheep and goats in Guizhou province, China. In the present study, we revealed the effect of altitude, genetics, and sex on fecal microbiota profiles and enterotypes in indigenous small ruminants of Guizhou province, China.

Methods

Fecal samples were collected from Hei and Qianbei Ma goats and Weining sheep in the Chinese province of Guizhou. 16S rRNA gene sequencing targeting the V3-V4 region was performed using the Illumina MiSeq platform. Sequences were processed using QIIME2, and the qualified sequences were processed using the plugin DADA2 to generate amplicon sequence variants (ASVs). The statistical analysis was performed using R studio.

Results

The fecal microbial profile was found to vary by herd (influenced by genetics/altitude) and sex. All samples were categorized into two enterotypes. The first enterotype is dominated by UCG-005, and the second enterotype is dominated by the Christensenellaceae_R-7_group, which may be highly driven by the host's genetics (breed). The predicted functional profiles of the fecal microbiota were also assigned to two clusters that corresponded exactly to the enterotypes. Cluster 1 of the functional profiling was characterized by biosynthesis pathways, and cluster 2 was characterized by energy metabolism pathways.

Discussion

Our findings may provide new insights into the fecal microbial community and enterotypes in small ruminants by herds, offering clues for understanding the mechanisms by which the fecal microbiota contribute to divergent host phenotypes in indigenous small ruminants in Guizhou.

Anaerobic membrane bioreactors for municipal wastewater: Progress in resource and energy recovery improvement approaches

Anaerobic membrane bioreactor (AnMBR) offer promise in municipal wastewater treatment, with potential benefits including high-quality effluent, energy recovery, sludge reduction, and mitigating greenhouse gas emissions. However, AnMBR face hurdles like membrane fouling, low energy recovery, etc. In light of net-zero carbon target and circular economy strategy, this work sought to evaluate novel AnMBR configurations, focusing on performance, fouling mitigation, net-energy generation, and nutrients-enhancing integrated configurations, such as forward osmosis (FO), membrane distillation (MD), bioelectrochemical systems (BES), membrane photobioreactor (MPBR), and partial nitrification-anammox (PN/A). In addition, we highlight the essential role of AnMBR in advancing the circular economy and propose ideas for the water-energy-climate nexus. While AnMBR has made significant progress, challenges, such as fouling and cost-effectiveness persist. Overall, the use of novel configurations and energy recovery strategies can further improve the sustainability and efficiency of AnMBR systems, making them a promising technology for future sustainable municipal wastewater treatment.

Switching from wet to dry anaerobic digestion of food waste with different dilution times under no mechanical mixing condition

Previous research on anaerobic digestion of food waste has primarily focused on either wet or dry anaerobic digestion (AD), typically accompanied by continuous mechanical mixing. However, the necessary dilution rates and the extent of mixing required have yet to be addressed. In this study, we investigated switching from wet to dry AD of food waste without mechanical mixing, employing different dilution rates. Lab-scale anaerobic reactors were operated with dilution rates of 10, 5, and 2 times during Phases I (0-56 days), II (57-121 days), and III (122-209 days), respectively. The methane production rates were not significantly different (p > 0.05) across the dilution rates decreased from 10 to 2 times. Remarkably, the methane production in the anaerobic reactors exhibited fluctuations due to variations in feeding, with the methane production rate ranging from 2.0 to 2.7 g CH4-COD/(L d), without mechanical mixing, as the solids content transitioned from wet to near-dry digestion conditions (15 %, food waste). The distribution of sludge volatile solids concentrations remained uniform in the reactor, even at high solids concentrations of up to 15 %. A dynamic microbial community response to changes in dilution rates, with a shift from aceticlastic to hydrogenotrophic methanogenesis pathways. Syntrophic acetate oxidization bacteria (the genus Syner-01 (4.2-8.9 %) and f_Synergistaceae (3.6-4.2 %)) were highly enriched as switching from wet AD to dry AD. The study's findings provide crucial operational insights for anaerobic food waste treatment, potentially resulting in decreased water usage and operational costs, particularly in scenarios with low dilution rates and without mechanical mixing.

Anaerobic sludge digestion elevates dissemination risks of bacterial antibiotic resistance in effluent supernatant

Anaerobic digestion following a variety of pretreatments is a promising technique for the reduction of excess sludge in municipal wastewater treatment plants (MWWTPs), and eliminations of possible pathogens, viruses, protozoa, and other disease-causing organisms. Notwithstanding a rapidly increasing health concern of antibiotic resistant bacteria (ARB) in MWWTPs, dissemination risks of ARB in anaerobic digestion processes are still poorly understood, especially in the digested supernatant. Taking the representative ARB with respect to the common tetracycline-, sulfamethoxazole-, clindamycin- and ciprofloxacin resistance, we investigated the compositions of ARB in the sludge and supernatant, and quantified their variations along the entire anaerobic sludge digestion process following ultrasonication-, alkali-hydrolysis- and alkali-ultrasonication pretreatments, respectively. Results showed that the abundance of ARB was diminished by up to 90% from the sludge along anaerobic digestion coupling with the pretreatments. Surprisingly, pretreatments clearly boosted the abundance of specific ARB (e.g., 2.3 × 10² CFU/mL of tetracycline-resistant bacteria) in the supernatant that otherwise remained relatively low value of 0.6 × 10² CFU/mL from the direct digestion. Measurements of the soluble-, loosely-bound- and tightly-bound extracellular polymeric substances components revealed a gradually intensified destruction of the sludge aggregates along the entire anaerobic digestion processes, which could be likely responsible to the increase of the ARB abundance in the supernatant. Furthermore, analysis of the bacterial community components showed that the ARB populations were strongly correlated with the occurrence of Bacteroidetes, Patescibacteria, and Tenericutes. Interestingly, intensified conjugal transfer (0.015) of antibiotic resistance genes (ARGs) was observed upon returning of the digested supernatant to the biological treatment system. It implies the likelihood of ARGs spreading and subsequent ecological risks upon anaerobic digestion towards reducing excess sludge, and therefore requires further attentions for the excess sludge treatments especially of supernatant.

Gut bacteria of adult and larval Cotinis nitida Linnaeus (Coleoptera: Scarabaeidae) demonstrate community differences according to respective life stage and gut region

The close association between bacteria and insect hosts has played an indispensable role in insect diversity and ecology. Thus, continued characterization of such insect-associated-microbial communities is imperative, especially those of saprophagous scarab beetles. The bacterial community of the digestive tract of adults and larvae of the cetoniine scarab species Cotinis nitida is characterized according to life stage, gut structure, and sex via high-throughput 16S rRNA gene amplicon sequencing. Through permutational ANOVAs of the resulting sequences, bacterial communities of the digestive system are shown to differ significantly between adults and larvae in taxon richness, evenness and relatedness. Significant bacterial community-level differences are also observed between the midgut and hindgut in adult beetles, while no significant host-sex differences are observed. The partitioning between bacterial communities in the larval digestive system is shown through significant differences in two distinct hindgut regions, the ileum and the expanded paunch, but not between the midgut and ileum portion of the hindgut region. These data further corroborate the hypothesis of strong community partitioning in the gut of members of the Scarabaeoidea, suggest hypotheses of physiological-digestive association, and also demonstrate the presence of a seemingly unusual non-scarab-associated taxon. These findings contribute to a general portrait of scarabaeoid digestive tract bacterial communities while illuminating the microbiome of a common new world cetoniine of the Gymnetini-a tribe largely neglected in scarab and beetle microbiome and symbiosis literature.

Substance bioconversion, hydrolases activity, and metagenomic analysis to unravel the enhanced biomethanation of corn stover with urea-hydrothermal pretreatment

Corn stover (CS) is a promising feedstock for producing biomethane, that can replace diminishing fossil fuels. However, the recalcitrant structure of CS resulted in low degradability in anaerobic digestion (AD). Numerous studies investigated the pretreatment of CS before AD, but the insight mechanism of biomethanation enhancement is not fully revealed. Therefore, this study advanced low-temperature urea-hydrothermal pretreatment of CS, and the biomethane production, substance bioconversion, hydrolase activity, and metagenomic analysis were conducted to unravel the intrinsic mechanisms of pretreatment for the enhanced biomethanation. The results showed that the pretreatment improved 11.5% of the specific surface area of CS, providing 111.5% higher total volatile fatty acids and 19.9% higher reducing sugars than the control, potentially enriching more anaerobic microorganisms. As a result, the pretreated CS achieved 19.1% higher biomethane yield, 9.1% higher volatile solid removal rate, and 3 days shorter digestion time. The mass balance and microbial community succession analysis indicated that the pretreatment reinforced the biomethane conversion from carbohydrate, which was attributed to the rapid enrichment of hydrolytic acidification bacteria (g__unclassified_o__Bacteroidales) (33.2%) and mixotrophic archaea (Methanosarcina) (72.3%). Meanwhile, the activity of cellulase and xylanase was enhanced up to 23.7% and 66.7%. Metagenomic analysis revealed that the combined pretreatment of CS promoted methanogenesis by enhancing various CAZymes secretion (such as oligosaccharide-degrading enzymes), and functional genes expression of hydrolytic, acidification and acetate-methane pathways at days 1-5. The study indicated that the combined pretreatment could influence microbial composition and function by changing the physicochemical properties of the CS, thereby improving methanogenic performance.

Dual utilization of aloe peel: Aloe peel-derived carbon quantum dots enhanced anaerobic co-digestion of aloe peel

Carbon materials have been widely used in anaerobic digestion (AD), but the role of zero-dimensional carbon quantum dots (CQDs) in anaerobic co-digestion (AcoD) has not yet been reported. In this work, the effect of aloe peel-derived CQDs (AP-CQDs) on the AcoD system of aloe peel and dairy manure was investigated. The addition of AP-CQDs accelerants increased the cumulative CH₄ yield from 201.14 to 266.92-339.64 mL/g VS and increased total chemical oxygen demand removal efficiency from 34.72 % to 48.77-57.87 %. The use of a digestate with 0.36 wt.% of AP-CQDs resulted in a thermogravimetric mass loss of 47.15 % and a promising total nutrient content of 46.65 g/kg. The excellent electron exchange capacity of AP-CQDs may facilitate direct interspecies electron transfer during the AD process. Moreover, the use of AP-CQDs can enrich methanogenic microorganisms (Methanosarcina and Methanobacterium). These findings provide a viable strategy for improving methane production and create awareness regarding the dual use of biomass waste.

Enhanced anaerobic digestion of kitchen waste at different solids content by alkali pretreatment and bentonite addition: Methane production enhancement and microbial mechanism

High solid anaerobic digestion (AD) has been considered as a promising and sustainable technology for treating kitchen waste. To enhance AD of kitchen waste, alkali pretreatment and bentonite addition treatment (AP/Be) was performed on kitchen waste, and microbial community was investigated at different total solids (TS) content (10%, 13%, 19%, 22% and 25%). The results indicated that after AP/Be treatment, methane yield was as high as 198 mL CH₄/g volatile solid (VS), which increased by 236% as the control. Moreover, microbial community analysis revealed that AP/Be treatment enriched bacterial microbial diversity. At TS of 10%, AP/Be treatment enhanced the hydrogenotrophic methanogens (Methanobacterium) significantly. In addition, the dominant methanogenic pathways changed at different TS content. These results demonstrated AP/Be treatment had a positive effect on methanogenesis during kitchen waste anaerobic digestion process. This study threw new insights towards enhancing kitchen waste anaerobic digestion, as well as the microbial mechanism.

Dissimilatory manganese reduction facilitates synergistic cooperation of hydrolysis, acidogenesis, acetogenesis and methanogenesis via promoting microbial interaction during anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) is commonly limited to poor synergistic cooperation of four stages including hydrolysis, acidogenesis, acetogenesis and methanogenesis. Dissimilatory metal reduction that induced by metal-based conductive materials is promising strategy to regulate anaerobic metabolism with the higher metabolic driving force. In this study, MnO₂ as inducer of dissimilatory manganese reduction (DMnR) was added into WAS-feeding AD system for mediating complicated anaerobic metabolism. The results demonstrated that main operational performances including volatile solid (VS) degradation efficiency and cumulative CH₄ production with MnO₂ dosage of 60 mg/g·VS reached up to maximum 53.6 ± 3.4% and 248.2 ± 10.1 mL/g·VS while the lowest operational performances in control group (38.5 ± 2.8% and 183.5 ± 8.5 mL/g·VS) was originated from abnormal operation of four stages. Furthermore, high-throughput 16 S rRNA pyrosequencing revealed that enrichment of dissimilatory manganese-reducing contributors and methanogens such as Thermovirga, Christensenellaceae_R_7_group and Methanosaeta performed the crucial role in short-chain fatty acids (SCFAs) oxidation and final methanogenesis, which greatly optimized operational environment of hydrolysis, acidogenesis and acetogenesis. More importantly, analysis of functional genes expression proved that abundances of genes encoding enzymes participated in acetate oxidation, direct interspecies electron transfer (DIET) and CO₂ reduction pathway were simultaneously up-regulated with the optimum MnO₂ dosage, suggesting that DMnR with SCFAs oxidation as electron sink could benefit stable operation of four stages via triggering effective DIET-based microbial interaction mode.

Research on the Gut Microbiota of Hainan Black Goat

The intestine of animals is a complex micro-ecosystem containing a large number of microbiomes, which is essential for the host's health development. The Hainan black goat with good resistance and adaptability is a unique species in Hainan, China. These unique physiological characteristics are inseparable from their intestinal microbiota. In this study, high-throughput sequencing was used to investigate bacterial communities in different segments of the intestinal tract of Hainan black goat. The results showed that the indices of Chao1 and ACE in the cecum and colon were significantly greater than those in the ileum (p = 0.007, 0.018). According to PCoA, the intestinal flora composition of the cecum and colon is almost equivalent. In contexts of the phylum, Firmicutes, Bacteroidota, and Pseudomonadota were the dominant phyla in the gut of the Hainan black goat. While in context of the genus, the dominant groups in the gut of black goats mainly include Ruminococcaceae_UCG-005, Bacteroides, Paeniclostridium, Christensenellaceae_R-7_group, Rikenellaceae_RC9_gut_group, and Eubacterium coprostanoligenes _group, Prevotella_1, they have different proportions in different intestinal segments. The gut microbiota of Hainan black goat is mainly Firmicutes, Bacteroidota, and Pseudomonadota. Influenced by the intestinal location where they colonize, the large intestine has a more complex intestinal flora than the small intestine. In contrast, there are only minor differences between the caecum and the colon in the large intestine.

Combined effects of liquid digestate recirculation and biochar on methane yield, enzyme activity, and microbial community during semi-continuous anaerobic digestion

The combined effects of liquid digestate recirculation (LDR) and biochar on methanogenesis and microbial communities were studied in semi-continuous anaerobic reactors fed with wheat straw and swine manure. The tolerated organic loading rate (OLR) was expanded from 5 g- volatile solids (VS)∙L-1∙d-1 in the control to higher than 6 g-VS∙L-1∙d-1 in the LDR. At the OLR of 5.0 g-VS∙L-1∙d-1, average special methane yield in LDR with biochar was 0.234 L∙g-VS-1, which was 5.4 % higher than that of the LDR alone. Moreover, enzyme activity and microbial community analysis indicated that LDR with biochar enhanced the processes of hydrolysis and methanogenesis, and balanced the pathway between hydrogenotrophic and acetoclastic methanogenesis. The co-application of LDR and biochar synergistically enhanced the degradation pathways of substrates and the loading shock resistance of anaerobic digestion system. This study could offer strategies for developing sustainable applications of full and continuous LDR in industrial biogas projects.

Dry Anaerobic Digestion of the Organic Fraction of Municipal Solid Waste: Biogas Production Optimization by Reducing Ammonia Inhibition

The aim of this work is to optimize biogas production from thermophilic dry anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) by comparing various operational strategies to reduce ammonia inhibition. A pilot-scale plug flow reactor (PFR) operated semi-continuously for 170 days. Three scenarios with different feedstock, namely solely OFMSW, OFMSW supplemented with structural material, and OFMSW altered to have an optimal carbon-to-nitrogen (C/N) ratio, were tested. Specific biogas production (SGP), specific methane production (SMP), the biogas production rate (GPR), and bioenergy recovery were evaluated to assess the process performance. In addition, process stability was monitored to highlight process problems, and digestate was characterized for utilization as fertilizer. The OFMSW and the structural material revealed an unbalanced content of C and N. The ammonia concentration decreased when the optimal C/N ratio was tested and was reduced by 72% if compared with feeding solely OFMSW. In such conditions, optimal biogas production was obtained, operating with an organic loading rate (OLR) equal to 12.7 gVS/(L d). In particular, the SGP result was 361.27 ± 30.52 NLbiogas/kgVS, the GPR was 5.11 NLbiogas/(Lr d), and the potential energy recovery was 8.21 ± 0.9 MJ/kgVS. Nevertheless, the digestate showed an accumulation of heavy metals and low aerobic stability.

Effect of biochar derived from biogas residue on methane production during dry anaerobic fermentation of kitchen waste

Kitchen wastes (KW) dramatically increasing with population and economy enhancing, and dry anaerobic fermentation was used to treat it. However, the large amount of biogas residue severely restricted the application of dry anaerobic fermentation, because the high total solid might lead to the system failure. Therefore, it is urgent to find appropriate way to improve the efficiency of dry anaerobic fermentation and reduce the great amount of biogas residue. In this study, a tentative experiment was conducted to investigate the effect of biochar prepared from biogas residue on the performance of dry anaerobic fermentation system. The results showed that almost half of the biogas residue was reduced and converted into biochar. At the presence of biochar, methane yield was 308.6 mL/gVS, which was 10.5% higher than that of control. Compared to the system without biochar, the highest volatile fatty acid (VFA) concentration was 19.3% higher and the percentage of acetate and valerate was 25.3％ and 12.8％, while it was 16.3% and 22.0% in the control, suggesting that biochar accelerated the degradation of VFA. Bacteria community diversity increased, Fastidiosipila and Proteiniphilum enriched at the presence of biochar, which might accelerate the hydrolysis and acidification of KW. Hydrogenotrophic methanogens was dominated and syntrophic acetate oxidation was the primary pathway to produce methane. This study developed a new recycle route for improving the efficiency of dry anaerobic fermentation while reducing the large amount of biogas residue generated from dry anaerobic fermentation.

Dynamic Variations in Rumen Fermentation Characteristics and Bacterial Community Composition during In Vitro Fermentation

This study aimed to explore the dynamic variations of rumen fermentation characteristics and bacterial community composition during a 24 h in vitro fermentation. A total of twenty-three samples were collected from original rumen fluid (ORF, n = 3), fermentation at 12 h (R12, n = 10), and fermentation at 24 h (R24, n = 10). Results showed that gas production, concentrations of microbial crude protein, ammonia nitrogen, and individual volatile fatty acids (VFA), as well as total VFA and branched-chain VFA concentrations, were higher in R24 when compared with R12 (p < 0.05). However, no significant differences were observed in acetate to propionate ratio and fermentation efficiency between R12 and R24 (p > 0.05). Bacterial diversity analysis found that Shannon index and Simpson index were higher in R24 (p < 0.05), and obvious clusters were observed in rumen bacterial community between R12 and R24. Taxonomic analysis at the phylum level showed that the abundances of Proteobacteria and Fibrobacteres were higher in R12 than that in R24, and inverse results were observed in Bacteroidetes, Firmicutes, Cyanobacteria, Verrucomicrobia, Lentisphaerae, and Synergistetes abundances. Taxonomic analysis at the genus level revealed that the abundances of Rikenellaceae RC9 gut group, Succiniclasticum, Prevotellaceae UCG-003, Christensenellaceae R-7 group, Ruminococcaceae UCG-002, Veillonellaceae UCG-001, and Ruminococcaceae NK4A214 group were higher in R24, whereas higher abundances of Succinivibrionaceae UCG-002, Ruminobacter, and Fibrobacter, were found in R12. Correlation analysis revealed the negative associations between gas production and abundances of Proteobacteria, Succinivibrionaceae UCG-002, and Ruminobacter. Moreover, the abundances of Firmicutes, Rikenellaceae RC9 gut group, Christensenellaceae R-7 group, and Ruminococcaceae UCG-002 positively correlated with VFA production. These results indicate that both rumen fermentation characteristics and bacterial community composition were dynamic during in vitro fermentation, whereas the fermentation pattern, efficiency, and bacterial richness remained similar. This study provide insight into the dynamics of rumen fermentation characteristics and bacterial composition during in vitro fermentation. This study may also provide a reference for decision-making for the sampling time point when conducting an in vitro fermentation for bacterial community investigation.

Metagenomics and network analysis elucidating the coordination between fermentative bacteria and microalgae in a novel bacterial-algal coupling reactor (BACR) for mariculture wastewater treatment

To achieve the goal of treating mariculture wastewater economically and efficiently, a novel bacterial-algal coupling reactor (BACR) integrating acidogenic fermentation and microalgae cultivation was firstly investigated for mariculture wastewater treatment. Volatile fatty acids (VFAs) generated in the dark chamber migrated into the photo chamber for microalgal utilization, which alleviated the pH drop and feedback inhibition of the acidogenic fermentation. The maximum dry cell weight (DCW) of microalgae was 1.46 g/L, and pollutants such as chemical oxygen demand (COD), ammonium (NH₄⁺-N) and total phosphorus (TP) in the BACR were effectively removed under the mixotrophic culture condition. Furthermore, bacterial community profiles and functional genes in the BACR and single acidogenic fermentation reactor were identified. Compared with the single acidogenic fermentation reactor, most of the fermentative bacteria (e.g., Ruminococcus, Christensenellaceae R-7 group, Exiguobacterium, Pseudomonas and Levilinea) were enriched by the BACR. From the genetic perspective, the abundances of dominant genes (ackA, acs and atoD) associated with acetic, propionic and butyric acid production were greatly enhanced in the BACR. In the fatty acid biosynthesis pathway (ko00061), three kinds of high-abundance acetyl-CoA carboxylase genes and eight kinds of downstream functional genes were up-regulated in the BACR. Finally, based on co-occurrence network analysis, the coordination between fermentative bacteria and microalgae in the BACR was revealed. This study provided a deep insight into the advantage and potential of the BACR in mariculture wastewater treatment.

Impact of temperature, inoculum flow pattern, inoculum type, and their ratio on dry anaerobic digestion for biogas production

This study is aimed to apply dry anaerobic digestion (DAD) for methane (CH₄) enriched biogas production from unsorted organic municipal solid waste (MSW). Cumulative biogas production was monitored for 35 days of operation in batch digesters at fixed feedstock to inoculum (F/I) ratio 2. Anaerobic sludge (AS) and cow manure (CM) were used as inoculum in single and mixed modes. Several process parameters such as inoculum flow pattern (single layer, multilayer, and spiral), digestion temperature (25 to 40 °C), inoculation modes (single and mixed mode), and inoculation proportion (AS:CM = 1:1, 1:2, 1:3, and 2:1) were investigated to determine the optimum DAD conditions to maximize the CH₄ laden biogas yield. The study of inoculum flow pattern showed that digester with multilayer inoculum configuration generated the maximum 555 mL cumulative biogas with the production rate of 195 mL/day (at 25 °C). Biogas production rate and cumulative biogas production were found to increase with a rise in temperature and the maximum values of 380 mL/day and 1515 mL respectively were observed at 37 °C. The mixed mode of inoculation containing AS and CM augmented the biogas yield at previously optimized conditions. Final results showed that digester with multilayer inoculum flow pattern at 37 °C produced 1850 mL cumulative biogas with 1256.58 mL CH₄/kg volatile solid (VS) when the mixed inoculum was used at the AS:CM—1:2 ratio. Biogas production with this significant amount of CH₄ justifies the use of the DAD process for energy (biogas) generation from widely available biomass feedstock (MSW), offering various advantages to the environment.

Anaerobic Digestion of the Organic Fraction of Municipal Solid Waste in Plug-Flow Reactors: Focus on Bacterial Community Metabolic Pathways

The aim of this study is to investigate the performance of a pilot-scale plug-flow reactor (PFR) as a biorefinery system to recover chemicals (i.e., volatile fatty acids (VFAs)), and biogas during the dry thermophilic anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW). The effects of the hydraulic retention time (HRT) on both outputs were studied, reducing the parameter from 22 to 16 days. In addition, VFA variation along the PFR was also evaluated to identify a section for a further valorization of VFA-rich digestate stream. A particular focus was dedicated for characterizing the community responsible for the production of VFAs during hydrolysis and acidogenesis. The VFA concentration reached 4421.8 mg/L in a section located before the end of the PFR when the HRT was set to 16 days. Meanwhile, biogas production achieved 145 NLbiogas/d, increasing 2.7 times when compared to the lowest HRT tested. Defluviitoga sp. was the most abundant bacterial genus, contributing to 72.7% of the overall bacterial population. The genus is responsible for the hydrolysis of complex polysaccharides at the inlet and outlet sections since a bimodal distribution of the genus was found. The central zone of the reactor was distinctly characterized by protein degradation, following the same trend of propionate production.

Engineering and microbial characteristics of innovative lab and pilot continuous dry anaerobic co-digestion system fed with cow dung and corn straw

Dry anaerobic digestion (dry-AD) allows high-solid digestion; however, dry-AD application is limited because it is prone to blockage and intermediate inhibition. Here, we reported innovative continuous dry co-digestion systems at both lab and pilot scales. The effects of digestate recirculation ratio, dry mass ratio of cow dung to corn straw (CD:CS), and TS content on the digestion performance were investigated. The effects of the three factors were ranked as follows: TS content > CD:CS > digestate recirculation ratio. The daily biogas production rate reached 0.386 NL/d/g VS with the optimal parameter combination, which was determined to be TS content of 30%, a substrate ratio of 1:3, and a digestate recirculation ratio of 40%. In addition, increasing the CD:CS and TS content increased digestate viscosity, which inhibited biogas production; however, increased abundance of Proteiniphilum and acetoclastic methanogens facilitated biogas production. This study provides empirical support for further application of dry-AD.

Novel insights of impacts of solid content on high solid anaerobic digestion of cow manure: Kinetics and microbial community dynamics

High solid anaerobic digestion has become the mainstream technology for sustainable on-farm treatment of solid wastes but has not been optimized with respect to increasing solid content in cow manure (CM). In the present study, CM was batch digested at total solid (TS) of 5%, 10%, 15% and 20% and microbial communities were investigated. The process remained stable up to 15% TS. The biomethane production rate at TS of 10% and 15% was reported to be 352.2 mL g^-1 VS and 318.6 mL g^-1 VS, reaching up to 83% and 75% of TS 5% biomethane, respectively. Kinetics results disclosed that the biodegradable organics could be utilized at increasing solid content with decreasing hydrolysis rate. The abundances of hydrogenotrophic and methylotrophic methanogens increased significantly with increasing solid content. This study is of great importance for understanding and application of high solid anaerobic digestion of cow manure.

Sustainable blueberry waste recycling towards biorefinery strategy and circular bioeconomy: A review

Waste valorization using biological methods for value addition as well as environmental management is becoming popular approach for sustainable development. The present review addresses the availability of blueberry crop residues (BCR), applications of this feedstock in bioprocess for obtaining range of value-added products, to offer economic viability, business development and market potential, challenges and future perspectives. To the best of our knowledge, this is the first article addressing the blueberry waste valorization for a sustainable circular bioeconomy. Furthermore, it covers the information on the alternative BCR valorization methods and production of biochar for environmental management through removal or mitigation of organic and inorganic pollutants from contaminated sites. The review also discusses the ample opportunities of strategic utilization of BCR to offer solutions for environmental sustenance, covers the emerging trends to produce multi-products and techno-economic prospective for sustainable agronomy.

Strategies to improve solid state anaerobic bioconversion of lignocellulosic biomass: an overview

Solid state anaerobic digestion (SSAD) of lignocellulosic biomass may be attractive solution for its valorisation. Compared to liquid state anaerobic digestion (LSAD), SSAD can handle higher organic loading rates (OLR), requires a less water and smaller reactor volume. It may require lower energy demand for heating or mixing and has higher volumetric methane productivity. Besides numerous benefits of SSAD processes and progress in system design, there are still obstacles, which need to be overcome for its successful implementations. This review aims to compile the recent trends in enhancing the bioconversion of agricultural stubbles in SSAD. Several pretreatment procedures used to breaking lignin and cellulose complex, method to overcome carbon to nitrogen ratio imbalance, use of carbon-based conducting materials to enhance Volatile Fatty Acids (VFA) conversion and additives for achieving nutrient balance will be discussed in this review. Leachate recirculation and its impacts on SSAD of agricultural stubbles are also discussed.

Effluent solids recirculation to municipal sludge digesters enhances long-chain fatty acids degradation capacity

BACKGROUND

Slow degradation kinetics of long-chain fatty acids (LCFA) and their accumulation in anaerobic digesters disrupt methanogenic activity and biogas production at high loads of waste lipids. In this study, we evaluated the effect of effluent solids recirculation on microbial LCFA (oleate) degradation capacity in continuous stirred-tank sludge digesters, with the overall aim of providing operating conditions for efficient co-digestion of waste lipids. Furthermore, the impacts of LCFA feeding frequency and sulfide on process performance and microbial community dynamics were investigated, as parameters that were previously shown to be influential on LCFA conversion to biogas.

RESULTS

Effluent solids recirculation to municipal sludge digesters enabled biogas production of up to 78% of the theoretical potential from 1.0 g oleate l-1 day-1. In digesters without effluent recirculation, comparable conversion efficiency could only be reached at oleate loading rates up to 0.5 g l-1 day-1. Pulse feeding of oleate (supplementation of 2.0 g oleate l-1 every second day instead of 1.0 g oleate l-1 every day) did not have a substantial impact on the degree of oleate conversion to biogas in the digesters that operated with effluent recirculation, while it marginally enhanced oleate conversion to biogas in the digesters without effluent recirculation. Next-generation sequencing of 16S rRNA gene amplicons of bacteria and archaea revealed that pulse feeding resulted in prevalence of fatty acid-degrading Smithella when effluent recirculation was applied, whereas Candidatus Cloacimonas prevailed after pulse feeding of oleate in the digesters without effluent recirculation. Combined oleate pulse feeding and elevated sulfide level contributed to increased relative abundance of LCFA-degrading Syntrophomonas and enhanced conversion efficiency of oleate, but only in the digesters without effluent recirculation.

CONCLUSIONS

Effluent solids recirculation improves microbial LCFA degradation capacity, providing possibilities for co-digestion of larger amounts of waste lipids with municipal sludge.

Performance of an anaerobic plug-flow reactor treating agro-industrial wastes supplemented with lipids at high organic loading rate

This study evaluated the performance of a plug-flow reactor (PFR) for high-rate anaerobic co-digestion of complex agro-industrial wastes and used cooking oil or animal fat. The PFR was successfully operated up to an organic loading rate (OLR) of 21 g L^-1 d^-1, yielding biogas at 0.35 L g^-1 chemical oxygen demand (COD) influent. During the study period, supernatant COD at the PFR effluent remained between 4 and 7 g L^-1, with negligible volatile fatty acids' concentrations (<500 mg L^-1) and no presence of foaming incidents. The biomass concentration inside the PFR, expressed as total suspended solids, remained between 30 and 60 g L^-1. Moreover, the above-mentioned anaerobic digestion technology has been currently scaled-up at 50 m³ PFR, while a full-scale facility of 240 kW-el is under construction in the region of north-eastern Greece.