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

Insights into high ammonia-resistant syntrophic microbiomes and metabolic pathways during continuous anaerobic digestion of cow manure

Understanding microbial responses to ammonia is critical for defining thresholds and ensuring stable operation of anaerobic digestion (AD); however, an understanding of the microbiome's resistance mechanisms to high-total-ammonia-nitrogen (TAN) conditions remains limited. This study determined a TAN threshold of 7 g/L for continuous cow manure AD with increasing TAN levels. TAN was identified as the most critical factor influencing the AD performance, with CH4 production decreasing by > 50 % beyond this level. Additionally, a highly TAN-resistant syntrophic microbiome was identified through network analysis, highlighting key bacteria, Thauera phenolivorans and Fermentimons spp., alongside hydrogenotrophic methanogens. Interestingly, shifts were observed within the hydrogenotrophic methanogen community, transitioning from Methanoculleus bourgensis to Methanoculleus chikugoensis, Methanocorpusculum spp. and Methanobacterium spp. under high-TAN conditions. Significant metabolic pathways specific to high-TAN environments were identified, providing insights into their roles in sustained operation of AD. These findings highlight the performance limitations and functional redundancy under high-TAN conditions.

Air nanobubble simultaneously enhances hydrolysis and methane yield of sludge temperature phased-anaerobic digestion

Nanobubble water (NBW) or temperature-phased anaerobic digestion assisted by microbial electrolysis cell (MEC-TPAD) can promote sludge hydrolysis and methanogenesis. However, the role of the combined application of NBW and MEC-TPAD in terms of anaerobic performance and related microbial properties remains unclear. This study investigated the impact of Air-NBW on hydrolysis and methanogenesis of dewatered sludge MEC-TPAD. Under different temperatures, NBW increased ammonia nitrogen by 7.8%-13.7% in the hydrolysis phase and ultimate methane yield by 23.3%-41.5%. NBW can significantly promote hydrolysis under mesophilic-mesophilic conditions, while it can promote substantially methanogenesis under thermophilic-thermophilic conditions. Moreover, NBW increased the diversity and richness of microorganisms in hydrolysis. As to bacteria, NBW increased the relative abundance (RA) of Firmicutes but decreased the RA of Proteobacteria. As to archaea, NBW increased the RA of Methanosarcina in hydrolysis but decreased it in methanogenesis. NBW synchronized with MEC-TPAD improved hydrolysis and methanogenesis of the dewatered sludge digestion process.

Waste valorization through anaerobic co-digestion in coffee and swine farms: CH4 yield optimization and farm-scale viability

Considering that the Brazilian southeast has several agricultural farms that produce coffee and raise swine, and that the waste generated in these farms has some complementary characteristics, the present study aimed to optimize the methane (CH4) yield in the batch anaerobic co-digestion of liquid swine manure (LSM), coffee wastewater (CFW), and coffee husk and pulp. The optimization occurred through a two-factor central composite rotational design with a variation of CFW percentage (8 to 22 %) in the mixing liquid substrates and the organic matter concentration (0.3 to 12 gCOD L-1). The optimized condition had an ideal nutritional condition (14 % of CFW, 86 % of LSM, 7.3 gCOD L-1 and COD/N ratio of 35) to obtain high CH4 production (971.7 mLCH4), yield (160.9 mLCH4 g-1VS), maximum specific production rate (1.6 mL h-1) and low lag phase (217.6 h).

An in-depth analysis of microbial response to exposure to high concentrations of microplastics in anaerobic wastewater fermentation

The impact of microplastics (MPs) in anaerobic wastewater treatment on microbial metabolism is significant. Anaerobic granular sludge (AS) and biofilm (BF) are two common ways, and their responses to microplastics will have a direct impact on their application potential. This study investigated the microbial reactions of AS and BF to three types of MPs: polyethylene (PE), polyvinyl chloride (PVC), and a mixture of both (MIX). Results exhibited that MPs reduced methane output by 44.65 %, 55.89 %, and 53.18 %, elevated short-chain fatty acid (SCFA) levels by 95.93 %, 124.49 %, and 110.78 %, and lowered chemical oxygen demand (COD) removal by 28.77 %, 36.78 %, and 33.99 % for PE-MP, PVC-MP, and MIX-MP, respectively, with PVC-MP showing the greatest inhibition. Meanwhile, microplastics also facilitated the relative production of reactive oxygen species (ROS, 40.29 %-96.99 %), lactate dehydrogenase (LDH, 20.01 %-75.02 %), and adenosine triphosphate (ATP, 26.64 %-43.80 %), while reducing cytochrome c (cyt c, 23.60 %-49.02 %) and extracellular polymeric substances (EPS, 17.44 %-26.58 %). AS and BF displayed distinct enzymatic activities under MPs exposure. Correspondingly, 16S-rRNA sequencing indicated that AS was mainly involved in acetate generation by Firmicutes, while BF performed polysaccharide degradation by Bacteroidota. Metatranscriptomic analysis showed AS to be rich in acetogens (Bacillus, Syntrophobacter) and methanogens (Methanothrix, Methanobacterium), while BF contained more fermentation bacteria (Mesotoga, Lentimicrobium) and electroactive microorganisms (Clostridium, Desulfuromonas) under MIX-MP. Moreover, BF exhibited higher glycolysis gene expression, whereas AS was more active in methane metabolism, primarily through the acetoclastic methanogenic pathway's direct acetate conversion. This study provides new insights into understanding the microbial response produced by microplastics during anaerobic wastewater digestion.

Role of polylactic acid microplastics during anaerobic co-digestion of cow manure and Chinese cabbage waste enhanced by nanobubble

With the increasing use of plastic products globally, environmental pollution by plastic waste is becoming increasingly problematic. This study investigated the impacts of two types of polylactic acid microplastics, clear microplastics and aluminised film microplastics, on methane yield, microbial community, and volatile fatty acid accumulation during anaerobic co-digestion of cow manure and Chinese cabbage waste under different temperature conditions. The influence of the addition of air nanobubbles on microplastic degradation in the anaerobic digestion system we also examined. The results revealed that under thermophilic conditions, clear and aluminised film microplastics increased the methane yield, with the latter resulting in greater improvement. Conversely, under mesophilic conditions, the presence of microplastics reduced the methane yield, but the addition of air-nanobubble partially mitigated this effect. Microplastics also affected the microbial community, with specific species showing correlations with methane yield. Methanothermobacter, which is linked to lactic acid conversion, was positively correlated with methane yield, whereas Methanomassiliicoccus levels increased in the presence of microplastics, particularly in the inhibited state of the digester. These results suggest that, under thermophilic conditions, microplastics may increase the cumulative methane yield by facilitating the degradation of lactic acid monomers. Furthermore, the aluminised film on microplastics could serve as an electrically conductive material during anaerobic digestion, potentially increasing the methane yield.

A cyclic shift-temperature operation method to train microbial communities of mesophilic anaerobic digestion

Temperature is the critical factor affecting the efficiency and cost of anaerobic digestion (AD). The current work develops a shift-temperature AD (STAD) between 35 °C and 55 °C, intending to optimise microbial community and promote substrate conversion. The experimental results showed that severe inhibition of biogas production occurred when the temperature was firstly increased stepwise from 35 °C to 50 °C, whereas no inhibition was observed at the second warming cycle. When the organic load rate was increased to 6.37 g VS/L/d, the biogas yield of the STAD reached about 400 mL/g VS, nearly double that of the constant-temperature AD (CTAD). STAD promoted the proliferation of Methanosarcina (up to 57.32 %), while severely suppressed hydrogenophilic methanogens. However, when the temperature was shifted to 35 °C, most suppressed species recovered quickly and the excess propionic acid was quickly consumed. Metagenomic analysis showed that STAD also promoted gene enrichment related to pathways metabolism, membrane functions, and methyl-based methanogenesis.

Alleviation of volatile fatty acids inhibition in anaerobic digestion of swine manure with nano-bubble water supplementation

Nano-bubble water (NBW) was applied to anaerobic digestion (AD) to alleviate volatile fatty acids (VFAs) inhibition, improve the buffering capacity and CH4 production in this work. Results indicated that NBW accelerated the consumption of VFAs and prevented inhibition due to VFAs accumulation. Additionally, NBW facilitated a rapid increase in partial alkalinity (PA) and total alkalinity (TA) as well as a corresponding rapid decrease in intermediate alkalinity (IA)/PA and VFA/TA, thereby improving buffering capacity and alleviating VFAs inhibition. Moreover, CH4 production improved by more than 12.2% by NBW. Similarly, the activities of the extracellular hydrolases and coenzyme F420 increased. Besides, NBW increased the abundance of microbial community and strengthened the metabolic pathways of hydrogenotrophic methanogens, which could be the intrinsic mechanism by which NBW alleviated VFAs inhibition, improved system stability, and increased CH4 production. This study demonstrates that NBW supplementation can be an effective method for mitigating frequent VFAs inhibition.

Enhanced anaerobic digestion of freezing and thawing pretreated cow manure with increasing solid content: kinetics and microbial community dynamics

High solid anaerobic digestion has proved the mainstream technology for the treatment of organic wastes. However, the high molecular weight and complex lignocellulosic structure of cow manure (CM) make it indigestible and inefficient, leading to limit the hydrolysis step of anaerobic digestion at high solid content. To mitigate this bottleneck, an improved cost-effective freezing and thawing pretreatment technique was proposed in this study. The freezing and thawing pretreatment of raw CM without any dilution was done for 20 days. The maximum cumulative methane yield (487 mL CH4 g- 1VS) was achieved at a total solid (TS) of 5% followed by TS of 10% and 15%, which was 13%, 20% and 21% higher than obtained from untreated CM, respectively. The kinetic results revealed that the biodegradable materials could be utilized at increasing TS with decreasing hydrolysis rate. The pretreatment significantly enhanced the methylotrophic methanogenic pathway during high solid anaerobic digestion, which was contrary to the general concept that the process is usually dominated by acetoclastic and hydrogenotrophic methanogens. This study is very important to understand the effect of solid content but also important to understand the effect of freezing and thawing pretreatment on process parameters and microbial community dynamics in high solid anaerobic digestion.

Nanobubble water promotes anaerobic digestion of high-solids cattle manure under mesophilic and thermophilic conditions

The gradual increase in cattle farming has led to a huge production of cattle manure (CM), but the conventional treatment methods are less efficient. In this study, the treatment method of anaerobic digestion (AD) of high-solids CM by combining nanobubble water (NBW) with different gases was proposed to present a new idea for the reduction, harmlessness, and resourcefulness of CM. It was found that the performance of the digester with added NBW was better than the control. Among them, the cumulative methane yield T-Air: 227.09 mL g-1 VSadded and T-CO2: 226.12 mL g-1 VSadded increased by 17.72 % and 17.22 %, respectively, compared with the control T: 192.90 mL g-1 VSadded under thermophilic conditions. Under mesophilic conditions, M-Air: 162.39 mL g-1 VSadded increased by 9.68 % compared with control M: 148.05 mL g-1 VSadded. Microbial communities analyzed at the genus level revealed that the relative abundance of bacteria favorable to hydrolysis and acid-producing processes, such as Defluviitalea, Haloplasma, and Bacillus, increased to varying degrees. Moreover, the relative abundance of archaea favorable for methanogenesis, such as Methanoculleus, Methanobrevibacter, and Methanosarcina, also increased to varying degrees. Therefore, the addition of NBW promoted the hydrolysis of high-solids CM, enhanced the stability of the reaction, improved the methanogenic performance, and increased the RA of favorable genera, which ultimately led to a better performance of the AD of high-solids CM.

Towards a circular bioeconomy to produce methane by co-digestion of coffee and brewery waste using a mixture of anaerobic granular sludge and cattle manure as inoculum

Coffee processing wastes, such as solid (pulp and husk) and wastewater, co-digested with industrial brewery wastewater, serve as excellent substrates for generating methane in the anaerobic digestion process. This study compared methane production using different compositions of cattle manure (CM) and granular sludge from an Upflow Anaerobic Sludge Blanket (UASB) reactor used in poultry wastewater treatment (GS). Four anaerobic batch reactors (500 mL) were assembled, A (50% CM and 50% GS), B (60% CM and 40% GS), C (70% CM and 30% of GS) and D (60% CM and 40% GS). Equal concentrations of substrates were added to all reactors: pulp and husk pretreated by hydrothermolysis (1 g L-1), coffee (10 g COD L-1) and brewery (1.5 g COD L-1) wastewaters. Assays A, B and C were supplemented with 2 g L-1 of yeast extract, except for assay D. The reactors were operated at 37 °C and pH 7.0. In assay B, the highest CH4 production of 759.15 ± 19.20 mL CH4 g-1 TS was observed, possibly favored by the synergistic interactions between cellulolytic bacteria Christensenellaceae_R-7_group and Methanosaeta archaea, as inferred by genes encoding enzymes related to acetoclastic methanogenesis (acetyl-CoA synthetase). Consequently, the electricity production potential of assay B (45614.08 kWh-1 year-1) could meet the energy demand of a farm producing coffee and beer, contributing to a positive energy balance concerning methane generation.

Synergistic effect and microbial community structure of waste-activated sludge and kitchen waste solids residue mesophilic anaerobic co-digestion

Anaerobic co-digestion was conducted on the solid residues after three-phase separation of kitchen waste (KWS) and waste-activated sludge (WAS), the synergistic effects and process performance were studied during co-digestion at different ratios of KWS to WAS. KWS and WAS mix ratios of 0:1, 1:4, 1:3, 1:2, 1:1, 2:1, 3:1, 4:1 and 1:0 (based on TS). The results showed that a ratio of KWS to WAS of 1:1 got a very high methane recovery with a methane yield of 310.45 ± 30.05 mL/g VSadded. The highest concentration of free ammonia among all reaction systems was only 70.23 ± 5.53 mg/L, which was not enough to produce ammonia inhibition in the anaerobic co-digestion system. However, when the KWS content exceeded 50%, methane inhibition and prolongation of the lag phase were observed due to the accumulation of volatile fatty acids (VFAs), and during the lag phase. Microbial community analysis showed that various bacterial groups involved in acid production and hydrolysis were mainly dominated by phylum Firmicutes, Chloroflexi, Proteobacteria and Bacteroidetes. Hydrogenotrophic methanogen was found to dominate all archaeal communities in the digesters. Co-digestion of KWS with WAS significantly increased the relative abundance of Methanobacterium compared with anaerobic digestion of WAS alone.

Enhanced hydrolysis and methane yield of temperature-phased dewatered sludge anaerobic digestion by microbial electrolysis cell

Temperature-phased anaerobic digestion (TPAD) and microbial electrolysis cell (MEC) are both able to improve hydrolysis and methane yield during anaerobic digestion (AD) of dewatered sludge. However, the effect of TPAD and MEC integration at different temperatures and different phases is unclear. This study investigated the effect of the integration of intermittent energization MEC in different phases of TPAD on the digestion of dewatered sludge. Thermophilic and MEC hydrolysis could release higher total ammonia nitrogen of 186.0% and 10.3% than control, mesophilic methanogenesis phase integrated with MEC relieved the ammonia inhibition and accelerated the acid utilization leading to the relief of acid accumulation. The ultimate methane yield of the TPAD integrated with MEC was increased by 118.9%, in which the relative abundance of Methanothermobacteria and Methanosarcina was increased. Therefore, intermittent energization MEC integrated TPAD synchronously improved the hydrolysis and methane yield.

Revolutionizing sanitation: Valorizing fecal slags through co-digesting food waste at high-solid content and dosing metallic nanomaterials for anaerobic digestion stability

To achieve the UN Sustainable Development Goals (SDGs) and the China Toilet Revolution on a global scale, it is crucial to implement a decentralized sanitation management system in developing countries. Fecal slags (FS) generated from septic tanks of toilets pose a challenge for remote villages. This study sought to resourcefully utilize FS through co-digesting with food waste (FW) under high-solid anaerobic co-digestion (HSAD). Besides, two metallic nanomaterials, nano-zerovalent iron (nZVI) and magnetite (Fe3O4), were employed to demonstrate the practical improvement of HSAD. The results showed that nZVI-dosed digesters produced the highest cumulative methane of 295.72 mL/gVS, 371.36 mL/gVS, 360.53 mL/gVS and 296.64 mL/gVS in 10%, 15%, 20% and 25% TS content, respectively, which was 1.15, 1.22, 1.16, 1.12 times higher than Fe3O4 dosed digesters. This increment could be ascribed to the simultaneous production of H2 from Fe2+ release from nZVI and the enrichment of homoacetogen. Changes in carbon degradation and methanogenic pathways, which facilitated stability under high TS contents, were observed. At low solid digestion (10% TS), Syntrophomonas cooperated with Methanosarcina and Methanobacterium to metabolize butyrate and propionate. However, due to the buildup of total ammonia nitrogen and volatile fatty acids, acetoclastic methanogens were inhibited in the high-solid digesters (15%, 20% and 25% TS). Consequently, a more resilient and highly tolerant Syntrophaceticus, alongside hydrogenotrophic methanogens such as Methanoculleus and Methanobrevibacter, maintained stability in the harsh environment.

A comprehensive study on anaerobic digestion of organic solid waste: A review on configurations, operating parameters, techno-economic analysis and current trends

The excessive discharge and accumulation of solid organic waste into the environment is of severe concern across the globe. Thus, an efficient waste management system is important to mitigate health risks to humans, minimize harmful impacts on the environment, and ensure a sustainable ecosystem. The organic waste is converted into value-added products either using microorganisms or heat energy; these methods are commonly known as biochemical and thermochemical techniques. The biochemical process has the advantage of higher selectivity of the products and lower processing temperatures. The principal conversion processes of this category are fermentation and anaerobic digestion (AD). This review article focuses on AD, a potential method for treating organic waste and creating a variety of products with added value. Here we present the digestibility of various organic wastes, the role of microorganisms, the decomposition process, co-substrates, digester designs, biogas yields, by-products, environmental impacts, and overall techno-economical effectiveness of the process. Further, this review offers insights into new directions for AD for waste treatment and future research without compromising the overall feasibility and environmental sustainability.

Microbiome assembly and stability during start-up of a full-scale, two-phase anaerobic digester fed cow manure and mixed organic feedstocks

Carbon transformations during anaerobic digestion are mediated by complex microbiomes, but their assembly is poorly understood, especially in full-scale digesters. Gene-centric metagenomics combining functional and taxonomic classification was performed for an on-farm digester during start-up. Cow manure and organic waste pre-treated in a hydrolysis tank were fed to the methane-producing digester and the volatile solids loading rate was slowly increased from 0 to 3.5 kg volatile solids m-3 d-1 over one year. The microbial community in the anaerobic digester exhibited a high ratio of archaea, which were dominated by hydrogenotrophic methanogens. Bacteria in the anaerobic digester had a high abundance of genes for ferredoxin cycling, H2 generation, and more metabolically complex fermentations than in the hydrolysis tank. In total, the results show that a functionally stable microbiome was achieved quickly during start-up and that the microbiome created in the low-pH hydrolysis tank did not persist in the downstream anaerobic digester.

Wood biochar enhances methanogenesis in the anaerobic digestion of chicken manure under ammonia inhibition conditions

The process of breaking down chicken manure through anaerobic digestion is an effective waste management technology. However, chicken manure can be a challenging feedstock, causing ammonia stress and digester instability. This study examined the impacts of adding wood biochar and acid-alkali-treated wood biochar to anaerobically digest chicken manure under conditions of ammonia inhibition. The results highlighted that only the addition of 5 % acid-alkali-treated wood biochar by volume can achieve cumulative methane production close to the typical methane potential range of chicken manure. The treated wood biochar also exhibited highest total ammonia nitrogen removal compared to the Control treatment. Scanning Electron Microscope revealed growing interactions between biochar and methanogens over time. Real-time polymerase chain reaction showed that treated wood biochar produced the highest number of bacterial biomass. In addition, 16S amplicon-based sequencing identified a more robust archaeal community from treated biochar addition. Overall, the acid-alkali treatment of biochar represents an effective method of modifying biochar to improve its performance in anaerobic digestion.

Pilot-scale investigation of conductive carbon cloth amendment for enhancing high-solids anaerobic digestion and mitigating antibiotic resistance

This study examined the effectiveness of introducing conductive carbon cloth into a pilot-scale high-solids anaerobic digestion (HSAD) system. Adding carbon cloth increased methane production by 22 % and improved the maximum methane production rate by 39 %. Microbial community characterization indicated a possible direct interspecies electron transfer-based syntrophic association among microbes. Using carbon cloth also enhanced microbial richness, diversity, and evenness. Carbon cloth effectively reduced the total abundance of antibiotic resistance genes (ARGs) by 44.6 %, mainly by inhibiting horizontal gene transfer, as shown by the significant decrease in the relative abundance of integron genes (particularly intl1). The multivariate analysis further demonstrated strong correlations of intl1 with most of the targeted ARGs. These findings suggest that carbon cloth amendment can promote efficient methane production and attenuate the spread of ARGs in HSAD systems.

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.

Influence of Molasses Residue on Treatment of Cow Manure in an Anaerobic Filter with Perforated Weed Membrane and a Conventional Reactor: Variations of Organic Loading and a Machine Learning Application

This study highlighted the influence of molasses residue (MR) on the anaerobic treatment of cow manure (CM) at various organic loading and mixing ratios of these two substrates. Further investigation was conducted on a model-fitting comparison between a kinetic study and an artificial neural network (ANN) using biomethane potential (BMP) test data. A continuous stirred tank reactor (CSTR) and an anaerobic filter with a perforated membrane (AF) were fed with similar substrate at the organic loading rates of (OLR) 1 to OLR 7 g/L/day. Following the inhibition signs at OLR 7 (50:50 mixing ratio), 30:70 and 70:30 ratios were applied. Both the CSTR and the AF with the co-digestion substrate (CM + MR) successfully enhanced the performance, where the CSTR resulted in higher biogas production (29 L/d), SMP (1.24 LCH4/gVSadded), and VS removal (>80%) at the optimum OLR 5 g/L/day. Likewise, the AF showed an increment of 69% for biogas production at OLR 4 g/L/day. The modified Gompertz (MG), logistic (LG), and first order (FO) were the applied kinetic models. Meanwhile, two sets of ANN models were developed, using feedforward back propagation. The FO model provided the best fit with Root Mean Square Error (RMSE) (57.204) and correlation coefficient (R2) 0.94035. Moreover, implementing the ANN algorithms resulted in 0.164 and 0.97164 for RMSE and R2, respectively. This reveals that the ANN model exhibited higher predictive accuracy, and was proven as a more robust system to control the performance and to function as a precursor in commercial applications as compared to the kinetic models. The highest projection electrical energy produced from the on-farm scale (OFS) for the AF and the CSTR was 101 kWh and 425 kWh, respectively. This investigation indicates the high potential of MR as the most suitable co-substrate in CM treatment for the enhancement of energy production and the betterment of waste management in a large-scale application.

The synergistic effect of chemical oxidation and microbial activity on improving volatile fatty acids (VFAs) production during the animal wastewater anaerobic digestion process treated with persulfate/biochar

Improving volatile fatty acid (VFA) production, rather than producing methane from the anaerobic digestion (AD) of waste, has become a new strategy of resource utilization. In regard to animal wastewater, the effectiveness of persulfate/biochar (potassium peroxodisulfate, PDS/BC) on the hydrolysis and acidogenesis stages and the reaction mechanisms are still unclear. In this study, the AD process on cow wastewater was controlled at the hydrolysis and acidification stages by setting the hydraulic retention time (HRT) at 25 days. The results showed that the contents of total solids (TS) and volatile solids (VS) were further reduced by PDS/BC treatment with 0.15 gPDS/gTS of PDS added. The VFAs production increased by 12.4 % from day 0 to 25 compared to the blank set. Based on our molecular analysis, the rate of increase for the dissolved organic matter with low molecular weight (0-10 kDa) was 699.5 mg/(L·d) in the first 10 days. The change rate increased nearly 2.1 times, leading to higher VFAs yield. Moreover, the activities of fermentative bacteria were enhanced and Anaerocella was determined to be the specific and critical genus. However, excessive PDS (0.3 gPDS/gTS) prolonged the acidification period and caused the inactivation of fermentative bacteria. Structural equation modeling demonstrated that PDS can directly affect VFAs yield and also had an indirect effect by influencing the decomposition of particulate matter and microbial activities. Therefore, the enhancement of VFAs production using the PDS/BC method could be due to synergistic chemical and microbial effects. Findings from this study can provide a practical strategy to enhance the VFAs production of AD technology for livestock wastewater and help reveal the reaction mechanism of PDS/BC treatment.

Effect of water regime on the dynamics of free ammonia during high solid anaerobic digestion of pig manure

Free ammonia (FAN) inhibition is commonly encountered during high solid anaerobic digestion (HSAD) of pig manure. The performance of HSAD is highly related to its operational water regime; however, little information is available regarding the dynamics of free ammonia with varied water regimes. In this work, four treatments were set with equal amount of water supply but varied addition frequencies, i.e. add once but at different times in treatments T1 and T2, add twice in T3 while it was three times in treatment T4. Results showed that the whole methanogenic process ran smoothly with the average methane gas production rate maintaining at 191.1 LCH₄/kgVS_added. Although a higher methane gas production rate of 217.4 LCH₄/kgVS_added was achieved in T1, one time water addition triggered a higher ammonia inhibition potential. Cumulative FAN release was 6.03 mgFAN/kgVS_added in T1 while the balance between FAN and ammonia tended to the fraction of FAN. In T4, cumulative FAN of 5.07 mgFAN/kgVS_added was evolved, which was lower than that in T1 but similar to the situation in T2. The lowest FAN was observed in T3, indicating that a moderate frequency of dilution might be conducive to alleviate free ammonia inhibition.

Carbon cloth amendment for boosting high-solids anaerobic digestion with percolate recirculation: Spatial patterns of microbial communities

The addition of conductive materials in anaerobic digestion (AD) is a promising method for boosting biomethane recovery from organic waste. However, conductive additives have rarely been investigated for the high-solids anaerobic digestion (HSAD). Here, the impact of adding carbon cloth in the solid phase of an HSAD system with percolate recirculation was investigated. Furthermore, spatial patterns of microbial communities in suspended biomass, percolate, and carbon cloth attached biofilm were assessed. Carbon cloth increased biomethane yield from source-separated organics (SSO) by 20% more than the unamended control by shortening the lag phase (by 15%) and marginally improving the methanogenesis rate constant (by ∼8%) under a batch operation for 50 days. Microbial community analysis demonstrated higher relative abundances of the archaeal population in the carbon cloth amended reactor than in unamended control (12%-21% vs. 5%-15%). Compared to percolate and suspension, carbon cloth attached microbial community showed higher enrichment of known electroactive Pseudomonas species along with Methanosarcina and Methanobacterium species, indicating the possibility of DIET-based syntrophy among these species.

Anaerobic co-digestion of Chinese cabbage waste and cow manure at mesophilic and thermophilic temperatures: Digestion performance, microbial community, and biogas slurry fertility

The objective of this study was to investigate the effects of mixing ratios of Chinese cabbage waste (CCW) and cow manure (CM) on methane yields, microbial community, and biogas slurry fertility during anaerobic co-digestion. Batch experiments were conducted at mesophilic and thermophilic temperatures with five different CCW/CM mixing ratios. Methane yields at mesophilic and thermophilic temperatures were 4.2-184.4 mL g^-1 Volatile solids_added (mL g^-1 VS_added) and 11.8-321.7 mL g^-1 VS_added, respectively. The richness and diversity of bacteria and archaea at mesophilic temperatures were higher than those at thermophilic temperatures. Compared with the unfertilized control, the dry weight of corn seedlings with the follow-up application of mesophilic or thermophilic biogas slurry increased by 12.3 %-73.4 % or 16.8 %-43.3 %, respectively. This study demonstrates that thermophilic temperatures are conducive to increasing methane yields, but mesophilic temperatures are conducive to improving the biogas slurry fertility.

Spatial distributions of granular activated carbon in up-flow anaerobic sludge blanket reactors influence methane production treating low and high solid-content wastewater

The impacts of granular activated carbon (GAC) spatial distributions in up-flow anaerobic sludge blanket (UASB) reactors treating different solid-content wastewater were evaluated in the present study. When treating high solid-content wastewater, the highest methane yield was observed for UASB supplemented with self-floating GAC (74.2 ± 3.7 %), which was followed by settled + self-floating GAC reactor (65.1 ± 3.8 %), then settled GAC reactor (58.3 ± 1.4 %). When treating low solid-content wastewater, all UASBs achieved improved methane yield, and settled + self-floating GAC reactor achieved the highest methane yield (83.4 ± 3.3 %). Self-floating GAC amended reactor showed the best performance for treating high solid-content wastewater, while settled + self-floating GAC amended reactor was optimal for treating medium and low solid-content wastewater. The spatial distributions of microbial communities differed in the reactors with settled GAC and floating GAC. This study underlines the importance of considering feedwater characteristics when adopting GAC-based UASB processes.

Recycling different textile wastes for methane production: Morphological and microstructural changes and microbial community dynamics

The dramatic increase of textile wastes has become a major global concern, which calls for alternative practices to alleviate severe environmental pollution and waste of resources due to their improper disposal and management. Anaerobic digestion (AD) is a cost-effective and eco-friendly technology that allows the bioconversion of organic wastes into clean energy (methane), which might be potentially useful for recycling textile wastes. In this study, AD was applied to 11 commonly available textile wastes in daily life to explore their feasibility, along with the methane production efficiency, biodegradability (B_D), degradation mechanism, and microbial community dynamics during AD. The results showed that all textile wastes presented an obvious decomposition from an integrated shape to fragmented pieces within 18 days except blue denim. The highest experimental methane production (EMP) of 356.0 mL/g volatile solids (VS) and B_D of 78.0 % were obtained with flax. The degradation mechanism could be concluded that predominant bacteria, especially Clostridium sensu stricto, first attached to the surface of textile waste and converted its main compositions cellulose and hemicellulose into acetate as the core intermediate. Then, acetate was utilized by the major methanogen, Methanothrix, through the acetoclastic methanogenesis pathway to produce methane. This study not only enriches the understanding of textile wastes degradation mechanisms during AD and provides very useful data on methane production from commonly available textile wastes but also proposes a promising method for efficiently recycling and utilizing the diverse range of textile wastes to reduce waste pollution and generate clean energy simultaneously.

Responses of Fermentation Characteristics and Microbial Communities to Vitamin B12 Supplementation in In Vitro Ruminal Cultures

Vitamin B12, an important cofactor involved in propionate formation, is synthesized exclusively by bacteria and archaebacteria. Humans need to intake vitamin B12 through food, and dairy products are generally the best source of vitamin B12. In the present study, the effects of vitamin B12 supplementation in diets on in vitro ruminal fermentation characteristics and microbial communities were investigated to provide a reference for increasing the vitamin B12 content in milk by dietary supplementation. A completely randomized design was carried out using the in vitro rumen culture technique, and 5 vitamin B12 dose levels (0, 0.5, 1.0, 2.0, and 4 mg/g of dry matter) were used. The results showed that vitamin B12 supplementation in diets decreased acetate: propionate ratio and butyrate concentration. The change in the acetate: propionate ratio can be attributed to the increased relative abundances of the Proteobacteria phylum and the Negativicutes class, both of which are involved in propionate metabolism. The decrease in butyrate concentration can likely be attributed to a reduction in relative abundance of species belonging to the Clostridia class, which are known as the predominant butyrate producers in the mammalian intestine. In addition, vitamin B12 supplementation in diets reduced the CH4 production by altering the species composition of the archaeal community. In conclusion, dietary supplementation of vitamin B12 resulted in rumen perturbation. In vivo studies should be conducted cautiously when evaluating the effects of vitamin B12 supplementation on the synthesis and absorption of it, as well as its content in milk.

Potential treatment of aged cow manure using spare capacity in anaerobic digesters treating a mixture of food waste and pig manure

With the increasing production of cow manure (CM) and the continuing decrease in the demand for manure compost, CM management has become an urgent and challenging task in Korea. In most cattle farms in Korea, CM mixed with bedding materials is left in pens exposed to the open air for several months before treatment, which makes CM an unsuitable feedstock for anaerobic digestion. This study examined the co-digestion of aged CM with a mixture of food waste and pig manure as the base substrate to assess the possibility of treating and valorizing CM using spare capacity in existing anaerobic digesters dealing with other wastes. The duplicate digesters initially fed with the base substrate were subjected to the addition of increasing amounts of CM (3-10% in the feed, w/v) over nine months. Co-feeding CM up to 5% in the feed (w/v) did not compromise the methanogenic degradation of the substrates, but adding more CM led to a significant performance deterioration likely related to the buildup of inhibitory free ammonia and H₂S. Adding CM substantially influenced the digester microbial communities, especially methanogenic communities, and induced a dominance shift from aceticlastic Methanothrix to hydrogenotrophic methanogens as the CM fraction increased. The overall results suggest that the CM fraction should not exceed 5% in the feed (w/v) for its stable treatment with the base substrate in the experimental digesters. Although further studies are needed, anaerobic treatment using spare capacity in existing digesters can be a useful strategy for the management of aged CM.

Novel insights from lignocellulosic waste to biogas through regulated dry-wet combined anaerobic digestion: Focusing on mining key microbes

Dry-wet combined anaerobic digestion is a novel approach for treating lignocellulosic waste by increasing the organic load of reactor while accelerating the conversion of organic acids. Here, we investigated the effect of regulated substrate ratios and initial pH in the dry acidogenesis stage on the bioconversion efficiency of dry-wet combined anaerobic digestion. Our data revealed microbial interactions and further identified key microbes based on microbial co-occurrence network analysis. On day three of acidification, the kinetic hydrolysis rate and acidification yield reached 1.66 and 60.07%, respectively; this was attributed to enhancement of the synergistic effect between Clostridiales and Methanosaeta, which increased the proportion of corn straw in the substrate or lowered the initial spray slurry pH to 5.5-6.5. With increased acidification capacity, acetoclastic methanogens were enriched in the wet methanogenesis stage; the syntrophic effect of Syntrophomonadales, Syntrophobacterales and Methanospirillum, meanwhile, was enhanced, leading to an overall improvement in biogas production.

Biochar amendment to advance contaminant removal in anaerobic digestion of organic solid wastes: A review

Anaerobic digestion (AD) has been widely applied to convert organic solid wastes into biogas, a renewable energy, and digestate, a bio-fertilizer, to sustain waste management. Nevertheless, several vexing contaminants in OSWs restrict digestate application in agriculture. Biochar has been evidenced to effectively improve AD by promoting organic biodegradation and alleviating the accumulation of inhibitory substances (e.g. ammonia and volatile fatty acids). Furthermore, biochar could advance contaminant removal in AD given its highly porous, conductive and alkaline features. Thus, this review aims to highlight the role of biochar amendment to advance contaminant removal in AD of OSWs. Key contaminants, such as antibiotics, heavy metals, microplastics, polycyclic aromatic hydrocarbons, furfural and 5-hydroxy methyl furfural (5-HMF) that ubiquitously present in OSWs were demonstrated. The underlying mechanisms of biochar to amend the removal of these contaminants by AD were discussed. Furthermore, future perspectives to the development of biochar-assisted AD for OSWs treatment were provided.

The relationship between the molecular composition of dissolved organic matter and bioavailability of digestate during anaerobic digestion process: Characteristics, transformation and the key molecular interval

In this study, swine wastewater (SW) and cow wastewater (CW) were used for anaerobic digestion (AD). We found the bioavailability of dissolved organic matter (DOM) was affected by the molecular weight ranges and molecular composition during the AD process. The organic substance in the small molecular range (0-5 kDa) accumulated due to a larger molecular fraction (>10 kDa) degradation, which enhanced the bioavailability of the DOM. Moreover, based on the excitation emission matrix-parallel factor (EEM-PARAFAC) analysis, the protein-like component in 0-5 kDa molecular size and humic-like component over 5 kDa are significantly positively correlated with DOM bioavailability. This study indicated that increasing the hydrolysis of larger organic matter and humification degree of molecular weights>5 kDa are critical solutions to improving the bioavailability of DOM. These conclusions can help explain the molecular mechanisms of DOM transformation and the AD process of livestock wastewater.

High-solids anaerobic digestion of organic fraction of municipal solid waste: Effects of feedstock to inoculum ratio and percolate recirculation time

The effects of feedstock to inoculum (F:I) ratio and percolate recirculation time (PRT) were studied for the high-solids anaerobic digestion (HSAD) of the organic fraction of municipal solid waste (OFMSW). Six mesophilic HSAD systems were operated at different F:I ratios (1 to 3 kg VS/kg VS; PRT = 2.5 h/d) and PRTs (1.5 to 3.5 h/d; F:I = 2 kg VS/kg VS). The F:I ratio of 1 provided up to 86% of the theoretical methane potential of OFMSW. In contrast, F:I ratio of 3 provided only 34% methane recovery due to volatile fatty acids (VFAs) accumulation and pH drop. Despite F:I ratio of 2 could provide 70% methane recovery, it could enable almost 45% higher organics processing capacity (VS basis) and lower solids washout during percolate recirculation, as compared to the F:I ratio of 1. However, different examined PRTs showed marginal impacts on methane yields with comparable changes in profiles of percolate characteristics.