Thermophilic Anaerobic Digestion Research Articles

Thermophilic and mesophilic anaerobic digestion of soybean molasses: A performance vs. stability trade-off

One of the factors that has a direct impact on anaerobic digestion is the applied organic loading rate (OLRA). Increasing OLRA can boost methane production but can also cause process failure. As a result, establishing the appropriate OLRA for the procedure is critical. This study evaluated the effect of increasing the OLRA using soybean molasses in a thermophilic anaerobic reactor (R-Thermo), as well as the effect of feeding strategy and co-processing with okara. Furthermore, the performance versus stability trade-off between R-Thermo and mesophilic anaerobic digestion (R-Meso) was investigated. The increase of OLRA from 10 to 15 and 20 kg-COD/m³/d led to a decrease in COD removal efficiency (90, 86, and 75%), methane yield (12.0, 11.6, and 9.9 mol-CH4/kg-COD) and an increase in total volatile acids concentration (251, 456, and 1393 mg-HAc/L, respectively). At 15 kg-COD/m³/d, R-Meso performed similarly to R-Thermo, and at 20 kg-COD/m3/d, R-Meso outperformed (81% COD removal efficiency, 9.3 mol-CH4/kg-CODrem and 154.5 mol-CH4/m3/d). Temperature greatly influenced the distribution of metabolic pathways, as shown by thermodynamic and kinetic analyses, thus impacting bacterial diversity. At 55 °C, amongst the bacterial genera, Tepidiphilus stood out (>28.2%), followed by Acetomicrobium, Coprothermobacter and Candidatus_Caldatribacterium. The OLRA clearly impacted the archaeal community; Methanothermobacter (77.4%) was favored over Methanosarcina (14.8%). Under thermophilic temperature, it seems that syntrophic acetate oxidation (SAO) bacteria might have competed for substrate with acetoclastic methanogens, while in R-Meso microorganisms responsible for the initial steps of organic matter breakdown, such as members of the Firmicutes and Proteobacteria phyla (at least 67%), were dominant. In summary, R-Meso, characterized by a more uniform distribution of metabolic pathways, as well as a diverse and well-adapted microbial consortium, have exhibited enhanced stability and outperformed R-Thermo at high-loads.

Anaerobic Degradation of Aromatic and Aliphatic Biodegradable Plastics: Potential Mechanisms and Pathways.

Biodegradable plastics (BDPs) have been widely used as substitutes for traditional plastics, and their environmental fate is a subject of intense research interest. Compared with the aerobic degradation of BDPs, their biodegradability under anaerobic conditions in environmental engineering systems remains poorly understood. This study aimed to investigate the degradability of BDPs composed of poly(butylene adipate-co-terephthalate) (PBAT), poly(lactide acid) (PLA), and their blends, and explore the mechanism underlying their microbial degradation under conditions of anaerobic digestion (AD). The BDPs readily depolymerized under thermophilic conditions but were hydrolyzed at a slow rate under conditions of mesophilic AD. After 45 days of thermophilic AD, a decrease in the molecular weight and significant increase in the production of methane and carbon dioxide production were observed. Network and metagenomics analyses identified AD as reservoirs of plastic-degrading bacteria that produce multiple plastic-degrading enzymes. PETase was identified as the most abundant plastic-degrading enzyme. A potential pathway for the anaerobic biodegradation of BDPs was proposed herein. The polymers of high molecular weight were subjected to abiotic hydrolysis to form oligomers and monomers, enabling subsequent microbial hydrolysis and acetogenesis. Ultimately, complete degradation was achieved predominantly via the pathway involved in the conversion of acetic acid to methane. These findings provide novel insight into the mechanism underlying the anaerobic degradation of BDPs and the microbial resources crucial for the efficient degradation of BDPs.

Unveiling the inactivation mechanisms of different viruses in sludge anaerobic digestion based on factors identification and damage analysis

Despite anaerobic digestion having potential for pathogen reduction in sewage sludge, the behaviors of viruses as the primary health concern are rarely studied. This study investigated the inactivation kinetics and mechanisms of four typical virus surrogates with different structures in mesophilic (MAD) and thermophilic (TAD) anaerobic digestion of sludge. Virus inactivation in MAD was virus-type-dependent correspondingly to different function loss. Temperature drove the faster inactivation proceeding for enveloped Phi6, while temperature and ammonia were the critical inactivation factors for nonenveloped MS2, causing genome degradation and protein functional damage. Interaction with sludge solids played critical role in DNA viruses T4 and Phix174 inactivation via inducing host binding function damage. By comparison, TAD enhanced viral protein denaturation, bringing efficient inactivation with reducing heterogeneity among nonenveloped viruses. These insights into unique virus behaviors in anaerobic digestion systems can provide guidance for developing more effective disinfection protocols and improving sludge biosafety.

Optimization of two-stage thermophilic anaerobic digestion of dairy wastewater: Effect of carrier material on process performance and microbial community

Although two-stage anaerobic digestion (TSAD) is not a novel process, information of the process stability, microbial community and effective operating conditions is limited and often contradictory. In this work, the influence of different carrier materials (polyurethane foam, carbon felt, Raschig rings and a combination of carbon felt and Raschig rings) on the performance of the thermophilic TSAD of dairy wastewater was studied. The organic loading rate (OLR) in the acidogenic reactor (RH) was gradually increased from 13.74 to 32.56 g COD/(L d), and from 0.64 to 11.46 g COD/(L d) in the methanogenic reactors by correspondingly reducing the hydraulic retention time (HRT). The highest hydrogen production rate of 1280.3 mL/(L·d) and hydrogen yield of 93.2 mL/g COD were achieved at OLR of 13.74 g COD/(L·d), but hydrogen production stopped at higher OLR. The main soluble metabolites in RH were butyrate and lactate, and the microbial community was dominated by Streptococcus, Thermoanaerobacterium, Veillonellales-Selenomonadales and Pseudomonas. The highest methane production rate (2674 mL/(L·d) at HRT of 0.5 days) was observed in the reactor with polyurethane foam, while the highest methane yield (305.5 mL/g COD at HRT of 1.5 days) was obtained in a reactor containing carbon felt. Spirochaetaceae, Desulfomicrobium, Anaerolineaceae, Candidatus Caldatribacterium and Cloacimonadaceae W5 were linked to these materials and explained the highest methanogenic performance.

Pilot-scale study of enhanced thermophilic anaerobic digestion of food waste with the addition of trace elements

Thermophilic anaerobic digestion (AD) offers many benefits for food waste treatment but is seldom adopted in industrial plants due to instability issue, particularly under higher loading conditions. This study thus conducted a 160-day continuous operation of a pilot-scale thermophilic AD system on-site. Results from the experiments showed that the system could operate under relatively lower loading but failed when the loading reached up to 5.69 kg·COD/(m3·d). Volatile fatty acids increased to 6000 mg/L at the corresponding hydraulic retention time of 15 days. Trace elements were then introduced, which restored higher process stability by reducing volatile fatty acids to 400 mg/L. The mass balance and materials decomposition resutls revealed the system’s strong resilience. Methanoculleus (92.52 %) and Methanomassiliicoccus (6.55 %) were the dominant methanogens, a phenomenon rarely observed in similar thermophilic systems. This system may tolerate more stressful conditions, as the loading limits had not been reached with the addition of trace elements.

Fate and effect of Polyamide-6 microplastics in mesophilic and thermophilic anaerobic digestion

Research has demonstrated that depending on the type and concentration, microplastics affect anaerobic digestion (AD). Owing to the high abundance of polyamide-6 (PA6) in wastewater treatment plants and limited understanding of its behavior, this study investigates PA6 microplastics' effect in AD. Biochemical methane potential experiments were performed under mesophilic (35 °C) and thermophilic (55 °C) conditions using PA6 at concentrations from 0 to 200 particles/g total solids (TS). Under both conditions, methane production increased in the highest (200 particles/g TS) PA6-dosed reactors, with thermophilic conditions having a statistically significant effect. Methane yield increased from 403.1 ± 5.3 mL/g VS to 436.6 ± 9.2 mL/g VS under thermophilic and from 332.1 ± 1.5 to 340.6 ± 6.6 mL/g VS under mesophilic conditions for the 200 particles/g TS dose, corresponding to increases of 8.3% and 2.6% respectively. PA6 crystallinity decreased from 32.8% to 27.1% and 26.8%, corresponding to decreases of 17.4% in mesophilic and 18.2% in thermophilic reactors compared to pristine control. Similarly, crystallinity decreased in PA6 microplastics collected from abiotic reactors, with thermophilic conditions showing a greater effect. The carbonyl index (CI) values were similar between biotic and abiotic reactors, but PA6 from all reactors had significantly higher CI than pristine PA6, suggesting abiotic factors also affect carbonyl bonds. Additionally, an increase in average PA6 mass was observed for mesophilic and thermophilic conditions by 22.0 % and 23.0 %, respectively. The study shows that temperature and other abiotic factors, like sludge chemistry, significantly influence the fate and effect of PA6 microplastics in digesters. Including abiotic reactors seems crucial for a full understanding of the impact of microbial and non-microbial factors in microplastic studies in the AD process. Studying the effects of microplastics on AD is only one part of the picture, whereas simultaneously examining their fate in digestion is necessary for a complete understanding.

Investigation of start-up strategies and temperature regulation on enhanced methanogenesis in anaerobic digestion of food waste

Temperature is a crucial parameter for anaerobic digestion of food waste. Thermophilic anaerobic digestion (TAD) exhibits high methane production but has a hard initiation. This study compared two start-up strategies, one-step temperature increase and step-wise temperature increase, for achieving rapid TAD initiation. Compared to the step-wise temperature increase, the one-step temperature increase demonstrated stronger stability with slight pH fluctuation. Thermophilic communities were established more rapidly by the one-step temperature increase. Its substrate degradation was more enhanced through improving carbohydrates and lipid metabolisms than step-wise’s. These indicated one-step temperature increase strategy can initiate TAD efficiently. Additionally, a comparative analysis between TAD and mesophilic anaerobic digestion (MAD) was conducted on multilevels. TAD produced 25.29% more methane than MAD. The increased Defluviitoga and Hydrogenispora in TAD promoted hydrogenotrophic methanogenesis. The enriched genes related to the decomposition of glucose and glycerol resulted in higher substrate hydrolysis, acidogenesis, and acetogenesis in TAD than that in MAD. This study elucidated the full stages of TAD, which strengthened methanogenesis.

Comparison of mesophilic and thermophilic anaerobic digestion of food waste: Focusing on methanogenic performance and pathogens removal

Food waste (FW) is a valuable and sustainable resource for anaerobic digestion (AD). However, mesophilic AD, commonly used, poses risks associated with pathogenic microorganisms, potentially limiting the safe utilization of digestion by-products. This study aimed to evaluate and compare methane production efficiencies and pathogen reduction capabilities between mesophilic and thermophilic AD processes. Methane production was assessed in batch and continuous operations under thermophilic conditions, resulting in methane yields of 380 and 360 mL/g VS, representing increases of 5.56 % and 22.44 % respectively compared to mesophilic conditions. Pathogen removal efficiency analysis showed that thermophilic AD achieved Escherichia coli elimination rates between 98.9 % and 99.9 %, with higher removal rates for Salmonella ranging from 77.9 % to 99.8 % compared to mesophilic processing. Overall, results from batch and continuous experiments demonstrate that thermophilic AD not only enhances methane production but also significantly improves pathogens reduction, offering a more effective approach for managing FW in AD systems. The findings of this study enhance the scientific understanding of anaerobic digestion technology in food waste treatment applications and provide crucial technical support and decision-making guidance for environmental management and renewable energy production.

Genome-centric metagenomics and methanogenic pathway analysis for acclimated anaerobic digestion of chicken manure with high ammonia stressed under thermophilic condition

Thermophilic anaerobic digestion (AD) of animal manure offers various environmental benefits but the process requires a microbial community acclimatized to high ammonia. In current study, a lab-scale continuous stirred tank reactor (CSTR) fed with chicken manure was operated under thermophilic condition for 450 days in total. Results showed that the volumetric methane production decreased from 445 to 328 and sharply declined to 153 mL L−1·d−1 with feeding total solid (TS) step increased from 5% to 7.5% and 10%, respectively. While, after a long-term stop feeding for 80 days, highly disturbed reactor was able to recover methane generation to 739 mL L−1·d−1 at feeding TS of 10%. Isotope analysis indicted acetate converted to methane through the syntrophic acetate oxidation and hydrogenotrophic methanogenesis (SAO-HM) pathway increased from 33% to 63% as the concentration of ammonium increased from 2493 to 6258 mg L−1. Significant different in the genome expression of the SAO bacterial from 0.09% to 1.23%, combining with main hydrogenotrophic partners (Methanoculleus spp. and Methanothermobacter spp.) contented of 2.1% and 99.9% during inhibitory and recovery stages, respectively. The highly expressed KEGG pathway in level 3 (enzyme genes) for the Recovery sludge combining with the extraordinary high abundance of genera Halocella sp. suggested that Halocella sp. might be a highly efficient hydrolytic and acidogenic microorganism and enhance the process of SAO during carbon metabolic flow to methane. This report will be a basis for further study of AD studies on high nitrogen content of poultry manure.

Study on the fate of per- and polyfluoroalkyl substances during thermophilic anaerobic digestion of sewage sludge and the role of granular activated carbon addition

Limited information is available on the removal of per- and polyfluoroalkyl substances (PFAS) in anaerobic digestion (AD). Τhe fate of six PFAS was studied in thermophilic bioreactors in the presence of granular activated carbon (GAC) and voltage application. Reactors with GAC exhibited lower concentrations of volatile fatty acids and higher methane production compared to those with and without the application of voltage. Analysis of PFAS in dissolved and solid phase showed that their distribution was dependent on perfluorocarbon chain length and functional group. Mass balances showed that PFAS were not removed during conventional AD or after applying voltage; however, significant removal (up to 61 ± 8 %) was observed in bioreactors with GAC for perfluoroheptanoic acid (PFHpA), perfluorooctanoic acid (PFOA), perfluorononanoic acid (PFNA), and perfluorooctane sulfonate (PFOS). Biomass characterization showed that in these bioreactors, the relative abundance of Acinetobacter and Pseudomonas was higher, indicating their potential role in PFAS biotransformation.

Enhanced thermophilic high-solids anaerobic digestion of organic fraction of municipal solid waste with spatial separation from conductive materials in a single reactor volume

Despite benefits such as lower water and working volume requirements, thermophilic high solids anaerobic digestion (THSAD) often fails due to the rapid build-up of volatile fatty acids (VFAs) and the associated drop in pH. Use of conductive materials (CM) can promote THSAD through stimulation of direct interspecies electron transfer (DIET), while the need for their constant dosing due to poor separation from effluent impairs economic feasibility. This study used an approach of spatially separating magnetite and granular activated carbon (GAC) from the organic fraction of municipal solid waste (OFMSW) in a single reactor for THSAD. GAC and magnetite addition could both mitigate the severe inhibition of methanogenesis after VFAs build-up to ∼28–30 g/L, while negligible methane production was observed in the control group. The highest methane yield (286 mL CH4/g volatile solids (VS)) was achieved in magnetite-added reactors, while the highest maximum CH4 production rates (26.38 mL CH4/g VS/d) and lowest lag-phase (2.83 days) were obtained in GAC-added reactors. The enrichment of GAC and magnetite biofilms with various syntrophic and potentially electroactive microbial groups (Ruminiclostridium 1, Clostridia MBA03, Defluviitoga, Lentimicrobiaceae) in different relative abundances indicates the existence of specific preferences of these groups for the nature of CM. According to predicted basic metabolic functions, CM can enhance cellular processes and signals, lipid transport and metabolism, and methane metabolism, resulting in improved methane production. Rearrangement of metabolic pathways, formation of pili-like structures, enrichment of biofilms with electroactive groups and a significant improvement in THSAD performance was attributed to the enhancement of the DIET pathway. Promising results obtained in this work due to the spatial separation of the bulk OFMSW and CM can be useful for modeling larger-scale THSAD systems with better recovery of CM and cost-effectiveness.

Enhancing the efficiency of high solid anaerobic digestion of empty fruit bunches under thermophilic conditions by particle size reduction and co-digestion with palm oil mill effluent

This study investigates the impact of thermophilic high solid anaerobic digestion (HS-AD) on biogas production from empty fruit bunches (EFB), focusing on the effects of total solids (TS) loading (5–40 %), particle size reduction (0.5, 3.25, and 6 cm), and co-digestion with palm oil mill effluent (POME) (10–30 % VS basis). The HS-AD at a 15–20 % TS loading has a methane yield of 103.4–105.3 mL CH4/g-VS with 24.6–25.1 % biodegradability. Particle size reduction to 0.5 cm enhanced methane yield by 54–61 % and improved hydrolysis rates by 45 % compared to the untreated EFB (6 cm) at a 15–20 % TS loading. Co-digestion of EFB with POME at a ratio of 31:1 based on VS basis led to a synergistic effect of 17.77 mL CH4/g-VS, increasing methane yield by 24–46.5 % and improving process stability, as evidenced by a 22.8–38.1 % reduction in volatile fatty acids (VFAs) accumulation. Microbial community analysis showed a 2-fold increase in the relative abundance of hydrogenotrophic methanogens (Methanothermobacter sp. and Methanoculleus sp.) during co-digestion, while the abundance of key cellulolytic bacteria (Clostridium sp. and Fibrobacter sp.) increased by 1.5-fold. The optimized HS-AD process achieved a maximum methane yield of 287.77 mL CH4/g-VS and a biodegradability of 61.2 % under thermophilic conditions, with a 20 % POME co-digestion addition (31:1 VS ratio) and 0.5 cm particle size. These findings demonstrate the potential of thermophilic HS-AD for the sustainable management of EFB and highlight the importance of process optimization and co-digestion strategies for enhanced biogas production from EFB.

Co-occurrence of dominant bacteria and methanogenic archaea and their metabolic traits in a thermophilic anaerobic digester.

Thermophilic anaerobic digestion (TAD) represents a promising biotechnology for both methane energy production and waste stream treatment. However, numerous critical microorganisms and their metabolic characteristics involved in this process remain unidentified due to the limitations of culturable isolates. This study investigated the phylogenetic composition and potential metabolic traits of bacteria and methanogenic archaea in a TAD system using culture-independent metagenomics. Predominant microorganisms identified in the stable phase of TAD included hydrogenotrophic methanogens (Methanothermobacter and Methanosarcina) and hydrogen-producing bacteria (Coprothermobacter, Acetomicrobium, and Defluviitoga). Nine major metagenome-assembled genomes (MAGs) associated with the dominant genera were selected to infer their metabolic potentials. Genes related to thermal resistance were widely found in all nine major MAGs, such as the molecular chaperone genes, Clp protease gene, and RNA polymerase genes, which may contribute to their predominance under thermophilic condition. Thermophilic temperatures may increase the hydrogen partial pressure of Coprothermobacter, Acetomicrobium, and Defluviitoga, subsequently altering the primary methanogenesis pathway from acetoclastic pathway to hydrogenotrophic pathway in the TAD. Consequently, genes encoding the hydrogenotrophic methanogenesis pathway were the most abundant in the recovered archaeal MAGs. The potential interaction between hydrogen-producing bacteria and hydrogenotrophic methanogens may play critical roles in TAD processes.

Denaturing Gradient Gel Electrophoresis Approach for Microbial Shift Analysis in Thermophilic and Mesophilic Anaerobic Digestions.

To determine the evolution of microbial community and microbial shift under anaerobic processes, this study investigates the use of denaturing gradient gel electrophoresis (DGGE). In the DGGE, short- and medium-sized DNA fragments are separated based on their melting characteristics, and this technique is used in this study to understand the dominant bacterial community in mesophilic and thermophilic anaerobic digestion processes. Dairy manure is known for emitting greenhouse gases (GHGs) such as methane, and GHG emissions from manure is a biological process that is largely dependent on the manure conditions, microbial community presence in manure, and their functions. Additional efforts are needed to understand the GHG emissions from manure and develop control strategies to minimize the biological GHG emissions from manure. To study the microbial shift during anaerobic processes responsible for GHG emission, we conducted a series of manure anaerobic digestion experiments, and these experiments were conducted in lab-scale reactors operated under various temperature conditions (28 °C, 36 °C, 44 °C, and 52 °C). We examined the third variable region (V3) of the 16S rRNA gene fingerprints of bacterial presence in anaerobic environment by PCR amplification and DGGE separation. Results showed that bacterial community was affected by the temperature conditions and anaerobic incubation time of manure. The microbial community structure of the original manure changed over time during anaerobic processes, and the community composition changed substantially with the temperature of the anaerobic process. At Day 0, the sequence similarity confirmed that most of the bacteria were similar (>95%) to Acinetobacter sp. (strain: ATCC 31012), a Gram-negative bacteria, regardless of temperature conditions. At day 7, the sequence similarity of DNA fragments of reactors (28 °C) was similar to Acinetobacter sp.; however, the DNA fragments of effluent of reactors at 44 °C and 52 °C were similar to Coprothermobacter proteolyticus (strain: DSM 5265) (similarity: 97%) and Tepidimicrobium ferriphilum (strain: DSM 16624) (similarity: 100%), respectively. At day 60, the analysis showed that DNA fragments of effluent of 28 °C reactor were similar to Galbibacter mesophilus (strain: NBRC 10162) (similarity: 87%), and DNA fragments of effluent of 36 °C reactors were similar to Syntrophomonas curvata (strain: GB8-1) (similarity: 91%). In reactors with a relatively higher temperature, the DNA fragments of effluent of 44 °C reactor were similar to Dielma fastidiosa (strain: JC13) (similarity: 86%), and the DNA fragments of effluent of 52 °C reactor were similar to Coprothermobacter proteolyticus (strain: DSM 5265) (similarity: 99%). To authors' knowledge, this is one of the few studies where DGGE-based approach is utilized to study and compare microbial shifts under mesophilic and thermophilic anaerobic digestions of manure simultaneously. While there were challenges in identifying the bands during gradient gel electrophoresis, the joint use of DGGE and sequencing tool can be potentially useful for illustrating and comparing the change in microbial community structure under complex anaerobic processes and functionality of microbes for understanding the consequential GHG emissions from manure.

Ammonia stripping by in situ biogas self-circulation to upgrade continuous thermophilic and mesophilic digestion of hydrothermal high-solid sludge

High-solid anaerobic digestion of hydrothermal sewage sludge has been developed. In order to upgrade the process by focusing on ammonia inhibition, a simply-equipped stripping system without additional alkali or heat supply was introduced by in situ biogas self-circulation. As the determined limit of total ammonia nitrogen at 1500 mg/L and 1000 mg/L for the mesophilic (MAD) and thermophilic anaerobic digestion (TAD) respectively and stripping rate at 5 L/min, continuous MAD and TAD was conducted in parallel. The stripping system successfully polished up the ammonia inhibition, and methanogenic capability of the TAD was promoted to approximately 90.0 % of the potential. Intermittent stripping mode proved usable. More frequent stripping was inevitable for the TAD as compared to the MAD. Hydraulic retention time below 20 d resulted in failure of the stripping mode due to rapid ammonia generation. Overall, this technology was practical in upgrading high-solid sludge digestion by effective ammonia control.

Different effects of mesophilic and thermophilic anaerobic digestion on subsequent pyrolysis: Characteristics, kinetics and thermodynamic analysis

Anaerobic digestion (AD) combined with pyrolysis has been used as an innovative sludge-to-energy process that enhances energy recovery. This study investigated the different impacts of mesophilic and thermophilic AD on subsequent pyrolysis. The pyrolysis characteristics, kinetics, and thermodynamics of raw sludge (RS) and digested sludge (DS) after both mesophilic and thermophilic AD were investigated using thermogravimetric analysis (TGA) and three kinetic models (Starink, Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose). TGA results indicated that thermophilic AD reduced the weight loss from 71.2-75.6 % for RS to 55.0–59.1 % for DS, more considerably than mesophilic AD (reducing from 70.6-75.2 % to 61.3–69.5 %), indicating higher organic matter removal. Kinetic analysis showed that the activation energy (E) of RS and DS pyrolysis ranged from 128.9 to 399.9 kJ/mol and 117.3 to 470.4 kJ/mol, respectively. Mesophilic AD generally increased the E, while thermophilic AD did not markedly alter E at low conversion rates (0.2–0.5) but increased it at high conversion rates (0.6–0.8), changing from 203.7–399.9 kJ/mol for RS to 248.9–470.4 kJ/mol for DS. Thermodynamic analysis showed that mesophilic AD generally enhanced pyrolysis favorability and reactivity but impeded activated complex formation. Thermophilic AD enhanced favorability and favored complex formation at low conversion rates but hindered it at high conversion rates. Reactivity varied with thermophilic AD extent, with excessive AD reducing reactivity. Pyrolysis–gas chromatography/mass spectrometry analysis confirmed that AD, on the one hand, decomposed biodegradable organics into low-molecular-weight organics, retained them in the DS, and facilitated subsequent pyrolysis. On the other hand, AD could accumulate large bio-refractory molecules, rendering DS less prone to pyrolytic decomposition. This effect of thermophilic AD was more significant than that of mesophilic AD. Given more favorable change was observed in kinetics and thermodynamics at lower conversion rates, low-temperature pyrolysis to produce biochar might be a promising strategy for DS treatment. The insights gained in this study may guide the optimization of AD conditions to maximize the efficiency of subsequent pyrolysis, providing more insight into the thermochemical conversion of DS and pyrolysis design.

Unveiling the bioaugmentation mechanism of Clostridium thermopalmarium HK1 enhancing methane production in thermophilic anaerobic digestion of food waste

To enhance methane production and ensure system stability in the thermophilic anaerobic digestion (TAD) of food waste (FW), bioaugmentation is a straightforward and effective strategy. This study investigated the bioaugmentation effect of Clostridium thermopalmarium HK1 on the TAD of FW. The results showed that the cumulative methane production (CMP) and the average methane content after bioaugmentation increased by 23.67% and 22.69%, respectively. Moreover, the addition of C. thermopalmarium HK1 contributed to alleviating ammonia inhibition and promoting volatile fatty acids (VFAs) conversion, thus maintaining the system stability. The dominant hydrolytic bacteria and hydrogenotrophic methanogens were enriched in the early stage after bioaugmentation. The genus Clostridium was positively correlated with methane content. Metagenomic analysis further suggested that methane metabolism was strengthened, and the relative abundances of genes encoding the key functional enzymes associated with hydrogenotrophic and acetoclastic methanogenic pathways were enhanced. Therefore, bioaugmentation with C. thermopalmarium HK1 had great potential for improving the efficiency of the TAD of FW.

Temperature-phased anaerobic sludge digestion effectively removes antibiotic resistance genes in a full-scale wastewater treatment plant

Sludge is a major by-product and the final reservoir of antibiotic resistance genes (ARGs) in wastewater treatment plants (WWTPs). Temperature-phased anaerobic digestion (TPAD), consisting of thermophilic anaerobic digestion (AD) (55 °C) and mesophilic AD processes (37 °C), has been implemented in WWTPs for sludge reduction while improving the biomethane production. However, the impact of TPAD on the ARGs' fate is still undiscovered in lab-scale experiments and full-scale WWTPs. This study, for the first time, investigated the fate of ARGs during the TPAD process across three seasons in a full-size WWTP. Ten typical ARGs and one integrase gene of class 1 integron (intI1) involving ARGs horizontal gene transfer were examined in sludge before and after each step of the TPAD process. TPAD reduced aac(6′)-Ib-cr, blaTEM, drfA1, sul1, sul2, ermb, mefA, tetA, tetB and tetX by 87.3–100.0 %. TPAD reduced the overall average absolute abundance of targeted ARGs and intI1 by 92.39 % and 92.50 %, respectively. The abundance of targeted ARGs in sludge was higher in winter than in summer and autumn before and after TPAD. During the TPAD processes, thermophilic AD played a major role in the removal of ARGs, contributing to >60 % removal of ARGs, while the subsequent mesophilic AD contributed to a further 31 % removal of ARGs. The microbial community analysis revealed that thermophilic AD reduced the absolute abundance of ARGs hosts, antibiotic resistant bacteria. In addition, thermophilic AD reduced the abundance of the intI1, while the intI1 did not reproduce during the mesophilic AD, also contributing to a decline in the absolute abundance of ARGs in TPAD. This study demonstrates that TPAD can effectively reduce the abundance of ARGs in sludge, which will suppress the transmission of ARGs from sludge into the natural environment and deliver environmental and health benefits to our society.

열적-알칼리성 전처리 유무에 따른 폴리하이드록시부티레이트의 고온 혐기성 소화 영향 연구

열적-알칼리성 전처리 유무에 따른 폴리하이드록시부티레이트의 고온 혐기성 소화 영향 연구

Optimisation of Biogas Production in the Co-Digestion of Pre-Hydrodynamically Cavitated Aerobic Granular Sludge with Waste Fats

The characteristics of excess aerobic granular sludge, related to its structure and chemical composition, limit the efficiency of anaerobic digestion. For this reason, pre-treatment methods and compositions with other organic substrates are used. In earlier work, no attempt was made to intensify the methane fermentation of the excess aerobic granular sludge by adding fatty waste materials. The aim of the research was to determine the effects of co-digestion of pre-hydrodynamically cavitated aerobic granular sludge with waste fats on the efficiency of methane fermentation under mesophilic and thermophilic conditions. The addition of waste fats improved the C/N ratio and increased its value to 19. Under mesophilic conditions, the highest effects were observed when the proportion of volatile solids from waste fats was 25%. The amount of biogas produced increased by 17.85% and CH4 by 19.85% compared to the control. The greatest effects were observed in thermophilic anaerobic digestion at 55 °C, where a 15% waste fat content in volatile solids was ensured. This resulted in the production of 1278.2 ± 40.2 mL/gVS biogas and 889.4 ± 29.7 mL/gVS CH4. The CH4 content of the biogas was 69.6 ± 1.3%. The increase in biogas and CH4 yield compared to pure aerobic granular sludge anaerobic digestion was 34.4% and 40.1%, respectively. An increase in the proportion of waste fats in the substrate had no significant effect on the efficiency of methane fermentation. Strong positive correlations (R2 > 0.9) were observed between biogas and CH4 production and the C/N ratio and VS concentration.