•Two TAD of food waste both had good performances at HRT 30 d in stable stage.•The VS destructions and methane yields of food waste in two TAD were comparable.•Two TAD recovered similar energy from ...the fed and destructed food waste.•Single stage TAD had higher energy recovery, rate and special rate of reaction for the four AD steps.•Two-stage TAD need to be adjusted its operational parameters to improve its AD efficiency.
The single-stage and two-stage thermophilic anaerobic digestion (TAD) in lab scale continuously stirred tank reactor systems fed semi-continuously with food waste (FW) were studied to compare their performance, energy balance and reaction processes. The experimental results showed the two TADs both had good performance at a sludge retention time (SRT) of 30 d. The VS destruction (83.22 ± 1.33%) of the single-stage TAD was comparable with the two-stage TAD (82.02 ± 1.25%) during the steady period. While the average biogas yield of the two-stage TAD (0.810 ± 0.13 L/g Added VS) was higher than that of single-stage TAD (0.775 ± 0.20 L/g Added VS) and the methane content of the former (59.1 ± 1.4%) was lower than that of the latter (61.6 ± 2.1%), the methane yields of the two TADs were similar. The single stage TAD had higher energy recovery, rate and specific rate of reaction for the four AD steps than the two-stage TAD. The two-stage TAD had to adjust its operational parameters to improve its AD efficiency.
As antibiotic resistance continues to spread globally, there is growing interest in the potential to limit the spread of antibiotic resistance genes (ARGs) from wastewater sources. In particular, ...operational conditions during sludge digestion may serve to discourage selection of resistant bacteria, reduce horizontal transfer of ARGs, and aid in hydrolysis of DNA. This study applied metagenomic analysis to examine the removal efficiency of ARGs through thermophilic and mesophilic anaerobic digestion using bench-scale reactors. Although the relative abundance of various ARGs shifted from influent to effluent sludge, there was no measureable change in the abundance of total ARGs or their diversity in either the thermophilic or mesophilic treatment. Among the 35 major ARG subtypes detected in feed sludge, substantial reductions (removal efficiency >90 %) of 8 and 13 ARGs were achieved by thermophilic and mesophilic digestion, respectively. However, resistance genes of aadA, macB, and sul1 were enriched during the thermophilic anaerobic digestion, while resistance genes of erythromycin esterase type I, sul1, and tetM were enriched during the mesophilic anaerobic digestion. Efflux pump remained to be the major antibiotic resistance mechanism in sludge samples, but the portion of ARGs encoding resistance via target modification increased in the anaerobically digested sludge relative to the feed. Metagenomic analysis provided insight into the potential for anaerobic digestion to mitigate a broad array of ARGs.
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.
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•TPAD reduced the average absolute abundance of targeted ARGs and intI1.•Thermophilic AD plays a major role in the ARGs and intI1 removals.•The removal of ARGs is related to the decreased abundance of ARB and intI1.•The abundance of targeted ARGs was higher in winter than in summer and autumn.
The policies and technological drivers to manage food waste in Asia have been shaped by the increasing awareness of the countries to this issue, their commitment to national and international ...development goals, their socio-economic constraints, and their recognition of the potency to recover nutrients and energy from food waste. The concept of reduce, reuse and recycle (the 3R principles) streamline the existing food waste management policies, and scrutinising the gaps and challenges led to a conclusion that most of the countries emphasise food waste segregation and treatment instead of prevention at source itself. Furthermore, a qualitative SWOT analysis of five prevailing treatment options led to a conclusion that animal feeding, incineration, and landfilling are unsustainable since they pose various health and environmental hazard risks. It was further concluded that anaerobic digestion was the preferred option than aerobic digestion (composting) considering the characteristics of the available food waste in Asia as well as the underlying environmental and economic benefits. Moreover, decentralised, community-scale, anaerobic digestion system has been gaining traction over centralised, large-scale system because of their lower energy footprint, ease of operation, need for lesser resources, lower operation and maintenance costs, and higher chances of public acceptance. It was also observed that the policy to gain energy from segregated food waste is a larger driving force for the efforts to promote anaerobic digestion and thereby manage food waste sustainably.
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•Policies and technological drivers to manage food waste in Asia have been reviewed.•Food waste reduction is not adequately prioritised in most Asian countries.•Community-scale systems are gaining traction for food waste management.•Policies, rather than technologies, are driving food waste management efforts in Asia.
This study investigates the development of a scale-down approach for anaerobic digesters. Geometric similarity, flow regime, dimensionless cavern size and power consumption per unit volume (P/V) were ...identified as scaling criteria to design the pilot digester. At this pilot scale, a Computational Fluid Dynamics (CFD) model was validated using Particle Image Velocimetry (PIV) measurements and extended to the industrial anaerobic digester to describe its hydrodynamics and evaluate the robustness of the scaling criteria. The pilot scale CFD simulations were able to capture the main features of the flow, and model P/V and cavern diameter (Dc) with an average error of 4% and 3%, respectively. The comparison between the pilot and the industrial scale showed a deviation of 9% for P/V and about 5% for Dc and similar distributions of the dimensionless velocities inside the cavern, validating the scaling approach.
•A scaled down approach to viscoplastic flows in anaerobic digesters was developed.•The numerical model was validated at pilot scale by Particle Image Velocimetry data.•The model was successfully applied to industrial scale to describe its hydrodynamics.•Constant Re, P/V and dimensionless cavern diameter were found to be robust criteria.
Livestock waste is a known reservoir of Escherichia coli (E. coli) carrying clinically important CTX-M-type extended-spectrum β-lactamase genes (blaCTX-M), however, the occurrence and transfer ...characteristics of blaCTX-M genes during anaerobic digestion (AD) remain unclear. Herein, four full-scale and two parallel lab-scale AD systems treating swine waste under ambient and mesophilic conditions were investigated by both molecular- and culture-based methods to reveal the occurrence and transfer behaviors of blaCTX-M genes during AD. Real-time TaqMan polymerase chain reaction revealed 1.3 × 104–6.8 × 105 and 3.0 × 104–7.0 × 105 copies/mL of blaCTX-M groups 1 and 9 in all feeding substrates. While AD reduced the absolute abundance of groups 1 and 9 by 0.63–2.24 and 0.08–1.30 log (P < 0.05), 5.0 × 102–4.1 × 103 and 1.1 × 104–3.5 × 104 copies/mL of groups 1 and 9 remained in the anaerobic effluent, respectively. In total, 141 blaCTX-M-carrying E. coli isolates resistant to cefotaxime were obtained from the AD reactors. Whole-genome sequencing showed that blaCTX-M-65 mainly carried by E. coli ST155 was the most frequently detected group 9 subtype in the feeding substrate; whereas blaCTX-M-14 associated with the dominant clones E. coli ST6802 and ST155 became the major subtype in AD effluent. Furthermore, blaCTX-M-14 was flanked by ΔIS26 upstream and ΔIS903B downstream. The ΔIS26-blaCTX-M-14-ΔIS903B element was mainly located on the IncHI2 plasmid in E. coli ST48 and ST6802 and also the IncFIB plasmid in ST155 in anaerobic effluent. Conjugation assays showed that the plasmids harboring blaCTX-M-14 could be successfully transferred at a frequency of 10−3–10−2 cells per recipient cell. This study revealed that blaCTX-M genes remained in both the full-scale and lab-scale AD effluents of swine waste. Thus, additional efforts should be implemented to block the discharge and spread of antibiotic resistance genes to the environment.
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•blaCTX-M groups 1 and 9 remained in swine waste effluent after anaerobic digestion.•Cefotaxime resistant E. coli was present in effluent after anaerobic digestion.•blaCTX-M-14 became the dominant group 9 subtype in E. coli in anaerobic effluent.•ΔIS26-blaCTX-M-14-ΔIS903B transferred among different E. coli clones and plasmids.•High conjugation frequency of IncHI2 plasmid favored blaCTX-M-14 propagation.
Considerable amounts of organic waste materials are disposed of in landfills or by incineration, creating the potential for environmental problems and missing opportunities for energy and material ...applications. The more digestible, valuable, and edible components (e.g. starch, lipids, edible fibers, and essential proteins for human health) are generally extracted first from biomass feedstocks, leaving the more recalcitrant, less beneficial, and unpleasant components. Due to higher-cost extractions, immature conversion techniques, and lower market demand, lignin and protein can become enriched in these biomass wastes: agricultural residues, dedicated (biofuel) plants, distillers' grains, seed cakes, forestry residues, food wastes, municipal solid wastes, sewage sludges, and digestate solids. These protein-rich lignocellulosic wastes still contain abundant fibrous polysaccharides (e.g. cellulose and hemicellulose) that have the potential for (further) conversion. In this review, each waste feedstock is evaluated for valorization by hydrolysis + fermentation, and anaerobic digestion, based on biomass composition, biomass degradation mechanisms, and yield/quality of the end products. Agricultural residues, dedicated plants, and distillers’ grains have the highest bioethanol yields compared to the other feedstocks. Stillage, sewage sludge, municipal solid waste, de-oiled seed cakes, and food wastes show higher activities in anaerobic digestion and produce greater biogas yields. Integration of hydrolysis + fermentation and anaerobic digestion may maximize the bioenergy recovery, and minimize residue generation, from most types of protein-rich lignocellulosic wastes. Screening wastes for different conversion methods enables greater chances for profitability while mitigating environmental risks within agricultural, industrial, and municipal sectors.
•Agricultural residues, dedicated plants, and DDGS favor ethanol production.•Stillage, sewage sludge, seedcakes, and food wastes favor methane production.•Serial integration of conversion methods maximizes feedstock utilization value.
Working towards a carbon-neutral future requires countries to decrease fossil fuel utilisation which is closely related to decreasing the natural gas dependence of countries. The utilisation of ...energy sources within the countries is preferable, for example by increasing biogas production and utilising materials used to a smaller extent. Such bioresources may be various freshwater macrophytes, especially such specimens that are quick to spread due to their invasive nature. If the atmospheric carbon dioxide bound by these macrophytes is not turned into methane via anaerobic digestion but the plants are used as base material, it could be a significant step to achieve a carbon-neutral future. In the article, various biogas production experiments with these plants are reviewed, focusing on the gas and methane yield from the mono- and co-digestion of the different plants, including the possible pre-treatment methods, and the disadvantages of their use. The biogas yield of freshwater macrophytes is generally between 200 and 400 Lgas/kgVS which can be increased with co-digestion in most cases. The use of aquatic plants after phytoremediation has not been fully explored yet, but the results so far indicate that biogas production is not hindered by the bound elements. The co-digestion experiments revealed that the addition of aquatic plants in 25–50 % to the base material produces the best gas and methane yield values. Various pre-treatment methods can be used to process aquatic plants before anaerobic digestion. Comminution and thermal treatment can increase gas yield but the effect of chemical pre-treatment depends on the type of chemical used. The utilisation of freshwater macrophytes in biogas production could be successfully up-scaled, as the pilot-sized experiments showed promising results.
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•The utilisation of freshwater macrophytes as a raw material for biogas is reviewed.•These are suitable for mono-digestion, optimally in 25–50 % w/w for co-digestion.•The biogas yield can be increased by pre-treating the aquatic plants.
•Effects of thermo-/mesophilic anaerobic digestion on sludge pyrolysis were studied.•Thermophilic anaerobic digestion removed more organics from sludge than mesophilic.•Thermophilic process retained ...small molecules but enriched more bio-refractory ones.•Kinetics and thermodynamics showed more favorable changes at lower conversion rates.•Ideal anaerobic digestion extent might maximize the subsequent pyrolytic efficiency.
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.
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•Recycling effluents boosts AD performance of lignocellulosic biomass. An 87 % improvement in specific biogas yield achieved by effluents recycled.•Methanol from pectin hydrolyzation ...involved into the methylotrophic methanogenesis.•Biogenic amine pathway contributed to methylamines production.
Effluents recycling proves to be an effective method for enhancing the performance of anaerobic digestion system for lignocellulosic-like biomass. However, a limited understanding exists regarding the regulation mechanisms of methanogens. This study aims to systematically assess the dynamic interactions between process parameters, microbial communities, and methanogenic activity throughout the AD process of corn stover with effluents recycling. The results show a significant increase in the specific biogas yields of corn stover to 369 ± 115 mL/g VS with effluents recycling, an 87 % improvement compared to the control system. Effluents recycling can recover nitrogen and selectively enrich the abundances of microorganisms such as Herbinix hemicellulosilytica, Bacteroides paurosaccharolyticus and Methanosarcina mazei, etc. Correspondingly, the strategy improves the abundance of CAZymes, enhancing the efficient depolymerization of cellulose, glucuronoarabinoxylan, xyloglucan and pectin. Methanol and methylamines originating from enhanced pectin hydrolyzation and the biogenic amine pathway boost methylotrophic methanogenesis. This study addresses knowledge gaps in producing-pathways of precursors for methylotrophic methanogenesis and elucidates the mechanism to boost methanogenesis process of lignocellulosic-like biomass with effluents recycling.
