A novel insight into the influence of thermal pretreatment temperature on the anaerobic digestion performance of floatable oil-recovered food waste: Intrinsic transformation of materials and microbial response

Anaerobic sludge digestion enhancement with bioelectrochemical and electrically conductive materials augmentation: A state of the art review

Excess biological sludge processing and disposal have a significant impact on the energy balance and economics of wastewater treatment operations, and on receiving environments. Anaerobic digestion is probably the most widespread in-plant sludge processing method globally, since it stabilizes and converts biosolids organic matter into biogas, allowing partial recovery of their embedded chemical energy. A considerable number of studies concerning applicable techniques to improve biogas production, both in quantity and quality, include pre-treatment strategies to promote biosolids disintegration aimed at the release and solubilization of intracellular energy compounds, inorganic/biological amendments aimed at improving process performance, and sludge thermal pre-treatment. As for in-process amendments, iron, micro and macro-nutrients, ashes from waste incineration and nanoparticles addition have been studied for the improvement of enzymatic reactions. Recently, use of electrically conductive materials has been credited with the possibility to accelerate and stabilize the conversion of organic substrates to methane. The possibility of increasing both biogas generation and its relative biomethane content by interfacing anaerobic digestion with bioelectrochemical systems was also postulated. This review addresses the research gap surrounding the integration of anaerobic digestion with novel technologies, particularly bioelectrochemical systems, to enhance biogas production and methane enrichment. While existing studies focus on pre-treatment and in-process amendments, the feasibility, mechanisms, and benefits of such integration remain underexplored. By critically evaluating the current state of the art, this review identifies the potential of bioelectrochemical integration to improve energy recovery and process stability, while highlighting key challenges and research needs for advancing these technologies toward practical implementation.

Advanced anaerobic digestion of household food waste pretreated by in situ-produced mixed enzymes via solid-state fermentation: Feasibility and application perspectives

Enzymatic pretreatment is an effective method which can improve the anaerobic digestion (AD) efficiency of household food waste (HFW). As an alternative to expensive commercial enzymes, mixed enzymes (MEs) produced in situ from HFW by solid-state fermentation (SSF) can greatly promote the hydrolysis rate of HFW and achieve advanced anaerobic digestion (AAD) economically sustainable. In this paper, strategies for improving the efficiency of the enzyme-production process and the abundance of MEs are briefly discussed, including SSF, fungal co-cultivation, and stepwise fermentation. The feasibility of using HFW as an applicable substrate for producing MEs (amylase, protease, and lignocellulose-degrading enzymes) and its potential advantages in HFW anaerobic digestion are comprehensively illustrated. Based on the findings, an integrated AAD process of HFW pretreated with MEs produced in situ was proposed to maximise bioenergy recovery. The mass balance results showed that the total volatile solids removal rate could reach 98.56%. Moreover, the net energy output could reach 2168.62 MJ/t HFW, which is 9.79% higher than that without in situ-produced MEs and pretreatment. Finally, perspectives for further study are presented.

Effect of combined bacteria on the flax dew degumming process: Substance degradation sequence and changes in functional bacteria taxa

In this study, 16S rDNA high-throughput sequencing, Fourier transform infrared spectroscopy, and two-dimensional correlation spectroscopy techniques were used to analyze the mechanisms driving the sequence of degradation of gummy substances by the microbial community and hydrolytic enzymes during the flax dew degumming process. The results revealed that the inoculation of combined bacteria induced quorum sensing, modulated hydrolytic enzyme production, and reshaped the community structure. Lignin-degraded genera (Pseudomonas and Sphingobacterium) were enriched, and the relative abundances of pectin- and cellulose-degraded genera (Chryseobacterium) decreased in the early degumming stages. Hemicellulose-degraded genera (Brevundimonas) increased over the degumming time. Moreover, the abundance of lignin hydrolytic enzymes improved in the early stages, while the abundance of pectin hydrolytic enzymes increased at the end of degumming. Various types of functional bacteria taxa changed the sequence of substance degradation. Electron scanning microscopy and differential scanning calorimetry results indicated that the degumming, facilitated by the inoculation of combined bacteria, was nearly completed by 21 d. The fibers exhibited smoother and more intact properties, along with higher thermal stability, as indicated by a melting temperature of 71.54 °C. This study provides a reference for selecting precise degumming bacterial agents to enhance degumming efficiency.

Temperature-dependent transformation of microbial community: A systematic approach to analyzing functional microbes and biogas production

The stability and effectiveness of the anaerobic digestion (AD) system are significantly influenced by temperature. While majority research has focused on the composition of the microbial community in the AD process, the relationships between functional gene profile deduced from gene expression at different temperatures have received less attention. The current study investigates the AD process of potato peel waste and explores the association between biogas production and microbial gene expression at 15, 25, and 35 °C through metatranscriptomic analysis. The production of total biogas decreased with temperature at 15 °C (19.94 mL/g VS), however, it increased at 35 °C (269.50 mL/g VS). The relative abundance of Petrimonas, Clostridium, Aminobacterium, Methanobacterium, Methanothrix, and Methanosarcina were most dominant in the AD system at different temperatures. At the functional pathways level 3, α-diversity indices, including Evenness (Y = 5.85x + 8.85; R2 = 0.56), Simpson (Y = 2.20x + 2.09; R2 = 0.33), and Shannon index (Y = 1.11x + 4.64; R2 = 0.59), revealed a linear and negative correlation with biogas production. Based on KEGG level 3, several dominant functional pathways associated with Oxidative phosphorylation (ko00190) (25.09, 24.25, 24.04%), methane metabolism (ko00680) (30.58, 32.13, and 32.89%), and Carbon fixation pathways in prokaryotes (ko00720) (27.07, 26.47, and 26.29%), were identified at 15 °C, 25 °C and 35 °C. The regulation of biogas production by temperature possibly occurs through enhancement of central function pathways while decreasing the diversity of functional pathways. Therefore, the methanogenesis and associated processes received the majority of cellular resources and activities, thereby improving the effectiveness of substrate conversion to biogas. The findings of this study illustrated the crucial role of central function pathways in the effective functioning of these systems.

Potential of hydrochar/pyrochar derived from sawdust of oriental plane tree for stimulating methanization by mitigating propionic acid inhibition in mesophilic anaerobic digestion of swine manure

VFAs accumulation in anaerobic digestion systems can lead to disturbance of the acid base balance, which has brought major challenges for methane production. Meanwhile, less research explored the potential of biochar derived from wood wastes of oriental plane tree (Platanus orientalis L.) for stimulating methanization in mesophilic anaerobic digestion. In this study, the effects of pyrochar and hydrochar derived from sawdust of oriental plane tree on mesophilic anaerobic digestion of swine manure were compared for the first time. Fourier infrared transform analysis indicated that more functional groups existed on the surface of hydrochar, whereas higher ash content and BET specific surface area were found in pyrochar. The maximum methane production rate during anaerobic digestion was observed in the pyrochar treatment, which increased by 59.5% compared with the control without biochar. Although stimulative effects on dissolved organic carbon and volatile fatty acids production were both observed in the pyrochar and hydrochar treatments, the pyrochar treatment was much easier to trigger multipath methanogenesis and direct interspecific electron transport and subdue propionic acid accumulation compared to the hydrochar treatment. Moreover, redundancy analysis indicated that the variations in acetic acid and dissolved organic carbon were mostly associated with microbial succession. These results suggest that pyrochar has better promoting effects than HC in terms of methane generation and propionic acid inhibition alleviation owing to its special porous structures, functional groups (e.g., C=O, C-O and O-H), and physicochemical properties. These excellent properties play a greater role in recruiting functional archaea and bacteria to regulate the levels of volatile fatty acids and dissolved organic carbon to enhance the methane yield of anaerobic digestion. This study provides novel and valuable information for further engineering applications of pyrochar and hydrochar derived from sawdust of oriental plane tree in energy production and environmental waste treatment.

Continuing progress in the field of two-dimensional correlation spectroscopy (2D-COS): Part III. Versatile applications

In this review, the comprehensive summary of two-dimensional correlation spectroscopy (2D-COS) for the last two years is covered. The remarkable applications of 2D-COS in diverse fields using many types of probes and perturbations for the last two years are highlighted. IR spectroscopy is still the most popular probe in 2D-COS during the last two years. Applications in fluorescence and Raman spectroscopy are also very popularly used. In the external perturbations applied in 2D-COS, variations in concentration, pH, and relative compositions are dramatically increased during the last two years. Temperature is still the most used effect, but it is slightly decreased compared to two years ago. 2D-COS has been applied to diverse systems, such as environments, natural products, polymers, food, proteins and peptides, solutions, mixtures, nano materials, pharmaceuticals, and others. Especially, biological and environmental applications have significantly emerged. This survey review paper shows that 2D-COS is an actively evolving and expanding field.

Novel insight on ferric ions addition to mitigate recalcitrant formation during thermal-alkali hydrolysis to enhance biomethanation

Thermal-chemical pre-treatment has proven to facilitate the solubilization of organics and improvement in biogas generation from the organic fraction of municipal solid waste (OFMSW). However, the production of recalcitrant is inevitable when OFMSW is pretreated at high temperatures and alkali dosage. This study develops a strategy to use Fe³⁺ to reduce the formation of recalcitrant compounds, i.e., 5-HydroxyMethyl Furfural (5-HMF), furfurals, and humic acids (HA) during thermal-alkali pre-treatment. It was postulated that the formation of the recalcitrant compound during pre-treatment can be reduced by Fe³⁺ dosing to oxidize intermediates of Maillard reactions. A decrease in 5-HMF (45-49%) and furfurals (54-66%) was observed during Fe³⁺ (optimum dose: 10 mg/L) mediated thermal-alkali pre-treatment owing to the Lewis acid behavior of FeCl₃. The Fe³⁺ mediated assays show a substantial improvement in VS removal (28%) and biogas yield, i.e., 31% (292 mL/gVS_added) in 150 °C + 3 g/L NaOH, 34% (316 mL/gVS_added) in 175 °C + 3 g/L NaOH, and 36% (205 mL/gVS_added) in 200 °C + 3 g/L NaOH assays, over their respective controls (no Fe³⁺ dosing). The reducing property of Fe³⁺ rendered a low ORP (-345 mV) in the system than control, which is beneficial to the anaerobic microbiome. Electrical conductivity (EC) also shows a three-fold increase in Fe³⁺ mediated assays over control, promoting direct interspecies electron transfer (DIET) amongst microbes involved in the electrical syntrophy. The score plot and loading plots from principal component analysis (PCA) showed that the results obtained by supplementing 10 mg/L Fe³⁺ at 150, 175, and 200 °C were significantly different. The correlation of the operational parameters was also mutually correlated. This work provides a techno-economically and environmentally feasible option to mitigate the formation of recalcitrant compounds and enhance biogas production in downstream AD by improving the degradability of pretreated substrate.

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

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

Comparison of microbial communities during anaerobic digestion of kitchen waste: Effect of substrate sources and temperatures

In this study, batch experiments were conducted to compare the effect of temperature and substrate source on microbial communities in the anaerobic digestion of kitchen waste. The results showed that the microbial communities of anaerobic digestion were not sensitive to varied sources of waste, but shifted with the change in operating temperatures. In the reactors operated at mesophilic conditions, Levilinea, Syntrophomonas, Methanothrix, and Methanosphaerula, etc. were the dominant microbes during the process. While in thermophilic reactors, Levilinea, Ornatilinea, Methanosphaerula and Methanomassiliicoccus, etc. prevailed. Meanwhile, an enrichment in Coprothermobacter, Defluviitoga, Defluviitalea, Tepidimicrobium, Lutispora and Fonticella were observed as the temperature changed from mesophilic to thermophilic, suggesting these genera could be selectively enriched at thermophilic conditions. The results provided fundamental understanding of the microbiology that could support the scale up of food waste anaerobic digestion.

Denitrification performance evaluation and kinetics analysis with mariculture solid wastes (MSW) derived carbon source in marine recirculating aquaculture systems (RAS)

Biological denitrification using mariculture solid wastes (MSW) carbon source is a promising solution for removing nitrate (NO₃^--N) and disposing MSW in marine recirculating aquaculture systems (RAS). To enhance denitrification performance, heating (HT), rhamnolipid (RL), alkali (AL), thermophilic bacteria (TB) pre-treated MSW acidogenic fermentation effluents were prepared as carbon sources. Profiles of soluble organics in four types of fermentation effluents were first evaluated. The highest volatile fatty acids (VFAs) yield (52.1%) was obtained from TB treated MSW after acidification. RL and TB treated MSW acidogenic fermentation effluents showed high NO₃^--N removal efficiency (NRE) (around 97%). Acidogenic fermentation effluent from TB treated MSW presented a high biodegradability, with the minimum effluent chemical oxygen demand (COD) amount (35 mg/L). Denitrification kinetics parameters were also analyzed; high fraction (74.5%) of the most readily biodegradable organics (S_S) demonstrated that TB treated MSW acidogenic fermentation effluent is a high-quality carbon source for enhancing denitrification.

Using an expended granular sludge bed reactor for advanced anaerobic digestion of food waste pretreated with enzyme: The feasibility and its performance

The high content of solid organics in food waste (FW) results in a low and unstable anaerobic digestion (AD) efficiency. Improving methane production rate and process stability is attracting much attention towards advanced AD of FW. The feasibility of advanced AD of FW pretreated with enzyme was investigated by batch experiments and 164 days running of an expanded granular sludge bed (EGSB) reactor. Simulation study based on the results of batch experiments indicates it is possible to treat enzymatically pretreated FW using an EGSB reactor. During the running of an EGSB reactor, the organic loading rate went up to 20 g chemical oxygen demand (COD)/L.d, and the total COD removal rate reached 88%. The significance of this study is to achieve an advanced AD of enzymatically pretreated FW with a stable and efficient methane production with biogas residue being reduced greatly.