Novel insights into the mechanism of dynamic changes in microstructure and physicochemical properties of corn straw pretreated by ball milling and feasibility analysis of anaerobic digestion

In this study, the effects of Ball milling (BM) pretreatment (0-240 min) on the microstructure, physicochemical properties and subsequent methanogenesis performance of corn straw (CS) were explored, and the feasibility analysis was carried out. The results showed that BM pretreatment destroyed the dense structure of the CS, and the particle size was significantly reduced (D50: 13.85 μm), transforming it into a cell-scale granular form. The number of mesopores increased, the pore volume (PV) (0.032 cm3/g) and specific surface area (SSA) (4.738 m2/g) considerably increased, and the water-absorbent property was improved. The crystalline order of cellulose was disrupted and the crystallinity (CrI) (8.61 %) and crystal size (CrS) (3.37) were remarkably reduced. The cross-links between lignocelluloses were broken, and the relative content and functional groups did not alter obviously. The bulk density (BD), repose angle (RA) and slip angle (SA) dramatically increased. As a result, CS was more readily accessible, attached and utilized by microorganisms and enzymes, causing the hydrolysis and acidification of AD to be greatly facilitated. Compared with the untreated group, the cumulative methane production (CMP) increased by 35.83 %-101.97 %, and the lag phase time (λ) was shortened by 33.04 %-71.17 %. The results of redundancy analysis, Pearson analysis and Mantel test showed that BM pretreatment affects the process of AD by changing the physicochemical factors of CS. The normalization analysis showed that particle size (D90) and BD can be used as direct indicators to evaluate the performance of AD and predict the threshold of biodegradation of CS. Energy analysis and energy conversion assessment showed that BM is a green and efficient AD pretreatment strategy. This result provides a theoretical basis for the industrial application of BM pretreatment towards more energy-efficient and sustainable development.

Chitosan-Fe3O4 composites enhance anaerobic digestion of liquor wastewater under acidic stress

Acid stress in the anaerobic digestion process of liquor wastewater leads to low anaerobic treatment efficiency. Herein, chitosan-Fe₃O₄ was prepared, and its effects on anaerobic digestion processes under acid stress were studied. Results showed that chitosan-Fe₃O₄ increased the methanogenesis rate of anaerobic digestion of acidic liquor wastewater by 1.5-2.3 times and accelerated the restoration of acidified anaerobic systems. The analysis of sludge characteristics showed that chitosan-Fe₃O₄ promoted the secretion of proteins and humic substances in extracellular polymeric substances and increased the electron transfer activity of the system by 71.4%. Microbial community analysis indicated that chitosan-Fe₃O₄ enriched the abundance of Peptoclostridium, and Methanosaeta participated in direct interspecies electron transfer. Chitosan-Fe₃O₄ could promote the direct interspecies electron transfer pathway to maintain stable methanogenesis. These methods and results regarding the use of chitosan-Fe₃O₄ could be referred to for improving the efficiency of anaerobic digestion of high concentration organic wastewater under acid inhibition.

Effects of incineration leachate on anaerobic digestion of excess sludge and the related mechanisms

Anaerobic digestion (AD) refers to a reliable channel for energy recovery from organics. However, the digestion efficiency of excess sludge (ES) has been unsatisfactory since there are defects relating to ES hydrolysis. Therefore, this study explored a method to improve the anaerobic digestion of ES, which could simultaneously treat ES and incineration leachate, and revealed the potential mechanism of AD process. As the investigation was conducted on the influences exerted by incineration leachate on the four phases (i.e., solubilization, methanogenesis, acidogenesis and hydrolysis) of ES anaerobic digestion, and the effect mechanism. According to obtained results, adding appropriate amounts of incineration leachate could facilitate the steps of solubilization, hydrolysis, acidogenesis and methanogenesis of ES. The hydrolysis and acidogenesis efficiency in the leachate added digesters were 5.7%-17.1% and 13%-45% higher than that of the control digester, respectively. Meanwhile, cumulative methane yields (CMY) were 27-86 mL/gVS higher than that in the control digester. Besides, the sludge floc stability was reduced by the leachate with the decrease in the median particle size (MPS) and apparent activation energy (AAE) of the sludge. According to microbial community and diversity analysis, adding incineration leachate increased the relative abundance of hydrolytic-acidification bacteria in the digesters and the relative abundance of Methanosaeta and Methanosarcina. Thus, the digestive performance exhibited by the leachate participated system was improved. These mentioned findings may provide an approach for treating ES and incineration leachate in practical engineering.

Mechanism study of improving anaerobic co-digestion performance of waste activated sludge and food waste by Fe3O4

A large amount of waste activated sludge (WAS) and food waste (FW) are produced every year in China. Anaerobic co-digestion is considered to be an effective way to solve this problem. This study applied FW/WAS mixture as co-substrate to create different digestive environment, aiming to understand the mechanism of Fe₃O₄ particles in promoting AD performance. The results showed that the addition of Fe₃O₄ presented various performances when facing different digestive acidification stress brought by different mixing ratios of WAS and FW. Methanogenic pathways and microbial communities varied with substrates' properties. For group A (WAS mono-digestion), the acetoclastic methanogens dominated, 20 mg/g VS (according to the iron element) Fe₃O₄ could promote methane production, while 200 mg/g VS Fe₃O₄ would inhibit microbial activity. The promoted methane production by Fe₃O₄ was attributable to the promotion of sludge hydrolysis. For group B (WAS: FW = 1:0.5, based on VS addition, similarly hereinafter), Fe₃O₄ triggered direct interspecific electron transfer (DIET) between bacteria and methanogens. For group C (WAS: FW = 1:1), the hydrogenotrophic methanogens dominated, bacteria excreted more non-conductive polysaccharides in EPS to resist unfavorable environment, thereby it prevented their contact with Fe₃O₄ particles. So, it was difficult for Fe₃O₄ to trigger DIET and promote the digestive performance of batch experiments in such condition.

The role of growth media on composition, bioconversion and susceptibility for mild thermal pre-treatment of waste activated sludge

The highly variable characteristics of waste activated sludge (WAS) hinder the comparison of experimental results on WAS bioconversion between the different studies that use excess sludge from different origin. Sludge grown under laboratory conditions with synthetic wastewater as feed showed high resistance to commonly applied pre-treatment techniques, such as thermal pre-treatment. However, a distinctly higher bioconversion of this sludge was recorded compared to WAS from a full-scale wastewater treatment plant (WWTP). The observed results casted concern on the suitability of the experimental laboratory-based data for practice. The physicochemical and biochemical characteristics of both WAS and lab-grown sludge are dependent on the wastewater characteristics or growth media on which the sludges were grown. The objective of this study was to formulate a growth medium that results into a lab-grown sludge which shows high similarity to the WAS coming from a specific full-scale WWTP in response to a pre-treatment technique. More specifically, in this study we targeted the formation of slowly-biodegradable lab-grown sludge that is similarly responsive to mild thermal pre-treatment with H₂O₂ addition. By comparing real and synthetic wastewaters, we discussed the various wastewater constituents that may lead to a higher degree of recalcitrance of the produced sludge. We then formulated a growth medium, which was fed to a lab-scale activated sludge reactor and evaluated the nutrient removal capacity, as well as the characteristics of the cultivated sludge before and after pre-treatment. Finally, the growth medium was modified to provoke a change in both the bioconversion and in the response to mild thermal pre-treatment. The growth medium proposed in this study resulted in a slowly-biodegradable sludge (195 ± 3.7 NLCH₄/kgVS_added) that after thermal pre-treatment resulted in an increase in methane production of 9 %, which was similar to the WAS coming from the full-scale WWTP. It was concluded that not only the bioconversion but also the response to mild thermal pre-treatment of lab-grown sludge was determined by the composition of the growth media.

Aged landfill leachate enhances anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) is considered as a sustainable pathway to recover energy from organic wastes, but the digestive efficiency for waste activated sludge (WAS) is not as expected due to the limitations in WAS hydrolysis. This study proposes an effective strategy to simultaneously treat WAS and landfill leachate, aiming to promote WAS hydrolysis and enhance organics converting to methane. The effects of landfill leachate on the four stages (i.e., solubilization, hydrolysis, acidogenesis, and methanogenesis) of AD of WAS, as well as the effect mechanisms were investigated. Results showed that adding appropriate amounts of landfill leachate could promote the steps of solubilization, hydrolysis and acidogenesis of WAS, but had no-effect on methanogenesis. The hydrolysis and acidogenesis efficiency in the leachate added digesters were 2.0%-8.4% and 35.2%-72.7% higher than the control digester. Mechanism studies indicated that humic acid (HA) contained in the leachate was conducive to the processes of both hydrolysis and acidogenesis, but detrimental to the methanogenesis. Effects of heavy metals (HMs) on AD of WAS was also dose-dependent. Digestive performance was inhibited by excessive HMs but promoted by moderate dosages. Humic acid and metal ions tend to interact to form complexes, and thus relieve their each inhibition effects. It is also found that the stability of sludge flocs was reduced by the leachate through reducing both apparent activation energy (AAE) and median particle size (MPS) of the sludge. Microbial community and diversity results revealed that the relative abundance of microbes responsible for hydrolysis and acidogenesis increased when landfill leachate was present. This research provides a more technically and economically feasible approach to co-treating and co-utilizing WAS and landfill leachate.

Biomethane production by typical straw anaerobic digestion: Deep insights of material compositions and surface properties

Straws as lignocellulosic agricultural biomass have a huge amount and are widely used for biomethane production by anaerobic digestion (AD). However, the mechanism of impacts of straw composition and surface properties on biomethane production remain unclear. Here, a lab-scale AD incubation experiment was conducted and the characterization of four types of straws (corn straw, wheat straw, sweet sorghum straw, and rice straw) were also determined. The straw compositions and net cumulative methane production showed significant difference. Although the relative contents of key organic components and carbon had no significant correlation to the biomethane production (r = -0.36, P > 0.05), there existed differences of non-polar characteristics, steric hindrance effect and special surface morphology in four types of straws, indicating that the surface characteristics affected anaerobic biomethane production process. In addition, the straw matrix associating with silicon might hinder the biotransformation.

Perspective on enhancing the anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) is an important bio-energy strategy that has been hindered by low conversion efficiency. This paper presents a comprehensive review of research on the sludge's property and enhancing AD of WAS, and proposes two perspectives of material structure and microbial activity on improving AD efficiency. In the first part of this review, the key principle problems for hindering AD efficiency are identified based on the concept of AD. Then, the possibility that the complex microstructure and composition of WAS are responsible for poor biodegradability is considered and main methods for enhancing AD are summarized. In the third part, according to the published works, the main knowledge gaps in research are recognized as the identification and specific activity adjustment of functional microbes, the understanding of key constituents of WAS and their interactions, the deciphering of complex structure of sludge organic substance, and the revealing of relationships between complex nature of WAS and biodegradability. Further discussions reveal that to enhance AD more studies should be centered on the sludge's structure and properties in future. However, this review is expected to provide the clear and accurate research directions for enhancing AD efficiency of WAS.

Effects of humic matter on the anaerobic digestion of sewage sludge: New insights from sludge structure

A sludge structure perspective of the effects of humic matter (HM) on the anaerobic digestion (AD) of sewage sludge was proposed and investigated in this study. It was observed that the net cumulative methane production of sludge was significantly lower in the presence of HM. The fractal dimension values of sludge samples were increased by the addition of HM, whilst their total surface energy values were decreased, indicating that HM reinforced the stability of the sludge structure. Analyses of the physicochemical properties of different sludge samples revealed that HM enhanced the structural stability of sludge organic substances and restricted its organic solubility by interacting with the biopolymers to form a dense granular aggregate, thereby reducing the random-coil degree of extracellular organic substances and its active sites for enzymes, which suggested that HM could inhibit AD by changing the sludge structure. It was further confirmed by the biochemical methane potential (BMP) assay process. The BMP tests and corresponding kinetic analyses revealed that HM can restrict the potentials of sludge organic solubilisation, hydrolysis and acidification processes, but not necessarily the rates of these processes by enhancing the structural stability of high molecular weight readily biodegradable organic matter (RBOM) and restricting the mobility of low molecular weight RBOM (i.e., hydrolysis products). These findings can enrich understanding of the AD of sludge and may thus aid in the development of more effective methods to improve sludge treatment.

Particle size reduction of rice straw enhances methane production under anaerobic digestion

The objective of this study was to investigate the effects of particle size reduction (20, 1, 0.15, and 0.075 mm) on biogas production from rice straw waste through batch anaerobic digestion experiments. To clarify the digestion mechanisms, the microbial community and rice straw properties including fractal dimension, dissolution abilities and the bio-liquefaction degree were determined. Particle size reduction of rice straw improved methane yield from 107 mL g^-1 VS to 197 mL g^-1 VS. The elevated digestion efficiency was attributed to the cellulose degradation (degradation rate from 27% to 93%) rather than hemicellulose or lignin. The comminution pretreatment improved the basic morphology, dissolution abilities and bio-liquefaction degree, which associated with the shifts in the bacterial community and the decreased bacterial diversity. These results suggested that particle size reduction of the rice straw in conjunction with optimized microbial growth could improve the methane yield in anaerobic digestion processes.

Trajectory of fine particles removal with diffusiophoresis and thermophoresis in a gas-liquid cross-flow array

A gas–liquid cross-flow array (GLCA) system is proposed for fine particles (diameter between 0.1 μm and 2.5 μm, simplified as PM2.5) removal in exhaust gas, where the continuous and smooth wastewater films, providing huge specific surface area, each act as independent traps to remove PM2.5. The removal efficiency of PM2.5 is important for evaluating the performance of a GLCA, and the trajectory across the films determines the migration and ultimate fate of PM2.5. An analytical model based on a single film is developed to analyze the critical removal trajectory with diffusiophoresis (DP) and thermophoresis (TP) in the thermal boundary layer to calculate the efficiency, where the role of each force is examined. And experiments with a lab-scale GLCA are carried out with different vapor concentration and temperature gradients to verify the model. They both reveal that the removal efficiency can be increase sharply by increasing the humidity gradient between the bulk gas and film surface, while it increases slowly as temperature gradient increasing. Thus DP and TP have important effects on PM2.5 removal in the GLCA, and DP has a much more important effect than TP. A GLCA with appropriate humidity and temperature gradient can remove PM2.5 in a costly and efficient manner.

A gas–liquid cross flow array system can significantly improve PM2.5 removal efficiency without additional energy input.

Magnetite Triggering Enhanced Direct Interspecies Electron Transfer: A Scavenger for the Blockage of Electron Transfer in Anaerobic Digestion of High-Solids Sewage Sludge

At present, high-solids anaerobic digestion of sewage sludge has drawn great attention due to the superiority of its small land area footprint and low energy consumption. However, a high organic loading rate may cause acids accumulation and ammonia inhibition, thus leading to an inhibited pseudo-steady state in which electron transfer through interspecies hydrogen transfer (IHT) between acetogens and methanogens is blocked. In this study, adding 50 mg/g TS (total solid) magnetite clearly reduced the accumulation of short-chain fatty acids and accelerated methane production by 26.6%. As demonstrated, the individual processes of anaerobic digestion could not be improved by magnetite when methanogenesis was interrupted. Analyzing stable carbon isotopes and investigating the methanogenesis pathways using acetate and H₂/CO₂ as substrates together proved that direct interspecies electron transfer (DIET) was enhanced by magnetite. Metatranscriptomic analysis and determination of key enzymes showed that IHT could be partially substituted by enhanced DIET, and acetate-dependent methanogenesis was improved after the blockage of electron transfer was scavenged. Additionally, the expression of both pili and c-type cytochromes was found to decrease, indicating that magnetite could replace their roles for efficient electron transfer between acetogens and methanogens; thus, a robust chain of electron transfer was established.

The influence of organic-binding metals on the biogas conversion of sewage sludge

The anaerobic conversion of sewage sludge to methane-rich biogas is an important bioenergy strategy that has been hindered by low conversion efficiency. The poorly understood mechanism of the influence of the key structural component in sludge is responsible for this. The influence of organic-binding metals (OBM), which account for a substantial proportion of metals in sludge, on biogas conversion of both sewage sludge and model sludge were explored in this study. It is observed that the net cumulative methane production of sludge decreased by 23% with the increase of OBM content, implying the crucial role of the OBM in anaerobic sludge digestion. Experimental results showed that the apparent activation energy of sludge organic solubilisation and the median particle size of sludge particulates increased with increasing OBM content, whereas the surface binding sites for enzymes decreased, indicating that the stability of the sludge floc was reinforced by the effect of OBM. Further analyses of the sludge structure revealed that a high OBM content (>2.5% total solids in the present study) compacted the sludge organic matter, restricted the molecular mobility and deteriorated the depolymerisation of the biopolymers by bridging and hydrogen-bonding interactions. This suggests that as a result of the effect of OBM, the hydrolysis and acidification of sludge particulate could be inhibited, resulting in poor biogas conversion. Moreover, it was further authenticated by the results from biochemical methane potential assay process. These findings can deepen the understanding of the role of OBM in sludge for biogas conversion and are important for the improvement of anaerobic sludge digestion.

Effect of the micron-sized silica particles (MSSP) on biogas conversion of sewage sludge

Micron-sized silica particles (MSSP), ranging from 3 μm to 50 μm, have been widely found in the sewage sludge. The inhibition of MSSP to biogas conversion of both excess sludge (ES) and model sludge (MS) are explored in this study. It is observed that with the effect of MSSP, the net cumulative methane production (NCMP) of ES and MS were decreased by 23.5% and 22.3%, respectively, and the apparent activation energy (AAE) of organic solubilisation of ES and MS were increased by 38.7% and 215%, respectively, which implies a crucial role for MSSP in anaerobic sludge digestion. Analysis of physicochemical properties of sludges before and after interaction with MSSP reveals that MSSP can bond with organic matter from sludge on the surface sites to form a larger bioinorganic-floc. Further analysis indicates that MSSP can increase the AAE of sludge organic solubilisation by reducing the surface site density, thus resulting in poor NCMP. Through characterizing the bioinorganic-floc, it is found that the protein in sludge is the main component that bonds with MSSP. Further research show that the interactions between protein and MSSP are mainly enthalpy-driven with exothermic (the enthalpy was about -10.93 ± 0.10 kJ/mol, at 25 °C), indicating that protein is more stable after non-covalent bonding. These findings can provide a new understanding of the characteristics of sludge and important references for the improvement of anaerobic sludge digestion.