Recent advances in conductive materials amended anaerobic co-digestion of food waste and municipal organic solid waste: Roles, mechanisms, and potential application

Biodegradation of sulfadiazine in anaerobic co-digestion of swine manure and food waste

This study explored the feasibility of employing the AcoD process for the removal of antibiotics and examined the impact of antibiotics on system performance. Sulfadiazine (SDZ), a prevalent broad-spectrum sulfonamide antibiotic in veterinary medicine, was selected as the model compound. Results showed that with the presence of SDZ at a concentration of 450 mg/kg total solids, the cumulative methane yield demonstrated a substantial decline of 79.2 % compared to the control group. The specific removal rate of SDZ was 47.5 % at 450 mg SDZ/kg total solids, surpassing those observed in traditional mono-anaerobic treatment processes. The elimination of SDZ by the AcoD system was predominantly ascribed to biodegradation. Within the AcoD system, cytochrome P450 enzyme (CYP450) served as the crucial enzyme in the biodegradation of SDZ. From a molecular point of view, the main interaction sites of SDZ with CYP450 enzyme were located as Thr258, Glu257, Pro428, Ala254, and Val318. Six transformation products were identified in the biodegradation process. Community diversity revealed that the predominant genera, Syntrophomonadaceae, Acinetobacter, AUTHM297, and Anaerolineaceae, were enriched in the AcoD process, which probably contributed to SDZ removal. In summary, the AcoD system may possess sufficient robustness to transform SDZ antibiotic.

Deciphering impacts of sulfadiazine on anaerobic co-digestion of pig manure and food waste for methane production

In this study, the impact of sulfadiazine (SDZ) at environmental and elevated levels on the integrated anaerobic digestion of pig manure with food waste was explored. Results showed that SDZ concentrations ranging from 50 and 450 mg/kg of total solids inhibited methane production by 3.3 % to 50.8 %. Moreover, SDZ hindered the acidification and methanogenesis processes but had minimal impact on the hydrolysis phase. Mechanism studies revealed that extracellular polymeric substances (EPS) absorbed and accumulated SDZ via amine, hydroxyl, and carboxyl groups. Then, SDZ incited microorganisms to secrete high amounts of EPS and reactive oxygen species. These compounds inhibited enzyme activity, thereby disrupting the processes of acidification and methanogenesis. Furthermore, SDZ remarkably reduced the populations of acidogenic bacteria and methanogens, rendering the microbial community inhospitable to methane generation. This study offers a noteworthy reference regarding the response of the anaerobic co-digestion system to various concentrations of SDZ.

Biochar@MIL-88A(Fe) accelerates direct interspecies electron transfer and hydrogen transfer in waste activated sludge anaerobic digestion: Exploring electron transfer and biomolecular mechanisms

Adding additives exogenously is an effective strategy to enhance methanogenic activity and improve AD stability. Corn straw-based biochar@MIL-88A(Fe) (BM) was synthesized herewith and used as an exogenous additive to boost methane (CH4) production. After adding BM at 250 mg/g WAS VS, the accumulative CH4 production and maximum CH4 yield increased by 1.2 and 1.9 times, respectively, with CH₄ comprising 88% of the biogas. BM accelerated electron transfer through its unsaturated sites and surface functional groups, while also enhancing metabolic functions for facilitating enzymatic activities and converting organic substrates. The abundance of syntrophic bacteria and methanogen were higher after BM addition. BM-mediated DIET and IHT pathways effectively oxidized propionate and butyrate, promoting methane generation. Higher expression of key genes involved in methane production correlated with shifts in microbial structure and increased CH4 yield after BM dosage. The invention of BM may provide more solutions for addressing low energy recovery during AD.

Enhancing reductive dechlorination of trichloroethylene in bioelectrochemical systems with conductive materials

The incorporation of conductive materials to enhance electron transfer in bioelectrochemical systems (BES) is considered a promising approach. However, the specific effects and mechanisms of these materials on trichloroethylene (TCE) reductive dechlorination in BES remains are not fully understood. This study investigated the use of magnetite nanoparticles (MNP) and biochars (BC) as coatings on biocathodes for TCE reduction. Results demonstrated that the average dechlorination rates of MNP-Biocathode (122.89 μM Cl·d-1) and BC-Biocathode (102.88 μM Cl·d-1) were greatly higher than that of Biocathode (78.17 μM Cl·d-1). Based on MATLAB calculation, the dechlorination rate exhibited a more significantly increase in TCE-to-DCE step than the other dechlorination steps. Microbial community analyses revealed an increase in the relative abundance of electroactive and dechlorinating populations (e.g., Pseudomonas, Geobacter, and Desulfovibrio) in MNP-Biocathode and BC-Biocathode. Functional gene analysis via RT-qPCR showed the expression of dehalogenase (RDase) and direct electron transfer (DET) related genes was upregulated with the addition of MNP and BC. These findings suggest that conductive materials might accelerate reductive dechlorination by enhancing DET. The difference of physicochemical characteristics (e.g. particle size and specific surface area), electron transfer enhancement mechanism between MNP and BC as well as the reduction of Fe(III) by hydrogen may explain the superior dechlorination rate observed with MNP-Biocathode.

Enhancement of anaerobic treatment of antibiotic pharmaceutical wastewater through the development of iron-based and carbon-based materials: A critical review

The extensive use of antibiotics has created an urgent need to address antibiotic wastewater treatment, posing significant challenges for environmental protection and public health. Recent advances in the efficacy and mechanisms of conductive materials (CMs) for enhancing the anaerobic biological treatment of antibiotic pharmaceutical wastewater are reviewed. For the first time, the focus is on the various application forms of iron-based and carbon-based CMs in strengthening the anaerobic methanogenic system. This includes the use of single CMs such as zero-valent iron (ZVI), magnetite, biochar (BC), activated carbon (AC), and graphene (GP), as well as iron-based and carbon-based composite CMs with diverse structures. These structures include mixed, surface-loaded, and core-shell combinations, reflecting the development of CMs. Iron-based and carbon-based CMs promote the rapid removal of antibiotics through adsorption and enhanced biodegradation. They also mitigate the inhibitory effects of toxic pollutants on microbial activity and reduce the expression of antibiotic resistance genes (ARGs). Additionally, as effective electron carriers, these CMs enrich microorganisms with direct interspecies electron transfer (DIET) functions, accelerate interspecies electron transfer, and facilitate the conversion of organic matter into methane. Finally, this review proposes the use of advanced molecular detection technologies to clarify microbial ecology and metabolic mechanisms, along with microscopic characterization techniques for the modification of CMs. These methods can provide more direct evidence to analyze the mechanisms underlying the cooperative anaerobic treatment of refractory organic wastewater by CMs and microorganisms.

Enhanced anaerobic co-digestion of food waste and sewage sludge by co-application of biochar and nano-Fe3O4

Conductive materials have been utilized to facilitate direct interspecies electron transfer (DIET) in anaerobic digestion (AD) to enhance methane production. However, the impact and efficacy of the co-application of biochar and nano-Fe3O4 have not been adequately elucidated, particularly their interaction on electron transfer efficiency. In this investigation, we examined the influence of simultaneously or independently adding biochar and nano-Fe3O4 to food waste (FW) and sewage sludge (SS) anaerobic co-digestion. A synergistic effect was observed under the co-application condition. Methane production reached 300.3 ± 19.8 mL/gCOD with the co-application of biochar and nano-Fe3O4, representing a 43.3%, 35.4%, and 5.4% increase compared to the sole Fe3O4, biochar, and nano-Fe3O4, respectively. Mechanistic analysis revealed that, in comparison to sole biochar and nano-Fe3O4, their co-occurrence significantly accelerated hydrolysis and acidogenesis, thereby enhancing the release of soluble organic components. Furthermore, the application of nano-Fe3O4 improved system stability and significantly promoted propionate degradation, maintaining a favorable condition for methane production. Additionally, the noteworthy increase in INT-ETS activity and cytochrome c concentration indicated that the co-application of biochar and nano-Fe3O4 stimulated electron transfer. Correspondingly, the activity of coenzyme F420, which indicates the performance of methanogenesis, exhibited a 2.44-fold increase compared to the control. This indicated that nano-Fe3O4 and biochar co-amendment can serve as a robust platform to strengthen DIET. This study provided a new insight regarding the application of biochar and nano-Fe3O4 in the AD system for strengthening electron transfer to promote methane production.

Electron-transfer-driven spatial optimisation of anaerobic consortia for efficient methanogenesis: Neglected inductive effect of conductive materials

The inductive effect of conductive materials (CMs) on enhancing methanogenesis metabolism has been overlooked. Herein, we highlight role of CMs in inducing the spatial optimisation of methanogenic consortia by altering the Lewis acid-base (AB) interactions within microbial aggregates. In the presence of CMs and after their removal, the methane production and methane proportion in biogas significantly increase, with no significant difference between the two situations. Analyses of interactions between CMs and extracellular polymer substances (EPSs) with and without D2O reveal that CMs promote release and transfer potential of electron in EPSs, which induce and enhance the role of water molecules being primarily as proton acceptors in the hydrogen bonding between EPSs and water, thereby changing the electron-donor- and electron-acceptor-based AB interactions. Investigations of succession dynamics of microbial communities, co-occurrence networks, and metagenomics further indicate that electron transfer drives the microbial spatial optimisation for efficient methanogenesis through intensive interspecies interactions.

Effects of carbon-based conductive materials on semi-continuous anaerobic co-digestion of organic fraction of municipal solid waste and waste activated sludge

Organic fraction of municipal solid waste (OFMSW) and waste activated sludge (WAS) are the most produced organic waste streams in urban centres. Their anaerobic co-digestion (AcoD) allows to generate methane (CH4) and digestate employable as renewable energy source and soil amendment, respectively, fully in accordance with circular bioeconomy principles. However, the widespread adoption of such technology is limited by relatively low CH4 yields that fail to bridge the gap between benefits and costs. Among strategies to boost AcoD of OFMSW and WAS, use of conductive materials (CMs) to promote interspecies electron transfer has gained increasing attention. This paper presents one of the few experimental attempts of investigating the effects of four different carbon(C)-based CMs (i.e., granular activated carbon - GAC, graphite - GR, graphene oxide - GO, and carbon nanotubes - CNTs) separately added in semi-continuous AcoD of OFMSW and thickened WAS. The presence of C-based CMs has been observed to improve CH4 yield of the control process. Specifically, after 63 days of operation (concentrations of GAC and GR of 10.0 g/L and of GO and CNTs of 0.2 g/L), 0.186 NL/gVS, 0.191 NL/gVS, 0.203 NL/gVS, and 0.195 NL/gVS of CH4 were produced in reactors supplemented with GAC, GR, GO, and CNTs, respectively, compared to 0.177 NL/gVS produced in the control process. Likewise, at the end of the test (i.e., after 105 days at concentrations of C-based CMs half of the initial ones), CH4 yields were 0.193 NL/gVS, 0.201 NL/gVS, 0.211 NL/gVS, and 0.206 NL/gVS in reactors supplemented with GAC, GR, GO, and CNTs, respectively, compared to 0.186 NL/gVS of the control process. Especially with regard to GR, GO, and CNTs, results obtained in the present study represent a significant advance of the knowledge on the effects of such C-based CMs to realistic and scalable AD process conditions respect to previous literature.

Biogas upgrading performance and underlying mechanism in microbial electrolysis cell and anaerobic digestion integrated system

The productivity and efficiency of two-chamber microbial electrolysis cell and anaerobic digestion integrated system (MEC-AD) were promoted by a complex of anaerobic granular sludge and iron oxides (Fe-AnGS) as inoculum. Results showed that MEC-AD with Fe-AnGS achieved biogas upgrading with a 23%-29% increase in the energy recovery rate of external circuit current and a 26%-31% decrease in volatile fatty acids. The energy recovery rate of MEC-AD remained at 52%-57%, indicating a stable operation performance. The selectively enriched methanogens and electroactive bacteria resulted in dominant hydrogenotrophic and acetoclastic methanogenesis in the cathode and anode chambers. Mechanistic analysis revealed that MEC-AD with Fe-AnGS led to specifically upregulated enzymes related to energy metabolism and electron transfer. Fe-AnGS as inoculum could improve the long-term operation performance of MEC-AD. Consequently, this study provides an efficient strategy for biogas upgrading in MEC-AD.

Enhancing methane production and interspecies electron transfer of anaerobic granular sludge by the immobilization of magnetic biochar

Supplementation of conductive materials has been proved to be a promising approach for enhancing microbial interspecies electron transfer (IET) in anaerobic digestion systems. In this study, magnetic bamboo-based biochar was prepared at temperatures of 400-800 °C via a ball milling/carbonization method, and it immobilized in mature anaerobic granular sludge (AGS) aimed to enhance methane production by improving the IET process between syntrophic microbial communities in the AGS. Results showed that the AGS with magnetic biochar immobilization demonstrated increased glucotrophic and acetotrophic methane production by 69.54-77.56 % and 39.96-54.92 %, respectively. Magnetic biochar prepared at 800 °C with a relatively higher Fe content (0.37 g/g magnetic biochar) displayed a stronger electron charge/discharge capacity (36.66 F/g), and its immobilization into AGS promoted methane production most. The conductivity of AGS increased by 52.13-87.32 % after incorporating magnetic biochar. Furthermore, the extracellular polymeric substance (EPS) of AGS showed an increased capacitance and decreased electron transfer resistance possibly due to the binding of magnetic biochar and more riboflavin secretion in EPS, which could contribute to the accelerated IET process in the inner AGS. In addition, the immobilization of magnetic biochar could promote the production of volatile fatty acids by 15.36-22.50 %. All these improvements may jointly lead to the enhanced methane production capacity of AGS. This study provided a fundamental understanding of the role of incorporated magnetic biochar in AGS in promoting anaerobic digestion performance.

A comprehensive review on food waste anaerobic co-digestion: Research progress and tendencies

Food waste (FW) anaerobic digestion systems are prone to imbalance during long-term operation, and the imbalance mechanism is complex. Anaerobic co-digestion (AcoD) of FW and other substrates can overcome the performance limitations of single digestion, allowing for the mutual use of multiple wastes and resource recovery. Research on the AcoD of FW has been widely conducted and successfully applied to a practical engineering scale. Therefore, this review describes the research progress of AcoD of FW with other substrates. By analyzing the problems and challenges faced by AcoD of FW, the synergistic effects and influencing factors of different biomass wastes are discussed, and improvement strategies to improve the performance of AcoD of FW are summarized from different reaction stages of anaerobic digestion. By combing the research progress of AcoD of FW, it provides a reference for the optimization and improvement of the performance of the co-digestion system.

Insights into the Occurrence, Fate, Impacts, and Control of Food Additives in Food Waste Anaerobic Digestion: A Review

The recovery of biomass energy from food waste through anaerobic digestion as an alternative to fossil energy is of great significance for the development of environmental sustainability and the circular economy. However, a substantial number of food additives (e.g., salt, allicin, capsaicin, allyl isothiocyanate, monosodium glutamate, and nonnutritive sweeteners) are present in food waste, and their interactions with anaerobic digestion might affect energy recovery, which is typically overlooked. This work describes the current understanding of the occurrence and fate of food additives in anaerobic digestion of food waste. The biotransformation pathways of food additives during anaerobic digestion are well discussed. In addition, important discoveries in the effects and underlying mechanisms of food additives on anaerobic digestion are reviewed. The results showed that most of the food additives had negative effects on anaerobic digestion by deactivating functional enzymes, thus inhibiting methane production. By reviewing the response of microbial communities to food additives, we can further improve our understanding of the impact of food additives on anaerobic digestion. Intriguingly, the possibility that food additives may promote the spread of antibiotic resistance genes, and thus threaten ecology and public health, is highlighted. Furthermore, strategies for mitigating the effects of food additives on anaerobic digestion are outlined in terms of optimal operation conditions, effectiveness, and reaction mechanisms, among which chemical methods have been widely used and are effective in promoting the degradation of food additives and increasing methane production. This review aims to advance our understanding of the fate and impact of food additives in anaerobic digestion and to spark novel research ideas for optimizing anaerobic digestion of organic solid waste.

Insight into application of phosphate-solubilizing bacteria promoting phosphorus availability during chicken manure composting

Understanding ecological roles of phosphate-solubilizing bacteria (PSB) is important to optimize composting systems. Illumina MiSeq sequencing, gene quantitation, and statistical analyses were employed to explore ecological mechanisms underlying available phosphorus (AP) facilitation during composting with the inoculation of PSB Pseudomonas sp. WWJ-22. Results displayed that the inoculation of PSB significantly increased AP from 0.83 to 1.23 g kg^-1, and notably increased abundances of phosphorus-cycling genes as well as numbers of PSB mineralizing phytate and lecithin. The PSB addition significantly affected compost bacterial community composition, and phosphorus factions and phosphorus-cycling genes independently explained 25.4 % and 25.0 % bacterial compositional dissimilarity. Stochastic and homogenizing processes affected more on bacterial community assembly, and rare bacteria potentially mediated organic phosphorus mineralization. These results emphasized that phosphorus fractions, PSB number, phosphorus-cycling gene abundance, and bacterial community composition contributed differently to phosphorus availability. Findings highlight ecological roles of exogenous PSB during chicken manure composting.

Influence of inoculum-to-substrate ratio on biogas enhancement during biochar-assisted co-digestion of food waste and sludge

High accumulation of volatile fatty acids (VFAs) is one of the major concerns during mesophilic anaerobic co-digestion of food waste (FW) and sewage sludge (SS). Therefore, improving the stability of the anaerobic digestion process could surpass quick acidification while accelerating methanogenesis. In this study, the suitability of biochar-assisted co-digestion was evaluated at different inoculum and substrate ratios (I/S ratios: 0.1, 0.3, 0.6, and 0.9). The maximum methane yield of 256.85 mL/gVSadd was observed at an I/S ratio of 0.6. The results indicated fast volatile solid removal (∼ 47.17% to 73%) and a critical role of biochar addition in alleviating the underlying inhibitions. Substantial changes in the microbial community composition including Methanosata, Methanobrevibacter, and Methanosarcina were also observed which predominated and stabilised the methanogenesis process at higher I/S ratios. These results emphasised that the anaerobic co-digestion of FW/sludge is a promising approach, wherein the biochar amendment at different I/S ratios should be well maintained to avoid inhibitions from excess microbial VFA acidification of organic waste feedstocks.

Enhanced stability of food waste anaerobic digestion under low inoculum to substrate ratio by using biochar

The influence of biochar on anaerobic digestion (AD) of organic waste have been widely studied. However, the effect of biochar on the mitigation of acidification and subsequently the stimulation of methanogenesis recovery during mono food waste (FW) digestion process under a low inoculum to substrate (I/S) ratio (i.e. a high organic loading) is rarely investigated. In this study, the benefit of biochar with respect to methane production from FW was explored in a mono FW AD system with four different additional amounts of biochar, i.e. 0, 5, 10 and 15 g/L. Results revealed that biochar boosted methane production in AD at a low I/S ratio by 390-530% through stimulating methanogenic activity, improving organics removal and enhancing process stability. The biochar dosage of 10 g/L demonstrated the highest biodegradability of 92.3% and the highest specific methane production of 553.0 mL/g VSremoved among all groups. Without biochar addition, volatile fatty acids (VFAs) accumulated to 20 g/L and the highest total ammonium-N (TAN) was > 1200 mg/L. The suppression of methanogenesis was significantly correlated with VFA and TAN (p < 0.05). Therefore, biochar addition presented a positive effect on VFAs degradation and buffering capacity which could be an effective approach to enhance methane production from FW digestion at a low inoculum to substrate ratio without the fear of system failure.

Revealing the mechanisms of polypyrrole (Ppy) enhancing methane production from anaerobic digestion of waste activated sludge (WAS)

Anaerobic digestion (AD) is a promising method for treating waste activated sludge (WAS), but the low methane yield limits its large-scale application. The addition of conductive nanomaterials has been demonstrated to enhance the activity of AD via promoting the direct interspecies electron transfer (DIET). In this study, novel conductive polypyrrole (Ppy) was prepared to effectively improve the AD performance of WAS. The results showed that the accumulative methane production was enhanced by 27.83% by Ppy, with both acidogenesis and methanogenesis being efficiently accelerated. The microbial community analysis indicated that the abundance of bacteria associated with acidogenesis process was significantly elevated by Ppy. Further investigation by metatranscriptomics revealed that fadE and fadN genes (to express the key enzymes in fatty acid metabolism) were highly expressed in the Ppy-driven AD, suggesting that Ppy promoted electron generation during acid production. For methanogenesis metabolism, genes related to acetate utilization and CO₂ utilization methanogenesis were also up-regulated by Ppy, illustrating that Ppy facilitates the utilization of acetate and electrons by methanogenic archaea, thus potentially promoting the methanogenesis through DIET.