Anaerobic Digestion Research Articles

New Horizons in Micro/Nanoplastic-Induced Oxidative Stress: Overlooked Free Radical Contributions and Microbial Metabolic Dysregulations in Anaerobic Digestion.

Excessive production of reactive oxygen species (ROS) induced by micro/nanoplastics (MPs/NPs) is highly toxic to microbes. However, the mechanisms underlying ROS generation and metabolic regulation within anaerobic guilds remain poorly understood. In this study, we investigated the effects of environmentally relevant levels of polypropylene (PP)-MPs/NPs on oxidative stress and microbial ecology during anaerobic digestion (AD). Electron paramagnetic resonance spectroscopy revealed that PP-MPs/NPs elevated the concentrations of environmentally persistent free radicals (EPFRs) and derived hydroxyl radicals (•OH). EPFRs were identified as the primary contributors to •OH generation, as evidenced by a high Spearman correlation coefficient (r = 0.884, p < 0.001) and free radical-quenching studies. The formation of •OH enhanced ROS production by 86.2-100.9%, resulting in decreased cellular viability and methane production (by 37.5-50.5%) at 100 mg/g TS PP-MPs/NPs. Genome-centric metagenomic and metatranscriptomic analyses suggested that PP-MPs/NPs induced the reassembly of community structures, re-evolution of functional traits, and remodeling of interspecies interactions. Specifically, PP-MPs/NPs induced a shift in methanogen consortia from hydrogenotrophic Methanofollis sp. to acetoclastic and hydrogenotrophic Methanothrix soehngenii, primarily because of the latter's diverse ingestion patterns, electron bifurcation complexes, and ROS-scavenging abilities. Downregulation of genes associated with antioxidative defense systems (i.e., sodN, katA, and osmC) and ROS-driven redox signal transduction pathways (c-di-AMP and phosphorylation signaling pathways) provided insights into the mechanisms underlying ROS-induced microbial metabolic dysregulation. Our findings enhance the understanding of microbial ecological and metabolic traits under MPs/NPs stressors, facilitating the control of MPs/NPs toxicity and the stabilization of AD processes.

Environment, Soil, and Digestate Interaction of Maize Silage and Biogas Production

In this study are presented the possibilities of using maize silage for biogas production. An experiment with maize silage took place over three years (2016–2018) in two localities, Ilandža, Alibunar municipality (L1—Locality 1) and Dolovo (L2—Locality 2), Serbia, and using two variants: a control with no digestate (C) and a variant with digestate, which was organic manure from biogas facilities (AD). In the AD variant, 50 t ha−1 of digestate was introduced into the soil just before sowing the maize. The following traits were examined: plant height (PH), biomass yield (BMY), biogas yield (BGY), and methane yield (MY). The effects of the studied factors (year, fertilization, and locality) on the biogas yield were significant (p &lt; 0.5). The most favorable year for biogas production was 2016 (207.95 m3 ha−1), while the highest values of maize plant height, biomass, and methane yield were recorded in 2018 (2.48 m, 51.15 t ha−1 dry matter, and 258.25 m3 ha−1). The digestate exerted a significant influence (p &lt; 0.5) on the values of all the tested maize parameters in all three experimental years. The biomass yield was positively associated with the plant height, biogas, and methane yield (r = 0.62 *; r = 0.70 *; r = 0.81 **) and positively but nonsignificantly associated with temperature (r = 0.42) and precipitation (r = 0.12). The application of the digestate before sowing improves the anaerobic digestion of maize silage and biogas production.

Interval Analysis of Volatile Solid (VS) and Biochemical Oxygen Demand (BOD) in Batch Anaerobic Fermentation of Selected Agrowaste

This study investigated the degradation of volatile solids (VS) and biochemical oxygen demand (BOD) during the anaerobic digestion of selected agro-waste, specifically cassava peels, maize husks, pig slurry, and the composite. The objective was to evaluate the biodegradability of these feedstocks, in line with their potential for biogas production. The Research was conducted at the National Centre for Energy Research and Development (NCERD) laboratory, University of Nigeria Nsukka (UNN). Batch reactors were used to conduct experiments over a period of 45 days. The digesters were operated under controlled conditions, and samples were analyzed at three (3) different points: 24 hours, peak of gas production (day 30), and at the end of the digestion (day 45). The results demonstrated that all the substrates exhibited a reduction in VS and BOD in the course of digestion. However, the most significant reduction in VS and BOD occurred around the peak of gas production. This entails that this period is critical for microbial activity and biogas yield. Corn husks had high initial VS and BOD values of 4.73±0.22 % and 92.80±3.26 mg/L respectively. However, that did not translate to higher biogas production compared with the composite that had initial values of 4.34±0.19 % and 88.00±5.50 mg/L respectively. By the end of the digestion, an average VS reduction of 65% and a BOD reduction of 70% were observed across all substrates, indicating a substantial degradation of organic material. Biogas production was better in the composite (72.580±4.53 %), an indication that higher VS and BOD do not automatically translate to higher gas yield. Thus, factors like multiple substrate interaction affect gas yield and process efficiency.

A life cycle assessment of cover crop ensiling and co-digestion with cattle manure

AbstractCover crops offer a potential biogas feedstock, and to enable continuous operation of the biogas plant, ensiling can be used for biomass preservation. The aim of the present study was to assess the environmental impacts of biogas production at an organic dairy farm for two modelled scenarios: (1) harvesting and ensiling of cover crops and cereal straw and co-digestion with cattle manure, or (2) mono-digestion of cattle manure and direct cover crop soil incorporation. The biogas scenarios were modelled in a consequential LCA in relation to a common baseline without anaerobic digestion, with cover crop soil incorporation and field application of raw cattle manure. Biogas scenarios resulted in decreased global warming impacts of (1) 250 and (2) 120 Mg CO2eq for a 1000-hectare example dairy farm due to substitution of natural gas. However, emissions from ensiling, biogas plant operation, and agronomic effects increased other environmental impacts. Agronomic modelling with the Daisy model showed a crop yield increase of 0.11 Mg DM ha−1 year−1 on a sandy loam soil with cover crop digestion, but also an increase in N leaching of 38% and a decrease in soil C stocks of 8.1 Mg C ha−1 over 100 years relative to the reference. Emissions of VOCs and NOx during ensiling increased ozone formation and negative impacts on human health and ecosystems, although further research is needed to better understand these emissions. In conclusion, this modelling study shows that greenhouse gas emissions can be reduced by using ensiled cover crops for co-digestion with manure when biogas is used to substitute fossil gas, although trade-offs with other environmental categories must be considered.

Opportunities for biogas production from algal biomass

Energy security is a critical global challenge in the transition to sustainable development. Anaerobic digestion (AD) offers a promising renewable energy solution that mitigates environmental impacts. Algae, as biomass feedstock, have shown significant potential for bioenergy production; however, their complex chemical composition poses challenges to the efficiency of the AD process. To address these limitations, various pretreatment methods have been applied to enhance biogas production. In this study, we performed a comprehensive meta‐analysis to evaluate the effects of different pretreatments on methane (CH₄) yields from both microalgae and macroalgae. Our results demonstrate that biological, physical, and combined chemical–physical pretreatments significantly improve CH₄ production in microalgae, with increases of up to 141%, 125%, and 151%, respectively. For macroalgae, physical pretreatments were the most effective, leading to a 129% increase in CH₄ yield. We also estimate that utilizing just 10% of the global algal biomass production (3.6 Mt) could generate over 5.5 TWh y−1 of energy. This potential could be doubled with the application of appropriate pretreatment strategies. These findings highlight the role of algae in advancing renewable energy production and contribute to the growing body of knowledge on optimizing AD processes for cleaner energy generation.

Coal-straw co-digestion-induced biogenic methane production: perspectives on microbial communities and associated metabolic pathways

This study assessed the impacts of wheat straw as a cosubstrate on coal biocoverion into methane and the associated mechanism within methane metabolic pathways. Co-digestion of coal with varying wheat straw concentrations resulted in a remarkable (1246.05%) increase in methane yield compared to that of the control (CK). Moreover, microbial analysis revealed a uniform distribution of Methanosarcinaceae (51.14%) and Methanobacteriaceae (39.90%) in the co-digestion of coal and wheat straw (CWS1) at a ratio of 3:1 (w/w) compared to other treatments such as coal and wheat straw (CWS2) at a ratio of 3:0.5. In addition, Hungatieclostridiaceae and Rhodobacteriaceae were abundant in both co-digesters, whereas the bacterial communities in the CK group were significantly different and more abundant than those in the Peptostreptococcaceae and Enterobacteriaceae groups. The key enzymes related to methanogenic metabolic pathways, including EC: 1.2.99.5 and EC: 2.1.1.86 (facilitating the conversion of CO2 into methane), and EC:1.12.98.1 exhibited significant abundance within CWS1. Aromatic compounds such as 4-(2-chloroanilino)-4-oxobutanoic acid and phthalic acid were substantially more abundant in CWS1 and CWS2 than in CK, indicating the increased bioavailability of coal to microbial activities. This novel approach demonstrates that wheat straw co-digestion with coal during anaerobic digestion modulates microbial communities and their metabolic pathways to enhance methane production from complex substrates such as coal.

The kinetics of a two-stage completely stirred tank reactor for treating tofu wastewater by anaerobic digestion

ABSTRACT Tofu wastewater is an inevitable by-product of the bean curd production process. In this study, we used a two-stage continuous stirred tank reactor (CSTR) to treat tofu wastewater. The kinetic analysis of two CSTR reactors was conducted utilizing the material accounting model, microbial accounting model, and first-order kinetic model. The results demonstrated that the methane production coefficient of reactor 1 was 0.00573 LCH4·mg−1 greater than that of reactor 2, which is 0.00533 L CH4·mg−1. The first-order kinetic constants of reactor 1 were poorer than those of reactor 2, and their values are 0.15700 d−1 and 0.11574 d−1, respectively. The microbial yield coefficients of reactors 1 and 2 were 0.009648 and 0.060704, respectively. Under different organic loading rates, the average microbial concentrations were 25.24000 and 2.706667 mg·L−1 for reactors 1 and 2, respectively. The operational performance of the two-stage CSTR anaerobic digestion system was evaluated in order to design a novel and unique method as well as provide a new perspective for enhancing the performance of anaerobic digestion.

Hydrogen production and elemental migration during supercritical water gasification of food waste digestate

The rapid growth of food waste (FW) is a huge challenge on a global scale, and anaerobic digestion is one of the most commonly used methods to deal with food waste, and the increasing amount of food waste digestate (FWD) produced by anaerobic digestion also poses a huge challenge to waste management. This paper explores supercritical water gasification (SCWG) as a valuable and innovative strategy for the conversion of FWD into H2-rich syngas. The research focuses on analyzing the effects of reaction temperature and residence time on syngas production, gas composition, element migration (C, N and P) during the SCWG process. Experimental results show that as the reaction temperature increases from 400 °C to 500 °C, the total syngas yield increases significantly, from 2.4 mol/kg to 9.7 mol/kg, especially the yields of H2, CO2, and CH4. As the reaction temperature increases and the residence time increases, the migration of carbon from the solid and liquid phases to the gas phase accelerates with increasing temperature and residence time, resulting in a higher proportion of carbon in the gas phase. In terms of liquid phase composition, nitrogenous compounds are significantly converted into ammonium (NH4+-N) at higher temperatures. In addition, the organic phosphorus is observed transferring into inorganic phosphorus, which are mainly apatite. This study explores the scalability of SCWG and its potential for the production of clean fuels, thereby contributing to the sustainable management of FWD.

Enrichment of Methanothrix species via riboflavin-loaded granular activated carbon in anaerobic digestion of high-concentration brewery wastewater amidst continuous inoculation of Methanosarcina barkeri

Effective treatment of high-concentration brewery wastewater through anaerobic digestion (AD) has always been a challenging issue. Enhancing direct interspecies electron transfer (DIET) was demonstrated to increase methane production during AD under high organic loading rate (OLR). Herein, the feasibility of enhancing DIET with the addition of riboflavin-loaded granular activated carbon (RF-GAC) as well as co-addition with Methanosarcina barkeri (Rf-GAC+M.barkeri) was investigated (M.barkeri is well-known to be capable of DIET with electroactive bacteria). During the whole process, the Rf-GAC and the Rf-GAC+M.barkeri group both achieved average COD removal rates above 97 %, which was 14 % higher than that of the control. The average methane production in the Rf-GAC group and the Rf-GAC+M.barkeri group respectively reached 0.334 ± 0.02 L(stp)/g COD and 0.345 ± 0.02 L(stp)/g COD, 1.35 and 1.39 times higher than the 0.247 ± 0.03 L(stp)/g COD reached by the control. The control reactor deteriorated at an OLR of 12 kg COD/(m3·d), whereas the Rf-GAC and the Rf-GAC+M.barkeri group maintained stable as the OLR reached as high as 17.5 kg COD/(m3·d) and the volatile fatty acids concentration was consistently below 10 mM. The RF-GAC performed better than Rf-GAC+M.barkeri in enriching Methanothrix, whose relative abundance was 60.6 % in the former group. Metabolic pathway analysis revealed the addition of RF-GAC upregulated genes related to DIET in Methanothrix species, including hdrA and fpoD. Furthermore, Methanothrix remained the dominant archaea even continuously inoculating pure strains of M.barkeri during the entire operational period. Pure culture experiments proved that GAC inhibited M.barkeri growth. The results of this study can be optimized for practical application of AD treating high-concentration brewery wastewater.

Residual ciprofloxacin in chicken manure inhibits methane production in an anaerobic digestion system

Anaerobic digestion (AD) is commonly used to dispose of laying hen manure. However, veterinary antibiotic residues present in chicken manure may affect the AD process. Here, the effects of three types of veterinary antibiotics commonly used in laying hen breeding on AD were explored. Manures containing antibiotics at two different concentrations were continuously added during AD for 5 days: amoxicillin (HAMX: 145.06 mg/kg, LAMX: 57.88 mg/kg), doxycycline (HDOC: 183.61 mg/kg, LDOC: 98.00 mg/kg), and ciprofloxacin (HCIP: 96.34 mg/kg, LCIP: 40.43 mg/kg). Compared with a control with no veterinary antibiotics, the amoxicillin and doxycycline groups presented no significant effects on biogas production, methane production, VFA concentration, acetic acid concentration or the pH of the AD system (P > 0.05). However, compared with the control, the ciprofloxacin groups presented significantly inhibited biogas and methane production during AD (P < 0.05), and the HCIP and LCIP groups presented significantly decreased biogas (47.82% and 45.37%, respectively) and methane (58.24% and 52.55%, respectively) production (P < 0.05). Moreover, the VFA and acetic acid concentrations of the ciprofloxacin groups were significantly higher than those of control during the entire AD period (P < 0.05), and the pH value at the withdrawal stage was significantly lower than that of the control group (P < 0.01), with no significant difference between the HCIP and LCIP groups (P > 0.05). Our results suggest that ciprofloxacin causes VFA and acetic acid accumulation in AD systems, thereby reducing the pH of the systems and inhibiting methanogen growth, ultimately reducing methane production in the AD systems. These findings contribute to a deeper understanding of the impact of ciprofloxacin on methane production in AD systems and offer some considerations for the application of AD systems.

Integrated urban wastewater management through on-site generation and application of ferrous carbonate

Integrated urban water management is an increasingly popular concept that cost-effectively maximizes system-wide performance by holistically considering all aspects of water and wastewater sectors. An innovative technology enabling production of high-quality bioenergy and an iron salt, ferrous carbonate (FeCO3), represents a significant opportunity for integrated urban water management. This study experimentally evaluates the effect of in-sewer FeCO3 dosing on the performance of sewers and the downstream wastewater treatment plants. Two continuous-flow laboratory-scale urban wastewater systems, each consisting of sewer reactors, a sequencing batch wastewater treatment reactor, and an anaerobic digester, were operated in parallel. After establishing comparable performance, one served as the control without any chemical dosing, while the other received a dosing of 10mgFe/L of FeCO3 in its sewer reactors. Compared to the control, the FeCO3-dosed experimental system reduced dissolved sulfide concentrations by 32.2±3.3% (at 0.58±0.05mgS/mgFe, or 1.0molFe/molS) in sewer reactors, decreased phosphate concentrations by 38.3%±3.2% (at 0.37±0.04mgP/mgFe, or 1.5molFe/molP) in sequencing batch reactors, and lowered dissolved sulfide concentrations by 72.0±4.2% (18.9±2.4mgS/L) in the anaerobic sludge digester. Iron accumulated in the sludge and improved sludge settleability by 33.9±5.5% and enhanced dewaterability of anaerobically digested sludge by 15.9±2.0%. The findings indicate multiple benefits from the integrated use of FeCO3, potentially being as a substitute for the currently used iron salts in urban wastewater systems.

Vacuum evaporation coupled with anaerobic digestion for process intensification and ammonia recovery: Model development, validation and scenario analysis

A mathematical model for vacuum evaporation process was developed, which was experimentally validated at different initial pHs and temperatures for ammonia removal from anaerobically digested sludge. Six scenarios were evaluated by combining vacuum evaporation process with anaerobic digestion using anaerobic digestion model 1. These scenarios included a control, a pretreatment by vacuum evaporation, a post-treatment by vacuum evaporation at pH 9, a post-treatment by conventional evaporation (100 °C), an intensification with vacuum-concentrated recycled digestate back to the digester, and a second intensification at pH 9. Results indicated that using the evaporator as post-treatment at pH 9 or for intensification at pH 9 were the most favorable options, recovering more than 76 % of the nitrogen present in influent sludge with no negative effect on methane production. An economic analysis showed that the intensification at pH 9 was cost-neutral, significantly higher than the net present value of the control scenario (–22 M$).

Investigating role of corn stover biochar supplementation on continuous pilot scale anaerobic digestion: Performance and microbial community dynamics

Biochar supplementation could facilitate microbial colonization and increase biogas and methane yield in anaerobic digestion (AD). This study investigated the impact of corn stover biochar (CSB) augmentation on the continuous pilot scale AD of rice straw. The CSB supplementation exhibited the daily average specific biogas and methane yield of 368.6 L/kg volatile solids (VS) and 230 L/kg VS, which were 35 % and 37 % higher than the control. Principle component analysis indicated that the VS reduction positively correlated with the daily average specific biogas and methane yield. Metagenomic analysis revealed that the CSB supplementation facilitated microbial colonization with the enrichment of unclassified genera from families Planococcaceae, Clostridiaceae, and Ruminococcaceae, and genus Clostridium and methanogenic archaea (Methanosarcina and Methanobacterium). The co-occurrence network revealed a notable shift in microbial interactions following the supplementation of CSB as an additive in AD. These results offer insights for enhancing the performance of the AD using CSB.

Improved volatile fatty acid production in anaerobic digestion via simultaneous temperature regulation and persulfate activation by biochar: Chemical and biological response mechanisms

Increasing volatile fatty acid (VFA) production via persulfate activation (i.e., chemical effect) in anaerobic digestion (AD) is an emerging resource utilization method. However, the reaction mechanisms responsible for improving VFA production in AD via simultaneous temperature regulation and persulfate activation by biochar remain unclear. In this study, three PB15 treatment systems of low temperature (15 °C), medium temperature (35 °C) and high temperature (55 °C) were set to explore the relationship between VFAs production and treatment temperature and the influence of temperature on the reaction mechanism. The results show that the improvement of hydrolysis and acidification efficiency of the system in the medium temperature system is the highest. The VFA yield and acid production rate in the treatment group at 35°C were 2.49 and 5.22 times higher than those in the control group, respectively. The chemical effect effectively initiated the anaerobic acid production process and maintained the dominant role of the biological effect. The activity of persulfate is too low at low temperature, and its decomposition is too fast at high temperature. Plenty of free radicals lead to enhanced oxidation of the system, which may kill the fermentation bacteria. The NCM model indicates that microbial stability is reduced in high temperature systems. The SEM model showed that temperature change mainly affected substrate degradation by hydrolytic bacteria and indirectly affected acid production by acid-producing bacteria. This study provides a new strategy for realizing pollutant recycling and increasing VFAs production in cold area.

Improved Anaerobic Digestion of Food Waste under Ammonia stress by Side-Stream Hydrogen Domestication

High ammonia concentration inhibits archaea's activity, causing the accumulation of H2 and acetate, which suppresses methane production in anaerobic digestion (AD). The study aimed to enhance microbial hydrogen metabolism through a side-stream hydrogen domestication (SHD) strategy, which involves applying hydrogen stimulation to a portion of the sludge separately. SHD maintained a stable methane yield of 407.5 mL/g VS at a high total ammonia nitrogen (TAN) concentration of 3.1 g/L. In contrast, the control group gradually decreased and stopped methane production at a TAN concentration of 2.3 g/L. Further analysis using enzyme activity assays, flow cytometry, and metagenomics explored the mechanisms underlying ammonia tolerance of SHD-treated group. SHD reshaped the microbial community, enriching homoacetogens and Methanosaeta-dominated methanogenic archaea. Key metabolic pathways including homoacetogenesis, butyrate degradation, propionate degradation, and methane production were enhanced. The activity of related enzymes also increased. Gene abundance in energy-generating pathways, such as glycolysis, was enhanced, ensuring adequate ATP production. Additionally, the high gene abundance of ion transport systems contributed to regulating proton imbalance and supplementing intracellular K+. This study provides important insights and practical guidance for developing novel techniques in the field of anaerobic digestion.

Enhancing effect of conductive materials and rumen microorganisms on the anaerobic digestion performance of a real traditional Chinese medicine wastewater

This work evaluated the effect of different concentrations of conductive materials and rumen microorganisms on the anaerobic digestion (AD) performance of real traditional Chinese medicine (TCM) wastewater. The experiments first determined the optimal organic concentration of 0.25 L/L for the AD system and removed suspended solids from real TCM wastewater to reduce the inhibition effect on methanogenesis. Analysis of the modified Gompertz model showed that the addition of activated carbon, biochar, cow manure, and rumen fluid at doses of 12 g/L, 12 g/L, 12 mL/L, and 125 mL/L, respectively, had the greatest effect on CH4 production, increasing the cumulative CH4 yield by 13.5 %, 10.4 %, 26.8 %, and 32.9 %. Through microbial community and metabolism pathway analyses, the conductive materials promoted direct interspecies electron transfer (DIET) between Clostridium and Methanothrix, and the acetoclastic methanogenic pathway dominated by acetyl-CoA synthase. Rumen microorganisms enhanced AD performance by promoting the growth of hydrogenotrophic methanogens and the abundance of genes dominated by formylmethanofuran dehydrogenase in the hydrogenotrophic methanogenic pathway, illustrating the relationship between microbial community and metabolism pathway. Rumen microorganisms increased CH4 production more than conductive materials in real TCM wastewater. This study helps to better understand the internal mechanisms by which different materials enhance AD performance.

Methylisothiazolinone modulates community assembly and improves syntrophic cooperation via adaptive evolution during sludge anaerobic digestion

Various antimicrobials could pose multifaced threats to anaerobic digestion (AD) of waste-activated sludge (WAS), yet it is still unclear how antimicrobials would affect the metabolic activity and assembly process of anaerobic microbiome. In this study, a typical antimicrobial, methylisothiazolinone (MIT), was introduced in AD and induced an initial lag effect on methane production that was eventually recovered. Although MIT disintegrated WAS to promote the release of bioavailable substrates from extracellular polymeric substances (EPS), it also caused adverse stress to the microbiome. The initial microbial metabolic activities and syntrophic relationships associated with methane generation were distinctly inhibited. Because the self-adaptive systems of two-component systems (e.g., ddpF, and ddpD) and quorum sensing (e.g., cheR, and cydB) were activated to restore the syntrophic interaction among functional anaerobes, metabolic activities were gradually recovered for methane generation. Ultimately, the methane-generation species (e.g., Methanobacterium, and Methanomassiliicoccus) were driven as core species by deterministic processes, while non-functional species (e.g., Nitrospirota) were marginalized by stochastic processes under MIT stress. Overall, this finding indicated the intrinsic drive of evolutionary adaptation, unraveled the community assembly process at phylogenetic level under antimicrobial stress, and gave direction on the assessment and mitigation of external contaminant-related ecological risks in WAS treatment.

Advanced anaerobic digestion by co-immobilization of anaerobic microbes and conductive particles in hydrogel for enhanced methane production performance

Recent research has increasingly focused on the enhancement of anaerobic digestion (AD) through direct interspecies electron transfer (DIET) facilitated by conductive particles (CP). Although this approach can significantly accelerate the AD process, the contact efficiency between CPs and AD microbes is relatively low due to the flow of water in a dispersed condition, leading to possible DIET inefficiency. In this study, a unique approach involving the “co-immobilization” of anaerobic microbes and multi-walled carbon nanotubes (MWCNTs) as CP into a hydrogel matrix was developed to improve the AD process. The advantages of this method include improved contact efficiency between microbes and CPs for enhanced DIET, and increased CP retention within the reactor, thereby omitting the need to compensate for CP washout. The methane production rate for the co-immobilized hydrogel was 2.5-fold and 1.9-fold faster than that of the control (dispersed sludge) and conventional DIET (dispersed sludge with MWCNT addition), respectively. Microbial analysis indicated the enrichment of functional microbes such as Anaerolineacea, Sedimentibacteraceae, Rhodocyclaceae, and Methanothrichaceae, which could be involved in the DIET under co-immobilized conditions. These results demonstrate the potential of the proposed method for realizing an advanced continuous AD process through improved DIET.

Mass flow and ecological risk of heavy metals in anaerobic digestion of food waste

Conversion of food waste (FW) to bioenergy via anaerobic digestion (AD) has become a topic of worldwide research interest. However, the content, distribution, and speciation of heavy metals (HMs) in feedstock and digestate samples throughout the AD process are currently unknown. Especially, the potential ecological risk (PER) of solid digestate with potential of land application was vague. Mass flow of HMs (Cr, Cu, Mn, Ni, Zn, As, Cd, Hg, and Pb) were estimated in a two-stage wet AD of FW plant in Shenzhen, China. Furthermore, the PER of the HMs in solid digestate (SD) was studied. The findings revealed that the variation in the HM contents of the intermediates, as well as the comparatively high HM contents of the SD, were mostly influenced by the total solid contents of the samples, since HMs tend to adhere to the solid matrix. As per mass balance, the Cr and Ni contents of the digestate samples were 4.7 and 5.5 times higher than in the feedstock samples, respectively. The remaining metals were within the range of 82.2126.1%. Furthermore, the enrichment of HMs in SD must be resolved before reclamation. Moreover, the PER of the HMs in SD was 279.6, considered as a considerable risk level, of which Hg and Cd contributed 46.3% and 33.6%, respectively. This research presents a case study for quantitatively assessing the distribution of typical pollutants throughout the AD process, and it offers preliminary data for potential environmental issues caused by digestion product recycling.

Enhancing energy recovery from expired rice wine via microbial electrolysis of supernatant and anaerobic digestion of concentrate after gravity settling

Makgeolli is a traditional Korean rice wine with a short shelf life, making the management of expired makgeolli (EM) challenging. Meanwhile, its organic- and nutrient-rich nature renders EM a promising bioconversion substrate. To explore the potential to enhance energy recovery and profitability from EM, the concentrate (MC) and supernatant (MS) separated from EM by gravity settling were utilized for hydrogen production in microbial electrolysis cells (MECs) and methane production through anaerobic digestion (AD), respectively. MS demonstrated superior electron and hydrogen recovery compared to EM in MECs, with greater enrichment of exoelectrogenic Geobacter. In the AD tests, MC exhibited faster and more stable methane production with higher yield than EM. This separate processing of MS and MC was predicted to produce 18–21% more energy (2,210 MJ/m3 EM) and 6–180% higher revenue (101 USD/m3 EM) than processing whole EM using either MEC or AD alone, warranting further research for optimization.