Enhanced anaerobic digestion of waste activated sludge digestion by the addition of zero valent iron

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.

Iron driven organic carbon capture, pretreatment, recovery and upgrade in wastewater: Process technologies, mechanisms, and implications

Wastewater treatment plants face significant challenges in transitioning from energy-intensive systems to carbon-neutral, energy-saving systems, and a large amount of chemical energy in wastewater remains untapped. Iron is widely used in modern wastewater treatment. Research shows that leveraging the coupled redox relationship of iron and carbon can redirect this energy (in the form of carbon) towards resource utilization. Therefore, re-examining the application of iron in existing wastewater carbon processes is particularly important. In this review, we investigate the latest research progress on iron for wastewater carbon flow restructuring. During the iron-based chemically enhanced primary treatment (CEPT) process, organic carbon is captured into sludge and its bioavailability is enhanced through iron-based advanced oxidation processes (AOP) pretreatment, further being recovered or upgraded to value-added products in anaerobic biological processes. We discuss the roles and mechanisms of iron in CEPT, AOP, anaerobic biological processes, and biorefining in driving organic carbon conversion. The dosage of iron, as a critical parameter, significantly affects the recovery and utilization of sludge carbon resources, particularly by promoting effective electron transfer. We propose a pathway for beneficial conversion of wastewater organic carbon driven by iron and analyze the benefits of the main products in detail. Through this review, we hope to provide new insights into the application of iron chemicals and current wastewater treatment models.

Perfluorooctanoic acid effect and microbial mechanism to methane production in anaerobic digestion

Perfluorooctanoic acid (PFOA) as emerging pollutants was largely produced and stable in nature environment. Its fate and effect to the wasted sludge digestion process and corresponding microbial mechanism was rarely reported. This study investigated the different dose of PFOA to the wasted sludge digestion process, where the methane yield and microbial mechanism was illustrated. The PFOA added before digestion were 0-10000 μg/L, no significant variation in daily and accumulated methane production between each group. The 9th day methane yield was significantly higher than other days (p < 0.05). The soluble protein was significantly decreased after 76 days digestion (p < 0.001). The total PFOA in sludge (R2 = 0.8817) and liquid (R2 = 0.9083) phase after digestion was exponentially correlated with PFOA dosed. The PFOA in liquid phase was occupied 54.10 ± 18.38% of the total PFOA in all reactors. The dewatering rate was keep decreasing with the increase of PFOA added (R2 = 0.7748, p < 0.001). The mcrA abundance was significantly correlated with the pH value and organic matter concentration in the reactors. Chloroflexi was the predominant phyla, Aminicenantales, Bellilinea and Candidatus_Cloacimonas were predominant genera in all reactors. Candidatus_Methanofastidiosum and Methanolinea were predominant archaea in all reactors. The function prediction by FAPROTAX and Tax4fun implied that various PFOA dosage resulted in significant function variation. The fermentation and anaerobic chemoheterotrophy function were improved with the PFOA dose. Co-occurrence network implied the potent cooperation among the organic matter degradation and methanogenic microbe in the digestion system. PFOA has little impact to the methane generation while affect the microbe function significantly, its remaining in the digested sludge should be concerned to reduce its potential environmental risks.

A holistic valorization of treasured waste activated sludge for directional high-valued products recovery: Routes, key technologies and challenges

Global energy shortages and environmental crises underscore the imperative for a circular economy to tackle resource scarcity and waste management. The circular economy model encourages the recovery and reuse of valuable materials, reducing reliance on finite natural resources and lessening the environmental impact of waste disposal. Among urban organic solid wastes, waste activated sludge (WAS) emerges as a potent reservoir of untapped resources (including various inorganic and organic ones) offering significant potential for recovery. This review delves into a comprehensive analysis of directional valorization of WAS to recover high-valued products, including the inorganic matters (i.e. phosphorus, ammonia nitrogen, and heavy metals), organic resources (i.e. extracellular polymers like alginate and protein, volatile fatty acid, methane, hydrogen, and plant growth hormones) and reutilization of WAS residues for the preparation of adsorbent materials - the biochar. Moreover, the main recovery methodologies associated influencing parameters, product application, and attendant challenges for those diverse recovered resources are unveiled. Future research are encouraged to prioritize the development of integrated multi-resource recovery approaches, the establishment of regulatory frameworks to support resource recovery and product utilization, and the systematic evaluation of disposal strategies to foster a more sustainable and resource-efficient future. This work illuminates avenues for sustainable WAS management with high-valued resource recovery towards circular economy.

Removal of organic matter from food wastewater using anaerobic digestion at low temperatures enhanced by exogenous signaling molecule N-hexanoyl-homoserine lactone enhancement: Insight to extracellular polymeric substances and key functional genes

This experiment aimed to study the effects of adding the exogenous signaling molecule N-hexanoyl-homoserine lactone (C6-HSL) on the anaerobic digestion of food wastewater at low temperature (15 °C). Daily addition of 0.4 μmol C6-HSL increased the average chemical oxygen demand removal from 45.98% to 94.92%, while intermittent addition (adding 2 μmol C6-HSL every five days) increased it from 45.98% to 72.44%. These two modes of C6-HSL addition increased protease and acetate kinase activity by 47.99%/8.04% and 123.26%/127.91% respectively, and increased coenzyme F420 concentrations by 15.79% and 63.16%, respectively. The regulation of loosely bound extracellular polymeric substances synthesis was influenced by C6-HSL, which increased protein and polysaccharide content in sludge. The relative abundance of Firmicutes and Bacteroidetes increased following addition of C6-HSL. After continuous addition of C6-HSL, the relative abundance of related functional genes such as amy, apgM, aceE, and accC increased, indicating that methanogens obtained sufficient substrate. The abundance of glycolysis-related functional genes such as glk, pfk, pgi, tpiA, gap, pgk, gpmA, eno, and pyk increased after the addition of C6-HSL, ensuring the efficient transformation and absorption of organic matter by anaerobic sludge at low temperatures. This study provides new comprehensive insights into the mechanism behind the enhancement of food wastewater anaerobic digestion by C6-HSL at low temperature.

Enhanced Cr(VI) removal and stabilization from bioleached wastewater by zero-valent iron coupled with hetero and autotrophic bacteria

Zero-valent iron (Fe0) usually suffers from organic acid complexation and ferrochrome layer passivation in Cr(VI) removal from bioleached wastewater of Cr slag. In this work, a synergetic system combined Fe0 and mixed hetero/autotrophic bacteria was established to reduce and stabilize Cr(VI) from bioleached wastewater. Due to bacterial consumption of organic acid and hydrogen, severe iron corrosion and structured-Fe(II) mineral generation (e.g., magnetite and green rust) occurred on biotic Fe0 surface in terms of solid-phase characterization, which was crucial for Cr(VI) adsorption and reduction. Therefore, compared with the abiotic Fe0 system, this integrated system exhibited a 6.1-fold increase in Cr(VI) removal, with heterotrophic reduction contributing 3.4-fold and abiotic part promoted by hydrogen-autotrophic bacteria enhancing 2.7-fold. After reaction, the Cr valence distribution and X-ray photoelectron spectroscopy indicated that most Cr(VI) was converted into immobilized products such as FexCr1-x(OH)3, Cr2O3, and FeCr2O4 by biotic Fe0. Reoxidation experiment revealed that these products exhibited superior stability to the immobilized products generated by Fe0 or bacteria. Additionally, organic acid concentration and Fe0 dosage showed significantly positive correlation with Cr(VI) removal within the range of biological adaptation, which emphasized that heterotrophic and autotrophic bacteria acted essential roles in Cr(VI) removal. This work highlighted the enhanced effect of heterotrophic and autotrophic activities on Cr(VI) reduction and stabilization by Fe0 and offered a promising approach for bioleached wastewater treatment.

Feedback of chain elongation microorganisms on iron-based conductive materials: Enhanced microbial functions and biotoxicity adaptation mechanisms

Despite the increased research efforts aimed at understanding iron-based conductive materials (CMs) for facilitating chain elongation (CE) to produce medium chain fatty acids (MCFAs), the impact of these materials on microbial community functions and the adaptation mechanisms to their biotoxicity remain unclear. This study found that the supply of zero-valent iron (ZVI) and magnetite enhanced the MCFAs carbon-flow distribution by 26 % and 52 %, respectively. Metagenomic analysis revealed the upregulation of fatty acid metabolism, pyruvate metabolism and ABC transporters with ZVI and magnetite. The predominant functional microorganisms were Massilibacterium and Tidjanibacter with ZVI, and were Petrimonas and Candidatus_Microthrix with magnetite. Furthermore, it was demonstrated that CE microorganisms respond and adapt to the biotoxicity of iron-based CMs by adjusting Two-component system and Quorum sensing for the first time. In summary, this study provided a new deep-insight on the feedback mechanisms of CE microorganisms on iron-based CMs.

Deep insights into the roles and microbial ecological mechanisms behind waste activated sludge digestion triggered by persulfate oxidation activated through multiple modes

Persulfate oxidation (PS) is widely employed as a promising alternative for waste activated sludge pretreatment due to the capability of generating free radicals. The product differences and microbiological mechanisms by which PS activation triggers WAS digestion through multiple modes need to be further investigated. This study comprehensively investigated the effects of persulfate oxidation activated through multiple modes, i.e., ferrous, zero-valent iron (ZVI), ultraviolet (UV) and heat, on the performance of sludge digestion. Results showed that PS_ZVI significantly accelerated the methane production rate to 12.02 mL/g VSS. By contrast, PS_Heat promoted the sludge acidification and gained the maximum short-chain fatty acids (SCFAs) yield (277.11 ± 7.81 mg COD/g VSS), which was 3.41-fold compared to that in PS_ZVI. Moreover, ferrous and ZVI activated PS achieved the oriented conversion of acetate, the proportions of which took 73% and 78%, respectively. MiSeq sequencing results revealed that PS_Heat and PS_UV evidently enriched anaerobic fermentation bacteria (AFB) (i.e., Macellibacteroides and Clostridium XlVa). However, PS_Ferrous and PS_ZVI facilitated the enrichment of Woesearchaeota and methanogens. Furthermore, molecular ecological network and mantel test revealed the intrinsic interactions among the multiple functional microbes and environmental variables. The homo-acetogens and sulfate-reducing bacterial had potential cooperative and symbiotic relationships with AFB, while the nitrate-reducing bacteria displayed distinguishing ecological niches. Suitable activation modes for PS pretreatments resulted in an upregulation of genes expression responsible for digestion. This study established a scientific foundation for the application of sulfate radical-based oxidation on energy or high value-added chemicals recovery from waste residues.

Unveiling the unique role of iron in the metabolism of methanogens: A review

Methanogens are the main participants in the carbon cycle, catalyzing five methanogenic pathways. Methanogens utilize different iron-containing functional enzymes in different methanogenic processes. Iron is a vital element in methanogens, which can serve as a carrier or reactant in electron transfer. Therefore, iron plays an important role in the growth and metabolism of methanogens. In this paper, we cast light on the types and functions of iron-containing functional enzymes involved in different methanogenic pathways, and the roles iron play in energy/substance metabolism of methanogenesis. Furthermore, this review provides certain guiding significance for lowering CH4 emissions, boosting the carbon sink capacity of ecosystems and promoting green and low-carbon development in the future.

Fulvic acid-mediated efficient anaerobic digestion for kitchen wastewater: Electrochemical and biochemical mechanisms

Fulvic acid, prevalent in humus derived from the anaerobic digestion of kitchen wastewater, is crucial in organic matter transformation. However, its effects and underlying mechanisms remain unclear. In this study, the fate of anaerobic digestion of artificial and kitchen wastewater with different fulvic acid contents was investigated. The results showed that 125 mg/L fulvic acid resulted in a 64.02 and 51.72 % increase in methane production in synthetic and kitchen wastewater, respectively. Fulvic acid acted as an electron mediator and increased substrate oxidation by boosting NAD and ATP levels, thereby increasing microbial metabolic rates and ensuring an adequate substrate for methane generation. Isotope analysis suggested that fulvic acid boosts the conversion of volatile fatty acids to methane via the interspecies electron transfer pathway. Gene expression analysis revealed that cytochrome c, FAD, and other electron transport coenzymes were upregulated by fulvic acid, thereby enhancing substrate utilisation and biogas quality. Fulvic acid presented a dual stimulatory and inhibitory effect on anaerobic digestion, with concentrations over 125 mg/L diminishing its positive impact. This dual effect may stem from the properties and concentrations of fulvic acid. This study revealed the effect mechanism of fulvic acid and provided insights into the humus performance in anaerobic digestion.

An overview of the occurrence, impact of process parameters, and the fate of antibiotic resistance genes during anaerobic digestion processes

Antibiotic resistance genes (ARGs) have emerged as a significant global health threat, contributing to fatalities worldwide. Wastewater treatment plants (WWTPs) and livestock farms serve as primary reservoirs for these genes due to the limited efficacy of existing treatment methods and microbial adaptation to environmental stressors. Anaerobic digestion (AD) stands as a prevalent biological treatment for managing sewage sludge and manure in these settings. Given the agricultural utility of AD digestate as biofertilizers, understanding ARGs' fate within AD processes is essential to devise effective mitigation strategies. However, understanding the impact of various factors on ARGs occurrence, dissemination, and fate remains limited. This review article explores various AD treatment parameters and correlates to various resistance mechanisms and hotspots of ARGs in the environment. It further evaluates the dissemination and occurrence of ARGs in AD feedstocks and provides a comprehensive understanding of the fate of ARGs in AD systems. This review explores the influence of key AD parameters such as feedstock properties, pretreatments, additives, and operational strategies on ARGs. Results show that properties such as high solid content and optimum co-digestion ratios can enhance ARG removal, while the presence of heavy metals, microplastics, and antibiotics could elevate ARG abundance. Also, operational enhancements, such as employing two-stage digestion, have shown promise in improving ARG removal. However, certain pretreatment methods, like thermal hydrolysis, may exhibit a rebounding effect on ARG levels. Overall, this review systematically addresses current challenges and offers future perspectives associated with the fate of ARGs in AD systems.

Recent progress and prospects for chain elongation of transforming biomass waste into medium-chain fatty acids

Chain elongation technology utilises microorganisms in anaerobic digestion to transform waste biomass into medium-chain fatty acids that have greater economic value. This innovative technology expands upon traditional anaerobic digestion methods, requiring abundant substrates that serve as electron donors and acceptors, and inoculating microorganisms with chain elongation functions. While this process may result in the production of by-products and elicit competitive responses, toxicity suppression of microorganisms by substrates and products remains a significant obstacle to the industrialisation of chain elongation technology. This study provides a comprehensive overview of existing research on widely employed electron donors and their synthetic reactions, competitive reactions, inoculum selection, toxicity inhibition of substrates and products, and increased chain elongation approaches. Additionally, it presents actionable recommendations for future research and development endeavours in this domain, intending to inspire and guide researchers in advancing the frontiers of chain elongation technology.

Enhancement of hydrogenotrophic methanogenesis for methane production by nano zero-valent iron in soils

Nanoscale zero-valent iron (nZVI) is attracting increasing attention as the most commonly used environmental remediation material. However, given the high surface area and strong reducing capabilities of nZVI, there is a lack of understanding regarding its effects on the complex anaerobic methane production process in flooded soils. To elucidate the mechanism of CH4 production in soil exposed to nZVI, paddy soil was collected and subjected to anaerobic culture under continuous flooding conditions, with various dosages of nZVI applied. The results showed that the introduction of nZVI into anaerobic flooded rice paddy systems promoted microbial utilization of acetate and carbon dioxide as carbon sources for methane production, ultimately leading to increased methane production. Following the introduction of nZVI into the soil, there was a rapid increase in hydrogen levels in the headspace, surpassing that of the control group. The hydrogen levels in both the experimental and control groups were depleted by the 29th day of culture. These findings suggest that nZVI exposure facilitates the enrichment of hydrogenotrophic methanogens, providing them with a favorable environment for growth. Additionally, it affected soil physicochemical properties by increasing pH and electrical conductivity. The metagenomic analysis further indicates that under exposure to nZVI, hydrogenotrophic methanogens, particularly Methanobacteriaceae and Methanocellaceae, were enriched. The relative abundance of genes such as mcrA and mcrB associated with methane production was increased. This study provides important theoretical insights into the response of key microbes, functional genes, and methane production pathways to nZVI during anaerobic methane production in rice paddy soils, offering fundamental insights into the long-term fate and risks associated with the introduction of nZVI into soils.

Unveiling the mechanisms of Fe(III)-loaded chitosan composite (CTS-Fe) in enhancing anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) is usually limited by the low generation efficiency of methane. Fe(III)-loaded chitosan composite (CTS-Fe) have been reported to effectively enhanced the digestion of WAS, but its role in promoting anaerobic sludge digestion remains unclear. In present study, the effects of CTS-Fe on the hydrolysis and methanogenesis stages of WAS anaerobic digestion were investigated. The addition of CTS-Fe increased methane production potential by 8%-23% under the tested conditions with the addition of 5-20 g/L CTS-Fe. Besides, the results demonstrate that the addition of CTS-Fe could effectively promote the hydrolysis of WAS, evidenced by lower protein or polysaccharides concentration, higher soluble organic carbon in rector adding CTS-Fe, as well as the increased activity of extracellular hydrolase with higher CTS-Fe concentration. Meanwhile, the enrichment of Clostridia abundance (iron-reducing bacteria (IRBs)) was observed in CTS-Fe adding reactor (8.9%-13.8%), which was higher than that in the control reactor (7.9%). The observation further suggesting the acceleration of hydrolysis through dissimilatory iron reduction (DIR) process, thus providing abundant substrates for methanogenesis. However, the presence of CTS-Fe was inhibited the acetoclastic and hydrogenotrophic methanogenesis process, which could be ascribed to the Fe(III) act as electron acceptor coupled to methane for anaerobic oxidation. Furthermore, coenzyme F420 activity in the CTS-Fe added reactor was 34.9% lower than in the blank, also abundance of microorganisms involved in hydrogenotrophic methanogenesis was decreased. Results from this study could provide theoretical support for the practical applications of CTS-Fe.

Simultaneous removal of antibiotic resistance genes and improved dewatering ability of waste activated sludge by Fe(II)-activated persulfate oxidation

Waste activated sludge properties vary widely with different regions due to the difference in living standards and geographical distribution, making a big challenge to developing a universally effective sludge dewatering technique. The Fe(II)-activated persulfate (S2O82-) oxidation process shows excellent ability to disrupt sludge cells and extracellular polymeric substances (EPS), and release bound water from sludge flocs. In this study, the discrepancies in the physicochemical characteristics of sludge samples from seven representative cities in China (e.g., dewaterability, EPS composition, surface charge, microbial community, relative abundance of antibiotic resistance genes (ARGs), etc.) were investigated, and the role of Fe(II)-S2O82- oxidation in enhancing removal of antibiotic resistance genes and dewatering ability were explored. The results showed significant differences between the EPS distribution and chemical composition of sludge samples due to different treatment processes, effluent sources, and regions. The Fe(II)-S2O82- oxidation pretreatment had a good enhancement of sludge dewatering capacity (up to 76 %). Microbial analysis showed that the microbial community in each sludge varied significantly depending on the types of wastewater, the wastewater treatment processes, and the regions, but Fe(II)-S2O82- oxidation was able to attack and rupture the sludge zoogloea indiscriminately. Genetic analysis further showed that a considerable number of ARGs were detected in all of these sludge samples and that Fe(II)-S2O82- oxidation was effective in removing ARGs by higher than 90 %. The highly active radicals (e.g., SO4-·, ·OH) produced in this process caused drastic damage to sludge microbial cells and DNA stability while liberating the EPS/cell-bound water. Co-occurrence network analysis highlighted a positive correlation between population distribution and ARGs abundance, while variations in microbial communities were linked to regional differences in living standards and level of economic development. Despite these variations, the Fe(II)-S2O82- oxidation consistently achieved excellent performance in both ARGs removal and sludge dewatering. The significant modularity of associations between different microbial communities also confirms its ability to reduce horizontal gene transfer (HGT) by scavenging microbes.

Mini-review: Nanoparticles for enhanced biogas upgrading

This mini-review is intended to explore the innovative applications of nanoparticles (NPs) in biogas upgrading, emphasizing their capacity to enhance biogas quality. Numerous studies underscore how NPs, when applied during anaerobic digestion, can boost not only the quantity but also the quality of the produced biogas, leading to reduce significantly the concentration of hydrogen sulphide or even to remove it completely. Moreover, NPs are proving to be excellent alternatives as adsorbent materials, achieving up to 400 mgH2S g-1 NPs. In addition, new studies are exploring the application of NPs to increase the efficiency of biological treatments thanks to their unique features. This review also emphasizes the potential benefits and addresses the challenges that need to be overcome for these technologies to reach their full potential, ultimately contributing to the development of a sustainable and environmentally friendly energy landscape.

Abiotic and biotic roles of metals in the anaerobic digestion of sewage sludge: A review

Anaerobic digestion (AD) is a promising technique for sludge treatment and resource recovery. Metals are very important components of sludge and can have substantial effects on its complex nature and microbial activity. However, systematic reviews have not addressed how metals in sludge affect AD and how they can be regulated to improve AD. This paper comprehensively reviews the effects of metals on the AD of sludge from both abiotic and biotic perspectives. First, we introduce the contents and basic characteristics (e.g., chemical forms) of intrinsic metals in sewage sludge. Then, we summarise the main mechanism by which metals influence sludge properties and the methods for removing metals and thus improving AD. Next, we analyze the effects of both intrinsic and exogenous metals on the enzymes and microbial communities involved in anaerobic bioconversion, focusing on the types, critical concentrations and valence states of the metals. Finally, we propose ideas for future research on the roles of metals in the AD of sludge. In summary, this review systematically clarifies the roles of metals in the AD of sludge and provides a reference for improving AD by regulating these metals.

Effect of zero-valent iron (ZVI) and biogas slurry reflux on methane production by anaerobic digestion of waste activated sludge

This study aimed to improve anaerobic digestion (AD) efficiency through the addition of zero-valent iron (ZVI) and biogas slurry. This paper demonstrated that methane production was most effectively promoted at a biogas slurry reflux ratio of 60%. The introduction of ZVI into anaerobic systems does not enhance its bioavailability. However, both biogas slurry reflux and the combination of ZVI with biogas slurry reflux increase the relative abundance of microorganisms involved in the direct interspecific electron transfer (DIET) process. Among them, the dominant microorganisms Methanosaeta, Methanobacterium, Methanobrevibacter, and Methanolinea accounted for over 60% of the total methanogenic archaea. The Tax4Fun function prediction results indicate that biogas slurry reflux and the combination of ZVI with biogas slurry reflux can increase the content of key enzymes in the acetotrophic and hydrotrophic methanogenesis pathways, thereby strengthening these pathways. The corrosion of ZVI promotes hydrogen production, and the biogas slurry reflux provided additional alkaline and anaerobic microorganisms for the anaerobic system. Their synergistic effect promoted the growth of hydrotrophic methanogens and improved the activities of various enzymes in the hydrolysis and acidification phases, enhanced the system's buffer capacity, and prevented secondary environmental pollution. PRACTITIONER POINTS: Optimal methane production was achieved at a biogas slurry reflux ratio of 60%. Biogas slurry reflux in anaerobic digestion substantially reduced discharge. ZVI addition in combination with biogas slurry reflux facilitates the DIET process.

Enhanced anaerobic digestion of human feces by ferrous hydroxyl complex (FHC): Stress factors alleviation and microbial resistance improvement

Anaerobic digestion (AD) offers a reliable strategy for resource recovery from source-separated human feces (HF), but is limited by a disproportionate carbon/nitrogen (C/N) ratio. Ferrous hydroxyl complex (FHC) was first introduced into the HF-AD system to mediate methanogenesis. Mono-digestion of undiluted HF was inhibited by high levels of volatile fatty acids (VFAs), ammonia, and hydrogen sulfide (H2S). FHC addition at optimum dosage (500-1000 mg/L) increased the cumulative methane (CH4) yield by 22.7%, enhanced the peak value of daily CH4 production by 60.5%, and shortened the lag phase by 24.7%. H2S concentration in biogas was also greatly decreased by FHC via precipitation. FHC mainly facilitated the hydrolysis, acidification, and methanogenesis processes. The production and transformation of VFAs were optimized in the presence of FHC, thus relieving acid stress. FHC elevated the activities of alkaline protease, cellulase, and acetate kinase by 32.3%, 18.2%, and 30.3%, respectively. Microbial analysis revealed that hydrogenotrophic methanogens prevailed in mono-digestion at high HF loading but were weakened after FHC addition. FHC also enriched Methanosarcina, thereby expanding the methanogenesis pathway and improving the resistance to ammonia stress. This work would contribute to improving the methanogenic performance and resource utilization for HF anaerobic digestion.

Hydrogen production promotion and energy saving in anaerobic co-fermentation of heat-treated sludge and food waste

The objective of this paper is to gain insights into the synergistic advantage of anaerobic co-fermentation of heat-treated sludge (HS) with food waste (FW) and heat-treated food waste (HFW) for hydrogen production. The results showed that, compared with raw sludge (RS) mixed with FW (RS-FW), the co-substrate of HS mixed with either FW (HS-FW) or HFW (HS-HFW) effectively promoted hydrogen production, with HS-HFW promoted more than HS-FW. The maximum specific hydrogen production (MSHP) and the maximum hydrogen concentration (MHC) of HS-HFW were 40.53 mL H2/g dry weight and 57.22%, respectively, and 1.21- and 1.45-fold as high as those from HS-FW. The corresponding fermentation was ethanol type for HS-HFW and butyric acid type for HS-FW. The net energy production from RS-FW and HS-FW was both negative, but it was positive (2.57 MJ) from 40% HFW addition to HS-HFW. Anaerobic fermentation was more viable for HS-HFW.

Influence of zero-valent iron nanoparticles on anaerobic digestion of swine manure: effects on methane yield

This study evaluated the effect of zero-valent iron nanoparticles (NZVI) on the anaerobic digestion of swine manure. A wide range of doses of NZVI was evaluated (5, 10, 15, 20, 25, 50, and 100 mgFe°/gVS). The maximum methane yield of 0.4506 L/gVSremoved was obtained with the concentration of 10 mgFe°/gVS representing an increase of 58.99% than the control system with 0.2834 L/gVSremoved, indicating that Fe° improves the methanogenic activity. However, when using doses greater than 20 mgFe°/gVS, there were decreases in the methane yield of 34.4-47.98%. Also, to observe the effect of NZVI in anaerobes was evaluated the activity in the electron transport system (ETS), where the control reactor showed an activity of 31.91 μg INTred/gVS•h, while in reactors with NZVI showed values of 39.48 μg INTred/gVS•h (10 Fe°mg/gVS), observing a stimulation of Fe° in microbial activity. However, the dose of 100 mgFe°/gVS showed the greatest decrease in methane yield (0.1474 L/gVSremoved) and a reduction in ETS was observed by 8.5% compared to the control. The effect on the composition of the volatile fatty acids was observed, where the control system obtained a maximum production of acetic acid of 639 mg/L, which was exceeded with the dose of 10 mg Fe°/gVS by 215% and a decrease of 41.15% with the inhibitory concentration of 100 mg Fe°/gVS. As a result, higher doses of NZVI affect the metabolic activity of anaerobes as well as the acetoclastic pathway causing a decrease in the methane production.

Characterisation of synthesised trimetallic nanoparticles and its influence on anaerobic digestion of palm oil mill effluent

The augmentation of biogas production can be achieved by incorporating metallic nanoparticles as additives within anaerobic digestion. The objective of this current study is to examine the synthesis of Fe-Ni-Zn and Fe-Co-Zn trimetallic nanoparticles using the co-precipitation technique and assess its impact on anaerobic digestion using palm oil mill effluent (POME) as carbon source. The structural morphology and size of the synthesised trimetallic nanoparticles were analysed using a range of characterization techniques, such as X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), and Energy-dispersive X-ray spectroscopy (EDX) . The average size of Fe-Ni-Zn and Fe-Co-Zn were 19-25.5 nm and 19.1-30.5 nm respectively. Further, investigation focused on examining the diverse concentrations of trimetallic nanoparticles, ranging from 0 to 50 mgL-1. The biogas production increased by 55.55% and 60.11% with Fe-Ni-Zn and Fe-Co-Zn trimetallic nanoparticles at 40 mgL-1 and 20 mgL-1, respectively. Moreover, the lowest biogas of 11.11% and 38.11% were found with 10 mgL-1 of Fe-Ni-Zn and Fe-Co-Zn trimetallic nanoparticles. The findings of this study indicated that the trimetallic nanoparticles exhibited interactions with anaerobes, thereby enhancing the degradation process of palm oil mill effluent (POME) and biogas production. The study underscores the potential efficacy of trimetallic nanoparticles as a viable supplement for the promotion of sustainable biogas generation.

Effects of hydrogen peroxide and calcium hypochlorite on chemical oxygen demand solubilization and disintegration of waste activated sludge by electro-chemical pretreatment

ABSTRACTThe combination of electrochemical (EC) pretreatment with hydrogen peroxide (HP) and calcium hypochlorite (CHC) was investigated in this study for their effect on soluble chemical oxygen demand (CODs) and disintegration degree (DD) of waste activated sludge (WAS). For this aim, response surface methodology (RSM) and Box-Behnken design (BBD) were applied the determination of the optimum operational conditions. Operational conditions were varied between 0.2 and 2.0 mmol/g SS for HP and CHC dosages, 1-5 A for the applied current, 2-10 for the initial pH, and 15-45 min for the treatment time. Obtained results for each treatment were accurate and significant with correlation coefficients (R2) of 0.8639% and 0.9189% for EC combined with HP pretreatment and EC combined with CHC pretreatment, respectively. According to the obtained results, CODs increased in comparison to the raw sludge (168 mg/L) noting that CODs for EC - CHC (1155 ± 21 mg/L) was higher than EC - HP (811.5 ± 15 mg/L) at optimized conditions (for EC-HP pretreatment: HP dosage: 0.34 mmol/g TSS, Applied current:5 A, Initial pH:10, Time: 45 min, For EC-CHC pretreatment: CHC dosage: 0.23 mmol/g TSS, Applied current:4.83 A, Initial pH:10, Time: 40 min). Besides, the DD in terms of COD, total nitrogen (TN) and total organic carbon (TOC) (DDCOD, DDTN and DDTOC) registered increased values after the application of the EC treatment with both oxidants. The highest DDCOD, DDTN and DDTOC values were obtained with EC-CHC pretreatment for 11.34%, 20.34% and 9.18% respectively compared to EC-HP pretreatment (DDCOD: 7.37%, DDTN: 15.18% and DDTOC: 6.94%).

Nanotechnology based technological development in biofuel production: Current status and future prospects

Depleting fossil fuels and net carbon emissions associated with their burning have driven the need to find alternative energy sources. Biofuels are near-perfect candidates for alternative energy sources as they are renewable and account for no net CO2 emissions. However, biofuel production must overcome various challenges to compete with conventional fuels. Conventional methods for bioconversion of biomass to biofuel include chemical, thermochemical, and biological processes. Substrate selection and processing, low yield, and total cost of production are some of the main issues associated with biofuel generation. Recently, the uses of nanotechnology and nanoparticles have been explored to improve the biofuel production processes because of their high adsorption, high reactivity, and catalytic properties. The role of these nanoscale particles and nanocatalysts in biomass conversion and their effect on biofuel production processes and yield are discussed in the present article. The applicability of nanotechnology in production processes of biobutanol, bioethanol, biodiesel, biohydrogen, and biogas under biorefinery approach are presented. Different types of nanoparticles, and their function in the bioprocess, such as electron transfer, pretreatment, hydrolysis, microalgae cultivation, lipid extraction, dark and photo fermentation, immobilization, and suppression of inhibitory compounds, are also highlighted. Finally, the current and potential applications of nanotechnology in biorefineries are also discussed.

Focus on Co-digestion of waste activated sludge and food waste via yeast pre-fermentation and biochar supplementation: The optimization and mechanism

Anaerobic digestion is a promising method to recover energy from waste, but the slow rate of fermentation hinders its application. Yeast pre-fermentation has been reported to enhance organic matter solubilization and ethanol production to promote syntrophic metabolism and methanogenesis. However, the pre-fermentation with yeast has not been optimized so far. In this study, the lab-scale experiment was conducted to optimize operational conditions, and a pilot-scale study was conducted to evaluate the combined strategy of yeast pre-fermentation and biochar supplementation. Results demonstrated that at a fermentation time of 6 h, temperature of 30 °C, and dry yeast dosage of 2‰, the highest ethanol production was achieved, which accounted for 6.2% of the total COD of pre-fermentation effluent of a mixture of waste-activated sludge and food waste. The methane yield of the pre-fermented waste averaged 161.3 mL/g VS/d, which was 18.7% higher than that of the control group without the yeast inoculation (135.8 mL/g VS/d). With supplementing biochar of 0.5 and 1 g/L, the average methane production was 27.8% and 36.4% higher than the control group, respectively. The volatile solid removal rate was over 10% higher than the control (58.2 ± 3.12%). Consistently, the electrochemical properties of sludge with biochar were significantly improved. A pilot-scale experiment further showed that the methane production with the yeast pre-fermentation and biochar supplementation reached 227 mL/g VS/d, 54.3% higher than that without yeast pre-fermentation and biochar. This study provided a feasible method to combine yeast pre-fermentation and biochar supplementation under optimal conditions, which effectively increased methane production during anaerobic digestion of organic waste.

Revealing the Adverse Impact of Additive Carbon Material on Microorganisms and Its Implications for Biogas Yields: A Critical Review

Biochar could be a brilliant additive supporting the anaerobic fermentation process. However, it should be taken into account that in some cases it could also be harmful to microorganisms responsible for biogas production. The negative impact of carbon materials could be a result of an overdose of biochar, high biochar pH, increased arsenic mobility in the methane fermentation solution caused by the carbon material, and low porosity of some carbon materials for microorganisms. Moreover, when biochar is affected by an anaerobic digest solution, it could reduce the biodiversity of microorganisms. The purpose of the article is not to reject the idea of biochar additives to increase the efficiency of biogas production, but to draw attention to the properties and ways of adding these materials that could reduce biogas production. These findings have practical relevance for organizations seeking to implement such systems in industrial or local-scale biogas plants and provide valuable insights for future research. Needless to say, this study will also support the implementation of biogas technologies and waste management in implementing the idea of a circular economy, further emphasizing the significance of the research.

Dosage effect of micron zero-valent iron during thermophilic anaerobic digestion of waste activated sludge: Performance and functional community

This work examined the performance and microbial traits in a thermophilic anaerobic digestion (TAD) of waste activated sludge that was impacted by micron zero valent iron (mZVI). Results showed that methane production was promoted by 0.8, 11.9, and 12.0 times, respectively, when mZVI was at dosages of 25, 100, and 250 mg/g total solid (TS). Also, the consumption of volatile fatty acids was increased by mZVI at higher dosages (100 and 250 mg/g TS). Furthermore, 16S rRNA sequencing demonstrated that microbial community stabilized after day 18 regardless of the dosage of mZVI, and that different dosages of mZVI induced different shifts in the functional community of the archaea rather than the bacteria involved in TAD. As a result, mZVI at 100 mg/g TS could increase the relative abundance of archaeal genera Methanothermobacter the most, increasing by 22.8% at the end of TAD compared to CK. Besides, redundancy analysis revealed that the physicochemical properties explained 79.65% and 89.10% of the variations of bacterial and archaeal abundance, respectively. Also, the findings of the correlation analysis revealed that total dissolved iron, ferrous iron, pH, and ammonium nitrogen, may be the key divers of altering functional communities, particularly archaea. Moreover, mZVI at 100 and 250 mg/g TS boosted the metabolic pathways of environmental information processing (ABC transporters) in bacteria and carbon metabolism and methane metabolism for archaea, as well as relative abundances of enzymes and their activities involved in various methanogenic pathways. This study provides new perspectives on the application of mZVI in solid wastes treatments.

Calcination of sewage sludge-based sulphoaluminate cement clinker: Mineral formation mechanism and heavy metal transition behaviors

Utilizing sewage sludge (SS) to calcinate sulphoaluminate cement (SAC) is a promising technology for low-carbon transition of cement industry, but the unclear effects of SS-contained heavy metals limit the application of this technology. In this study, the effects of SS addition on the calcination of SAC clinker and the transformation of heavy metals were studied from the aspects of mineral phase change, microstructure evolution and heavy metal speciation respectively, covering the mineral formation temperature 900-1250 °C. The results show that the added SS will reduce the formation temperature and change the reaction pathways of mineral phases. When the content of SS increases from 10% to 25%, the compositions of mesophases CaO·Al2O3 and 4CaO·2SiO2·CaSO4 increase by 6.33% and 9.73%, respectively. Meanwhile, the formation of minerals will solidify Zn, Ni, Mn, Cu, Cr, and convert them into a more stable fraction (residual fraction), indicating a lower probability to harm the environment. Moreover, heavy metals present different migration behaviors. After calcination, Mn migrates from SS to 4CaO·Al2O3·Fe2O3 (52.48%), while Zn prefers to enter 3CaO·3Al2O3·CaSO4 (43.74%) and 4CaO·Al2O3·Fe2O3 (38.06%). This study offers new insights into the mineral formation mechanism and heavy metal transition behaviors of sewage sludge-based SAC.

Promoting short-chain fatty acids production from sewage sludge via acidogenic fermentation: Optimized operation factors and iron-based persulfate activation system

Promoting short-chain fatty acids (SCFAs) production and ensuring the stability of SCFAs-producing process are becoming the two major issues for popularizing the acidogenic fermentation (AF). The key controlling operating and influencing factors during anaerobic fermentation process were thoroughly reviewed to facilitate better process performance prediction and to optimize the process control of SCFAs promotion. The wide utilization of iron salt flocculants during wastewater treatment could result in iron accumulating in sewage sludge which influenced AF performance. Additionally, appropriate ferric chloride (FC) could promote the SCFAs accumulation, while poly ferric sulfate (PFS) inhibited the bioprocess. Iron/persulfate (PS) system was proved to effectively enhance the SCFAs production while mechanism analysis revealed that the strong oxidizing radicals remarkably enhanced the solubilization and hydrolysis. Moreover, the changes of oxidation-reduction potential (ORP) and pH caused by iron/PS system exhibited more negative effects on the methanogens, comparing to the acidogenic bacteria. Furthermore, performance and mechanisms of different iron species-activating PS, organic chelating agents and iron-rich biochar derived from sewage sludge were also elucidated to extend and strengthen understanding of the iron/PS system for enhancing SCFAs production. Considering the large amount of generated Fe-sludge and the multiple benefits of iron activating PS system, carbon neutral wastewater treatment plants (WWTPs) were proposed with Fe-sludge as a promising recycling composite to improve AF performance. It is expected that this review can deepen the knowledge of optimizing AF process and improving the iron/PS system for enhancing SCFAs production and provide useful insights to researchers in this field.

Enhancing anaerobic digestion of sulphate wastewater by adding nano-zero valent iron

In this paper, the effects of nano-zero valent iron (nZVI) on anaerobic digestion of sulphate wastewater with different SO 4 2 - /COD ratios, including the COD removal rate, methane yield, intermediate products and the change of microbial community structure, were investigated. The results showed that nZVI could effectively enhance the treatment efficiency and methane yield. Compared with the control group without nZVI, the methane yield increased from 348.6833 to 1007.05 mL CH4/gCODremoval with 4 g nZVI loading at SO 4 2 - /COD = 0.1. nZVI could make electron flow from sulphate reduction to methane production, which increased methane yield even at high sulphate concentration. The microbial community analysis showed that adding nZVI could increase the abundance of acetoclastic methanogens, which accelerated hydrolysis acidification.

Stimulating anaerobic digestion to degrade recalcitrant organic pollutants: Potential role of conductive materials-led direct interspecies electron transfer

This review aims to provide a comprehensive understanding of the potential of CMs-dominated DIET in the degradation of recalcitrant organic pollutants in AD. The review covers the mechanisms and efficiencies of recalcitrant organic pollutant degradation by CMs-dominated DIET, the comparison of degradation pathways between DIET and chemical treatment, recent insights on DIET-enhanced degradation, and the evaluation of the potential and future development of CMs-dominated DIET. The review emphasizes the importance of coupled syntrophic microorganisms, electron flux, and physicochemical properties of CMs in enhancing the degradation performance of AD. Additionally, it highlights the advantages of DIET-led syntrophic metabolism over traditional oxidation technologies in terms of environmental friendliness and efficiency. Finally, the review acknowledges the potential risks associated with introducing CMs into AD systems and provides guidance for waste treatment and energy recovery.

A review of application of combined biochar and iron-based materials in anaerobic digestion for enhancing biogas productivity: Mechanisms, approaches and performance

Strengthening direct interspecies electron transfer (DIET), via adding conductive materials, is regarded as an effective way for improving methane productivity of anaerobic digestion (AD). Therein, the supplementation of combined materials (composition of biochar and iron-based materials) has attracted increasing attention in recent years, because of their advantages of promoting organics reduction and accelerating biomass activity. However, as far as we known, there is no study comprehensively summarizing the application of this kind combined materials. Here, the combined methods of biochar and iron-based materials in AD system were introduced, and then the overall performance, potential mechanisms, and microbial contribution were summarized. Furthermore, a comparation of the combinated materials and single material (biochar, zero valent iron, or magnetite) in methane production was also evaluated to highlight the functions of combined materials. Based on these, the challenges and perspectives were proposed to point the development direction of combined materials utilization in AD field, which was hoped to provide a deep insight in engineering application.

Responses of applied voltages on the archaea microbial distribution in sludge digestion

As the development of urban population led to the increase of domestic water consumption, consequently the generation of surplus sludge (SS) produced increasingly during sewage treatment processes. In order to enhance the SS resource utilization efficiency, an electricity-assisted anaerobic digestion (EAAD) system was employed to examine the alterations in the digestion broth and the characteristics of gas production. Additionally, the response of applied voltages on the distribution of archaeal community near various electrodes within the sludge was explored. The results revealed that the application of high voltages exceeding 3.0 V hindered the CH4 production but stimulated the CO2 generation. Subsequently, both CH4 and CO2 production were impeded by the applied voltages. Furthermore, the increased voltages significantly decreased the abundance of Methanomicrobia, Methanosaeta, and Methanosarcina, which were crucial determinants of CH4 content in biogas. Notably, the excessively high voltages intensities caused the AD process to halt and even inactivate the microbial flora. Interestingly, the distribution characteristics of archaeal community were influenced not only by the voltages intensity but also exhibited variations between the anode and cathode regions. Moreover, as the applied voltage intensified, the discrepancy of responses between the cathode and anode regions became more pronounced, offering novel theoretical and technical foundations for the advancement of electricity-assisted with AD technology.

Improved net energy recovery in a sludge anaerobic digestion process by coupling an electrochemical system: electrode material and its impact on suspended microbial community

Despite its great potential to recover energy from waste sludge, anaerobic digestion (AD) still needs to solve issues such as slow hydrolysis and H2 inhibition. This study investigated the effects of coupling microbial electrolysis cell (MEC) with AD on the CH4 yield. Results and analysis show that the CH4 yield was significantly improved in MEC-AD reactors by two factors, i.e., enhanced and accelerated hydrolysis and acidogenesis, and enrichment of hydrogenotrophic methanogens in suspended culture. Compared with graphite rod and carbon fiber brush, carbon felt (CF) as an electrode showed the best performance in terms of net energy output. The CH4 yield of MEC-AD-CF was 40.2 L CH4/kg VS, 92.3% higher than in the control group, and the VS removal rate was also increased by 47.2%. Acetoclastic methanogens were dominant in the control AD reactor, while the relative abundance of Methanobacterium, which is electroactive and known as hydrogenotrophic methanogen, increased to 24.6% in MEC-AD with CF as electrodes.

Trace elements' deficiency in energy production through methanogenesis process: Focus on the characteristics of organic solid wastes

Excessive or insufficient supplementation of trace elements (TEs) limits the progression of anaerobic digestion. The main reason for this is the lack of sufficient understanding of digestion substrate characteristics, which significantly affects the demand for TEs. In this review, the relationship between TEs requirements and substrate characteristics is discussed. We mainly focus on three aspects. 1) The basis for TE optimization and existing problems: The optimization of TEs often based on the total solids (TS) or volatile solids (VS) of substrates, does not fully consider substrate characteristics. 2) TE deficiency mechanisms for different types of substrates: nitrogen-rich, sulfur-rich, TE-poor, and easily hydrolyzed substrates are the four main types of substrates. The mechanisms underlying TEs deficiency in the different substrates are investigated. 3) Regulation of TE bioavailability: characteristics of substrates affect digestion parameters, which disturb the bioavailability TE. Therefore, methods for regulating bioavailability of TEs are discussed.

Deciphering different effects of ZVI and NaOH on metabolic characteristics in the process of methanogenesis recovery from VFA suppression

Dosing zero valent iron (ZVI) or sodium hydroxide (NaOH) is the common method of addressing acidification in anaerobic digestion (AD) systems; however, few studies have discussed and compared their effects on microbial metabolism. In the present study, microbial syntrophy and metabolic pathways under ZVI and NaOH regulation are comparatively analyzed through microbial network analysis and metagenomic/metaproteomic analyses. CH₄ yield in the ZVI reactor was 414 mL/gVS, an increase of 23% when compared with that in the reactor with NaOH dosing (336 mL/gVS). The methanogenesis recovery period in the ZVI reactor (37 days) was shorter than that in the NaOH reactor (48 days). Co-occurrence networks indicated that ZVI promoted Methanoculleus and Methanosarcina to establish a complex syntrophic association with SAO bacteria (Syntrophaceticus and Aminobacterium) and syntrophic acetogens (Syntrophomonas), strengthening SAO-hydrogenotrophic methanogenesis (HM) and acetoclastic methanogenesis (AM) pathways simultaneously. Metagenomic analysis showed that the relative abundance of mcrA and fwdB in the ZVI reactor was higher 27% than that in the NaOH reactor. Furthermore, through metaproteomics analysis, much more enzymes related to glucose degradation, bioconversion of butyric acid and pyruvate, conversion of formate and acetate to CO₂, and production of CH₄ from acetate and CO₂ were significantly upregulated under ZVI regulation than under NaOH regulation (fold change relative to control [FC] > 1.5, p < 0.05). The results of the present study enhance our understanding of methanogenic mechanisms under the regulation of ZVI, providing a theoretical basis for its practical application in AD systems experiencing VFA suppression.

Advanced treatment and valorization of food waste through staged fermentation and chain elongation

A novel valorization approach of food waste via staged fermentation and chain elongation was proposed. Food waste was moderately saccharified, saccharification effluent was fermented to produce ethanol and saccharification residue was hydrolyzed and acidified to produce VFAs. The yeast fermentation effluent and hydrolytic acidification effluent were sequentially performed for chain elongation. Ethanol and volatile fatty acids from staged fermentation were suitable for direct chain elongation and the n-caproate production was 184.69 mg COD/g VS when yeast fermentation effluent to hydrolytic acidification effluent ratio was 2:1. Food waste was deeply utilized with an organic conversion of 80%. The relative abundance of Clostridium sensu stricto increased during chain elongation, which might be responsible for the improvement of n-caproate production. A profit of 10.65 USD/t was estimated for chain elongation of food waste staged fermentation effluent. This study provided a new technology to achieve advanced treatment and high-valued utilization of food waste.

Long-term transformation of nanoscale zero-valent iron explains its biological effects in anaerobic digestion: From ferroptosis-like death to magnetite-enhanced direct electron transfer networks

Nanoscale zero-valent iron (nZVI) has been extensively used for environmental remediation and wastewater treatment. However, the biological effects of nZVI remain unclear, which is no doubt a result of the complexity of iron species and the dynamic succession of microbial community during nZVI aging. Here, the aging effects of nZVI on methanogenesis in anaerobic digestion (AD) were consecutively investigated, with an emphasis on deciphering the causal relationships between nZVI aging process and its biological effects. The addition of nZVI in AD led to ferroptosis-like death with hallmarks of iron-dependent lipid peroxidation and glutathione (GSH) depletion, which inhibited CH₄ production during the first 12 days of exposure. With prolonged exposure time, a gradual recovery (12-21 days) and even better performance (21-27 days) in AD were observed. The recovery performance of AD was mainly attributed to nZVI-enhanced membrane rigidity via forming siderite and vivianite on the outer surface of cells, protecting anaerobes against nZVI-induced toxicity. At the end of 27-days exposure, the significantly increased amount of conductive magnetite simulated direct interspecies electron transfer among syntrophic partners, improving CH₄ production. Metagenomic analysis further revealed that microbial cells gradually adapted to the aging of nZVI by upregulating functional genes related to chemotaxis, flagella, conductive pili and riboflavin biosynthesis, in which electron transfer networks likely thrived and the cooperative behaviors between consortium members were promoted. These results unveiled the significance of nZVI aging on its biological effects toward multiple microbial communities and provided fundamental insights into the long-term fates and risks of nZVI for in situ applications.

Effect of riboflavin and carbon black co-modified fillers coupled with alkaline pretreatment on anaerobic digestion of waste activated sludge

Additional various carbon and free riboflavin could improve anaerobic digestion of waste activated sludge (WAS). However, these substances were not reused. In this study, a reusable riboflavin and carbon black (RCB) co-modified filler was developed and combined with alkaline pretreatment for enhancing the production of volatile fatty acids (VFAs) and methane during anaerobic digestion of WAS. The results showed that RCB-modified fillers exhibited a promoting effect on the reduction of alkali-pretreated WAS. The amounts of the accumulated VFAs mainly containing acetate and the produced methane rose with the increased concentration of immobilized riboflavin (0-0.75 g/L) in the presence of 4 g/L carbon black. When the alkaline pretreatment time of WAS increased from 3 d to 8 d, the amount of methane production increased from 22.8% to 63.9% in the presence of 0.75 g/L riboflavin and 4 g/L carbon black compared with that without RCB-modified fillers. Moreover, 0.75 g/L riboflavin and 4 g/L carbon black had a synergetic effect on promoting methane production via broadening extracellular electron transfer pathways. During this process, microbial dehydrogenase activity, electron transport system activity and coenzyme F420 were enhanced. Microbial community analysis showed that RCB-modified filler addition promoted the enrichment of Syntrophomonas and Pseudomonas involved in direct interspecies electron transfer (DIET). These results indicated that DIET establishment was accelerated. Meanwhile, the populations of acetic acid-producing bacteria including Rikenellaceae_RC9_gut_group and Proteiniphilum, aceticlastic and acid-tolerant methanogenic archaea including Methanosarcina and Methanosaeta, RumEn_M2 were increased. These results indicate that RCB-modified fillers coupled with alkaline pretreatment is an effective method to promote the production of methane during anaerobic digestion of WAS.

Optimizing waste activated sludge disintegration by investigating multiple electrochemical pretreatment conditions: Performance, mechanism and modeling

The complex and rigid floc structure often limits the reutilization of waste activated sludge (WAS). Electrochemical pretreatment (EPT) is one of the most effective technologies that can enhance WAS disintegration. But a comprehensive investigation into how multiple EPT conditions work was rarely reported. The study evaluated the effects of multiple EPT conditions, i.e., different electrolytes (NaCl, Na₂SO₄, and CaCl₂), electrolytes dosage (0 g/L, 0.5 g/L, 1.0 g/L, and 3.0 g/L), EPT current (0 A, 0.5 A, 1.0 A, and 3.0 A) and EPT time (0 min, 30 min, 60 min, and 90 min) on WAS disintegration. The results showed that NaCl was outstanding from other electrolytes in promoting more WAS disintegration. Besides, a relatively higher NaCl dosage, a higher EPT current, and a longer EPT time promoted more reactive chlorine species (RCS), thus enhancing WAS disintegration in terms of extracellular polymeric substances (EPS) structure destruction and biodegradable organic matter release. After EPT for 60 min at NaCl dosage of 1.0 g/L and current of 1.0 A, the EPS multilayer structure destruction, biodegradable organic matters release, and soluble chemical oxygen demand (SCOD) increase in the supernatant were enhanced by 17.2 %, 130.5 %, and 238.7 %, respectively. Then a predictive quadratic model was established and the impact significance of the above EPT factors for enhancing WAS disintegration followed dosage of NaCl > current > EPT time. Furthermore, response surface methodology (RSM) suggested NaCl dosage of 2.75 g/L, current of 2.0 A, and EPT time of 30 min were the optimal EPT conditions, bringing a 42.0 % increase in the net economic benefit of WAS treatment compared to without EPT.

Inhibition of aged microplastics and leachates on methane production from anaerobic digestion of sludge and identification of key components

Large amounts of microplastics (MPs) accumulate in the sludge anaerobic digestion system after being treated by the wastewater treatment plants, inevitably leading to aging and chemicals leaching. However, no information is available about the effects of aged MPs and leachates on the anaerobic digestion of sludge. In this study, the effects of different aged MPs ((polyethylene (PE), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polylactic acid (PLA)) and leachates on anaerobic methanogenesis of sludge were investigated. PLA-related treatments caused no adverse effects on anaerobic digestion. While PE-, PET-, and PVC-related treatments significantly inhibited methane production with an order of leachates (26.4-42.4 %) > MPs (16.1-22.9 %) > aged MPs (2.4-11.8 %). For different leachates, PET leachate caused the strongest inhibitory effects. The same order was found for the methane potential and hydrolysis coefficient. These results suggest that the inhibition of MPs on methanogenesis is mainly caused by the leachates. Based on biochemical and microbial community analysis, the primary mechanism is that the leachates induce oxidative stress, damaging microbial cells and reducing microbial activity, consequently inhibiting methanogenesis. Furthermore, via effect-directed analysis, methyl benzoate (MB), dimethyl phthalate (DMP), and 2,4-Di-tert-butylphenol (DTBP) were identified as key components in the PET-leachate inhibiting anaerobic methanogenesis.

The coupling system of magnetite-enhanced thermophilic hydrolysis-acidification and denitrification for refractory organics removal from anaerobic digestate food waste effluent (ADFE)

The aim of this study was to remove the refractory organics from high-temperature anaerobic digestate food waste effluent by the coupling system of hydrolysis-acidification and denitrification. Iron-based materials (magnetite, zero-valent iron, and iron-carbon) were used to enhance the performance of thermophilic hydrolysis-acidification. Compared with the control group, magnetite had the best strengthening effect, increasing volatile fatty acids concentration and fluorescence intensity of easily biodegradable organics in the effluent by 47.6 % and 108.4 %, respectively. The coupling system of magnetite-enhanced thermophilic hydrolysis-acidification and denitrification achieved a nitrate removal efficiency of 91.2 % (influent NO₃^--N was 150 mg L^-1), and reduced the fluorescence intensity of refractory organics by 33.8 %, compared with influent. Microbiological analysis indicated that magnetite increased the relative abundance of thermophilic hydrolytic acidifying bacteria, and coupling system enriched some genera simultaneously removing nitrate and refractory organics. This study provided fresh information on refractory organics and nitrogen removal of thermophilic wastewater biologically.

Synergistic effect of extracellular polymeric substances and carbon layer on electron utilization of Fe@C during anaerobic treatment of refractory wastewater

Nano zero-valent iron (NZVI) has been widely used to improve refractory wastewater treatment. However, the rapid dissolution of NZVI causes a waste of resources and an unstable bioaugmentation. Herein, to verify the essential role of slow release of NZVI on biological systems, a core-shell structured Fe@C composite was developed to demonstrate the long-term feasibility of Fe@C for enhancing azo dye biodegradation in comparison to a mixture of NZVI and carbon powder (Fe+C). The 150 days of long-term reactor operation showed that, although both Fe@C and Fe+C enhanced azo dye degradation, the former achieved a better performance than the latter. The strengthening effect of Fe@C was also more durable and stable than Fe+C. It may be due to the fact that the carbon layer of Fe@C could interact with extracellular polymeric substances (EPS) through physical adsorption and chemical bonding to form a stable buffer to regulate NZVI dissolution. The buffer layer could not only regulate the attack of H⁺ on NZVI to reduce its dissolution rate but also complex released Fe²⁺ and neutralize OH^- to alleviate the passivation layer formed on the NZVI surface. Moreover, microbial community analysis indicated that both Fe@C and Fe+C increased the abundance of fermentative bacteria (e.g., Bacteroidetes_vadinHA17, Propionicicella) and methanogens (e.g., Methanobacterium), but only Fe@C promoted the growth of azo dye degraders (e.g., Clostridium, Geobacter). Metatranscriptomic analysis further revealed that only Fe@C could substantially stimulate the expression of azoreductase and redox mediator (e.g., riboflavin, ubiquinone) biosynthesis involved in the extracellular degradation of azo dye. This work provides novel insights into the bioaugmentation of Fe@C for refractory wastewater treatment.

Mechanisms, performance, and the impact on microbial structure of direct interspecies electron transfer for enhancing anaerobic digestion-A review

Direct interspecies electron transfer (DIET) has been received tremendous attention, recently, due to the advantages of accelerating methane production via organics reduction during anaerobic digestion (AD) process. DIET-based syntrophic relationships not only occurred with the existence of pili and some proteins in the microorganism, but also can be conducted by conductive materials. Therefore, more researches into understanding and strengthening DIET-based syntrophy have been conducted with the aim of improving methanogenesis kinetics and further enhance methane productivity in AD systems. This study summarized the mechanisms, application and microbial structures of typical conductive materials (carbon-based materials and iron-based materials) during AD reactors operation. Meanwhile, detail analysis of studies on DIET (from substrates, dosage and effectiveness) via conductive materials was also presented in the study. Moreover, the challenges of applying conductive materials in boosting methane production were also proposed, which was supposed to provide a deep insight in DIET for full scale application.

The Effects of Nanoparticles- Zerovalent Iron on Sustainable Biomethane Production through Co-Digestion of Olive Mill Wastewater and Chicken Manure

The impacts of nanoparticles-zerovalent iron (NP-ZVI) on anaerobic co-digestion (AcoD) were assessed. The production of biogas and methane (CH4), as well as the removal efficiency of volatile solids (VS) and contaminants were investigated in the AcoD of chicken manure (CM) and olive mill wastewater (OMWW) with the addition of NP-ZVI at different concentrations (10–50 mg/g VS) and different sizes resulting from various mixing volume ratios (MVR) of NaBH4:FeSO4.7H2O. The results show that NP-ZVI ≤ 30 mg/g VS at MVR-2:1, MVR-4:1, and MVR-6:1 improves the AcoD. In contrast to 40–50 mg/g VS of NP-ZVI, which caused an inhibitory impact in all of the AcoD stages, as well as a decrease in the contaminant’s removal efficiency, the concentration of 10–30 mg NP-ZVI/g VS at MVR-4:1 achieved a maximum improvement of CH4 by 21.09%, 20.32%, and 22.87%, respectively, and improved the biogas by 48.14%, 55.0%, and 80.09%, respectively, vs. the 0 additives. Supplementing AcoD with NP-ZVI at a concentration of 30 mg/g VS at MVR-4:1 resulted in maximum enhancement of the contaminant removal efficiency, with a total oxygen demand (TCOD) of up to 73.99%, turbidity up to 79.07%, color up to 53.41%, total solid (TS) up to 59.57%, and volatile solid (VS) up to 74.42%. It also improved the hydrolysis and acidification percentages by up to 86.67% and 51.3%, respectively.

Mechanism insights into enhanced treatment of wasted activated sludge by hydrogen-mediated anaerobic digestion

In the current study, different forms of added gas including H₂, CO₂, and mixed gas (V_H2:V_CO2 = 4:1), as well as different hydrogen partial pressures (0.10, 0.30, and 0.50 atm) were investigated for the influence on anaerobic performance in waste activated sludge (WAS) treatment. The mixed gas significantly improved methane production by over 20%, which positively correlated with the hydrogen partial pressure. However, pure H₂ (0.5 atm) heavily inhibited methane production by 76.5%. Combined with the microbial metabolic activity study, H₂ accelerated the hydrolysis process. Afterward, mixing with CO₂ accelerated H₂ and organic consumption, thus promoting WAS degradation and methane production. Based on the most extra release of organics, the mixed group exerted the superior performance with hydrogen partial pressure at 0.3 atm. The microbial community analysis evidenced that mixed gas enriched proteolytic and homoacetogenic bacteria and hybrid-trophic methanogens. By metagenomics study, hydrolysis, acetogenic, and methanogenesis pathways were all enhanced via the exogenous addition of H₂ and CO₂, sustainably transforming WAS towards CH₄. This study discovered the mechanism of the enhanced conversion from WAS to CH₄ by exogenous H₂ and provided a promising approach for WAS reduction and energy recovery.

Optimization of the Fabrication of Sustainable Ceramsite Adsorbent from Coal Fly Ash/Waterworks Sludge/Waste Glass for Decolorization of Malachite Green

As a traditional dye, malachite green (MG) poses a threat to our environment and health. To decolorize MG, a composite ceramsite adsorbent composed of coal fly ash (CFA), sewage treatment sludge (STS), and waste glass (WG) with a quality ratio of 3 : 3 : 4, respectively, was prepared. The optimal preparation parameters were determined as follows: preheating $\text{temperature}=600°\text{C}$ , sintering $\text{temperature}=1157°\text{C}$ , and sintering $\text{time}=17\min$ . Under optimal conditions, scanning electron microscopy (SEM) images show that the X-Com-ceramsite sample exhibits rough features and a porous structure. The obtained X-Com-ceramsite has a good MG decolorization effect (92% decolorization rate with an initial MG concentration of 56.876 mg/L). The $q_{\max }$ value of MG can reach up to 37.6 mg/g. The retention degree of MG in the X-Com-ceramsite with a relatively higher pH is stronger, and the adsorption process is spontaneous and endothermic. Synchronous fluorescence, two-dimensional correlation spectroscopy (2D-COS), and Fourier transform infrared spectroscopy (FT-IR) proved that the sensitivity of the C-O/C-O-O functional groups of the carbohydrates on the surface of the X-Com-ceramsite has a higher binding affinity toward MG as the initial concentration of MG changes.

Optimization of iron-enhanced anaerobic digestion of agro-wastes for biomethane production and phosphate release

In this study, the optimization of additives (polypyrrole magnetite nanocomposites (Ppy/Fe₃O₄) and antagonists (humic acid and arsenic oxide)) for simultaneous recovery of biomethane and phosphate release from enhanced anaerobic co-digestion of okra waste and pig manure was investigated. The pre-determined dosages of additives from our previous studies were used for the batch anaerobic digestion at different ratios under mesophilic conditions based on the two level-four factors central composite design (CCD) response surface methodology (RSM). After the anaerobic digestion processes, the biomethane yields were recorded and the digestates were characterized to determine the quantity of soluble phosphates. Both the independent variables and the responses were used to model and optimize the biogas yield and phosphate release conditions. The result showed that the maximum biomethane yield and P release were respectively 502.743 mLCH4/gVS and 168.674 mg/L at the optimum conditions of Ppy/Fe₃O₄ (20.0014 mg/L), HA (5.0018 mg/L), As (1.448 mg/L) and co-digestion (25.0001%). The response models predicted biomethane yield and P release to be 528.635 mLCH4/gVS and 164.405 mg/L respectively. All the response models were highly significant with appropriate goodness of fit and had prediction differences of 4.90% and 2.597% respectively for both biomethane yield and P release. Although both the accelerants and antagonists had influences on the anaerobic digestion processes by achieving enhanced biomethane production and P release, the influence of long exposure of anaerobic digestion processes to these additives on both responses is recommended for further investigation.

Elucidation of microbial interactions, dynamics, and keystone microbes in high pressure anaerobic digestion

High-pressure anaerobic digestion (HPAD) is a promising technology for producing biogas enriched with high methane content in a single-step process. To enhance HPAD performance, a comprehensive understanding of microbial community dynamics and their interactions is essential. For this, mesophilic batch high-pressurized anaerobic reactors were operated under 3 bars (H3) and 6 bars (H6). The experimental results showed that the effect of high-pressure (up to 6 bar) on acidification was negligible while methanogenesis was significantly delayed. Microbial analysis showed the predominance of Defluviitoga affiliated with the phylum Thermotogae and the reduction of Thiopseudomonas under high-pressure conditions. In addition, the microbial cluster pattern in H3 and H6 was significantly different compared to the CR, indicating a clear shift in microbial community structure. Moreover, Methanobacterium, Methanomicrobiaceae, Alkaliphilus, and Petrimonas were strongly correlated in network analysis, and they could be identified as keystone microbes in the HPAD reactor.

Effects of synthesized AlFe2O4 and MgFe2O4 nanoparticles on biogas production from anaerobically digested sugar refinery wastewater

The application of magnetic nanoparticles (MNPs) toward sustainable water economy and bioenergy production has become a subject of great interest. Anaerobic digestion (AD) has been widely exploited in wastewater treatment settings, whereby utilization of MNP additives by microorganism response for degradation of organics into biogas is seen to be eco-friendly and economically viable. The present study investigated the impact of two synthesized MNPs such as aluminum ferrite (AlFe₂O₄) and magnesium ferrite (MgFe₂O₄) on biogas production via biochemical methane potential (BMP) tests. A BMP set-up of three (3) bioreactors was tested with a working volume of 0.8 L which comprises 0.5 L wastewater (WW) and 0.3 L activated sludge (AS) with 1.5 g of the MNPs and a control system without MNPs. The degradation of chemical oxygen demand (COD) was increased with set-ups that contained MNPs as compared to the control system, MgFe₂O₄ (93.96%) > AlFe₂O₄ (85.95%) > control (68.83%). In terms of biogas production, the methane yield was also recorded as MgFe₂O₄ (85.7%) > AlFe₂O₄ (84.3%) > control (65.7%). The physical and chemical stability of MNPs makes them more advantageous for application in biogas production. In the prospects of biogas enhancement and biodegradability, integrating MNPs in an anaerobic digestion system will result in a more efficient anaerobic process performance for biogas production.