Zerovalent Iron Effectively Enhances Medium-Chain Fatty Acids Production from Waste Activated Sludge through Improving Sludge Biodegradability and Electron Transfer Efficiency

Plasticizers inhibit food waste anaerobic digestion performance by affecting microbial succession and metabolism

The widely existed plastic additives plasticizers in organic wastes possibly pose negative influences on anaerobic digestion (AD) performance, the direct evidence about the effects of plasticizers on AD performance is still lacking. This study evaluated the influencing mechanism of two typical plasticizers bisphenol A (BPA) and dioctyl phthalate on the whole AD process. Results indicated that plasticizers addition inhibited methane production, and the inhibiting effects were reinforced with the increase of concentration. By contrast, 50 mg/L BPA exhibited the strongest inhibition on methane production. Physicochemical analysis showed plasticizers inhibited the metabolism efficiency of soluble polysaccharide and volatile fatty acids. Microbial communities analyses suggested that plasticizers inhibited the direct interspecies electron transfer participators of methanogenic archaea (especially Methanosarcina) and syntrophic bacteria. Furthermore, plasticizers inhibited the methane metabolisms, key coenzymes (CoB, CoM, CoF420 and methanofuran) biosynthesis and the metabolisms of major organic matters. This study shed light on the effects of plasticizers on AD performance and provided new insights for assessing the influences of plasticizers or plastic additives on the disposal of organic wastes.

Deciphering the reduction of antibiotic resistance genes (ARGs) during medium-chain fatty acids production from waste activated sludge: Driven by inhibition of ARGs transmission and shift of microbial community

Medium-chain fatty acids (MCFAs) production from waste activated sludge (WAS) by chain extension (CE) is a promising technology. However, the effects and mechanisms of CE process on the fate of antibiotic resistance genes (ARGs) remain unclear. In this study, the results showed that the removal efficiency of ARGs was 81.15 % in CE process, suggesting its efficacy in reducing environmental risks. Further, the observed decrease in mobile genetic elements (MGEs) indicated that CE process restricted the horizontal gene transfer (HGT). Complementing this, the increase in soluble organic matters and extracellular 16 S rDNA confirmed that MCFAs production caused bacterial damage. Decreased intracellular ARGs and increased extracellular ARGs further revealed that MCFAs production impaired ARGs hosts, thereby limiting the vertical gene transfer (VGT) of ARGs. Shift of microbial community combined with co-occurrence network analysis demonstrated that functional bacteria without host potential for ARGs were enriched, but potential ARGs and MGEs hosts decreased, showing the role of functional bacterial phylogeny and selection pressure of MCFAs in reducing ARGs. Finally, partial least squares path model was used to systematic verify the mechanism of ARGs removal in CE process, which was attributed to the inhibition of ARGs transmission (HGT and VGT) and shift of microbial community.

Significant in situ sludge yield reduction in an acidic activated sludge system

Minimizing sludge generation in activated sludge systems is critical to reducing the operational cost of wastewater treatment plants (WWTPs), particularly for small plants where bioenergy is not recovered. This study introduces a novel acidic activated sludge technology for in situ sludge yield reduction, leveraging acid-tolerant ammonia-oxidizing bacteria (Candidatus Nitrosoglobus). The observed sludge yield (Yobs) was calculated based on the cumulative sludge generation and COD removal during 400 d long-term operation. The acidic process achieved a low Yobs of 0.106 ± 0.004 gMLSS/gCOD at pH 4.6 to 4.8 and in situ free nitrous acid (FNA) of 1 to 3 mg/L, reducing sludge production by 58 % compared to the conventional neutral-pH system (Yobs of 0.250 ± 0.003 gMLSS/gCOD). The acidic system also maintained effective sludge settling and organic matter removal over long-term operation. Mechanism studies revealed that the acidic sludge displayed higher endogenous respiration, sludge hydrolysis rates, and higher soluble microbial products and loosely-bounded extracellular polymer substances, compared to the neutral sludge. It also selectively enriched several hydrolytic genera (e.g., Chryseobacterium, Acidovorax, and Ottowia). Those results indicate that the acidic pH and in situ FNA enhanced sludge disintegration, hydrolysis, and cryptic growth. Besides, a lower intracellular ATP content was observed for acidic sludge than neutral sludge, suggesting potential decoupling of catabolism and anabolism in the acidic sludge. These findings collectively demonstrate that the acidic activated sludge technology could significantly reduce sludge yield, contributing to more cost- and space-effective wastewater management.

Synergistic mechanisms of denitrification in FeS2-based constructed wetlands: Effects of organic carbon availability under day-night alterations

In constructed wetlands (CWs), carbon source availability profoundly affected microbial metabolic activities engaged in both iron cycle and nitrogen metabolism. However, research gaps existed in understanding the biotransformation of nitrogen and iron in response to fluctuations in organic carbon content under day-night alterations. Results demonstrated increased removal efficiency of NO3--N (95.7 %) and NH4+-N (75.70 %) under light conditions, attributed to increased total organic carbon (TOC). This enhancement promoted the relative abundance of bacteria involved in nitrogen and iron processes, establishing a more stable microbial network. Elevated TOC content also upregulated genes for iron metabolism and glycolysis, facilitating denitrification. Spearman correlation analysis supported the synergistic mechanisms between FeS2-based autotrophic denitrification and TOC-mediated heterotrophic denitrification under light conditions. The significant impact of carbon sources on microbial activities underscores the critical role of organic carbon availability in enhancing nitrogen removal efficiency, providing valuable insights for optimizing FeS2-based CWs design and operation strategies.

A Novel Sustainable and Self-Sufficient Biotechnological Strategy for Directly Transforming Sewage Sludge into High-Value Liquid Biochemicals

Sewage sludge, as a carbon-rich byproduct of wastewater treatment, holds significant untapped potential as a renewable resource. Upcycling this troublesome waste stream represents great promise in addressing global escalating energy demands through its wide practice of biochemical recovery concurrently. Here, we propose a biotechnological concept to gain value-added liquid bioproducts from sewage sludge in a self-sufficient manner by directly transforming sludge into medium-chain fatty acids (MCFAs). Our findings suggest that yeast, a cheap and readily available commercial powder, would involve ethanol-type fermentation in chain elongation to achieve abundant MCFA production from sewage sludge using electron donors (i.e., ethanol) and acceptors (i.e., short-chain fatty acids) produced in situ. The enhanced abundance and transcriptional activity of genes related to key enzymes, such as butyryl-CoA dehydrogenase and alcohol dehydrogenase, affirm the robust capacity for the self-sustained production of MCFAs. This is indicative of an effective metabolic network established between yeast and anaerobic microorganisms within this innovative sludge fermentation framework. Furthermore, life cycle assessment and techno-economic analysis evidence the sustainability and economic competitiveness of this biotechnological strategy. Overall, this work provides insights into sewage sludge upgrading independent of additional carbon input, which can be applied in existing anaerobic sludge fermentation infrastructure as well as to develop new applications in a diverse range of industries.

Hexanoic acid production and microbial community in anaerobic fermentation: Effects of inorganic carbon addition

Carbon dioxide (CO2) plays a crucial role in carbon chain elongation with ethanol serving as an electron donor. In this study, the impacts of various carbonates on CO2 concentration, hexanoic acid production, and microbial communities during ethanol-butyric acid fermentation were explored. The results showed that the addition of MgCO3 provided sustained inorganic carbon and facilitated interspecific electron transfer, thereby increasing hexanoic acid yield by 58%. MgCO3 and NH4HCO3 inhibited the excessive ethanol oxidation and decreased the yield of acetic acid by 51% and 42%, respectively. The yields of hexanoic acid and acetic acid in the CaCO3 group increased by 19% and 15%, respectively. The NaHCO3 group exhibited high headspace CO2 concentration, promoting acetogenic bacteria enrichment while reducing the abundance of Clostridium_sensu_stricto_12. The batch addition of NaHCO3 accelerated the synthesis of hexanoic acid and increased its production by 26%. The relative abundance of Clostridium_sensus_stricto_12 was positively correlated with hexanoic acid production.

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.

Comprehensive review of microbial production of medium-chain fatty acids from waste activated sludge and enhancement strategy

Microbial production of versatile applicability medium-chain fatty acids (MCFAs) (C6-C10) from waste activated sludge (WAS) provides a pioneering approach for wastewater treatment plants (WWTPs) to achieve carbon recovery. Mounting studies emerged endeavored to promote the MCFAs production from WAS while struggling with limited MCFAs production and selectivity. Herein, this review covers comprehensive introduction of the transformation process from WAS to MCFAs and elaborates the mechanisms for unsatisfactory MCFAs production. The enhancement strategies for biotransformation of WAS to MCFAs was presented. Especially, the robust performance of iron-based materials is highlighted. Furthermore, knowledge gaps are identified to outline future research directions. Recycling MCFAs from WAS presents a promising option for future WAS treatment, with iron-based materials emerging as a key regulatory strategy in advancing the application of WAS-to-MCFAs biotechnology. This review will advance the understanding of MCFAs recovery from WAS and promote sustainable resource management in WWTPs.

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.

L-tryptophan anaerobic fermentation for indole acetic acid production: Bacterial enrichment and effects of zero valent iron

Indole acetic acid (IAA) as a plant hormone, was one of the valuable products of anaerobic fermentation. However, the enriching method remained unknown. Moreover, whether zero valent iron (ZVI) could enhance IAA production was unexplored. In this work, IAA producing bacteria Klebsiella (63 %) was enriched successfully. IAA average production rate and concentration were up to 3 mg/L/h and 56 mg/L. With addition of 1 g/L ZVI, IAA average production rate and concentration was increased for 2 and 3 folds. Mechanisms indicated ZVI increased Na+K+-ATP activity and electron transport activity for 2 folds and 1 fold. Moreover, macro transcription determined indole pyruvate pathway activity like primary-amine oxidase, indole pyruvate decarboxylase and aldehyde dehydrogenase were increased for 146 %, 187 %, and 557 %, respectively. Therefore, ZVI was suitable for enhancement IAA production from mixed culture anaerobic fermentation.

Influence of adding strategy of biochar on medium-chain fatty acids production from sewage sludge

Production of medium-chain fatty acids (MCFAs) from sewage sludge has dual effects on valuable sludge disposal and renewable energy generation, while low efficiency limits its application. Biochar addition is considered an effective method to improve MCFAs production. In this study, the influence of biochar adding strategies (i.e., adding biochar in acidification or chain elongation (CE) processes) on MCFAs production was explored. Results showed that by adding biochar in the acidification process, MCFAs accumulation increased by over 114%, accompanied by the highest carbon conversion efficiency (134.66%) and electron transfer efficiency of MCFAs (94.22%) by the terminal CE. Adding biochar before the acidification process better enriched CE bacteria (e.g., Paraclostridium) and strengthened the dominant metabolic pathway. In contrast, the biochar added before the CE process priorly enriched the bacteria capable of degrading organics, like unclassified_f__Dysgonomonadaceae, norank_f__norank_o__OPB41, and Acetobacterium. The differences in excessive ethanol oxidation and short-chain fatty acids accumulation induced by varied adding strategies might be responsible for this.

Research on the Resource Recovery of Medium-Chain Fatty Acids from Municipal Sludge: Current State and Future Prospects

The production of municipal sludge is steadily increasing in line with the production of sewage. A wealth of organic contaminants, including nutrients and energy, are present in municipal sludge. Anaerobic fermentation can be used to extract useful resources from sludge, producing hydrogen, methane, short-chain fatty acids, and, via further chain elongation, medium-chain fatty acids. By comparing the economic and use values of these retrieved resources, it is concluded that a high-value resource transformation of municipal sludge can be achieved via the production of medium-chain fatty acids using anaerobic fermentation, which is a hotspot for future research. In this study, the selection of the pretreatment method, the method of producing medium-chain fatty acids, the influence of the electron donor, and the technique used to enhance product synthesis in the anaerobic fermentation process are introduced in detail. The study outlines potential future research directions for medium-chain fatty acid production using municipal sludge. These acids could serve as a starting point for investigating other uses for municipal sludge.

Enhancement of medium-chain fatty acids production from sludge anaerobic fermentation liquid under moderate sulfate reduction

In recent years, there has been a marked increase in the production of excess sludge. Chain-elongation (CE) fermentation presents a promising approach for carbon resource recovery from sludge, enabling the transformation of carbon into medium-chain fatty acids (MCFAs). However, the impact of sulfate, commonly presents in sludge, on the CE process remains largely unexplored. In this study, batch tests for CE process of sludge anaerobic fermentation liquid (SAFL) under different SCOD/SO42- ratios were performed. The moderate sulfate reduction under the optimum SCOD/SO42- of 20:1 enhanced the n-caproate production, giving the maximum n-caproate concentration, selectivity and production rate of 5.49 g COD/L, 21.4% and 4.87 g COD/L/d, respectively. The excessive sulfate reduction under SCOD/SO42- ≤ 5 completely inhibited the CE process, resulting in almost no n-caproate generation. The variations in n-caproate production under different conditions of SCOD/SO42- were all well fitted with the modified Gompertz kinetic model. Alcaligenes and Ruminococcaceae_UCG-014 were the dominant genus-level biomarkers under moderate sulfate reduction (SCOD/SO42- = 20), which enhanced the n-caproate production by increasing the generation of acetyl-CoA and the hydrolysis of difficult biodegradable substances in SAFL. The findings presented in this work elucidate a strategy and provide a theoretical framework for the further enhancement of MCFAs production from excess sludge.

Performance and mechanisms of zero valent iron enhancing short-chain fatty acids production during thermophilic anaerobic fermentation of waste activated sludge

This work first explored the feasibility and possible mechanisms of zero valent iron (ZVI) pretreatment on the generation of short-chain fatty acids (SCFAs) during thermophilic anaerobic fermentation of waste activated sludge (WAS). Results showed that ZVI enhanced the quantity of SCFAs. On Day 6, the SCFAs production reached 455.84 ± 47.88 mg COD/g VSS at 5 g/L of ZVI addition, which increased by 63.80 % relative to control. The presence of ZVI can effectively promote butyric-based fermentation. ZVI accelerated the destruction of extracellular polymeric substances (EPS) and interior sludge cells, as well as improved biodegradation of soluble organics. Also, ZVI enhanced key enzyme activities (i.e., BK and CoA-), thus promoting degradation rates of acidogenesis (6.30 ± 0.84 mg/(gVSS·h) in glucose) and acetogenesis (74.63 ± 0.29 mg/(gVSS·h) in butyrate). Compared to Fe(III), the contribution of Fe(II) was higher among the decomposition products of ZVI. Besides, ZVI favored Proteobacteria and Actinobacteria, which enhanced acetate formation and organic compounds disassimilation of the process, respectively. The abundance of Tepidiphilus, Thermobrachium and Tepidimicrobium was increased, indicating promoting the system stability of SCFAs production in thermophilic anaerobic fermentation.

Permanganate (PM) pretreatment improves medium-chain fatty acids production from sewage sludge: The role of PM oxidation and in-situ formed manganese dioxide

Medium-chain fatty acids (MCFAs) production from sewage sludge is mainly restricted by the complex substrate structure, competitive metabolism and low electron transfer rate. This study proposes a novel permanganate (PM)-based strategy to promote sludge degradation and MCFAs production. Results show that PM pretreatment significantly increases MCFAs production, i.e., attaining 12,036 mg COD/L, and decreases the carbon fluxes of electron acceptor (EA)/electron donor (ED) to byproducts. Further analysis reveals that PM oxidation enhances the release and biochemical conversion of organic components via disrupting extracellular polymers (EPS) structure and reducing viable cells ratio, providing directly available EA for chain elongation (CE). The microbial activity positively correlated with MCFAs generation are apparently heightened, while the competitive metabolism of CE (i.e., methanogensis) can be completely inhibited. Accordingly, the functional bacteria related to critical bio-steps and dissimilatory manganese reduction are largely enriched. Further mechanism exploration indicates that the main contributors for sludge solubilization are 1O2 (61.6 %) and reactive manganese species (RMnS), i.e., Mn(V)/Mn(VI) (22.3 %) and Mn(III) (∼16.1 %). As the main reducing product of PM reaction, manganese dioxide (MnO2) can enable the formation of microbial aggregates, and serve as electron shuttles to facilitate the carbon fluxes to MCFAs during CE process. Overall, this strategy can achieve simultaneous hydrogen recovery, weaken competitive metabolisms and provide electron transfer accelerator for CE reactions.

Biosynthesis mechanisms of medium-chain carboxylic acids and alcohols in anaerobic microalgae fermentation regulated by pH conditions

Valorization of microalgae into high-value products and drop-in chemicals can reduce our dependence on non-renewable fossil fuels in an environmentally sustainable way. Among the valuable products, medium-chain carboxylic acids (MCCAs) and alcohols are attractive building blocks as fuel precursors. However, the biosynthetic mechanisms of MCCAs and alcohols in anaerobic microalgae fermentation and the regulating role of pH on the microbial structure and metabolism interaction among different functional groups have never been documented. In this work, we systematically investigated the roles of pH (5, 7, and 10) on the production of MCCAs and alcohols in anaerobic microalgae fermentation. The gene-centric and genome-centric metagenomes were employed to uncover the dynamics and metabolic network of the key players in the microbial communities. The results indicated that the pH significantly changed the product spectrum. The maximum production rate of alcohol was obtained at pH 5, while pH 7 was more beneficial for MCCA production. Metagenomic analysis reveals that this differential performance under different pH is attributed to the transformation of microbial guild and metabolism regulated by pH. The composition of various functional groups for MCCA and alcohol production also varies at different pH levels. Finally, a metabolic network was proposed to reveal the microbial interactions at different pH levels and thus provide insights into bioconversion of microalgae to high-value biofuels.IMPORTANCECarboxylate platforms encompass a biosynthesis process involving a mixed and undefined culture, enabling the conversion of microalgae, rich in carbohydrates and protein, into valuable fuels and mitigating the risks associated with algae blooms. However, there is little known about the effects of pH on the metabolic pathways of chain elongation and alcohol production in anaerobic microalgae fermentation. Moreover, convoluted and interdependent microbial interactions encumber efforts to characterize how organics and electrons flow among microbiome members. In this work, we compared metabolic differences among three different pH levels (5, 7, and 10) in anaerobic microalgae fermentation. In addition, genome-centric metagenomic analysis was conducted to reveal the microbial interaction for medium-chain carboxylic acid and alcohol production.

Critical Functions of Soil Components for In Situ Persulfate Oxidation of Sulfamethoxazole: Inherent Fe(II) Minerals-Coordinated Nonradical Pathway

Naturally occurring iron (Fe) minerals have been proved to activate persulfate (PS) to generate reactive species, but the role of soil-inherent Fe minerals in activating PS as well as the underlying mechanisms remains poorly understood. Here, we investigated sulfamethoxazole (SMX) degradation by PS in two Fe-rich soils and one Fe-poor soil. Unlike with the radical-dominant oxidation processes in Fe-poor soil, PS was effectively activated through nonradical pathways (i.e., surface electron-transfer) in Fe-rich soils, accounting for 68.4%-85.5% of SMX degradation. The nonradical mechanism was evidenced by multiple methods, including electrochemical, in situ Raman, and competition kinetics tests. Inherent Fe-based minerals, especially those containing Fe(II) were the crucial activators of PS in Fe-rich soils. Compared to Fe(III) minerals, Fe(II) minerals (e.g., ilmenite) were more liable to form Fe(II) mineral-PS* complexes to initiate the nonradical pathways, oxidizing adjacent SMX via electron transfer. Furthermore, mineral structural Fe(II) was the dominant component to coordinate such a direct oxidation process. After PS oxidation, low-crystalline Fe minerals in soils were transformed into high-crystalline Fe phases. Collectively, our study shows that soil-inherent Fe minerals can effectively activate PS in Fe-rich soils, so the addition of exogenous iron might not be required for PS-based in situ chemical oxidation. Outcomes also provide new insights into the activation mechanisms when persulfate is used for the remediation of contaminated soils.

Percarbonate-strengthened ferrate pretreatment for enhancing short-chain fatty acids production from sewage sludge

Sewage sludge management poses a pressing environmental challenge, demanding the implementation of sustainable solutions to facilitate resource recovery. Short-chain fatty acids (SCFAs) serve as valuable chemicals and renewable energy sources, underscoring the importance of maximizing their production to achieve sustainable waste management. Therefore, this study proposes a novel and green strategy, i.e., percarbonate-strengthened ferrate pretreatment to enhance SCFAs synthesis from sewage sludge, because percarbonate could activate ferrate oxidation through providing (bi) carbonate and hydrogen peroxide. Results show that percarbonate largely reduces the required ferrate dosage for fermentation improvement, and their combination exhibits obvious synergistic effects on SCFAs accumulation and sludge reduction. Under the optimal pretreatment conditions, SCFAs production is promoted to 3670.2 mg COD/L, representing a remarkable increase of 5512.4 %, 156.0 % or 395.1 % compared to the control, percarbonate alone or ferrate alone, respectively. Mechanism explorations demonstrate that percarbonate-strengthened ferrate pretreatment significantly enhances sludge solubilization, elevates substrate biodegradability, and alters the physiochemical properties of sludge to favor organics fermentation. The synergistic effects on solid organics release and sludge properties can be attributed to the combined mechanisms of enhanced oxidation and alkaline hydrolysis. Further investigations on metabolic pathways reveal that the combination substantially improves key enzyme activities associated with hydrolysis and SCFAs formation, while severely inhibits that of SCFAs consumption. These findings are further supported by the functional genes coding relevant enzymes. Moreover, the combination alters microbial structures and compositions, leading to the screening and enrichment of key microbes that facilitate SCFAs accumulation. This innovative strategy holds significant promise in advancing sewage sludge management towards a more circular and resource-efficient paradigm.

The performance and microbial response of zero valent iron alleviating the thermal-alkaline stress and enhancing hydrolysis-acidification of primary sludge

The biological thermal-alkaline hydrolysis-acidification (BTAHA) could promote sludge disintegration, which was conducive to producing volatile fatty acids (VFAs). However, high temperature and strong alkali could reduce the BTAHA effluent quality. Because high temperature denatures proteins and significantly changes the material and energy metabolism of bacteria, while strong alkali inhibits fermentation microorganisms (especially acid-producing microorganisms). This study investigated the internal mechanism of zero valent iron (ZVI) and magnetite (Mag.) alleviating temperature and alkali stress and improving the quality of hydrolysis-acidification effluent. At pH 7-10, compared with the control and magnetite, ZVI increased the average effluent VFAs by 24.0%-40.1% and 11.6%-18.1%, respectively. At pH 9, ZVI could provide an ecological niche for acidifying bacteria that preferred neutral and weakly alkaline conditions, with a 49.8% proportion of VFAs to soluble chemical oxygen demand (SCOD). At pH 12, the fluorescence intensity ratio of easy to difficult biodegradable organic matter in control, RMag., and RZVI were 0.63, 0.62, and 1.31, respectively. It indicated ZVI effectively alleviated high temperature and strong alkali stress. This study provides a reference for improving the quality of BTAHA effluent.

Metatranscriptome analysis unveils the mechanisms of zero-valent iron enhancing reactivation of starvation hydrolysis acidification sludge by inducing high-level gene expression

Hydrolysis acidification (HA) is a promising method for wastewater treatment and resource recovery. However, the extended time required for bacterial reactivation after starvation or a change in living conditions often poses a challenge to the efficient operation of the system. Although the addition of zero-valent iron (ZVI) could enhance HA performance, its effects on sludge reactivation in the HA process are not fully understood. In this study, ZVI was employed to accelerate sludge reactivation and its involved genetic mechanisms were unveiled. The results demonstrated that ZVI addition activated the sludge within 35 days with stable HA performance. Sludge characteristics revealed that ZVI improved active biomass, enzyme activity (by 11.4 % ∼ 26.7 %), ETS activity (by 566 %), and cell viability, with a higher concentration of MLVSS, live cells, more microbial byproducts in EPS, and relative abundance of HA bacteria (63.41 %). Moreover, metatranscriptome analysis showed that ZVI upregulated the expression of genes related to key enzymes in carbohydrate degradation metabolism, biosynthesis of electron transfer media such as heme and ubiquinone, and biosynthesis of vital cofactors like vitamin B12 and folate during microbial growth and metabolism. These findings suggest that ZVI enhanced electron transfer, bacterial growth, and metabolism, resulting in effective starch conversion and VFAs generation. Overall, these results deepen our understanding of the mechanism by which ZVI enhanced HA sludge reactivation, providing valuable information for addressing sludge starvation issues in HA systems.

In-situ production of lactate driving the biotransformation of waste activated sludge to medium-chain fatty acid

Medium-chain fatty acids (MCFAs) have drawn great attention due to their high energy density and superior hydrophobicity. Waste activated sludge (WAS) has been documented as a renewable feedstock for MCFAs production via anaerobic fermentation. However, MCFAs production from WAS depends on exogenous addition of electron donor (ED, e.g., lactate) for chain elongation (CE) bioprocess, which results in increased economic cost and limited practical application. In this study, a novel biotechnology was proposed to produce MCFAs from WAS with in-situ self-formed lactate by inoculating Yoghurt starter powder containing with Lactobacillales cultures. The batch experimental results revealed that the lactate was in-situ generated from WAS and the maximum production of MCFAs increased from 1.17 to 3.99 g COD/L with the increased addition of Lactobacillales cultures from 6✕107 to 2.3✕108 CFU/mL WAS. In continuous long-term test over 97 days, average MCFA production reached up to 3.94 g COD/L with a caproate yield of 82.74% at sludge retention time (SRT) 12 days, and the average MCFA production increased to 5.87 g COD/L with 69.28% caproate and 25.18% caprylate at SRT 15 days. A comprehensive analysis of the metagenome and metatranscriptome demonstrated that the genus of Lactobacillus and Streptococcus were capable of producing lactate from WAS and upgrading to MCFAs. Moreover, another genus, i.e., Candidatus Promineofilum, was firstly revealed that it might be responsible for lactate and MCFAs production. Further investigation of related microbial pathways and enzyme expression suggested that D-lactate dehydrogenase and pyruvate ferredoxin oxidoreductase contributed to lactate and acetyl-CoA production, which were the crucial steps for MCFAs generation and were most actively expressed. This study provides a conceptual framework of MCFAs from WAS with endogenous ED, potentially enhancing the energy recovery from WAS treatment.

Co-electron donors driven medium-chain fatty acids production: Roles of electron donors, reaction kinetics and metabolic pathways

Energy conversion of waste activated sludge alkaline fermentation liquor (WASAFL) to medium-chain fatty acids (MCFAs) is promising for sludge treatment and carbon recovery. However, the single electron donor (ED) fermentation for MCFAs production has irreparable defects. To resolve the respective shortcomings of single electron donor (ED) and improve the MCFAs production efficiency from WASAFL, a novel biotechnical process utilizing ethanol and lactate as co-EDs within different combination ratios were investigated. The results verified that MCFAs production was highest with ethanol to lactate ratio of 1:3 (6988.54 ± 208.18 mg COD/L), being 1.46 and 1.87 times of that with ethanol and lactate as single ED. The kinetic analysis results confirmed that ethanol to lactate ratio of 1:3 resulted in the highest MCFAs yield and formation rate. The microbial taxa results uncovered that the relative abundance of Sphaerochaeta and Haloimpatiens showed positive correlation with MCFAs production. The metabolic pathway analysis indicated that the ethanol oxidization, lactate oxidization, acrylate pathway, reverse β oxidization and fatty acid biosynthesis pathway might take place in the WASAFL fermentation system, contributing to the WASAFL-to-MCFAs conversion.

Advances in understanding entire process of medium chain carboxylic acid production from organic wastes via chain elongation

Chain elongation is an environmentally friendly biological technology capable of converting organic wastes into medium chain carboxylic acids (MCCAs). This review aims to offer a comprehensive analysis of MCCA production from organic wastes via chain elongation. Seven kinds of organic wastes are introduced and classified as easily degradable and hardly degradable. Among them, food waste, fruit and vegetable waste are the most potential organic wastes for MCCA production. Combined pretreatment technologies should be encouraged for the pretreatment of hardly degradable organic wastes. Furthermore, the mechanisms during MCCA production are analyzed, and the key influencing factors are evaluated, which affect the MCCA production and chain elongation efficiency indirectly. Extracting MCCA simultaneously is the most important way to improve MCCA production efficiency, and technologies for sequentially extracting different kinds of MCCAs are recommended. Finally, some perspectives for future chain elongation researches are proposed to promote the large-scale application of chain elongation.

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.

In situ reductive dehalogenation of groundwater driven by innovative organic carbon source materials: Insights into the organohalide-respiratory electron transport chain

In situ bioremediation using organohalide-respiring bacteria (OHRB) is a prospective method for the removal of persistent halogenated organic pollutants from groundwater, as OHRB can utilize H₂ or organic compounds produced by carbon source materials as electron donors for cell growth through organohalide respiration. However, few previous studies have determined the suitability of different carbon source materials to the metabolic mechanism of reductive dehalogenation from the perspective of electron transfer. The focus of this critical review was to reveal the interactions and relationships between carbon source materials and functional microbes, in terms of the electron transfer mechanism. Furthermore, this review illustrates some innovative strategies that have used the physiological characteristics of OHRB to guide the optimization of carbon source materials, improving the abundance of indigenous dehalogenated bacteria and enhancing electron transfer efficiency. Finally, it is proposed that future research should combine multi-omics analysis with machine learning (ML) to guide the design of effective carbon source materials and optimize current dehalogenation bioremediation strategies to reduce the cost and footprint of practical groundwater bioremediation applications.

Integrated microbial electrolysis with high-alkali pretreated sludge digestion: Insight into the effect of voltage on methanogenesis and substrate metabolism

Integrated microbial electrolysis with anaerobic digestion is proved to be an effective way to improve methanogenesis efficiency of waste activated sludge (WAS). WAS requires pretreatment for efficient improvement of acidification or methanogenesis efficiency, but excessive acidification may inhibit the methanogenesis. In order to balance these two stages, a method for efficient WAS hydrolysis and methanogenesis has been proposed in this study by high-alkaline pretreatment integrated with microbial electrolysis system. The effects of pretreatment methods and voltage on the normal temperature digestion of WAS have also been further investigated with emphasis on the effects of voltage and substrate metabolism. The results show that compared to low-alkaline pretreatment (pH = 10), high-alkaline pretreatment (pH > 14) can double the SCOD release and promote the VFAs accumulation to 5657 ± 392 mg COD/L, but inhibit the methanogenesis process. Microbial electrolysis can alleviate this inhibition effectively through the rapid consumption of VFAs and speeding up of the methanogenesis process. The optimal methane yield of the integrated system is 120.4 ± 8.4 mL/g VSS at the voltage of 0.5 V. Enzyme activities, high-throughput and gene function prediction analysis reveal that the cathode and anode maintain the activity of methanogens under high substrate concentrations. Voltage positively responded to improved methane yield from 0.3 to 0.8 V, but higher than 1.1 V is found to be unfavorable for cathodic methanogenesis and results in additional power loss. These findings provide a perspective idea for rapid and maximum biogas recovery from WAS.

Electricity-Driven Microbial Metabolism of Carbon and Nitrogen: A Waste-to-Resource Solution

Electricity-driven microbial metabolism relies on the extracellular electron transfer (EET) process between microbes and electrodes and provides promise for resource recovery from wastewater and industrial discharges. Over the past decades, tremendous efforts have been dedicated to designing electrocatalysts and microbes, as well as hybrid systems to push this approach toward industrial adoption. This paper summarizes these advances in order to facilitate a better understanding of electricity-driven microbial metabolism as a sustainable waste-to-resource solution. Quantitative comparisons of microbial electrosynthesis and abiotic electrosynthesis are made, and the strategy of electrocatalyst-assisted microbial electrosynthesis is critically discussed. Nitrogen recovery processes including microbial electrochemical N₂ fixation, electrocatalytic N₂ reduction, dissimilatory nitrate reduction to ammonium (DNRA), and abiotic electrochemical nitrate reduction to ammonia (Abio-NRA) are systematically reviewed. Furthermore, the synchronous metabolism of carbon and nitrogen using hybrid inorganic-biological systems is discussed, including advanced physicochemical, microbial, and electrochemical characterizations involved in this field. Finally, perspectives for future trends are presented. The paper provides valuable insights on the potential contribution of electricity-driven microbial valorization of waste carbon and nitrogen toward a green and sustainable society.

Effect of granular activated carbon and chloroform on chain elongation with simple substrate ethanol and acetate

Chain elongation is a promising technology for production of medium-chain fatty acids (MCFAs). Granular activated carbon (GAC) is commonly used in anaerobic fermentation. Low level CHCl₃ can inhibit methanogenesis and homoacetogenesis at the same time. However, the effect of them on chain elongation performance with highly enriched consortia and simple substrate (i.e., ethanol and acetate) was still unclear. Hence, the effects of CHCl₃ and on MCFAs production and the microbial community was studied here. CHCl₃ displayed fatal effect on chain elongation system when its concentration was higher than 0.1% v/v. 0.05% v/v CHCl₃ was enough to inhibit homoacetogens and further decreased the caproate production efficiency without altering the core bacteria tremendously. GAC was found to be adverse for chain elongation with simple substrate (i.e., ethanol and acetate) and highly enriched microbial consortia dominated by Clostridium sensu stricto, less than 20% electrons were finally distributed in caproate. It might be attributed to other electron consuming activities induced by GAC.

Deciphering the role and mechanism of nano zero-valent iron on medium chain fatty acids production from CO2 via chain elongation in microbial electrosynthesis

The integrated system of microbial electrosynthesis (MES) coupled with chain elongation has been considered a promising platform for carboxylic acids production. However, this biotechnology is still in its infancy, and many limitations are needed to be transcended, such as low electron transfer efficiency between cathode and microbes. In this study, nano zero-valent iron (NZVI) was employed to improve carboxylic acid production in the integrated system, and the promotion mechanisms were revealed. Results suggested that the highest production concentrations of acetate, butyrate, and caproate were observed at 7.5 g/L optimized NZVI dosage, increasing the total yield and coulomb efficiency by 23.7 % and 40.3 % compared to the control. Mechanism studies indicated that the hydrogen and electron released by the anaerobic corrosion of NZVI could be used as additional reducing equivalents, thereby enhancing the electron transfer performance. Besides, NZVI was also proven to facilitate the formation of electroactive biofilms according to the results of biofilm characterization and total DNA. In functional microbes' respect, the moderate NZVI enriched the chain elongator in biofilm, like Clostridium_sensu-stricto_12, and upregulated the activities of key enzymes of homoacetogenesis and chain elongation metabolic pathways, like carbon-monoxide dehydrogenase and hydroxyacyl-CoA dehydratase. This study provided the evidence and revealed how NZVI assisted carboxylic acid production from CO₂ via chain elongation in MES.

Zero-valent iron effectively enhances valuable products generated from wastewater containing 2-bromo-4,6-dinitroaniline during hydrolysis acidification process: Performance and mechanisms

Treatment to remove 2-bromo-4,6-dinitroaniline (BDNA) from wastewater is urgently needed owing to its carcinogenicity, mutagenicity, and teratogenicity. Hydrolysis acidification (HA) is widely used to treat wastewater to improve biodegradability and resource utilization. Thus, a zero-valent iron (ZVI)-coupled HA system was operated to treat BDNA-containing wastewater for the first time, with emphasis on the performance and enhanced mechanisms. The improved results for BDNA removal efficiency and B/C ratio and the decreased acute toxicity suggested that ZVI addition benefited the formation of advantageous products for subsequent biological treatment. The volatile fatty acids (VFAs) ratio (C_HAc:C_HPr:C_HBu) was optimized from 21:5:4 to 29:5:6, which benefited the utilization of wastewater resources for lipid generation. ZVI characterization, density functional theory (DFT) calculations, extracellular polymeric substances (EPS) analysis, molecular ecological network analysis (MENA), and redundancy analysis (RDA) of the microbial community further revealed that the enhanced mechanisms were summarized as beneficial interactions between ZVI and microorganisms. The ZVI was protected from excessive corrosion and lowered the oxidation-reduction potential (ORP), a key environmental factor, resulting in differences in microbial communities. These differences were presented as the enrichment of keystone species (e.g., Lactococcus), which function in BDNA reduction and VFAs generation. Moreover, ZVI promoted electron transfer, as proven by the high electron transfer capacity (ETC) of 0.452 and 0.361 μmol e^-/g VSS in the R_ZVI and blank systems, respectively.

A novel strategy for efficiently transforming waste activated sludge into medium-chain fatty acid using free nitrous acid

The global energy crisis is approaching due to rapid population growth and overexploitation of fossil fuels. Therefore, the development and use of new and renewable energy sources is already in the extreme urgency. This work developed a novel technology to efficiently produce renewable liquid bioenergy from discarded wastes, by effectively transforming sewage sludge into high-value medium chain fatty acids (MCFA). The maximum MCFA yield in the anaerobic sludge fermentation was revealed to be 10.6 times of control when utilizing sewage sludge with 1.78 mg-N/L free nitrous acid (FNA) pretreatment. The carbon flow from sewage sludge into MCFA in the fermentation system was significantly enhanced with appropriate levels (0.71-1.78 mg-N/L) of FNA pretreatment. Compared to FNA pretreatment, however, its direct addition severely inhibited total products (i.e., carboxylates and complex alcohols) generation because of the toxicity on live cells (decreasing to 8.3 %-13.9 %) in sludge. Kinetic models (one-substrate and two-substrate) were utilized to investigate the mechanism of MCFA promotion by FNA pretreatment on anaerobic sludge fermentation, in which linear relationship analysis between FNA-derived organic release and the fitted parameters were also performed. The results indicated that the conversion of refractory materials into rapidly bioavailable substrates for MCFA production contributed to increasing MCFA production rate and potential. Moreover, the relative abundances of functional microorganisms related to hydrolysis-acidification and chain elongation process increased under FNA pretreatment, further favoring the MCFA production. This study provides a novel and effective technology of sludge energy recovery that can achieve the next-generation sustainable sewage sludge management.

Nonmonotonic effect of CuO nanoparticles on medium-chain carboxylates production from waste activated sludge

The growing applications of CuO nanoparticles (NPs) in industrial and agriculture has increased their concentrations in wastewater and subsequently accumulated in waste activated sludge (WAS), raising concerns about their impact on reutilization of WAS, especially on the medium-chain carboxylates (MCCs) production from anaerobic fermentation of WAS. Here we showed that CuO NPs at 10-50 mg/g-TS can significantly inhibit MCCs production, and reactive oxygen species generation was revealed to be the key factor linked to the phenomena. At lower CuO NPs concentrations (0.5-2.5 mg/g-TS), however, MCCs production was enhanced, with a maximum level of 37% compared to the control. The combination of molecular approaches and metaproteomic analysis revealed that although low dosage CuO NPs (2.5 mg/g-TS) weakly inhibited chain elongation process, they displayed contributive characteristics both in WAS solubilization and transport/metabolism of carbohydrate. These results demonstrated that the complex microbial processes for MCCs production in the anaerobic fermentation of WAS can be affected by CuO NPs in a dosage-dependent manner via regulating microbial protein expression level. Our findings can provide new insights into the influence of CuO NPs on anaerobic fermentation process and shed light on the treatment option for the resource utilization of CuO NPs polluted WAS.

Dioctyl phthalate enhances volatile fatty acids production from sludge anaerobic fermentation: Insights of electron transport and metabolic functions

As one of the most widely used phthalate plasticizers, dioctyl phthalate (DOP) has been detected in wastewater and accumulates in sludge through wastewater treatment, which may adversely affect further sludge treatment. However, the role of DOP on sludge anaerobic fermentation and its mechanism are not yet clear. Therefore, this study focused on the effect of DOP on the volatile fatty acids (VFAs) generation via the anaerobic fermentation of sludge. The results demonstrated that the presence of DOP had a considerable contribution to the generation of VFAs, and the maximum production of VFAs reached 4769 mg COD/L at 500 mg/kg DOP, which was 1.57 folds that of the control. Mechanistic investigation showed that DOP mainly enhanced the hydrolysis, acidification and related enzymes activities of sludge. VFAs-producing microorganisms (e.g., Clostridium and Conexibacter) were also enriched under DOP exposure. Importantly, the presence of DOP increased the electron transfer activity by 26 %, consequently facilitating the organics conversion and fermentation process. Notably, the functional gene expressions involved in substrate metabolism and VFAs biosynthesis were enhanced with DOP, resulting in increased VFAs production from sludge. The results obtained in this study offered a new strategy for the control of pollutants and the recycling of valuable products from sludge.

Applying Dynamic Magnetic Field To Promote Anaerobic Digestion via Enhancing the Electron Transfer of a Microbial Respiration Chain

Electrochemical methods have been reported to strengthen anaerobic digestion, but the continuous electrical power supply and the complicated electrode installed inside the digester have restricted it from practical use. In this study, a dynamic magnetic field (DMF) was placed outside a digester to induce an electromotive force to electrically promote anaerobic digestion. With the applied DMF, an electromotive force of 0.14 mV was generated in the anaerobic sludge, and a 65.02% methane increment was obtained from the anaerobic digestion of waste-activated sludge. Experiments on each stage of anaerobic digestion showed that acidification and methanogenesis that involve electron transfer of respiration chains were promoted with the DMF, while solubilization and hydrolysis less related to respiration chains were not enhanced. Further analysis indicated that the induced electromotive force polarized the protein-like substances in the sludge to increase the conductivity and capacitance of the sludge. Electrotrophic methanogens (Methanothrix) and exoelectrogens (Exiguobacterium) were enriched with DMF. The kinetic isotope effect test confirmed that electron transfer was accelerated with DMF. Consistently, the concentration ratio of co-enzymes (NADH/NAD⁺ and F₄₂₀H₂/F₄₂₀₎ that reflects the electron exchange with respiration chains significantly increased. Applying the DMF seemed a more accessible strategy to electrically strengthen anaerobic digestion.

Clostridium kluyveri enhances caproate production by synergistically cooperating with acetogens in mixed microbial community of electro-fermentation system

As a chain elongation (CE) model strain, Clostridium kluyveri has been used in the studies of bioaugmentation of caproate production. However, its application in the novel electro-fermentation CE system for bioaugmentation is still unclear. In this study, the CE performances, with or without bioaugmentation and in conventional or electro-fermentation systems were compared. And the mechanism of electrochemical-bioaugmentation by constructing a co-culture of Acetobacterium woodii and Clostridium kluyveri were further verified. Results demonstrated that the bioaugmentation treatments have better CE performance, especially in electro-fermentation system, with a highest caproate concentration of 4.68 g·L^-1. Mechanism analysis revealed that C. kluyveri responded to the electric field and emerged synergy with the acetogens, which was proved by the increases of C. kluyveri colonization and the acetogens abundance in biofilm and supported by the co-culture experiment. This study provides a novel insight of microbial synergy mechanism of C. kluyveri during CE bioaugmentation in electro-fermentation system.

One-stage anaerobic fermentation of excess sludge for caproate production by supplementing chain elongation enrichments with ethanol as electron donor

Medium chain fatty acids (MCFAs) production from excess sludge have recently received great research interest due to higher energy densities, easy-separation capability and high economic benefits. Here, the addition of chain elongation (CE) enrichments with ethanol as electron donor was used to enhance caproate production from one-stage sludge fermentation. Compared with 0.20 g/L of controls, caproate production reached 9.00 g/L by supplementing CE enrichments with ethanol/acetate ratio of 3:1 after 7 days of acidification of organic matter in pretreated sludge fermentation. Clostridium_sensu_stricto_12, that refers to CE, was enriched in the first and second transfer of the sludge microbial consortium. Maintaining the stability of the microbial consortium would be the key that enables stable and efficient caproate production from sludge fermentation by supplementing CE enrichments.

Effects of sponge iron dosage on nitrogen removal performance and microbial community structure in sequencing batch reactors

The application of sponge iron (SI) carriers can improve the biochemical treatment performance of sequencing batch reactors (SBR) during wastewater treatment. This study used SBR reactors to explore the effects of SI dosage on the nitrogen removal performance and reactor stability and microbial community structure under low temperature and ultra-low load. In contrast to conventional SBR, the average removal rate of total nitrogen (TN) in the biological sponge iron system (BSIS) was increased by 5.38 % for 45 g/L, 18.93 % for 90 g/L, and 13.52 % for 135 g/L, respectively. The nitrogen removal performance and reactor stability showed the best performance under the SI dosage of 90 g/L. The addition of SI formed the anaerobic-anoxic-aerobic microenvironments, which facilitate the propagation of denitrifying bacteria (Saccharimonadales, Hydrogenophaga) and iron bacteria (Rhodoferax and Acinetobacter) in the BSIS. This study provides a new insight on the application of SI in the wastewater treatment.

Roles of zero-valent iron in anaerobic digestion: Mechanisms, advances and perspectives

With the rapid growth of population and urbanization, more and more bio-wastes have been produced. Considering organics contained in bio-wastes, to recover resource from bio-wastes is of great significance, which can not only achieve the resource recycle, but also protect the environment. Anaerobic digestion (AD) has been proved as one of the most promising strategies to recover bio-energy from bio-wastes, as well as to realize the reduction of bio-wastes. However, the conventional interspecies electron transfer is sensitive to environmental shocks, such as high ammonia, organic pollutants, metal ions, etc., which lead to instability or failure of AD. The recent findings have proved that the introduction of zero-valent iron (ZVI) in AD system can significantly enhance methane production from bio-wastes. This review systematically highlighted the recent advances on the roles of ZVI in AD, including underlying mechanisms of ZVI on AD, performance enhancement of AD contributed by ZVI, and impact factors of AD regulated by ZVI. Furthermore, current limitations and outlooks have been analyzed and concluded. The roles of ZVI on underlying mechanisms in AD include regulating reaction conditions, electron transfer mode and function of microbial communities. The addition of ZVI in AD can not only enhance bio-energy recovery and toxic contaminants removal from bio-wastes, but also have the potential to buffer adverse effect caused by inhibitors. Moreover, the electron transfer modes induced by ZVI include both interspecies hydrogen transfer and direct interspecies electron transfer pathways. How to comprehensively evaluate the effects of ZVI on AD and further improve the roles of ZVI in AD is urgently needed for practical application of ZVI in AD. This review aims to provide some references for the introduction of ZVI in AD for enhancing bio-energy recovery from bio-wastes.

Polystyrene microplastics and nanoplastics distinctively affect anaerobic sludge treatment for hydrogen and methane production

Microplastics and nanoplastics generally accumulated in waste activated sludge (WAS) after biological wastewater treatment. Currently, researches mainly focused on how plastics affected a particular sludge treatment method, without the comparison of different sludge systems. Herein, distinct responses of hydrogen-producing and methane-producing sludge systems were comprehensively evaluated with polystyrene microplastics (PS-MPs) and nanoplastics (PS-NPs) existence. Experimental results showed that PS particles would stimulate inhibition on anaerobic gas production except that PS-MPs were conducive to hydrogen accumulation, which was caused by the enhanced solubilization. Mechanistic investigation demonstrated that severe inhibition of PS-NPs to hydrogen production was derived from the excessively inhibitory hydrolysis despite of improving solubilization. Varying degrees of inhibition to acidification and methanation collectively contributed to reduced methane accumulation with exposure to PS-MPs and PS-NPs. Excessive oxidative stress would be generated in the presence of PS-MPs or PS-NPs, deteriorating microbial activities and richness of species responsible for hydrogen or methane production.

Strategy of electron acceptors for ethanol-driven chain elongation from kitchen waste

A two-phase kitchen waste (KW) fermentation was proposed in the current study to enhance medium-chain fatty acids (MCFAs) production from kitchen waste. In particular, effect of acetate to butyrate ratio (ABR) on MCFAs production was investigated which can be regulated by different pH and organic loading during the acidification phase. Medium ABR (1.00) was obtained when pH is 5.5 and organic loading is 20 g VS/L in FW acidification fermentation. Subsequent chain elongation fermentation demonstrated that the highest yield of caproate 9.67 g/L with selectivity of 79 %, and highest ethanol conversion efficiency of 1.11 was achieved in medium ABR system. Microbial community study showed that medium ABR significantly enrich the functional bacteria especially Clostridium kluyveri. The study provides a new method for chain elongation enhancement without addition of other additives in kitchen waste fermentation system and gives a guide for the regulation of the short-chain fatty acids distribution in its acidification phase.

Deciphering the role of microplastic size on anaerobic sludge digestion: Changes of dissolved organic matter, leaching compounds and microbial community

Here the role of microplastic size on dissolved organic matter, leaching compounds and microbial community during anaerobic sludge digestion was evaluated. Compared to that without the addition of polyvinyl chloride (PVC), during the 30 days' incubation, the anaerobic sludge digestion by adding PVC at the size of 75 μm and the concentration of 2.4 g/g volatile solids (VS) showed a 8.5% lower cumulative methane production, while a 17.9% higher cumulative methane production was noted by adding PVC at the size of 3000 μm and the concentration of 2.4 g/g VS. A long-term fed-batch laboratory-scale fermenter test for 147 days further testified, that higher removal efficiencies of total solids, volatile solids, and total chemical oxygen demand, and higher methane production were noted by adding PVC (2.4 g/g VS, 3000 μm) into the fermenter. More interestingly, higher concentrations of proteins, polysaccharides, volatile fatty acids, and soluble microbial by-products component were noted in the liquid phase of sludge drawn from the fermenter added with PVC since the biomass therein showed higher efficiencies of solubilization, hydrolysis, acidification, and methanogenesis. Moreover, as identified from the fermenter added with PVC, dibutyl phthalate (DBP) was the most predominant leaching phthalates compound, although the biomass therein showed a 93.4% anaerobic biodegradability of DBP. The leaching of DBP drove the predominance of microbial community towards Synergistota and Methanosaeta. More irregular elliptical shallow dimples were noted on the PVC surface after 147 days' incubation, accompanied with abundances of Proteobacteria, Actinobacteriota, Chloroflexi, Methanosaeta and Methanobacterium. The results from this study showed that the size of microplastic was a crucial factor in evaluating its impact on anaerobic sludge digestion.

New insight into mechanisms of ferroferric oxide enhancing medium-chain fatty acids production from waste activated sludge through anaerobic fermentation

Medium chain fatty acids (MCFAs) production from waste activated sludge (WAS) is restricted by poor biodegradability of WAS and low electron transfer efficiency. Herein, a novel ferroferric oxide (Fe₃O₄) technique was proposed. Results indicated that the MCFAs yield and selectivity were respectively enhanced by 155.4% and 66.7% in the Fe₃O₄-mediated WAS. Mechanistic studies disclosed that Fe₃O₄ promoted substrates degradation through conducting dissimilatory iron reduction (DIR) and stimulating hydrolase activity, providing precursors for chain elongation (CE). Generally, Fe₃O₄ improved the key processes for MCFA production at different degrees, i.e., hydrolysis, acidification and CE. Interestingly, MCFAs yield enhancement was primarily ascribed to facilitated electron transfer rather than DIR or produced ferrous iron, which could be supported by the analyses of electrochemical properties, electron transfer system activity and morphology. Further, Fe₃O₄ shifted the key microorganisms in favor of MCFAs production. Overall, this strategy could improve MCFAs production, sludge dewatering and phosphorus removal, concurrently.

An assessment of how the properties of pyrochar and process thermodynamics impact pyrochar mediated microbial chain elongation in steering the production of medium-chain fatty acids towards n-caproate

Microbial chain elongation fermentation is an alternative technology for medium-chain fatty acid (MCFA) production. This paper proposed the addition of pyrochar and graphene in chain elongation to improve MCFA production using ethanol and acetate as substrates. Results showed that the yield of, and selectivity towards, C₆ n-caproate were significantly enhanced with pyrochar addition. At the optimal mass ratio of pyrochar to substrate of 2 g/g, the maximum n-caproate yield of 13.67 g chemical oxygen demand/L and the corresponding selectivity of 56.8% were obtained; this represents an increase of 115% and 128% respectively as compared with no pyrochar addition. Such improvements were postulated as due to the high electrical conductivity and surface redox groups of pyrochar. The optimal ethanol to acetate molar ratio of 2 mol/mol achieved the highest MCFA yield under pyrochar mediated chain elongation conditions. Thermodynamic calculations modelled an energy benefit of 93.50 kJ/mol reaction for pyrochar mediated n-caproate production.

Performance and mechanism of sodium percarbonate (SPC) enhancing short-chain fatty acids production from anaerobic waste activated sludge fermentation

A novel pretreatment technique (i.e., using Sodium percarbonate, SPC) to improve the short-chain fatty acids (SCFA) production waste activated sludge (WAS) was proposed in this study. Results indicated that the maximum SCFA production of 1605.7 mg COD/L and acetic acid of 52.9% were attained at 0.2 g SPC/g TSS, being 8.4 and 2.8 times of the control (191.3 mg COD/L and 19%), respectively. Meanwhile, the optimal time for SCFA accumulation was decreased from 6d (control) to 4d (0.2 g/g TSS). Mechanism explorations unraveled that SPC largely accelerated WAS solubilization and enhanced the bioavailability of organics released from WAS. It improved enzymatic activities related to hydrolysis and acidogenesis, while suppressed the Coenzyme F₄₂₀ responsible for SCFA consumption. Illumina MiSeq sequencing analysis showed that SPC substantially enhanced the relative abundances of hydrolytic and/or acid-forming microbes. Furthermore, CO₃^- and O₂^- were the key factors to production enhancement in SPC-involved sludge fermentation.

Unveiling the distinctive role of titanium dioxide nanoparticles in aerobic sludge digestion

Aerobic digestion is considered to be a common process for the stabilization of waste activated sludge (WAS) in the small-sized wastewater treatment systems, while the broad application of titanium dioxide nanoparticles (TiO₂ NPs) results in their unavoidable existence in WAS aerobic digestion, with its role in aerobic sludge digestion being never documented. This study set up a series of aerobic sludge digesters to evaluate the previously unknown role of TiO₂ NPs on the performance of the digesters. The volatile solids (VS) degradation percentage increased from 21.9 ± 0.6% to 26.9 ± 0.1% - 30.0 ± 0.3% with the different contents of TiO₂ NPs (0, 1, 20 and 50 mg/L). Similarly, the total inorganic nitrogen production increased from 23.1 ± 0.3 to 31.0 ± 0.1 mg N/g VS with the rising TiO₂ NPs concentrations from 0 to 50 mg/L. The microbial analysis suggested that TiO₂ NPs contributed to the accumulation of specific microbes correlated with the degradation of organic substances and the conversion of nitrogen compounds. Model-based analysis showed the higher biodegradability and hydrolysis rate of sludge with TiO₂ NPs. Further mechanistic studies indicated that the enhancement of WAS solubilization and the degradation of recalcitrant substances (e.g., humic acid and cellulose) contributed to the better performance of experimental aerobic digesters, which was confirmed by the fourier transformation infrared spectroscopy (FTIR) indicating the converting of these materials into biodegradable substrates for digestion with TiO₂ NPs. It could be inferred from this investigation that aerobic digestion rather than anaerobic digestion would be a more suitable treatment method for sludge containing TiO₂ NPs.

Medium-chain carboxylates production by co-fermentation of sewage sludge and macroalgae

The co-fermentation of sewage sludge and macroalgae at different mixing ratios was performed for medium-chain carboxylates (MCCs) production. The results showed that MCCs production was enhanced in co-fermentation groups due to the abundant readily available organics supplied by macroalgae and the alkaline buffer capacity provided by sewage sludge. Highest MCCs concentration of 112.7 mmol C/L (25.5 mmol C/g VS_added) was obtained in the co-fermentation group with sludge/macroalgae ratio of 4:6, which was higher than MCCs produced from the mono-fermentation of sewage sludge (41.7 mmol C/L, 9.4 mmol C/g VS_added) or macroalgae (79.9 mmol C/L, 18.1 mmol C/g VS_added). Microbial analysis showed that species from genus Romboutsia, Terrisporobacter, Clostridium_sensu_stricto_12, Paraclostridium, unclassified_f_Peptostreptococcaceae and Caproiciproducens were significantly positively correlated with MCCs production. Metabolic pathways analysis demonstrated that the co-fermentation promoted the chain elongation process by stimulating the rate-limiting steps involved in the conversion of ethanol to Acetyl-CoA and circular fatty acid biosynthesis pathway.

Corncob ash boosts fermentative hydrogen production from waste activated sludge

With the increasing demand for sustainable development, the recycling and utilization of wastes has received widespread attention. This study proposed a green method of using one waste, corncob ash, to boost microbial the production of hydrogen from another waste, waste activated sludge, during anaerobic fermentation. The corncob ash dosage and the fermentative hydrogen production was positively correlated, and the maximum production of hydrogen reached up to 46.8 ± 1.0 mL/g VS, which was about 3.5 times that of the control group without corncob ash dosage (17.0 ± 0.9 mL/g VS). Mechanistic studies found that corncob ash was beneficial to the solubilization, hydrolysis and acetogenesis processes involved in fermentative hydrogen production process. The microbial community analysis indicated that corncob ash enriched more hydrolytic microorganisms (e.g., Bacteroides sp. and Leptolinea sp.), and has less impact on acidifying microorganisms, compared to the control group. The strategy of using corncob ash to boost the production of hydrogen during anaerobic waste activated sludge fermentation proposed in this study might provide a new waste-control-waste paradigm, making sludge disposal and wastewater treatment more sustainable.

Metatranscriptomic insight into the effects of antibiotic exposure on performance during anaerobic co-digestion of food waste and sludge

Antibiotics are inevitably entered into anaerobic co-digestion (AcoD) system of food waste (FW) and sludge along with the addition of abundant antibiotic-containing activated sludge. However, the in-depth insights into antibiotics affecting AcoD performance have not comprehensively studied. In present study, the results showed that tetracycline (TC), sulfamethoxazole (SMZ) and erythromycin (ERY) inhibited and delayed methane production except for 5 mg/L ERY. By comparison, TC and SMZ significantly inhibited the cumulative methane yields (one-way ANOVA, p < 0.01), and the inhibition effects were magnified as the antibiotic level increased. Physicochemical and methane yield analysis indicated antibiotics inhibited hydrolysis process and delayed methanogenesis process, which was in line with the declined abundance of acetogenic Proteiniphilum and hydrogenotrophic Methanobacterium during AcoD. Furthermore, metatranscriptomic analysis demonstrated the microbial activities of major organic and energy metabolism were down-regulated under antibiotics exposure, thereby down-regulating the expressions of key coenzymes (coenzymes M, F420, methanofuran) biosynthesis for methanogenesis and methane metabolism. The declined methanogenesis activity was completely consistent with the inhibited activity of dominant Methanosarcina and methane production, proving the importance of Methanosarcina on methane production. This study provides new metatranscriptomic evidence into the effects of antibiotics on methanogenesis during AcoD.

Transforming waste activated sludge into medium chain fatty acids in continuous two-stage anaerobic fermentation: Demonstration at different pH levels

Bioenergy recovery in the form of medium-chain fatty acids (MCFAs) from waste activated sludge (WAS) is increasingly attractive, which are valuable building blocks for fuel production. This study experimentally demonstrated the long-term MCFAs (C6-C8) production from WAS in two-stage anaerobic sludge fermentation at different pH conditions, using continuously operated bench-scale anaerobic reactors. The WAS was continuously converted to short chain fatty acids (SCFAs, 3500-3800 mg chemical oxygen demand (COD)/L) at the first stage via alkaline anaerobic fermentation, which was directly fed into the second stage as both substrates and inoculum for MCFAs production through chain elongation (CE). The productions of MCFAs at the second stage were continuously studied under three different pH conditions (i.e., 10, 7 and 5.5). The results demonstrated that there was no significant MCFAs production at pH 10 during the steady state, whereas the MCFAs productions were clearly observed at both pH 7 and pH 5.5, with much higher MCFAs production from WAS at pH 7 (i.e., 10.32 g COD/L MCFAs) than that at pH 5.5 (i.e., 8.73 g COD/L MCFAs) during the steady state. A higher MCFAs selectivity of 62.3% was also achieved at pH 7. The relatively lower MCFAs production and selectivity at pH 5.5 was likely due to the higher undissociated MCFAs generated at pH 5.5, which would pose toxicity impact on CE microbes and thus inhibit the CE process. Microbial community analysis confirmed that the relative abundances of CE related microbes (e.g., Clostridium sensu stricto 12 sp. and Clostridium sensu stricto 1) increased at pH 7 compared to those at pH 5.5, which enabled more efficient MCFAs production from WAS.