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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.

Heterotrophic-sulfur autotrophic denitrification (HAD) has been proposed to be a prospective nitrogen removal process. In this work, the potential of fermentation liquid (FL) from waste-activated sludge (WAS) as the electron donor for denitrification in the HAD system was explored and compared with other conventional carbon sources. Results showed that when FL was used as a carbon source, over 99% of NO 3 - -N was removed and its removal rate exceeded 14.00 mg N/g MLSS/h, which was significantly higher than that of methanol and propionic acid. The produced sulfate was below the limit value and the emission of N 2 O was low (1.38% of the NO 3 - -N). Microbial community analysis showed that autotrophic denitrifiers were predominated in the HAD system, in which Thiobacillus (16.4%) was the dominant genus. The economic analysis showed the cost of the FL was 0.062 €/m 3 , which was 30% lower than that in the group dosed with methanol. Our results demonstrated the FL was a promising carbon source for the HAD system, which could reduce carbon emission and cost, and offer a creative approach for waste-activated sludge resource reuse., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Microbial internal storage processes have been demonstrated to occur and play an important role in activated sludge systems under both aerobic and anoxic conditions when operating under dynamic conditions. High-rate anaerobic reactors are often operated at a high volumetric organic loading and a relatively dynamic profile, with large amounts of fermentable substrates. These dynamic operating conditions and high catabolic energy availability might also facilitate the formation of internal storage polymers by anaerobic microorganisms. However, so far information about storage under anaerobic conditions (e.g., anaerobic fermentation) as well as its consideration in anaerobic process modeling (e.g., IWA Anaerobic Digestion Model No. 1, ADM1) is still sparse. In this work, the accumulation of storage polymers during anaerobic fermentation was evaluated by batch experiments using anaerobic methanogenic sludge and based on mass balance analysis of carbon transformation. A new mathematical model was developed to describe microbial storage in anaerobic systems. The model was calibrated and validated by using independent data sets from two different anaerobic systems, with significant storage observed, and effectively simulated in both systems. The inclusion of the new anaerobic storage processes in the developed model allows for more successful simulation of transients due to lower accumulation of volatile fatty acids (correction for the overestimation of volatile fatty acids), which mitigates pH fluctuations. Current models such as the ADM1 cannot effectively simulate these dynamics due to a lack of anaerobic storage mechanisms.

[Display omitted] • Metabolic pathways and corresponding functional microbes are presented. • Main challenges for restricting MCCs are introduced. • Application and suitable operational conditions for enhancing MCCs are summarized. • Future research perspective to expand MCCs production scale are discussed. The fast increase in fossil-based energy consumption has induced continuously rising energy prices and the urgent need for alternative chemicals. Producing medium-chain carboxylates (MCCs) through anaerobic fermentation from organic wastes provides an economical and sustainable strategy to generate renewable chemicals. To better improve MCCs production from open-culture fermentation, a comprehensive analysis of understanding MCCs produce, metabolism, functional microbes, restriction factors and corresponding resolve methods is currently needed. To this end, the formation pathways and functional microbes of MCCs were comprehensively introduced. Then, the restrictive factors and competitive reactions for MCCs production were summarized, followed by the corresponding resolve strategies with appropriate application conditions. The potential research need and future perspective were last suggested according to this review. [ABSTRACT FROM AUTHOR]

Zinc oxide nanoparticles (ZnO NPs) has been documented to accumulate in waste activated sludge (WAS). Nevertheless, its potential effect on hydrogen accumulation during anaerobic fermentation has never been reported. Such impact exists in anaerobic sludge treatment systems but is unrecognized before. Therefore, this work aims to explore the effect of ZnO NPs on hydrogen accumulation during WAS fermentation while avoiding hydrogen consumption by maintaining pH around 10. Deep insights into the mechanisms of the enhanced hydrogen production after dosing ZnO NPs can then be provided. The 21.1 to 74.0 mL-H 2 /g-VS hydrogen yields were obtained after dosing 0–150 mg/g-TS ZnO NPs into the system, which is 2.5 times higher than the control. The zinc ions released from ZnO NPs did not pose obvious cytotoxic effect on organisms based on the mechanism exploration of this study. The reactive oxygen species (ROS) induced by ZnO NPs facilitated sludge disintegration and improved the biodegradability of released organics, but restrained the bio-processes of hydrolysis and acetogenesis simultaneously. This was further verified by the microbial community analysis, as the abundance of key microbes related to hydrolysis and acetogenesis was reduced evidently in the presence of 150 mg/g-TS ZnO NPs. This study would provide a green and economic strategy for the sustainable engineering application of WAS containing ZnO NPs. [Display omitted] • ZnO NPs at 150 mg/g-TS enhanced hydrogen production by 3.5 folds of control. • ZnO NPs improved sludge disintegration and organics biodegradability. • ZnO NPs reduced the abundance of key hydrolytic microbes and acetogens. • ROS played an essential role in the cytotoxicity rather than the released Zn2+. [ABSTRACT FROM AUTHOR]

This work provided a novel anaerobic fermentation paradigm of inoculating yeast to internally generate ethanol from FW as ED for subsequent MCFAs production. [Display omitted] • High MCFAs production from FW was achieved with self-produced ethanol instead of external electron donor. • Ethanol was on-site generated from FW by the inoculation of yeast. • A coordinated metabolic route related to MCFAs production from FW was established. • This novel biotechnology does not require major changes to existing anaerobic fermentation infrastructure but reduce the cost. As food waste (FW) is a largely available and carbon-rich feedstock, bulk production of medium-chain fatty acids (MCFAs) via chain elongation (CE) from FW is of great interest to biotechnology. However, the development of this emerging biotechnology is limited by the high cost from external electron donor (ED) input. To solve this bottleneck, replacing external ED input with self-produced ED in the system is a key. Therefore, this study provided a novel anaerobic fermentation paradigm of inoculating yeast to internally generate ethanol from FW as ED for subsequent MCFAs production. Batch experimental results demonstrated that cumulative 1540 mg COD/L of MCFAs was produced from FW with endogenous ethanol when inoculating 4.50 × 107 cells/ml-FW Saccharomyces cerevisiae in the fermenter over 20 days. In contrast, only a very small amount of MCFAs were detected in the control (58.98 mg COD/L). In continuous operation over 61 days, around 1300 mg COD/L of MCFAs was steadily obtained in the long-term fermenter with yeast assisted, which was almost 1.49 times higher than that from the control. The enriching S. cerevisiae responsible for on-site ethanol generation and chain-elongating bacteria including Caproiciproducens and Oscillibacter jointly promoted FW-derived MCFAs productions. Higher abundance of genes encoding substrate degradation, ethanol biosynthesis and CE further ensured the higher MCFAs production in the yeast-assisted system. This biotechnology does not require major changes to the design and operation of existing anaerobic fermentation infrastructure. Instead, the utilization of FW was enlarged through producing high-value MCFAs while waiving the cost of external ED by using inexpensive cheap yeast. [ABSTRACT FROM AUTHOR]

Volatile fatty acids (VFA) from sewage sludge represent an excellent recovered resource from wastewater treatment. This study investigated four sludge pre-treatments (namely, potassium permanganate - KMnO 4 , initial pH = 10, initial pH = 2.5 and low-temperature thermal hydrolysis) by operating batch reactors under acidogenic fermentation conditions. Results revealed that 0.1 g KMnO 4 /g of total suspended solids represents the best pre-treatment obtaining up to 2713 mgCOD L−1 and 452 mgCOD/g of volatile suspended solids. These results also paralleled metataxonomic analysis highlighting changes in prokaryotic microbial structures of sewage sludge of the batch fermentations subjected to the different pre-treatments. [Display omitted] • Sewage sludge acidogenic fermentation was performed in batch scale reactors. • Potassium permanganate, pH and thermal hydrolysis were applied as pre-treatments. • Potassium permanganate increased VFA production by 45200% compared to control test. • VFA accounted for 95 ± 4 % of the soluble COD produced. • Comamonadaceae and Chlamydiae percentage abundance was reduced by permanganate. [ABSTRACT FROM AUTHOR]

The recovery of phosphorus (P) through vivianite crystallization offers a promising approach for resource utilization in wastewater treatment plants. However, this process encounters challenges in terms of small product size and low purity. The study aimed to assess the feasibility of using quartz sand as a seed material to enhance P recovery and vivianite crystal characteristics from anaerobic fermentation supernatant. Various factors, including seed dosage, seed size, Fe/P ratio, and pH, were systematically tested in batch experiments to assess their influence. Results demonstrated that the effect of seed enhancement on vivianite crystallization was more pronounced under higher seed dosages, smaller seed sizes, and lower pH or Fe/P ratio. The addition of seeds increased P recovery by 4.43% in the actual anaerobic fermentation supernatant and also augmented the average particle size of the recovered product from 19.57 to 39.28 μm. Moreover, introducing quartz sand as a seed material effectively reduced co-precipitation, leading to a notable 12.5% increase in the purity of the recovered vivianite compared to the non-seeded process. The formation of an ion adsorption layer on the surface of quartz sand facilitated crystal attachment and growth, significantly accelerating the vivianite crystallization rate and enhancing P recovery. The economic analysis focused on chemical costs further affirmed the economic viability of using quartz sand as a seed material for P recovery through vivianite crystallization, which provides valuable insights for future research and engineering applications. [Display omitted] • Quartz sand as seed material increased P recovery in anaerobic fermentation supernatant. • Seed-induced vivianite crystallization was enhanced at higher seed doses, smaller seed sizes, and lower pH or Fe/P ratios. • Quartz sand as seed material improved vivianite purity through reduced co-precipitation. • Quartz sand as seed material for vivianite crystallization was economically feasible. [ABSTRACT FROM AUTHOR]

[Display omitted] • The study firstly reviews MCFAs production from sewage sludge fermentation. • MCFAs synthesis can be suppressed by competitive metabolisms and sludge hydrolysis. • Competition and cooperation among microbes co-exist in sludge CE system. • Pretreatments and semi-/conductive materials are the main reinforcement strategies. • Pretreatments are efficient to impede byproducts formation and avoid BES dosing. Huge sludge volumes and increased energy demands are the main challenges faced by wastewater treatment plants (WWTPs) currently, while it stimulates organics transformation and carbon recovery from sludge treatment process. Due to higher energy density and greater hydrophobicity, medium-chain fatty acids (MCFAs) exhibit easier extraction and more widespread application than traditional short-chain fatty acids (SCFAs). Therefore, MCFAs synthesis from sewage sludge via chain elongation (CE) has recently attracted growing attention in anaerobic open culture fermentation. Nevertheless, a comprehensive and systematic review focused on MCFAs synthesis from sludge is not available so far. This study provides a critical review on sewage sludge-derived MCFAs synthesis. The metabolic pathways, potential influencing factors, and cooperative and competitive relationships among various microorganisms are first elucidated to understand the mechanisms, limitations, and microbial interactions involved in MCFAs production via open culture fermentation of sludge. Unfortunately, MCFAs synthesis from sewage sludge is mainly hampered by low organics accessibility, inferior electron transfer efficiency and presence of competitive microbes. To foster sludge degradation and high-value MCFAs production, various techniques based on pretreatment and facilitation of electron transfer have recently been proposed, which are comprehensively reviewed with the corresponding mechanisms meticulously discussed. Ultimately, knowledge gaps and new opportunities for sludge-derived MCFAs production are prospected. This review will offer theoretical and technological guidelines for the subsequent investigations and future engineering applications of MCFAs production from sewage sludge. [ABSTRACT FROM AUTHOR]

In this work, a low-cost alternative method (i.e., adding nitrate into WAS) to significantly enhance hydrogen production was reported. Experimental results showed that with an increase of nitrate addition from 0 to 362 mg/L, the maximal hydrogen production from acidic (pH 5.5) fermentation of WAS obviously increased from 12.6 ± 0.5 to 19.3 ± 0.9 mL per gram volatile suspended solids (VSS). The mechanism investigations illustrated more substrates were provided for subsequent hydrogen production. Although the nitrate added inhibited all the biological processes, its inhibition to the hydrogen consumption processes was much severer than that to the hydrogen production processes. The enzyme analyses on the long-term semi-continuous fermenters showed that the nitrate addition slightly inhibited the relative activities of protease, butyrate kinase, acetate kinase, CoA-transferase, and [FeFe] hydrogenase but largely suppressed the relative activities of coenzyme F 420 , carbon monoxide dehydrogenase, and adenylyl sulfate reductase. Image 1 •The presence of nitrate enhanced hydrogen production from waste activated sludge. •Nitrate accelerated the disruption of extracellular polymeric substances. •Nitrate inhibited all the biological processes in anaerobic fermentation. •Inhibition to hydrogen consumption was severer than that to hydrogen production. •A new "closed-loop" concept for the energy cycle of WWTPs was proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • CaO 2 mediated microalgal-bacterial consortium (MBC) system was first proposed. • CaO 2 enhanced SCFAs production from anaerobic fermentation for MBC system. • CaO 2 effectively promoted the breakdown of MBC sludge cells for organics releasing. • Phosphorus and COD removal of MBC system were promoted. Microalgal-bacterial consortium (MBC) is an effective way to meet increasingly stringent standards in wastewater treatment, with advantages in nutrient removal, resource recovery, and energy conservation. The efficient and stable operation of the MBC sewage treatment system is generally limited by the insufficient and unstable carbon source in the influent. In this study, a calcium peroxide-mediated anaerobic fermenter was configured to transfer the MBC sludge into the preferred carbon source, short-chain fatty acids (SCFAs) in-situ, to alleviate the limitations. Experimental results showed that after 4-day 0.15 g/g VS calcium peroxide mediated anaerobic fermentation, the production of SCFAs and the proportion of acetic acid reached 514.8 mg COD/g VS and 59 %, respectively. Scanning electron microscopy, COD mass balance analysis, and lactate dehydrogenase assays jointly showed that calcium peroxide effectively promoted the breakdown of MBC sludge cells, which led to the release of large amounts of organic matter and could be attributed to the released OH–, ·OH and ·O 2 – from calcium peroxide, thereby providing more bioavailable substrates for SCFAs production. Microbial community analysis indicated that the anaerobes associated with hydrolysis and degradation of refractory organic matters were enriched in the mediated fermenter and had positive correlation with SCFAs production. Using such fermentation liquid as an additional carbon source for influent sewage, the MBC process performance was significantly promoted, with phosphorus and COD removal and sludge settleability reaching 84.7 %, 96.1 %, and 33 %, respectively. Configuring calcium peroxide-mediated anaerobic fermentation may thus become a sustainable strategy to strengthen the MBC sewage treatment system, thus providing new ideas for wastewater treatment to reduce inputs and improve performance. [ABSTRACT FROM AUTHOR]

Productions of medium-chain fatty acids (MCFAs) from waste streams via chain elongation (CE) platforms are conducive to the development of a sustainable society. Open-culture anaerobic fermentation is a promising and cost-effective technique to reclaim MCFAs from waste. Nevertheless, due to the complex microbial structure and the intertwined metabolic interactions, unwanted competing reactions including methanogenesis, excessive ethanol oxidation (EEO) and acrylate pathways always persist in the fermentation systems. Successful production of MCFAs requires the suppression of these competing reactions that could consume the substrate or byproduct used for CE process. Aiming to enhance the production of such higher-value liquids, multiple strategies have been proposed to hinder the occurrence of these undesired reactions in open-culture systems. Compared to the MCFAs productions, the roles of competitive reactions in CE processes are less well elucidated and have never been specifically reviewed before. To this end, this review first gave a general overview of CE pathways at first, followed by the introduction of the common competitive reactions that existed in the fermentation systems. Both conventional and emerging strategies to prevent competing reactions from occurring were examined in detail from the aspects of mechanisms, practicability, and the potential for improvements. The outlook for strategies with better technical feasibility was then put forward to ensure more carbon is shunted to MCFAs productions rather than wasted via competitive reactions. [Display omitted] • Overview concerning chain elongation is provided. • Competitive reactions for producing MCFAs are introduced. • Novel strategies to prevent competitive reactions from occurring are discussed. • Potential strategies with better technique feasibility are proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Food waste-derived MCFAs was synthesised predominantly at pH 5. • Chain elongators were enriched maximally at pH 5. • Chain elongation pathways were improved at pH 5. • Restricted EEO limited the ethanol loss during MCFAs production. Using food waste (FW) as a sustainable feedstock for the production of valuable medium-chain fatty acids (MCFAs) would alleviate greenhouse gas emissions while allowing the efficient recovery of carbon contained in this biowaste. The fermentation pH is a crucial factor controlling the type and yields of the MCFAs derived from FW-fed fermentation by steering the biological reactions involved. However, the mechanism shifts that occur at varying pH values are not well understood. Therefore, this study assessed the potential role of fermentation pH (i.e., 5, 7, and 10) on the carbon recovery from FW as MCFAs and the associated mechanisms. Predominantly MCFAs (5.52 g COD/L) were synthesized at pH 5 with caproate formed as the main high-value product. Abundant but a bit fewer MCFAs (1.08 g COD/L) were yielded at pH 7, in which caproate also was the prevalent MCFAs. However, no obvious MCFAs were synthesized under pH 10. The pH also drove the change of key microbial communities that served as chain elongators, which were maximally enriched at pH 5 but were scarce at pH 10. More importantly, mildly acidic pH (i.e., 5) drove less ethanol oxidization, ensuring the effective usage of electron donor for higher MCFAs yield. This study revealed how pH steers the metabolic pathways of MCFAs production from FW in open-culture anaerobic fermentation from a metagenomic viewpoint. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Calcium peroxide affected hydrogen production in fermentation system. • Calcium peroxide facilitated solubilization and release of biodegradable organics. • Calcium peroxide inhibited methanogenesis process. • Contributions of ingredients were in the order of alkali > O 2 − > OH. Abstract Calcium peroxide (CaO 2), one of versatile and harmless inorganic peroxy compounds, has been recently used as a highly effective oxidant to degrade pollutants in sludge and a method to achieve sludge resource utilization. However, its impact on hydrogen production has never been studied before. This investigation therefore aims to fill this knowledge gap. Results indicated that with CaO 2 increased from 0.05 to 0.25 g/g VSS (volatile suspended solids), the maximum hydrogen yield increased from 0.77 to 10.55 mL/g VSS. The mechanism studies revealed that CaO 2 accelerated the breakage and death of sludge cells. Excitation emission matrix (EEM) analyses further indicated that CaO 2 increased the biodegradability of the released substances, which could provide more biodegradable organics for the following reactions. Although CaO 2 caused inhibitions to some extents to all the tested microbes, its inhibition to hydrogen consumers was much severer than that to hydrolytic microbes and hydrogen producers. The enzyme analyses also demonstrated that the suppression of calcium peroxide to the activities of enzymes related to hydrogen consumption process was much severer than that to the activities of the activities of enzymes related to hydrogen production process. Further investigations exhibited that alkali, O 2 − and OH radicals, were the major intermediate products of CaO 2 decomposition. All of them were verified to contribute the increased hydrogen production, and their contributions were in the order of alkali > O 2 − > OH. This is the first work demonstrating that CaO 2 can enhance hydrogen production from WAS. The findings reported in our paper not only expand the application field of CaO 2 but also deepen the understanding of the CaO 2 -involved sludge fermentation process. [ABSTRACT FROM AUTHOR]

Abstract Simultaneous sludge fermentation and nitrite removal is an effective approach to enhance nutrient removal from low carbon-wastewater. It was found in this work that the presence of nitrite largely promoted hydrogen production from acidic fermentation of waste activated sludge (WAS). The results showed that with an increase of nitrite from 0 to 250 mg/L, the maximal hydrogen yield increased from 8.5 to 15.0 mL/g VSS at pH 5.5 fermentation and 8.1–13.0 mL/g VSS at pH 6 fermentation. However, the maximal hydrogen yield from WAS fermentation at pH 8 remained almost constant (2.9–3.7 mL/g VSS) when nitrite was in the range of 0–250 mg/L. Further analyses revealed that free nitrous acid (FNA) rather than nitrite was the major contributor to the promotion of hydrogen yield. The mechanism investigations showed that FNA not only accelerated the disruption of sludge cells but also promoted the biodegradability of organics released, thereby provided more biodegradable substrates for subsequent hydrogen production. Although FNA inhibited activities of all microbes involved in the anaerobic fermentation, its inhibitions to hydrogen consumers were much severer than those to hydrolytic microorganisms and hydrogen producers. Further investigations with microbial community showed that FNA increased the abundances of hydrogen producers (e.g., Citrobacter sp.) and denitrifiers (e.g., Dechloromonas sp.), but reduced the abundances of hydrogen consumers (e.g., Clostridium_aceticum). This work demonstrated for the first time that FNA in WAS fermentation systems enhanced hydrogen production. The findings obtained expand the application field of FNA and may provide supports for sustainable operation of wastewater treatment plants. Graphical abstract Image 1 Highlights • Nitrate addition promoted hydrogen production from anaerobic fermentation of sludge. • Free nitrous acid (FNA) rather than nitrite was the major contributor to the promotion of hydrogen yield. • FNA not only accelerated the disruption of sludge cells but also promoted the biodegradability of organics released. • FNA increased the abundances of hydrogen producers and denitrifiers, but reduced the abundances of hydrogen consumers. [ABSTRACT FROM AUTHOR]

Ammonium and/or free ammonia (the unionized form of ammonium) are generally thought to inhibit the activities of microbes involved in anaerobic digestion of waste activated sludge. It was found in this work, however, that the presence of ammonium (NH 4 + -N) largely enhanced dark fermentative hydrogen production from alkaline pretreated-sludge. With the increase of initial NH 4 + -N level from 36 to 266 mg/L, the maximal hydrogen production from alkaline (pH 9.5) pretreated-sludge increased from 7.3 to 15.6 mL per gram volatile suspended solids (VSS) under the standard condition. Further increase of NH 4 + -N to 308 mg/L caused a slight decrease of hydrogen yield (15.0 mL/g VSS). Experimental results demonstrated that free ammonia instead of NH 4 + -N was the true contributor to the enhancement of hydrogen production. It was found that the presence of free ammonia facilitated the releases of both extracellular and intracellular constituents, which thereby provided more substrates for subsequent hydrogen production. The free ammonia at the tested levels (i.e., 0–444 mg/L) did not affect acetogenesis significantly. Although free ammonia inhibited all other bio-processes, its inhibition to the hydrogen consumption processes (i.e., homoacetogenesis, methanogenesis, and sulfate-reducing process) was much severer than that to the hydrolysis and acidogenesis processes. Further investigations with enzyme analyses showed that free ammonia posed slight impacts on protease, butyrate kinase, acetate kinase, CoA-transferase, and [FeFe] hydrogenase activities but largely suppressed the activities of coenzyme F420, carbon monoxide dehydrogenase, and adenylyl sulfate reductase, which were consistent with the chemical analyses performed above. [ABSTRACT FROM AUTHOR]

In this work, a mathematical model was developed to describe the dynamics of fermentation products in sludge alkaline fermentation systems for the first time. In this model, the impacts of alkaline fermentation on sludge disintegration, hydrolysis, acidogenesis, acetogenesis, and methanogenesis processes are specifically considered for describing the high-level formation of fermentation products. The model proposed successfully reproduced the experimental data obtained from five independent sludge alkaline fermentation studies. The modeling results showed that alkaline fermentation largely facilitated the disintegration, acidogenesis, and acetogenesis processes and severely inhibited methanogenesis process. With the pH increase from 7.0 to 10.0, the disintegration, acidogenesis, and acetogenesis processes respectively increased by 53%, 1030%, and 30% while methane production decreased by 3800%. However, no substantial effect on hydrolysis process was found. The model also indicated that the pathway of acetoclastic methanogenesis was more severely inhibited by alkaline condition than that of hydrogentrophic methanogenesis. [ABSTRACT FROM AUTHOR]

[Display omitted] • Metabolic pathways for producing LCAs are presented. • Competitive reactions for producing LCAs are introduced. • Suitable operational conditions for enhancing desired products are discussed. • Challenges and strategies to improve LCAs productions are evaluated. The continuously rising energy prices and the growth in petroleum consumption have induced the pertinent pursuit of producing alternative fuels. Utilizing long-chain alcohols (LCAs) yielded from anaerobic bioprocess as renewable energy is a promising strategy to achieve the sustainable production of biofuels. Anaerobic fermentation is a sustainable process for reclaiming biodiesel from the organic wastes, which has received increasing attention due to its potential in producing renewable energy. Open-culture fermentation is preferred over single-species fermentation to yield the said alcohols because of its lower operating and capital costs. To better explore the LCAs productions from open-culture fermentation, a comprehensive understanding of this subject is currently needed but not available yet. To this end, the formations of LCAs and their possible precursors, medium-chain fatty acids (MCFAs), were systematically evaluated at first time with the focus on metabolic platforms, interspecies interactions, and competing microbial reactions. Suitable operational conditions and challenges were then synthesized, followed by the discussion on the viability of adopting current strategies towards higher alcohols productivities. The potential opportunities for enhancing LCAs outputs via biological processes were then suggested based on the review. [ABSTRACT FROM AUTHOR]

The growing global demand for petroleum-derived products and alarms concerning to depletion of crude oil have encouraged the conversion of organic wastes into medium-chain fatty acids (MCFAs), the precursor molecules biofuels. To this end, anaerobic fermentation-based technology has received a great deal of interests, as such eco-friendly technique can produce these value-added chemicals efficiently and sustainably. Open-culture fermentation is preferred to generate the said carboxylates given to its lower capital and operating costs than axenic systems. However, the underlying microbial pathways and the microbial interactions are not well understood. Therefore, a comprehensive understanding of the MCFAs productions from open-culture fermentation would benefit the valorisation of wastes by forming products with higher commercial value. To this end, this review article firstly covered a systematic introduction regarding the MCFAs formations through open-culture fermentation from the aspects of metabolic platforms and competitive bio-reactions. Suitable operational conditions and challenges are then scrutinized to discuss the feasibility of up-to-date strategies towards higher productivities. The potential opportunities for improving MCFAs productions biologically are finally proposed based on the content of the review. [Display omitted] • Metabolic pathways for producing MCFAs from biowastes are introduced. • Competitive reactions for producing MCFAs from biowastes are presented. • Suitable operational conditions for enhancing MCFAs from biowastes are discussed. • Challenges and strategies to improve MCFAs productions are scrutinized. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • MCFAs production from sludge was achieved in continuous two-stage system. • MCFAs production was completely suppressed at pH 10 at s stage. • Higher MCFAs production was observed at pH 7 than at pH 5.5. • Higher undissociated MCFAs at pH 5.5 might inhibit the CE microbes. • Key CE related microbes were enriched more at pH 7 than those at pH 5.5. [ABSTRACT FROM AUTHOR]

[Display omitted] • Coconut shell ash enhanced SCFAs production from algae. • Coconut shell ash promoted solubilization, hydrolysis and acetogenesis. • Coconut shell ash increased alkalinity to tolerate higher acidic environments. • Coconut shell ash enriched microbes related to hydrolysis and acetogenesis. This study proposed a novel method to enhance short-chain fatty acids (SCFAs) production from anaerobic algae fermentation by using coconut shell ash. The maximum SCFAs production was 683.0 mg COD/g VS at the ash dosage of 1.2 g/g TS, which was about 1.4-folds that of the control, and the enhancement of acetate production was the main path for the promotion of SCFAs. Coconut shell ash increased the pH and alkalinity of digestate, thereby reducing the use of alkaline reagents and being more resistant to acidic environments. Coconut shell ash promoted the processes of solubilization, hydrolysis and acetogenesis, and enriched hydrolytic microorganisms (e.g., Candidatus Microthrix) and acidifying microorganisms with acetate as substrate (e.g., Caldilinea and Proteiniphilum). Anaerobic fermentation residue with ash containing inorganic elements has the potential to be used as fertilizer, making this waste-control-waste strategy with more economic and environmental benefits for potential practical applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • TCS was partially degraded in anaerobic fermentation process. • TCS inhibited hydrogen production from anaerobic fermentation of sludge. • The presence of TCS inhibited acidogenesis process. • High level of TCS changed the relative abundances of functional microbes. Triclosan (TCS), a widely used antibacterial agent, was accumulated at significant levels in waste activated sludge (WAS). To date, however, the interaction between TCS and sludge anaerobic fermentation was rarely reported. Hence, this work aimed to deeply understanding the degradation of TCS in sludge anaerobic fermentation and its impact on hydrogen production. Experimental results showed that ~ 45% of TCS was degraded in long-term anaerobic fermentation, with 2,4-dichlorophenol as its main intermediate. Based on the information from high performance liquid chromatography–mass spectrometry analysis, three pathways i.e., dechlorination, hydroxylation, and cleavage of ether bonds, were proposed for TCS degradation. It was found that the maximum hydrogen yield decreased from 18.6 to 12.8 mL/g VSS with the increase of TCS from 12 to 487 mg/kg TSS. One possible reason for the decreased hydrogen yield was that a part of hydrogen generated might serve as electron donors for TCS dechlorination. Besides, the presence of TCS significantly suppressed acidogenesis (an important step responsible for hydrogen generation). This inhibition to acidogenesis is likely due to that the high-affinity functional groups of TCS such as hydroxyl groups could bind to the active sites of acetate kinase (AK, a key enzyme in acidogenesis), which reduced the active sites available for original fermentation substrates. Microbial analysis revealed that TCS increased the relative abundances of potential contaminant decomposers such as Guggeheimella but inhibited the populations of hydrogen producers such as Proteiniborus , which was consistent with the results obtained by chemical analyses. [ABSTRACT FROM AUTHOR]

Tetracycline (TC), a widely used antibiotic, was enriched in waste activated sludge (WAS) at significant levels. However, the TC impact on WAS anaerobic fermentation are still poorly understood. This work aims to analyze the effect of TC to the WAS anaerobic fermentation by investigating the differences of sludge properties, short-chain fatty acids (SCFAs) production and microbial community abundance. The results showed that the environmental level of TC had no effect on the SCFAs production, but with the further increase of the content of TC to 60 mg/kg TSS, the maximum SCFAs yield decreased from 125.1 ± 3.2 to 90.8 ± 1.7 mg COD/g VSS. Mechanism exploration indicated that TC had no significant effect on solubilization, hydrolysis and homoacetogenesis processes, but severely inhibited acidogenesis, acetogenesis and methanogenesis processes. Microbial analysis showed that the presence of TC reduced the diversity of microbial communities and the abundance of functional microorganisms relevant to SCFAs production and complex organic degradation, such as Proteiniclasticun and Novosphingobium. This negative effect was persistent because only a small amount of TC can be degraded in the anaerobic fermentation process. Hence, CaO 2 was proposed and studied as a regulation strategy that can reduce the toxicity of TC on anaerobic fermentation. [Display omitted] • When the TC level were 5, 30 and 60 mg/kg TSS, the SCFAs production decreased by 11.0%, 18.2%, and 27.4%, respectively. • TC severely inhibited acidogenesis, acetogenesis and methanogenesis processes. • The most of TC was stuck after anaerobic fermentation of WAS. • CaO 2 improved the SCFAs production by degrading TC and disintegrating sludge. [ABSTRACT FROM AUTHOR]

• Pretreatment with 0.1 g/g TSS KMnO 4 increased SCFAs yield from WAS to 7.4 times. • KMnO 4 pretreatment promoted the degradation of humus and lignocellulose. • KMnO 4 reduced the species and total detection frequency of organic contaminants. • KMnO 4 pretreatment enhanced the inactivation of the fecal coliforms. Potassium permanganate (KMnO 4), one typical strong oxidant, is used extensively in various applications. However, its impact on anaerobic fermentation of waste activated sludge is unknown. This paper therefore aims to investigate its effect on generation of short-chain fatty acids (SCFAs) and qualities of fermentation liquor and fermented solid sludge. Experimental results showed that when KMnO 4 increased from 0 to 0.1 g/TSS, the highest SCFAs production enhanced from 33.9 to 251.8 mg COD/g VSS. The mechanism analysis revealed that KMnO 4 addition increased the disintegration of sludge cells and the degradation of substantial recalcitrant organics in sludge such as humus and lignocellulose, thereby provided more substances for SCFAs generation. Ultraviolet absorption spectroscopy analysis showed that KMnO 4 addition destroyed unsaturated conjugated bonds and decreased organics aromaticity, thereby promoting humus and lignocellulose degradation. GC/MS analyses showed that several micro-molecular substances (e.g., acid-like, alkane-like, and alcohol-like organics) were generated from KMnO 4 pretreated humus and lignocellulose, and some of them were demonstrated to serve as substrates to produce SCFAs. The enzyme activity and model-based analyses showed that although KMnO 4 restrained all the microorganisms to some extends, the activities of SCFAs generators were much higher than those of SCFAs consumers. In addition, the sludge mixture was separated into liquid and solid fraction after anaerobic fermentation, and the components in fermentation liquid and solid sludge were tested. It was found that the addition of KMnO 4 effectively reduced the categories and total detected frequency of refractory organic pollutants in fermentation liquid, which improved the quality of the produced SCFAs. KMnO 4 addition also increased the inactivation of fecal coliforms and degradation of emerging contaminants in fermented sludge, which were helpful to its final disposal. [ABSTRACT FROM AUTHOR]

The utilization of waste activated sludge (WAS) to recover energy in the form of methane or short-chain fatty acids (SCFAs) is generally restricted by the low energy density of products and poor degradability of WAS. Herein, this study reported a novel alternative WAS fermentation technology to produce high-energy medium-chain fatty acids (MCFAs) from WAS in a one-stage anaerobic fermentation system using ethanol as electron donor. The MCFAs production and WAS degradation at different ethanol levels were investigated. The increased ethanol levels resulted in increasing MCFAs production (from 1875 to 6115 mg chemical oxygen demand (COD)/L) and selectivity (from 30.3 to 56.2%). The main MCFAs products were n-caproate and n-caprylate at lower level of ethanol, while n-caproate was the sole MCFA product at higher level of ethanol with longer chain alcohol (i.e., n-hexanol) produced as well. The ethanol markedly increased WAS degradation, with the greatest degradation (0.72 g COD/g volatile solids (VS)) being 1.9 times of that without ethanol (0.38 mg COD/mg VS, at 0 mmol/L), which was ascribed to the advancement of sludge solubilization, hydrolysis and acidification. Microbial community revealed that the ethanol participation induced the community shift to the favorable direction for hydrolysis-acidification and chain elongation in anaerobic WAS fermentation. Image 1 • MCFAs production from WAS and enhanced WAS degradation were simultaneously achieved. • Sludge solubilization, hydrolysis and acidification were enhanced by ethanol. • MCFAs production and selectivity were positively related to the ethanol addition. • Hydrolytic, acidification and CE microbes were enriched for MCFAs production. [ABSTRACT FROM AUTHOR]

• Medium-chain fatty acid (MCFA) and Long-chain alcohol (LCA) co-production was gained. • MCFA and LCA co-production is positively related to ethanol content. • High level of ethanol could improve the selectivity of MCFA but reduce that of LCA. • Key functional microbes for co-production of MCFA and LCA were enriched. Traditional bioenergy recovery in the form of short chain fatty acids (SCFAs) from waste activated sludge (WAS) is generally limited by economic unattractiveness and complexity of products separation. Herein, a novel biotechnology process of two-stage anaerobic fermentation for converting the WAS into high energy density, easy-separated medium chain fatty acids (MCFAs) and long-chain alcohols (LCAs) was evaluated. In this process, the WAS was first converted to WAS alkaline fermentation liquid (WASAFL), serving as electron acceptors (EAs) and inoculum, then adding ethanol as electron donor (ED) for chain elongation (CE). The co-production of MCFAs and LCAs during CE were studied under three different ED to EA ratios, i.e., 3:1, 4:1 and 5:1. Experimental results demonstrated that when the ratio of ED to EA increased from 3:1 to 5:1, the production of MCFA and LCAs respectively increased from 5.57 ± 0.17 and 2.58 ± 0.18 to7.67 ± 0.48 and 4.21 ± 0.19 g COD/L. A similar observation was made in the total product electron efficiency, increasing from 59.9% to 72.1%. However, the highest total product selectivity (i.e., 68.0%) and highest products production yield (i.e., 59.77%) were not achieved at the ED to EA ratio of 5:1 due to toxicity caused by higher accumulation of n-caproate. The kinetic analysis further confirmed that high ratio of ED to EA induced improvement in product maximum yield, production rate for both MCFAs and LCAs. Microbial community analysis indicated that Clostridium, Caproiciproducens, Acinetobacter, Exilispira , and Oscillibacter were clearly enriched in the CE reactor and had positive correlation with MCFAs and LCAs production. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Due to the poor biodegradability of released organics and the rapid consumption of hydrogen, hydrogen production from the untreated waste activated sludge (WAS) and/or inocula is still limited. In this study, it was found that the dark fermentative hydrogen production was largely enhanced from WAS pretreated by freezing in the presence of nitrite. With an increase of nitrite addition from 100 to 400 mg NO 2 −-N/L during freezing pretreatment (−5 °C for 4 h), the maximal hydrogen yield increased from 7.96 to 19.40 mL/g VS (volatile solids), which was 5.5–13.4 times of that in the control (without freezing and nitrite addition). Mechanism explorations revealed that the proposed pretreatment not only accelerated the disintegration of sludge but also promoted the proportion of biodegradable organics released, thereby provided more bio-available substrates for subsequent hydrogen production. Proposed pretreatment severely suppressed the sludge microorganisms responding to homoacetogenesis (−32.1%), methanogenesis (−58.4%), and sulfate-reducing process (−51.5%), inhibited the consumption of hydrogen. Moreover, there was more acetic and butyric (76% versus 57.5%) but less propionic acid (22.6% versus 13.4%) in this pretreated fermenter, which was in correspondence with the theory of fermentation type affecting hydrogen production. Long-term fermentation experiments indicated that the proposed pretreatment boosted the [FeFe]-hydrogenase activities while suppressed the activities of carbon monoxide dehydrogenase, coenzyme F420, and adenylyl sulfate reductase. Image 1 • FNP enhanced dark fermentative hydrogen production from WAS by 5.5–13.4 times. • FNP accelerated sludge disintegration and promoted organics biodegradability. • FNP suppressed the hydrogen-consuming bacteria by 32.1–58.4%. • FNP induced the shift of fermentation to butyrate-type. [ABSTRACT FROM AUTHOR]

Effective sludge treatment with bioenergy production is attracting increasing interests as large quantities of waste activated sludge (WAS) are produced during the wastewater treatment. In this study, a new biotechnical process for converting the WAS alkaline fermentation liquor (WASAFL) into valuable, easy-separated medium chain fatty acids (MCFAs) through chain elongation (CE) was investigated, which may provide a new insight into sludge treatment. In the process, ethanol was served as the electron donor (EDs) and WASAFL were main electron acceptors (EAs). The MCFAs productions were investigated under three different ED to EA ratios (i.e., 1:2, 1:1 and 2:1). The result showed that MCFAs production was increased from 2.88 ± 0.01 to 5.28 ± 0.18 g COD/L with the increase of ED to EA ratio. However, the highest MCFA selectivity was achieved at 72.9% when the ED to EA ratio was 1:1. The decrease in the selectivity at high ED:EA ratio is mainly due to the production of higher alcohol (i.e., n-butanol and n-hexanol). The thermodynamic analysis confirmed all CE processes for MCFAs production from WASAFL were exothermic reactions, with the spontaneity and energy release of the reactions increased with the ethanol level. The microbial community analysis showed that the relative abundances of Clostridium, Oscillibacter , Leptolinea and Exilispira were positively correlated with the MCFAs production. The metagenomic analysis suggested that both the reverse β-oxidization pathway and fatty acid biosynthesis pathway contributed to the CE process in the studied system. The functional enzymes were mainly associated within Clostridium, with Clostridium Kluyveri , Clostridium botulinum and Clostridium magnum being likely the key species responsible for the CE process. Image 1 • MCFAs production from WAS alkaline fermentation liquid was achieved. • MCFA production is positively related to the level of ethanol as electron donor. • A high level of ethanol could reduce the selectivity of MCFA production. • Key functional microbes involved C. Kluyveri , C. botulinum and C. magnum. • Production pathways included reverse β-oxidation and fatty acid biosynthesis. [ABSTRACT FROM AUTHOR]