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[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]

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]

[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]

[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]

[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]

[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]

[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]

• 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]