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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] • US-enhanced PAA pretreatment remarkably improved SCFAs yields from WAS. • US-enhanced PAA oxidation accelerated sludge disruption and organic matter release. • US-enhanced PAA pretreatment altered protein secondary structure. • 1O 2 was the dominant reactive species during US-enhanced PAA oxidation. • Acidogenic microorganisms were enriched by US-enhanced PAA pretreatment. This study proposed a new technology, i.e. , ultrasound (US)-enhanced peracetic acid (PAA) pretreatment, to enhance resource recovery from anaerobic sludge fermentation. Results show that the optimum operation condition was achieved with US-enhanced 10 mg PAA/g TSS (total suspended solids), leading to a remarkable 31.72-fold increase in short-chain fatty acids (SCFAs) production compared to the control group (with PAA contribution in SCFAs deducted). Mechanism explorations prove that US-enhanced PAA pretreatment significantly promoted sludge disruption and organic matter release, creating a substantial amount of soluble substances (i.e. , tyrosine-like and tryptophan-like substances) for subsequent fermentation. Fourier transform infrared (FTIR) analysis indicates that US-enhanced PAA pretreatment induced alterations in the protein secondary structure and facilitated its transformation. Quenching experiments imply that the contribution of reactive species during sludge oxidation followed the order: 1O 2 > OH > CH 3 C(O)O /CH 3 C(O)OO. These reactive species played essential roles in the inherent mechanisms responsible for sludge disintegration and organic conversion. On the other hand, the analysis of key enzyme activities reveals a severe inhibition of the enzymes associated with SCFAs consumption. Further microbial community investigation confirms the enrichment of acidogenic microorganisms, such as Clostridium_sensu_stricto_10 sp. , norank_f__norank_o__Bacteroidales sp. , etc. Overall, US-enhanced PAA pretreatment can facilitate energy recovery and enhance the efficiency of anaerobic fermentation, offering significant benefits in sustainable sludge management. [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]

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. [Display omitted] • MCFA production from WAS with in-situ self-formed lactate was achieved. • MCFA production was positively related to inoculated Lactobacillales cultures level. • The SRT of 15 days was preferable for MCFA production than the SRT of 12 days. • Key microorganisms were revealed to contribute to lactate and MCFA productions. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Sophorolipid enhanced the SCFA production from anaerobic fermentation of WAS. • Sophorolipid facilitated the WAS solubilization. • Sophorolipid enhanced release of biodegradable organics. • Inhibition of methanogens by sophorolipid led to accumulation of SCFA. • Sophorolipid increased the abundance of hydrolytic microbes and SCFA producers. Abstract It was found in this study that the presence of sophorolipid (SL) enhanced the production of short-chain fatty acid (SCFA) from anaerobic fermentation of waste activated sludge (WAS). Experimental results showed that with an increase of SL addition from 0 to 0.1 g/g TSS, the maximal SCFA yield increased from 50.5 ± 4.9 to 246.2 ± 7.5 mg COD/g VSS. The presence of SL reduced the surface tension between hydrophobic organics and fermentation liquid, which thereby accelerated the disintegration of WAS and improved the biodegradability of the released organics. SL promoted the carbon/nitrogen ratio of the fermentation system, enhancing the conversion of proteins in WAS. Moreover, SL suppressed severely the activities of methanogens, probably due to the drop of pH caused by SL addition. Amplicon sequencing analyses revealed that SL increased the abundance of hydrolytic microbes such as Bacteroides sp. and Macellibacteroides sp., and SCFA producers (e.g., Acinetobacter sp.). [ABSTRACT FROM AUTHOR]

• The combination of heat pretreatment with FA promoted hydrogen yield from WAS. • The accumulative hydrogen yield was in the order of blank < heat < FA < heat + FA. • The combination of heat pretreatment with FA accelerated WAS disintegration. • The combined heat pretreatment with FA severely inhibited hydrogen consumers. Controlling free ammonia in an anaerobic fermenter at pertinent levels is reported recently to be an economically attractive and practically feasible approach to enhance hydrogen yield from waste activated sludge (WAS). This paper reports a new technology for WAS dark fermentation, i.e., using heat pretreatment (70 °C for 60 min) to assist free ammonia for further improving hydrogen yield. The experimental results showed that the accumulative hydrogen production from combined reactors was promoted from 12.3 to 19.2 mL/g VSS (volatile suspended solids), the maximum of which was 1.8, 2.7, and 7.1 times of that from sole free ammonia (131.9 mg NH 3 -N/L), sole heat, and blank reactors, respectively. Mechanism explorations showed that the combination strategy significantly enhanced WAS disintegration, providing more substrates for hydrogen production. Moreover, the combination suppressed activities of all microbes associated with anaerobic fermentation, but its inhibition to hydrogen consumers was much severer than that to other microbes. [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]

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]

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]

Hydrogen production from waste activated sludge (WAS) was widely considered and intensively investigated as a promising technology to recover energy from wastewater treatment plants. To date, no efforts have been made on either systematic summarization or critical thinking of the application niche of hydrogen production from WAS treatment. It is therefore time to evaluate whether and how to recover hydrogen in a future paradigm of WAS treatment. In this critical review, the principles and potentials, microorganisms, possible technologies, and process parameters of hydrogen generation were analyzed. Microbial electrolysis cell shows high theoretical hydrogen yield and could utilize a variety of organic compounds as substrates, which is regarded as a prospective technology for hydrogen production. However, the poor organics utilization and rapid consumptions of produced hydrogen hindered hydrogen recovery from WAS. Based on the analysis of the current state of the literatures, the opportunities and challenges of hydrogen production from WAS are rethought, the detailed knowledge gaps and perspective of hydrogen production from WAS were discussed, and the probable solutions of hydrogen recovery from WAS treatment are figured out. To guide the application and development of hydrogen recovery, a more promising avenue through rational integration of the available technologies to form a hybrid process is finally proposed. The integrated operational paradigm of WWTPs could achieve substantial technical, environmental and economic benefits. In addition, how this hybrid process works is illustrated, the challenges of this hybrid process and future efforts to be made in the future are put forward. Image 1 • H 2 is mainly produced from the acidogenesis process of WAS dark fermentation. • Microbial electrolysis cell is a prospective technology for H 2 production. • The poor organics utilization and rapid H 2 consumption hinder hydrogen recovery. • A promising avenue through integration of the available technologies was proposed. • The hybrid process could achieve substantial technical and economic benefits. [ABSTRACT FROM AUTHOR]

• First study using K 2 FeO 4 as additive to improve microalage anaerobic fermentation. • K 2 FeO 4 addition enhanced HAc-enriched SCFAs production from microalage. • K 2 FeO 4 facilitated microalgae disintegration and organics solubilization. • K 2 FeO4 enriched hydrolytic and SCFA-forming microbes. • K 2 FeO 4 severely inhibited methanogenesis process. Anaerobic fermentation of microalgae was always hindered by its rigid cell wall structure. This paper reports a novel technique, i.e., adding potassium ferrate (K 2 FeO 4) into microalgae fermentation systems to enhance short-chain fatty acids (SCFAs) production. The results showed that the maximum SCFAs production and acetic acid proportion were 732.6 mg COD/g VS and 54.6% at a dosage of 112.8 mg Fe(VI)/g VS, which were 168% and 208% of those in the control, respectively. Mechanism studies revealed that K 2 FeO 4 effectively destroyed surface morphology and cell structure, and thus facilitated microalgae solubilization, providing a large number of biodegradable substrates for subsequent SCFA production. Although K 2 FeO 4 inhibited all the microbial activities relevant to hydrolysis, acidification and methanogenesis processes to some degree, its inhibition to methanogens was much severer than that to other microbes. Illumina MiSeq sequencing analyses revealed that K 2 FeO 4 addition increased the relative abundance (from 9.45% to 50.4%) of hydrolytic and SCFAs-forming bacteria. [ABSTRACT FROM AUTHOR]

• CaO 2 effectively eliminated the inhibitory effect of grease on SCFAs production. • SCFAs yield from food waste fermentation was further promoted with CaO 2. • CaO 2 facilitated biodegradable organics release and improved grease degradation. • OH– and Ca2+ played a vital role in SCFAs production improvement. • O 2 – and OH radicals were the main contributors to grease degradation. Deterioration of anaerobic fermentation can occur with the presence of grease in food waste, but little information on eliminating this deterioration is currently available. In this study, it was found that the presence of 10 g/L grease decreased SCFAs production from 16.97 to 13.32 g COD/L and prolonged the optimal fermentation time to 7 days, but could be respectively recovered to 39.10 g COD/L and 4 days with 0.02 mg/g VS (volatile solids) calcium peroxide addition. Mechanism investigations indicated that calcium peroxide facilitated biodegradable organics release and improved grease degradation, thereby providing enough nutrients and better growth environments to microbes for SCFAs-producing, which could be further supported by the elevated enzymes activities responding to hydrolysis and acidification process. Further investigations revealed that among the main derivates of calcium peroxide, OH– and Ca2+ played vital role in SCFAs production promotion, O 2 – and OH radicals were the main contributors to grease degradation. [ABSTRACT FROM AUTHOR]

Alkaline pretreatment was demonstrated to be effective in the enhancement of hydrogen production. However, the sludge solubilization rate of alkaline pretreatment is still limited. This study reports a new strategy of K 2 FeO 4 + pH 9.5 for sludge mesophilic anaerobic fermentation. Experimental results showed that the combination of K 2 FeO 4 /pH 9.5 pretreatment had a greater hydrogen yield than the individual K 2 FeO 4 and pH 9.5. The maximum hydrogen yield was 19.2 mL per gram volatile suspended solids (VSS) under the optimal condition (0.02 g per gram total suspended solids K 2 FeO 4 + pH 9.5). Kinetic analysis showed that the highest hydrogen production potential of 19.9 mL/g VSS was obtained in the combined reactor, which well fitted the first-order kinetic model (R2 = 0.9925). Besides, the fermentation type was mainly acetic and butyric in the combined reactor, which contributed to hydrogen production. Further analyses showed that the combined pretreatment reduced hydrogen sulfide yield, providing an environmentally friendly method to sludge treatment. Unlabelled Image • pH 9.5 + K 2 FeO 4 treatment promoted hydrogen yield from waste activated sludge. • pH 9.5 + K 2 FeO 4 treatment benefited the lysis of sludge cells. • The fermentation type in the combined reactor was mainly acetic and butyric. • pH 9.5 + K 2 FeO 4 treatment inhibited all the microbes in anaerobic fermentation. [ABSTRACT FROM AUTHOR]

Alkaline (especially pH 10) anaerobic fermentation of waste activated sludge (WAS) has been reported to be an effective approach for hydrogen production through inhibiting the homoacetogenesis and methanogenesis. However, the potential effect of the widespread microplastics in sludge on the performance of hydrogen production has never been reported. To fill this knowledge gap, the dominant polyethylene terephthalate (PET) microplastics in WAS were selected as the model microplastics to evaluate their influences on hydrogen production during alkaline anaerobic fermentation of WAS as well as the key mechanisms involved. Experimental results demonstrated that hydrogen production from WAS decreased in the presence of PET microplastics (i.e., 10, 30 and 60 particles/g-TS) compared to the control, with the hydrogen yield at 60 particles/g-TS being only 70.7 ± 0.9% of the control. Although the hydrogen consumption (i.e., homoacetogenesis and methanogenesis) was restrained under alkaline (pH 10) condition, PET microplastics inhibited hydrolysis, acidogenesis and acetogenesis in alkaline WAS anaerobic fermentation, leading to the inhibitory effect on hydrogen production. This was further confirmed by the microbial analysis, which clearly showed PET microplastics caused the shift of the microbial community toward the direction against hydrolysis-acidification. Mechanism studies revealed that PET microplastics carried on their negative influence mainly through leaching the toxic di-n-butyl phthalate (DBP). The reactive oxygen species (ROS) and live/dead staining tests indicated that the increased ROS was induced by PET microplastics, causing more cells dead, which further resulted in the decreased production of hydrogen. Image 1 • PET microplastics decreased hydrogen production from alkaline (pH 10) anaerobic fermentation of WAS. • PET microplastics inhibited sludge hydrolysis, acidogenesis and acetogenesis processes. • PET microplastics caused the shift of the microbial community toward the direction against hydrolysis-acidification. • PET microplastics carried on their negative effect mainly through the toxic DBP leached and ROS induced by PET microplastics. [ABSTRACT FROM AUTHOR]

• Ultrasonic aids FNA pretreatment enhanced the yield of hydrogen. • Ultrasonic aids FNA pretreatment accelerated sludge solubilization. • Ultrasonic aids FNA pretreatment enhanced the release of biodegradable organics. • Ultrasonic aids FNA pretreatment severely inhibited hydrogen consumers. This study presents a novel and effective method, i.e., adding nitrite into acidic fermentations after ultrasonic (US) pretreatment to form free nitrous acid (FNA), to further enhance hydrogen yield. Experimental results showed that when 180 mg/L nitrite was added into the US (2 W/mL, 15 min) pretreated waste activated sludge (WAS), the maximal hydrogen yield of 24.81 ± 1.24 mL/g VSS (volatile suspended solids) was obtained under acidic fermentation (1.0 mg/L FNA was initially formed under this condition), which was 2.21-folds (or 1.36-folds) of that from US pretreatment (or FNA treatment) alone. This combination approach caused a positive synergy on sludge disintegration and enhanced the transformation of the released organics from non-biodegradable to biodegradable. Further study showed that the inhibiting effect of this combination method on hydrogen consuming microorganism was severer. Considering its pollution free, this combination strategy is an attractive technology for hydrogen recovery from WAS. [ABSTRACT FROM AUTHOR]