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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]

• 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 • TCC enhanced hydrogen production from waste activated sludge. • TCC could be degraded during anaerobic fermentation system. • TCC promoted solubilization and acidogenesis. • TCC inhibited methanogenesis and homoacetogenesis. Abstract This work aims to investigate whether and how TCC affects hydrogen production using both experimental and model approaches. Experimental results showed that the exposure of TCC not only enhanced the hydrogen production yield but also promoted the hydrogen yield potential and hydrogen production rate. The maximum hydrogen production yield and hydrogen production rate increased from 10.1 ± 0.2 to 14.2 ± 0.2 mL/g VSS and 0.09 to 0.13 mL/g VSS·h, respectively, when TCC level increased from 0 to 1403 ± 150 mg/kg TSS. Mechanism exploration showed that the presence of TCC significantly promoted the release of substances and observably facilitated the acidification process but seriously inhibited the methonogenesis and homoacetogenesis processes. Further investigations with enzyme analysis revealed that TCC importantly increased the activities of acetate kinase and [FeFe] hydrogenase but seriously inhibited the activities of carbon monoxide dehydrogenase and Coenzyme F420. [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]

This study reported a new low-cost and high-efficient combined method of CaO 2  + free ammonia (FA) pretreatment for sludge anaerobic fermentation. Experimental results showed that the optimal short-chain fatty acids (SCFA) yield of 338.6 mg COD/g VSS was achieved when waste activated sludge (WAS) was pretreated with 0.05 g/g VSS of CaO 2  + 180 mg/L of FA for 3 d, which was 2.5-fold of that from CaO 2 pretreatment and 1.5-fold of that from FA pretreatment. The mechanism investigations exhibited that the CaO 2  + FA could provided more biodegradable substrates, this combination accelerated the disintegration of sludge cells, which thereby providing more organics for subsequent SCFA production. It was also found that the combination of CaO 2 and FA inhibited the specific activities of hydrolytic microbes, SCFA producers, and methanogens to some extents, but its inhibition to methanogens was much severer than that to the other two types of microbes. [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]

Triclocarban (TCC), one typical antibacterial agent being widely used in various applications, was found to be present in waste activated sludge at significant levels. To date, however, its effect on anaerobic fermentation of sludge has not been investigated. This work therefore aims to fill this knowledge gap. Experimental results showed that when TCC content in sludge increased from 26.7 ± 5.3 to 520.5 ± 12.6 mg per kilogram total suspended solids, the maximum concentration of short-chain fatty acids (SCFA) increased from 32.6 ± 2.5 to 228.2 ± 3.6 (without pH control) and from 211.7 ± 2.4 to 378.3 ± 3.2 mg COD/g VSS (initial pH 10), respectively. The large promotion of acetic acid was found to be the major reason for the enhancement of total SCFA production. Although a significant level of TCC was degraded in the fermentation process, SCFA was neither produced from TCC nor affected by its major intermediates at the relevant levels. It was found that TCC facilitated solubilization, acidogenesis, acetogenesis, and homoacetogenesis processes but inhibited methanogenesis process. Microbial analysis revealed that the increase of TCC increased the microbial community diversity, the abundances of SCFA (especially acetic acid) producers, and the activities of key enzymes relevant to acetic acid production. [ABSTRACT FROM AUTHOR]

This study is the first to report a new free ammonia (FA) pretreatment technique to recover short-chain fatty acids (SCFAs) from primary sludge (PS). The results showed FA pretreatment (240 mg N/L) largely enhanced the SCFAs production from PS to 355.8 mg chemical oxygen demand (COD)/g VSS, which was about 4.5-fold that of control (i.e., 80 mg COD/g VSS). Mechanism investigations showed that FA pretreatment greatly accelerated the PS solubilization and destroyed the integrity of EPS, and the easily biodegradable substrates in the fermentation broth increased significantly, providing microorganisms with more organics to convert into SCFAs. Further analyses displayed that FA greatly inhibited the methane production during fermentation, resulting in further accumulation of SCFAs. Given that the NH 4 +-N released during the fermentation can be converted into FA in situ , this could facilitate the transition of wastewater treatment plants operation from linear to circular mode. [Display omitted] • FA greatly increased the production of SCFAs by about 4.5 times that of the control. • The optimum concentration of FA pretreatment was 240 mg/L with the SCFAs yield of 355.8 mg COD/g VSS. • FA facilitated the disintegration and biodegradability of PS and inhibited methane production. • FA pretreatment could be applied to PS alone and a mixture of WAS and PS. [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]

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]

Low voltage electric field is an important stimulation condition for biochemical metabolic of microorganism. But few literatures were available related to the effect of low voltage electric field on hydrogen production from anaerobic digestion of waste activated sludge (WAS). This study aims to explore such influencing thus carried a series experiments under 35 ± 1 °C and pH 7.0 ± 0.2. The experimental results showed that the hydrogen production increased from 28.1 to 32.5 mL/g VSS with electric field strengthening from 0 to 40 V/m. The mechanism explorations revealed that the yield of volatile fatty acids (VFAs) yield could reach 1.16-fold of control group when the highest-level electric field (40 V/m) forced in the anaerobic fermentation system with dextran as model substrate. Further analysis of relative activities of functional enzymes, such as NADH, acetate kinase, butyrate kinase and OAATC, showed that it was promoted by electric field stimulation as 2.09, 1.52, 1.28 and 1.16-fold of the control test, respectively. Meanwhile, the conductivity of fermentation liquor in presence of low voltage electric field stimulation increased 83% compared with the control group. This work verified the promotion of low voltage electric field stimulation on hydrogen production from anaerobic digestion of WAS and might provide a new sight for the green energy generation. Unlabelled Image • Low voltage electric field stimulated VFA generation from sludge digestion system. • Low voltage electric field enhanced hydrogen production from anaerobic digestion. • Low voltage electric field influenced each stage of anaerobic digestion system. • Low voltage electric field improved relative activities of functional enzymes. • Low voltage electric field disturbed cell membrane permeability. [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]

Cationic polyacrylamide, a flocculation powder widely used in wastewater pretreatment and sludge dewatering, was highly accumulated in waste activated sludge. However, its effect on short-chain fatty acids (SCFAs) accumulation from anaerobic fermentation of waste activated sludge has not been investigated. This work therefore aims to deeply unveil how cationic polyacrylamide affects SCFAs production, through both long-term and batch tests using either real waste activated sludge or synthetic wastewaters as fermentation substrates. Experimental results showed that the presence of cationic polyacrylamide not only significantly decreased the accumulation of SCFAs but also affected the composition of individual SCFA. The concentration of SCFAs decreased from 3374.7 to 2391.7 mg COD/L with cationic polyacrylamide level increasing from 0 to 12 g/kg of total suspended solids, whereas the corresponding percentage of acetic acid increased from 45.2% to 55.5%. The mechanism studies revealed that although cationic polyacrylamide could be partially degraded to produce SCFAs during anaerobic fermentation, cationic polyacrylamide and its major degradation metabolite, polyacrylic acid, inhibited all the sludge solubilization, hydrolysis, acidogenesis, acetogenesis and homoacetogenesis processes to some extents. As a result, the accumulation of SCFAs in the cationic polyacrylamide added systems decreased rather than increased. However, the inhibition to acetogenesis and homoacetogenesis was slighter than that to acidogenesis, leading to an increase of acetic acid to total SCFAs. It was further found that cationic polyacrylamide had stronger ability to adhere to protein molecules surface, which inhibited the bioconversion of proteins more severely. Illumina MiSeq sequencing analyses showed that cationic polyacrylamide decreased microbial community diversity, altered community structure and changed activities of key enzymes responsible for SCFAs accumulation. Graphical abstract Image 1 Highlights • cPAM decreased the yield of SCFAs from anaerobic fermentation of WAS. • cPAM affected the composition of individual SCFA. • cPAM showed different inhibition on microbes involved in the anaerobic fermentation. • cPAM's degradation product polyacrylic acid also suppressed anaerobic fermentation. • cPAM decreased microbial community diversity and altered community structure. [ABSTRACT FROM AUTHOR]

Abstract Peroxymonosulfate (PMS) has been recently used as an additive to pretreat waste activated sludge (WAS) to enhance short-chain fatty acids (SCFAs) production. However, the mechanisms of how PMS enhances SCFAs production remain largely unknown. This work therefore aims to explore the mechanisms through deeply understanding its impact on the disintegration of sludge cells, the biodegradability of organics released and the bioprocesses involved in anaerobic fermentation, and differentiating the contributions of its degradation intermediates to SCFAs production. This was demonstrated by a series of batch fermentation tests using either real sludge or model organic compounds as fermentation substrates. Experimental results showed that the maximal SCFAs yield increased from 29.69 to 311.67 mg COD/g VSS with PMS level increasing from 0 to 0.09 g/g TSS. No obvious increase in SCFAs yield was observed when PMS further increased. The mechanism explorations revealed that PMS pretreatment not only enhanced the disintegration of sludge cells but also promoted the biodegradability of organics released, thereby providing more biodegradable substrates for subsequent SCFAs production. PMS pretreatment decreased the percentages of fulvic acid-like and humic acid-like substances in the released organics. Moreover, the species and total detection frequency of other recalcitrant organics such as cyclopentasiloxane, heptasiloxane, and ethylene glycol, which were hardly degraded in ordinary anaerobic condition, also decreased remarkably. Although PMS caused harms to some extents to all the microbes in the anaerobic fermentation, its inhibitions to SCFAs consumers were much severer than that to SCFAs producers, probably due to the less tolerance of methanogens. Further analyses exhibited that 1O 2 , SO 4 •- and •OH were the major contributors to the increased SCFAs production, and their contributions were in the order of 1O 2 > SO 4 •- > •OH. The findings obtained in this work provide insights into PMS-involved sludge fermentation process and might have important implication for further manipulation of WAS treatment in the future. Graphical abstract Image 1 Highlights • PMS pretreatment enhanced the SCFAs production from anaerobic fermentation of WAS. • PMS facilitated the disintegration of WAS and the biodegradability of organics released. • PMS improved the degradation of the refractory organic pollutants. • Contributions of ingredients were in the order of 1O 2 > SO 4.•- > •OH [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]

In this study, the impact of diclofenac (DCF), an antiinflammatory drug being extensively used in human health care and veterinary treatment, on the production of volatile fatty acids (VFAs) from anaerobic fermentation of waste activated sludge (WAS) was investigated for the first time. Experimental results showed that when DCF concentration increased from 2.5 to 25 mg/kg total suspended solid (TSS), the maximum production of VFAs increased from 599 to 1113 mg COD/L, but further increase of DCF to 47.5 mg/kg TSS decreased VFAs yield to 896 mg COD/L. The mechanism investigation revealed that DCF had no effect on the hydrolysis process, promoted the process of acidogenesis, acetogenesis, and homoacetogenesis, but severely inhibited methanogenesis, leading to the accumulation of VFAs. Microbial community analysis showed that the addition of DCF could promote the relative abundance of VFAs (especially acetic acid) producers, which was well consistent with the results obtained above. [ABSTRACT FROM AUTHOR]

Poly aluminum chloride (PAC) is accumulated in waste activated sludge at high levels. However, details of how PAC affects short-chain fatty acids (SCFA) production from anaerobic sludge fermentation has not been documented. This work therefore aims to fill this knowledge gap by analyzing the impact of PAC on the aggregate of sludge flocs, disruption of extracellular polymeric substances (EPS), and the bio-processes of hydrolysis, acidogenesis, and methanogenesis. The relationship between SCFA production and different aluminum species (i.e., Ala, Alb, and Alc) was also identified by controlling different OH/Al ratio and pH in different fermentation systems. Experimental results showed that with the increase of PAC addition from 0 to 40 mg Al per gram of total suspended solids, SCFA yield decreased from 212.2 to 138.4 mg COD/g volatile suspended solids. Mechanism exploration revealed that PAC benefited the aggregates of sludge flocs and caused more loosely- and tightly-bound extracellular polymeric substances remained in sludge cells. Besides, it was found that the hydrolysis, acidiogenesis, and methanogenesis processes were all inhibited by PAC. Although three types of Al species, i.e., Ala (Al monomers, dimer, and trimer), Alb (Al 13 (AlO 4 Al 12 (OH) 24 (H 2 O)7 + 12), and Alc (Al polymer molecular weight normally larger than 3000 Da), were co-existed in fermentation systems, their impacts on SCFA production were different. No correlation was found between SCFA and Ala, whereas SCFA production decreased with the contents of Alb and Alc. Compared with Alb, Alc was the major contributor to the decreased SCFA production (R 2  = 0.5132 vs R 2  = 0.98). This is the first report revealing the underlying mechanism of how PAC affects SCFA production and identifying the contribution of different Al species to SCFA inhibition. [ABSTRACT FROM AUTHOR]

Short-chain fatty acid (SCFA) production from waste activated sludge (WAS) anaerobic fermentation is often limited by the slow hydrolysis rate and poor substrate availability, thus a long fermentation time is required. This paper reports a new pretreatment approach, i.e., using free nitrous acid (FNA) to pretreat sludge, for significantly enhanced SCFA production. Experimental results showed the highest SCFA production occurred at 1.8 mg FNA/L with time of day 6, which was 3.7-fold of the blank at fermentation time of day 12. Mechanism studies revealed that FNA pretreatment accelerated disruption of both extracellular polymeric substances and cell envelope. It was also found that FNA pretreatment benefited hydrolysis and acidification processes but inhibited the activities of methanogens, thereby promoting the yield of SCFA. In addition, the FNA pretreatment substantially stimulated the activities of key enzymes responsible for hydrolysis and acidification, which were consistent with the improvement of solubilization, hydrolysis and acidification of WAS anaerobic fermentation. [ABSTRACT FROM AUTHOR]

• Various pretreatments caused distinctive DOM pattern in WAS fermentation. • DOM biodegradability and reactivity were improved by pretreatment. • Large DOM concentration with high O/C ratio favored VFAs promotion. • Substrate paradigm shift stimulated microbial community and metabolism. • The metabolic synergy of anaerobes and biodegradable DOM drove organic matter conversion to VFAs. Various pretreatments are commonly adopted to facilitate dissolved organic matter (DOM) release from waste activated sludge (WAS) for high-valued volatile fatty acids (VFAs) promotion, while the interplay impact of DOM dynamics transformation on microbial population and metabolic function traits is poorly understood. This work constructed "DOM-microorganisms-metabolism-VFAs" symbiotic ecologic networks to disclose how DOM dynamics variation intricately interacts with bacterial community networks, assembly processes, and microbial traits during WAS fermentation. The distribution of DOM was altered by different pretreatments, triggering the release of easily biodegradable compounds (O/C ratio > 0.3) and protein-like substance. This alteration greatly improved the substrates biodegradability (higher biological index) and upregulated microbial metabolism capacity (e.g. , hydrolysis and fatty acid synthesis). In turn, microbial activity modifications augment substance metabolism level and expedite the conversion of highly reactive compounds (proteins-like DOM) to VFAs, leading to 1.6–4.2 fold rise in VFAs generation. Strong correlations were found between proteins-like DOM and topological properties of DOM-bacteria associations, suggesting that high DOM availability leads to more intricate ecological networks. A change in the way communities assemble, shifting from stronger uniform selection in pH10 and USp reactors to increased randomness in heat reactor, was linked to DOM composition alterations. The ecologic networks further revealed metabolic synergy between hydrolytic-acidogenic bacteria (e.g., Bacteroidota and Firmicutes) and biodegradable DOM (e.g., proteins and amino sugars) leading to higher VFAs generation. This study provides a deeper knowledge of the inherent connections between DOM and microbial traits for efficient VFAs biosynthesis during WAS anaerobic fermentation, offering valuable insights for effective WAS pretreatment strategies. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • CTAB conditioning improved anaerobic fermentation sludge dewaterability. • The dewatering mechanism was raised by analyzing the properties of solid and liquid. • CTAB conditioning reduced surface energy of EPS surface and damaged sludge cells. • CTAB conditioning reduced liquid composition contents through specific interactions. Cetyl trimethyl ammonium bromide (CTAB) is considered a promising sludge dewatering conditioner, but its potential dewatering mechanism has not been thoroughly understood. This study, therefore, aims to fully investigate how CTAB improves sludge dewaterability. Experimental results showed that after sludge was conditioned by 75 mg/g TSS CTAB, capillary suction time (CST), specific resistance to filtration (SRF), and the water content of sludge cake (WC SC) by vacuum filtration decreased from 164.1 ± 10.7 s, 1.90 ± 0.05 × 1012 m/kg, and 97.36 ± 0.12 % to 32.1 ± 1.4 s, 0.18 ± 0.01 × 1012 m/kg, and 70.55 ± 0.21 %, respectively. Mechanism revelations showed that CTAB could neutralize the negative charges on EPS surface and reduce the repulsive force between sludge particles to reduce the Zeta potential; CTAB could also enhance the hydrophobicity and the adhesion among the sludge particles to reduce the surface energy, and CTAB could release the intracellular water by damaging the outer membrane of sludge cells. The presence of CTAB also decreased the concentrations of PO 4 3- , humic acid, and proteins in the liquid phase through electrostatic attractive or hydrophobic interactions. All these CTAB-caused variations in either the solid or liquid phase of sludge could result in the enhancement of sludge dewaterability. The findings deepen our understanding of CTAB affecting sludge dewatering and also might guide engineers to exploit promising strategies to facilitate sludge dewatering in the future. [ABSTRACT FROM AUTHOR]

Hydrogen production from waste-activated sludge (WAS) anaerobic fermentation is considered to be an effective method of resource recovery. However, the presence of a large number of complex organic compounds in sludge will affect the biological hydrogen production process. As an extensively applied prevalent anti-inflammatory drug, diclofenac (DCF) is inevitably released into the environment. However, the effect of diclofenac on hydrogen production from WAS anaerobic fermentation has not been fully investigated. This work therefore aims to comprehensively investigate the removal efficiency of DCF in mesophilic anaerobic fermentation of WAS and its effect on hydrogen yield. Experiment results showed that 32.5%–38.3% of DCF was degraded in the fermentation process when DCF concentration was ranged from 6 to 100 mg/kg TSS (total suspended solids). DCF at environmental level inhibited hydrogen production, the maximal hydrogen yield decreased from 24.2 to 15.3 mL/g VSS (volatile suspended solids) with an increase of DCF addition from 6 to 100 mg/kg TSS. This is because the presence of DCF caused inhibitions to acetogenesis and acidogenesis, the processes responsible for hydrogen production, probably due to that the polar groups of DCF (i.e., carboxyl group) could readily bind to active sites of [FeFe]- Hydrogenase. Besides, the microbial analysis revealed that DCF increased the microbial diversity but had few influences on the microbial structure. [Display omitted] • Approximately 32.5%–38.3% of DCF was degraded during anaerobic fermentation system. • DCF reduced the hydrogen production from WAS anaerobic fermentation. • DCF led to inhibition to hydrolysis, acidogenesis, and acetogenesis. • DCF decreased the numbers of anaerobes relevant to hydrogen producers. [ABSTRACT FROM AUTHOR]

Anaerobic fermentation of waste activated sludge (WAS) is often limited by low disintegration rates of WAS and quick consumptions of short-chain fatty acids (SCFAs) by methanogens. This work proposed a novel, efficient and harmless pretreatment approach i.e., using potassium ferrate (PF) combined with free ammonia (FA), to enhance the disintegration of WAS and the accumulation of SCFAs from WAS anaerobic fermentation. It was found in this work, the best pretreating condition of WAS is 0.10 g PF/g VSS + 180 mg FA/L, under which the SCFAs consistence reached its maximum value (342.5 mg COD/g VSS), which was respectively 7.45-fold, 3.50-fold and 2.06-fold of that from the Blank, FA and PF reactor. Further in-depth mechanism study showed that the disintegration of WAS promoted and the biodegradability of fermentation broth enhanced under the PF + FA pretreatment condition. Moreover, the populations of fecal coliforms in WAS reduced significantly when PF + FA was adopted. Considering that FA could be produced in situ during the ammoniating process, the findings in this work promoted the utilization of PF-based pretreatment method in actual operations. [Display omitted] • Applicable addition of FA not only promoted SCFAs production but also reduced PF dosage. • The combined method accelerated the disintegration of sludge cells and provided more biodegradable substrates. • The combined method inhibited methanogenesis processes. • The combined method benefited the inactivation of pathogens in fermented sludge. [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]

• BZC exhibited a dose-dependent effect on the production of SCFAs. • Novel methods revealed protein denaturation diversity between liquid-solid phases. • EPS secreted and the solubilization of sludge was enhanced with BZC. • The hydrolysis stage was promoted but the methanogenesis stage was inhibited. • Microbial structure changed and functional bacteria enriched with BZC. Benzethonium chloride (BZC) is viewed as a promising disinfectant and widely applied in daily life. While studies related to its effect on waste activated sludge (WAS) anaerobic fermentation (AF) were seldom mentioned before. To understand how BZC affects AF of WAS, production of short chain fatty acids (SCFAs), characteristics of WAS as well as microbial community were evaluated during AF. Results manifested a dose-specific relationship of dosages between BZC and SCFAs and the optimum yield arrived at 2441.01 mg COD/L with the addition of 0.030 g/g TSS BZC. Spectral results and protein secondary structure variation indicated that BZC denatured proteins in the solid phase into smaller proteins or amino acids with unstable structures. It was also found that BZC could stimulate the extracellular polymeric substances secretion and reduce the surface tension of WAS, leading to the enhancement of solubilization. Beside, BZC promoted the hydrolysis stage (increased by 7.09 % to 0.030 g/g TSS BZC), but inhibited acetogenesis and methanogenesis stages (decreased by 6.85 % and 14.75 % to 0.030 g/g TSS BZC). The microbial community was also regulated by BZC to facilitate the enrichment of hydrolytic and acidizing microorganisms (i.e. Firmicutes). All these variations caused by BZC were conducive to the accumulation of SCFAs. The findings contributed to investigating the effect of BZC on AF of WAS and provided a new idea for the future study of AF mechanism. [Display omitted] [ABSTRACT FROM AUTHOR]

The presence of endocrine disruptor compounds (EDCs) in wastewater treatment plants has attracted widespread attention, but their potential impact on anaerobic fermentation of waste activated sludge (WAS) remains unclear. Therefore, this study aims to reveal the effect of typical EDC octylphenol (OP) on the recovery of volatile fatty acids (VFAs) in anaerobic fermentation. The results show that OP has a positive effect on the recovery of VFAs from WAS. The presence of 200 mg/kg dry sludge of OP increased the cumulative amount of VFAs from 3245 in the control (without OP) to 6828 mg COD/L. The increase in VFA production was mainly attributed to the accumulation of acetic acid, which rose from 1511 to 4425 mg COD/L, almost tripled. Further research found that OP promoted solubilization and hydrolysis by improving the biodegradability of WAS, and severely inhibited the methanogenesis process by inhibiting the activity of coenzyme F420, thereby significantly increasing the accumulation of acetic acid. These findings are of great significance to clarify the role of OP in anaerobic fermentation, and provide theoretical basis and guidance for the selection of target products in anaerobic fermentation of WAS containing OP. Unlabelled Image • Octylphenol improves volatile fatty acid recovery from waste activated sludge. • Octylphenol has a positive effect on acetic acid accumulation. • Octylphenol enhances biodegradability of sludge. • Octylphenol inhibits methanogenesis by reducing F420 activity. [ABSTRACT FROM AUTHOR]

• NaDCC contributed to pathogens inactivation and VFAs promotion cost-effectively. • NaDCC caused WAS disintegration and organics release for microbial metabolism. • The microbial community and metabolic traits for VFAs were selectively upregulated. • Microbial adaptive systems were activated to counteract unfavorable NaDCC stress. Harmless and resourceful treatment of waste activated sludge (WAS) have been the crucial goal for building environmental-friendly and sustainable society, while the synergistic realization approach is currently limited. This work skillfully utilized the disinfectant sodium dichloroisocyanurate (NaDCC) to simultaneously achieve the pathogenic potential inactivation (decreased by 60.1 %) and efficient volatile fatty acids (VFAs) recovery (increased by 221.9 %) during WAS anaerobic fermentation in rather cost-effective way (Chemicals costs:0.4 USD/kg VFAs versus products benefits: 2.68 USD/kg chemical). Mechanistic analysis revealed that the C=O and N Cl bonds in NaDCC could spontaneously absorb sludge (binding energy -4.9 kJ/mol), and then caused the sludge disintegration and organic substrates release for microbial utilization due to the oxidizability of NaDCC. The disruption of sludge structure along with the increase of bioavailable fermentation substrates contributed to the selectively regulation of microbial community via enriching VFAs-forming microorganisms (e.g., Pseudomonas and Streptomyces) and reducing VFAs-consuming microorganisms, especially aceticlastic methanogens (e.g., Methanothrix and Methanospirillum). Correspondingly, the metabolic functions of membrane transport, substrate metabolism, pyruvate metabolism, and fatty acid biosynthesis locating in the central pathway of VFAs production were all upregulated while the methanogenic step was inhibited (especially acetate-type methanogenic pathway). Further exploration unveiled that for those enriched functional anaerobes were capable to activate the self-adaptive systems of DNA replication, SOS response, oxidative stress defense, efflux pump, and energy metabolism to counteract the unfavorable NaDCC stress and maintain high microbial activities for efficient VFAs yields. This study would provide a novel strategy for synergistic realization of harmless and resourceful treatment of WAS, and identify the interrelations between microbial metabolic regulations and adaptive responses. [Display omitted] [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]

Graphical abstract Highlights • Adding TR into WAS enhanced VFA production and shortened fermentation time. • TR + WAS promoted sludge solubilization, hydrolysis and acidogenesis processes. • TR + WAS posed a positive synergetic effect on lignocellulose hydrolysis. • Appropriate amounts of mineral elements in TR benefited solubilization. • Iron and calcium in TR contribute to the phosphorus removal in fermentation liquor. Abstract In this study, an economical and eco-friendly strategy (i.e., adding tofu residue (TR) into waste activated sludge (WAS)) to enhance volatile fatty acid (VFA) production was reported. Experimental results indicated that the maximal VFA yield at T/W ratio (TR/WAS, the ratio of the volatile suspended solids (VSS)) of 0.64 on 5 d was 240.8 mg COD/g VSS, which was 10.2 and 1.1-fold of that in sole WAS and sole TR, respectively. The feasible fermentation time was shortened by 2 days, as compared with sole WAS or sole TR. Mechanism investigation showed that the addition of TR promoted solubilization, hydrolysis, and acidogenesis processes. The synergistic effect of microorganisms contained in TR and WAS may be responsible for the enhancement of lignocellulose hydrolysis and VFA generation. Appropriate amounts of mineral elements in TR benefited solubilization, the soluble iron and calcium in TR could contribute to the phosphorus removal in fermentation liquor. [ABSTRACT FROM AUTHOR]

Quaternary ammonium compounds (QACs) was frequently detected in wastewater treatment plants and leads to potential toxicity to the related biological processes. In this study, the effect of benzalkonium bromide (BK) on anaerobic sludge fermentation process for short chain fatty acid (SCFAs) production was investigated. Batch experiments indicated that BK exposure significantly enhanced the SCFAs production from anaerobic fermentation sludge and the maximum concentration of total SCFAs increased from 474.40 ± 12.35 mg/L to 916.42 ± 20.35 mg/L with BK increasing from 0 to 8.69 mg/g VSS. Mechanism exploration exhibited that the presence of BK enhanced much more bioavailable organic matters release, little affected on hydrolysis, acidification, but seriously inhibited methanogenesis. Microbial community investigation revealed that BK exposure importantly enhanced the relative abundances of hydrolytic-acidifying bacteria and also improved the metabolic pathways and functional genes for sludge lysis. This work further supplement the information for environmental toxicity of emerging pollutants. [Display omitted] • BK exposure promoted SCFAs production, particularly acetate. • BK significantly promoted soluble microbial by-products. • BK impacted microbial community structure and gene expression. [ABSTRACT FROM AUTHOR]

[Display omitted] • 0.25 g/g VSS CaO 2 reduced by 80.5% of H 2 S production from anaerobic fermentation. • CaO 2 addition destroyed sludge EPS structure and caused cell lysis. • The abundance and activity of S2− producers were reduced by CaO 2 addition. • CaO 2 addition promoted metal ions release and changed sulfur transformation. • Organic sulfur hydrolysis and inorganic sulfate reduction were inhibited. Calcium peroxide (CaO 2) addition is widely regarded as a high-efficient sludge treatment method to degrade pollutants and enhance resource recovery. In this study, it was further found that CaO 2 addition changed sulfur distribution during sludge anaerobic treatment, which promoted sulfur flow from sludge solid to liquid phase but significantly inhibited toxic hydrogen sulfide (H 2 S) gas production. With CaO 2 addition increased from 0 to 0.25 g/g VSS, the accumulated H 2 S yield decreased from 314.6 × 10−4 to 61.3 × 10−4 mg/g VSS while the accumulated SCFAs and hydrogen yields increased from 46.2 to 266.5 mg COD/g VSS, and 0.9 to 10.2 mL/g VSS. Mechanism analysis showed that the released OH– and oxidizing substances (e.g., ·OH and ·O 2 –) firstly destroyed functional groups and hydrogen bonding networks contained in sludge EPS, with the contents of C=O group, β-sheet and α-Helix being reduced by 24%, 19.7%, and 23.2%, respectively. This caused the cracking of sludge EPS and releases of soluble metal ions (e.g., Al, Mg, and Fe). The released metal ions in alkaline environment promoted the transformation of H 2 S gas and dissolved sulfide to metal sulfide, which was one important reason to reduce H 2 S gas release. Then, the free radicals further attacked sludge cells without EPS protection and caused cell lysis. Flow cytometry analysis showed that 0.25 g/g VSS CaO 2 reduced the rate of living cells from 90.7% to 81.4%. Microbial analysis unveiled that CaO 2 reduced the content of S2− producers (e.g., Desulfobulbus and Desulfovibrio) and the expression of related genes (e.g., PepD and Sat). Moreover, the activities of S2− producers, e.g., methionine lyase and adenosine-5′-phosphosulfate reductase also reduced by 41.3% and 64.8% with 0.25 g/g VSS CaO 2 addition. These findings provide new insights into CaO 2 -based sludge treatment and might have significant implications for sludge treatment. [ABSTRACT FROM AUTHOR]

In this study, the impact of sulfamethazine (SMZ), a typical Pharmaceuticals and Personal Care Products (PPCPs) existed in waste activated sludge (WAS), on the production of short-chain fatty acids (SCFA) during sludge anaerobic fermentation was investigated for the first time. Experimental results showed that the production of SCFA was positively affected by SMZ, which increased from 571 mg COD/L in the blank to 989 mg COD/L in the 24 mg/kg VSS SMZ reactor, causing 1.73-fold of increase. Mechanism exploration revealed that SMZ enhanced the disruption of both extracellular polymeric substances (EPS) and cell envelop, thereby benefiting the solubilization of sludge. It was also found that SMZ benefited the acidogenesis and acetogenesis processes but inhibited methonogenesis process, which was another reason for the enhanced SCFA production. Microbial analysis revealed that the abundance of anaerobic functional microorganisms in the SMZ reactor was more advantageous to acetic acid production. [ABSTRACT FROM AUTHOR]

This study reports a new strategy, i.e. combination of free nitrite acid (FNA) and biosurfactant Alkyl polyglucose (APG), for the production of short chain fatty acids (SCFA) from sludge anaerobic fermentation. The results showed that when FNA concentration was 1.54 mg/L, the maximum yield of SCFA was 354.6 mg/g, which was significantly higher than that of FNA or APG alone. The combination of FNA and APG also shortened the optimal fermentation time to 5 d. Mechanism studies showed that FNA combined with APG can promote the sludge disintegration. By detecting the degradation of the simulated compounds in the synthetic water, it is found that the FNA combined with APG can synergistically promote sludge hydrolysis and acidification but seriously inhibit the methanogenesis process. The enzyme activity analysis was also consistent with the above experimental results. The combination of FNA and APG in this study provides a promising strategy for enhancing sludge anaerobic fermentation. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) is present in significant levels in waste activated sludge, but its potential toxicities on anaerobic fermentation of sludge remain largely unknown. This work therefore aims to provide such support. Experimental results showed that the impact of Cd on short-chain fatty acids (SCFA) production from sludge anaerobic fermentation was dose-dependent. The presence of environmentally relevant level of Cd (e.g., 0.1 mg/g VSS) enhanced SCFA production by 10.6%, but 10 mg/g VSS of Cd caused 68.1% of inhibition. Mechanism exploration revealed that although all levels of Cd did not cause extra leakage of intracellular substrates, 0.1 mg/g VSS Cd increased the contents of both soluble and loosely-bound extracellular polymeric substances (EPS), thereby benefitting sludge solubilization. On the contrary, 10 mg/g VSS Cd decreased the levels of all EPS layers, which reduced the content of soluble substrates. It was also found that 0.1 mg/g VSS Cd benefited both the hydrolysis and acidogenesis but 10 mg/g VSS Cd inhibited all the hydrolysis, acidogenesis, and methanogenesis processes. Further investigations with microbial community and enzyme analysis showed that the pertinent presence of Cd enhanced the activities of protease, acetate kinase, and oxaloacetate transcarboxylase whereas 10 mg/g VSS Cd decreased the microbial diversity, the abundances of functional microbes, and the activities of key enzymes. Finally, one strategy that could effectively mitigate the adverse impact of high Cd levels on SCFA production was proposed and examined. This work provides insights into Cd-present sludge fermentation systems, and the findings obtained may guide engineers to manipulate sludge treatment systems in the future. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe0 catalyzing sulfite pretreatment promoted SCFAs production from sludge. • Acetate proportion was promoted by 275 % at 500 mg S/L and 1.25 ZVI/sulfite ratio. • Enhanced disintegration of sludge and loosen structure of protein were observed. • ZVI/sulfite pretreatment promoted hydrolysis, acidogenesis and acetogenesis. • Functional genes related to acetate formation were enriched by ZVI/sulfite pretreatment. The quality and quantity of short chain fatty acids (SCFAs) production from anaerobic fermentation of sewage sludge is generally constrained by low organics solubilization and poor substrate-availability. This study provided an environmentally friendly strategy with zero valent iron (ZVI) activating sulfite pretreatment to overcome this limitation and advance the sludge anaerobic fermentation. Experimental results showed that the maximal SCFAs production and acetate proportion promoted by 5.7 and 2.75 times respectively at 500 mg S/L and the ZVI/sulfite ratio of 1.25. Mechanism analysis elucidated that the free radicals of S O 4 - · and ·OH produced from ZVI-sulfite pretreatment accelerated the disintegration of sludge-flocs and decreased the value of α-helix and ratio of α-helix/ (β-sheet + random coil) from 21.64 % and 44.23 % to 17.80 % and 38.67 %, respectively, which provided more easily- and bio- available substance for functional anaerobes. The ZVI-sulfite pretreatment enhanced the ability of functional anaerobes for SCFAs production processes as hydrolysis, acidogenesis and acetogenesis and reduced the SCFAs consumption as methanogenesis, which ultimately promoted SCFAs accumulation. Microbial community illustrated that ZVI-sulfite pretreatment enriched the relative abundance of functional anaerobes related to hydrolysis, acidogenesis and acetogenesis bioprocesses. PICRUSt analysis demonstrated that the pretreatment boosted the functional genes abundance involved in pathway of butyrate and acetate formation. Moreover, the enrichment genes of acetate formation were consistent with the increase of acetate proportion. [ABSTRACT FROM AUTHOR]

Free ammonia (FA), the unionized form of ammonium, is presented in anaerobic fermentation of waste activated sludge (WAS) at high levels. However, its potential role in sulfur transformation, especially H 2 S production, during WAS anaerobic fermentation process was unrecognized previously. This work aims to unveil how FA affects anaerobic sulfur transformation in WAS anaerobic fermentation. It was found that FA significantly inhibited H 2 S production. With an increase of FA from 0.04 to 159 mg/L, H 2 S production reduced by 69.9%. FA firstly attacked tyrosine-like proteins and aromatic-like proteins in sludge EPSs, with C O groups being responded first, which decreased the percentage of α-helix/(β-sheet + random coil) and destroyed hydrogen bonding networks. Cell membrane potential and physiological status analysis showed that FA destroyed membrane integrity and increased the ratio of apoptotic and necrotic cells. These destroyed sludge EPSs structure and caused cell lysis, thus strongly inhibited the activities of hydrolytic microorganisms and sulfate reducing bacteria. Microbial analysis showed that FA reduced the abundance of functional microbes (e.g., Desulfobulbus and Desulfovibrio) and genes (e.g., MPST , CysP , and CysN) involved in organic sulfur hydrolysis and inorganic sulfate reduction. These findings unveil an actually existed but previously overlooked contributor to H 2 S inhibition in WAS anaerobic fermentation. [Display omitted] • Free ammonia in situ reduced H 2 S production during sludge anaerobic treatment. • Free ammonia inhibited the activities of hydrolytic microorganisms and SRBs. • Free ammonia destroyed membrane integrity and led to apoptosis. • Free ammonia reduced the relative abundance of S2- producers. [ABSTRACT FROM AUTHOR]

[Display omitted] • Citric acid (CA) exhibits dose-dependent effects on the SCFAs promotion. • CA is mainly converted to acetic acid in the anaerobic fermentation system. • CA removes multivalent cations from extracellular polymeric substances (EPS). • CA facilitates WAS hydrolysis by disrupting EPS and microbial cells. • Microbial functional traits related to SCFAs production are regulated by CA. Numerous effective chemical strategies have been explored for short-chain fatty acids (SCFAs) production from waste activated sludge (WAS), but many technologies have been questioned due to the chemical residues. This study proposed a citric acid (CA) treatment strategy for improving SCFAs production from WAS. The optimum SCFAs yield reached 384.4 mg COD/g VSS with 0.08 g CA/g TSS addition. Meanwhile, CA biodegradation occurred and its contribution to the yield of total SCFAs, especially acetic acid, cannot be ignored. Intensive exploration indicated the sludge decomposition, the biodegradability of fermentation substrates, as well as the abundance of fermenting microorganisms were definitely enhanced in the existence of CA. The optimization of SCFAs production techniques based on this study deserved further study. This study comprehensively revealed the performance and mechanisms of CA enhancing biotransformation of WAS into SCFAs and the findings promotes the research of carbon resource recovery from sludge. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carbamazepine at environmentally relevant levels increased rather than decreased hydrogen production. • The processes relevant to hydrogen consumption were inhibited by carbamazepine. • Hydrogen bonds were bound between carbamazepine and sulfite reductase protein. • The corresponding genes with hydrogen consumption was reduced in the presence of carbamazepine. The widespread use of pharmaceuticals sparked concerns about their potential environmental effects. Previous studies demonstrated that most of the pharmaceuticals investigated had detrimental effects on wastewater or sludge treatment. However, it was found in this work that carbamazepine at environmentally relevant levels increased hydrogen production from anaerobic fermentation of waste activated sludge (WAS). Experimental results exhibited that with an increment of carbamazepine from 7 to 51 mg/kg VSS, the maximal hydrogen yield increased from 14.9 ± 0.6 to 23.5 ± 0.3 mL/g VSS. Mechanism analysis showed that carbamazepine accelerated the release of organics from solid to liquid phase, offering more substrates for hydrogen production. Carbamazepine did not affect hydrogen producing processes but inhibited the processes relevant to hydrogen consumption. Molecular docking demonstrated that carbamazepine and sulfite reductase protein were joined by hydrogen bond and ASP-597 and VAL-598 were the active sites. These contributed to the contact between carbamazepine and sulfite reductase, thereby affecting the hydrogen consumption process. The reduced α -helix ratio and α -helix / (β -plate + random coil) values indicated that the presence of carbamazepine led to the destroy of the hydrogen network, leading the collapse of protein structure. Further investigation revealed that carbamazepine reduced the corresponding genes with hydrogen consumption, especially involved in the steps of CO 2 → fmdA → Formyl-MFR → 5,10-Methylere-THMPT → mer → 5,10-Methyl-THM(S)PT (CO 2 -reduction), 5,10-Methyl-THM(S)PT → mtrD/E → Methyl-CoM → mcrA/B → CH 4 (CO 2 -reduction and aceticlastic reaction) and Sulfate → sat → APS → aprB → Sulfite → cysJ → Sulfide (sulfate reduction). [ABSTRACT FROM AUTHOR]

[Display omitted] • Pretreatment with FeS/sulfite increased the quality and quantity of SCFAs. • The generated free radicals •OH, SO 4 •-, . O 2 , and 1O 2 decomposed sludge flocs. • FeS/sulfite pretreatment facilitated the conversion of propionate to acetate. • FeS/sulfite pretreatment decreased the refractory compounds in fermentation mixture. Maximizing resource recovery by improving the anaerobic treatment efficiency of waste activated sludge (WAS) is critical to reducing carbon emissions. In this study, a concept of "co-treating" WAS with sulfite-laden wastes (sulfite) and mackinawite (FeS) was proposed to improve the anaerobic fermentation performance of WAS. Experimental results showed that the maximal short-chain fatty acids (SCFAs) yield of 2461.4 mg COD/L and corresponding acetate proportion (55.9 %) were obtained with a sulfite/FeS ratio of 0.8 at 500 mg S/L sulfite. It was found that •OH, SO 4 •-, . O 2 , and 1O 2 free radicals produced by FeS/sulfite pretreatment could decompose the sludge flocs significantly, resulting in reduced particle size and viable cell ratio, thereby providing more available organics for SCFAs production. Mechanism investigations revealed that FeS/sulfite pretreatment enhanced the production of SCFAs by increasing key hydrolytic enzymes, improving the abundance of microorganisms related to hydrolysis and acidification and altering the functional traits of the corresponding microbes. FeS/sulfite pretreatment also facilitated the conversion of propionate and butyrate acid to acetate during anaerobic fermentation, probably due to FeS/sulfite-induced iron reduction and sulfate reduction. The GC–MS results suggested that FeS/sulfite pretreatment promoted the quality of the fermentation mixture by decreasing the refractory compounds. This work provided a practical approach to the final disposal of WAS with added economic and environmental value. [ABSTRACT FROM AUTHOR]

[Display omitted] • CBZ at environmental levels slightly promoted the production of SCFAs. • The abundance of some functional traits changed in the presence of CBZ. • Persulfate-based treatment was effective for CBZ removal. • Hydroxylation and deamination reaction were the main pathways for CBZ degradation. The effects of pollutants on anaerobic fermentation of waste activated sludge (WAS) have been widely studied. Nevertheless, little attention was paid to how to mitigate the adverse effects of pollutants on anaerobic fermentation. In this work, representative pollutant carbamazepine (CBZ) was chosen to explore the potential effects of CBZ on WAS anaerobic fermentation. Experimental results showed that CBZ at environmental levels slightly promoted the production of short-chain fatty acids (SCFAs), which increased from 3858.2 to 4143.8 mg COD/L as the CBZ dose increased from 8 to 28 mg/kg VSS, but further increasing the CBZ dose to 88 mg/kg VSS decreased the production of SCFAs. Context of homologous group database revealed that the abundance of some functional traits changed, such as RNA processing and modification, energy production and conversion, and amino acid transport. This adverse effect was sustained because CBZ could not be degraded during anaerobic fermentation. Therefore, a regulation strategy was proposed, e.g., persulfate was used to attenuate the toxicity of CBZ to WAS anaerobic fermentation, and the results showed that the removal of CBZ reached 83.7 % in the persulfate-treated reactor. Based on density functional theory and metabolic intermediates determination, hydroxylation and deamination reactions were the two main metabolic pathways for CBZ degradation. [ABSTRACT FROM AUTHOR]

The poor dewaterability of fermented sludge is an important factor limiting the development of anaerobic fermentation applications. Herein we reported an efficient strategy, i.e., using acidic regulation to stimulate the release of indigenous enzymes, to enhance the hydrolysis and dewatering of fermented sludge. The results showed that after acidic regulation at pH 4.0 for 1 day, the activity of protease and α-glucosidase were improved by 131.4% and 146.0%, while the capillary suction time and specific resistance to filtration were decreased by 93.8% and 69.5%, respectively. Mechanism study revealed that the method firstly destroyed the slime and bound EPS and cells of fermented sludge, causing the release of indigenous enzymes (i.e., protease and α-glucosidase) contained in. Then, the released enzymes directly accelerated the hydrolysis and acidification of fragmentized extracellular polymeric substances, thereby benefited the release of bound water in sludge particles. Finally, such acidic condition decreased the electrostatic repulsive interactions between destroyed sludge particles, further improving their flocculation. The findings not only deepen the understanding of indigenous enzymes contained in fermented sludge affecting sludge dewatering, but also might guide engineers to develop promising strategies to facilitate fermented sludge dewatering and fermentation liquid recovery in the future. [Display omitted] • Acidic treatment (AT) improved dewaterability of anaerobically fermented sludge. • The minimal capillary suction time was obtained after AT at pH 4.0 for 1 day. • AT destroyed sludge cells and facilitated the release of hydrolytic enzymes. • Acid-driven indigenous enzymatic hydrolysis was major contributor to dewatering. • AT reduced the electrostatic repulsive interactions between sludge particles. [ABSTRACT FROM AUTHOR]

Diclofenac (DCF), a nonsteroidal anti-inflammatory drug, is one of the most commonly detected pharmaceuticals in wastewater treatment plants. However, the fate of DCF in waste activated sludge (WAS) anaerobic fermentation has not been well-understood so far. This work therefore aims to comprehensively reveal whether and how DCF is transformed in WAS mesophilic anaerobic fermentation through both experimental investigation and density functional theory (DFT) calculation. Experimental results showed that ∼28.8% and 45.8% of DCF were respectively degraded during the batch and long-term fermentation processes. Based on the detected intermediates and DFT-predicted active sites, three metabolic pathways, i.e., chlorination, hydroxylation, and dichlorination, responsible for DCF transformation were proposed. DFT calculation also showed that the Gibbs free energy (ΔG) of the three transformation pathways was respectively 19.0, −4.3, and −19.3 kcal/mol, suggesting that the latter two reactions (i.e., hydroxylation and dichlorination) were thermodynamically favorable. Illumina MiSeq sequencing analyses revealed that DCF improved the populations of complex organic degradation microbes such as Proteiniclasticum and Tissierellales , which was in accord with the chemical analyses above. This work updates the fundamental understanding of the degradation of DCF in WAS anaerobic fermentation process and enlightens engineers to apply theoretical calculation to the field of sludge treatment or other complex microbial ecosystems. [Display omitted] • Approximately 28.8% of DCF was degraded during anaerobic fermentation system. • DCF reduced the diversity and complexity of microbial community. • Hydroxylation and dichlorination reactions were thermodynamically favorable. • Proteiniclasticum and Tissierellales may be microbes related to DCF degradation. [ABSTRACT FROM AUTHOR]

Short-chain fatty acids (SCFAs) are the valuable products derived from the anaerobic fermentation of organic solid waste. However, SCFAs yield was limited by the worse solubilization and hydrolysis of particulate organic matter, and rapid consumption of organic acid by methanogens. In this study, an efficient and green strategy, i.e. adding biosurfactant alkyl polyglycosides (APG) into anaerobic fermentation system, was applied to enhance SCFAs production from food waste. Experimental results showed that APG not only greatly improved SCFAs production but also shortened the fermentation time for the maximum SCFAs accumulation. The SCFAs yield at optimal APG dosage 0.2 g/g TS (total solid) reached 37.2 g/L, which was 3.1-fold of that in blank. Meanwhile, the time to accumulate the maximum SCFAs in the presence of APG was shortened from day 14 to day 6. The activities of key enzymes such as hydrolytic and acid-forming enzymes were greatly promoted due to the presence of APG. These results demonstrated that the enhanced mechanism of SCFAs production should be attributed to the acceleration of solubilization and hydrolysis, enhancement of acidification and inhibition of methanogenesis by APG. [ABSTRACT FROM AUTHOR]

• Rhamnolipid significantly increased H 2 S production from sludge fermentation. • Rhamnolipid increased the release of organic sulfurs and inorganic sulfate. • The added and in situ generated rhamnolipid damaged the structure of organic sulfurs. • Rhamnolipid promoted H 2 S production from the hydrolysis of organic sulfurs. • Rhamnolipid facilitated H 2 S production from dissimilatory sulfate reduction. Rhamnolipid (RL), one representative biosurfactant, is widely regarded as an economically feasible and environmentally beneficial additive to improve fermentation efficiency and resource recovery from waste activated sludge (WAS). However, its potentially detrimental impact on WAS fermentation such as H 2 S generation was overlooked previously. This study therefore aims to fill the gap through exploring whether and how the presence of RL affects H 2 S generation from WAS anaerobic fermentation. Experimental results showed that when RL increased from 0 to 40 mg/g total suspended solids (TSS), the cumulative H 2 S yield enhanced from 323.6 × 10−4 to 620.3 × 10−4 mg/g volatile suspended solids (VSS). Mechanism analysis showed that RL reduced WAS surface tension, which benefited transformations of organic sulfurs (e.g., aliphatic-S and sulfoxide) and inorganic sulfate from solid to liquid phase. The presence of RL not only reduced the ratio of α-helix/(β-sheet + random coil) and damaged the hydrogen bonding networks of organic sulfurs but also promoted substrate surface charges and cell membrane permeability. These facilitated the contact between hydrolase and organic sulfurs, thereby increasing sulfide production from organic sulfurs hydrolysis. Further investigations showed that RL promoted the expression of key genes (e.g., aprA/B and dsrA/B) involved in the dissimilatory sulfate reduction, which accelerated the reaction of adenosine 5'-phosphosulfate (APS) → sulfite → sulfide. Meanwhile, RL inhibited the corresponding key genes such as CysH , and Sir , responsible for assimilatory sulfate reduction (APS → 3'-phosphoadenosine-5'phosphosulfate → organosulfur), which reduced substrate competition in favor of H 2 S production from dissimilatory sulfate reduction. Besides, RL decreased the fermentation pH, which benefited the transformation of H S − to H 2 S. [Display omitted] [ABSTRACT FROM AUTHOR]

• The FC–N pretreatment increased the yield of SCFAs in WAS anaerobic fermentation. • The FC–N integrated pretreatment enhanced the conversion rate of propionic acid. • The FC–N pretreatment greatly improved the biodegradability of the sludge. • The FC–N pretreatment vastly increased the proportion of acid-producing bacteria. • FC could be applied as a substitute for HCl to promote free nitrite formation. The production of short-chain fatty acids (SCFAs) via anaerobic fermentation of waste activated sludge (WAS) is often limited with poor quality of SCFAs and long fermentation time. To overcome these issues, we provided an efficient strategy by using ferric chloride (FC) to aid nitrite pretreatment. Experimental results showed that the maximal SCFAs production of 211.3 ± 3.1 mg COD/g VS was achieved with 4 mmol/L of FC integrated with 250 mg/L of nitrite pretreatment on day 5, which was 4.1-fold higher than that of the blank control (52 ± 5 mg COD/g VS, day 7). Besides, the enrichment of acetic acid was observed in the combined system, which accounted for 54.6 ± 3.5% of total SCFAs, while the proportion was only 31.5 ± 4.9% in the blank control. Propionic acid, isobutyric acid, n-butyric acid, n-valeric acid and isovaleric acid accounted for 14.7 ± 1.5%, 6.9 ± 1.4%, 7.4 ± 1.5%, 13.1 ± 1.0%, and 3.3 ± 1.5% of total SCFAs in the combined system and 22.8 ± 4.0%, 11.9 ± 3.0%, 6.7 ± 3.1%, 17.6 ± 2.0%, and 9.5 ± 3.9% of total SCFAs in the blank control, respectively. It was found that soluble proteins and carbohydrates in the combined system were higher than those in the blank control, suggesting that FC and nitrite pretreatment was beneficial for WAS disintegration. The fluorescence spectrum results suggested that FC and nitrite pretreatment improved the biodegradability of released organics, which provided more biodegradable substances for the subsequent SCFAs production. This was because the addition of FC induced the formation of free nitrous acid from nitrite. Besides, FC-induced iron reduction also promoted the conversion of recalcitrant organics to biodegradable organic matter. Microbial community structure analysis demonstrated that the functional bacteria involved in acetogenesis process such as Enterococcus, Proteiniclasticum , and Petrimonas were highly enriched due to the pretreatment of FC and nitrite, indicating this method could improve the relative abundance of SCFAs producers. Overall, this study revealed that the pretreatment of FC and nitrite promoted the formation of free nitrous acid and increased the yield of SCFAs, which provided a novel method for wastewater treatment plants to ameliorate the sewage treatment craft and rationally use the existing substances in WAS to enhance resource recovery. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Sulfite-based pretreatment remarkably promoted VFAs production from microalgae. • Acetate proportion of 54.5% was obtained at 200 mg S/L with time of day 8. • More and relatively easy biodegradable organics were released into liquid phase. • Sulfite-induced sulfate-reducing bacteria facilitated the acetate generation pathway. • β-glucanase, β-glucosidase, and acetate kinase activities were accelerated. Volatile fatty acids (VFAs) production from anaerobic fermentation of microalgae is generally constrained by low organics solubilization and poor substrate-availability. In this study, sulfite-based pretreatment was developed to overcome such situation. Experimental results showed that the maximum concentration of VFAs (467.5 mg COD/g VSS) and corresponding acetate proportion (54.5%) was obtained at 200 mg sulfite-S/L with fermentation time of day 8, which was respectively 2.1- and 1.9-fold of control. It was found that after sulfite pretreatment, more and relatively easy biodegradable organics were released into liquid phase, providing available substrate for acid-producing bacteria. The rigid cell wall of microalgae was destroyed, evidenced by the decreased particle size and increased surface area, which made the microalgae more accessible for subsequent hydrolysis and acidification. Meanwhile, the sulfite-induced sulfate-reducing bacteria facilitated the acetate generation pathway. The accelerated activities of β-glucanase, β-glucosidase, and acetate kinase involved in anaerobic fermentation further validated the above results. [ABSTRACT FROM AUTHOR]

An efficient and green strategy, i.e. adding nano zero-valent iron into anaerobic fermentation systems to remarkably stimulate the accumulation of short-chain fatty acids from waste activated sludge via accelerating the solubilization and hydrolysis processes has been developed. In the presence of nano zero-valent iron, not only the short-chain fatty acids production was significantly improved, but also the fermentation time for maximal short-chain fatty acids was shortened compared with those in the absence of nano zero-valent iron. Mechanism investigations showed that the solubilization of sludge, hydrolysis of solubilized substances and acidification of hydrolyzed products were all enhanced by addition of nano zero-valent iron. Also, the general microbial activity of anaerobes and relative activities of key enzymes with hydrolysis and acidification of organic matters were improved than those in the control. 454 high-throughput pyrosequencing analysis suggested that the abundance of bacteria responsible for waste activated sludge hydrolysis and short-chain fatty acids production was greatly enhanced due to nano zero-valent iron addition. [ABSTRACT FROM AUTHOR]

Highlights: [•] SRT or temperature increase benefited the hydrolysis of fermentation substrates. [•] SCFAs, especially propionic acid, accumulated most at SRT 8d and 37°C. [•] The activities of key enzymes were in accordance with SCFAs production. [•] The ratio of Bacteria to Archaea was improved at SRT 8d and 37°C. [ABSTRACT FROM AUTHOR]