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[Display omitted] • 102.1 mg/g-VS humic substances reduced the methane production rate by 33.8 % in AD. • 0.6 V MEC-AD maintained stable when 102.1 mg/g-VS humic substances were added. • Humic substances acted as carbon sources and electron shuttles in 0.6 V MEC-AD. • The newly formed humic substances have lower aromaticity and higher reducibility. • The abundance of hydrogenotrophic methanogens on the cathode reached 42.9 %. Humic substances as major components of waste activated sludge are refractory to degrade and have inhibition in traditional anaerobic digestion (AD). This study for the first time investigated the feasibility and mechanism of microbial electrolysis cell assisted anaerobic digestion (MEC-AD) to break the recalcitrance and inhibition of humic substances. The cumulative methane production of AD decreased from 134.7 to 117.6 mL/g-VS with the addition of humic acids and fulvic acids at 25.2–102.1 mg/g-VS. However, 0.6 V MEC-AD maintained stable methane production (155.5–158.2 mL/g-VS) under the effect of humic substances. 0.6 V MEC-AD formed electrical stimulation on microbial cells, provided anodic oxidation and cathodic reduction transformation pathways for humic substances (acting as carbon sources and electron shuttles), and aggregated functional microorganisms on electrodes, facilitating the degradation of humic substances and generation of methane. This study provides a theoretical basis for improving the energy recovery and system stability of sludge treatment. [ABSTRACT FROM AUTHOR]

This study reports an innovative strategy known as stepwise pH fermentation, developed to enhance the production of short chain volatile fatty acids (SCFA) from waste activated sludge (WAS) anaerobic fermentation. Experimental results confirmed the optimal pH for WAS disruption and acidification was 11 and 9, respectively, and corresponding optimal time was, respectively, 5 d and 2 d. In this scenario, the optimal SCFA yield was 2356 mg chemical oxygen demand (COD)/L, which was much higher than that derived from alkaline fermentation system. Investigation of the mechanism indicated that pH 11 could accelerate the disruption of WAS and inhibit the activities of methanogens; furthermore, pH 9 was beneficial to the activity of acid-producing bacteria, resulting in more SCFA production. Stepwise pH fermentation integrated with sodium chloride (NaCl) present in WAS had synergistic impacts on WAS anaerobic fermentation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Refractory substances are primary but often ignored factor related to WAS biodegradability. • The presence and role of refractory substances in WAS biodegradability are reviewed. • Inhibition mechanisms of refractory substances in WAS are analyzed. • Mechanisms of pretreatment strategies to enhance WAS biodegradability are discussed. • Challenges of WAS enhanced biodegradability technology are prospected. Anaerobic digestion (AD) is a low-cost technology widely used to divert waste activated sludge (WAS) to renewable energy production, but is generally restricted by its poor biodegradability which mainly caused by the endogenous and exogenous refractory substances present in WAS. Several conventional methods such as thermal-, chemical-, and mechanical-based pretreatment have been demonstrated to be effective on organics release, but their functions on refractory substances conversion are overlooked. This paper firstly reviewed the presence and role of endogenous and exogenous refractory substances in anaerobic biodegradability of WAS, especially on their inhibition mechanisms. Then, the pretreatment strategies developed for enhancing WAS biodegradability by facilitating refractory substances conversion were comprehensively reviewed, with the conversion pathways and underlying mechanisms being emphasized. Finally, the future research needs were directed, which are supposed to improve the circular bioeconomy of WAS management from the point of removing the hindering barrier of refractory substances on WAS biodegradability. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Pretreatment of polyacrylamide-flocculated WAS (PF-WAS) was conducted. • Hydrolysis rate of PF-WAS was improved through thermal-alkaline pretreatment (TAP) • Biochemical methane potential of PF-WAS was improved through TAP. • Optimal TAP for methane yield of PF-WAS was 75 °C and pH 11.0 for 17.5 h. • Optimal TAP improved PAM's degradation and avoided acrylamide's accumulation. Abstract Deterioration of anaerobic digestion can occur with the presence of polyacrylamide (PAM) in waste activated sludge, and little information on mitigating this deterioration is currently available. In this study, simultaneous mitigation of PAM negative effects and improvement of methane production was accomplished by thermal-alkaline pretreatment. Under the optimized pretreatment conditions (i.e., 75 °C, pH 11.0 for 17.5 h), the biochemical methane potential of PAM-flocculated sludge increased from 100.5 to 210.8 mL/g VS and the hydrolysis rate increased from 0.122 to 0.187 d−1. Mechanism investigations revealed that the pretreatment not only broke the large firm floccules, improved the degradation of PAM, but also facilitated the release of biodegradable organics from sludge, which thereby provided better growth environment and enough nutrients to anaerobic microbes for methane production. The activities of key enzymes responsible for methane production and PAM degradation were greatly improved in pretreated reactor, with the accumulation of acrylamide being avoided. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • CFHP was applied to improve methane production from WAS for the first time. • Hydrolysis rate of WAS are improved through CFHP. • Biochemical methane potential of WAS are improved through CFHP. • CFHP is an attractive technology for the application in real-world situations. Abstract Anaerobic digestion of waste activated sludge (WAS) is often limited by low hydrolysis efficiencies and poor methane potentials. This work presents a novel pretreatment technology for WAS anaerobic digestion, i.e., combining free ammonia with heat pretreatment (CFHP). Experimental results showed that compared with control, solo free ammonia (135.4 mg NH 3 -N/L) and solo heat (70 °C) pretreatment, the combined free ammonia and heat (135.4 mg NH 3 -N/L with 70 °C) obtained 52.2%, 25.5% and 30.2% faster in hydrolysis rate and 25.2%, 17.9% and 16.5% higher in biochemical methane potential, respectively. Mechanism investigations showed that the combined pretreatment not only largely facilitated the disintegration of WAS but also increased the proportion of biodegradable organic matters, thereby providing better contract between biodegradable organics and the anaerobic microbes for methane production. Considering its effectiveness and renewability, the combined pretreatment is an attractive technology for the application in real-world situations. [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]

[Display omitted] • Non-antibiotic pharmaceutical also promoted the widespread enrichment of ARGs. • Low concentrations of carbamazepine enriched ARGs more than high concentrations. • Carbamazepine promoted the aggregation of microorganisms and conjugation of ARGs. • TCS, QS and T4SS played important roles in the horizontal transfer of ARGs. • Carbamazepine increased the potential for pathogens to acquire antibiotic resistance. Antimicrobial resistance is a global health security issue of widespread concern. Recent studies have unveiled the potential contribution of non-antibiotics to the emergence of antimicrobial resistance. This study investigated the effect of carbamazepine, a non-antibiotic pharmaceutical, on the fate of antibiotic resistance genes (ARGs) during anaerobic digestion. The results, as revealed by both metagenomic sequencing and absolute quantification, demonstrated that carbamazepine induced the enrichment of ARGs and increased the abundance of ARGs hosts by 1.2–2.1 times. Carbamazepine facilitated microbial aggregation and intercellular communication by upregulating functional genes associated with two-component systems, quorum sensing and type IV secretion systems, thereby increasing the frequency of ARGs conjugation. Furthermore, carbamazepine induced the acquisition of ARGs by pathogens and elevated the overall pathogenic abundance. This study revealed the mechanisms of microbial self-regulation and ARGs transmission under carbamazepine stress, highlighting the potential health risks posed by non-antibiotic pharmaceuticals during the safe disposal of sludge. [ABSTRACT FROM AUTHOR]

[Display omitted] • CaO effectively promoted kitchen waste dry anaerobic digestion, and the maximum biogas was 656.84 mL/g. • CaO facilitated the production of dissolved organic matter and the transformation of VFA, thereby avoiding its negative impact on biogas production. • Metagenomics revealed the microbial mechanism underlying the CaO enhanced kitchen waste dry anaerobic digestion. In light of the characteristics of excessive acidification and low biogas yield during kitchen waste (KW) dry digestion, the impact of the calcium oxide (CaO) on KW mesophilic dry digestion was investigated, and the enhanced mechanism was revealed through metagenomic approach. The results showed that CaO increased the biogas production, when the CaO dosage was 0.07 g/g (based on total solid), the biogas production reached 656.84 mL/g suspended solids (VS), approximately 8.38 times of that in the control. CaO promoted the leaching and hydrolysis of key organic matter in KW. CaO effectively promoted the conversion of volatile fatty acid (VFA) and mitigated over-acidification. Macrogenome analysis revealed that CaO increased the microbial diversity in KW dry digestion and upregulated the abundance of genes related to amino acid and carbohydrates metabolism. This study provides an effective strategy with potential economic benefits to improve the bioconversion efficiency of organic matter in KW. [ABSTRACT FROM AUTHOR]

• Triclosan had a tendency to restrain methane production from sludge. • Triclosan could be degraded by anaerobic microorganism. • Triclosan at higher levels had seriously negative effect on biological processes. • Triclosan exposure could greatly change the richness and diversity of populations. Triclosan (TCS), as an antimicrobial agent, is considered as a representative emerging contaminant and was frequently detected in excess sludge. This study investigated the effect of TCS on activate wastewater sludge (WAS) digestion through laboratory methane production experiment. It was concluded that TCS had a tendency to restrain methane production from sludge with its exposure level increasing. The results displayed that the yields of final maximum cumulative methane production were similar about 108.4 mL/g VSS at TCS level lower 200 mg TCS/kg TSS, while the values were approximately 95.2 mL/g VSS with TCS level over 550 mg TCS/kg TSS. Although TCS could be degraded, its intermediates in this study had no effect on sludge digestion. In addition, TCS at higher levels had seriously negative effect on the solubilization, hydrolysis, acidification, and methanogenesis processes. Microbial community was further analyzed to understand the TCS's effect on digestion system from a micro perspective. [ABSTRACT FROM AUTHOR]

• Microwave pretreatment (MP) benefited the degradation of PAM. • MP accelerated the release and hydrolysis of organics from PF-WAS. • MP facilitated the breaking of large firm "PAM-sludge" floccules. • Hydrolysis rate of PF-WAS was accelerated through MP. • Biochemical methane potential of PF-WAS was improved through MP. Deterioration of anaerobic digestion can occur with the presence of polyacrylamide (PAM) in waste activated sludge, but the information on alleviating this deterioration is still limited. In this study, the simultaneous alleviation of negative effect of PAM and improvement of methane production during anaerobic digestion was accomplished by microwave pretreatment. Experimental results showed that with the microwave pretreatment times increased from 0 to 12 min, the biochemical methane potential of PAM-flocculated sludge (12 g PAM/kg total solids) asymptotically increased from 123.1 to 242.5 mL/g volatile solids, hydrolysis rate increased from 0.06 to 0.13 d−1. Mechanism analysis indicated that the microwave pretreatment accelerated the release and hydrolysis of organic substrates from PAM-flocculated sludge, facilitated the breaking of large firm "PAM-sludge" floccules, and benefited the degradation of PAM, which alleviated the PAM inhibitory impacts on digestion and meanwhile provided better contact between the released organic substrates and anaerobic bacteria for methane production. [ABSTRACT FROM AUTHOR]

Highlights • A new strategy was developed to obtain high polyhydroxyalkanoates accumulation. • ∼46.5% nitrite accumulation was obtained in the N-SBR with FNA treatment. • FNA treatment reduced the abundance and activity of nitrite oxidizing bacteria. • Nitrite-based denitrifying phosphorus removal was achieved in AAO-SBR. Abstract A novel strategy to achieve substantial polyhydroxyalkanoates (PHA) accumulation in waste activated sludge (WAS) was developed, which was conducted in a two-sludge system consisted of an anaerobic/anoxic/oxic reactor (AAO-SBR) and a nitrifying reactor (N-SBR), where the nitrifying-sludge was treated by free nitrous acid (FNA). Initially, 0.98 ± 0.09 and 1.46 ± 0.10 mmol-c/g VSS of PHA were respectively determined in the control-SBR and AAO-SBR. When 1/16 of nitrifying sludge was daily treated with 1.49 mg N/L FNA for 24 h, ∼46.5% of nitrite was accumulated in the N-SBR, ∼2.43 ± 0.12 mmol-c/g VSS of PHA was accumulated in WAS in AAO-SBR without deteriorating nutrient removal. However, nutrient removal of control-SBR was completely collapsed after implementing the same FNA treatment. Further investigations revealed that the activity and abundance of nitrite oxidizing bacteria (NOB) was decreased significantly after FNA treatment. Finally, sludge with high PHA level to generate more methane was confirmed. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • CaO 2 + ultrasonic synergistically improved the intensity of free radicals. • CaO 2 + ultrasonic facilitated solubilization and release of biodegradable organics. • CaO 2 + ultrasonic improved the hydrolysis rate and biochemical methane potential. • CaO 2 + ultrasonic enhanced the removal of refractory organic pollutants. Abstract This study reported a novel and high-efficient pretreatment method for anaerobic digestion, i.e., combining calcium peroxide (CaO 2) with ultrasonic (US), by which not only the methane production was remarkably improved but also the removal of refractory organic contaminants was enhanced. Experimental results showed the optimum condition for methane production was achieved at 0.1 g CaO 2 /g VSS combined with US (1 W/ml, 10 min). Under this condition, the maximal methane yield of 211.90 ± 2.6 L CH 4 /kg VSS was obtained after 36 d of anaerobic digestion, which was respectively 1.36-fold, 1.19-fold and 1.26-fold of that from the control, solo US (1 W/ml, 10 min) and solo CaO 2 (0.1 g/g VSS). Mechanism investigations revealed that CaO 2 + US not only improved the disintegration of waste activated sludge (WAS) but also increased the proportion of biodegradable organic matters. Moreover, the total frequency of recalcitrant contaminants contained in WAS decreased significantly when CaO 2 + US was applied. [ABSTRACT FROM AUTHOR]

This study reported a new, renewable and high-efficient strategy for anaerobic fermentation, i.e., using free ammonia (FA) to pretreat waste activated sludge (WAS) for 1 d and then combining with rhamnolipid (RL), by which the short-chain fatty acids (SCFA) production was remarkably improved. Experimental results showed the maximal SCFA production of 324.7 ± 13.9 mg COD/g VSS was achieved at 62.6 mg FA/L pretreatment combined with 0.04 g RL/g TSS, which was respectively 5.95-fold, 1.63-fold and 1.41-fold of that from control, FA pretreatment and RL pretreatment. Mechanism investigations revealed that FA + RL enhanced sludge solubilization and hydrolysis, providing more organics for subsequent SCFA production. It was also found that the combined method inhibited acidogenesis and methanogenesis, but the inhibition to methanogenesis was much severer than that to acidogenesis. Finally, the feasibility of NH 4 + -N and PO 3− 4 -P, released in fermentation liquor, being recovered as magnesium ammonium phosphate (MAP) was confirmed. [ABSTRACT FROM AUTHOR]

In this study, waste activated sludge (WAS) was used as feedstock to generate hydrochars at different temperatures (220 °C and 260 °C) and their effect on sludge anaerobic digestion was evaluated. Experimental results showed that the maximum yield of short-chain fatty acid (SCFA) enhanced by hydrochar (220 °C) and hydrochar (260 °C) were 507.33 and 270.80 mg chemical oxygen demand (COD)/L respectively, which were much higher than that in blank (141.49 mg COD/L). Mechanism investigation confirmed that hydrochar remarkably accelerated the solubilization and hydrolysis of organic matters, enhanced the acidification of hydrolyzed products, and inhibited the activity of methanogenic bacteria as well as promoted the activities of key enzymes. Meanwhile, the organic matters especially humic substances existed in the hydrochar played an important role during anaerobic digestion. [ABSTRACT FROM AUTHOR]

[Display omitted] • AITC was degraded by –N C S and C C–C– isomerization in anaerobic co-digestion. • AITC inhibited methane production from anaerobic co-digestion of KW and WAS. • AITC inhibited acetotrophic methanogenesis by reducing the activity of AK enzyme. • AITC enriched complex organic degradation microbes such as Bacillus. Producing methane from anaerobic co-digestion of kitchen waste and waste activated sludge has been widely implemented in real-world situations. However, the fate and impact of allyl isothiocyanate (AITC), a main active component in cruciferous vegetables, in the anaerobic co-digestion has never been documented. This study therefore aims to provide such supports. Experimental results exhibited that AITC was degraded completely by microorganisms and served as a substrate to produce methane. As AITC increased from 0 to 60 mg/L, the maximum methane production decreased from 285.1 to 35.8 mL/g VS, and the optimum digestion time was also prolonged. The mechanism study demonstrated that AITC induced cell apoptosis by modifying the physicochemical properties of cell membrane, which resulted in inhibitions to the procedure of anaerobic co-digestion. The high-throughput sequencing showed that AITC enriched the microorganism for degradation of complex organic compounds such as Bacillus , but lessened anaerobes involved in hydrolysis, acidogenesis, and methanogenesis. [ABSTRACT FROM AUTHOR]

[Display omitted] • The intrinsic drawbacks of WAS for efficient AD process are disclosed. • The main inhibitory mechanisms of interfering pollutants for WAS AD were analyzed. • Potential strategies for mitigating the restrictions on WAS digestion are revealed. • The future directions for the improvement of WAS digestion are proposed. Anaerobic digestion (AD) is an effective approach for waste activated sludge (WAS) disposal with substantial recovery of valuable substrates. Previous studies have extensively explored the correlations of common operational parameters with AD efficiency, but the impacts of intrinsic characteristics of WAS on the AD processes are generally underestimated. This study focused on disclosing the association of intrinsic drawbacks in WAS with AD performance, and found that the cemented WAS structure, low fraction of biomass and various high levels of inhibitory pollutants (e.g. , organic pollutants and heavy metals), as the integral parts of WAS all greatly restricted the AD performance. The main potential strategies and underlying mechanisms to mitigate the restrictions for efficient WAS digestion, including the practical pretreatment methods, bioaugmentation and aided substances addition, were critically analyzed. Also, future directions for the improvement of WAS digestion were proposed from the perspectives of technical, management and economic aspects. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sodium lauroyl sarcosinate improved methane yield in anaerobic digestion. • Methane can be produced during the degradation of sodium lauroyl sarcosinate. • Sodium lauroyl sarcosinate promoted the release of organic matters. • Sodium lauroyl sarcosinate improved the activities of hydrolytic enzymes. In this study, sodium lauroyl sarcosinate (SLS) was used to promote anaerobic digestion of waste activated sludge for producing methane. It was found maximum cumulative methane production increased from 98.1 ± 3.1 to 166.0 ± 4.3 mL/g Volatile Suspended Solids (VSS) with dosage increasing from 0 (control) to 40 mg SLS/g TSS. But the addition of SLS (>10 mg SLS/g Total Suspended Solids (TSS)) resulted in prolonged lag phase time. Microbiological analysis showed that Syntrophobacter and Syntrophomonas both got enriched in reactors fed with SLS. Furthermore, hydrogenotrophic methanogens genus got more enrichment in contrast to acetoclastic methanogens. Mechanism analysis indicated that addition SLS could decrease surface tension, and promote release of organic matters as well as improve activities of hydrolytic enzymes. Besides, SLS could be nearly degraded completely within 3 days, and its degradation intermediates could be further transformed into methane gradually, thus enhancing methane production eventually. [ABSTRACT FROM AUTHOR]

• The impact of S-mZVI on the anaerobic co-digestion of WAS and FW was evaluated. • The cumulative CH 4 yield within 15 days reached to 264.78 mL/g-VS at S-mZVI 10 g/L. • S-mZVI had significant influence on the methanogenesis stage of co-digestion. • The addition of S-mZVI improved the direct interspecies electron transfer (DIET). The enhancement of zerovalent iron (ZVI) on anaerobic digestion (AD) has been proved, but there are still some problems that constrain the large-scale application of ZVI, such as the destruction of cell membrane and the inhibition of methanogenesis led by rapid H 2 accumulation. Aiming at these problems, sulfidated microscale zerovalent iron (S-mZVI) was employed to evaluate its effect on anaerobic co-digestion (AcoD) of waste activated sludge (WAS) and food waste (FW). Experimental results showed that S-mZVI promoted the direct interspecies electron transfer (DIET) between specific bacteria and methanogens, resulting in higher methane yield. At S-mZVI 10 g/L, the cumulative methane yield and ETS activity reached 264.78 mL/g-VS and 24.62 mg INTF/(g-TS h), which was 1.33 and 1.83 times that of blank. Microbiological analysis demonstrated that the abundance of DIET-related microorganisms such as Syntrophomonas , Methanosarcina and Methanobacterium increased with the increasing dosage of S-mZVI. [ABSTRACT FROM AUTHOR]

• Biochar enhanced high-quality biomethane production from PS. • Biochar accelerated PS hydrolysis and enhanced methane production potential of PS. • Biochar incresed buffering capacity, conductivity and alleviated NH 3 inhibition. • Biochar enhanced sludge reduction and reduced sludge disposal costs. • Biochar could be applied to anaerobic digestion of the mixture of WAS and PS. This study conducted batch and continuous tests to reveal the feasibility of corn stover biochar on improving anaerobic digestion of primary sludge (PS). Dosing biochar (1.82, 2.55 and 3.06 g/g Total Solids (TS)) in digester improved methane content increasing from 67.5% to 81.3–87.3% and enhanced methane production by 8.6–17.8%. Model analysis indicated that biochar accelerated PS hydrolysis and enhanced methane potential of PS. The mechanistic studies showed that biochar enhanced process stability provided by strong buffering capacity and alleviated NH 3 inhibition. In continuous test over 116 days, the volatile solids (VS) destruction in the biochar-dosed digester increased by 14.9%, resulting in a 14% reduction in the volume of digestate for disposal. Biochar changed microbial community in an expected direction for anaerobic digestion. This work suggests that biochar technology would apply to co-digestion of WAS and PS to maximize the energy recovery and sludge reduction from the two sludge streams. [ABSTRACT FROM AUTHOR]

• Impact of pyrochar (PC) and hydrochar (HC) on the anaeribic digestion of WAS was compared. • The highest accumulative methane yield was observed at group HC180. • Pyrochar showed a slightly negative effect on methane production. • Specific DIET-based syntrophic microbes were enriched with HCs addition. This study compared the effects of sewage sludge-derived pyrochar (PC300, PC500, and PC700) and hydrochar (HC180, HC240, and HC300) on mesophilic anaerobic digestion of waste activated sludge (WAS). It was demonstrated that hydrochar can better promote the methane production compared with pyrochar. The highest accumulative methane yield of 132.04 ± 4.41 mL/g VS added was obtained with HC180 addition. In contrast, the PC500 and PC700 showed a slightly negative effect on methane production. Sludge-derived HC not only accelerated the solubilization and hydrolysis of sludge floc, but also improved the production of acetic acid and propionate, further resulting in improved methane production. Simultaneously, the syntrophic microbes facilitating direct interspecies electron transfer (DIET) such as Syntrophomonas, Peptococcaceae, Methanosaeta and Methanobacterium bred rapidly with the addition of HCs. These results indicated that the hydrochar is more ideal as the accelerant to promote the methane production from mesophilic anaerobic digestion of WAS than the pyrochar. [ABSTRACT FROM AUTHOR]