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• 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]

Graphic abstract Highlights • FA aids ultrasound pretreatment enhanced the yield of SCFA. • FA aids ultrasound pretreatment accelerated sludge solubilization. • FA aids ultrasound pretreatment enhanced the release of biodegradable organics. • FA aids ultrasound pretreatment severely inhibited methanogenesis process. Abstract Ultrasound pretreatment was proved to be effective in accelerating waste activated sludge (WAS) disintegration and promoting anaerobic fermentative short-chain fatty acids (SCFA) production. However, due to the high activities of SCFA consumers, SCFA yield is limited. Herein we reported an efficient strategy, i.e., using free ammonia (FA) to aid ultrasound pretreatment, to enhance SCFA accumulation. Experimental results showed that the greatest SCFA yield of 316.7 ± 14.9 mg COD/g VSS was obtained at 2 w/mL ultrasound pretreatment for 15 min followed by initial 60 mg/L FA pretreatment for 2 d, which was 2.2 times (1.7 times) of that from sole ultrasound (FA) pretreatment. The mechanism study revealed that this combination method accelerated sludge solubilization, enhanced the release of biodegradable organics, and caused severe inhibitions to SCFA consumption. Moreover, this method has inhibitory effects on specific activities of hydrolytic microorganisms, SCFA-producing bacteria and methanogens, but the inhibitions to methanogens were severe. [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]

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 • 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]

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]

The effect of Fe(II)-activated peroxymonosulfate (Fe(II)-PMS) oxidation on the waste activated sludge (WAS) dewatering and its mechanisms were investigated in this study. The capillary suction time (CST), specific resistance to filterability (SRF) of sludge and water content (W C ) of dewatered sludge cake were chosen as the main parameters to evaluate the sludge dewaterability. Experimental results showed that Fe(II)-PMS effectively disintegrated sludge and improved sludge dewaterability. High CST and SRF reduction (90% and 97%) was achieved at the optimal conditions of PMS ( HSO 5 - ) 0.9 mmol/gVSS, Fe(II) 0.81 mmol/gVSS, and pH 6.8. Extracellular polymeric substances (EPS) and three-dimensional excitation–emission matrix (3D-EEM) fluorescence spectroscopy before and after Fe(II)-PMS oxidation were determined to explain the enhanced dewatering mechanism. The release of EPS-bound water induced by the destruction of EPS was the primary reason for the improvement of sludge dewaterability during Fe(II)-PMS oxidation. [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]

[Display omitted] • UHP pretreatment enhanced the production of SCFAs in WAS anaerobic fermentation. • UHP pretreatment contributed to WAS disintegration significantly. • UHP pretreatment facilitated the generation and transformation of intermediates. • OH, O 2 and urea were the major contributors to the increased WAS disintegration. • Functional microbials and genes related with acetate production were upregulated. Improving the anaerobic treatment performance of waste activated sludge (WAS) to achieve resource recovery is an indispensable requirement to reduce carbon emissions, minimize and stabilize biosolids. In this study, a novel strategy by using urea hydrogen peroxide (UHP) to enhance SCFAs production through accelerating WAS disintegration, degrading recalcitrant substances and alleviating competitive suppression of methanogens. The SCFAs production and acetate proportion rose from 436.9 mg COD/L and 31.3% to 3102.6 mg COD/L and 54.1%, respectively, when UHP grew from 0 to 80 mg/g TSS. Mechanism investigation revealed that OH, O 2 and urea were the major contributors to accelerate WAS disintegration with the sequence of OH> O 2 > urea. Function microbes related to acidification and genes associated with acetate production ([EC:2.3.1.8] and [EC:2.7.2.1]) were upregulated while genes encoding propionic acid production ([EC:6.4.1.3] and [EC:6.2.1.1]) were downregulated. These results raised the application prospects of UHP in WAS resource utilization. [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]

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]

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]

Short-chain fatty acids (SCFA), raw substrates for biodegradable plastic production and preferred carbon source for biological nutrients removal, can be produced from anaerobic fermentation of waste activated sludge (WAS). This paper reports a new, high-efficient and eco-friendly strategy, i.e., using free nitrous acid (FNA) pretreatment combined with Tea saponin (TS), to enhance SCFA production. Experimental results showed 0.90 mg/L FNA pretreatment and 0.05 g/g total suspended solids TS addition (FNA + TS) not only significantly increased SCFA production to 315.3 ± 8.8 mg COD/g VSS (5.52, 1.76 and 1.93 times higher than that from blank, solo FNA and solo TS, respectively) but also shortened fermentation time to 4 days. Mechanism investigations revealed that FNA pretreatment combined with TS cause a positive synergetic effect on sludge solubilization, resulting in more release of organics. It was also found that the combination benefited hydrolysis and acidogenesis processes but inhibited the methanogenesis. [ABSTRACT FROM AUTHOR]

Excessive discharge of phosphate (P) into the surface water is the key factor to cause the eutrophication, so its removal has aroused much attention in recent years. In this study, different iron modification (chemical co-precipitation of Fe 3+ /Fe 2+ or FeCl 3 impregnation) was used to improve the phosphate adsorption capacity of waste activated sludge (WAS)-based biochar. Comparative tests demonstrated that the FeCl 3 -impregnated WAS-based biochar exhibited much superior phosphate adsorption capacity (111.0 mg/g) in all as-prepared samples and performed well even under the interferences with pH and coexisting ions. X-ray diffraction (XRD) analyzes indicated that the iron in FeCl 3 -impregnated WAS-based biochar existed mainly in amorphous phase, as hematite and amorphous hydroxides forms, which was of great benefit to the phosphate adsorption. Besides, ligand exchange plays important role in the adsorption of phosphate. The WAS-based biochar kept over 60% phosphate removal efficiency after five recycles. [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] • 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] • 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] • CTC promotes volatile fatty acid production from waste activated sludge. • 10 mg/kg TSS CTC induces the highest volatile fatty acid accumulation. • CTC stimulates the secretion of extracellular polymeric substances. • CTC enhances acidification and inhibits methanation by regulating enzyme activity. One of the key challenges of wastewater treatment today is to understand the potential effect of residual pollutants on the management of waste activated sludge (WAS). This study aims to clarify the effect of chlortetracycline (CTC) as a residual antibiotic on the anaerobic fermentation of WAS to produce volatile fatty acids (VFAs). The results show that CTC with a concentration of 10 mg/kg total suspended solids enhances the VFA production by 21.1%. Mechanistically, CTC was found to prompt the secretion of extracellular polymeric substances to provide more substrates for anaerobic fermentation. Meanwhile, CTC stimulates acidification by increasing the activity of acetate kinase, and inhibits methanogenesis by reducing F 420 activity, thereby increasing the accumulation of VFAs. This article provides new insights into the behavior of CTC in WAS fermentation, which is essential for resource recovery from WAS containing CTC. [ABSTRACT FROM AUTHOR]

• CA pretreatment at an acidic environment could result in outer EPS destruction accompanied by cell lysis. • CA pretreatment could improve sludge dewatering degree but could not increase sludge dewatering speed. • CA have deflocculation property which could influence EPS removal and cell lysis promotion. • Help to understand the use and promotion of CA in practical applications. This paper investigated the effects of citric acid (CA) on extracellular polymer destruction and cell lysis in sludge at different initial pH by measuring capillary suction time (CST), extracellular polymeric substances (EPS) and intracellular bound water. The results indicated that under CA concentration at 0.05 g/g suspended solids (SS) and initial pH 4, the CST value decreased from 175.5 s to 112.3 s, slime extracellular polymeric substances (S-EPS) and loosely bound EPS (LB-EPS) content respectively to increase from 4.92 to 41.43, 2.27 to 5.49 mg/g volatile suspended solids (Vss), while tightly bound EPS (TB-EPS) content to decrease from 12.35 to 5.01 mg/g (Vss), which suggested CA could disrupt outer EPS effectively. Intracellular bound water content decreased from 1.23 g/g to 0.41 g/g dry solid (DS). As a result, CA could release intracellular bound water effectively, thereby improving sludge dewatering degree. [ABSTRACT FROM AUTHOR]

• Clarithromycin promoted volatile fatty acids production from sludge fermentation. • Clarithromycin was partly degraded during the anaerobic fermentation process. • Clarithromycin enhanced sludge solubilization process. • Clarithromycin inhibited all the other process of anaerobic fermentation. This study investigated the effect of Clarithromycin (CLA) on volatile fatty acids (VFAs) production during waste activated sludge (WAS) anaerobic fermentation for the first time. Experimental results showed that when CLA concentration in WAS increased from 0 to 1000 mg/kg TSS, the maximum yield of VFAs increased from 27.7 to 35.7 mg COD/g VSS (without pH pretreatment) and from 59.3 to 65.6 mg COD/g VSS (initial pH 9 pretreatment), respectively. Mechanism exploration revealed that CLA facilitated the disruption of extracellular polymeric substances, thus promoting WAS solubilization. CLA inhibited all the other anaerobic fermentation processes. However, its inhibition to acetogenesis and methanogenesis was severer than that to hydrolysis and acidogenesis, resulting in the decrease in VFAs consumption. Microbial analysis showed that CLA slightly increased the abundance of microorganisms responsible for hydrolysis and acidogenesis whereas decreased the proportion of VFAs-consuming microorganisms. [ABSTRACT FROM AUTHOR]