Promotion of short-chain fatty acids production and fermented sludge properties via persulfate treatments with different activators: Performance and mechanisms

Peroxydisulfate activation and versatility of defective Fe3O4@MOF-808 for enhanced carbon and phosphorus recovery from sludge anaerobic fermentation

Although being viewed as a promising technology for reclamation of carbon and phosphorus from excess sludge, anaerobic fermentation (AF) grapples with issues such as a low yield of volatile fatty acids (VFAs) and high phosphorus recovery costs. In this study, we synthesized Fe3O4@MOF-808 (FeM) with abundant defects and employed it to simultaneously enhance VFAs and phosphorus recovery during sludge anaerobic fermentation. Through pre-oxidization of sludge catalyzed by FeM-induced peroxydisulfate, the soluble organic matter increased by 2.54 times, thus providing ample substrate for VFAs production. Subsequent AF revealed a remarkable 732.73 % increase in VFAs and a 1592.95 % increase in phosphate. Factors contributing to the high VFAs yield include the non-biological catalysis of unsaturated Zr active sites in defective FeM, enhancing protein hydrolysis, and the inhibition of methanogenesis due to electron competition arising from the transformation between Fe(III) and Fe(II) under Zr influence. Remarkably, FeM exhibited an adsorption capacity of up to 92.64 % for dissolved phosphate through ligand exchange and electrostatic attractions. Furthermore, FeM demonstrated magnetic separation capability from the fermentation broth, coupled with excellent stability and reusability in both catalysis and adsorption processes.

Contribution identification of hydrolyzed products of potassium ferrate on promoting short-chain fatty acids production from waste activated sludge

Potassium ferrate (K2FeO4) has been extensively employed to promote short-chain fatty acids (SCFAs) production from anaerobic fermentation of waste activated sludge (WAS) because of its potent oxidizing property and formation of alkaline hydrolyzed products (potassium hydroxide, KOH and ferric hydroxide, Fe(OH)3). However, whether K2FeO4 actually works as dual functions of both an oxidizing agent and an alkalinity enhancer during the anaerobic fermentation process remains uncertain. This study aims to identify the contributions of hydrolyzed products of K2FeO4 on SCFAs production. The results showed that K2FeO4 did not execute dual functions of oxidization and alkalinity in promoting SCFAs production. The accumulation of SCFAs using K2FeO4 treatment (183 mg COD/g volatile suspended solids, VSS) was less than that using either KOH (192 mg COD/g VSS) or KOH & Fe(OH)3 (210 mg COD/g VSS). The mechanism analysis indicated that the synergistic effects caused by oxidization and alkalinity properties of K2FeO4 did not happen on solubilization, hydrolysis, and acidogenesis stages, and the inhibition effect caused by K2FeO4 on methanogenesis stage at the initial phase was more severe than that of its hydrolyzed products. It was also noted that the inhibition effects of K2FeO4 and its hydrolyzed products on the methanogenesis stage could be relieved during a longer sludge retention time, and the final methane yields using KOH or KOH & Fe(OH)3 treatment were higher than that using K2FeO4, further confirming that dual functions of K2FeO4 were not obtained. Therefore, K2FeO4 may not be an alternative strategy for enhancing the production of SCFAs from WAS compared to its alkaline hydrolyzed products. Regarding the strong oxidization property of K2FeO4, more attention could be turned to the fates of refractory organics in the anaerobic fermentation of WAS.

Metagenomic approach reveals the mechanism of calcium oxide improving 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.

Promoting short-chain fatty acids production from sewage sludge via acidogenic fermentation: Optimized operation factors and iron-based persulfate activation system

Promoting short-chain fatty acids (SCFAs) production and ensuring the stability of SCFAs-producing process are becoming the two major issues for popularizing the acidogenic fermentation (AF). The key controlling operating and influencing factors during anaerobic fermentation process were thoroughly reviewed to facilitate better process performance prediction and to optimize the process control of SCFAs promotion. The wide utilization of iron salt flocculants during wastewater treatment could result in iron accumulating in sewage sludge which influenced AF performance. Additionally, appropriate ferric chloride (FC) could promote the SCFAs accumulation, while poly ferric sulfate (PFS) inhibited the bioprocess. Iron/persulfate (PS) system was proved to effectively enhance the SCFAs production while mechanism analysis revealed that the strong oxidizing radicals remarkably enhanced the solubilization and hydrolysis. Moreover, the changes of oxidation-reduction potential (ORP) and pH caused by iron/PS system exhibited more negative effects on the methanogens, comparing to the acidogenic bacteria. Furthermore, performance and mechanisms of different iron species-activating PS, organic chelating agents and iron-rich biochar derived from sewage sludge were also elucidated to extend and strengthen understanding of the iron/PS system for enhancing SCFAs production. Considering the large amount of generated Fe-sludge and the multiple benefits of iron activating PS system, carbon neutral wastewater treatment plants (WWTPs) were proposed with Fe-sludge as a promising recycling composite to improve AF performance. It is expected that this review can deepen the knowledge of optimizing AF process and improving the iron/PS system for enhancing SCFAs production and provide useful insights to researchers in this field.

Bacterial antioxidant mechanism in calcium peroxide aided sludge anaerobic fermentation

Although calcium peroxide (CaO₂) can enhance the short-chain fatty acids (SCFAs) production in sludge anaerobic fermentation, the microbiological mechanisms underlying this process remain unclear. In this study, it is aimed to elucidate the bacterial protective mechanisms in response to the oxidative stress induced by CaO₂. Results show that extracellular polymeric substance (EPS) and anti-oxidant enzymes play vital roles in protecting bacterial cells from CaO₂. The addition of CaO₂ resulted in increased relative abundances of genes exoP and SRP54, which are associated with EPS secretion and transportation. Superoxide dismutase (SOD) played a crucial in alleviating oxidative stress. The dosage of CaO₂ significantly influences the succession of the bacterial community in the anaerobic fermentation system. With 0.3 g CaO₂/g VSS, the net income was approximately 4 USD/ton of sludge treated. The CaO₂-assisted anaerobic fermentation process has the potential to recover more resources from sludge and thus, benefit the environment.

Mitigation of N2O emissions via enhanced denitrification in a biological landfill leachate treatment using external carbon from fermented sludge

The effects of an external carbon source (C-source) on the mitigation of N₂O gas (N₂O_(g)) emissions from landfill leachate were investigated via enhanced denitrification using anaerobically fermented sewage sludge. Anaerobic fermentation of sewage sludge was conducted under thermophilic conditions with progressively increasing organic loading rates (OLR). Optimal conditions for fermentation were determined based on the efficiency of hydrolysis and the concentrations of sCOD and volatile fatty acids (VFAs) as follows: at an OLR of 40.48 ± 0.77 g COD/L·d with 1.5 days of solid retention time (SRT), 14.68 ± 0.59% of efficiency of hydrolysis, 14.42 ± 0.30 g sCOD/L and 7.85 ± 0.18 g COD/L of VFAs. Analysis of the microbial community in the anaerobic fermentation reactor revealed that degradation of sewage sludge might be potentially affected by proteolytic microorganisms producing VFAs from proteinaceous materials. Sludge-fermentate (SF) retrieved from the anaerobic fermentation reactor was used as the external C-source for denitrification testing. The specific nitrate removal rate (K_NR) of the SF-added condition was 7.54 mg NO₃^-N/g VSS·hr, which was 5.42 and 2.43 times higher than that of raw landfill leachate (LL) and a methanol-added condition, respectively. In the N₂O_(g) emission test, the liquid phase N₂O (N₂O-^N_(l)) of 20.15 mg N/L was emitted as N₂O_(g) of 19.64 ppmv under only LL-added condition. On the other hand, SF led to the specific N₂O_(l) reduction rate (K_N2O) of 6.70 mg N/g VSS hr, resulting in mitigation of 1.72 times the N₂O_(g) emission compared to under the only-LL-added condition. The present study revealed that N₂O_(g) emissions from biological landfill leachate treatment plants can be attenuated by simultaneous reduction of NO₃^-N and N₂O_(l) during enhanced denitrification via a stable supply of an external C-source retrieved from anaerobically fermented organic waste.

Role of pretreatment type and microbial mechanisms on enhancing volatile fatty acids production during anaerobic fermentation of refinery waste activated sludge

This study compared the effects of alkaline, thermal, thermal-peroxymonosulfate (PMS), and alkyl polyglucose (APG) pretreatments on volatile fatty acids (VFAs) production from refinery waste activated sludge (RWAS), including VFAs yield, composition, organics components, microbial communities, and the potential improvement of mechanisms. All pretreatments effectively enhanced the bioconversion of RWAS and consequently promoted the hydrolysis process, which inhibited the methanogenesis process. However, the release of lignin/carboxyl-rich alicyclic molecules (CRAM)-like compounds and tannin substances in thermal-PMS and APG groups significantly influenced the acidogenesis and acetogenesis processes. Among all pretreatments, alkaline pretreatment showed the highest VFAs yield of 95.06 mg/g volatile solids (VS) and VS removal of 17%. This result could be associated with the enrichment of functional hydrolytic-acidification bacteria, such as Planococcus and Soehngenia, and increased metabolism of amino acids, carbohydrates, and nucleotides. By considering an economical and efficient perspective, this study recommended the alkaline pretreatment for the anaerobic fermentation of RWAS.

Performance and mechanisms of citric acid improving biotransformation of waste activated sludge into short-chain fatty acids

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.

Sludge disintegration and phosphorus migration in anaerobic fermentation of waste activated sludge by the addition of EDTA-2Na

The effects of the addition of EDTA-2Na on sludge disintegration and phosphorus (P) migration during anaerobic fermentation (AF) of waste activated sludge (WAS) are investigated. The efficiency of sludge disintegration was positively correlated with the dose of EDTA-2Na from 0.5-2.0 g/g SS, and an enormous quantity of P was liberated into the aqueous phase, accompanied by sludge disintegration. The proper dose of EDTA-2Na for P release from WAS was 1.5 g/g SS, with an orthophosphate concentration of 394.72 mg/L. P release was more consistent with the pseudo second-order kinetic model. The migration of P species during AF with EDTA-2Na addition was also studied. Orthophosphate was the main species in both of the liquid phase and the loosely bound extracellular polymeric substances (EPS), but organic P (OP) was much more abundant in tightly bound EPS. Inorganic P (IP) was the dominant P speciation in the solid and was mainly distributed in the fraction of non-apatite IP, which accounted for more than 62.8% of IP in the presence of EDTA-2Na. In addition, both IP and OP in the solid contributed to the accumulation of P and the former was outperformed. Furthermore, the increased total dissolved P mainly came from cells. However, the fermented sludge tended to be smaller and to have low compressibility, which is detrimental to its further treatment.

In-situ sulfite treatment enhanced the production of short-chain fatty acids from waste activated sludge in the side-stream anaerobic fermentation

Sulfite-based technology could enhance methane production from anaerobic sludge digestion. However, its potential for in-situ direct sludge treatment without anaerobic sludge addition in the side-stream remains unclear. This study investigated the feasibility of using in-situ sulfite treating sludge for short-chain fatty acids (SCFAs) production via anaerobic fermentation of waste activated sludge (WAS) as a side-stream treatment. In-situ sulfite direct sludge treatment enhanced SCFAs and acetic acid production by 2.03 and 4.89 times at 500 mg S/L compared to the control. With in-situ sulfite treatment, WAS hydrolysis and acidification were enhanced while methanogenesis was spontaneously hindered. The in-situ sulfite treatment inactivated pathogens and improved the sludge dewatering properties. The relative abundances of SCFAs-production microbial were stimulated, facilitating the sludge bioconversion. The produced SCFAs from in-situ sulfite side-stream treatment could be applied as an "internal carbon source" to enhance biological nutrient removal to improve economic and environmental value from sludge treatment.

Comprehensive understanding of DOM reactivity in anaerobic fermentation of persulfate-pretreated sewage sludge via FT-ICR mass spectrometry and reactomics analysis

Understanding the composition and reactivity of dissolved organic matter (DOM) at molecular level is vital for deciphering potential regulators or indicators relating to anaerobic process performance, though it was hardly achieved by traditional analyses. Here, the DOM composition, molecular reactivity and transformation in the enhanced sludge fermentation process were comprehensively elucidated using high-resolution mass spectrometry measurement, and data mining with machine learning and paired mass distance (PMD)-based reactomics. In the fermentation process for dewatered sludge, persulfate (PDS) pretreatment presented its highest performance in improving volatile fatty acids (VFAs) production with the increase from 2,711 mg/L to 3,869 mg/L, whereas its activation in the presence of Fe (as well as the hybrid of Fe and activated carbon) led to the decreased VFAs production performance. In addition to the conventional view of improved decomposition and solubilization of N-containing structures from sludge under the sole PDS pretreatment, the improved VFAs production was associated with the alternation of DOM molecular compositions such as humification generating molecules with high O/C, N/C, S/C and aromatic index (AI_mod). Machine learning was capable of predicting the DOM reactivity classes with 74-76 % accuracy and found that these molecular parameters in addition to nominal oxidation state of carbon (NOSC) were among the most important variables determining the generation or disappearance of bio-resistant molecules in the PDS pretreatment. The constructed PMD-based network suggested that highly connected molecular network with long path length and high diameter was in favor of VFAs production. Especially, -NH related transformation was found to be active under the enhanced fermentation process. Moreover, network topology analysis revealed that CHONS compounds (e.g., C₁₃H₂₇O₈N₁S₁) can be the keystone molecules, suggesting that the presence of sulfur related molecules (e.g., cysteine-like compounds) should be paid more attention as potential regulators or indicators for controlling sludge fermentation performance. This study also proposed the non-targeted DOM molecular analysis and downstream data mining for extending our understanding of DOM transformation at molecular level.

A novel strategy and mechanism for high-quality volatile fatty acids production from primary sludge: Peroxymonosulfate pretreatment combined with alkaline fermentation

To obtain high-quality VFAs production from primary sludge, a novel strategy that combined peroxymonosulfate (PMS) pretreatment and alkaline fermentation (i.e., PMS & pH9) was proposed in the study. The results showed that PMS & pH9 was efficient in sludge solubilization and hydrolysis, resulting in a maximal VFAs yield of 401.2 mg COD/g VSS, which was 7.3-, 2.1-, and 8.8-fold higher than the sole PMS, sole pH9, and control, respectively. Acetate comprised 87.6% of VFAs in this integration system. Mechanism investigations revealed that sulfate and free radicals produced by PMS play roles in improving VFAs yield under alkaline conditions. Besides, sulfate also aided in C3∼C5 VFAs converting to acetate under alkaline conditions depending on the increase of incomplete-oxidative sulfate-reducing bacteria (iso-SRB) (i.e., Desulfobulbus and Desulfobotulus). Moreover, the relative abundances of acid-forming characteristic genera (i.e., Proteiniborus, Proteinilcasticum, and Acetoanaerobium) were higher in PMS & pH9.

Potassium permanganate pretreatment effectively improves methane production from anaerobic digestion of waste activated sludge: Reaction kinetics and mechanisms

As a powerful oxidizing agent, potassium permanganate (KMnO₄) has attracted widespread interest in sludge treatment and contaminant removal. However, its effect on the anaerobic digestion of waste activated sludge (WAS) is ambiguous. This investigation was designed to provide perspectives into this problem. In comparison with the control, 0.3 g KMnO₄/g TSS pretreatment enhanced the methane production by 78.82 %. Model analysis demonstrated that the KMnO₄ pretreatment enhanced the biochemical methane potential (B₀) of WAS. Mechanistic studies elucidated that the KMnO₄ pretreatment process generated reactive radicals such as ·OH, ·O₂^- and ¹O₂, which contributed to sludge disintegration and biodegradation process of dissolved substances, thus resulting in more substances available for subsequent methane generation. Enzyme activity analysis indicated that KMnO₄ pretreatment facilitated the activities of key enzymes associated with anaerobic digestion to various degrees. Microbial analysis illustrated that the relative abundance of functional microorganisms was significantly elevated after KMnO₄ pretreatment, which was conducive to methane production.

Application of persulfate-based oxidation processes to address diverse sustainability challenges: A critical review

Over the past years, persulfate (PS) is widely applied due to their high versatility and efficacy in decontamination and sterilization. While treatment of organic chemicals, remediation of soil and groundwater, sludge treatment, disinfection on pathogen microorganisms have been covered by most published reviews, there are no comprehensive and specific reviews on its application to address diverse sustainability challenges, including solid waste treatment, resources recovery and regeneration of ecomaterials. PS applications mainly rely on direct oxidation by PS itself or the reactive sulfate radical (SO₄^•-) or hydroxyl radical (^•OH) from the activation of peroxodisulfate (PDS, S₂O₈^2-) or peroxymonosulfate (PMS, HSO₅^-) in SO₄^•--based advanced oxidation processes (SO₄^•--AOPs). From a broader perspective of environmental cleanup and sustainability, this review summarizes the various applications of PS except pollutant decontamination and elaborates the possible reaction mechanisms. Additionally, the differences between PS treatment and conventional technologies are highlighted. Challenges, research needs and future prospect are thus discussed to promote the development of the applications of PS-based oxidation processes in niche environmental fields. In all, this review is a call to pay more attention to the possibilities of PS application in practical resource reutilization and environmental protection except widely reported pollutant degradation.

Co-impacts of the microplastic polyamide and sertraline on the denitrification function and microbial community structure in SBRs

The co-impacts of microplastics (MPs) and organic pollutants on activated sludge have attracted extensive attention. In this study, microplastic polyamide (PA) and sertraline (SER) were respectively or simultaneously added to sequencing batch reactors (SBRs), and the impacts of these pollutants on activated sludge were investigated. The results showed that NH₄⁺-N and TN removal efficiencies significantly decreased with the simultaneous adding of the two pollutants. The coexistence of PA and SER could observably decrease the settling ability of activated sludge, and more proteins and polysaccharides were generated to reduce the combined toxicity. The microbial diversity, especially the denitrification microorganism, was restrained and the metabolic function and the key enzyme involved in nitrogen metabolism pathways were observably decreased, due to the combined toxicity of this two pollutants. Furthermore, the effective SER interception by PA in SBR could induce the SER enrichment in activated sludge and enhance the biotoxicity toward sludge microorganisms.

Upgrading volatile fatty acids production from anaerobic co-fermentation of orange peel waste and sewage sludge: Critical roles of limonene on functional consortia and microbial metabolic traits

Orange peel waste (OPW) and sewage sludge (SS) valorization for volatile fatty acids (VFAs) production from anaerobic co-fermentation are attractive and feasible. The highest VFAs reached 11996.3 mg COD/L within 10 d at the mass ratio (TS/TS) of 1:1, which was approximately 30-fold of that in sole SS fermentation. The OPW provided plenty of organic substrates and facilitated the fermentation processes by disintegrating SS structure and inhibiting methanogenesis due to the abundant limonene. Also, the OPW feeds reshaped the microbial community and enriched fermentative bacteria, especially those saccharolytic ones (i.e. Prevotella-7). The key genes involved in membrane transport (i.e. ptsG), glycolysis (i.e. pgk), pyruvate metabolism (i.e. ace), and fatty acid biosynthesis (i.e. accA), which are associated with VFAs biosynthesis, were up-regulated in OPW/SS reactors. Overall, it was the increase in bioavailable organic matter and functional microorganisms, and the simultaneous enhancement of metabolic activity that improved the efficient VFAs production.

How does the polyhexamethylene guanidine interact with waste activated sludge and affect the metabolic functions in anaerobic fermentation for volatile fatty acids production

Antibacterial agents are frequently used to ensure public hygiene. Most of the massively consumed chemicals are discarded and accumulated in waste activated sludge (WAS), which might influence the subsequent anaerobic fermentation process for WAS treatment. This study mainly investigated the impacts of polyhexamethylene guanidine (PHMG, considered as a safe and efficient broad-spectrum antibacterial agent) on the volatile fatty acids (VFAs) production derived from WAS anaerobic fermentation and disclosed the key mechanisms. Results demonstrated that low level of PHMG evidently increased the VFAs accumulation as well as the acetic acid proportion, while the excessive dose posed evident negative effects. Further analysis found that appropriate PHMG synchronously stimulated the solubilization/hydrolysis and acidification processes but inhibited methanogenesis. Mechanistic exploration revealed that PHMG firstly absorbed on WAS due to electric attraction but then interacted with WAS to promote extracellular polymeric substance (EPS) disintegration and organics release (especially proteinaceous matter). Moreover, PHMG affected the microbial community structure and metabolic functions. The low level of PHMG evidently enriched functional VFAs producers (i.e., Desulfobulbus, Macellibacteroides and Sporanaerobacter) and upregulated the critical genes expression responsible for substrates metabolism (particularly the proteins) and VFAs biosynthesis (i.e., aldehyde dehydrogenase (NAD+) (K00128) and molybdopterin oxidoreductase (K00184)). This study provides an in-depth understanding of emerging pollutant impacts on WAS fermentation and provides insightful guidance on WAS disposal.

Thiosulfate-assisted Fe2+/persulfate pretreatment effectively alleviating iron dose and enhancing biotransformation of waste activated sludge into high-value liquid products

Ferrous-based acidogenic fermentation (AF) as a means to treat waste activated sludge (WAS) and produce short-chain fatty acids (SCFAs) has drawn increasing attention, but the massive amount of "iron sludge" that it produces not only significantly increases costs and difficulty of disposal but also poses risks to the environment and human health. This study explored a novel approach to not only reduce the iron dosage required by AF but also to improve its performance by introducing a thiosulfate (TS)-assisted Fe²⁺/persulfate (TAFP) pretreatment. Effects of the TAFP pretreatment on WAS disintegration and biodegradability, SCFA production, and microbial community structure with different persulfate-Fe²⁺-thiosulfate molar ratios at 4:4:0 (R1), 4:3:1 (R2), 4:2:2 (R3) and 4:1:3 (R4) were investigated. The results showed that the TAFP pretreatment by a remarkable margin promoted the disintegration of WAS as well as the biodegradability of the organics released, owing to the production of robust free radicals (SO₄•^- and •OH) triggered by the thiosulfate and Fe²⁺ cycles. 48-day AF tests further showed maximum SCFA production, ranging roughly between 1283 and 1395 mg COD/L in the TAFP pretreated samples, much higher than Control (<120 mg/L) and R1 (around 593 mg COD/L). At the meantime, the Fe²⁺ dosage was reduced by 50% in R3 than that of R1. However, a prolonged lag phase of SCFA generation was observed between days 7 and 25, which was ascribable to the acidic conditions (pH < 4.5) closely related to impaired metabolic activities as well as electron transfer efficiencies and limited activities of acidogenic enzymes (i.e., pyruvate-ferredoxin oxidoreductase). Despite such lag phase, the economic and environmental assessment of AF of TAFP-pretreated WAS had a higher net SCFA yield and less "iron sludge" than either without any pretreatment or with Fe²⁺/persulfate-only pretreatment.

Removal of 4-nonylphenol in activated sludge by peroxymonosulfate activated with sorghum distillery residue-derived biochar

The presence of 4-nonylphenol (4-NP), an endocrine disrupting chemical, waste activated sludge (WAS) or biosolids at elevated content requires effective method for 4-NP reduction in total sludge management. Herein, sorghum distillery residue-based biochar-activated peroxymonosulfate (SDRBC/PMS) system was studied as pretreatment of WAS. Results indicated 91% of 4-NP removal at pH 6.0 in the presence of 3.1 × 10^-6 M and 0.8 g L^-1 PMS and SDRBC500, individually. The synergetic effects of singlet oxygen (¹O₂) and the abundant functional sites (C = O/C-O content) of SDRBC significantly improved 4-NP degradation. The decreased fluorescent dissolved organic matter (DOM) in the sludge also enhanced the pretreatment efficiency. Moreover, the enrichment of the Nitrospira functional bacteria in the microbial community yielded the highest 4-NP degradation in the SDRBC/PMS-pretreated sludge. The SDRBC/PMS system functions mainly via nonradical-mediated oxidation pathway in pretreating WAS in particular and potentially by combined advanced oxidation and biodegradation processes for wastewater treatment in general.

Persulfate-based strategy for promoted acesulfame removal during sludge anaerobic fermentation: Combined chemical and biological effects

The acesulfame (ACE) degradation in waste activated sludge (WAS) via direct anaerobic fermentation is difficult and the efficient elimination techniques are imperative for the ultimate safe WAS disposal. Persulfate (PS)-based approach was developed to promote the ACE removal during WAS anaerobic fermentation. Results demonstrated the effectiveness of PS-based treatments on ACE degradation, and the ACE removal efficiency was respectively 48.2% and 96.2% in the PS and PS/Fe-treated reactors while it was only 6.0% in the control reactor. Mechanism explorations revealed that the active free radicals (i.e. OH• and SO₄^•-) generated in the PS-based reactors were the key oxidative species for the ACE degradation. However, such effects were interfered by the released soluble substrates (i.e. protein, carbohydrate and inorganic ions) during anaerobic fermentation by competing and/or quenching free radicals, which caused the deceleration of the ACE removal efficiency. Moreover, the PS-based treatment facilitated the enrichment of functional microorganisms (i.e. Phyllobacteriaceae and Bradyrhizobiaceae) and upregulated the critical genes (i.e. pncB and nadE) involved in the ACE degradation. Based on the density functional theory (DFT) and metabolic intermediates analysis, the hydroxylation and oxidative ring-opening were the two main proposed metabolic pathways for ACE degradation. Overall, the combined chemical and biological metabolism effects collectively contributed to the efficient ACE degradation, and it provided a novel and effective strategy for refractory pollutants removal during WAS anaerobic fermentation.

Effect of potassium persulphate addition on sludge disintegration of a mesophilic anaerobic fermentation system

Persulphates, an advanced oxidation process, has been recently used as an alternative pretreatment method to enhance short-chain fatty acids (SCFAs) yield from waste-activated sludge (WAS) anaerobic fermentation (AF). But so far, the effects of peroxydisulphate (PDS) dosages on mesophilic anaerobic fermentation are still not studied fully. Herein, we explored the influences of potassium PDS addition on mesophilic AF of WAS. Notably, the addition of PDS could drastically accelerate WAS solubilization and hydrolysis, which was proportional to the amount of PDS. The maximal total SCFAs yield of 249.14 mg chemical oxygen demand/L was obtained with 120 mg PDS/g suspended solids addition at 6 days of AF, which was 2.2-fold that of the control one. Tightly bound extracellular polymeric substances (EPSs) were transformed into loosely bound EPS and dissolved organic matters, and aromatic proteins and humic-like substances of EPSs were disintegrated, which were caused by the devastating effects of PDS treatments on EPSs disruption. The intracellular constituents of microbial cells in the sludge were released accordingly. As a result, there was release of soluble substrates derived from the disintegration of both EPSs and cells, the amounts of which were proportional to the dose of PDS. Moreover, microbial diversity and richness were both decreased in the presence of PDS, and the relative abundance of phyla Actinobacteria increased with the increase of the PDS dosage. In addition, the stability of sludge flocs was destroyed in the presence of PDS, the distribution of particle size tended to be small and dispersive, and dewaterability of the sludge was deteriorated.

Treatment of fracturing wastewater by anaerobic granular sludge: The short-term effect of salinity and its mechanism

The effects of salinity shock on the anaerobic treatment of fracturing wastewater regarding chemical oxygen demand (COD) removal performance, sludge characteristics and microbial community were investigated. Results showed COD removal efficiency decreased from 76.0% to 69.1%, 65.6%, 33.7% and 21.9% with the increase of salinity from 2.5 g/L to 10, 15, 25 and 45 g/L, respectively. The cumulative biogas production decreased by 13.8%-81.1% when salinity increased to 15-85 g/L. The increase of salinity led to the decline in particle size of granular sludge, and the activity of granular sludge, including SMA, coenzyme F₄₂₀ and dehydrogenase, was inhibited significantly. Flow cytometry indicated the percentage of damaged cells in granular sludge gradually increased with the increase of salinity. Sequence analysis illustrated that microbial community structure in anaerobic digestion reactor was influenced by the salinity, high salinity reduced the diversity of archaea and decreased the abundance of methanogens, especially Methanosaeta.

Unraveling the role of polyferric chloride in anaerobic digestion of waste activated sludge

This study explored the effect of polyferric chloride (PFC) as a flocculant on waste activated sludge anaerobic digestion. The results verified that PFC has an inhibitory effect on methane production during anaerobic digestion. PFC with a concentration of 40 g/kg total suspended solids reduced methane production from 195 ± 2.10 to 156 ± 1.50 L/kg volatile suspended solids, a decrease of 20.0 ± 0.09%. PFC released hydroxyl polymers and Fe(III). Hydroxy polymers aggregated sludge flocs and hindered the release of dissolved organic matter. Fe(III) induced dissimilar iron reduction processes to contend with methyl-CoM for electrons, thereby further reducing methane production. In addition, PFC enriched iron-reducing bacteria and reduced the abundance of methanogens, resulting in microbial communities that are not conducive to methane production. This article puts forward innovative insights on the role of PFC in biological sludge treatment, which is expected to guide the flocculant selection during wastewater treatment.

Revealing the intrinsic drawbacks of waste activated sludge for efficient anaerobic digestion and the potential mitigation strategies

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.

Effects of persulfate treatment on the fates of antibiotic resistance genes in waste activated sludge fermentation process and the underlying mechanism

The occurrence of antibiotic resistance genes (ARGs) in waste activated sludge (WAS) fermentation was investigated with persulfate (PS)-based treatment. ARGs affiliated with multidrug (mexP), macrolide (bla_OXA-129), tetracycline (tetB), sulfonamide (sul1), and vancomycin (vanRG) types were significantly decreased by PS/Fe treatment. Mechanistic investigations revealed that PS/Fe possessed oxidating potential and exhibited devastating effects on WAS fermentation. First, PS/Fe promoted cell structure damage, which facilitated ARGs release from potential hosts. A co-occurrence network analysis indicated that Fe/PS suppressed the proliferation of potential host bacteria. In addition, the PS/Fe treatment induced the decreased abundance of certain functional genes involved in pathways associated with ARGs dissemination. Finally, variation partitioning analysis demonstrated that the microbial community structure exhibited more vital effects on ARGs fates than physicochemical factors (i.e., pH and ORP) and gene expression (i.e., two-component system). This work provided a deeper understanding of the critical factors used to determine ARGs fates during WAS fermentation.

New insight into enhanced short-chain fatty acids production from waste activated sludge through pretreatment of cation exchange resin coupled NaCl addition

In this study, a novel pretreatment of cation exchange resin (CER) coupled NaCl addition was proposed to enhance waste activated sludge (WAS) hydrolysis and promote short-chain fatty acids (SCFAs) production in the anaerobic fermentation process. At the optimal pretreatment condition of 3 g/g SS CER and 15 g/L NaCl, considerable SCOD (i.e. 5107 mg/L, 35.4% of TCOD) was released after 2-day coupled treatment, which provided sufficient organic substance for the subsequent SCFAs production. The sludge hydrolysis mechanism was illustrated, i.e. CER triggered extracellular polymeric substances (EPS) disruption and NaCl induced microbial cells lysis. The synergistic interaction between CER and NaCl pretreatment was investigated and application potential of fermentative liquid was evaluated after the coupled pretreatment-enhanced anaerobic fermentation. In the presence of abundant biodegradable substrates in the fermentative liquid, 4742 mg COD/L (i.e. 388 mg COD/g VSS) of SCFAs production was achieved within 6-day anaerobic fermentation, mainly composed of acetic and propionic acids (70.4% of total SCFAs).

Unveiling the dual faces of chitosan in anaerobic digestion of waste activated sludge

In this study, the roles of chitosan (CTS) in anaerobic digestion of Waste activated sludge (WAS) were investigated. The results show that the methane production potential of WAS is positively correlated with the CTS content. The presence of 30 g/kg total suspended solids CTS increased the cumulative methane production from 215 ± 1.52 to 272 ± 1.83 mL/g volatile suspended solids. The positively charged amino groups in CTS neutralize the hydroxyl and carboxyl groups of extracellular polymeric substances, which reduces the negative charge on the surface of sludge and promotes sludge agglomeration, thereby inhibiting the release of organic matter. CTS also inhibits hydrolysis and acidification by immobilizing hydrolases and acidulase enzymes. However, CTS flocculates humus to avoid its interference with electron transfer, thereby enhancing the activity of coenzyme F420 and methanogenesis. In addition, CTS increases the abundance of methanogens, which also contributes to methane production.

Integrating mechanistic and deep learning models for accurately predicting the enrichment of polyhydroxyalkanoates accumulating bacteria in mixed microbial cultures

The enrichment of polyhydroxyalkanoates (PHA) accumulating bacteria (PAB) in mixed microbial cultures (MMC) is extremely difficult to be predicted and optimized. Here we demonstrate that mechanistic and deep learning models can be integrated innovatively to accurately predict the dynamic enrichment of PAB. Well-calibrated activated sludge models (ASM) of the PAB enrichment process provide time-dependent data under different operating conditions. Recurrent neural network (RNN) models are trained and tested based on the time-dependent dataset generated by ASM. The accurate prediction performance is achieved (R² > 0.991) for three different PAB enrichment datasets by the optimized RNN model. The optimized RNN model can also predict the equilibrium concentration of PAB (R² = 0.944) and corresponding time, which represents the end of the PAB enrichment process. This study demonstrates the strength of integrating mechanistic and deep learning models to predict long-term variations of specific microbes, helping to optimize their selection process for PHA production.

Applicability of two-stage anoxic/oxic shortcut nitrogen removal via partial nitrification and partial denitrification for municipal wastewater by adding sludge fermentation products continuously

Partial nitrification and partial denitrification combined with anammox is a promising process for sewage treatment. In this study, real municipal wastewater was treated in a continuous two-stage anoxic/oxic (A/O) reactor. External mixed sludge fermentation products were added in the anoxic zone, simultaneously achieving partial nitrification and partial denitrification and achieving a high and relatively stable accumulation of nitrite. The maximum accumulation rates of NO₂^--N in A1.2 and A2.1-A2.4 zones of the reactor reached 70% and 61%-37%, respectively, which improved denitrification efficiency and created conditions that supported the coupling of subsequent anammox. The influent nitrogen load of the system was 0.078 kg/(m³•d), and the mean influent and effluent total nitrogen were 51 and 12 mg/L, respectively. The mean total nitrogen removal rate reached 76%. Further analysis revealed that Hyphomicrobium (incomplete denitrifiers) and Nitrosomonas (ammonia oxidizing bacteria) were enriched, which may have facilitated the high nitrite accumulation. Moreover, the batch test showed that adding sludge fermentation during denitrification significantly suppressed nitrite reduction, resulting in the nitrite accumulation.

Facilitating biofilm formation of Pseudomonas aeruginosa via exogenous N-Acy-L-homoserine lactones stimulation: Regulation on the bacterial motility, adhesive ability and metabolic activity

The N-Acy-L-homoserine lactones (AHLs) mediated quorum sensing (QS) system exhibited important ecological significance in bacterial biofilm formation. However, the previous studies mainly focused on indigenous AHLs while the role of exogenous AHLs has remained unclear. This study evaluated the roles of exogenous AHLs on the biofilm formation of Pseudomonas aeruginosa. Both the C6-HSL and C8-HSL promoted the biofilm formation of P. aeruginosa with an enhancement of 2.47 and 1.88 times, respectively. Further analysis showed that exogenous AHLs contributed greatly to the adhesive ability instead of growth rate. Also, the bacterial motility and metabolic activities were significantly improved by AHLs. Moreover, the microbial functional genes (i.e. lasI, lasR, rhlI and rhlR) involved in regulating the biofilm formation were highly expressed in AHLs reactors. These findings expanded the knowledge of AHLs functions in mediating biofilm formation, and provided insightful guidance on the biofilm regulation in the wastewater treatment via biofilm technology.

Alkali activated persulfate mediated extracellular organic release on enzyme secreting bacterial pretreatment for efficient hydrogen production

The present investigation is proposed to assess the competency of Sodium Persulphate (SPS) induced enzyme secreting bacterial pretreatment in enhancing the generation of biohydrogen from waste activated sludge (WAS). Alkali activated SPS of dosage 0.015 g/g SS has been opted to disseminate the floc structure to fortify the release of Extracellular polymeric substance (EPS) into aqueous phase. This removal of EPS enhances the bacterial disintegration fostering 18.71% of suspended solids reduction and 21% of COD solubilization which was comparatively higher than bacterially pretreated (BP) and control (C) sludge. Biohydrogen production of control (C), bacterially pretreated (BP) and SPS mediated bacterially pretreated (SPS-BP) sludge were found to be 32.2 mLH₂/g COD, 48.3 mLH₂/g COD and 103.8 mLH₂/g COD respectively. The net energy production of SPS - BP is 0.01 kWh which is higher than the C and BP sample during the entire treatment and obtained energy ratio greater than 1.

Insights into the accelerated venlafaxine degradation by cysteine-assisted Fe2+/persulfate: Key influencing factors, mechanisms and transformation pathways with DFT study

The effective removal of refractory antidepressant in wastewater is challenging. In this study, a novel strategy of cysteine-assisted Fe²⁺/persulfate system (Fe²⁺/Cys/PS) was applied for the venlafaxine (Ven, as a typical antidepressant) degradation. The obtained results revealed that the Ven removal was evidently accelerated and enhanced in Fe²⁺/Cys/PS process, and achieved complete degradation in 5 min with optimal dosage. Further analysis indicated that the Ven degradation efficiency was associated with the chemical concentrations (i.e. Fe²⁺, Cys and PS) and operational conditions (i.e. pH and temperature). Moreover, the reactions were not impacted by the co-occurring organic matters (i.e. fulvic acid) and inorganic ions (i.e. Cl^-) potentially existing in real wastewater matrices. Mechanistic explorations demonstrated that the presence of Cys promoted the Fe³⁺/Fe²⁺ redox cycle, and thus enhanced the reactive oxygen species yields (ROS). The OH was considered as the primary ROS in Fe²⁺/Cys/PS process for Ven degradation via the radical scavenger verification. Also, the main intermediates of Ven degradation were identified, and the possible transformation pathway was proposed, in which the hydroxylation attacked by the OH was the main reaction. Moreover, the active reaction sites in Ven were calculated with the density function theory (DFT), which was consistent with the observed metabolic routes.

Recent Development in Sludge Biochar-Based Catalysts for Advanced Oxidation Processes of Wastewater

Sewage sludge as waste of the wastewater treatment process contains toxic substances, and its conversion into sludge biochar-based catalysts is a promising strategy that merges the merits of waste reutilization and environmental cleanup. This study aims to systematically recapitulate the published articles on the development of sludge biochar-based catalysts in different advanced oxidation processes of wastewater, including sulfate-based system, Fenton-like systems, photocatalysis, and ozonation systems. Due to abundant functional groups, metal phases and unique structures, sludge biochar-based catalysts exhibit excellent catalytic behavior for decontamination in advanced oxidation systems. In particular, the combination of sludge and pollutant dopants manifests a synergistic effect. The catalytic mechanisms of as-prepared catalysts in these systems are also investigated. Furthermore, initial solution pH, catalyst dosage, reaction temperature, and coexisting anions have a vital role in advanced oxidation processes, and these parameters are systematically summarized. In summary, this study could provide relatively comprehensive and up-to-date messages for the application of sludge biochar-based catalysts in the advanced oxidation processes of wastewater treatment.

Degradation of organic contaminants in marine sediments by peroxymonosulfate over LaFeO3 nanoparticles supported on water caltrop shell-derived biochar and the associated microbial community responses

Sediment is an important final repository of persistent organic pollutants such as polycyclic aromatic hydrocarbons (PAHs). Herein, a novel catalyst of LaFeO₃ nanoparticles supported on biochar was synthesized from water caltrop shell by chemical precipitation. The composite (LFBC) was used as peroxymonosulfate (PMS) activator to oxidize PAHs in real marine sediments. Systematic surface characterization confirmed the immobilization of well crystalline nano LaFeO₃ particles onto the biochar surface. Under optimal conditions, i.e., [PMS] = 3 × 10^-4 M, [LFBC] = 0.75 g/L, pH 6.0, and seawater, the total PAH degradation efficiency was 90%, while that of 2-, 3-, 4-, 5-, and 6-ring PAHs was 52%, 61%, 66%, 56%, and 29%, respectively, in 24 h. The Langmuir-Hinshelwood equation better predicted the PAHs degradation kinetics over LFBC by PMS. Interactions between surface oxygen species at LaFeO₃ defective sites and the graphitized biochar network facilitated the PAHs degradation. Furthermore, changes in the bacterial community during the LFBC/PMS treatment were highlighted to assess the sustainable development of the sediment ecosystem. The LFBC/PMS process enhanced the biological richness and diversity of sediment eco-systems. The major phylum was Proteobacteria initially, while Hyphomonas was the genera after LFBC/PMS treatment of the sediment.

Effects of low- and high-temperature thermal-alkaline pretreatments on anaerobic digestion of waste activated sludge

To compare the effects of low- and high-temperature thermal-alkaline pretreatments (LTTAP, 60 ± 1 °C, pH 12.0 ± 0.1, 30 min and HTTAP, 160 ± 1 °C, pH 12.0 ± 0.1, 30 min, respectively) on anaerobic digestion (AD) of waste activated sludge, long-term and semi-continuous experiments were conducted in three laboratory continuous stirred tank reactors. The experimental results showed that the two pretreatments increased the methane yield of sludge from 89.20 ± 2.41 mL/g added volatile solids (VS) to 117.50 ± 5.27 mL/g added VS (LTTAP) and 156.40 ± 2.99 mL/g added VS (HTTAP). After AD, the reduction of sludge (volatile solid) increased from 32.91 ± 0.27% to 44.17 ± 1.53% (LTTAP), and 50.86 ± 1.18% (HTTAP), and the abundance of pathogenic bacteria decreased from 6.53% to 0.38% (LTTAP) and 0.14% (HTTAP). LTTAP enhanced both hydrogentrophic and acetoclastic methanogenis and HTTAP only enhanced acetoclastic methanogenis. Additionally, the energy efficiency of HTTAP and its subsequent AD was lower than that of LTTAP and its subsequent AD.

Distribution patterns of microbial community and functional characteristics in full-scale wastewater treatment plants: Focusing on the influent types

The impacts of the influent type in wastewater treatment plants (WWTPs) on the distribution patterns of the microbial community and functional characteristics were investigated. The obtained results indicated that the influent types exhibited evident influences on the microbial distribution patterns. The diversity and richness of functional microbes in HI-WWTP (with a ratio of >30% industrial wastewater in influents) were evidently decreased compared with those in HM- (with 70-90% municipal wastewater in influents) and M-WWTPs (with >90% municipal wastewater in influents). The core functional bacteria included denitrifiers, anaerobic fermentation bacteria (AFB), organic degrading bacteria (ODB), phosphorus accumulating organisms (PAO) and nitrite oxidizing bacteria (NOB), but they exhibited distinct abundances in WWTPs receiving different categories of wastewater. The denitrifiers in HI-WWTPs was 15.6-32.5% higher than that in other WWTPs, while PAOs had higher abundances in M - and HI-WWTPs (28.9% and 39.3%, respectively) compared with HM-WWTPs. Clear co-occurrence relationships were found among the main functional microbes with similar metabolic characteristics. Moreover, information on functional genes related to carbon, nitrogen and phosphorus metabolism, which is closely associated with pollutant removal efficiency, was obtained. M-WWTPs had higher abundances of genetic expressions for organic matters degradation (i.e. amino acid (10.42%) and carbohydrate (9.86%) metabolisms). Nar, Nir and Nor showed lowest abundances in HM-WWTPs, causing the low nitrogen removal (63.04-65.79%). However, influent type had little effect on genetic expression related with phosphorus removal. This work provided new insights into the interrelationship among bacterial co-occurrence, microbial activity and pollutant removal in WWTPs with different influent types.

The synergistic effect of potassium ferrate and peroxymonosulfate application on biogas production and shaping microbial community during anaerobic co-digestion of a cow manure-cotton straw mixture

Anaerobic co-digestion of a cow manure-cotton straw mixture (CCM) has been shown to promote methanogenesis, but the recalcitrant crystal structure of organic polymers in CCM hinders its hydrolysis during anaerobic digestion (AD). Here, the efficacy of different pretreatment methods based on potassium ferrate (PF) and peroxymonosulfate (PMS) was evaluated to facilitate CCM decomposition and methanogenesis during AD. The maximum lignocellulosic removal rate (62.5%), the highest volatile fatty acids (VFAs) (7769.6 mg/L), and cumulative methane yield (109.4 mL CH₄/g VS) were both achieved in PF-pretreated samples after the digestion process. The dominant bacterial populations in PF-pretreated CCM were affiliated with Sideroxydans, Herbinix, Clostridium, and Smithella, which played an important role in the hydrolysis and acidification of CCM. The enrichment of Methanosarcina and Methanobacterium and highly-effective acidogenesis might account for the highest methane yield in the PF-pretreated group.

An innovative cation regulation-based anaerobic fermentation strategy for enhancing short-chain fatty acids production from waste activated sludge: Metal ion removal coupled with Na+-regulation

This study proposed a cation-regulation strategy based on metal ion removal coupled Na⁺-regulation for enhancing anaerobic fermentation of waste activated sludge. The optimal treatment condition was: cation-exchange resin dosage of 1.75 g/g SS for 1-day treatment, followed by Na⁺-enhanced anaerobic fermentation at NaCl concentration of 20 g/L. The CER induced sludge solubilization and the Na⁺-regulation treatment triggered secondary hydrolysis of CER-solubilized sludge, causing remarkable sludge disintegration and extracellular polymeric substance (EPS) disruption. Numerous SCOD of 6588 mg/L (SCOD/TCOD = 40.6%) was released within 2 days, and the short-chain fatty acids (SCFAs) of 439.9 mg COD/g VSS was produced through 4-day anaerobic fermentation. More than 59% of the SCFAs was composed of acetate and propionate. Nitrogen-free organic matters (i.e. SCFAs and carbohydrates) accounted for 77.9% of SCOD, while considerable sludge solid reduction (51.6% of total VSS) was achievable, which was beneficial for fermentative liquid utilization and sludge disposal.

Influence of volatile fatty acids in anaerobic bioleaching of potentially toxic metals

Potentially toxic metals are common contaminants associated with sewage sludge, and limited information is available on migration and transformation behavior of potentially toxic metals during anaerobic digestion (AD) process. The aim of this study was to reveal the influence of volatile fatty acids (VFAs) on the solubilization of metals through VFAs-metal complexation. Addition of readily biodegradable extra carbon source at organic loading rate (OLR) of 17.65 gVS/L.d resulted in accumulation of 67,255 mg chemical oxygen demand (COD)/L as VFAs. Low pH values due to VFAs accumulation enhanced the solubilization of Ni and more than 22% of its total concentration became soluble. Subsequent to consumption of VFAs and increase of pH to neutral levels (~7.5), solubility of Ni decreased below 10% of its total concentration. Contrarily, the solubility of Cr reached to 25% of its total concentration at neutral pH values. Presumably the complexation of Cr with dissolved organic matter (DOM) have increased its concentration in the liquid fraction at neutral pH values. Fractionation analysis of metals revealed that AD process altered Cu and Zn speciation between organically-bound and residual fractions, and hence solubility of Zn and Cu remained consistently low over the entire period of the AD process.

A preliminary metatranscriptomic insight of eggshells conditioning on substrates metabolism during food wastes anaerobic fermentation

The anaerobic treatment of food wastes (FW) for resource recovery has been extensively studied. However, the information on the traits of functional genes and enzymes for substrates metabolisms and their associations with microbial community are little. In this study, the influences of eggshells conditioning on the substrates metabolism for volatile fatty acids production (VFAs) in the process of FW fermentation were investigated at genetic levels by using the metatranscriptomic approach. The obtained results suggested that the critical genes involved in the carbohydrate and protein metabolisms (i.e. pgmB, GPI, glsA, pyrB and etc.) were up-regulated in the eggshell-conditioned reactor, which were beneficial to the bioconversion of macromolecule organics during FW fermentation. Moreover, the functional genes related with the intermediate products metabolism (i.e. pyruvate acid, butanoate) also exhibited differential genetic expression levels, which resulted in the alteration of microbial metabolic pathways and contributed to the acetic and butyric acids accumulation. In addition, a preliminary association of microbial distribution and genetic expressions was analyzed. The distinct distribution of microbial community in different FW fermentation systems affected the corresponding microbial contribution to those genetic expression levels of metabolic enzymes involved in VFAs production. This study would provide new insights of the underlying mechanism of VFAs promotion in the eggshell-conditioned FW fermentation process from the perspectives of substrates metabolisms at genetic and functional traits.

Evaluating the physicochemical properties of refuse with a short-term landfill age and odorous pollutants emission during landfill mining: A case study

A field excavation of refusewitha short-termlandfillage from the Qingdao Xiaojianxi municipal solid waste (MSW) landfill was conducted. The physical composition and chemical properties of refuse with landfill ages of 1-4 years were studied, and the emission characteristics of odorous pollutants during the excavation period were monitored. The refuse aged 1-2 years has a higher proportion of combustible material than that the refuse aged 3-4 years, and the volatile content and calorific value in refuse aged 1-2 years were also higher than those in refuse aged 3-4 years, indicating that the refuse with a short-term landfill age was more suitable for incineration than refuse with a long-term landfill age. The pH and availablephosphorus (AP) gradually increased with increasing landfill age, while the total Kjeldahlnitrogen (TKN) and organic matter (OM) decreased. The contents of the heavy metals Cu, Zn, Ni, Pb and As generally decreased with landfill age, especially in refuse aged 2-4 years, whereas the Cr content showed no significant differences in refuse aged 1-4 years. The main odorous pollutants emitted during the excavation and screening periods were ammonia (NH₃) and carbon disulfide (CS₂), and the odor intensity of excavated refuse aged 1-3 years was higher than that of refuse aged 4 years. Under the condition of a small excavation area and continuous deodorization, the pollution intensity can meet the discharge standards of the factory boundary.

Distinct effects of hypochlorite types on the reduction of antibiotic resistance genes during waste activated sludge fermentation: Insights of bacterial community, cellular activity, and genetic expression

The effectiveness of hypochlorites (NaClO and Ca(ClO)₂) on the reduction of antibiotic resistance genes (ARGs) during waste activated sludge (WAS) fermentation was determined by the quantitative PCR. NaClO and Ca(ClO)₂ exhibited distinct effects on ARGs fates. Ca(ClO)₂ was effective in removing all investigated ARGs, and the efficiency was highly dose-dependent. Unexpectedly, the NaClO treatment attenuated ARGs with lower efficiency and even caused the propagation of certain ARGs (i.e., aadA1 and tetQ) at higher doses. The extracellular polymeric substances dissolution and membrane integrity suggested that unstable NaClO had acute effects on bacteria initially, while it was ineffective to further attenuate ARGs released from hosts due to the rapid consumption of oxidative ClO^-. Without lasting and strong oxidative stress, the microbial activities of tolerant ARGs hosts will partially recover and then contribute to the ARGs dissemination across genera. In contrast, solid-state Ca(ClO)₂ was slowly released and exhibited prolonged effects on bacteria by disrupting cell membranes and removing the susceptible ARGs released from hosts. Furthermore, bacterial taxa-ARG network analysis indicated that Ca(ClO)₂ reduced the abundance of potential hosts, and the metabolic pathway and gene expression related to ARGs propagation were significantly downregulated by Ca(ClO)₂, which contributed to efficient ARGs attenuation.

Activation of peroxymonosulfate using metal-free in situ N-doped carbonized polypyrrole: A non-radical process

Metal-free catalysts are widely considered as promising alternatives to traditional metal-based catalysts, which can effectively activate peroxymonosulfate (PMS). In this study, a novel metal-free catalyst, carbonized polypyrrole (CPPy) was synthesized through high-temperature carbonization of PPy, easily achieving the in situ N doping without the addition of nitrogen sources. Tetracycline (TC) was selected as the target contaminant to assess the catalytic activity of the CPPy/PMS system. Enhanced catalytic activity was observed in CPPy/PMS over a wide pH range (3.0-9.0), and the removal rate of TC by CPPy-3/PMS reached 91.3% after 10 min. After regenerating the used catalyst, the catalytic activity was refreshed, implying its stability and recyclability. The catalytic degradation of TC by CPPy/PMS was mainly attributed to a non-radical process. CPPy, as an intermediary, grabbed electrons from the electron-donating groups of TC and transferred them to the PMS molecule. Thereafter, TC that lost electrons was oxidized and degraded, and the O-O bonds of PMS were destroyed by the transferred electrons to form SO₄^2- and OH^-. Moreover, O⋅₂^- and ¹O₂ were involved in TC degradation. TC degradation pathway was investigated through HPLC-MS analysis. These findings provide a promising strategy for the construction of catalysts for PMS and environmental remediation.

Triclosan facilitates the recovery of volatile fatty acids from waste activated sludge

The emergence of triclosan (TCS) in the environment has caused extensive concern, but its role in waste activated sludge (WAS) anaerobic fermentation (AF) is still uncertain. This work investigated the impact of TCS on volatile fatty acids (VFAs) recycling from WAS. The results showed that TCS of 200 mg/kg TSS increased the maximum VFA accumulation from 7284 to 15,083 mg COD/L. The increase in total VFA production is attributed to the massive increase in acetic acid. Mechanism exploration showed that TCS promotes WAS solubilization by facilitating cell breakage and extracellular polymeric substances disruption, and stimulates AF by enhancing the activity of key enzymes among all stages. TCS promotes acidification stronger than methanogenesis, which makes VFA production faster than consumption, leading to increased VFA accumulation. These findings provide novel insights for revealing the role of TCS in WAS resource recovery, and offer thoughts for the selective production of final recycling products of TCS-containing WAS.

Effect of fluoxetine on enhanced biological phosphorus removal using a sequencing batch reactor

In this work, the potential impact of emerging pollutant Fluoxetine (FLX) on enhanced biological phosphorus removal (EBPR) was systematically investigated using the sequencing batch reactor. The experimental results showed that even 200 μg/L FLX had no significant effect on EBPR during the short-term exposure. However, in the long-term exposure test, high dosage of FLX inhibited EBPR. 200 μg/L FLX induced biological phosphorus removal efficiency dropped to 71.3 ± 2.1%, significantly lower than that of the blank. The mechanism investigation showed that high concentration of FLX reduced anaerobic phosphorus release and oxic phosphorus absorption, and the consumption of organic matter during the anaerobic period. In addition, FLX decreased the synthesis of intracellular polymer polyhydroxyalkanoates (PHA), but promoted the metabolism of glycogen and polyhydroxyvalerate. FLX reduced the activity of key enzymes in EBPR and the relative abundance of Accumulibacter, but improved the relative abundance of Candidatus Competibacter.

Iron coupling with carbon fiber to stimulate biofilms formation in aerobic biological film systems for improved decentralized wastewater treatment: Performance, mechanisms and implications

Iron coupling with carbon fiber (ICF) as carriers to stimulate the biofilms formation for decentralized wastewater treatment was proposed. The typical pollutants removal was accelerated and enhanced (increased by 13.65% for chemical oxygen demand, 19.68% for ammonia nitrogen and 32.66% for phosphate) in ICF compared with the traditional carbon fiber (CF) system. Mechanism explorations indicated that the iron coupling improved the surface properties of carbon fibers and contributed to the attachment and growth of biomass significantly. The components of biomass were changed with increasing proteins proportion in ICF, which was beneficial to the biofilms formation and stability. The microbial community was altered with the enrichment of functional microorganisms (i.e. Pseudomonas and Thauera). Moreover, the microbial metabolic functions (i.e. enzymatic activities and encoding genes) involved in pollutants removal derived from decentralized wastewater were highly expressed in ICF. This work provided an effective strategy to enhance the decentralized wastewater treatment.

Deterioration of biological pollutants removal induced by linear alkylbenzene sulphonates in sequencing batch reactors: Insight of sludge characteristics, microbial community and metabolic activity

Linear alkylbenzene sulfonates (LAS) are widely detected in wastewater, and pose potential risks to environment. The influences of LAS on the typical pollutants removal in sequencing batch reactors (SBRs) were evaluated. The results indicated that the removal efficiency of COD, NH₄⁺ and PO₄^3- was respectively reduced by 10.5-27.5%, 5.4-7.3% and11.6-28.9% with the exposure of 10-50 mg/L LAS. Mechanisms investigation found that LAS disrupted the sludge structure and reduced the biomass in reactors due to the saponification effects. Also, the presence of LAS altered the microbial community of activated sludge, and reduced the abundances of functional bacterial responsible for pollutants removal (i.e.Candidatus Accumulibacter, Nitrospira, Denitratisoma and etc.). Moreover, the LAS exhibited negative impacts on the microbial activity with increased LDH release but decreased ATP concentration. The genes expressions for microbial metabolism (i.e. carbohydrate metabolisms, energy metabolism) and typical pollutants removal (i.e. electron transport, phosphonate transport) were all downregulated in LAS-exposed SBRs.