Mechanisms and Strategies of Advanced Oxidation Processes for Membrane Fouling Control in MBRs: Membrane-Foulant Removal versus Mixed-Liquor Improvement.

Membrane bioreactors (MBRs) are well-established and widely utilized technologies with substantial large-scale plants around the world for municipal and industrial wastewater treatment. Despite their widespread adoption, membrane fouling presents a significant impediment to the broader application of MBRs, necessitating ongoing research and development of effective antifouling strategies. As highly promising, efficient, and environmentally friendly chemical methods for water and wastewater treatment, advanced oxidation processes (AOPs) have demonstrated exceptional competence in the degradation of pollutants and inactivation of bacteria in aqueous environments, exhibiting considerable potential in controlling membrane fouling in MBRs through direct membrane foulant removal (MFR) and indirect mixed-liquor improvement (MLI). Recent proliferation of research on AOPs-based antifouling technologies has catalyzed revolutionary advancements in traditional antifouling methods in MBRs, shedding new light on antifouling mechanisms. To keep pace with the rapid evolution of MBRs, there is an urgent need for a comprehensive summary and discussion of the antifouling advances of AOPs in MBRs, particularly with a focus on understanding the realizing pathways of MFR and MLI. In this critical review, we emphasize the superiority and feasibility of implementing AOPs-based antifouling technologies in MBRs. Moreover, we systematically overview antifouling mechanisms and strategies, such as membrane modification and cleaning for MFR, as well as pretreatment and in-situ treatment for MLI, based on specific AOPs including electrochemical oxidation, photocatalysis, Fenton, and ozonation. Furthermore, we provide recommendations for selecting antifouling strategies (MFR or MLI) in MBRs, along with proposed regulatory measures for specific AOPs-based technologies according to the operational conditions and energy consumption of MBRs. Finally, we highlight future research prospects rooted in the existing application challenges of AOPs in MBRs, including low antifouling efficiency, elevated additional costs, production of metal sludge, and potential damage to polymeric membranes. The fundamental insights presented in this review aim to elevate research interest and ignite innovative thinking regarding the design, improvement, and deployment of AOPs-based antifouling approaches in MBRs, thereby advancing the extensive utilization of membrane-separation technology in the field of wastewater treatment.

Tannic Acid Modulation of Substrate Utilization, Microbial Community, and Metabolic Traits in Sludge Anaerobic Fermentation for Volatile Fatty Acid Promotion.

Anaerobic fermentation is a crucial route to realize effective waste activated sludge (WAS) resource recovery and utilization, while the overall efficiency is commonly restrained by undesirable disruptors (i.e., chemical dewatering agents). This work unveiled the unexpectedly positive effects of biodewatering tannic acid (TA) on the volatile fatty acids (VFAs) biosynthesis during WAS anaerobic fermentation. The total VFAs yield was remarkably increased by 15.6 folds with enriched acetate and butyrate in TA-occurred systems. TA was capable to disintegrate extracellular polymeric substances to promote the overall organics release. However, TA further modulated the soluble proteins structure by hydrogen bonding and hydrophobic interactions, resulting in the decrease of proteins bioavailability and consequential alteration of metabolic substrate feature. These changes reshaped the microbial community and stimulated adaptive regulatory systems in hydrolytic-acidogenic bacteria. The keystone species for carbohydrate metabolism (i.e., Solobacterium and Erysipelotrichaceae) were preferentially enriched. Also, the typical quorum sensing (i.e., enhancing substrate transport) and two-component systems (i.e., sustaining high metabolic activity) were activated to promote the microbial networks connectivity and ecological cooperative behaviors in response to TA stress. Additionally, the metabolic functions responsible for carbohydrate hydrolysis, transmembrane transport, and intracellular metabolism as well as VFA biosynthesis showed increased relative abundance, which maintained high microbial activities for VFAs biosynthesis. This study underscored the advantages of biodewatering TA for WAS treatment in the context of resource recovery and deciphered the interactive mechanisms.

Deep insights into the roles and microbial ecological mechanisms behind waste activated sludge digestion triggered by persulfate oxidation activated through multiple modes.

Persulfate oxidation (PS) is widely employed as a promising alternative for waste activated sludge pretreatment due to the capability of generating free radicals. The product differences and microbiological mechanisms by which PS activation triggers WAS digestion through multiple modes need to be further investigated. This study comprehensively investigated the effects of persulfate oxidation activated through multiple modes, i.e., ferrous, zero-valent iron (ZVI), ultraviolet (UV) and heat, on the performance of sludge digestion. Results showed that PS_ZVI significantly accelerated the methane production rate to 12.02 mL/g VSS. By contrast, PS_Heat promoted the sludge acidification and gained the maximum short-chain fatty acids (SCFAs) yield (277.11 ± 7.81 mg COD/g VSS), which was 3.41-fold compared to that in PS_ZVI. Moreover, ferrous and ZVI activated PS achieved the oriented conversion of acetate, the proportions of which took 73% and 78%, respectively. MiSeq sequencing results revealed that PS_Heat and PS_UV evidently enriched anaerobic fermentation bacteria (AFB) (i.e., Macellibacteroides and Clostridium XlVa). However, PS_Ferrous and PS_ZVI facilitated the enrichment of Woesearchaeota and methanogens. Furthermore, molecular ecological network and mantel test revealed the intrinsic interactions among the multiple functional microbes and environmental variables. The homo-acetogens and sulfate-reducing bacterial had potential cooperative and symbiotic relationships with AFB, while the nitrate-reducing bacteria displayed distinguishing ecological niches. Suitable activation modes for PS pretreatments resulted in an upregulation of genes expression responsible for digestion. This study established a scientific foundation for the application of sulfate radical-based oxidation on energy or high value-added chemicals recovery from waste residues.

Indole-3-acetic acid regulating the initial adhesion of microalgae in biofilm formation.

Regulating the microalgal initial adhesion in biofilm formation is a key approach to address the challenges of attached microalgae cultivation. As a type of phytohormone, Indole-3-acetic acid (IAA) can promote the growth and metabolism of microalgae. However, limited knowledge has been acquired of how IAA can change the initial adhesion of microalgae in biofilm formation. This study focused on investigating the initial adhesion of microalgae under different IAA concentrations exposure in biofilm formation. The results showed that IAA showed obvious hormesis-like effects on the initial adhesion ability of microalgae biofilm. Under exposure to the low concentration (0.1 mg/L) of IAA, the initial adhesion quantity of microalgae on the surface of the carrier reached the highest value of 7.2 g/m2. However, exposure to the excessively high concentration (10 mg/L) of IAA led to a decrease in the initial adhesion capability of microalgal biofilms. The enhanced adhesion of microalgal biofilms due to IAA was attributed to the upregulation of genes related to the Calvin Cycle, which promoted the synthesis of hydrophobic amino acids, leading to increased protein secretion and altering the surface electron donor characteristics of microalgal biofilms. This, in turn, reduced the energy barrier between the carriers and microalgae. The research findings would provide crucial support for the application of IAA in regulating the operation of microalgal biofilm systems.

Mixotrophic denitrification of waste activated sludge fermentation liquid as an alternative carbon source for nitrogen removal: Reducing N2O emissions and costs.

Heterotrophic-sulfur autotrophic denitrification (HAD) has been proposed to be a prospective nitrogen removal process. In this work, the potential of fermentation liquid (FL) from waste-activated sludge (WAS) as the electron donor for denitrification in the HAD system was explored and compared with other conventional carbon sources. Results showed that when FL was used as a carbon source, over 99% of NO3--N was removed and its removal rate exceeded 14.00 mg N/g MLSS/h, which was significantly higher than that of methanol and propionic acid. The produced sulfate was below the limit value and the emission of N2O was low (1.38% of the NO3--N). Microbial community analysis showed that autotrophic denitrifiers were predominated in the HAD system, in which Thiobacillus (16.4%) was the dominant genus. The economic analysis showed the cost of the FL was 0.062 €/m3, which was 30% lower than that in the group dosed with methanol. Our results demonstrated the FL was a promising carbon source for the HAD system, which could reduce carbon emission and cost, and offer a creative approach for waste-activated sludge resource reuse.

Dose-dependent effects of different parabens on food waste biorefinery for volatile fatty acids production: Insight into specific fermentation processes, substrates transformation and microbial metabolic traits.

Parabens are largely concentrated in food waste (FW) due to their large consumption as the widely used preservative. To date, whether and how they affect FW resource recovery via anaerobic fermentation is still largely unknown. This work unveiled the hormesis-like effects of two typical parabens (i.e., methylparaben and n-butylparaben) on VFAs production during FW anaerobic fermentation (i.e., parabens increased VFAs by 6.73-14.49 % at low dose but caused 82.51-87.74 % reduction at high dose). Mechanistic exploration revealed that the parabens facilitated the FW solubilization and enhanced the associated substrates' biodegradability. The low parabens enriched the functional microorganisms (e.g., Firmicutes and Actinobacteria) and upregulated those critical genes involved in VFAs biosynthesis (e.g., GCK and PK) by activating the microbial adaptive capacity (i.e., quorum sensing and two-component system). Consequently, the metabolism rates of fermentation substrates and subsequent VFAs production were accelerated. However, due to increased biotoxicity of high parabens, the functional microorganisms and relevant metabolic activities were depressed, resulting in the significant reduction of VFAs biosynthesis. Structural equation modeling clarified that microbial community was the predominant factor affecting VFAs generation, followed by metabolic pathways. This work elucidated the dose-dependent effects and underlying mechanisms of parabens on FW anaerobic fermentation, providing insights for the effective management of FW resource recovery.

Time-dependent interference of surfactants and CeO2/Fe2O3 nanoparticles co-occurrence on the volatile fatty acids biosynthesis during semi-continuous sludge fermentation.

Various exogenous contaminants typically coexist in waste activated sludge (WAS), and the long-term impacts of these co-occurring contaminants on WAS anaerobic fermentation and associated mechanisms remain largely unknown. This study reveals that the co-occurrence of surfactants and nanoparticles (NPs, i.e., Fe2O3 and CeO2, frequently detected in sludge) exhibited time-dependent impacts on the volatile fatty acids (VFAs) biosynthesis. Surfactants triggered WAS decomposition and enhanced NPs dispersion, leading to increased exposure of functional anaerobes to NPs toxicity, negatively affecting them. Consequently, key fermentation processes, acidogenic bacterial abundance, and metabolic functions were inhibited in co-occurrence reactors compared to those containing only surfactants in the early stage (before 56 d). Surprisingly, the fermentation systems containing surfactants collapsed subsequently, with VFAs yield at 72 d decreasing by 48.59-71.27 % compared to 56 d. The keystone microbes (i.e., Acidobacteria (16 d) vs Patescibacteria (56 d)) were reshaped, and metabolic traits (i.e., proB involved in intracellular metabolism) were downregulated by 0.05-78.02 % due to reduced microbial adaptive capacity (i.e., quorum sensing (QS)). Partial least squares path modeling (PLS-PM) analysis suggests that the microbial community was the predominant factor influencing VFAs generation. This study provides new insights into the long-term effects of co-contaminants on the biological treatment of WAS.

Sulfate-reducing bacteria decreases fractional pressure of H2 to accelerate short-chain fatty acids production from waste activated sludge fermentation assisted with zero-valent iron activated sulfite pretreatment.

The production of short-chain fatty acids (SCFAs) is constrained by substrate availability and the increased fractional pressure of H2 emitted by acidogenic/fermentative bacteria during anaerobic fermentation of waste activated sludge (WAS). This study introduced a novel approach employing zero-valent iron (ZVI)-activated sulfite pretreatment combined with H2-consuming sulfate-reducing bacteria (SRB) mediation to improve SCFAs, especially acetate production from WAS fermentation. Experimental results showed that the combined ZVI-activated sulfite and incomplete-oxidative SRB (io-SRB) process achieved a peak SCFAs production of 868.11 mg COD/L, with acetate accounting for 80.55 %, which was 7.90- and 2.18-fold higher than that obtained from raw WAS fermentation, respectively. This could be firstly attributed to the SO4- and OH generated by ZVI-activated sulfite, which significantly promoted WAS decomposition, e.g., soluble proteins and carbohydrates increased 14.3- and 10.8-fold, respectively, over those in raw WAS. The biodegradation of dissolved organic matter was subsequently enhanced by the synergistic interaction and H2 transfer between anaerobic fermentation bacteria (AFB) and io-SRB. The positive and negative correlations among AFB, nitrate-reducing bacteria (NRB) and the io-SRB consortia were revealed by molecular ecological network (MEN) and Mantel test. Moreover, the expression of functional genes was also improved, for instance, in relation to acetate formation, the relative abundances of phosphate acetyltransferase and acetate kinase was 0.002 % and 0.005 % higher than that in the control test, respectively. These findings emphasized the importance of sulfate radicals-based oxidation pretreatment and the collaborative relationships of multifunctional microbes on the value-added chemicals and energy recovery from sludge fermentation.

Parachlorometaxylenol stress caused multidrug-type antibiotic resistance genes proliferation via simultaneously reshaping microbial community and interfering metabolic traits during wastewater treatment process.

Despite biological wastewater treatment processes (e.g., sequencing batch reactors (SBR)) being able to reduce the dissemination of antibiotic resistance genes (ARGs), the variation of ARGs under exogenous pollutant stress is an open question. This work investigated the impacts of para-chloro-meta-xylenol (PCMX, typical antibacterial contaminants) on ARGs spread in long-term SBR operation. Although the SBR process inherently decreased ARGs abundance, the presence of PCMX substantially amplified both the prevalence (mainly multidrug) and abundance of total ARGs (1.17-fold of the control). Further analysis demonstrated that PCMX disintegrated sludge structures as well as increased membrane permeability, facilitating the release of mobile genetic elements and subsequent horizontal transfer of ARGs. In addition, PCMX selectively enriched potential ARG hosts, notably Nitrospira and Candidatus Accumulibacter, which predominantly served as multidrug ARG hosts. Concurrently, the self-adaptive functions of ARGs hosts in the PCMX-exposed SBR system were activated via quorum sensing, two-component regulatory system, ATP-binding cassette transporters, and bacterial secretion system. The upregulation of these metabolic pathways also contributed to the dissemination of ARGs.

Disinfectant polyhexamethylene guanidine triggered simultaneous efflux pump antibiotic- and metal-resistance genes propagation during sludge anaerobic digestion.

The environmental transmission of antibiotic resistance genes (ARGs) and metal resistance genes (MRGs) exerted devastating threats to global public health, and their interactions with other emerging contaminants (ECs) have raised increasing concern. This study investigated that the abundances of ARGs and MRGs with the predominant type of efflux pump were simultaneously increased (8.4-59.1%) by disinfectant polyhexamethylene guanidine (PHMG) during waste activated sludge (WAS) anaerobic digestion. The aggregation of the same microorganisms (i.e., Hymenobacter and Comamonas) and different host bacteria (i.e., Azoarcus and Thauera) were occurred upon exposure to PHMG, thereby increasing the co-selection and propagation of MRGs and ARGs by vertical gene transfer. Moreover, PHMG enhanced the process of horizontal gene transfer (HGT), facilitating their co-transmission by the same mobile genetic elements (20.2-223.7%). Additionally, PHMG up-regulated the expression of critical genes (i.e., glnB, trpG and gspM) associated with the HGT of ARGs and MRGs (i.e., two-component regulatory system and quorum sensing) and exocytosis system (i.e., bacterial secretion system). Structural equation model analysis further verified that the key driver for the simultaneous enrichment of ARGs and MRGs under PHMG stress was microbial community structure. The study gives new insights into the aggravated environmental risks and mechanisms of ECs in sludge digestion system, providing guidance for subsequent regulation and control of ECs.

Simultaneous volatile fatty acids promotion and antibiotic resistance genes reduction in fluoranthene-induced sludge alkaline fermentation: Regulation of microbial consortia and cell functions.

The impact and mechanism of fluoranthene (Flr), a typical polycyclic aromatic hydrocarbon highly detected in sludge, on alkaline fermentation for volatile fatty acids (VFAs) recovery and antibiotic resistance genes (ARGs) transfer were studied. The results demonstrated that VFAs production increased from 2189 to 4272 mg COD/L with a simultaneous reduction of ARGs with Flr. The hydrolytic enzymes and genes related to glucose and amino acid metabolism were provoked. Also, Flr benefited for the enrichment of hydrolytic-acidifying consortia (i.e., Parabacteroides and Alkalibaculum) while reduced VFAs consumers (i.e., Rubrivivax) and ARGs potential hosts (i.e., Rubrivivax and Pseudomonas). Metagenomic analysis indicated that the genes related to cell wall synthesis, biofilm formation and substrate transporters to maintain high VFAs-producer activities were upregulated. Moreover, cell functions of efflux pump and Type IV secretion system were suppressed to inhibit ARGs proliferation. This study provided intrinsic mechanisms of Flr-induced VFAs promotion and ARGs reduction during alkaline fermentation.

Dynamic microbiome disassembly and evolution induced by antimicrobial methylisothiazolinone in sludge anaerobic fermentation for volatile fatty acids generation.

In the post-COVID-19 pandemic era, various antimicrobials have emerged and concentrated in waste-activated sludge (WAS), affecting the biological treatment of WAS. However, there is still a knowledge gap in the dynamic response and adaptive mechanism of anaerobic microbiome under exogenous antimicrobial stress. This study found that methylisothiazolinone (MIT, as a typic antimicrobial) caused an interesting lag effect on the volatile fatty acids (VFAs) promotion in the WAS anaerobic fermentation process. MIT was effective to disintegrate the extracellular polymeric substances (EPS), and those functional anaerobic microorganisms were easily exposed and negatively impacted by the MIT interference after the loss of protective barriers. Correspondingly, the ecological interactions and microbial metabolic functions related to VFA biosynthesis (e.g., pyruvate metabolism) were downregulated at the initial stage. The syntrophic consortia gradually adapted to the interference and attenuated the MIT stress by activating chemotaxis and resistance genes (e.g., excreting, binding, and inactivating). Due to the increased bioavailable substrates in the fermentation systems, the dominant microorganisms (i.e., Clostridium and Caloramator) with both VFAs production and MIT-tolerance functions have been domesticated. Moreover, MIT disrupted the syntrophic interaction between acetogens and methanogens and totally suppressed methanogens' metabolic activities. The VFA production derived from WAS anaerobic fermentation was therefore enhanced due to the interference of antimicrobial MIT stress. This work deciphered dynamic changes and adaptive evolution of anaerobic syntrophic consortia in response to antimicrobial stress and provided guidance on the evaluation and control of the ecological risks of exogenous pollutants in WAS treatment.

Novel calcium oxide activated peroxymonosulfate system for methylene blue removal: Identification of key influencing factors, transformation pathway and toxicity assessment.

The activation of peroxymonosulfate (PMS) has gained significant interest in the removal of organic pollutants. However, traditional methods usually suffer from drawbacks such as secondary contamination and high energy requirements. In this study, we propose a green and cost-effective approach utilizing calcium oxide (CaO) to activate PMS, aiming to construct a simple and reliable PMS based advanced oxidation processes (AOPs). The proposed CaO/PMS system achieved fast degradation of methylene blue (MB), where the degradation rate of CaO/PMS system (0.24 min-1) was nearly 2.67 times that of PMS alone (0.09 min-1). Under the optimized condition, CaO/PMS system exhibited remarkable durability against pH changes, co-exists ions or organic matters. Furthermore, singlet oxygen (1O2) was identified as the dominant reactive oxygen species by electron paramagnetic resonance (EPR) and quenching tests. Accordingly, the degradation pathways of MB are proposed by combing the results of LC/MS analysis and density functional theory (DFT) calculations, and the predicted ecotoxicity of the generated byproducts evaluated by EOCSAR could provide systematic insights into the fates and environmental risks of MB. Overall, the study provides an eco-friendly and effective strategy for treating dyeing wastewater, which should shed light on the application of PMS based AOPs.

Untangling the interplay of dissolved organic matters variation with microbial symbiotic network in sludge anaerobic fermentation triggered by various pretreatments.

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