Enhancing acidogenic fermentation of waste activated sludge via isoelectric-point pretreatment: Insights from physical structure and interfacial thermodynamics

A novel strategy to promote sludge solubilization and short-chain fatty acid production by coupling thermal hydrolysis and sodium thiosulfate pretreatment

Thermal hydrolysis (TH) technology is promising for sludge pretreatment, but the high cost and the generation of refractory substances limit its application. In this study, sodium thiosulfate (STS) was innovatively combined with TH pretreatment to improve the anaerobic fermentation efficiency of sludge. TH-STS pretreatment (140 °C, 0.132 g/g TSS) increased sludge solubility by 33.4 % and increased short-chain fatty acid (SCFA) production to 1.86 times that of the control group. TH effectively stripped the extracellular polymeric substances, and STS subsequently lysed the cells through its reducing power. TH-STS pretreatment promoted SCFA accumulation by increasing the activity of key enzymes and enriching hydrolyzing and acidifying bacteria. In addition, TH-STS pretreatment increased the activity of the electron transport system, which positively promoted the biotransformation of SCFAs. This study reveals that STS and TH pretreatment have a synergistic effect, providing an effective method for improving sludge pretreatment and resource recycling.

Choline chloride-assisted thermal hydrolysis pretreatment improves short-chain fatty acid recovery from waste activated sludge

Recovery of short-chain fatty acids (SCFAs) from waste activated sludge (WAS) by anaerobic fermentation (AF) is limited by the slow rate of WAS hydrolysis. Here, choline chloride (ChCl)-assisted thermal hydrolysis pretreatment (THP) was proposed to improve the hydrolysis rate and SCFA recovery from WAS. ChCl-assisted THP (140 °C, 30 min, 0.60 g/g TSS) increased the SCFA yield by 139 % compared to the control (unpretreated) group, rising from 2629 ± 190 mg COD/L to 6276 ± 153 mg COD/L, and raised the percentage of acetic acid from 28.7 % to 58.0 %. The hydrogen-bonding effect of ChCl facilitates sludge destruction by THP and induces oxidative stress through quaternary ammonium groups to destroy cells, thus creating a synergistic effect on WAS hydrolysis. In addition, ChCl-assisted THP promotes the production of SCFAs by increasing the activity of key enzymes and enriching hydrolyzing and acidifying bacteria. This article provides a new approach to enhance SCFA recovery from WAS.

Unraveling temperature effects on caproate and caprylate production from waste activated sludge

This study explored the impact of different temperatures on the continuous production of medium-chain fatty acids (MCFAs) from waste activated sludge (WAS). Experimental results showed that there was almost no MCFAs accumulation at 55 °C. Both 25 °C and 37 °C were suitable for MCFAs production, with 25 °C favoring high-value caprylate production. The metagenomic and metatranscriptomic analysis highlighted reverse β-oxidization as the main chain elongation (CE) cycle. The lack of CE-related microorganisms and enzymes at 55 °C hindered MCFAs production, in contrast to the heightened activity observed at 25 °C and 37 °C, with peak activity at 25 °C leading to increased longer-chain MCFAs synthesis. 37 °C promoted hydrolysis and acidification, resulting in a accumulation of higher short-chain fatty acids, but further elongation to MCFAs would be hindered by product toxicity. This research concludes that 25 °C is the most effective temperature for the production of WAS-derived MCFAs, offering significant economic advantages.

Investigating enhancement of protease and lysozyme combination pretreatment on hydrolysis of sludge organics under humic acid inhibition

This study investigated the impact of humic acid (HA) on enzymatic pretreatment efficiency, focusing on sludge properties and HA molecular structure. The results showed that enzymatic pretreatment alleviates HA inhibition, improving hydrolysis efficiency. In the presence of HA, soluble proteins and polysaccharides in the enzyme-cocktail group reached 27.7 mg/L and 23.9 mg/L, 1.4 and 1.3 times higher than the blank group, respectively. The enzyme-cocktail group also had the highest soluble DNA concentration (19.4 mg/L) and the lowest viable cell proportion (69.3 %), indicating effective cell lysis. Enzyme-cocktail pretreatment reduced electrostatic repulsion, enhancing the mobility of extracellular organics. Enzyme interactions with HA released internal hydrolases and decreased amide groups on the HA surface, increasing the availability of biodegradable substrates. Overall, enzymatic pretreatment proves effective in mitigating HA-induced inhibition, thereby improving sludge biodegradation and enhancing carbon recovery in anaerobic fermentation.

The effects of headspace volume reactor on the microbial community structure during fermentation processes for volatile fatty acid production

The transition from traditional wastewater treatment plants to biorefineries represents an environmentally and economically sustainable approach to extracting valuable compounds from waste. Sewage sludge produced from wastewater treatment is incinerated or disposed of in specific landfills. Repurposing this waste material to recover valuable resources could help lower disposal costs and reduce environmental impact by producing other beneficial polymers. Microorganisms present in the sewage sludge can ferment organic pollutants, producing volatile fatty acids (VFA), precursors for biopolymers that could be used as an alternative to petroleum-derived plastics. To boost VFA production during sewage sludge fermentation, it is necessary to understand the operating microbial community and its metabolic capacities in anaerobic conditions. This study presents the influence of the headspace volume on the microbial community and the VFA production to define the best operational parameters in a 225 L pilot plant fermenter. The wasted sewage sludge was withdrawn from an oxic-settling-anaerobic plant that collected wastewater from the canteen and dormitory of the UNIPA Campus (Palermo University, Italy) and incubated using a 40% and a 60% headspace volume. The microbial community was analysed before and after the fermentation process through metataxonomic analysis, and VFA yields were determined by gas chromatography analysis. Our results showed that the 40% headspace volume induced a tenfold higher VFA production than the 60% headspace volume, modulating the microbial community's efforts to establish a VFA-producing factory. Notably, at 40% headspace, the relative abundance of bacteria, like Proteobacteria, Firmicutes, Actinobacteria, and Chloroflexi, significantly increased, as well as the relative abundance of Bacteroidetes and Verrucomicrobia decreased during the fermentation process. This result is consistent with the selection of efficient VFA-producing bacteria that lead to increased VFA yields that are not obtained at 60% headspace. Thus, reducing headspace is a promising strategy that can be implemented, even in full-scale plants, to optimise the wastewater reuse process and maximise VFA production to produce bioplastics, like polyhydroxyalkanoate, for the transition from linear wastewater treatment plants to circular biorefineries.

Neglected sludge solid phase in sludge pretreatment process: Physicochemical characterization and mechanism study of its role in anaerobic degradation

The low anaerobic digestion efficiency of the solid phase separated from pre-treated sludge indicates the need to explore other suitable resource utilization pathways for sludge solid phase. However, there is a lack of comprehensive and in-depth research on the physicochemical properties of sludge solid phase. This study comprehensively analyzes the characteristics of sludge solid phase and elucidates the mechanism of sludge solid phase in the anaerobic degradation of toxic wastewater. The results show that the surface free energy of sludge solid phase after different pre-treatments is mainly contributed by Lewis acid-base hydration free energy. The distribution of proteins on the surface of sludge solid phase plays a major role in the adhesion between sludge solid particles. Metal ions in the sludge solid phase are mainly present in the exchange state, followed by the carbonate state and the organics-bound state. The sludge solid phase obtained by sludge pH 12 + 150 °C treatment has the highest conductivity (1.36 mS/m) and capacitance (25.51 μF/g), mainly due to the presence of melanoidins in the sludge solid phase, which has similar semiquinone radicals to humic acids, thus increasing conductivity. The addition of sludge solid phase promotes an increase in cumulative methane production and rate of methane production. The sludge solid phase might play a role of an auxiliary carbon source acting as an adsorbent to buffer against toxicity inhibition and facilitate electron transfer. This study reveals the characteristics of sludge solid phase and its role in anaerobic digestion, providing theoretical guidance for finding suitable resource utilization pathways for sludge solid phase.

Enhancing volatile fatty acids production from waste activated sludge: The role of pretreatment by N,N-bis(carboxymethyl)-l-glutamate (GLDA)

N,N-bis(carboxymethyl)-l-glutamate (GLDA) is an eco-friendly chelating agent that effectively extracts multivalent metal ions from waste activated sludge (WAS) flocs, which could potentially alter their structure. However, the effect of GLDA on the production of volatile fatty acids (VFAs) from WAS is not well known. Here, we demonstrate that pretreatment with GLDA at a concentration of 200 mmol per kg VSS results in a significant increase of 142% in extractable extracellular polymeric substances and enhances the total VFAs yield by 64% compared to untreated samples. We reveal GLDA's capability to mobilize organic-binding multivalent metal ions within sludge flocs. Specifically, post-pretreatment analyses showed the release of 69.1 mg L-1 of Ca and 109.8 mg L-1 of Fe ions from the flocs, leading to a more relaxed floc structure and a reduced apparent activation energy (10.6 versus 20 kJ mol-1) for WAS solubilization. Molecular dynamic simulations further demonstrate GLDA's preferential binding to Fe3+ and Ca2+ over Mg2+. Our study suggests that GLDA pretreatment causes minimal disruption to reactor stability, thereby indicating the stability of microbial community composition. GLDA has emerged as a viable pretreatment agent for enhancing volatile fatty acids production from waste activated sludge.

Electrogenerated singlet oxygen and reactive chlorine species enhancing volatile fatty acids production from co-fermentation of waste activated sludge and food waste: The key role of metal oxide coated electrodes

Electrochemical pretreatment (EPT) has shown to be superior in improving acidogenic co-fermentation (Co-AF) of waste activated sludge (WAS) and food waste (FW) for volatile fatty acids (VFAs). However, the influence of EPT electrode materials on the production of electrogenerated oxidants (such as singlet oxygen (1O2) and reactive chlorine species (RCS)), as well as their effects on properties of electrodes, the microbial community structure and functional enzymes remain unclear. Therefore, this study investigated the effects of various metal oxide coated electrodes (i.e., Ti/PbO2, Ti/Ta2O5-IrO2, Ti/SnO2-RuO2, and Ti/IrO2-RuO2) on EPT and subsequent Co-AF of WAS-FW. The results showed that EPT with Ti/PbO2, Ti/Ta2O5-IrO2, Ti/SnO2-RuO2 and Ti/IrO2-RuO2 electrodes generated 165.3-848.2 mg Cl2/L of RCS and 5.643 × 1011-3.311 × 1012 spins/mm3 of 1O2, which significantly enhanced the solubilization and biodegradability of WAS-FW by 106.4 %-233.6 % and 177.3 %-481.8 %, respectively. Especially with Ti/Ta2O5-IrO2 as the electrode material, an appropriate residual RCS (2.0-10.4 mg Cl2/L) remained in Co-AF step, resulted in hydrolytic and acidogenic bacteria (e.g., Prevotella_7, accounting for 78.9 %) gradually become dominant rather than methanogens (e.g., Methanolinea and Methanothrix) due to their different tolerance to residual RCS. Meanwhile, the functional gene abundances of hydrolytic and acidogenic enzymes increased, while the methanogenic enzymes deceased. Consequently, this reactor produced the highest VFAs up to 545.5 ± 36.0 mg COD/g VS, which was 101.8 % higher than that of the Control (without EPT). Finally, the economic analysis and confirmatory experiments further proved the benefits of WAS-FW Co-AF with EPT.

Electron-transfer-driven spatial optimisation of anaerobic consortia for efficient methanogenesis: Neglected inductive effect of conductive materials

The inductive effect of conductive materials (CMs) on enhancing methanogenesis metabolism has been overlooked. Herein, we highlight role of CMs in inducing the spatial optimisation of methanogenic consortia by altering the Lewis acid-base (AB) interactions within microbial aggregates. In the presence of CMs and after their removal, the methane production and methane proportion in biogas significantly increase, with no significant difference between the two situations. Analyses of interactions between CMs and extracellular polymer substances (EPSs) with and without D2O reveal that CMs promote release and transfer potential of electron in EPSs, which induce and enhance the role of water molecules being primarily as proton acceptors in the hydrogen bonding between EPSs and water, thereby changing the electron-donor- and electron-acceptor-based AB interactions. Investigations of succession dynamics of microbial communities, co-occurrence networks, and metagenomics further indicate that electron transfer drives the microbial spatial optimisation for efficient methanogenesis through intensive interspecies interactions.

Mechanistic insight into the aggregation ability of anammox microorganisms: Roles of polarity, composition and molecular structure of extracellular polymeric substances

The chemical characteristics of extracellular polymeric substances (EPS) of anammox bacteria (AnAOB) play a crucial role in the rapid enrichment of AnAOB and the stable operation of wastewater anammox processes. To clarify the influential mechanisms of sludge EPS on AnAOB aggregation, multiple parameters, including the polarity distribution, composition, and molecular structure of EPS, were selected, and their quantitative relationship with AnAOB aggregation was analyzed. Compared to typical anaerobic sludge (anaerobic floc and granular sludge), the anammox sludge EPS exhibited higher levels of tryptophan-like substances (44.82-56.52 % vs. 2.57-39.81 %), polysaccharides (40.02-53.49 mg/g VSS vs. 30.22-41.69 mg/g VSS), and protein structural units including α-helices (20.70-23.98 % vs. 16.48-19.32 %), β-sheets (37.43-42.98 % vs. 25.78-36.72 %), and protonated nitrogen (Npr) (0.065-0.122 vs. 0.017-0.061). In contrast, it had lower contents of β-turns (20.95-27.39 % vs. 28.17-39.04 %). These biopolymers were found to originate from different genera of AnAOB. Specifically, the α-helix-rich proteins were mainly derived from Candidatus Kuenenia, whereas the extracellular proteins related to tryptophan and Npr were closely associated with Candidatus Brocadia. Critically, these EPS components could drive anammox aggregation through interactions. Substantial amounts of tryptophan-like substances facilitated the formation of β-sheet structures and the exposure of internal hydrophobic clusters, which benefited the anammox aggregation. Meanwhile, extracellular proteins with high Npr content played a pivotal role in the formation of mixed protein-polysaccharide gel networks with the electronegative regions of polysaccharides, which could be regarded as the key component in the maintenance of anammox sludge stability. These findings provide a comprehensive understanding of the multifaceted roles of EPS in driving anammox aggregation and offer valuable insights into the development of EPS regulation strategies aimed at optimizing the anammox process.

Soluble carbon source recovery using preconditioning coagulants for applicable short-term fermentation of waste activated sludge in WWTPs

A huge production of waste activated sludge (WAS) has been a burden for wastewater treatment plants (WWTPs) with high disposal cost and little benefit back to wastewater purification. The short-chain fatty acids (SCFAs) produced by a short-term acidogenic fermentation of WAS before methane production have been proven to be a high-quality carbon source available for microbial denitrification process. The dual purpose of full recovery of fermentation liquid products and facilitating disposal of residual solid waste necessitate an efficient solid-liquid separation process of short-term fermentation liquid. The transformation and loss of various soluble carbon sources between solid and liquid are very important issues for carbon recovery efficiency when combining short-term fermentation and sludge dewatering in WWTPs. Here we testified the three conventional preconditioning coagulants, Polyferric Sulfate (PFS), Poly Aluminum Chloride (PAC) and Polyacrylamide (PAM), to improve the efficiency of subsequent solid-liquid separation. The results show that conversion yield of SCFAs in the liquid phase of sludge after short-term fermentation was 195 mg COD/g VSS, when using the coagulants PFS, PAC, and PAM for recovery, the recovery ratio was 79.5%, 82.0%, and 85.9%, respectively, while the dewaterability could be improved after preconditioning short-term fermentation sludge. The complexation of Al3+/Fe3+ in metal coagulants with carboxyl groups of SCFA demonstrated by Density Functional Theory calculation led to small part of soluble carbons co-migration to the solid phase, mainly a loss of high molecular weight organic compounds (carbohydrate, proteins, humic acids), while the application of PAM had little impact on carbon recovery. Economic calculations further showed PAM preconditioning short-term fermentation liquid of WAS could achieve higher recovery benefits.

Ascorbic acid reduction pretreatment enhancing metal regulation to improve methane production from anaerobic digestion of waste activated sludge

Conversion of waste activated sludge (WAS) to methane by anaerobic digestion (AD) is often limited by the slow rate of hydrolysis, and the presence of metal ions in sludge is regarded as a critical factor hindering sludge hydrolysis. This study developed a novel strategy to remove Fe from WAS by using ascorbic acid (VC) as a reducing agent under acidic conditions. The feasibility of reduction pretreatment in improving methane production of AD and its intrinsic mechanism were investigated. Results indicate that, under VC doses of 100 mmol/L and pH of 3.50, pretreatment removed 47.60 % of Fe, 59.88 % of Ca, and 51.86 % of Mg contained in the sludge. The removal of metal ions facilitated the disruption of sludge flocculation structure and extracellular polymeric substance (EPS) layers, leading to a 14.78 % increase in cell lysis and a decrease in fractal dimension values to 2.08. Batch AD experiments showed that VC pretreatment improved methane production, with an optimized net methane yield of 190.22 mL/g·VS, an increase of 134.75 % compared to raw WAS. The pretreatment affected the interfacial interaction energy of the sludge, leading to a transformation in the sludge surfaces from hydrophilic to hydrophobic, reducing the interaction between sludge molecules and increasing the number of binding sites available for enzymatic reactions. According to a study of microbial communities, it was found that VC pretreatment caused an increase in the presence of essential functional microbes responsible for hydrolysis, acidification, and methanation. This increase in acetoclastic and hydrogenotrophic methanogens resulted in a substantial enhancement in methane production. These results can be used to develop better pretreatment methods to enhance AD performance.

Effects of Mixtures of Engineered Nanoparticles and Cocontaminants on Anaerobic Digestion

The widespread application of nanotechnology inevitably leads to an increased release of engineered nanoparticles (ENPs) into the environment. Due to their specific physicochemical properties, ENPs may interact with other contaminants and exert combined effects on the microbial community and metabolism of anaerobic digestion (AD), an important process for organic waste reduction, stabilization, and bioenergy recovery. However, the complicated interactions between ENPs and other contaminants as well as their combined effects on AD are often overlooked. This review therefore focuses on the co-occurrence of ENPs and cocontaminants in the AD process. The key interactions between ENPs and cocontaminants and their combined influences on AD are summarized from the available literature, including the critical mechanisms and influencing factors. Some sulfides, coagulants, and chelating agents have a dramatic "detoxification" effect on the inhibition effect of ENPs on AD. However, some antibiotics and surfactants increase the inhibition of ENPs on AD. The reasons for these differences may be related to the interactive effects between ENPs and cocontaminants, changes of key enzyme activities, adenosine triphosphate (ATP) levels, reactive oxygen species (ROS) production, and microbial communities. New scientific opportunities for a better understanding of the coexistence in real world situations are converging on the scale of nanoparticles.

Enzyme-enhanced acidogenic fermentation of waste activated sludge: Insights from sludge structure, interfaces, and functional microflora

Anaerobic fermentation is widely installed to recovery valuable resources and energy as CH4 from waste activated sludge (WAS), and its implementation in developing countries is largely restricted by the slow hydrolysis, poor efficiency, and complicate inert components therein. In this study, enzyme-enhanced fermentation was conducted to improve sludge solubilization from 283 to 7728 mg COD/L and to enhance volatile fatty acids (VFAs) yield by 58.6 % as compared to the conventional fermentation. The rapid release of organic carbon species, especially for tryptophan- and tyrosine-like compounds, to outer layer of extracellular polymeric substance (EPS) occurred to reduce the structural complexity and improve the sludge biodegradability towards VFAs production. Besides, upon enzymatic pretreatment the simultaneous exposure of hydrophilic and hydrophobic groups on sludge surfaces increased the interfacial hydrophilicity. By quantitative analysis via interfacial thermodynamics and XDLVO theory, it was confirmed that the stronger hydrophilic repulsion and energy barriers in particle interface enhanced interfacial mass transfer and reactions involved in acidogenic fermentation. Meanwhile, these effects stimulate the fermentation functional microflora and predominant microorganism, and the enrichment of the hydrolytic and acid-producing bacteria in metaphase and the proliferation of acetogenic bacteria, e.g., Rubrivivax (+9.4 %), in anaphase also benefits VFAs formation. This study is practically valuable to recovery valuable VFAs as carbon sources and platform chemicals from WAS and agriculture wastes.

Cation exchange resins enhance anaerobic digestion of sewage sludge: Roles in sequential recovery of hydrogen and methane

The recovery of renewable bioenergy from anaerobic digestion (AD) of sludge is a promising method to alleviate the energy problem. Although methane can be effectively recovered through sludge pretreatment by cation exchange resin (CER), the simultaneous enhancement of hydrogen and methane generation from AD using CER has not been extensively investigated. Herein, the effect of CER on the sequential recovery of hydrogen and methane and the corresponding mechanisms were investigated. When CER is introduced, the maximum increases for the hydrogen and methane production are 104.7 % and 35.3 %, respectively, confirming the sequential enhancement effects of CER on the hydrogen and methane production. Analyses of the variations in the main biochemical components with and without the effect of CER demonstrate that CER promotes sludge organic solubilisation, hydrolysis, and acidification in both hydrogen- and methane-production stages. Moreover, investigations of variations in the solid-liquid interfacial thermodynamics and removal rates of main multivalent metals of sludge reveal that the ion exchange reactions between the CER and sludge in the hydrogen-production stage provide the direct driving force of effective contact between bacteria and organic particulates. Additionally, the residual effect of the CER during methane production reduces the energy barrier for mass transfer and provides a driving force for this transfer. Further analyses of the microbial community structure and metagenomics indicate that CER directly drives the enrichment of hydrogen-producing bacteria (+ 15.1 %) and key genes encoding enzymes in the hydrogen-production stage. Moreover, CER indirectly induces the enrichment of methane-producing anaerobes (e.g. Methanosaeta: + 16.7 %, Methanosarcina: + 316.5 %); enhances the bioconversion of different substrates into methyl-coenzyme M; and promotes the metabolism pathway of acetoclastic process and CO2 reduction in the methane-production stage. This study can provide valuable insights for simultaneously enhancing the production of hydrogen and methane from AD through sequential recovery.

Revealing How Polyvinyl Chloride Microplastic Physicochemically Affect the Anaerobic Digestion of Waste Activated Sludge

Recent studies have shown that the microplastics in waste activated sludge (WAS) can directly reduce the microbial activity and influence the performance of anaerobic digestion. Unfortunately, few studies paid attention on the interactions between WAS and MPs, since MPs could impact the contact between sludge flocs and microorganisms. We found that PVC-MP changed the interfacial energy properties of the WAS surface and affected methane production. Low concentration (40 mg/L) of PVC-MP changed the water affinity and greatly reduced the energy barrier of interfacial reaction. Simultaneously, WAS surface charge characteristics changed with increasing MPs concentration, which made the sludge difficult to contact with microorganisms. The change process of WAS surface functional groups also indicated that PVC-MP first cover the sludge surface to prevent from being utilized by microorganisms, and then affect the surface protein structure before toxic substances leaching. Our study provides new insights into how MPs affect anaerobic digestion.

Organic binding iron formation and its mitigation in cation exchange resin assisted anaerobic digestion of chemically enhanced primary sedimentation sludge

Fe based chemically enhanced primary sedimentation (CEPS) is an effective method of capturing the colloidal particles and inorganic phosphorous (P) from wastewater but also produces Fe-CEPS sludge. Anaerobic digestion is recommended to treat the sludge for energy and phosphorus recovery. However, the aggregated sludge flocs caused by the coagulation limited sludge hydrolysis and P release during anaerobic digestion process. In this study, cation exchange resin (CER) was employed during anaerobic digestion of Fe-CEPS sludge with aims of prompting P release and carbon recovery. CER addition effectively dispersed the sludge flocs. However, the greater dispersion of sludge flocs could not translate to higher sludge hydrolysis. The maximum hydrolysis and acidification achieved at lower CER dosage of 0.5 g CER/g TS. It was observed that the extents of sludge hydrolysis and acidification had a strongly negative correlation with the organic binding iron (OBI) concentration. The presence of CER during anaerobic digestion favored Fe(III) reduction to Fe(II), and then further induced iron phase transformation, leading to the OBI formation from the released organic matters. Meanwhile, higher CER dosage resulted in higher P release efficiency and the maximum efficiency at 4 g CER/g TS was four times than that of the control. The reduction of BD-P, NaOH-P and HCl-P in solid phase contributed most P release into the supernatant. A new two-stage treatment process was further developed to immigrate the OBI formation and improve the carbon recovery efficiency. Through this process, approximately 45% of P was released, and 63% of carbon was recovered as methane from Fe-CEPS sludge via CER pretreatment.

Volatile fatty acids bio-production using extracellular polymeric substances disengaged from sludge for carbon source recycling

Excessive waste-activated sludge (WAS) and insufficient carbon source (CS) for biological nitrogen removal (BNR) often coexist in municipal sewage treatment. Although the production of volatile fatty acids (VFAs) from WAS has been recognized as a promising solution, the development is limited by low VFAs production efficiency and dewatering deterioration of sludge. This study extracted the extracellular polymeric substances (EPS) from sludge by low-temperature thermal-hydrolysis (LTH) and high-speed hydro-cyclone (HSHC) pretreatment and recovered it for high-quality VFAs bio-production in thermophilic fermentation. Microbial mechanism analysis disclosed that interspecific interaction networks composed of functional flora, which accumulate VFAs by bio-converting EPS primarily and supplemented by EPS synthesis, guaranteed the efficient bio-production of VFAs. This process scheme shows promise in providing alternative denitrification CSs and avoiding deterioration of sludge dewaterability.

Enhanced co-digestion of sewage sludge and food waste using novel electrochemical anaerobic membrane bioreactor (EC-AnMBR)

Membrane fouling remains a big challenge hindering the wide-application of anaerobic membrane bioreactor (AnMBR) technology. In this study, an electrochemical anaerobic membrane bioreactor (EC-AnMBR) was developed by coupling electrochemical regulation to enhance co-digestion of sewage sludge and food waste and mitigate membrane fouling. The highest methane production (0.12 ± 0.02 L/L_reactor/day) and net energy recovery (31.82 kJ/day) were achieved under the optimum conditions of 0.8 V, hydraulic retention time of 10 days and solids retention time of 50 days. Electrochemical regulation accelerated the mineralization of high-molecular-weight organics and reinforced the membrane antifouling ability by inducing electrostatic repulsive force and electrochemical oxidation. Besides, symbiotic relationships among functional microorganisms (Spirochaetes, Methanolinea, etc.) were enhanced, improving the hydrolysis and methanogenesis processes of complex organics and the long-term stability. This study confirms the technical feasibility of EC-AnMBR in treating high-solid biowastes, and provides the fundamental data to support its application in real-world scenarios.

The modified properties of sludge-based biochar with ferric sulfate and its effectiveness in promoting carbon release from particulate organic matter in rural household wastewater

The scarcity of carbon sources presents a significant challenge for the bio-treatment of rural domestic wastewater (RDW). This paper presented an innovative approach to address this issue by investigating the supplementary carbon source through in-situ degradation of particulate organic matter (POM) facilitated by ferric sulfate modified sludge-based biochar (SBC). To prepare SBC, five different contents of ferric sulfate (0%, 10%, 20%, 25%, and 33.3%) were added to sewage sludge. The results revealed that the pore and surface of SBC were enhanced, providing active sites and functional groups to accelerate the biodegradation of protein and polysaccharide. During the 8-day hydrolysis period, the concentration of soluble chemical oxidation demand (SCOD) increased and peaked (1087-1156 mg L^-1) on the fourth day. The C/N ratio increased from 3.50 (control) to 5.39 (25% ferric sulfate). POM was degraded the five dominant phyla, which were Actinobacteriota, Firmicutes, Synergistota, Proteobacteria, and Bacteroidetes. Although the relative abundance of dominant phyla changed, the metabolic pathway remained unchanged. The leachate of SBC (<20% ferric sulfate) was beneficial for microbes, but an excessive amount of ferric sulfate (33.3% ferric sulfate) could have inhibition effects on bacteria. In conclusion, ferric sulfate modified SBC holds the potential for the carbon degradation of POM in RDW, and further improvements should be made in future studies.

Redirecting carbon to recover VFA to facilitate biological short-cut nitrogen removal in wastewater treatment: A critical review

Wastewater treatment plants (WWTPs) are facing a great challenge to transition from energy-intensive to carbon-neutral and energy-efficient systems. Biological nutrient removal (BNR) can be severely impacted by carbon limitation, particularly for wastewater with a low carbon-to-nitrogen (C/N) ratio, which can significantly increase the operational costs. Waste activated sludge (WAS) is a valuable byproduct of WWTPs, as it contains high levels of organic matter that can be utilized to improve BNR management by recovering and reusing the fermentative volatile fatty acids (VFAs). This review provides a comprehensive examination of the recovery and reuse of VFAs in wastewater management, with a particular focus on advancing the preferable biological short-cut nitrogen removal process for carbon-insufficient municipal wastewaters. First, the method of carbon redirection for recovering VFAs was reviewed. Carbon could be captured through the two-stage A/B process or via sludge fermentation with different sludge pretreatment and process control strategies to accelerate sludge hydrolysis and inhibit methanogens to enhance VFA production. Second, VFAs can support the metabolism of autotrophic N-cycling microorganisms involved in wastewater treatment, such as AOB, NOB, anammox, and comammox bacteria. However, VFAs can also cause inhibition at high concentrations, leading to the partition of AOB and NOB; and can promote partial denitrification as an efficient carbon source for heterotrophic denitrifiers. Third, the lab- and pilot-scale engineering practices with different configurations (i.e., A²O, SBR, UASB) were summarized that have shown the feasibility of utilizing the fermentate to achieve superior nitrogen removal performance without the need for external carbon addition. Lastly, the future perspectives on leveraging the relationships between mainstream and sidestream, nitrogen and phosphorus, autotrophs and heterotrophs were given for sustainable and efficient BNR management.

Co-treatment with free nitrous acid and calcium peroxide regulates microbiome and metabolic functions of acidogenesis and methanogenesis in sludge anaerobic digestion

Wasted activated sludge (WAS) is a promising feedstock for carbon management because of its abundance and carbon-neutral features. Currently, the goal is to maximize the energy in WAS and avoid secondary toxic effects or accumulation of harmful substances in the environment. Chemical pretreatment is an effective strategy for enhancing WAS disintegration and production of short chain fatty acids (SCFAs). However, the role of pretreatment in shaping the core microbiome and functional metabolism of anaerobic microorganisms remains obscure. Here, the mechanisms of SCFA synthesis and microbiome response to free nitrous acid (FNA) and calcium peroxide (CaO₂) co-treatment during sludge anaerobic digestion (AD) were investigated. The combination of FNA and CaO₂ enriched acidogenic Macellibacteroides, Petrimonas, and Sedimentibacter to a relative abundance of 15.0%, 10.3%, and 7.3%, respectively, resulting in an apparent increase in SCFA production. Metagenome analysis indicated that FNA + CaO₂ co-treatment facilitated glycolysis, phosphate acetyltransferase-acetate kinase pathway, amino acid metabolism, and acetate transport, but inhibited CO₂ reduction and common pathway of methanogenesis compared with the untreated control. This work provides theoretical insights into the functional activity and interaction of microorganisms with ecological factors.

Optimizing waste activated sludge disintegration by investigating multiple electrochemical pretreatment conditions: Performance, mechanism and modeling

The complex and rigid floc structure often limits the reutilization of waste activated sludge (WAS). Electrochemical pretreatment (EPT) is one of the most effective technologies that can enhance WAS disintegration. But a comprehensive investigation into how multiple EPT conditions work was rarely reported. The study evaluated the effects of multiple EPT conditions, i.e., different electrolytes (NaCl, Na₂SO₄, and CaCl₂), electrolytes dosage (0 g/L, 0.5 g/L, 1.0 g/L, and 3.0 g/L), EPT current (0 A, 0.5 A, 1.0 A, and 3.0 A) and EPT time (0 min, 30 min, 60 min, and 90 min) on WAS disintegration. The results showed that NaCl was outstanding from other electrolytes in promoting more WAS disintegration. Besides, a relatively higher NaCl dosage, a higher EPT current, and a longer EPT time promoted more reactive chlorine species (RCS), thus enhancing WAS disintegration in terms of extracellular polymeric substances (EPS) structure destruction and biodegradable organic matter release. After EPT for 60 min at NaCl dosage of 1.0 g/L and current of 1.0 A, the EPS multilayer structure destruction, biodegradable organic matters release, and soluble chemical oxygen demand (SCOD) increase in the supernatant were enhanced by 17.2 %, 130.5 %, and 238.7 %, respectively. Then a predictive quadratic model was established and the impact significance of the above EPT factors for enhancing WAS disintegration followed dosage of NaCl > current > EPT time. Furthermore, response surface methodology (RSM) suggested NaCl dosage of 2.75 g/L, current of 2.0 A, and EPT time of 30 min were the optimal EPT conditions, bringing a 42.0 % increase in the net economic benefit of WAS treatment compared to without EPT.

Citric acid treatment directly on anaerobic digestor sludge alleviates the inhibitory effect of in-situ generated humic acids by their deconstruction and redistribution

Humic acids were known to inhibit sewage sludge anaerobic digestion (AD) and many studies have been investigated to deal with the inhibitory effect of humic acids (HA) in raw sludge feeding the digestor. However, HA could also be in-situ produced during the sludge AD process with a significant amount, but the relieving of the inhibitory effect posed by the in-situ produced HA has been historically ignored. Thus, this study attempted to use citric acid (CA), an easily biodegradable polyatomic acid, to directly treat sludge in the anaerobic digestor instead of the feeding sludge, for alleviating inhibition caused by the in-situ produced HA. Results showed that methane production of AD batches with CA pretreatment at 0.06∼0.10 g/g TS on digestor sludge were effectively enhanced, with the highest production being 2.89 times of that observed in control AD without CA treatment and 2.08 times of that achieved by contrast AD with CA pretreatment on raw sludge. However, the AD batches with CA treatment at low dosages of 0.02∼0.04 g/g TS had lower methane production, during which sludge solubilization and hydrolysis were enhanced initially associated with released HA surfactant and hydrolytic enzymes; then later inhibited since higher structurally sound HA in SP easily rebind proteinaceous substrates and enzymes again. Whilst CA treatment at 0.06∼0.10 g/g TS could promote sludge solubilization and hydrolysis throughout the sludge AD process. This dosage-dependent effect of CA treatment can be caused by its discriminating impact on HA structure and distribution. At low dosages, complexes of HA-proteins (including hydrolases) can be decrosslinked and transferred to the soluble phase (SP), thus enhancing solubilization and hydrolysis initially. However, the released HA in SP could rebind more organics and hydrolases during AD, resulting in augmented inhibition. While the dosage increased, HA were drastically deconstructed with aromatic cores in SP migrating to extracellular polymeric substances, preventing binding effects on organics and hydrolases. Accordingly, a smart mutex-switch operational strategy of CA treatment was proposed. The results suggested, to alleviate the inhibitory effect of HA on sludge AD, more attention should be given to HA produced and accumulated in AD rather than raw sludge so that the CA treatment directly on sludge in the digestor can be a promising method.

Carbon source recovery from waste sludge reduces greenhouse gas emissions in a pilot-scale industrial wastewater treatment plant

Carbon cycle regulation and greenhouse gas (GHG) emission abatement within wastewater treatment plants (WWTPs) can theoretically improve sustainability. Currently, however, large amounts of external carbon sources used for deep nitrogen removal and waste sludge disposal aggravate the carbon footprint of most WWTPs. In this pilot-scale study, considerable carbon was preliminarily recovered from primary sludge (PS) through short-term (five days) acidogenic fermentation and subsequently utilized on-site for denitrification in a wool processing industrial WWTP. The recovered sludge-derived carbon sources were excellent electron donors that could be used as additional carbon supplements for commercial glucose to enhance denitrification. Additionally, improvements in carbon and nitrogen flow further contributed to GHG emission abatement. Overall, a 9.1% reduction in sludge volatile solids was achieved from carbon recovery, which offset 57.4% of external carbon sources, and the indirect GHG emissions of the target industrial WWTP were reduced by 8.05%. This study demonstrates that optimizing the allocation of carbon mass flow within a WWTP has numerous benefits.

Successive choline addition enhancing the methanogenesis of waste activated sludge anaerobic digestion: Insight from hydrophilicity, electrochemical performance and microbial community

Anaerobic digestion (AD) is a promising technology to treat waste-activated sludge, previous study proved that methane production could be enhanced with the addition of choline, this work aimed to solve the problem of rapid biodegradability of choline in the AD process by changing its dosing method. With 0.75 g/L as the optimal choline dosing concentration, experimental results showed that successive choline dosing during the first 3-6 days of AD (experimental groups, EGs) performed better than the single dosing. The accumulative biogas production in EGs was increased by 35.55-36.73%, which could be caused by the simultaneous promotion of hydrolysis-acidification and methanogenesis processes. Especially, the electron exchange capacity of digested sludge in EGs was increased by 16.71-34.58%. In addition, the surface Gibbs free energy (△G_SL) of sludge in EGs was 105.51-172.21% higher (corresponding to stronger hydrophilicity and repulsion), which might help disperse sludge flocs and improve mass transfer efficiency, and the △G_SL values were positively correlated with the accumulative methane production (R² = 0.7029). Microbiological analysis showed that microbial communities in EGs were richer and Methanosaeta was regarded as the dominant species with 15.93-30.08% higher relative abundance with choline addition. According to Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, EGs were found to be more active in metabolism clusters. Collectively, these findings demonstrated that successive choline dosing during the first 3-6 days is an effective and novel method to enhance methane production in AD process.

Untangling the effect of solids content on thermal-alkali pre-treatment and anaerobic digestion of sludge

Total solids (TS) content is critical for thermal hydrolysis and anaerobic digestion (AD) performance, but its role in thermal-alkaline pre-treatment (TAP) is unclear. Therefore, this study aimed to reveal the key role of TS content in TAP and AD of waste activated sludge. The results showed that the optimum TS content of TAP (at 90 °C for 1 h, pH = 10) was 8 %. Sludge disintegration and methane production increased from 19.7 ± 2.2 % to 34.3 ± 2.9 % and from 167.4 ± 4.2 to 246.0 ± 6.2 mL/g volatile solids, respectively, when TS content were increased from 2 % to 8 %. A high TS content will likely promote sludge disintegration since it will reduce heat loss and improve heating efficiency. Additionally, increasing TS content from 2 % to 10 % minimized the production of intracellular reactive oxygen species by 30.4 ± 0.7 % and increased cell viability by 11.5 ± 2.6 %. In contrast, excessive TS content (i.e., ≥10 %) deteriorated the fluidity of sludge, which prevents it from disintegration. Once TS reached 10 %, the accumulation of ammonia nitrogen and volatile fatty acids reached 812.7 ± 27.4 and 1932.0 ± 5.3 mg/L, respectively, which reduced the activity of acidulase and coenzyme F420 and shifted the archaeal community from acetylotrophic to hydrogenotrophic methanogens. This article provides new insights into the TS content in TAP and AD technology.

Enhanced anaerobic digestion of waste activated sludge with periodate-based pretreatment

The potential of periodate (PI) in sludge anaerobic digestion is not tapped, although it has recently attracted great research interest in organic contaminants removal and pathogens inactivation in wastewater treatment. This is the first work to demonstrate significant improvement in methane generation from waste activated sludge (WAS) with PI pretreatment and to provide underlying mechanisms. Biochemical methane potential tests indicated that methane yield enhanced from 100.2 to 146.3 L per kg VS (VS, volatile solids) with PI dosages from 0 to 100 mg per g TS (TS, total solids). Electron spin resonance showed PI could be activated without extra activator addition, which might be attributed to the native transition metals (e.g., Fe²⁺) in WAS, thereby generating hydroxyl radical (•OH), superoxide radicals (•O₂ ^-), and singlet oxygen (¹O₂). Further scavenging tests demonstrated all of them synergistically promoted WAS disintegration, and their contributions were in the order of •O₂ ^- > •OH > ¹O₂, leading to the release of substantial biodegradable substances (i.e., proteins and polysaccharides) into the liquid phase for subsequent biotransformation. Moreover, fluorescence and ultraviolet spectroscopy analyses indicated the recalcitrant organics (especially lignocellulose and humus) could be degraded by reducing their aromaticity under oxidative stress of PI, thus readily for methanogenesis. Microbial community analysis revealed some microorganisms participating in hydrolysis, acidogenesis, and acetoclastic methanogenesis were enriched after PI pretreatment. The improved key enzyme activities and up-regulated metabolic pathways further provided direct evidence for enhanced methane production. This research was expected to broaden the application scope of PI and provide more diverse pretreatment choices for energy recovery through anaerobic digestion.

Impact of food waste composition on acidogenic co-fermentation with waste activated sludge

The impact of food waste (FW) composition on co-fermentation performance was studied to elucidate if adjusting FW composition can be used to drive the fermentation yield and profile, which is relevant for biorefinery applications. First, the impact of individual FW components (i.e., fruit, vegetables, pasta, rice, meat, fish, and cellulose) was assessed. Subsequently, the effect of mixing a protein-rich component and a carbohydrate-rich component was studied (i.e., fish/fruit and fish/cellulose, and meat/rice and meat/vegetable). All experiments were carried out in mesophilic batch assays using waste activated sludge (WAS) as main substrate, the same mixture ratio (70 % WAS +30 % FW on VS basis), and no pH control. Results showed that each FW component had a distinct effect on VFA yield and profile, with protein-rich components reaching the highest VFA yields; 502 and 442 mgCOD/gVS for WAS/Fish and WAS/Meat, respectively. A positive interaction on VFA yield was observed when mixing a protein-rich and a carbohydrate-rich component. This interaction was not proportional to the co-substrates proportion in the mixtures. On the other hand, the VFA profile was clearly driven by the components in the mixture, including both WAS and FW composition. Overall, these results indicate that predicting the VFA yield of WAS/FW co-fermentation is not just related to FW composition, but FW composition could be used to adjust the VFA profile to a certain extent.

Deciphering the underlying mechanism of MOF-808-based abiotic catalysis enhancing biodegradability of waste activated sludge: Insights from the effects on bioconversion of extracellular organic substances into methane

Extracellular organic substances (EOSs) usually control sludge biodegradability. Metal-organic framework-808 (MOF-808), a catalyst with a high ability of catalytic hydrolysis and proton transfer, has great potential to enhance biodegradability of EOSs. In this study, the underlying mechanism of MOF-808 enhancing the biodegradability of EOSs via abiotic catalysis was investigated. Experimental results showed that protein-like and humic acid-like substances were the main organic components in EOSs, and the MOF-808 enhanced the disintegration of protein-like substances rather than humic acid-like substances. Analyses of the changes in the functional group, the secondary structure of protein-like substances, and the electron transfer of catalytically degraded EOS samples revealed that the MOF-808 enhanced the hydrolysis of EOSs protein-like substances, loosened their secondary structure, and improved the electron transfer in EOSs. Further analyses of the MOF-808 before and after the catalysis reaction revealed that the coordination of Zr sites with protein-like substance-specific active sites (such as ZrN) in the MOF-808 substantially contributed to the high-efficiency catalysis. Biochemical methane potential assays confirmed that MOF-808-based abiotic catalysis promoted the generation efficiency of methane from the EOSs. These findings can elucidate the abiotic catalytic effect of MOF-808 on sludge biodegradability during the anaerobic digestion process.

Enhancing phosphorus recovery from efficient acidogenic fermentation of waste activated sludge with acidic cation exchange resin pretreatment: Insights from occurrence states and transformation

Achieving phosphorus (P) recovery during treatment and disposal of waste activated sludge (WAS) by anaerobic-based processes has received increasing attention. To solve the problem of low phosphorus release efficiency, anaerobic fermentation (AF) combined with acidic cation exchange resin (ACER) pretreatment was first proposed in this study. Results showed that the isoelectric point pretreatment with ACER increased the recoverable phosphorus content by 2.3 times compared to that without ACER pretreatment. Phosphorus transformation was systematically analyzed from a whole-process perspective, and the results visually revealed that the release of phosphorus during the conventional AF process (without ACER pretreatment) was limited by insufficient phosphorus release from extracellular polymeric substances (EPS) and mineral precipitation, as well as the reprecipitation of soluble phosphorus with metals. ACER enabled effective dissolution of mineral phosphorus by acidifying WAS. On the other hand, ACER adsorbed metals to promote EPS disintegration and hydrolysis, thereby enhancing the release of EPS-bound P, which also reduced the reprecipitation of soluble phosphorus during AF. Furthermore, ACER pretreatment increased volatile fatty acids production by >2-fold with enhanced sludge hydrolysis. This finding has important implications for both non-renewable phosphorus recovery and sludge resource recovery.

Comprehensive insights into the impact of pretreatment on anaerobic digestion of waste active sludge from perspectives of organic matter composition, thermodynamics, and multi-omics

Although various pretreatments have been applied to promote the anaerobic digestion of waste active sludge (WAS), the mechanisms regarding the impact of pretreatment on anaerobic digestion have not been well addressed. In this study, the effects of acid, alkali, and thermal pretreatments on anaerobic digestion of WAS were comprehensively investigated from the perspectives of organic matter composition, thermodynamics, and multi-omics. Results showed acid, alkali, and thermal pretreatments increased the methane production potential of WAS by 53.7%, 98.2%, and 101.8%, respectively, compared with the control. The protein secondary structure was disrupted after pretreatment, with a shift from α-helix and β-sheet to random coil and antiparallel β-sheet/aggregated strands. Thermodynamically, the WAS flocculation process was controlled by the short-range interfacial interactions described by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, which was positively correlated (R = 0.97, p < 0.05) with the organic matter solubilization of the WAS. After pretreatment, the flocculation energy barrier of pretreated WAS was 4.1 (acid), 7.0 (alkali) and 7.1 (thermal) times higher than that of the control group, respectively. Multi-omics analysis confirmed that pretreatment promoted amino acids (tryptophan, tyrosine, phenylalanine, aspartate, glutamate) metabolism, energy metabolism (ABC transporters) and vitamin metabolism. Moreover, the comparison of upregulated differentially expressed proteins (DEPs) revealed that for amino acid metabolism, thermal treatment had the best promotion effect; for carbohydrate metabolism, alkali treatment had the best promotion effect; and for lipid metabolism, acid treatment was more advantageous, resulting in different anaerobic digestion efficiencies. This study provides an in-depth understanding of the impact of different pretreatments on WAS anaerobic digestion and has practical implication for the choice of proper pretreatment technology for biosolids.

A novel application of dissolved ozone flotation on sewage sludge thickening: Performance and mechanism investigation

Sludge thickening in sewage treatment plants is an essential step to reduce the sewage sludge volume and provide space for collaborative anaerobic digestion of sludge and other urban organic wastes. Dissolved ozone flotation (DOF) is a novel choice worthy of application in the field of sludge thickening. In order to investigate the effect of DOF thickening, the total solid content (TS %) was used to characterize the thickening performance under different O₃ dosage. The optimal condition was determined to be polyacrylamide (PAM) dosage = 3 ‰ TS, air floatation time = 2 h and O₃ dosage = 12 mg/g TS, under which the TS % of raw-sewage sludge (RS) increased from 0.33 ± 0.01 % to 8.03 ± 0.06 %. In this study, the relationship between the sludge thickening performance, physicochemical properties, and the changes of organic matter (content, structure and molecular weight) in extracellular polymers (EPS) was systematically clarified. The results indicated that the DOF couple with PAM could change the sludge surface properties, destroy the sludge floc structure, release intracellular organic matter, and increase moisture fluidity. The surface hydrophilicity/hydrophobicity, protein (PN) secondary structure and moisture distribution were mainly responsible for sludge thickening performance. Moreover, the change of TS % during the DOF thickening process was mainly caused by the variations of the organic matter content in EMPS layer. The identification of key influencing factors was conducive to the further regulation and upgrading of the novel application for enhanced sludge thickening in sewage treatment plants.

Metagenomic insights into improving mechanisms of Fe0 nanoparticles on volatile fatty acids production from potato peel waste anaerobic fermentation

The management of potato peel waste (PPW) has been a challenge faced by the potato industry. This investigation assessed the feasibility of PPW for volatile fatty acids (VFAs) production via anaerobic fermentation, and investigated the impact of Fe⁰ nanoparticles (Fe⁰ NPs) supplementation on the VFAs production. It is found that PPW is a potential feedstock for producing VFAs, achieving a yield of 480.4 mg COD/g-vS Meanwhile, the supplementation of Fe⁰ NPs significantly promoted the VFAs productivity and quality. The higher enrichment of VFAs-producing bacteria, including Clostridium, Proteiniphilum, Fonticella and Pygmaiobacter, contributed to the promotion of the VFAs yield. Furthermore, metagenomic analysis revealed that the encoding genes responsible for carbohydrate metabolism (especially starch), membrane transport, glycolysis and the formation of acetic and butyric acids were remarkably up-regulated,which could be the essential reason for the enhanced metabolic activity and VFAs productivity. This work provides a promising strategy for recycling PPW.

Electrochemical pretreatment enhancing co-fermentation of waste activated sludge and food waste into volatile fatty acids: Performance, microbial community dynamics and metabolism

Waste activated sludge (WAS) has low biodegradability that restricts acidogenic fermentation (AF), thereby limiting the high-value volatile fatty acids (VFAs) production. This study investigated an alternative electrochemical pretreatment (EPT) approach that can facilitate AF of WAS and food waste (FW) and therefore enhance VFAs production. The results showed through introducing 50 % volatile solid basis of FW (containing massive chloride) into WAS, a 60-min EPT produced reactive chlorine species (RCS), which diffused into WAS-FW inner layers resulting in cell lysis, therefore significantly promoted and accelerated WAS-FW disintegration, contributing to more soluble and biodegradable dissolved organic matter (DOM). Then during the subsequent 15-day acidogenic co-fermentation (Co-AF), the residual RCS (approximate 5 mg Cl₂/L) also caused acidogenic bacteria (including Prevotella_7, Lactobacillus and Veillonella) gradually outcompeted methanogens due to their different tolerance to residual RCS. Consequently, the maximum VFAs yield of the WAS-FW Co-AF with EPT was 40.8 % higher than WAS-AF without EPT.

Regulation of exogenous acyl homoserine lactones on sludge settling performance: Monitoring via ultrasonic time-domain reflectometry

Despite extensive studies, a comprehensive solution for sludge bulking has not yet been found. This study improves the sludge settling performance via quorum sensing (QS) by adding exogenous acyl homoserine lactones (AHLs). First, a novel approach based on ultrasonic time-domain reflectometry, which can automatically and in-situ assess a sludge volume index (SVI), was developed using the displacement in the ultrasonic spectra as a feasible indicator (R² = 0.98, p < 0.01). Next, the effects of typical AHLs, i.e., 3OC6-HSL, C12-HSL, and 3OC14-HSL, on sludge settling properties were investigated. Results indicated that the three AHLs significantly promoted the sludge settleability by 1.90, 2.03, and 1.62 times, respectively. The regulation mechanisms were investigated from the perspective of sludge physicochemical properties and biological community interactions. The draining degree of water to extracellular polymeric substances (EPS) significantly increased (p < 0.05) with all three AHLs. Meanwhile, the hydrophobic tryptophan content increased with the addition of 3OC6-HSL and C12-HSL. Hence, EPS hydrophobicity was promoted, which is conducive to microbial aggregation. In addition, molecular ecological networks of activated sludge (AS) indicated that bacterial community structures were more complex and species interactions were more intense when adding 3OC6-HSL and C12-HSL. Meanwhile, additional keystones were identified, with the proportion of QS species increasing by 63.6% and 22.2%, respectively. Exogenous 3OC6-HSL eventually decreased the gross relative abundance of filamentous bacteria by 2.37%. Overall, appropriate AHLs could enhance community stability and microbial cooperation by strengthening the communication hub role of QS species, thereby suppressing the overgrowth of filamentous bacteria and improving the sludge settleability. This study provides an effective strategy to determine the appropriate AHL to rapidly eliminate filamentous bulking problems.

Simultaneous enhancing phosphorus recovery and volatile fatty acids production during anaerobic fermentation of sewage sludge with peroxydisulfate pre-oxidation

Phosphorus release and sludge hydrolysis are the keys for phosphorus and carbon recovery from sewage sludge via anaerobic process. In this study, iron-rich sludge (a common phosphorus-rich sewage sludge) was pre-oxidized by heat-activated peroxydisulfate (PDS) to enhance volatile fatty acids (VFAs) production and iron-bound phosphorus (Fe-P) release during anaerobic fermentation (AF). With low-dosage PDS pre-oxidation (33.75 mg/g total solids), the concentration of recoverable phosphorus increased by 49.3% than that noted in control along with enhanced VFAs production after 4 days. This is mainly because PDS oxidation not only effectively disintegrated sludge, but also generated sulfate simultaneously. Sludge disintegration enhanced organic matter hydrolysis, promoting VFAs yield, while sulfate was reduced to sulfide during AF and precipitated with iron, leading to Fe-P release. The application of PDS pre-oxidation on iron-rich sludge could not only improve the resourcefulness of sludge but also reduce secondary pollution (sulfate or hydrogen sulfide).

Novel anaerobic digestion of waste activated sludge via isoelectric-point pretreatment: Ultra-short solids retention time and high methane yield

The prolonged reaction period and low methane yield have become a pivotal bottleneck in the anaerobic digestion (AD) of waste activated sludge (WAS), severely limiting its use in bioenergy. This study evaluated the long-term semi-continuous AD of WAS with isoelectric-point (pI) pretreatment in terms of the reaction period, methane yield, material resource recovery, and economic and environmental benefits. The experimental results show that after pI pretreatment, at the same solids retention time (SRT), the average methane yield and methane content in the biogas of WAS increased by 120.9% and 15.5%, respectively. The average methane yield from AD with pI pretreatment at a 5-day SRT was 1.6 times that of the AD without pI pretreatment at a 15-day SRT, indicating that pI pretreatment significantly (P < 0.001) increased the methane yield from the AD of WAS at different SRTs, even at an ultra-short SRT. Statistical analyses of the changes in the concentrations of soluble organic matter in the AD process with and without pI pretreatment confirmed that pI pretreatment enhanced the biochemical reactions related to the transformation of sludge organic matter during the whole AD process, but not the initial organic solubilisation and hydrolysis of the sludge. Furthermore, pI pretreatment recovered the NaHCO₃, Na₂S, and multivalent metals from the sludge, indicating that AD with pI pretreatment efficiently recovered both bioenergy and material resources from the sludge. Economic and environmental analyses of AD with and without pI pretreatment further indicated that with pI pretreatment, the annual economic benefit of AD increases by 162% and carbon emissions decreases by approximately 2.63 × 10⁷ kg CO₂/year compared with conventional AD. These findings serve as an important reference for the development of a novel AD technology for sludge that incorporates second-generation pretreatment and operates at an ultra-short SRT.

Polydimethyldiallylammonium chloride induces oxidative stress in anaerobic digestion of waste activated sludge

The effects and key mechanisms of polydimethyldiallylammonium chloride on anaerobic digestion of waste activated sludge were investigated. Polydimethyldiallylammonium chloride at 38.1 g/kg total solids substantially reduced cumulative methane production from 138.2 ± 5.5 to 49.4 ± 5.0 L CH₄/kg volatile solids added, a reduction of 64.3 ± 0.2%. The quaternary ammonium groups on polydimethyldiallylammonium chloride agglomerated sludge flocs by neutralizing negatively charged amino groups in in extracellular polymeric substances, which hindered the release of organic matter. Quaternary ammonium groups induce oxidative stress by inducing the production of reactive oxygen species, thereby inhibiting the activity of anaerobic digestive enzymes. In addition, quaternary amine groups reduced the abundance of hydrolyzing bacteria, acidifying bacteria, and acetylotrophic methanogens. Oxidative stress could be an underappreciated mechanism that quaternary ammonium groups deteriorate anaerobic digestion, which could be transformative for understanding the potential risks of quaternary ammonium cationic flocculants in biological sludge treatment.

Cation exchange resin pretreatment enhancing methane production from anaerobic digestion of waste activated sludge

The application of anaerobic digestion (AD) to treat waste activated sludge (WAS) still exhibits some limitations, such as low methane production. In this study, cation exchange resin (CER) pretreatment was explored to enhance the efficiency of the AD of WAS. Based on the response surface methodology, the optimal conditions for CER pretreatment were reaction time of 7.4 h, 33.8 g CER (wet weight) /g volatile solids and sludge total solids of 2.4%. Under these optimal CER pretreatment conditions, approximately 30% of metals were removed from the WAS, particularly organic-binding metals. This metal removal disrupted the structures of extracellular polymer substances and led to sludge deflocculation, thereby releasing large amounts of organic substances from the sludge solids. Batch AD experiments showed that CER pretreatment increased the maximal production of volatile fatty acids and methane by 565.7% and 80.5%, respectively. Additionally, CER pretreatment promoted each stage of AD (i.e. solubilisation, hydrolysis, acidification and methanation) and the corresponding activities of key enzymes. Experimental results for semi-continuous AD further confirmed that CER pretreatment enhanced the proportion of methane in the biogas (from 62.75 ± 2.14% to 73.96 ± 0.99%) and the production of methane. An analysis of changes in the microbial communities demonstrated that CER pretreatment enhanced the abundance of microorganisms involved in hydrolysis, acidification and acetification and changed the major methanogenic pathway from acetoclastic methanogens to methylotrophic methanogens. These findings are expected to provide a reference for developing new pretreatment methods for enhancing anaerobic biodegradability of organic matters.

Enhancing short-term ethanol-type fermentation of waste activated sludge by adding saccharomycetes and the implications for bioenergy and resource recovery

Ethanol-type sludge fermentation has recently attracted much attention because it can enhance direct interspecies electron transfer and thus improve the anaerobic digestion of waste activated sludge (WAS). In this paper, the enhancement of short-term ethanol-type fermentation of WAS via adding Saccharomyces was investigated. The experimental results show that the maximum ethanol production of 1030.8 ± 20.6 mg/L was achieved, with the optimum fermentation conditions of a pH of 5.1, temperature of 26.0 ℃ and time of 8.0 hr. Although the content of volatile fatty acid (VFA) increased within 10 hr, it is one order of magnitude lower than the content of ethanol, indicating that the VFA generation did not affect the efficient production of ethanol. The analyses of changes in the microbial community during the fermentation process demonstrate that the greatest Saccharomyces activity occurred in the first 8 hr and it can play an important role in ethanol production even at a very low relative abundance. Meanwhile, most typical acid-producing bacteria were inhibited, but the hydrogenotrophic methanogens (i.e., Methanobacterium) were enriched to a certain extent. Further statistical analyses reveal that the Rhodobacter, Thermomonas, Terrimonas and Saccharomyces are responsible for ethanol production during the fermentation. However, these findings not only provide a reference for the development of enhancing ethanol-type fermentation of sludge, but also are expected to provide a new way of thinking for the efficient bioenergy and resource recovery from sludge.

Implications for mitigation of antibiotic resistance: Differential response of intracellular and extracellular antibiotic resistance genes to sludge fermentation coupled with thermal hydrolysis

Thermal hydrolysis pretreatment (THP) can effectively remove the antibiotic resistance genes (ARGs) from sewage sludge, but the rebounding effects in the subsequent anaerobic fermentation are often observed. The purpose of this study was to elucidate the distribution and fate of intracellular and extracellular ARGs (iARGs and eARGs) in the sludge acidogenic fermentation process coupled with THP. Our results revealed that THP significantly reduced the absolute abundance of total ARGs in raw sludge but increased eARGs by 0.3-1.4 log units under practical conditions (140 °C for 30 min). There is no significant difference in the removal of total ARGs between the two acidogenic fermenters receiving raw and hydrolyzed sludge, with iARGs prevailing in the produced biosolids. The succession of bacterial community and the co-occurrence relationships among ARG type, mobile genetic elements and bacterial taxa were observed, suggesting a phylogenetic basis for the iARGs patterns in fermented sludge. However, eARGs were susceptible to biodegradation with a half-life of 2.34 h and they contributed limitedly to the ARGs propagation through transformation. These findings suggest an emphasis on the mitigation of iARGs during the acidogenic fermentation of sludge, which would be achieved by lowering the richness and physicochemical destruction of potential hosts.

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).

Secondary acidogenic fermentation of waste activated sludge via voltage supplementation: Insights from sludge structure and enzymes activity

Microbial electrolysis cells were integrated with the anaerobic digestion at different fermentation stage (0th day and 30th day) to explore the bio-electrochemical enhancement of acidogenic fermentation from waste activated sludge. Results showed that significant increases in volatile fatty acid production can be achieved by electrically-assisted acidogenic fermentation (0th day to 12th day). In comparison, volatile fatty acid production during secondary acidogenic fermentation (30th day to 42nd day) via voltage supplementation was also investigated. The concentrations of soluble total organic carbon, soluble protein, soluble polysaccharide via voltage supplementation during the secondary acidogenic fermentation process were improved from 69.9, 50.3, and 18.8 mg/L to 260.6, 135.6, and 43.8 mg/L, respectively. Meanwhile, fractal dimension (D_f) value was decreased via voltage supplementation along with the significantly improving of protease and α-glucosidase activities. These results suggest that the presence of voltage brought a secondary solubilization and hydrolysis of sludge via loosening sludge structure and promoting corresponding enzymes activities, thus improved the secondary acidogenic fermentation performance of sludge.

Propionic acid-rich fermentation (PARF) production from organic wastes: A review

Nowadays, increasing attention has been drawn to biological valorization of organic wastes. Wherein, propionic acid-rich fermentation (PARF) has become a focal point of research. The objective of this review is to make a thorough investigation on the potential of PARF production and give future outlook. By discussing the key factors affecting PARF including substrate types, pH, temperature, retention time, etc., and various improving methods to enhance PARF including different pretreatments, inoculation optimization and immobilization, a comprehensive summary on how to achieve PARF from organic waste is presented. Then, current application of PARF liquid is concluded, which is found to play an essential role in the efficient denitrification and phosphorus removal of wastewater and preparation of microbial lipids. Finally, the environmental performance of PARF production is reviewed through life cycle assessment studies, and environmentally sensitive sectors are summarized for process optimization, providing a reference for waste management in low carbon scenarios.

Development of a novel partial nitrification, fermentation-based double denitrification bioprocess (PN-F-Double/DN) to simultaneous treatment of mature landfill leachate and waste activated sludge

Introducing fermentation technology into sewage treatment is a sustainable development concept, but future application still faces many challenges. A novel partial nitrification, fermentation-based double denitrification bioprocess (PN-F-Double/DN) was achieved in three separated SBR type reactors, simultaneously treating high ammonia (1766.6 mg/L) mature landfill leachate and external waste activated sludge (WAS, MLSS = 20.6 g/L). Firstly, NH₄⁺-N was oxidized to NO₂^--N in partial nitrification reactor (PN-SBR), with nitrite accumulation ratio (NAR) of 96.5%. Next, the PN-SBR effluent (NO₂^--N = 1529.8 mg/L) coupled with the WAS were introduced to an anoxic reactor for integrated fermentation-denitrification (IFD-SBR). The occurrence of fermentation was mainly attributed to free nitrous acid (FNA, nitrite protonate form) promoting the splitting decomposition of sludge spatial configuration and interfacial forces. The released volatile fatty acids (VFAs) were utilized in situ during the denitrification process (NO₂^--N→N₂), obtaining 0.6 kg/m³•d nitrogen removal rate and 3.3 kg/m³•d sludge reduction rate. Finally, undesirable fermentation by-products from IFD-SBR (NH₄⁺-N = 119.2 mg/L) were further removed in the endogenous post-denitrification reactor (EPD-SBR) through operational strategy of anaerobic/aerobic/anoxic by residual VFAs as the carbon source. In the EPD-SBR, Defluviicoccus (0.9%) and Candidatus Competibacter (5.8%) dominated carbon source storage and nitrogen removal, acting as a typical denitrifying glycogen-accumulating organism (DGAO), with an intracellular carbon storage efficiency of 83.1% and nitrogen removal contribution of 93.7%. After 200 days of operation, the PN-F-Double/DN process provided effluent containing, on average, 1.86 mg/L NH₄⁺-N and 5.5 mg/L NO_x^--N, with 98.5% TN removal. Compared with traditional bioprocesses, PN-F-Double/DN allowed up to 25% saving in aeration energy consumption, 100% decrease in carbon source demand, and achieve 46.1% external WAS reduction.

Enhancing methanogenic fermentation of waste activated sludge via isoelectric-point pretreatment: Insights from interfacial thermodynamics, electron transfer and microbial community

The usefulness of waste activated sludge (WAS) as an energy source is limited by the poor generation efficiency of methane from WAS, which is mainly due to the complex nature of sludge and low abundance of functional microbes. In this study, the interfacial thermodynamics, electron transfer and microbial community of sludge were investigated to reveal the enhancing effects of isoelectric-point (pI) pretreatment on the efficiency of methane generation from WAS. Experimentally, after pI pretreatment, the methane production potential, maximum methane production rate and maximum methane proportion in the biogas increased by 122.2%, 154.4% and 17.4%, respectively, indicating that pI pretreatment enhanced the generation efficiency of methane. Analyses of changes in the solid-liquid interfacial non-covalent interaction energy, electron transfer capacity (ETC) and reductive peak potential values of sludge samples with and without pI pretreatment during a 170-day methanogenic fermentation period revealed that pI pretreatment enhanced the self-driven solid-liquid interfacial hydrophobic attractions of sludge, increased the abiotic driving forces of interfacial enzymatic reactions, promoted the electron transfer efficiency and lowered the barrier of the reduction reaction. It was thus hypothesised that these changes would be responsible for increasing methane production, which was confirmed by the correlation analyses between the interfacial free energy (IFE) and ETC versus daily methane production. Moreover, statistical analyses of the differences between the microbial communities of sludge samples with and without pI pretreatment during fermentation demonstrated that pI pretreatment significantly (P < 0.05) improved the relative abundances of the main functional microbes with respect to hydrolysis, acidification and methanation. A further investigation of the relationships of IFE and ETC with the relative abundances of the main genera of methanogens indicated that the hydrophobic attraction of sludge surface and a high ETC are conducive to the enrichment of hydrogenotrophic methanogens (+29.9%). These findings are expected to provide a conceptual framework for developing second-generation pretreatment methods and provide a methodological reference for revealing the details of the 'black-box' anaerobic digestion process.

Compact wastewater treatment process based on abiotic nitrogen management achieved high-rate and facile pollutants removal

Chemically enhanced primary treatment (CEPT), ammonium ion exchange and regeneration (AIR) and membrane bioreactor (MBR) were coupled as CAIRM to treat domestic wastewater compactly and efficiently. CAIRM achieved efficient removal of chemical oxygen demand, ammonia nitrogen, total nitrogen (TN) and total phosphorus with total hydraulic retention time of 4.6 h, and obtained 2.3 ± 0.9 mg/L TN in the effluent. CEPT removed phosphate and impurities and prevented AIR from pollution. AIR maintained excellent nitrogen removal with a slight decrease in the exchange capacity of ion exchangers. MBR polished the effluent from AIR, and the larger particle size and better dewaterability of sludge mitigated the membrane fouling. Many heterotrophic genera, such as Rhodobacter and Defluviimonas, were enriched in the oligotrophic MBR. This study demonstrates the viability and stability of CAIRM in efficient wastewater treatment, which will address critical challenges in insufficient nitrogen removal and high land occupancy of current processes.

AHLs-mediated quorum sensing threshold and its response towards initial adhesion of wastewater biofilms

Quorum sensing (QS) plays a crucial role during initial biofilm formation, however the QS threshold and the response of biofilm formation towards N-acyl-homoserine lactones (AHLs) remains largely unknown due to the limitation of nondestructive online methods for monitoring bacterial adherence and the complexity of QS system, which limits the application of QS signal reagents in biofilm reactors. In this study, bacterial QS threshold and its response of biofilm formation to AHLs in purely cultured Sphingomonas rubra biofilm as well as in three different wastewater biofilms #1-3 were investigated via real time cell analysis (RTCA). The main perspective was to study the biomass adherence in response to 12 different forms of AHLs at different concentrations. Results showed that bacterial adhesion was significantly improved by exogenous AHLs with the maximum increase of 2.26-, 2.36-, 2.52-, and 2.80- times biomass production in the four respective biofilms. Although the preferred form of AHL differed for various biofilms, the long-chain AHLs (12-14 carbons) resulted in an overall improvement of bacterial adhesion due to their stronger hydrophobicity and hydrolysis resistance. In addition, bacterial QS threshold of AHLs was observed to have a wide range of concentration from 10 ng/L to 10 μg/L. Meanwhile, QS response time to AHLs also showed a significant difference in different biofilms. Biofilm #2 inoculated with bulking sludge had lower QS threshold of 10 ng/L and faster response to most AHLs that is less than 6 h. Thus, considering the improvement of biofilm adhesion by AHLs, 10 ng/L of C12-HSL, 10 ng/L of C12-HSL, and 10 ng/L of C6-HSL were preferentially selected for wastewater biofilms #1-3 respectively. Unexpectedly, adding high-concentration of AHLs detected in sludges did not significantly improved the bacterial adhesion. Infact the addition of these AHLs at low concentrations or even undetected concentrations substantially improved bacterial adhesion, which could be explained by bacterial communities composition. According to the Pearson correlation analysis, 62% of the top 50 most abundant genera in bacterial communities were significantly negatively related to the response time of multiple AHLs, representing their fast QS response. The QS bacteria, Dechloromonas and Nitrospira have fast QS response for C4-HSL and C8-HSL while, Comamonadaceae has fast QS response for 3OC8-HSL, 3OC10-HSL, 3OC12-HSL, and 3OC14-HSL. In contrast, the rest 38% of the top most abundant genera, such as Ferruginibacter, Hyphomicrobium, and Terrimonas quickly responded to only one AHL, showing significant negative relationship with the response time of C6-HSL. Overall, this study provides an effective and convenient means to select appropriate AHL reagents to promote bacterial adhesion in biofilm systems. Moreover, it also suggests that exogenous AHLs may be useful in improving the settling property of bulking sludge.

The influence of a stepwise pH increase on volatile fatty acids production and phosphorus release during Al-waste activated sludge fermentation

In this study, the performance of volatile fatty acids (VFAs) production and phosphorus (P) release during Al-waste activated sludge (Al-WAS) anaerobic fermentation with stepwise pH increases from 8 to 11 was investigated via a long-term acclimation strategy. As results, VFAs concentration increased with increasing pH and the maximum yield of VFAs was 358.03 mg-COD/g VS at pH 11, which was much higher than at pH 8. P was also released during the process, and the P concentration increased gradually from 26 mg/L at pH 8 to 186 mg/L at pH 11, accounting for 35.8% of the total P in the Al-WAS. The P distribution results demonstrated the dissolution of non-apatite inorganic phosphorus (NAIP) and organic P in the sludge contributed to release P into the liquid at pH 8, 9, and 10, while inorganic P release originated from the dissolution of NAIP at pH 11.