How Does Chitosan Affect Methane Production in Anaerobic Digestion?

Interaction of poly dimethyl diallyl ammonium chloride with sludge components: Anaerobic digestion performance and adaptive changes of anaerobic microbes

The wide utilization of poly dimethyl diallyl ammonium chloride (polyDADMAC) in industrial conditions leads to its accumulation in waste activated sludge (WAS), thereby affecting subsequent WAS treatment processes. This work investigated the interaction between polyDADMAC and WAS components from the perspective of anaerobic digestion (AD) performance and anaerobes adaptability variation. The results showed that polyDADMAC decreased the content of biodegradable organic substrates (i.e., soluble protein and carbohydrate) by binding with the functional groups and then settling to the solid phase, thus impeding the subsequent utilization. Higher concentrations of polyDADMAC prompted an initial protective response of excreting organic substrates into extracellular environment, but its toxicity to archaea was irreversible. Consequently, polyDADMAC inhibited the processes of AD and induced a 30 % reduction in methane production with 0.05 g polyDADMAC/g total suspended solid (TSS) addition. Changes in microbial community structure indicated that archaea involved in methane production (e.g., Anaerolineaceae sp. and Methanosaeta sp.) were inhibited when exposed to polyDADMAC. However, several adaptive bacteria with the ability of utilizing complex organics and participating in nitrogen cycle (e.g., Aminicenantales sp. and Ellin6067 sp.) were enriched with the above dosage. Specifically, the decreased abundance of genes relevant to methane metabolism pathway (i.e., mer and cdh) and increased abundance of genes involved in metabolism of cofactors and vitamins (e.g., nad and thi) indicated the toxicity of polyDADMAC and the irritant response of microflora. Moreover, polyDADMAC underwent degradation in AD system, resulting in a 12 % reduction in 15 days, accompanied by an increase in the -NO2 functional group. In general, this study provided a thorough understanding of the interaction between polyDADMAC and WAS components, raising concerns regarding the elimination of endogenous pollutants during AD.

Combination of magnetite and sodium percarbonate to enhance acetate-enriched short-chain fatty acids production during sludge anaerobic fermentation

Large amounts of waste activated sludge are generated daily worldwide, posing significant environmental challenges. Anaerobic fermentation is a promising method for sludge disposal, but it has two technical bottlenecks: the availability of short-chain fatty acids (SCFAs)-producing substrates and SCFAs consumption by methanogenesis. This study proposes a pretreatment strategy combining sodium percarbonate (SPC) and magnetite (Fe3O4) to address these issues. Under optimized conditions (20 mg Fe3O4/g TSS and 15 mg SPC/g TSS), SCFAs production increased to 3244.10 ± 216.31 mg COD/L, about 3.06 times the control (1057.29 ± 35.06 mg COD/L) and surpassing reported treatments. The combined pretreatment enhanced the disruption of extracellular polymeric substances, increased the release of biodegradable matters, improved acidogenesis enzyme activities, and inhibited methanogenesis. Additionally, it increased NH4+-N release in favor of the recovery of phosphorus from sludge residual. This study demonstrates an efficient pretreatment for high SCFAs production and resource recovery from WAS.

Tris (2-chloroethyl) phosphate presence inhibits methane production from anaerobic digestion: Alterations in organic matter transformation, cell physiological status, and microbial community

Organophosphate flame retardants (OPFRs) are widely used in consumer products, leading to their unavoidable release into the environment, especially accumulation in anaerobic environments and posing potential risks. This study focused on Tris(2-chloroethyl) phosphate (TCEP), a representative OPFR, to investigate its effects on carbon transformation and methane production in anaerobic digestion. Increasing TCEP concentrations from control to 16 mg/L resulted in decreased cumulative methane yield (from 235.4 to 196.3 mL/g COD) and maximum daily methane yield (from 40.8 to 16.17 mL/(g COD·d)), along with an extended optimal anaerobic digestion time (from 15 to 20 days). Mechanistic analysis revealed TCEP binding to tyrosine-like proteins in extracellular polymeric substances, causing cell membrane integrity impairment. The TCEP-caused alteration of the physiological status of cells was demonstrated to be a significant contribution to the inhibited bioprocesses including acidogenesis, acetogenesis, and methanogenesis. Illumina Miseq sequencing showed TCEP decreasing the relative abundance of acidogens (58.8 % to 46.0 %) and acetogens (7.1 % to 5.0 %), partly shifting the methanogenesis pathway from acetoclastic to hydrogenotrophic methanogenesis. These findings enhance understanding of TCEP's impact on anaerobic digestion, emphasizing the environmental risk associated with its continued accumulation.

Exposure of polyethylene microplastics affects sulfur migration and transformation in anaerobic system

Polyethylene (PE) microplastic, which is detected in various environmental media worldwide, also inevitably enters wastewater treatment plants, which may have an impact on anaerobic processes in wastewater treatment. In this work, the effect of PE microplastics on anaerobic sulfur transformation was explored. Experimental results showed that PE microplastics addition at 0.1%- 0.5% w/w promoted H2S production by 14.8%-27.4%. PE microplastics enhanced the release of soluble organic sulfur and inorganic sulfate, and promoted the bioprocesses of organosulfur compounds hydrolysis and sulfate reduction. Mechanism analysis showed that PE microplastics increased the content of electroactive components (e.g., protein and humic acids) contained in extracellular polymeric substances (EPS). In particular, PE microplastics increased the proportion and the dipole moment of α-helix, an important component involved in electron transfer contained in extracelluar protein, which provided more electron transfer sites and promoted the α-helix mediated electron transfer. These enhanced the direct electron transfer ability of EPSs, which might explain why PE microplastics facilitated the bioprocesses of organosulfur compounds hydrolysis and sulfate reduction. Correspondingly, metagenomic analysis revealed that PE microplastics increased the relative abundance of S2- producers (e.g., Desulfobacula and Desulfonema) and the relative abundance of functional genes involved in anaerobic sulfur transformation (e.g., PepD and cysD), which were beneficial to H2S production in anaerobic system.

Natural flocculant chitosan inhibits short-chain fatty acid production in anaerobic fermentation of waste activated sludge

Chitosan (CTS) serves as an excellent natural flocculant in wastewater purification and sludge conditioning, but its potential impact on anaerobic fermentation of waste-activated sludge is unclear. The current study investigated the role of CTS in short-chain fatty acids (SCFAs) generation via sludge alkaline anaerobic fermentation. The results showed a drastic reduction in SCFA production with CTS, showing a maximum inhibition of 33 % at 6 mg/g of total suspended solids. CTS hindered sludge solubilization through flocculation, and acted as a humus precursor, promoting humus formation, and consequently reduced the amount of available substrates. Further, CTS promoted free ammonia production, posing a challenge to enzymes and cell viability. Additionally, CTS increased the population of Rikenellaceae sp. and weakened the dominance of hydrolyzing and acidifying bacteria. This study deepens the understanding of the potential impact of CTS on anaerobic fermentation and provides a theoretical basis for reducing the risk of polymeric flocculants.

Probing the Surface Layer Modulation on Archaeal Mechanics and Adhesion at the Single-Cell Level

Addressing the challenge of understanding how cellular interfaces dictate the mechanical resilience and adhesion of archaeal cells, this study demonstrates the role of the surface layer (S-layer) in methanogenic archaea. Using a combination of atomic force microscopy and single-cell force spectroscopy, we quantified the impact of S-layer disruption on cell morphology, mechanical properties, and adhesion capabilities. We demonstrate that the S-layer is crucial for maintaining cell morphology, where its removal induces significant cellular enlargement and deformation. Mechanical stability of the cell surface is substantially compromised upon S-layer disruption, as evidenced by decreased Young's modulus values. Adhesion experiments revealed that the S-layer primarily facilitates hydrophobic interactions, which are significantly reduced after its removal, affecting both cell-cell and cell-bubble interactions. Our findings illuminate the S-layer's fundamental role in methanogen architecture and provide a chemical understanding of archaeal cell surfaces, with implications for enhancing methane production in biotechnological applications.

The Impact of Bisphenol A on the Anaerobic Sulfur Transformation: Promoting Sulfur Flow and Toxic H2S Production

Bisphenol A (BPA), as a typical leachable additive from microplastics and one of the most productive bulk chemicals, is widely distributed in sediments, sewers, and wastewater treatment plants, where active sulfur cycling takes place. However, the effect of BPA on sulfur transformation, particularly toxic H2S production, has been previously overlooked. This work found that BPA at environmentally relevant levels (i.e., 50-200 mg/kg total suspended solids, TSS) promoted the release of soluble sulfur compounds and increased H2S gas production by 14.3-31.9%. The tryptophan-like proteins of microbe extracellular polymeric substances (EPSs) can spontaneously adsorb BPA, which is an enthalpy-driven reaction (ΔH = -513.5 kJ mol-1, ΔS = -1.60 kJ mol-1K -1, and ΔG = -19.52 kJ mol-1 at 35 °C). This binding changed the composition and structure of EPSs, which improved the direct electron transfer capacity of EPSs, thereby promoting the bioprocesses of organic sulfur hydrolysis and sulfate reduction. In addition, BPA presence enriched the functional microbes (e.g., Desulfovibrio and Desulfuromonas) responsible for organic sulfur mineralization and inorganic sulfate reduction and increased the abundance of related genes involved in ATP-binding cassette transporters and sulfur metabolism (e.g., Sat and AspB), which promoted anaerobic sulfur transformation. This work deepens our understanding of the interaction between BPA and sulfur transformation occurring in anaerobic environments.

Research progress on the origin, fate, impacts and harm of microplastics and antibiotic resistance genes in wastewater treatment plants

Previous studies reported microplastics (MPs), antibiotics, and antibiotic resistance genes (ARGs) in wastewater treatment plants (WWTPs). There is still a lack of research progress on the origin, fate, impact and hazards of MPs and ARGs in WWTPs. This paper fills a gap in this regard. In our search, we used "microplastics", "antibiotic resistance genes", and "wastewater treatment plant" as topic terms in Web of Science, checking the returned results for relevance by examining paper titles and abstracts. This study mainly explores the following points: (1) the origins and fate of MPs, antibiotics and ARGs in WWTPs; (2) the mechanisms of action of MPs, antibiotics and ARGs in sludge biochemical pools; (3) the impacts of MPs in WWTPs and the spread of ARGs; (4) and the harm inflicted by MPs and ARGs on the environment and human body. Contaminants in sewage sludge such as MPs, ARGs, and antibiotic-resistant bacteria enter the soil and water. Contaminants can travel through the food chain and thus reach humans, leading to increased illness, hospitalization, and even mortality. This study will enhance our understanding of the mechanisms of action among MPs, antibiotics, ARGs, and the harm they inflict on the human body.

A novel method for production of nitrogen fertilizer with low energy consumption by efficiently adsorbing and separating waste ammonia

Recovering waste NH3 to be used as a source of nitrogen fertilizer or liquid fuel has recently attracted much attention. Current methods mainly utilize activated carbon or metal-organic frameworks to capture NH3, but are limited due to low NH3 adsorption capacity and high cost, respectively. In this study, novel porous materials that are low cost and easy to synthesize were prepared as NH3 adsorbents by precipitation polymerization with acid optimization. The results showed that adsorption sites (‒COOH, -OH, and lactone) which form chemical adsorption or hydrogen bonds with NH3 were successfully regulated by response surface methods. Correspondingly, the dynamic NH3 adsorption capacity increased from 5.45 mg g-1 to 129 mg g-1, which is higher than most known activated carbon and metal-organic frameworks. Separation performance tests showed that NH3 could also be separated from CO2 and CH4. The findings in this study will advance the industrialization of NH3 polymer adsorbents and provide technical support for the recycling of waste NH3.

Unveiling the mechanisms of Fe(III)-loaded chitosan composite (CTS-Fe) in enhancing anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) is usually limited by the low generation efficiency of methane. Fe(III)-loaded chitosan composite (CTS-Fe) have been reported to effectively enhanced the digestion of WAS, but its role in promoting anaerobic sludge digestion remains unclear. In present study, the effects of CTS-Fe on the hydrolysis and methanogenesis stages of WAS anaerobic digestion were investigated. The addition of CTS-Fe increased methane production potential by 8%-23% under the tested conditions with the addition of 5-20 g/L CTS-Fe. Besides, the results demonstrate that the addition of CTS-Fe could effectively promote the hydrolysis of WAS, evidenced by lower protein or polysaccharides concentration, higher soluble organic carbon in rector adding CTS-Fe, as well as the increased activity of extracellular hydrolase with higher CTS-Fe concentration. Meanwhile, the enrichment of Clostridia abundance (iron-reducing bacteria (IRBs)) was observed in CTS-Fe adding reactor (8.9%-13.8%), which was higher than that in the control reactor (7.9%). The observation further suggesting the acceleration of hydrolysis through dissimilatory iron reduction (DIR) process, thus providing abundant substrates for methanogenesis. However, the presence of CTS-Fe was inhibited the acetoclastic and hydrogenotrophic methanogenesis process, which could be ascribed to the Fe(III) act as electron acceptor coupled to methane for anaerobic oxidation. Furthermore, coenzyme F420 activity in the CTS-Fe added reactor was 34.9% lower than in the blank, also abundance of microorganisms involved in hydrogenotrophic methanogenesis was decreased. Results from this study could provide theoretical support for the practical applications of CTS-Fe.

Formation of autotrophic nitrogen removal granular sludge driven by the dual-partition airlift internal circulation: Insights from performance assessment, community succession, and metabolic mechanism

Granular sludge has been recognized as an effective method for the application and industrialization of the anammox-based process due to its good biomass retention capacity and environmental tolerance. In this study, a one-stage autotrophic nitrogen removal (ANR) dual-partition system with airlift internal circulation was implemented for 320 days. A high nitrogen removal efficiency of 84.6% was obtained, while the nitrogen removal rate reached 1.28 g-N/L/d. ANR granular sludge dominated by Nitrosomonas and Candidatus Brocadia was successfully cultivated. Results showed that activity and abundance of functional flora first increased with granulation process, but eventually declined slightly when particle size exceeded the optimal range. Total anammox activity was observed to be significantly correlated with protein content (R2 = 0.9623) and nitrogen removal performance (R2 = 0.8796). Correlation network revealed that AnAOB had complex interactions with other bacteria, both synergy for nitrogen removal and competition for substrate. Changes in abundances of genes encoding the Carbohydrate Metabolism, Energy Metabolism, and Membrane Transport suggested energy production and material transfer were possibly blocked with further sludge granulation. Formation of ANR granular sludge promoted the interactions and metabolism of functional microorganisms, and the complex nitrogen metabolic pathways improved the performance stability. These results validated the feasibility of granule formation in the airlift dual-partition system and revealed the response of the ANR system to sludge granulation.

A comprehensive review on the preparation of biochar from digestate sources and its application in environmental pollution remediation

The preparation of biochar from digestate is one of the effective ways to achieve the safe disposal and resource utilization of digestate. Nevertheless, up to now, a comprehensive review encompassing the factors influencing anaerobic digestate-derived biochar production and its applications is scarce in the literature. Therefore, to fill this gap, the present work first outlined the research hotspots of digestate in the last decade using bibliometric statistical analysis with the help of VOSviewer. Then, the characteristics of the different sources of digestate were summarized. Furthermore, the influencing factors of biochar preparation from digestate and the modification methods of digestate-derived biochar and associated mechanisms were analyzed. Notably, a comprehensive synthesis of anaerobic digestate-derived biochar applications is provided, encompassing enhanced anaerobic digestion, heavy metal remediation, aerobic composting, antibiotic/antibiotic resistance gene removal, and phosphorus recovery from digestate liquor. The economic and environmental impacts of digestate-derived biochar were also analyzed. Finally, the development prospect and challenges of using biochar from digestate to combat environmental pollution are foreseen. The aim is to not only address digestate management challenges at the source but also offer a novel path for the resourceful utilization of digestate.

Synergic effects of free ammonia and sodium percarbonate for enhancing short-chain fatty acid production during sludge fermentation: Effectiveness assessment and mechanism elucidation

The production of short-chain fatty acids (SCFAs) from waste activated sludge (WAS) via anaerobic fermentation is typically restricted by poor sludge decomposition capacity and low substrate-availability. Therefore, the free ammonia (FA)‑sodium percarbonate (SPC) technology was presented to successfully overcome the limitation while addressing unsatisfactory acidogenic fermentation pretreated with sole FA or SPC. It revealed that FA + SPC co-pretreatment could boost the SCFA concentration to 347.1 mg COD/g VSS at 180 mg/L FA and 0.15 g/g TSS SPC. In-depth studies demonstrated that FA + SPC pretreatment greatly improved sludge disintegration, biodegradability of substrates and acidification of hydrolysis products. Furthermore, FA + SPC co-pretreatment stimulated the activity of hydrolytic and acidogenic enzymes but inhibited methanogenic enzymes while changing the microbial structure and promoting the enrichment of fermentation microorganisms. The synergistic effect of FA and SPC in this work improves the yield of SCFAs from WAS and facilitates the study of WAS carbon resource recovery.

Distinct effects of chemical- and bio- flocculants on the sludge acidogenic fermentation for volatile fatty acids production by affecting the acidogenic steps, microbial community structure and metabolic functions

Flocculants play crucial roles in sludge treatment, while the specific impact of chemical and bio-flocculants on sludge anaerobic fermentation was unknown. This study unveiled the contrasting effects of chitosan (CTS) and poly‑aluminum chloride (PAC) on volatile fatty acids (VFAs) generation during sludge fermentation. CTS supplementation resulted in 17.2 % increase in VFAs production, while PAC exposure led to 7.6 % reduction compared to the control. Further investigation revealed that CTS facilitated sludge solubilization and hydrolysis, thus providing sufficient organic substrates for VFAs generation. Additionally, environmental-friendly CTS exposure positively influenced the abundance and activity of functional anaerobes, as well as the expression of genes associated with VFAs biosynthesis. In contrast, PAC exposure resulted in the formation of larger sludge flocs, which hindered WAS solubilization and hydrolysis. Meanwhile, its potential microbial toxicity also impeded the microbial metabolic activity (i.e., genetic expressions), resulting in unsatisfactory VFAs production.

Efficient nitrogen removal from municipal wastewater by an autotrophic-heterotrophic coupled anammox system: The up-regulation of key functional genes

The metabolic pathways based on key functional genes were innovatively revealed in the autotrophic-heterotrophic coupled anammox system for real municipal wastewater treatment. The nitrogen removal performance of the system was stabilized at 88.40 ± 3.39 % during the treatment of real municipal wastewater. The relative abundances of the nitrification functional genes ammonia oxidase (amoA/B/C), hydroxylamine oxidoreductase (hao), and nitrite oxidoreductases (nxrA/B) were increased by 1.2-2.4 times, and these three nitrification functional genes were mostly contributed by Nitrospira that dominated the efficient nitrification of the system. The relative abundance of anammox bacteria Candidatus Brocadia augmented from 0.35 % to 0.75 %, accompanied with the increased expression of hydrazine synthase (hzs) and hydrazine dehydrogenase (hdh), resulting in the major role of anammox (81.24 %) for nitrogen removal. The expression enhancement of the functional genes nitrite reductase (narG/H, napA/B) that promoted partial denitrification (PD) of the system weakened the adverse effects of the sharp decline in the population of PD microbe Thauera (from 5.7 % to 2.2 %). The metabolic module analysis indicated that the carbon metabolism pathways of the system mainly included CO2 fixation and organic carbon metabolism, and the stable enrichment of autotrophic bacteria ensured stable CO2 fixation. Furthermore, the enhanced expression of the glucokinases (glk, GCK, HK, ppgk) and the abundant pyruvate kinase (PK) achieved stable hydrolysis ability of organic carbon metabolism function of the system. This study offers research basics to practical application of the mainstream anammox process.

Revealing the Adverse Impact of Additive Carbon Material on Microorganisms and Its Implications for Biogas Yields: A Critical Review

Biochar could be a brilliant additive supporting the anaerobic fermentation process. However, it should be taken into account that in some cases it could also be harmful to microorganisms responsible for biogas production. The negative impact of carbon materials could be a result of an overdose of biochar, high biochar pH, increased arsenic mobility in the methane fermentation solution caused by the carbon material, and low porosity of some carbon materials for microorganisms. Moreover, when biochar is affected by an anaerobic digest solution, it could reduce the biodiversity of microorganisms. The purpose of the article is not to reject the idea of biochar additives to increase the efficiency of biogas production, but to draw attention to the properties and ways of adding these materials that could reduce biogas production. These findings have practical relevance for organizations seeking to implement such systems in industrial or local-scale biogas plants and provide valuable insights for future research. Needless to say, this study will also support the implementation of biogas technologies and waste management in implementing the idea of a circular economy, further emphasizing the significance of the research.

Metagenomic approach reveals the mechanism of calcium oxide improving kitchen waste dry anaerobic digestion

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

Effects of different types of granular activated carbon on methanogenesis of carbohydrate-rich food waste: Performance, microbial communities and optimization

Granular activated carbon (GAC) supplementation is an efficient method for enhancing methane production during the anaerobic digestion of food waste, but it remains unclear which type of GAC is optimal and what potential mechanisms are involved with different types of GAC, particularly for the methanogenic system of carbohydrate-rich food waste. This study selected three commercial GAC (GAC#1, GAC#2, GAC#3) with very distinct physical and chemical properties, and investigated their impacts on the methanogenesis of carbohydrate-rich food waste with an inoculation/substrate ratio of 1. Results indicated that Fe-doped GAC#3 had a lower specific surface area but higher conductivity, yet exhibited superior performance in facilitating methanogenesis compared with GAC#1 and GAC#2, which possessed larger specific surface areas. The addition of 10 g/L GAC#3 enhanced the methane yield by 10-folds through regulating pH levels, alleviating volatile fatty acids-induced stress, enhancing key enzymatic activity, as well as enriching direct interspecies electron transfer-mediated syntrophic partner of Syntrophomonas with Methanosarcina. Furthermore, GAC#1, which had the largest specific surface area but exhibited the poorest performance, was chemically modified to enhance its ability in promoting methanogenesis. The resulting material, named MGAC#1 (Fe3O4-loaded GAC#1), exhibited superior electro-conductivity and high methane production efficiency. The methane yield of 588 mL/g-VS showed a remarkable increase of 468 % compared with GAC#1, and a modest increase of 13 % compared with GAC#3, surpassing most values reported in literature. These findings suggested that the Fe3O4-loaded GAC with lager specific surface area, was the optimal choice for the methanogenesis of sole readily acidogenic waste, providing valuable insights for the preparation of superior-quality GAC for application in biogas industry.

A review of application of combined biochar and iron-based materials in anaerobic digestion for enhancing biogas productivity: Mechanisms, approaches and performance

Strengthening direct interspecies electron transfer (DIET), via adding conductive materials, is regarded as an effective way for improving methane productivity of anaerobic digestion (AD). Therein, the supplementation of combined materials (composition of biochar and iron-based materials) has attracted increasing attention in recent years, because of their advantages of promoting organics reduction and accelerating biomass activity. However, as far as we known, there is no study comprehensively summarizing the application of this kind combined materials. Here, the combined methods of biochar and iron-based materials in AD system were introduced, and then the overall performance, potential mechanisms, and microbial contribution were summarized. Furthermore, a comparation of the combinated materials and single material (biochar, zero valent iron, or magnetite) in methane production was also evaluated to highlight the functions of combined materials. Based on these, the challenges and perspectives were proposed to point the development direction of combined materials utilization in AD field, which was hoped to provide a deep insight in engineering application.

From anaerobic digestion to single cell protein synthesis: A promising route beyond biogas utilization

The accumulation of a large amount of organic solid waste and the lack of sufficient protein supply worldwide are two major challenges caused by rapid population growth. Anaerobic digestion is the main force of organic waste treatment, and the high-value utilization of its products (biogas and digestate) has been widely concerned. These products can be used as nutrients and energy sources for microorganisms such as microalgae, yeast, methane-oxidizing bacteria(MOB), and hydrogen-oxidizing bacteria(HOB) to produce single cell protein(SCP), which contributes to the achievement of sustainable development goals. This new model of energy conversion can construct a bioeconomic cycle from waste to nutritional products, which treats waste without additional carbon emissions and can harvest high-value biomass. Techno-economic analysis shows that the SCP from biogas and digestate has higher profit than biogas electricity generation, and its production cost is lower than the SCP using special raw materials as the substrate. In this review, the case of SCP-rich microorganisms using anaerobic digestion products for growth was investigated. Some of the challenges faced by the process and the latest developments were analyzed, and their potential economic and environmental value was verified.

Sewage sludge pretreatment: current status and future prospects

Sewage sludge is regarded by wastewater treatment plants as problematic, from a financial and managerial point of view. Thus, a variety of disposal routes are used, but the most popular is methane fermentation. The proportion of macromolecular compounds in sewage sludges varies, and substrates treated in methane fermentation provide different amounts of biogas with various quality and quantity. Depending on the equipment and financial capabilities for methane fermentation, different methods of sewage sludge pretreatment are available. This review presents the challenges associated with the recalcitrant structure of sewage sludge and the presence of process inhibitors. We also examined the diverse methods of sewage sludge pretreatment that increase methane yield. Moreover, in the field of biological sewage sludge treatment, three future study propositions are proposed: improved pretreatment of sewage sludge using biological methods, assess the changes in microbial consortia caused with pretreatment methods, and verification of microbial impact on biomass degradation.

Surfactant enhances anaerobic fermentative hydrogen sulfide production: Changes in sulfur-containing organics structure and microbial community

The presence of surfactants in waste activated sludge (WAS) system is generally regarded as beneficial to sludge treatment such as enhancing sludge dewatering and improving value-added fermentation products generation. However, in this study, it was firstly found that sodium dodecylbenzene sulfonate (SDBS, a typical surfactant) obviously increased toxic hydrogen sulfide (H₂S) gas production from WAS anaerobic fermentation at environmentally relevant concentrations. Experimental results showed that H₂S production from WAS significantly increased from 53.24 × 10^-3 to 111.25 × 10^-3 mg/g volatile suspended solids (VSS) when SDBS level increased from 0 to 30 mg/g total suspended solid (TSS). It was found that SDBS presence destroyed WAS structure and enhanced sulfur containing organics release. SDBS reduced the proportion of α-helix structure, damaged disulfide bridges and protein conformation, and effectively destroyed protein structure. SDBS promoted sulfur containing organics degradation and provided more readily hydrolyzed micro-molecule organics for sulfide production. Microbial analysis showed that SDBS addition enhanced the abundance of functional genes encoding protease, ATP-binding cassette transporters, and amino acids lyase, enhanced the activities and abundance of hydrolytic microbes, thus increased sulfide production from the hydrolysis of sulfur containing organics. Compared with the control, 30 mg/g TSS SDBS increased organic sulfurs hydrolysis and amino acids degradation by 47.1 % and 63.5 %, respectively. Key genes analysis further showed that SDBS addition promoted sulfate transport system and dissimilatory sulfate reduction. SDBS presence also lowered fermentation pH, promoted the chemical equilibrium transformation of sulfide, thus increased H₂S gas release.

Revealing an unrecognized role of free ammonia in sulfur transformation during sludge anaerobic treatment

Free ammonia (FA), the unionized form of ammonium, is presented in anaerobic fermentation of waste activated sludge (WAS) at high levels. However, its potential role in sulfur transformation, especially H₂S production, during WAS anaerobic fermentation process was unrecognized previously. This work aims to unveil how FA affects anaerobic sulfur transformation in WAS anaerobic fermentation. It was found that FA significantly inhibited H₂S production. With an increase of FA from 0.04 to 159 mg/L, H₂S production reduced by 69.9%. FA firstly attacked tyrosine-like proteins and aromatic-like proteins in sludge EPSs, with CO groups being responded first, which decreased the percentage of α-helix/(β-sheet + random coil) and destroyed hydrogen bonding networks. Cell membrane potential and physiological status analysis showed that FA destroyed membrane integrity and increased the ratio of apoptotic and necrotic cells. These destroyed sludge EPSs structure and caused cell lysis, thus strongly inhibited the activities of hydrolytic microorganisms and sulfate reducing bacteria. Microbial analysis showed that FA reduced the abundance of functional microbes (e.g., Desulfobulbus and Desulfovibrio) and genes (e.g., MPST, CysP, and CysN) involved in organic sulfur hydrolysis and inorganic sulfate reduction. These findings unveil an actually existed but previously overlooked contributor to H₂S inhibition in WAS anaerobic fermentation.

Insights into the role of ·OH generated in Fe2+/CaO2/coal slime system for efficient extracellular polymeric substances degradation to improve dewaterability of sewage sludge

The disposal of massive sewage sludge and coal slime is a problem facing municipalities in China. A hypothesis for the co-disposal of sludge and coal slime is proposed to improve dewaterability by utilizing the beneficial role of coal slime as a filter assist and CaO₂ enhanced system in this research. Results showed that capillary suction time, specific resistance to filtration and water content decreased dramatically from 49.3 s, 13.2 × 10¹² m/kg and 84.85% to 19.1 s, 1.0 × 10¹² m/kg and 50.07%, respectively, under the optimal conditions with 0.3/0.1/0.3-Fe²⁺/CaO₂/coal slime g/g DS. The hydroxyl radicals generated in the Fe²⁺/CaO₂ process acted on extracellular polymeric substances (EPS), resulting in a drop in the ratio of α-helix/(β-sheet + random coil) in the secondary structure of EPS proteins and a reduction in the concentration of aromatic proteins and tryptophan-like substances in TB-EPS, thereby enhancing the sludge dewaterability. Furthermore, coal slime as the skeleton building material induced a rise in sludge particle size and contact angle, lowering the hydrophilicity, compressibility of sludge and providing more channels to facilitate water flow. This work verified the promising application prospect of the Fe²⁺/CaO₂/coal slime combined system in the enhancement of sludge dewaterability.

Enhanced hydrolysis/acidogenesis and potential mechanism in thermal-alkali-biofilm synergistic pretreatment of high-solid and low-organic-content sludge

Improving the anaerobic digestion (AD) of high-solid and low-organic-content sludge is imperative for sustainable waste activated sludge (WAS) management. Here, a thermal-alkali-biofilm pretreatment (TAB) was established to treat high-solid and low-organic-content sludge and compared with thermal and thermal-alkali methods. The results showed that TAB drastically improved WAS reduction, hydrolysis/acidogenesis efficiency, and biochemical methane potential. TAB possessed the lowest sludge particle size and the highest surface charge due to the stimulated proteolysis and WAS solubilization, supported by the protease activity test and secondary substrate identification. In addition, the biofilm assistance noticeably accelerated the elimination of autochthonous bacteria in WAS (e.g., Proteobacteria) and facilitated the enrichment of specialized fermentative microorganisms (e.g., Firmicutes) along with relevant functional genes, lying molecular foundation for the enhanced hydrolysis/acidogenesis in TAB. These findings could expand the application of biofilm in the AD of WAS and provide new insight into the pretreatment strategy of high-solid and low-organic-content sludge.

Effect and mechanism of benzalkonium bromide on short chain fatty acid production from anaerobic sludge fermentation process

Quaternary ammonium compounds (QACs) was frequently detected in wastewater treatment plants and leads to potential toxicity to the related biological processes. In this study, the effect of benzalkonium bromide (BK) on anaerobic sludge fermentation process for short chain fatty acid (SCFAs) production was investigated. Batch experiments indicated that BK exposure significantly enhanced the SCFAs production from anaerobic fermentation sludge and the maximum concentration of total SCFAs increased from 474.40 ± 12.35 mg/L to 916.42 ± 20.35 mg/L with BK increasing from 0 to 8.69 mg/g VSS. Mechanism exploration exhibited that the presence of BK enhanced much more bioavailable organic matters release, little affected on hydrolysis, acidification, but seriously inhibited methanogenesis. Microbial community investigation revealed that BK exposure importantly enhanced the relative abundances of hydrolytic-acidifying bacteria and also improved the metabolic pathways and functional genes for sludge lysis. This work further supplement the information for environmental toxicity of emerging pollutants.

Insights into high-solids anaerobic digestion of food waste concomitant with sorbate: Performance and mechanisms

High-solids anaerobic digestion (HS-AD) of food waste is increasingly applied commercially. Sorbate, a food preservative extensively used in the food industry, induces potential environmental risks. Results indicated sorbate at 0-10 mg/g VS slightly inhibited methane production, and the cumulative methane yield suggested a negative correlation with 25 mg/g VS sorbate, with a reduction of 15.0% compared to the control (from 285.7 to 253.6 mL CH₄/g VS). The reduction in methane yield could be ascribed to the promotion of solubilization and inhibition of acidogenesis and methanogenesis with sorbate addition. Excessive sorbate (25 mg/g VS) resulted in the inhibition of aceticlastic metabolism and the key enzymes activities (e.g., acetate kinase and coenzyme F₄₂₀). This study deeply elucidated the response mechanism of HS-AD to sorbate, supplemented the potential ecological risk assessment of sorbate, and could provide insights to further prevent the potential risk of sorbate in anaerobic digestion of FW.

Insights into the Occurrence, Fate, Impacts, and Control of Food Additives in Food Waste Anaerobic Digestion: A Review

The recovery of biomass energy from food waste through anaerobic digestion as an alternative to fossil energy is of great significance for the development of environmental sustainability and the circular economy. However, a substantial number of food additives (e.g., salt, allicin, capsaicin, allyl isothiocyanate, monosodium glutamate, and nonnutritive sweeteners) are present in food waste, and their interactions with anaerobic digestion might affect energy recovery, which is typically overlooked. This work describes the current understanding of the occurrence and fate of food additives in anaerobic digestion of food waste. The biotransformation pathways of food additives during anaerobic digestion are well discussed. In addition, important discoveries in the effects and underlying mechanisms of food additives on anaerobic digestion are reviewed. The results showed that most of the food additives had negative effects on anaerobic digestion by deactivating functional enzymes, thus inhibiting methane production. By reviewing the response of microbial communities to food additives, we can further improve our understanding of the impact of food additives on anaerobic digestion. Intriguingly, the possibility that food additives may promote the spread of antibiotic resistance genes, and thus threaten ecology and public health, is highlighted. Furthermore, strategies for mitigating the effects of food additives on anaerobic digestion are outlined in terms of optimal operation conditions, effectiveness, and reaction mechanisms, among which chemical methods have been widely used and are effective in promoting the degradation of food additives and increasing methane production. This review aims to advance our understanding of the fate and impact of food additives in anaerobic digestion and to spark novel research ideas for optimizing anaerobic digestion of organic solid waste.

Substance bioconversion, hydrolases activity, and metagenomic analysis to unravel the enhanced biomethanation of corn stover with urea-hydrothermal pretreatment

Corn stover (CS) is a promising feedstock for producing biomethane, that can replace diminishing fossil fuels. However, the recalcitrant structure of CS resulted in low degradability in anaerobic digestion (AD). Numerous studies investigated the pretreatment of CS before AD, but the insight mechanism of biomethanation enhancement is not fully revealed. Therefore, this study advanced low-temperature urea-hydrothermal pretreatment of CS, and the biomethane production, substance bioconversion, hydrolase activity, and metagenomic analysis were conducted to unravel the intrinsic mechanisms of pretreatment for the enhanced biomethanation. The results showed that the pretreatment improved 11.5% of the specific surface area of CS, providing 111.5% higher total volatile fatty acids and 19.9% higher reducing sugars than the control, potentially enriching more anaerobic microorganisms. As a result, the pretreated CS achieved 19.1% higher biomethane yield, 9.1% higher volatile solid removal rate, and 3 days shorter digestion time. The mass balance and microbial community succession analysis indicated that the pretreatment reinforced the biomethane conversion from carbohydrate, which was attributed to the rapid enrichment of hydrolytic acidification bacteria (g__unclassified_o__Bacteroidales) (33.2%) and mixotrophic archaea (Methanosarcina) (72.3%). Meanwhile, the activity of cellulase and xylanase was enhanced up to 23.7% and 66.7%. Metagenomic analysis revealed that the combined pretreatment of CS promoted methanogenesis by enhancing various CAZymes secretion (such as oligosaccharide-degrading enzymes), and functional genes expression of hydrolytic, acidification and acetate-methane pathways at days 1-5. The study indicated that the combined pretreatment could influence microbial composition and function by changing the physicochemical properties of the CS, thereby improving methanogenic performance.

Microplastics decrease the toxicity of cadmium to methane production from anaerobic digestion of sewage sludge

Microplastics (MPs) and Cd have been proven to inhibit methane production from anaerobic digestion of sewage sludge. However, the published studies mainly focused on their single inhibition. This cannot reflect the real-world situations where MPs and Cd co-exist. This study therefore aims to reveal the combined effect of MPs and Cd on anaerobic digestion of sewage sludge. Experimental results showed that PVC-MPs at environmentally relevant levels (e.g., 1, 10 particles/g total solids (TS)) did not affect methane yield but decrease the toxicity of Cd. When PVC-MPs were 30 particles/g TS, the cumulative methane production recovered from 58.8 % (in the presence of 5 mg Cd/g TS) to 89.7 % of the control. Organic fluxes were significantly increased compared with the control, particularly affecting the content of dissolved substances and short-chain fatty acids during anaerobic digestion. Mechanistic exploration showed that the adsorption of Cd by PVC-MPs was higher than that of sludge-substrate, which reduced the bioavailability of Cd by anaerobes, as evidenced by the increased anaerobes driven carbon flux from solid-phase to bio-methane during anaerobic digestion. Overall, these findings identified important factors in determining the toxicity of pollutants on anaerobic digestion process, providing precise data for toxicity evaluation of MPs and metals in anaerobic environment.

1-Butyl-3-methylimidazolium Chloride Affects Anaerobic Digestion through Altering Organics Transformation, Cell Viability, and Microbial Community

1-Butyl-3-methylimidazolium chloride (BmimCl), an imidazolium-based ionic liquid, is considered the representative emerging persistent aquatic pollutant, and its environmental toxicity has attracted a growing concern. However, most of the investigations focused on monocultures or a single organism, with little information available on the complex syntrophic consortium that dominates the complex and successional biochemical processes, such as anaerobic digestion. In this study, the effect of BmimCl at environmentally relevant levels on glucose anaerobic digestion was therefore investigated in several laboratory-scale mesophilic anaerobic digesters to provide such support. Experimental results showed that BmimCl at 1-20 mg/L inhibited the methane production rate by 3.50-31.03%, and 20 mg/L BmimCl inhibited butyrate, hydrogen, and acetate biotransformation by 14.29%, 36.36%, and 11.57%, respectively. Toxicological mechanism studies revealed that extracellular polymeric substances (EPSs) adsorbed and accumulated BmimCl through carboxyl, amino, and hydroxyl groups, which destroyed the EPSs' conformational structure, thereby leading to the inactivation of microbial cells. MiSeq sequencing data indicated that the abundance of Clostridium_sensu_stricto_1, Bacteroides, and Methanothrix decreased by 6.01%, 7.02%, and 18.45%, respectively, in response to 20 mg/L BmimCl. Molecular ecological network analysis showed that compared with the control, the lower network complexity, fewer keystone taxa, and fewer associations among microbial taxa were found in the BmimCl-present digester, indicating the reduced stability of the microbial community.

Enhancing methane production from waste activated sludge with heat-assisted potassium ferrate (PF) pretreatment: Reaction kinetics and mechanisms

This work proposed a novel strategy via heat-assisted potassium ferrate (PF) pretreatment to enhance methane production from waste activated sludge (WAS) during anaerobic digestion. In this research, five dosages of PF (i.e., 0, 0.05, 0.1, 0.3 and 0.5 g/g VSS) at two temperatures (i.e., 25 °C and 55 °C) were explored. Biochemical methane potential experiments illustrated that heat-assisted PF pretreatment improved cumulative methane production with the maximum yield up to 163.93 mL CH₄/g VSS, 149.0 %, 119.6 % and 121.0 % of that in the control, individual 0.5 g PF/g VSS and individual heat (i.e., 55 °C) pretreatment digesters, respectively. The maximum methane potential (B₀) was promoted by 63.2 % with heat-assisted PF pretreatment compared to the control, while the hydrolysis rate (k) changed slightly. Mechanism analysis revealed that heat-assisted PF pretreatment accelerated WAS solubilization and enhanced the biodegradability of released substances, providing more available matrix for bacteria during the following anaerobic digestion processes. Microbial community analysis exhibited that several microbes such as Proteiniclasticum sp., Sedimentibacter sp. and Methanosaeta sp. associated with hydrolysis, acidification and methanogenesis respectively were improved after heat-assisted PF pretreatment. In addition, the relative bioactivities of protease, butyrate kinase and acetate kinase were also increased. Furthermore, variation of dominant genes associated with methane production indicated that acetate-dependent methanogenesis was the main pathway while CO₂-dependent methanogenesis pathway was inhibited by heat-assisted PF pretreatment.

Insights into the role of chitosan in hydrogen production by dark fermentation of waste activated sludge

Chitosan is widely used as a dewatering flocculant, but whether it affects hydrogen production from sludge anaerobic fermentation is unclear. This study aimed to elucidate the role of chitosan in the dark fermentation of waste activated sludge for hydrogen production. The results showed that chitosan had a negative effect on hydrogen production from sludge. Chitosan at 30 g/kg total suspended solids reduced hydrogen accumulation by 56.70 ± 1.22 % from 3.94 ± 0.12 to 1.71 ± 0.10 mL/g volatile suspended solids. Chitosan hindered the solubilization of sludge by flocculation, which reduced the available substrate for anaerobic fermentation. In addition, chitosan interfered with the electron transport system by reducing cytochrome C and caused lipid peroxidation by inducing reactive oxygen species, thereby inhibiting the activity of enzymes involved in anaerobic fermentation. Hydrogen production was reduced because hydrogen-producing processes (i.e., hydrolysis, acidification, and acetification) were inhibited more strongly than hydrogen-consuming processes (i.e., methanogenesis, sulfate reduction, and homoacetogenesis). Furthermore, chitosan enriched the abundance of Spirochaetaceae sp. and Holophagaceae sp., which occupied the survival space of hydrogen-producing microorganisms. This study reveals the potential impact of chitosan on hydrogen production in dark fermentation of sludge and provide direct evidence that chitosan triggers oxidative stress in anaerobic fermentation.

Polycarbonate microplastics induce oxidative stress in anaerobic digestion of waste activated sludge by leaching bisphenol A

Polycarbonate (PC) microplastics are frequently detected in waste activated sludge. However, understanding the potential impact of PC microplastics on biological sludge treatment remains challenging. By tracking the changes in methane production under different concentrations of PC microplastics, a dose-dependent effect of PC microplastics on anaerobic digestion of sludge was observed. PC microplastics at 10-60 particles/g total solids (TS) improved methane production by up to 24.7 ± 0.1 % (at 30 particles/g TS), while 200 particles/g TS PC microplastics reduced methane production by 8.09 ± 0.1 %. Bisphenol A (BPA) leached from 30 particles/g TS PC microplastics (1.26 ± 0.18 mg/L) down-regulated intracellular reactive oxygen species (ROS) production, thereby enhancing enzyme activity, biomass viability, and abundance of methanogenic (Methanobacterium sp. and Methanosarcina sp.), ultimately boosting methane production. Conversely, BPA leached from 200 particles/g TS PC microplastics (4.02 ± 0.15 mg/L) stimulated ROS production, resulting in decreased biomass viability and even apoptosis. Modulation of oxidative stress by leaching monomeric BPA is an underappreciated transformative mechanism for improving the mastery of the potential behavior of microplastics in biological sludge treatment.

Iron-rich digestate biochar toward sustainable peroxymonosulfate activation for efficient anaerobic digestate dewaterability

In this study, a suite of Fe-rich biochars derived from Fenton-like treated digestate (Fe-BC) were fabricated under different pyrolysis temperatures (300, 500, and 800 °C), which were firstly utilized as peroxymonosulfate (PMS) activators for promoting digestate dewaterability with wide applicability. Results showed that compared to the Fe-BC₃₀₀/Fe-BC₅₀₀ + PMS treatments, Fe-BC₈₀₀ + PMS process performed superior digestate dewaterability in which specific resistance to filtration reduction and water content reduction improved by > 12.5% and > 130%, respectively, under the optimal conditions. Mechanistic results demonstrated that in Fe-BC₈₀₀ + PMS system, HO• and SO₄^•- oxidation played a pivotal role on promoted digestate dewaterability, while HO• and ¹O₂ oxidation was dominated in Fe-BC₃₀₀/Fe-BC₅₀₀ + PMS treatments. Fe-BC₈₀₀ containing higher Fe and CO contents could efficiently interact with PMS to generate numerous HO• and SO₄^•- via iron cycle. These highly reactive oxygen species proficiently reduced the hydrophilic biopolymers, protein molecules, and amino acids in extracellular polymeric substances, leading to remarkable decrease in particle size, hydrophilicity, adhesion, network strength, and bound water of digestate. Consequently, the flowability and dewaterability of digestate could be significantly enhanced. The cost-benefit result indicated the Fe-BC + PMS treatment possessed desirable reusability, applicability, and economic viability. Collectively, the Fe-BC + PMS is a high-performance and eco-friendly technique for digestate dewatering, which opens a new horizon towards a closed-loop of digestate reutilization.

Impacts of organic loading rate and hydraulic retention time on organics degradation, interspecies interactions and functional traits in thermophilic anaerobic co-digestion of food waste and sewage sludge

This study provided novel insights into the effects of organic loading rate (OLR) and hydraulic retention time (HRT) on thermophilic anaerobic co-digestion of food waste and sewage sludge. The obtained maximum methane (CH₄) yield of 328 ± 4 mL CH₄/g COD_fed at HRT of 15 days (OLR = 5.8 g VS/L/d) was partly attributable to the enhanced acidogenesis, acetogenesis, and methanogenesis phases. The increased key enzyme activities, particularly acetate kinase (improved by 5.2-fold), providing substantial methanogenic substrates for efficient CH₄ production. The functional syntrophs that were related to syntrophic decarboxylation, novel acetate oxidation & reductive acetyl-CoA, and β-oxidation pathways could drive trophic interactions with methanogens. This markedly stimulated hydrogenotrophic Methanoculleus thermophilus metabolism and concomitantly enriched mixotrophic Methanosarcina thermophila. The distinctive cross-feeding interspecies interactions significantly affected the assembly and dynamics of thermophilic consortia. These findings shed light on the physicochemical and microbial mechanisms of HRT- and OLR-dependent enhancement of methanogenesis.

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

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

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.

Unveiling the Mechanism of Urine Source Separation-Derived Pretreatment on Enhancing Short-Chain Fatty Acid Yields from Anaerobic Fermentation of Waste Activated Sludge

A novel strategy employing urine wastewater derived from source separation technology, to pretreat waste activated sludge (WAS) for promoting yields of short-chain fatty acids (SCFAs), has been proposed in this study. It was found experimentally that SCFA production could ascend up to 305.4 mg COD/g VSS (volatile suspended solids) with a urine volumetric proportion of 1:2 to the whole reaction system, being 8.8 times that produced in the control. Exploration of the mechanism indicated that WAS disintegration was significantly enhanced due to the synergistic effect of urea and free ammonia (FA). Degradation rates of model organic substrates and measurements of critical enzymatic activities demonstrated that hydrolysis and acidogenesis were inhibited under high urine content (urine proportion of 1:2), while not significantly affected under low urine content (i.e., 1:4), which might be attributed to metal ions existing in urine wastes alleviating the inhibition induced by FA. In contrast, methanogenesis was negatively suppressed by any urine concentration owing to its higher sensitivity to the environmental variations. Shift of microbial population further elucidated the abundance of hydrolytic and acidogenic microbes were enriched in the fermenters with urine addition. The findings provide a new thought for recovering resources from wastes, potentially reducing the pressure of sewage and sludge treatment in wastewater treatment plants.

The fate of diclofenac in anaerobic fermentation of waste activated sludge

Diclofenac (DCF), a nonsteroidal anti-inflammatory drug, is one of the most commonly detected pharmaceuticals in wastewater treatment plants. However, the fate of DCF in waste activated sludge (WAS) anaerobic fermentation has not been well-understood so far. This work therefore aims to comprehensively reveal whether and how DCF is transformed in WAS mesophilic anaerobic fermentation through both experimental investigation and density functional theory (DFT) calculation. Experimental results showed that ∼28.8% and 45.8% of DCF were respectively degraded during the batch and long-term fermentation processes. Based on the detected intermediates and DFT-predicted active sites, three metabolic pathways, i.e., chlorination, hydroxylation, and dichlorination, responsible for DCF transformation were proposed. DFT calculation also showed that the Gibbs free energy (ΔG) of the three transformation pathways was respectively 19.0, -4.3, and -19.3 kcal/mol, suggesting that the latter two reactions (i.e., hydroxylation and dichlorination) were thermodynamically favorable. Illumina MiSeq sequencing analyses revealed that DCF improved the populations of complex organic degradation microbes such as Proteiniclasticum and Tissierellales, which was in accord with the chemical analyses above. This work updates the fundamental understanding of the degradation of DCF in WAS anaerobic fermentation process and enlightens engineers to apply theoretical calculation to the field of sludge treatment or other complex microbial ecosystems.

Enhanced Methane Production from Sludge Anaerobic Digestion with the Addition of Potassium Permanganate

This work aims to reveal the effect of potassium permanganate (KMnO₄) on the sludge anaerobic digestion process, as well as the relevant mechanisms. Experimental data showed that the biomethane production was gradually increased from 159.3 ± 3.0 to 211.5 ± 5.1 mL/g VSS (volatile suspended solids) when the KMnO₄ content was increased from 0 to 0.08 g/g VSS, with an increasing rate of 32.8%. A further increase in the KMnO₄ dosage however resulted in the decline of the methane yield. First-order kinetic model analysis indicated that higher methane production potentials and hydrolysis rates were achieved in KMnO₄-added reactors than in the control. Mechanism analysis demonstrated that KMnO₄ not only efficiently disintegrated the sludge flocs, which resulted in the increased contents of dissolved organics, but also enhanced the proportion of biodegradable substances in the sludge liquor. Meanwhile, the biodegradabilities of recalcitrant humus and lignocellulose substances were found to be promoted by KMnO₄ treatment as higher methane yields were attained from KMnO₄-treated model substrates. 16S rRNA analysis illustrated that the functional microbes participated in anaerobic digestion were largely enriched in the KMnO₄-pretreated digestor. Furthermore, efficient inactivation of the fecal coliform was achieved by KMnO₄ pretreatment.

Optimization of dissolution and fermentation acid production of rhamnolipid-alkali-heat synergistic pretreatment of sludge

With the development of the urban sewage treatment industry, the sludge output has increased year by year, and it has the characteristics of large output and high organic content, and has great potential for resource recovery. However, the biodegradability of sludge is poor, resulting in low added value of resource products. Therefore, it is necessary to treat sludge efficiently and improve the utilization. Based on this, the effects on sludge characteristics and acid-producing fermentation were investigated, and optimal conditions were determined by response surface method. The results showed that: The optimal conditions for experimental optimization are rhamnolipid (RL: 40 mg/gVS) alkali (Alk: 35 mg/gVS), heat: 80 °C. Response surface design optimization results are RL (28.44 mg/gVS), NaOH (35 mg/gVS), heat: 80 °C. In the process of RL-Alk-Heat pretreatment, the organic matter dissolution is Heat > Alk. Also, RL, Alk and Heat all promoted the content of fluorescent substances. From the results of the optimal combination verification test, it showed that SC (Soluble carbohydrate) and SP (Soluble protein) increase. Among them, three-factor treatment is higher than two-factor treatment than single-factor treatment.

Peracetic acid promotes biohydrogen production from anaerobic dark fermentation of waste activated sludge

Peracetic acid (PAA), a widely used organic peroxide with strong disinfection and oxidizing effect, has recently attracted research interest in waste activated sludge (WAS) treatment to achieve sludge reduction and resource utilization. However, its impact on hydrogen accumulation from WAS dark fermentation has not been documented. This study therefore is intended to fill in this knowledge gap and clarify the underlying mechanism of PAA-promoted hydrogen generation. Batch experiments revealed that when raised PAA dosage from 0 to 8 mg/g TSS (total suspended solids), cumulative hydrogen production within 168 h fermentation increased from 1.3 to 14.2 mL/g VSS (volatile suspended solids), however, further increase PAA dosage to 10 mg/g TSS resulted in a slight decrease in hydrogen yield. Mechanism studies revealed that PAA was beneficial to sludge disintegration (10 mg/g TSS PAA increased SCOD (soluble chemical oxygen demand) by 254 %). Although PAA inhibited the activity of all microorganism involved in dark fermentation, the inhibitory effect on hydrogen consumers were much more serious than that on hydrogen producers (-45.8 % versus -5.1 % and - 7.3 %). The fermentation was found to shift from propionate type to acetate and butyrate type, favoring hydrogen production. Moreover, the methane production process was effectively inhibited by PAA, which meant less hydrogen consumption. Microbial community analysis results unveiled that PAA increased the abundances of hydrolytic bacteria (e.g., norank_f__Saprospiraceae) and hydrogen producers (e.g., Clostridium_sensu_stricto_10). These findings obtained in this work provide new insights into oxidants-involved sludge treatment process and might have important implication for WAS treatment and bioenergy production in the future.

Insight into effects of polyethylene microplastics in anaerobic digestion systems of waste activated sludge: Interactions of digestion performance, microbial communities and antibiotic resistance genes

The environmental risks of microplastics (MPs) have raised an increasing concern. However, the effects of MPs in anaerobic digestion (AD) systems of waste activated sludge (WAS), especially on the fate of antibiotic resistance genes (ARGs), have not been clearly understood. Herein, the variation and interaction of digestion performance, microbial communities and ARGs during AD process of WAS in the presence of polyethylene (PE) MPs with two sizes, PE MPs-180μm and PE MPs-1mm, were investigated. The results showed that the presence of PE MPs, especially PE MPs-1mm, led to the increased hydrolysis of soluble polysaccharides and proteins and the accumulation of volatile fatty acids. The methane production decreased by 6.1% and 13.8% in the presence of PE MPs-180μm and PE MPs-1mm, respectively. Together with this process, hydrolytic bacteria and acidogens were enriched, and methanogens participating in acetoclastic methanogenesis were reduced. Meanwhile, ARGs were enriched obviously by the presence of PE MPs, the abundances of which in PE MPs-180μm and PE MPs-1mm groups were 1.2-3.0 times and 1.5-4.0 times higher than that in the control by the end of AD. That was associated with different co-occurrence patterns between ARGs and bacterial taxa and the enrichment of ARG-hosting bacteria caused by the presence of PE MPs. Together these results suggested the adverse effects of PE MPs on performance and ARGs removal during AD process of WAS through inducing the changes of microbial populations.

Insights into N2O turnovers under polyethylene terephthalate microplastics stress in mainstream biological nitrogen removal process

The ubiquitous microplastics in wastewater have raised growing concerns due to their unintended effects on microbial activities. However, whether and how microplastics affect nitrous oxide (N₂O) (a potent greenhouse gas) turnovers in mainstream biological nitrogen removal (BNR) process remain unclear. This work therefore aimed to fill such knowledge gap by conducting both long-term and batch tests. After over 100 days of feeding with wastewater containing polyethylene terephthalate (PET) microplastics (0-500 μg/L), the long-term results showed that both production and reduction of N₂O during denitrification were reduced, as well as the N₂O production during nitrification. Accordingly, 60% reduction in N₂O accumulation and 70% reduction in N₂O production were observed in the denitrification and nitrification batch tests, respectively. Nevertheless, the long-term N₂O emission factors under PET microplastics stress were comparable to that in the control reactor, mainly because PET microplastics led to more nitrite accumulation in anoxic period. With the aid of online N₂O sensors and site-preference analysis, it was demonstrated that the heterotrophic bacteria pathway and ammonia oxidizing bacteria denitrification pathway for N₂O production were negatively affected by PET microplastics, whereas a clear increase in the contribution of hydroxylamine pathway (+ 22.9%) was observed. Further investigation revealed that PET microplastics even at environmental level (i.e. 10 μg/L) significantly reshaped the BNR sludge characteristics (e.g. much larger particle size) and microbial communities (e.g. Thauera, Rhodobacte and Nitrospira) as well as the nitrogen metabolism pathways, which were chiefly responsible for the changes of N₂O turnovers and N₂O production pathways under the PET microplastics stress.

Advances in pretreatment strategies to enhance the biodegradability of waste activated sludge for the conversion of refractory substances

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

Hormesis-Like Effects of Tetrabromobisphenol A on Anaerobic Digestion: Responses of Metabolic Activity and Microbial Community

Tetrabromobisphenol A (TBBPA) has extensive applications in various fields; its release into ecosystems and the potential toxic effects on organisms are becoming major concerns. Here, we investigated the effects of TBBPA on anaerobic digestion, whose process is closely related to the carbon cycles under anaerobic conditions. The results revealed that TBBPA exhibited dose-dependent hormesis-like effects on methane production from glucose, i.e., the presence of 0.1 mg/L TBBPA increased the methane production rate by 8.79%, but 1.0-4.0 mg/L TBBPA caused 3.45-28.98% of decrement. We found that TBBPA was bound by the tyrosine-like proteins of the extracellular polymeric substances of anaerobes and induced the increase of reactive oxygen species, whose slight accumulation stimulated the metabolism activities but high accumulation increased the apoptosis of anaerobes. Owing to the differences between individual anaerobes in tolerance, TBBPA at 0.1 mg/L stimulated the acidogenesis and hydrogenotrophic methanogenesis, whereas higher levels (i.e., 1.0-4.0 mg/L) severely restrained all of the processes of acidogenesis, acetogenesis, and methanogenesis. Along with the accumulation of bisphenol A (BPA) produced from TBBPA by Longilinea sp. and Pseudomonas sp., the methanogenic pathway was partly shifted from acetate-dependent to hydrogen-dependent direction, and the activities of carbon monoxide dehydrogenase and acetyl-CoA decarbonylase/synthase were inhibited, while acetate kinase and F420 were hormetically affected. These findings elucidated the mechanism of anaerobic syntrophic consortium responses to TBBPA, supplementing the potential environmental risks of brominated flame retardants.

Methane production from peroxymonosulfate pretreated algae biomass: Insights into microbial mechanisms, microcystin detoxification and heavy metal partitioning behavior

This study investigated the methane production potential of algal biomass by anerobic digestion with the addition of peroxymonosulfate (PMS), the removal of microcystin were analyzed and discussed. The microcystin concentration in the collected algal sludge was 1.20 μg/L in the liquid phase and 1393 μg/g in the algal sludge before anaerobic fermentation. The microcystin concentration decreased to 0.20-0.35 μg/L in the liquid phase and 4.16-11.51 μg/g in the sludge phase after 60 days of digestion. The initial PMS dose and residue microcystin concentration could be simulated with a logarithmic decay model (R² > 0.87). Anaerobic digestion could recover energy from algal source in the form of methane gas, which was not affected in the presence of microcystin, and the microcystin removal rate was >99%. Digestion decreased the total contents of Cd and Zn in the liquid phase and increased the total contents of Cr and Pb in the liquid phase. The microbial community and function prediction results indicated that the PMS0.1 system had the highest methane production, which was attributed to the high abundance of Mechanosaeta (40.52%). This study provides insights into microbial mechanisms, microcystin detoxification and the heavy metal partitioning behavior of the algal biomass during methane production.

Microplastics aging in wastewater treatment plants: Focusing on physicochemical characteristics changes and corresponding environmental risks

Microplastics (MPs) have been frequently detected in effluent wastewater and sludge in wastewater treatment plants (WWTPs), the discharge and agricultural application of which represent a primary source of environmental MPs contamination. As important as quantitative removal is, changes of physicochemical characteristics of MPs (e.g., shapes, sizes, density, crystallinity) in WWTPs are crucial to their environmental behaviors and risks and have not been put enough attention yet. This review is therefore to provide a current overview on the changes of physicochemical characteristics of MPs in WWTPs and their corresponding environmental risks. The changes of physicochemical characteristics as well as the underlying mechanisms of MPs in different successional wastewater and sludge treatment stages that mainly driven by mechanical (e.g., mixing, pumping, filtering), chemical (e.g., flocculation, advanced oxidation, ultraviolet radiation, thermal hydrolysis, incineration and lime stabilization), biological (e.g., activated sludge process, anaerobic digestion, composition) and their combination effects were first recapitulated. Then, the inevitable correlations between physicochemical characteristics of MPs and their environmental behaviors (e.g., migration, adsorption) and risks (e.g., animals, plants, microbes), are comprehensively discussed with particular emphasis on the leaching of additives and physicochemical characteristics that affect the co-exist pollutants behavior of MPs in WWTPs on environmental risks. Finally, knowing the summarized above, some relating unanswered questions and concerns that need to be unveiled in the future are prospected. The physicochemical properties of MPs change after passing through WWTP, leading to subsequent changes in co-contaminant adsorption, migration, and toxicity. This could threaten our ecosystems and human health and must be worth investigating.

Contributions of MOF-808 to methane production from anaerobic digestion of waste activated sludge

The bioconversion of waste activated sludge (WAS) into methane is usually limited by the poor hydrolysis of sludge and/or poor syntrophic methanogenesis during anaerobic digestion (AD). In this study, the underlying mechanism of MOF-808 enhancing hydrolysis and syntrophic methanogenesis during AD process of WAS was investigated. Experimentally, with the effects of MOF-808 (150 mg MOF-808/g Volatile Solid (VS)), the methane production and the proportion of methane in biogas increased by approximately 26.7% and 15.6%, respectively, and the lag phase of methanogenesis decreased by 50.8%, which indicate that MOF-808 enhanced the generation efficiency of methane. The changes in activities of main hydrolytic enzymes with and without MOF-808 (150 mg MOF-808/g VS) during AD process revealed that MOF-808 improved the enzymatic hydrolysis of sludge, and the abiotic hydrolysis of sludge extracellular organic substances by MOF-808 shows that the maximum proportion and the initial increasing rate of low-molecular weight fractions increased by 60% and 583.7%, respectively, indicating that MOF-808 can greatly enhance the hydrolysis degree and rate of sludge via abiotic effect. These demonstrate that MOF-808 enhanced both biological and abiotic hydrolysis of sludge during AD. In addition, changes in the concentrations of acetate kinase and volatile fatty acids (VFAs) with and without MOF-808 (150 mg MOF-808/g VS) during AD process showed that MOF-808 accelerated the bioconversion of VFAs to methane, suggesting MOF-808 has a positive effect on syntrophic metabolism for methanogenesis. Moreover, further analyses of the microbial community structure of sludge samples with and without MOF-808 (150 mg MOF-808/g VS) showed that MOF-808 enriched hydrogen-producing bacteria and mixotrophic methanogens (i.e. Methanosarcina), and changed the methanogenic pathway via accelerating proton transfer between syntrophic anaerobes, especially improving the reduction of CO₂ to methane, and resulting in highly efficient syntrophic methanogenesis. These findings, however, may provide an important reference for enhancing AD efficiency of WAS based on MOF-like materials.