Changes in the characteristics of dissolved organic matter during sludge treatment: A critical review

Thermodynamics and explainable machine learning assist in interpreting biodegradability of dissolved organic matter in sludge anaerobic digestion with thermal hydrolysis

Dissolved organic matter (DOM) is essential in biological treatment, yet its specific roles remain incompletely understood. This study introduces a machine learning (ML) framework to interpret DOM biodegradability in the anaerobic digestion (AD) of sludge, incorporating a thermodynamic indicator (λ). Ensemble models such as Xgboost and LightGBM achieved high accuracy (training: 0.90-0.98; testing: 0.75-0.85). The explainability of the ML models revealed that the features λ, measured m/z, nitrogen to carbon ratio (N/C), hydrogen to carbon ratio (H/C), and nominal oxidation state of carbon (NOSC) were significant formula features determining biodegradability. Shapley values further indicated that the biodegradable DOM were mostly formulas with λ lower than 0.03, measured m/z value higher than 600 Da, and N/C ratios higher than 0.2. This study suggests that a strategy based on ML and its explainability, considering formula features, particularly thermodynamic indicators, provides a novel approach for understanding and estimating the biodegradation of DOM.

Characterizing properties and environmental behaviors of organic matter in sludge using liquid chromatography organic carbon detection and organic nitrogen detection: A mini-review

The presence of organic matter in sludge plays a significant role in sludge dewatering, anaerobic sludge digestion, resource (i.e., protein) recovery and pollutants removal (i.e., heavy metals) from sludge, as well as post-application of sludge liquid and solid digestate. This study summarized the current knowledge on using liquid chromatography organic carbon detection and organic nitrogen detection (LC-OCD-OND) for characterization and quantification of organic matter in sludge samples related with sludge treatment processes by fractionating organic matter into biopolymers, building blocks, humic substances, low molecular weight (LMW) acids, low LMW neutrals, and inorganic colloids. In addition, the fate, interaction, removal, and degradation of these fractions in different sludge treatment processes were summarized. A standardized extraction procedure for organic components in different extracellular polymeric substances (EPS) layers prior to the LC-OCD-OND analysis is highly recommended for future studies. The analysis of humic substances using the LC-OCD-OND analysis in sludge samples should be carefully conducted. In conclusion, this study not only provides a theoretical foundation and technical guidance for future experiments and practices in characterizing sludge organic matter using LC-OCD-OND, but also serves as a valuable resource for consulting engineers and other professionals involved in sludge treatment.

A systematic investigation of peracetic acid oxidation and polymeric coagulants re-flocculation to enhance activated sludge dewatering: Multi-porous skeleton structures

In this research, the effects of peracetic acid (PAA), polymeric flocculants, and their combined conditioning on improving the dewatering performance were comprehensively evaluated. The results showed that sludge cake moisture content, capillary suction time (CST), and specific resistance to filtration (SRF) were 70.6%, 48.1 s, and 3.42 × 1012 m/kg after adding 0.10 g/gMLSS PAA for 50 min, representing reductions of 12.60%, 40.32%, and 33.98%, respectively. Additionally, conditioning of sludge with polyferric sulfate (PFS), polyaluminum chloride (PAC), and cationic polyacrylamide (CPAM) enhanced sludge properties in the following order: CPAM > PAC > PFS. After the PAA oxidation and re-flocculation process, the optimal dosages of PFS, PAC, and CPAM were reduced to 1.5 g/L, 0.9 g/L, and 0.04 g/L, respectively. The sludge dewatering performance significantly improved, with sludge cake moisture content measuring 65.8%, 66.3%, and 61.7%, respectively. Moreover, the spatial multi-porous skeleton structures were formed via re-flocculation to improve the sludge dewatering. Furthermore, economic evaluation validated that the pre-oxidation and re-flocculation process could be considered an economically viable option. These research findings could serve as a valuable reference for practical engineering applications.

External redox couple enhanced anammox sludge activity at low temperature: Insight into intracellular resource synthesis

Anaerobic ammonium oxidation (anammox), an energy-efficient deamination biotechnology, faces operational challenges in low-temperature environments. Enhancing the metabolic activity of anammox bacteria (AnAOB) is pivotal for advancing its application in mainstream municipal wastewater treatment. Inspired by the metabolic adaptability of AnAOB and based on our previous findings, this work investigated the enhancement of intracellular ATP and NADH synthesis through the exogenous supply of reduced humic acid (HAred) and H2O2 redox couple, aiming to augment AnAOB activity under low-temperature conditions. Our experimental setup involved continuous dosing of 0.0067 μmol g-1 volatile suspended solid of H2O2 and 10 mg g-1 volatile suspended solid of HAred into a mainstream anammox reactor operated at 15 °C with an influent TN content of 60 mg/L. The results showed that HAred / H2O2 couple succeeded in maintaining the effluent TN at 10.72 ± 0.91 mg l-1. The specific anammox activity, ATP and NADH synthesis levels of sludge increased by 1.34, 2.33 and 6.50 folds, respectively, over the control setup devoid of the redox couple. High-throughput sequencing analysis revealed that the relative abundance of Candidatus Kuenenia after adding HAred / H2O2 couple reached 3.65 % at the end of operation, which was 5.14 folds higher than that of the control group. Further metabolomics analysis underscored an activation in the metabolism of amino acids, nucleotides, and phospholipids, which collectively enhanced the availability of ATP and NADH for the respiratory processes. These findings may provide guidance on strategy development for improving the electron transfer efficiency of AnAOB and underscore the potential of using redox couples to promote the mainstream application of anammox technology.

Mechanistic insights into chemical conditioning on transformation of dissolved organic matter and plant biostimulants production during sludge aerobic composting

Inorganic coagulants (aluminum and iron salt) are widely used to improve sludge dewaterability, resulting in numerous residues in dewatered sludge. Composting refers to the controlled microbial process that converts organic wastes into fertilizer, and coagulant residues in dewatered sludge can affect subsequent compost efficiency and resource recycling, which remains unclear. This work investigated the effects of two typical metal salt coagulants (poly aluminum chloride [PAC] and poly ferric sulfate [PFS]) conditioning on sludge compost. Our results revealed that PAC conditioning inhibited composting with decreased peak temperature, microbial richness, enzymatic reaction intensities, and compost quality, associated with decreased pH and microbial toxicity of aluminum. Nevertheless, PFS conditioning selectively enriched Pseudoxanthomonas sp. and resulted in more fertile compost with increased peak temperature, enzymatic reaction intensities, and humification degree. Spectroscopy and mass difference analyses indicated that PFS conditioning enhanced reaction intensities of labile biopolymers at the thermophilic stage, mainly comprising hydrolyzation (H2O), dehydrogenation (-H2, -H4), oxidation (+O1H2), and other reactions (i.e., +CH2, C2H4O1, C2H6O1). Unlike the common composting process primarily conducts humification at the cooling stage, PFS conditioning changed the main occurrence stage to the thermophilic stage. Non-targeted metabolomics revealed that indole (a humification intermediate) is responsible for the increased humification degree and indoleacetic acid content in the PFS-conditioned compost, which then promoted compost quality. Plant growth experiments further confirmed that the dissolved organic matter (DOM) in PFS-conditioned compost produced the maximum plant biomass. This study provided molecular-level evidence that PFS conditioning can promote humification and compost fertility during sludge composting, enabling chemical conditioning optimization for sustainable management of sludge.

Phosphorus release and realignment in anaerobic digestion of thermal hydrolysis pretreatment sludge - Masking effects from high ammonium

Waste activated sludge (WAS) is a significant phosphorus (P) repository, and there is a growing interest in P recovery from WAS. Typically, the commercial technology for treating WAS involves thermal hydrolysis pretreatment (THP) coupled with anaerobic digestion (AD). However, there is ongoing debate regarding the transformation and distribution of P throughout this process. To address this, a long-term THP-AD process was operated in this study to comprehensively investigate P transformation and distribution. The results revealed that a substantial biodegradation of dissolved organic nitrogen (DON) raised the pH of the digestate to 8.3 during the AD process. This increased pH facilitated the dissolution of Al, leading to a reduction of 6.92 mg/L of NaOH-P. Simultaneously, sulfate reduction contributed to a decrease of 11.04 mg/L of Bipy-P in the solid. However, the reduction of Bipy-P and NaOH-P in the solid did not result in an improved P release to the supernatant. Conversely, a decrease of 23.60 mg/L P in the aqueous phase was observed after anaerobic digestion. The disappeared P was primarily precipitated with Mg and Ca, driven by the increased pH, and it contributed to the increase of HCl-P in the solid from 107.80 to 144.52 mg/L. These findings were further confirmed by results obtained from scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and solid-state 31P nuclear magnetic resonance (NMR) spectroscopy. This study provides valuable insights into the mechanisms of P transformation during THP-AD process that is nearly opposite from conventional AD system.

Formation mechanism of persistent free radicals during pyrolysis of Fenton-conditioned sewage sludge: Influence of NOM and iron

The present study provided an innovative insight into the formation mechanism of persistent free radicals (PFRs) during the pyrolysis of Fenton-conditioned sludge. Fenton conditioners simultaneously improve the dewatering performance of sewage sludge and catalyze the pyrolysis of sewage sludge for the formation of PFRs. In this process, PFRs with a total number of spins of 9.533×1019 spins/g DS could be generated by pyrolysis of Fenton-conditioned sludge at 400°C. The direct thermal decomposition of natural organic matter (NOM) fractions contributed to the formation of carbon-centered radicals, while the Maillard reaction produced phenols precursors. Additionally, the reaction between aromatic proteins and iron played a crucial role in the formation of phenoxyl or semiquinone-type radicals. Kinetics analysis using discrete distributed activation energy model (DAEM) demonstrated that the average activation energy for pyrolysis was reduced from 178.28 kJ/mol for raw sludge to 164.53 KJ/mol for Fenton conditioned sludge. The reaction factor (fi) indicated that the primary reaction in Fenton-conditioned sludge comprised of 27 parallel first-order reactions, resulting from pyrolysis cleavage of the NOM fractions, the Maillard reaction, and iron catalysis. These findings are significant for understanding the formation process of PFRs from NOM in Fenton-conditioned sludge and provide valuable insight for controlling PFRs formation in practical applications.

Exogenous additive ferric sulfate regulates sulfur-oxidizing bacteria in cow manure composting to promote carbon fixation

Enhancing carbon fixation in the composting process was of great significance in the era of massive generation of organic solid waste. In this study, the experimental results showed that the contents of dissolved organic matter (DOM) in the experimental group (CT) were 37.58% higher than those in the control group (CK). The CO2 emission peaked on day 5, and the value of CK was 1.34 times that of CT. Significant differences were observed between the contents of sulfur fractions in CT and CK. This phenomenon may be due to the suppression of sulfur-reducing gene expression in CT. On day 51 of composting, the abundance of sulfur-oxidizing bacteria (SOB) Rhodobacter (5.33%), Rhodovulum (14.76%), and Thioclava (23.83%) in CT was higher than that in CK. In summary, the composting fermentation regulated by Fe2(SO4)3 increased the sulfate content, enhanced the expression of sulfur-oxidizing genes and SOB, and ultimately promoted carbon sequestration during composting.

Enhancing bioavailable carbon sources and minimizing ammonia emissions in distillery sludge and distiller's grains waste co-composting through deep eutectic solvent addition

This study introduced two deep eutectic solvents, ChCl/oxalic acid (CO) and ChCl/ethylene glycol (CE), into a 34-day co-composting process of distillery sludge and distiller's grains waste to address challenges related to NH3 emissions. The addition of DES increased dissolved organic carbon by 68% to 92%, offering more utilizable carbon for microorganisms. SYTO9/PI staining and enzyme activity tests showed the CE group had higher bacterial activity and metabolic levels during the thermophilic phase than the control. Bacterial community analysis revealed that early dominance of Lactobacillus and Lysinibacillus in CE accelerated the onset of the thermophilic phase, reduced pile pH, and significantly decreased urease production by reducing Ureibacillus. Consequently, CE treatment substantially dropped NH3 emissions by 73% and nitrogen loss by 54%. Besides, CE fostered a more abundant functional microbial community during the cooling and maturation phases, enhancing deep degradation and humification of organic matter.

Dearomatization drives complexity generation in freshwater organic matter

Dissolved organic matter (DOM) is one of the most complex, dynamic and abundant sources of organic carbon, but its chemical reactivity remains uncertain1-3. Greater insights into DOM structural features could facilitate understanding its synthesis, turnover and processing in the global carbon cycle4,5. Here we use complementary multiplicity-edited 13C nuclear magnetic resonance (NMR) spectra to quantify key substructures assembling the carbon skeletons of DOM from four main Amazon rivers and two mid-size Swedish boreal lakes. We find that one type of reaction mechanism, oxidative dearomatization (ODA), widely used in organic synthetic chemistry to create natural product scaffolds6-10, is probably a key driver for generating structural diversity during processing of DOM that are rich in suitable polyphenolic precursor molecules. Our data suggest a high abundance of tetrahedral quaternary carbons bound to one oxygen and three carbon atoms (OCqC3 units). These units are rare in common biomolecules but could be readily produced by ODA of lignin-derived and tannin-derived polyphenols. Tautomerization of (poly)phenols by ODA creates non-planar cyclohexadienones, which are subject to immediate and parallel cycloadditions. This combination leads to a proliferation of structural diversity of DOM compounds from early stages of DOM processing, with an increase in oxygenated aliphatic structures. Overall, we propose that ODA is a key reaction mechanism for complexity acceleration in the processing of DOM molecules, creation of new oxygenated aliphatic molecules and that it could be prevalent in nature.

Application of mid-infrared spectroscopy to the prediction and specification of pesticide sorption: A promising and cost-effective tool

The cocktail of pesticides sprayed to protect crops generates a miscellaneous and generalized contamination of water bodies. Sorption, especially on soils, regulates the spreading and persistence of these contaminants. Fine resolution sorption data and knowledge of its drivers are needed to manage this contamination. The aim of this study is to investigate the potential of Mid-Infrared spectroscopy (MIR) to predict and specify the adsorption and desorption of a diversity of pesticides. We constituted a set of 37 soils from French mainland and West Indies covering large ranges of texture, organic carbon, minerals and pH. We measured the adsorption and desorption coefficients of glyphosate, 2,4-dichlorophenoxyacetic acid (2,4-D) and difenoconazole and acquired MIR Lab spectra for these soils. We developed Partial Least Square Regression (PLSR) models for the prediction of the sorption coefficients from the MIR spectra. We further identified the most influencing spectral bands and related these to putative organic and mineral functional groups. The prediction performance of the PLSR models was generally high for the adsorption coefficients Kdads (0.4 < R2 < 0.9 & RPIQ >1.8). It was contrasted for the desorption coefficients and related to the magnitude of the desorption hysteresis. The most significant spectral bands in the PLSR differ according to the pesticides indicating contrasted interactions with mineral and organic functional groups. Glyphosate interacts primarily with polar mineral groups (OH) and difenoconazole with hydrophobic organic groups (CH2, CC, COO-, C-O, C-O-C). 2,4-D has both positive and negative interactions with these groups. Finally, this work suggests that MIR combined with PLSR is a promising and cost-effective tool. It allows both the prediction of adsorption and desorption parameters and the specification of these mechanisms for a diversity of pesticides including polar active ingredients.

Relation between hydrophilic/hydrophobic characteristics of sludge extracellular polymeric substances and sludge moisture-holding capacity in hot-pressing drying

Sludge poses a serious threat to the environmental health. Hot-pressing drying has been proven efficient in sludge treatment because of the reduced thermal contact resistance, rapid increase in sludge temperature, and high drying rate. Sludge extracellular polymeric substances (EPS) significantly influence moisture transfer. However, whether in hot-pressing or traditional thermal drying, the effect of EPS on sludge moisture-holding capacity is rarely reported. Thereby, this study investigated the relationship between hydrophilic/hydrophobic characteristics of EPS and sludge moisture-holding capacity at various drying time and mechanical compression using XAD resin fractionation. Thermodynamic analysis indicated that sludge moisture desorption isotherms, net isosteric heat of desorption, and differential entropy presented a downward trend with the increase in drying time and mechanical compression, suggesting reduced sludge moisture-holding capacity. EPS analysis showed that at the same drying time, applying 25 kPa mechanical compression increased sludge temperature by 16 % and protein content by 13.8 %. At the same sludge temperature, protein content rose by 7.3 % compared to the drying without mechanical compression. It was concluded that the fast rise in sludge temperature and the mechanical extrusion facilitated the destruction of sludge microbial flocs, accelerating the release of intracellular and EPS-bound moisture and contributing to the decrease in moisture-holding capacity. Besides, tryptophan protein-like substances were the major source of hydrophilic/hydrophobic organic matter, compared to polysaccharide and humic acid-like substances. The gradually reduced sludge moisture-holding capacity was divided into three stages. Below 67 °C, the moisture desorption was dominated by the release of intracellular moisture. Below 85 °C, the increase in protein and the enhanced exposure of hydrophobic functional groups in protein improved the hydrophobicity of EPS. Above 85 °C, protein consumption due to thermal decomposition and browning reaction facilitated the desorption of EPS-bound moisture. Hence, this study provided novel insights into the mechanism of sludge drying.

Synergistic effect of organic matter-floc size-bound water and multifactorial quantitative model of optimal reagent demand in sewage sludge conditioning process prior to dewatering

The high amount of densely hydrated organic substance present in sewage sludge impedes its filterability, thus restricting sludge disposal. Although chemical conditioning can facilitate filtration, the diverse sludge properties complicate the quantitative control of conditioning process. Investigating how to accurately quantify the optimal reagent demand (ORD) based on the critical physicochemical properties of the target sludge is an effective way to address the current issue. This study focused on the sewage and stockpiled sludge with varying properties, and their ORD under different chemical conditioning. The results showed that organic content, floc size, and bound water synergistically influenced conditioning process. The quantitative models were established between their coupling indicators and ORD, with coupling indicators including the ratio of organic content to floc size, the ratio of flow viscosity to floc size, and the ratio of the product of organic content and bound water to floc size. The linear correlation of the coupling indicator with ORD was higher than that of the traditional single-factor indicator. Furthermore, the inherent filterability of the sludge was somewhat separate from the adjustability of its filtration. A "dual-system" impact model was proposed to characterized the conditioning and filtration processes. These results provide theoretical guidance for the quantitative regulation of conditioning and filtration processes of sludge with complex characteristics.

Landfill bacteriology: Role in waste bioprocessing elevated landfill gaseselimination and heat management

This study delves into the critical role of microbial ecosystems in landfills, which are pivotal for handling municipal solid waste (MSW). Within these landfills, a complex interplay of several microorganisms (aerobic/anaerobic bacteria, archaea or methanotrophs), drives the conversion of complex substrates into simplified compounds and complete mineralization into the water, inorganic salts, and gases, including biofuel methane gas. These landfills have dominant biotic and abiotic environments where various bacterial, archaeal, and fungal groups evolve and interact to decompose substrate by enabling hydrolytic, fermentative, and methanogenic processes. Each landfill consists of diverse bio-geochemical environments with complex microbial populations, ranging from deeply underground anaerobic methanogenic systems to near-surface aerobic systems. These kinds of landfill generate leachates which in turn emerged as a significant risk to the surrounding because generated leachates are rich in toxic organic/inorganic components, heavy metals, minerals, ammonia and xenobiotics. In addition to this, microbial communities in a landfill ecosystem could not be accurately identified using lab microbial-culturing methods alone because most of the landfill's microorganisms cannot grow on a culture medium. Due to these reasons, research on landfills microbiome has flourished which has been characterized by a change from a culture-dependent approach to a more sophisticated use of molecular techniques like Sanger Sequencing and Next-Generation Sequencing (NGS). These sequencing techniques have completely revolutionized the identification and analysis of these diverse microbial communities. This review underscores the significance of microbial functions in waste decomposition, gas management, and heat control in landfills. It further explores how modern sequencing technologies have transformed our approach to studying these complex ecosystems, offering deeper insights into their taxonomic composition and functionality.

Characteristics of dissolved organic matter (DOM) in Chinese farmland soils under different climate zone types: A molecular perspective

Interactions between dissolved organic matter (DOM) and surrounding environments are highly complex. Understanding DOM at the molecular level can contribute to the management of soil pollution and safeguarding agricultural fields. Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) has enabled a molecular-level understanding of DOM. Accordingly, in this study, we investigated soil samples from 27 different regions of mainland China with various soil types and climatic characteristics. Based on the geographical features of the four typical climatic zones in mainland China (temperate monsoon, temperate continental, subtropical monsoon, and Qinghai-Tibet Plateau climates), we employed high-resolution mass spectrometry to determine the molecular diversity of DOM under different climatic conditions. The results indicated that lignin and tannin-like substances were the most active categories of DOM in the soils. Collectively, the composition and unsaturation of DOM molecules are influenced by sunlight, precipitation, temperature, and human activity. All climatic regions contained a substantial number of characteristic molecules, with CHO and CHON constituting over 80%, and DOM containing nitrogen and sulfur was relatively more abundant in the monsoon regions. The complex composition of DOM incorporates various active functional groups, such as -NO2 and -ONO2. Furthermore, soil DOM in the monsoon regions showed higher unsaturation and facilitated various (bio) biochemical reactions in the soil.

The microbial oxidation of pharmaceuticals in an anaerobic aqueous environment: Effect of dissolved organic matter fractions from different sources

Previous studies have demonstrated the importance of dissolved organic matter (DOM) on the biodegradation of trace organic contaminants occurred in the hyporheic zone. However, the role of diverse DOM fractions with distinct physicochemical properties on the biodegradation of pharmaceuticals under reducing conditions is scarcely known. To address this knowledge gap, DOMs derived from road-deposited sediment, soil, and active sludge (namely allochthonous DOM) and algae (namely autochthonous DOM) were collected and isolated into different fractions. Thereafter, the effect of DOM fractions on the anaerobic microbial oxidation of two typical pharmaceuticals, i.e., ritonavir (RTV) and tetracycline (TC) was explored by using simulated anaerobic microcosms. Mechanistic insights into how DOM fractions from different sources influence pharmaceutical biodegradation processes were provided by optical and electrochemical analyses. Results showed that humic acid and fulvic acid fractions from allochthonous DOM could enhance the biodegradation of TC (12.2 % per mgC/L) and RTV (14.5 % per mgC/L), while no significant impact was observed for that of hydrophilic fractions. However, autochthonous DOM promoted the biodegradation of TC (4.17 % per mgC/L) and inhibited that of RTV. Mechanistic analysis showed that the higher of humification and aromatization level of DOM components, the stronger their promotive effect on the biodegradation of TC and RTV. Further, the promotive mechanism could be attributed to the response of quinone moieties in DOM as extracellular electron acceptors that yields more energy to support microbial metabolism. These results provide a more comprehensive understanding of diverse DOM fractions mediating microbial anaerobic oxidation of trace organic pollutants, and extend our insights into contamination control and remediation technologies.

Natural zeolite enhances anaerobic digestion of waste activated sludge: Insights into the performance and the role of biofilm

Anaerobic digestion is widely employed for the treatment of waste activated sludge (WAS) due to its advantages like simultaneous energy recovery and sludge stabilization, promoting carbon-neutral operation of wastewater treatment plants. Natural zeolite, a low-cost and eco-friendly additive, has the potential to improve methane production from anaerobic digestion. This study investigated the effects of natural zeolite on anaerobic digestion when the substrate was WAS. It was found that methane production potential in response to natural zeolite was dosage-dependent. The optimal dosage was 0.1 g zeolite/g volatile suspended solids (VSS), with a methane yield of 181.89 ± 6.75 mL/g VSS, which increased by 20.1% compared to that of the control. Although the methane yields with other dosages of natural zeolite were higher than that of control, they were lesser than that with 0.1 g zeolite/g VSS. Natural zeolite affected transfer and conversion of proteins much more than polysaccharides in liquid phase and extracellular polymeric substances. In anaerobic digestion, natural zeolite had with little effects on WAS solubilization, while it improved hydrolysis, acidification, and methanogenesis. The dosages of natural zeolite did have significant effects on bacterial communities in biofilm rather than suspension, while the archaeal communities in biofilm and suspension were all greatly related to natural zeolite dosages. The developed biofilms promoted richness and functionality of microbial communities. The syntrophic metabolism relationships between methanogens and bacteria were improved, which was proved by selective enrichment of Methanosarcina, Syntrophomonas, and Petrimonas. The findings of this work provided some new solutions for promoting methane production from WAS, and the roles of natural zeolite in anaerobic digestion.

Binding of Dissolved Organic Matter to RNA and Protection from Nuclease-Mediated Degradation

The persistence of RNA in environmental systems is an important parameter for emerging applications, including ecological surveys, wastewater-based epidemiology, and RNA interference biopesticides. RNA persistence is controlled by its rate of biodegradation, particularly by extracellular enzymes, although the specific factors determining this rate have not been characterized. Due to prior work suggesting that nucleic acids-specifically DNA-interact with dissolved organic matter (DOM), we hypothesized that DOM may bind RNA and impede its biodegradation in natural systems. We first adapted a technique previously used to assess RNA-protein binding to differentiate RNA that is bound at all sites by DOM from RNA that is unbound or partially bound by DOM. Results from this technique suggested that humic acids bound RNA more extensively than fulvic acids. At concentrations of 8-10 mgC/L, humic acids were also found to be more effective than fulvic acids at suppressing enzymatic degradation of RNA. In surface water and soil extract containing DOM, RNA degradation was suppressed by 39-46% relative to pH-adjusted controls. Due to the ability of DOM to both bind and suppress the enzymatic degradation of RNA, RNA biodegradation may be slowed in environmental systems with high DOM concentrations, which may increase its persistence.

Carbon dots/TiO2 enhanced visible light-assisted photocatalytic of leachate: Simultaneous effects and Mechanism insights

The persistence and potential fouling risks associated with humic substances and bacteria present in leachate have gained increasing attention. Therefore, developing efficient and environmentally compatible technologies for their removal is essential. This study presented the hydrothermal synthesis of a photocatalyst by coupling carbon dots (CDs) and bulk TiO2 (P25). The incorporation of CDs increased the photocatalytic performance by enhancing visible light absorption and facilitating the separation of electrons/holes. Compared to P25, the CDs/P25 exhibited optimal photocatalytic activity for humic acid (HA), fulvic acid (FA), and leachate, with 1.64, 1.02, and 1.12 times higher activity, respectively. Remarkedly, the CDs/P25 accelerated the conversion of large HA molecules into small molecules at a faster rate and higher amount than the bulk P25, due to the increase of hydroxyl radicals, monoclinic oxygen radicals, and superoxide radicals. Additionally, the CDs/P25 demonstrated better bacterial-deactivation ability than the P25, with dead bacteria percentages of 83.3% and 34.6%, respectively. This study provides a promising strategy for efficiently applying CDs/P25 photocatalysis to leachate treatment.

Spatiotemporal evolution of dissolved organic matter (DOM) and its response to environmental factors and human activities

The South-to-North Water Diversion East Project (SNWDP-E) is an effective way to realize the optimal allocation of water resources in China. The North Dasha River (NDR) is the reverse recharge section that receives water from the Yufu River to the Wohushan Reservoir transfer project line in the SNWDP. However, the dissolved organic matter (DOM) evolution mechanism of seasonal water transfer projects on tributary waters has not been fully elucidated. In this paper, the NDR is the main object, and the changes in the composition and distribution of spectral characteristics during the winter water transfer period (WT) as well as during the summer non-water transfer period (NWT) are investigated by parallel factor analysis (PARAFAC). The results showed that the water connectivity caused by water transfer reduces the environmental heterogeneity of waters in the basin, as evidenced by the ammonia nitrogen (NH4+-N) and total phosphorus (TP) in the water body were significantly lower (p<0.05, p<0.01) during the water transfer period than the non-water transfer period. In addition, the fluorescence intensity of DOM was significantly lower in the WT than the NWT (p<0.05) and was mainly composed of humic substances generated from endogenous sources with high stability. While the NWT was disturbed by anthropogenic activities leading to significant differences in DOM composition in different functional areas. Based on the redundancy analysis (RDA) and multiple regression analysis, it was found that the evolution of the protein-like components is dominated by chemical oxygen demand (COD) and NH4+-N factors during the WT. While the NWT is mainly dominated by total nitrogen (TN) and TP factors for the evolution of the humic-like components. This study helps to elucidate the impact of water transfer projects on the trunk basin and contribute to the regulation and management of inter-basin water transfer projects.

Characteristics in dissolved organic matter and disinfection by-product formation during advanced treatment processes of municipal secondary effluent with Orbitrap mass spectrometry

Dissolved organic matter (DOM) is reported to be a precursor to disinfection by-products (DBPs), which have adverse effects on human health. Therefore, it is crucial to effectively remove DOM before water disinfection. Characteristics of DOM and DBPs formation during advanced treatment processes including coagulation, adsorption, ultraviolet (UV) irradiation, and ozone (O3) oxidation in municipal secondary effluent were investigated in this research. DOM was characterized by Fourier transform infrared spectroscopy (FTIR), excitation-emission matrix fluorescence spectroscopy (EEM), and Orbitrap mass spectrometry (Orbitrap MS). Moreover, DBPs formation potential under different advanced treatment processes was also discussed. FTIR results indicated that various functional groups existing in DOM may react with the disinfectant to form toxic DBPs. EEM analysis indicated that DOM in all water samples was dominated by soluble microbial product-like (SMPs) and humic acid-like (HA) substances. The municipal secondary effluent was abundant with DOM and rich in carbon, hydrogen, oxygen, and nitrogen atoms, contained a certain dosage of phosphorus and sulfur atoms, and the highest proportion is lignin. Most of the precursors (CHO features) had positive double bond equivalent subtracted oxygen per carbon [(DBE-O)/C] and negative carbon oxidation state (Cos) in all four different advanced treatment processes. DBPs formation potential (DBPFP) of coagulation, adsorption, UV irradiation, and O3 oxidation advanced treatment processes were 487 μg L-1, 586 μg L-1, 597 μg L-1, and 308 μg L-1, respectively. And the DBPs precursors removal efficiency of coagulation, adsorption, UV irradiation, and O3 oxidation advanced treatment processes were 50.8%, 40.8%, 39.8%, and 69.0%, respectively. This study provides in-depth insights into the changes of DOM in municipal secondary effluent at the molecular level and the removal efficiency of DBPs precursors during coagulation, adsorption, UV irradiation, and O3 oxidation advanced treatment processes.

Behavior of organic components and the migration of heavy metals during sludge dewatering by different advanced oxidation processes via optical spectroscopy and molecular fingerprint analysis

A comparative study of the different advanced oxidation processes (Fe(II)-Oxone, Fe(II)-H2O2, and Fe(II)-NaClO) was carried out herein to analyze the characteristics of organic components and the migration of heavy metals in waste activated sludge. With the Fe(II)-Oxone and Fe(II)-H2O2 treatments, sludge dewaterability was significantly improved, however, sludge dewaterability was deteriorated by the Fe(II)-NaClO treatment. The enhanced sludge dewaterability by the Fe(II)-Oxone and Fe(II)-H2O2 treatments was strongly correlated with the shifted organic components, particularly proteins, in soluble extracellular polymeric substances (S-EPS), while the deteriorated sludge dewaterability by the Fe(II)-NaClO treatment was strongly correlated with the over release of organic components from bound EPS (B-EPS) to S-EPS. For both the Fe(II)-Oxone and Fe(II)-H2O2 treatments, the radicals preferentially attacked humic acid-like organic components over the protein-like organic components in S-EPS, while for the Fe(II)-NaClO treatment, interestingly, the radicals preferentially attacked the protein-like organic components in both S-EPS and B-EPS. The hydrophilic functional groups like phenolic OH and CO of polysaccharides may be more preferentially migrated to S-EPS of sludge by the Fe(II)-NaClO treatment compared to the other two treatments. With the Fe(II)-Oxone and Fe(II)-H2O2 treatments, the proportion of aliphatic compounds as well as the much oxygenated organic components with a low desaturation and a low molecular weight increased. While with the Fe(II)-NaClO treatment, the proportion of low oxygenated organic components with a high desaturation and a high molecular weight increased. The concentration of total organic carbon, particularly the concentration of proteins, may be the key factor determining the shift of Zn and Cu from sludge solid to liquid phase, along with the high oxidation extent of organic components and close binding to CHOS and CHON compounds as indicated by density functional theory (DFT) calculation. This study systematically revealed the simultaneous sludge dewatering and migration of heavy metals when the role of organic components was factored into herein.

Potential of fluorescent tracers to appraise biochar amendment strategies for pesticide mitigation - insights from comparative sorption

Mitigation of pesticide dispersion in soil and water is required to protect ecosystem health and the anthropic uses of water bodies. Biochar amendments have been suggested to reduce pesticide dispersion due to their high sorption potentials. Nevertheless, appraisals at different scales have been limited by the costs of pesticide analyses. The aim of this study was to evaluate the potential of two fluorescent tracers, uranine (UR) and sulforhodamine B (SRB), for use as pesticide proxies in the context of biochar amendments used for mitigation purposes. Therefore, we compared the sorption processes of both fluorescent tracers and those of three pesticides, glyphosate, 2,4-D, and difenoconazole for soils; three wood biochars (pine, oak, and beech/charm blend); and soil/biochar mixtures representing agricultural usages. The results showed that the sorption of glyphosate by soil was unaffected by amendment with the tested pine, oak, and wood blend biochars. In contrast, the sorption coefficients of UR, SRB, 2,4-D, and difenoconazole were significantly increased with these biochar amendments. SRB, in particular, exhibited sorption behavior similar to that of the hydrophobic fungicide difenoconazole. This indicates promise for the use of SRB as a proxy for hydrophobic pesticides, in testing biochar amendments.

Treatment of atrazine-containing wastewater by algae-bacteria consortia: Signal transmission and metabolic mechanism

Atrazine is a toxic endocrine disruptor. Biological treatment methods are considered to be effective. In the present study, a modified version of the algae-bacteria consortia (ABC) was established and a control was simultaneously set up to investigate the synergistic relationship between bacteria and algae and the mechanism by which atrazine is metabolized by those microorganisms. The total nitrogen (TN) removal efficiency of the ABC reached 89.24% and the atrazine concentration was reduced to below the level recommended by the Environment Protection Agency (EPA) regulatory standards within 25 days. The protein signal released from the extracellular polymeric substances (EPS) secreted by the microorganisms triggered the resistance mechanism of the algae, and the conversion of humic acid to fulvic acid and electron transfer constituted the synergistic mechanism between the bacteria and algae. The mechanism by which atrazine is metabolized by the ABC mainly consists of hydrogen bonding, H-pi interactions, and cation exchange with atzA for hydrolysis, followed by a reaction with atzC for decomposition to non-toxic cyanuric acid. Proteobacteria was the dominant phylum for bacterial community evolution under atrazine stress, and the analysis revealed that the removal of atrazine within the ABC was mainly dependent on the proportion of Proteobacteria and the expression of degradation genes (p < 0.01). EPS played a major role in the removal of atrazine within the single bacteria group (p < 0.01).

Biosolids-derived fertilisers: A review of challenges and opportunities

Soil application of biosolids as an organic fertiliser continues to be a cost-effective way to beneficially utilise its carbon and nutrient contents to maintain soil fertility. However, ongoing concerns over microplastics and persistent organic contaminants means that land-application of biosolids has come under increased scrutiny. To identify a way forward for the ongoing future use of biosolids-derived fertilisers in agriculture, the current work presents a critical review of: (1) contaminants of concern in biosolids and how regulatory approaches can address these to enable on-going beneficial reuse, (2) nutrient contents and bioavailability in biosolids to understand agronomic potential, (3) developments in extractive technologies to preserve and recover nutrients from biosolids before destructive dissipation when the biosolids are thermally processed to deal with persistent contaminants of concern (e.g. microplastics), and (4) use of the recovered nutrients, and the biochar produced by thermal processing, in novel organomineral fertilisers that match specific equipment, crop and soil requirements of broad-acre cropping. Several challenges were identified and recommendations for prioritisation of future research and development are provided to enable safe beneficial reuse of biosolids-derived fertilisers. Opportunities include more efficient technologies to preserve, extract and reuse nutrients from sewage sludge and biosolids, and the production of organomineral fertiliser products with characteristics that enable reliable widespread use across broad-acre agriculture.

Sludge source-redox mediators obtainment and availability for enhancing bioelectrogenesis and acidogenesis: Deciphering characteristics and mechanisms

Anaerobic biological treatment was regarded as one of promising options for realizing concurrent WAS reduction, stabilization and bioenergy/bioresource recycle. But the relatively low treatment efficiency limited its spreading application toward larger scale considerably in China. Aimed at such barrier, this study offered a novel enhancing strategy for achieving high-efficiency of bioenergy/bioresource recycle from WAS anaerobic treatment via improving bioelectrogenesis/acidogenesis using sludge source-redox mediators (SSRMs). SSRMs not only facilitated bioeletrogenesis with an increasing efficiency of 36% for voltage output and 39% for bioelectricity bioconversion, but also enhanced acidogenesis of WAS with a mean elevating efficiency of 37.5% of volatile fatty acids (VFAs) production within 5 d Mechanistic investigations indicated that SSRMs had a potential influence on improving the protein and carbohydrate metabolisms-related genes' expression for enhancing bioelectrogenesis and acidogenesis. Moreover, SSRMs exerted roles of electrochemical "catalysts" or as terminal electron acceptors with affecting functional proteins of complexes of Ⅰ and Ⅳ in electron transfer chains for improving electron transfer efficiency. Meanwhile, the core members' abundance, microbial diversity and community distributive evenness were prompted concurrently for carrying out superior bioelectrogenesis and acidogenesis. A schematic illustration was established for demonstrating the mechanism of SSRMs for enhancing bioelectrogenesis and acidogenesis via changing microbial metabolism functions, enhancing electron transfer efficiency, and regulating functional genes' expression of functional proteins (up-regulating cytochrome c oxidase and down-regulating-NADH dehydrogenase). This study provided an effective enhancing strategy for facilitating WAS bioconversion to bioenergy/bioresource with well-process sustainability.

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

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

Characterizing molecular transformation of dissolved organic matter during high-solid anaerobic digestion of dewatered sludge using ESI FT-ICR MS

In this study, the effects of anaerobic digestion (AD) on molecular characteristics of dissolved organic matter (DOM) in the dewatered sludge has been described by advanced electrospray ionization combined with Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) technology. With the progress of AD, molecular amounts in DOM samples increased with the lowering in the carbon atom number of average molecular formula and average double bond equivalent (DBE). CHON and CHONS groups are the two main organic substances in sludge with their relative DOM proportions of 29.64% and 32.56%, respectively. The resistants (i.e., refractory organic matter) mainly consist of the proteins regions of CHO groups as well as the proteins/lignin regions of CHON groups. The contrasting temporal trends in protein contents (e.g., decrease (CHO and CHON) vs. increase (CHONS)) may imply differences in their degradation characteristics. Likewise, the multi-N (N₃, N₄) and S₂ organic groups in the sludge are converted to N₂ and S₁ molecules, while the relative abundance of O atoms (in Ox molecules) tends to increase. In addition, the resistants in sludge DOM contain high oxidizing C and low unsaturation. The overall results of this research are expected to provide the theoretical basis for further optimization of the sludge AD process.

Molecular composition and chemodiversity of dissolved organic matter in wastewater sludge via Fourier transform ion cyclotron resonance mass spectrometry: Effects of extraction methods and electrospray ionization modes

High-resolution mass spectrometry was extensively applied in molecular composition and transformation pathways of dissolved organic matter (DOM) in wastewater sludge treatments. Sample pretreatment methods and electrospray ionization (ESI) modes significant affect the accuracy of molecular characterization for DOM. This study investigated the effects of pretreatment methods (styrene divinyl benzene polymer (PPL), octadecyl (C18), and electrodialysis (ED)) on molecular characteristics of DOM in two typical wastewater sludges (waste activated sludge (WAS) and anaerobic digestion sludge (ADS)) analyzed by FT-ICR MS in both positive ESI (ESI (+)) and negative ESI (ESI (-)) modes. The results indicated that ED pretreatment exhibited the highest recovery rate of 70% ‒ 95% for sludge-derived DOM. ED and PPL performed well in recovering the different sludge-derived DOM with a high similarity of molecular characteristics (e.g., lipids, proteins/aliphatic, and lignins/CRAM-like), and the C18 method was ineffective in extracting carbohydrates, unsaturated hydrocarbons, and amino sugars. In addition, compared with single ESI (-) analysis mode, the molecular number identified by ESI (+) analysis mode was increased by 200%, especially, more unsaturated hydrocarbons and N-containing compounds were detected. Except for biogenic DOM, plenty of emerging containments (ECs) in sludge-derived DOM were identified; ESI (-) mode was more effectively in recognizing the alkyl benzene sulfonic acids (e.g., anionic surfactants); and ESI (+) mode was more effectively for plasticizers identification, for example, dioctyl terephthalate and dibutyl phthalate. This study illustrated that ED pretreatment coupled with FT-ICR MS in dual ESI modes could give more insights in complexed molecular information for DOM in wastewater sludge, and provides a theoretical basis for subsequent sludge treatments and disposals.

Biostimulants in dissolved organic matters recovered from anaerobic digestion sludge with alkali-hydrothermal treatment: Nontarget identification by ultrahigh-resolution mass spectrometry

Recovering high-value biomaterials from anaerobic digestion sludge (ADS) has attracted considerable attention. However, the molecular features and biological effects of abundant dissolved organic matters (DOMs) in ADS are still unclear, which limits the efficient recycling and application of these bioproducts. This study investigated the molecular composition and transformation of DOMs recovered from ADS through a mild-temperature alkali-hydrothermal treatment (AHT) with ultrahigh-resolution mass spectrometry and energy spectroscopy, and the fertilizing effects of DOMs were evaluated by rice hydroponics. The results indicated that AHT processes significantly promoted the solubilization and release of DOMs from ADS, where most of DOMs molecules remained unchanged and mainly consisted of N-containing compounds with 1-3 N atoms, featuring aromatic or N-heterocyclic rings. Furthermore, AHT processes at pH of 9-10 induced the hydrolysis of partial protein-like substances in DOMs, which was accompanied by formation of heterocyclic-N compounds. Under AHT at pH of 11-12, protein-like and heterocyclic-N substances were increasingly decomposed into amino-N compounds containing 1 or 5 N atoms, while numerous oxygenated aromatic substances with phytotoxicity were degraded and removed from DOMs. Rice hydroponic test verified that ADS-derived DOMs recovered by AHT process at pH of 12 exhibited the highest bioactivity for rice growth, which was attributed to the abundance of amino compounds and humic substances. This study proposed a novel process for the recovery of high-quality liquid organic fertilizer from ADS through AHT process, which can further enrich the technical options available for the safe utilization of sludge resources.

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

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

Fractionation, molecular composition, and biological effects of organic matter in bio-stabilization sludge with implication to land utilization

Bioactive organic compounds (BOCs) contained in bio-stabilized products of waste activated sludge (WAS) have attracted considerable attention, as they can enhance the fertilizing effect of WAS in land applications. This study investigated the molecular composition and plant-growth-promoting mechanisms of various BOCs in the bio-stabilized products of WAS. After stepwise fractionation, aerobic composting sludge (ACS) and anaerobic digestion sludge (ADS) were chemically fractioned into five subcomponents, namely dissolved organic matter (DOM) (C1), weakly interacted organic matter (OM) (C2), metal-bonded OM (C3), NaOH-extracted OM (C4), and strongly interacted OM (C5), in sequence. The results showed that fatty acids and carboxylic acid (CAs) present in ACS C2 promoted plant growth and enhanced the ability of plants against stresses by upregulating pathways related to "carbohydrate metabolism," "lipid metabolism," "amino acid metabolism," and "phenylpropanoid biosynthesis." However, in ACS C4, plenty of amino acids could promote plant growth via upregulating "carbohydrate metabolism" and "amino acid metabolism" pathways. As an important precursor, aromatic amino acids inside ACS C4 also stimulated the production of indoleacetic acids. In ADS C1, amino sugar and phytohormone were the major BOCs causing the up-regulation of "carbohydrate metabolism" and AAA catabolism in "amino acid metabolism" pathways. CAs enriched in ADS C2 stimulated plant growth through "amino acid metabolism" pathway. In summary, alkali extraction can recycle a large proportion of BOCs with low environmental risk from the bio-stabilization products of WAS. The results from this study provide scientific guidance for safe and value-added resource utilization of bio-stabilization products of WAS in land applications.

Microbial mechanisms of refractory organics degradation in old landfill leachate by a combined process of UASB-A/O-USSB

A combined process of anaerobic digestion (UASB), shortcut nitrification-denitrification (A/O), and semi-anoxic co-metabolism (operated by an up-flow semi-anoxic sludge bed; USSB) was constructed for the treatment of old landfill leachate (>10 years). The performance and mechanism of refractory organics degradation by the combined process (UASB-A/O-USSB) were investigated. The results showed that the semi-anoxic co-metabolism contributes 57 % of the totally degraded refractory organics. Specific microorganisms and their corresponding metabolic functions drive the degradation of refractory organics in each unit of the UASB-A/O-USSB process. In detail, organics with simple molecular structures were preferentially degraded by anaerobic digestion and shortcut denitrification, whereas those with complex structures were subsequently degraded in the oxic tanks and USSB reactor by shortcut nitrification and semi-anoxic co-metabolism. The structural equation model showed that the combined process of shortcut nitrification and semi-anoxic co-metabolism had a better effect on the degradation of recalcitrant organics than the single process. These findings provide information on how refractory organics are metabolically degraded in a combined process.

Deciphering the role of microplastic size on anaerobic sludge digestion: Changes of dissolved organic matter, leaching compounds and microbial community

Here the role of microplastic size on dissolved organic matter, leaching compounds and microbial community during anaerobic sludge digestion was evaluated. Compared to that without the addition of polyvinyl chloride (PVC), during the 30 days' incubation, the anaerobic sludge digestion by adding PVC at the size of 75 μm and the concentration of 2.4 g/g volatile solids (VS) showed a 8.5% lower cumulative methane production, while a 17.9% higher cumulative methane production was noted by adding PVC at the size of 3000 μm and the concentration of 2.4 g/g VS. A long-term fed-batch laboratory-scale fermenter test for 147 days further testified, that higher removal efficiencies of total solids, volatile solids, and total chemical oxygen demand, and higher methane production were noted by adding PVC (2.4 g/g VS, 3000 μm) into the fermenter. More interestingly, higher concentrations of proteins, polysaccharides, volatile fatty acids, and soluble microbial by-products component were noted in the liquid phase of sludge drawn from the fermenter added with PVC since the biomass therein showed higher efficiencies of solubilization, hydrolysis, acidification, and methanogenesis. Moreover, as identified from the fermenter added with PVC, dibutyl phthalate (DBP) was the most predominant leaching phthalates compound, although the biomass therein showed a 93.4% anaerobic biodegradability of DBP. The leaching of DBP drove the predominance of microbial community towards Synergistota and Methanosaeta. More irregular elliptical shallow dimples were noted on the PVC surface after 147 days' incubation, accompanied with abundances of Proteobacteria, Actinobacteriota, Chloroflexi, Methanosaeta and Methanobacterium. The results from this study showed that the size of microplastic was a crucial factor in evaluating its impact on anaerobic sludge digestion.

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.

The role of dissolved organic matter during Per- and Polyfluorinated Substance (PFAS) adsorption, degradation, and plant uptake: A review

The negative effects of polyfluoroalkyl substances (PFAS) on the environment and health have recently attracted much attention. This article reviews the influence of soil- and water-derived dissolved organic matter (DOM) on the environmental fate of PFAS. In addition to being co-adsorped with PFAS to increase the adsorption capacity, DOM competes with PFAS for adsorption sites on the surface of the material, thereby reducing the removal rate of PFAS or increasing water solubility, which facilitates desorption of PFAS in the soil. It can quench some active species and inhibit the degradation of PFAS. In contrast, before DOM in water self-degrades, DOM has a greater promoting effect on the degradation of PFAS because DOM can complex with iron, iodine, among others, and act as an electron shuttle to enhance electron transfer. In soil aggregates, DOM can prevent microorganisms from being poisoned by direct exposure to PFAS. In addition, DOM increases the desorption of PFAS in plant root soil, affecting its bioavailability. In general, DOM plays a bidirectional role in adsorption, degradation, and plant uptake of PFAS, which depends on the types and functional groups of DOM. It is necessary to enhance the positive role of DOM in reducing the environmental risks posed by PFAS. In future, attention should be paid to the DOM-induced reduction of PFAS and development of a green and efficient continuous defluorination technology.

Elucidating the dialogue between arbuscular mycorrhizal fungi and polyamines in plants

The most dominant arbuscular mycorrhizal (AM) symbiont can be established on roots of most terrestrial plants by beneficial AM fungi. A type of polycationic and aliphatic compounds, polyamines (PAs), are involved in plant physiological activities including stress responses. Interestingly, small amounts of PAs such as putrescine (Put) and spermidine (Spd) were found in AM fungal spores, and they are considered to be a component involved in mycorrhizal development, including mycorrhizal colonization, appressoria formation, spore germination and mycelial growth. Thus, PAs are regulatory factors in plant-AM symbiosis. Inoculation of AM fungi also affects the metabolism of endogenous PAs in host plants, including PAs synthesis and catabolism, thus, regulating various physiological events of the host. As a result, there seems to be a dialogue between PAs and AM fungi. Existing knowledge makes us understand that endogenous or exogenous PAs are an important regulator factor in the growth of AM fungi, as well as a key substance to colonize roots, which further enhances mycorrhizal benefits in plant growth responses and root architecture. The presence of AM symbiosis in roots alters the dynamic balance of endogenous PAs, triggering osmotic adjustment and antioxidant defense systems, maintaining charge balance and acting as a stress signalling molecule, which affects various physiological activities, such as plant growth, nutrient acquisition, stress tolerance and improvement of root architecture. This review mainly elucidated (i) what is the role of fungal endogenous PAs in fungal growth and colonization of roots in host plants? (ii) how AM fungi and PAs interact with each other to alter the growth of fungi and plants and subsequent activities, providing the reference for the future combined use of AM fungi and PAs in agricultural production, although there are still many unknown events in the dialogue.

The Influence of Municipal Wastewater Treatment Technologies on the Biological Stabilization of Sewage Sludge: A Systematic Review

Various wastewater treatment technologies are available today and biological processes are predominantly used in these technologies. Increasing wastewater treatment systems produces large amounts of sewage sludge with variable quantities and qualities, which must be properly managed. Anaerobic and aerobic digestion and composting are major strategies to treat this sludge. The main indicators of biological stabilization are volatile fatty acids (VFAs), volatile solids (VS), the carbon/nitrogen (C/N) ratio, humic substances (HS), the total organic carbon (TOC), the carbon dioxide (CO2) evolution rate, the specific oxygen uptake rate (SOUR), and the Dewar test; however, different criteria exist for the same indicators. Although there is no consensus for defining the stability of sewage sludge (biosolids) in the research and regulations reviewed, controlling the biological degradation, vector attraction, and odor determines the biological stabilization of sewage sludge. Because pollutants and pathogens are not completely removed in biological stabilization processes, further treatments to improve the quality of biosolids and to ensure their safe use should be explored.

"Humic substances" measurement in sludge dissolved organic matter: A critical assessment of current methods

The role of humic substances (HS) during sludge treatment has been the focus in recent years. Quantification of HS in sludge dissolved organic matter (DOM) and the chemical and structural characterization of HS data are the prerequisite for understanding their role during different sludge treatment processes. Currently, a number of published articles inadequately acknowledge fundamental principles of analysis methods both in terms of experimental approach and data analysis. Therefore, a more comprehensive and detailed description of the experimental methods and the data analysis are needed. In this study, the current used methods for HS quantification in DOM of sludge had been tested for different calibration and sludge DOM samples. The results indicated that the current methods showed overestimated and contradictory results for HS quantification in sludge DOM. To be specific, using the modified Lowry method, different values were obtained depending on the humic acids used for calibration, and false negative results were observed for some sludge samples. By using the relative amount of HS (based on dissolved organic carbon (DOC)) to total sludge DOM (based on DOC), variations among the results of different analysis methods for the same sample were high. According to the calculated Bray-Curtis dissimilarity indexes, the results for HS quantification obtained by three-dimensional excitation emission matrix (3D-EEM), either with spectra analysis methods by peak picking, fluorescence region integration (both region volume and area integration), or PARAllel FACtor analysis showed higher degrees of dissimilarity to those quantified by size exclusion liquid chromatography or XAD-8 method. The selection of fluorescence regions for HS seemed to be the determining factor for overestimation obtained by the 3D-EEM technique. In future work, strategies, like a consistent terminology of HS, the use of an internal standard sample, and the related standardized operation for HS quantification in sludge DOM need to be established.

Enhanced dewaterability of waste-activated sludge with zero-valent iron-activated persulfate oxidation under mild hydrothermal conditions

A novel technique to enhance sludge dewaterability with zero-valent iron-activated persulfate (ZVI/PDS) and hydrothermal treatment (HT) under mild temperature is proposed in this study. Key operating parameters were considered to study their influences on the dewaterability of sludge. Comparative studies of organic matter, especially extracellular polymeric substances (EPS), were analyzed carefully to reveal the mechanisms involved. The results indicated that the specific resistance to filtration declined by 86.72%, and the capillary suction time reduced by 72.35% compared with the raw sludge under optimal conditions of 100 mg/g TSS ZVI and 200 mg/g TSS PDS doses at 120 °C. Soluble protein and fulvic acid-like in EPS were the key components affecting the sludge dewatering performance. The disappearance of the peak in the amide III led to a decrease in hydrophilic functional groups, which helped to improve sludge dewaterability.

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).

Mechanism insights into hydrothermal dewatering of food waste digestate for products valorization

Poor dewaterability is a bottleneck of the disposal of digestate from food waste (DFW). However, the dewatering mechanism remains unclear due to the complex composition of DFW. Understanding the dewatering mechanism, as well as the transformation of organic/inorganic matters is essential for the DFW management and valorization. In this study, the distribution, transformation, and complex interplay of organic and inorganic matters at different Hydrothermal treatment (HTT) temperatures were comprehensively analyzed to explore the hydrothermal dewatering mechanism of DFW. When HTT was conducted in the temperature range of 120-180 °C, the interstitial water was released as surface or free water because of membrane breaking and size reduction of the solid substrate. Releasing divalent cations increased the Zeta potential of the bulk solution. The weaker electrostatic repulsion between suspended particles made them easier to settle as the centrifugation cake. When the temperature of HTT was above 180 °C, polymerization and aromatization reactions took place gradually for organic matters, and the bound water was further removed. The generated humic substances were more hydrophobic than the raw material. In addition, the humic substance could combine with cationic metals, which decreased the zeta potential of the bulk solution but promoted the aggregation of nanoparticles and enhance the dewaterability of DFW.

Effect of mixed primary and secondary sludge for two-stage anaerobic digestion (AD)

As an energy-efficient and eco-friendly sludge treatment process, two-stage anaerobic digestion (AD) is widely employed to recovery biomass energy from waste sludge. However, the effect of primary and secondary sludge for two-stage AD was not clear. In this study, two-stage AD of mixed sludge in different volume ratio was investigated. The maximum cumulative H₂ yield (100.5 ml) and CH₄ yield (2643.6 ml) were obtained in volume ratio of 1:3 (primary sludge: secondary sludge). In two-phase AD, mixed sludge could induce positive effect on both organics releasing in extracellular polymeric substances (EPS) and the utilization of volatile fatty acids (VFAs). By investigating the compositional characteristics of dissolved organic matters (DOM) through excitation-emission matrix (EEM) coupling with fluorescence regional integration (FRI), it revealed more degradable substances utilization in mixture of sludge. Results from this work suggest that two-phase AD with mixed sludge is efficient for renewable energy recovery.

Comprehensive insights into the organic fractions on solid-liquid separation performance of anaerobic digestates from food waste

Anaerobic digestion (AD) is an effective approach for food waste treatment and valorization. However, AD is limited by proper disposal of the food waste digestates in megacities. The objective of this study was to elucidate the dominant factors on solid-liquid separation performance of the digestates from food wastes. Results indicated that the composition differences in organic fractions showed significant impacts on solid-liquid separation performance by comparing the digestates from different cities in China. Dissolved organic matter (DOM), especially for proteins and carbohydrates, varied between the digestates in various cities, which may lead to the difference in solid-liquid separation performance. Digestate with lowest proteins (33.86 mg/L) from Shanghai's food waste AD plant had the best dewatering performance. In contrast, digestate from Nanjing's food waste AD plant showed the worst dewatering performance. Pearson's correlation analysis indicated that there is a significant correlation between soluble protein and the dewaterability of digestates (R² > 0.9573, p < 0.03), the lower soluble protein achieved higher dewaterability of digestates.

Molecular mechanisms of interaction between enzymes and Maillard reaction products formed from thermal hydrolysis pretreatment of waste activated sludge

Thermal hydrolysis pretreatment (THP) is often used to improve the anaerobic digestion performance of waste activated sludge (WAS) in wastewater treatment plants (WWTPs). During the THP process, the proteins and polysaccharides in the biomass will undergo hydrolysis and Maillard reaction, producing biorefractory organic substances, such as recalcitrant dissolved organic nitrogen (rDON) and melanoidins. In this study, a series of spectroscopy methods were used to quantitatively analyze the Maillard reaction of glucose and lysine, and the interaction mechanisms of the Maillard reaction products (MRPs) and lysozyme were investigated. Results showed that the typical aromatic heterocyclic structures in MRPs, such as pyrazine and furan, were found to quench molecular fluorescence of lysozyme, resulting in an unfolding of standard protein structure and increase in lysozyme hydrophobicity. Significant loss of enzyme activity was detected during this process. Thermodynamic parameters obtained from isothermal titration calorimetry (ITC) confirmed that the interaction between MRPs and lysozyme occurred both exothermically and spontaneously. Density functional theory (DFT) calculations suggested that the molecular interactions of MRPs and protein included parallel dislocation aromatic stacking, T-shaped vertical aromatic stacking, H-bond and H-bond coupled to aromatic stacking.

Application of Internal Carbon Source from Sewage Sludge: A Vital Measure to Improve Nitrogen Removal Efficiency of Low C/N Wastewater

Biological nitrogen removal from wastewater is widely used all over the world on account of high efficiency and relatively low cost. However, nitrogen removal efficiency is not optimized when the organic matter has inadequate effect for the lack of a sufficient carbon source in influent. Although addition of an external carbon source (e.g., methanol and acetic acid) could solve the insufficient carbon source problem, it raises the operating cost of wastewater treatment plants (WWTPs). On the other hand, large amounts of sludge are produced during biological sewage treatment, which contain high concentrations of organic matter. This paper reviews the emerging technologies to obtain an internal organic carbon resource from sewage sludge and their application on improving nitrogen removal of low carbon/nitrogen wastewater of WWTPs. These are methods that could solve the insufficient carbon problem and excess sludge crisis simultaneously. The recovery of nitrogen and phosphorus from treated sludge before recycling as an internal carbon source should also be emphasized, and the energy and time consumed to treat sludge should be reduced in practical application.

Tracing sewage contamination based on sterols and stanols markers within the mainland aquatic ecosystem: a case study of Linggi catchment, Malaysia

Sewage contamination is a principal concern in water quality management as pathogens in sewage can cause diseases and lead to detrimental health effects in humans. This study examines the distribution of seven sterol compounds, namely coprostanol, epi-coprostanol, cholesterol, cholestanol, stigmasterol, campesterol, and β-sitosterol in filtered and particulate phases of sewage treatment plants (STPs), groundwater, and river water. For filtered samples, solid-phase extraction (SPE) was employed while for particulate samples were sonicated. Quantification was done by using gas chromatography-mass spectrometer (GC-MS). Faecal stanols (coprostanol and epi-coprostanol) and β-sitosterol were dominant in most STP samples. Groundwater samples were influenced by natural/biogenic sterol, while river water samples were characterized by a mixture of sources. Factor loadings from principal component analysis (PCA) defined fresh input of biogenic sterol and vascular plants (positive varimax factor (VF)1), aged/treated sewage sources (negative VF1), fresh- and less-treated sewage and domestic sources (positive VF2), biological sewage effluents (negative VF2), and fresh-treated sewage sources (VF3) in the samples. Association of VF loadings and factor score values illustrated the correlation of STP effluents and the input of biogenic and plant sterol sources in river and groundwater samples of Linggi. This study focuses on sterol distribution and its potential sources; these findings will aid in sewage assessment in the aquatic environment.

Integrated Microbiome and Metabolome Analysis Reveals a Positive Change in the Intestinal Environment of Myostatin Edited Large White Pigs

Myostatin (MSTN) functional inactivation can change the proportion of lean meat and fat content in pigs. While both genotype and microbial composition are known to affect the host phenotype, so far there has been no systematic study to detect the changes in the intestinal microbial composition and metabolome of MSTN single copy mutant pigs. Here, we used 16S rDNA sequencing and metabolome analysis to investigate how MSTN gene editing affects changes in the microbial and metabolome composition in the jejunum and the cecum of Large White pigs. Our results showed that Clostridium_sensu_stricto_1, Bifidobacterium, Lachnospiraceae_UCG-007, Clostridium_sensu_stricto_6, Ruminococcaceae_UCG-002, and Ruminococcaceae_UCG-004 were significantly upregulated; while Treponema_2 and T34_unclassified were significantly downregulated in the jejunum of MSTN pigs. Similarly, Phascolarctobacterium, Ruminiclostridium_9, Succinivibrio, Longibaculum, and Candidatus_Stoquefichus were significantly upregulated, while Barnesiella was significantly downregulated in the cecum of MSTN pigs. Moreover, metabolomics analysis showed significant changes in metabolites involved in purine, sphingolipid and tryptophan metabolism in the jejunum, while those associated with glycerophospholipid and pyrimidine metabolism were changed in the cecum. Spearman correlation analysis further demonstrated that there was a significant correlation between microflora composition and metabolites. Our analyses indicated the MSTN editing affects the composition of metabolites and microbial strains in the jejunum and the cecum, which might provide more useable nutrients for the host of MSTN± Large White pigs.