Effect of lignin on short-chain fatty acids production from anaerobic fermentation of waste activated sludge

Antibiotic fermentation residue for biohydrogen production: Inhibitory mechanisms of the inherent antibiotic

Antibiotic fermentation residue, which is generated from the microbial antibiotic production process, has been a troublesome waste faced by the pharmaceutical industry. Dark fermentation is a potential technology to treat antibiotic fermentation residue in terms of renewable H2 generation and waste management. However, the inherent antibiotic in antibiotic fermentation residue may inhibit its dark fermentation performance, and current understanding on this topic is limited. This investigation examined the impact of the inherent antibiotic on the dark H2 fermentation of Cephalosporin C (CEPC) fermentation residue, and explored the mechanisms from the perspectives of bacterial communities and functional genes. It was found that CEP-C in the antibiotic fermentation residue significantly inhibited the H2 production, with the H2 yield decreasing from 17.2 mL/g-VSadded to 12.5 and 9.6 mL/g-VSadded at CEP-C concentrations of 100 and 200 mg/L, respectively. CEP-C also prolonged the H2-producing lag period. Microbiological analysis indicated that CEP-C remarkably decreased the abundances of high-yielding H2-producing bacteria, as well as downregulated the genes involved in hydrogen generation from the"pyruvate pathway" and"NADH pathway", essentially leading to the decline of H2 productivity. The present work gains insights into how cephalosporin antibiotics influence the dark H2 fermentation, and provide guidance for mitigating the inhibitory effects.

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.

Exploring the Relationships Between Bacterial Community, Taste-Enhancing Peptides and Aroma in Thai Fermented Fish (Pla-ra)

As a traditional Thai condiment, Pla-ra is used to add flavor and richness to dishes. Nine treatment combinations of Pla-ra formulations created from 3 types of fish (Mor fish, Kradee fish, and Mor + Kradee fish) and 4 different carbohydrate sources (none, rice bran, roasted rice, and rice bran─roasted rice mixture) were studied through a 12 month fermentation period (1, 3, 5, 7, 8, 9, 10, 11, and 12 months). 16S rRNA Next Generation Sequencing (NGS) and LC-MS/MS techniques were used to analyze the microbial diversity and identify taste-enhancing peptides. Descriptive sensory analysis was performed on the extracts of the 108 Pla-ra samples mixed in a model broth. Koku perception and saltiness-enhancing attributes were clearly perceived and dominant in all samples, even though glutamyl peptides, including γ-Glu-Val-Gly, were found at subthreshold levels. The samples from mixed fish and Mor fish fermented with roasted ground rice and rice bran for 12 months had the most typical Pla-ra odors and tastes and had high taste-enhancing activities. NGS analysis revealed the presence of bacteria containing a large number of protease and aminopeptidase genes in the samples. Bacillus spp., Gallicola spp., and Proteiniclasticum spp. correlated well with the generation of glutamyl and arginyl peptides and typical odors in the samples. These results confirmed the typical sensory quality of Pla-ra depended on protein sources, carbohydrate sources, and bacteria communities. Further optimization of the microbial composition found could lead to the development of starter cultures to control and promote flavor development in fermented fish products.

Deep eutectic solvents pretreatment enhances methane production from anaerobic digestion of waste activated sludge: Effectiveness evaluation and mechanism elucidation

Anaerobic digestion (AD) is a prevalent waste activated sludge (WAS) treatment, and optimizing methane production is a core focus of AD. Two DESs were developed in this study and significantly increased methane production, including choline chloride-urea (ChCl-Urea) 390% and chloride-ethylene glycol (ChCl-EG) 540%. Results showed that ChCl-Urea mainly disrupted extracellular polymeric substances (EPS) structures, aiding in initial sludge solubilization during pretreatment. ChCl-EG, instead, induced sludge self-driven organic solubilization and enhanced hydrolysis and acidification processes during AD process. Based on the extent to which the two DESs promoted AD for methane production, the AD process can be divided into stage Ⅰ and stage Ⅱ. In stage Ⅰ, ChCl-EG promoted methanogenesis more significantly, microbiological analysis showed both DESs enriched aceticlastic methanogens-Methanosarcina. Notably, ChCl-Urea particularly influenced polysaccharide-related metabolism, whereas ChCl-EG targeted protein-related metabolism. In stage Ⅱ, ChCl-Urea was more dominant than ChCl-EG, ChCl-Urea bolstered metabolism and ChCl-EG promoted genetic information processing in this stage. In essence, this study investigated the microbial mechanism of DES-enhanced sludge methanogenesis and provided a reference for future research.

Analysis of landfill leachate promoting efficient application of weathered coal anaerobic fermentation

This research aimed to develop a new method for clean utilization and treatment of landfill leachate and solid waste weathered coal. Landfill leachate and weathered coal were adopted for combined anaerobic fermentation for methane production. The characteristics of microbial community, mechanism of biological methane production, and utilization characteristics of fermentation broth and solid residue for co-fermentation were analyzed through metagenomics, soluble organic matter detection and thermogravimetric (TG) analysis. The obtained results revealed that combined anaerobic fermentation increased methane production by 80.1%. Syntrophomonas, Salipiger, Methanosaeta and Methanothrix were highly correlated. Gene abundances of 2-oxoacid ferredoxin oxidoreductase and enolase were increased in methane conversion pathway mainly by acetic acid. Pyruvate-ferroredoxin oxidoreductase, 2-oxoglutarate synthase and succinate dehydrogenase acetate synthase intensified electron transfer pathways among microorganisms. Fulvic acid, tyrosine and tryptophan contents were high in fermentation broth. Volatile decomposition temperature, ignition point and residual char combustion temperature of residual coal were decreased and combustion was more stable. The obtained results showed that the co-fermentation of landfill leachate and weathered coal improved biological methane gas production, degraded weathered coal and improved combustion performance, which provided a new idea for weathered coal clean utilization.

Response of rumen microorganisms to pH during anaerobic hydrolysis and acidogenesis of lignocellulose biomass

Rumen microorganisms can efficiently degrade lignocellulosic wastes to produce volatile fatty acids (VFAs). pH is a key factor in controlling the type and yield of VFAs by affecting the microorganisms involved in rumen fermentation. However, the effects of different pH on rumen microbial diversity, communities, and mechanisms are unclear. In this study, the hydrolysis and acidogenesis of corn straw and diversity, communities, and mechanisms of rumen microorganisms were explored at different initial pHs. Results showed that the highest hemicellulose, cellulose, and lignin degradation efficiency of corn straw was 55.2 %, 38.3 %, and 7.01 %, respectively, and VFA concentration was 10.2 g/L at pH 7.0. Low pH decreased the bacterial diversity and increased the fungal diversity. Rumen bacteria and fungi had different responses to initial pHs, and the community structure of bacteria and fungi had obviously differences at the genus level. The core genera Succiniclasticum, Treponema, and Neocallimastix relative abundance at initial pH 7.0 samples were significantly higher than that at lower initial pHs, reaching 6.01 %, 1.61 %, and 5.35 %, respectively. The bacterial network was more complex than that of fungi. pH, acetic acid, and propionic acid were the main factors influencing the bacterial and fungal community structure. Low pH inhibited the expression of functional genes related to hydrolysis and acidogenesis, explaining the lower hydrolysis and acidogenesis efficiency. These findings will provide a better understanding for rumen fermentation to produce VFAs.

Anaerobic co-fermentation of waste activated sludge with corn gluten meal enhanced phosphorus release and volatile fatty acids production: Critical role of corn gluten meal dosage on fermentation stages and microbial community traits

The anaerobic co-fermentation of iron bound phosphorus (P) compounds (FePs)-bearing sludge with corn gluten meal (CGM) and the underlying mechanisms associated with P release and volatile fatty acids (VFAs) production were investigated. The optimal CGM dosage for P release was 0.6 g chemical oxygen demand (COD)/g total suspended solid (TSS), which resulted in an increase in efficiency from 7 % (control sample) to 39 %. However, the optimal CGM dosage for VFAs production was 0.4 g COD/g TSS, and the yield increased from 37.4 (control sample) to 331.7 mg COD/g volatile suspended solid. The addition of CGM enhanced hydrolysis and acidogenesis by supplying abundant organic substrates to promote the growth of hydrolytic and acidogenic bacteria. A higher VFAs/ammonium-nitrogen ratio resulted in a lower pH, which promoted greater FePs dissolution and P release from the sludge. This study provides novel insights into the effects of CGM on P release and 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.

Performance and mechanisms of urea exposure for enhancement of biotransformation of sewage sludge into volatile fatty acids

Herein, a cost-effective method for improving the anaerobic fermentation performance of sewage sludge (SS) is proposed. The highest volatile fatty acids (VFAs) reached up to 5550 mg COD/L with the supplementation of 0.2 g urea/g total suspended solids (TSS). Intensive exploration showed that SS decomposition was profoundly triggered by urea and the free ammonia generated due to the hydrolysis of urea, providing adequately bioaccessible substrates for acidogenic reactions and thus contributing to VFAs formation. Microbial composition analysis indicated that functional bacteria (i.e., Tissierella and Clostridium) associated with VFAs generation were enriched. Moreover, the metabolic activities of functional flora (i.e., membrane transport and fatty acid synthesis) were up-regulated due to the stimulation of urea. In general, the increase in bioavailable organic matter and functional microbes, and thus the increased microbial metabolic activities, improved the efficient production of VFAs. This study could provide a cost-effective approach for resource recovery from SS.

Enhancing acidogenic fermentation of waste activated sludge via urea hydrogen peroxide pretreatment: Performance and mechanisms

Improving the anaerobic treatment performance of waste activated sludge (WAS) to achieve resource recovery is an indispensable requirement to reduce carbon emissions, minimize and stabilize biosolids. In this study, a novel strategy by using urea hydrogen peroxide (UHP) to enhance SCFAs production through accelerating WAS disintegration, degrading recalcitrant substances and alleviating competitive suppression of methanogens. The SCFAs production and acetate proportion rose from 436.9 mg COD/L and 31.3% to 3102.6 mg COD/L and 54.1%, respectively, when UHP grew from 0 to 80 mg/g TSS. Mechanism investigation revealed that OH, O2 and urea were the major contributors to accelerate WAS disintegration with the sequence of OH> O2 > urea. Function microbes related to acidification and genes associated with acetate production ([EC:2.3.1.8] and [EC:2.7.2.1]) were upregulated while genes encoding propionic acid production ([EC:6.4.1.3] and [EC:6.2.1.1]) were downregulated. These results raised the application prospects of UHP in WAS resource utilization.

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.

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.

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.

Valorization of food waste into short-chain fatty acids via enzymatic pretreatment: Effects of fermentation-pH on acid-producing processes and microbial metabolic functions

Food waste (FW) has been widely considered as an essential resource for the production of short-chain fatty acids (SCFAs), an important class of chemicals with wide applications and over 20 million tons of annual market demand, by anaerobic fermentation. Although enzymatic pre-treatment could improve the FW biodegradation efficiency, resulting in enhanced efficiency of solubilization and hydrolysis, the influence of fermentation-pH on the SCFAs production and the metabolic functions, have rarely been reported. This study demonstrated that the uncontrolled pH could efficiently lead to an increase in the SCFAs production (33011 mgCOD/L) during long-term fermentation of FW (mainly consisting of 48.8% carbohydrates, 20.6% proteins, and 17.4% lipids) after enzymatic pre-treatment compared to the control (16413 mgCOD/L). Meanwhile, the acid-producing processes (i.e., solubilization, hydrolysis, and acidification) were synchronously enhanced by the enzymatic pre-treatment and no control over fermentation-pH. Metagenomic analysis revealed that the acid-forming microorganisms (i.e., Olsenella sp. and Sporanaerobacter) were significantly accumulated, and the corresponding genetic expressions related to extracellular hydrolysis (i.e., aspB and gltB), membrane transport (i.e., metL and glnH), and intracellular material metabolism (i.e., pfkA and ackA) were evidently stimulated, thereby promoting ultimate SCFAs generation. Although the alkaline conditions could further slightly increase the SCFAs yield slightly (37100 mgCOD/L) and also stimulate the metabolic activities, it might not be suitable for large-scale practical applications due to additional costs associated with alkaline chemical additives.

Dose-response and type-dependent effects of antiviral drugs in anaerobic digestion of waste-activated sludge for biogas production

In the context of the COVID-19 pandemic, antiviral drugs (AVDs) were heavily excreted into wastewater and subsequently enriched in sewage sludge due to their widespread use. The potential ecological risks of AVDs have attracted increasing attention, but information on the effects of AVDs on sludge anaerobic digestion (AD) is limited. In this study, two typical AVDs (lamivudine and ritonavir) were selected to investigate the responses of AD to AVDs by biochemical methane potential tests. The results indicated that the effects of AVDs on methane production from sludge AD were dose- and type-dependent. The increased ritonavir concentration (0.05-50 mg/kg TS) contributed to an 11.27-49.43% increase in methane production compared with the control. However, methane production was significantly decreased at high lamivudine doses (50 mg/kg TS). Correspondingly, bacteria related to acidification were affected when exposed to lamivudine and ritonavir. Acetoclastic and hydrotropic methanogens were inhibited at a high lamivudine dose, while ritonavir enriched methylotrophic and hydrotropic methanogens. Based on the analysis of intermediate metabolites, the inhibition of lamivudine and the promotion of ritonavir on acidification and methanation were confirmed. In addition, the existence of AVDs could affect sludge properties. Sludge solubilization was inhibited when exposed to lamivudine and enhanced by ritonavir, perhaps caused by their different structures and physicochemical properties. Moreover, lamivudine and ritonavir could be partially degraded by AD, but 50.2-68.8% of AVDs remained in digested sludge, implying environmental risks.

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.

Boosting short-chain fatty acids production from co-fermentation of orange peel waste and waste activated sludge: Critical role of pH on fermentation steps and microbial function traits

The anaerobic co-fermentation of orange peel waste (OPW) and waste activated sludge (WAS) for useful short-chain fatty acids (SCFAs) generation presents an environmentally friendly and efficient method for their disposal. This study amied to investigate the effects of pH regulation on OPW/WAS co-fermentation, and found that the alkaline pH regulation (pH 9) significantly enhanced the promotion of SCFAs (11843 ± 424 mg COD/L), with a high proportion of acetate (51%). Further analysis revealed that alkaline pH regulation facilitated solubilization, hydrolysis, and acidification while simultaneously inhibiting methanogenesis. Furthermore, the functional anaerobes, as well as the expressions of corresponding gene involved in SCFAs biosynthesis, were generally improved under alkaline pH regulation. Alkaline treatment might played a critical role in alleviating the toxicity of OPW, resulting in improving microbial metabolic activity. This work provided an effective strategy to recover biomass waste as high-value products, and insightful understanding of microbial traits during OPW/WAS co-fermentation.

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.

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.

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.

Enhanced mesophilic fermentation of waste activated sludge by integration with in-situ nitrate reduction

This study investigated the reduction of nitrate in a mesophilic waste activated sludge (WAS) fermentation system and determined the effect of nitrate reduction on the hydrolysis, acidogenesis and acetogenesis. Experimental results showed that the initial nitrate concentrations of 100, 200 and 400 mg/L were completely reduced in 1, 2 and 7 days, respectively. The destruction of volatile suspended solids was 1.2, 1.8 and 2.8 times, respectively, that without nitrate, demonstrating nitrate promoted the release of organic matter in sludge and enhanced the biodegradability of sludge organics. Moreover, batch tests using model substrates illustrated nitrate reduction promoted sludge hydrolysis and acetogenesis, but slightly inhibited acidogenesis. This study offers a feasible method to address two major problems currently faced by biological wastewater treatment plants, i.e. the overabundance of WAS and the lack of carbon sources for the denitrification process.

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.

Complexation modelling and oxidation mechanism of organic pollutants in cotton pulp black liquor during iron salt precipitation and electrochemical treatment

Removal behavior of organic pollutants such as lignin in cotton pulp black liquor (CPBL) was investigated in precipitation followed by electrochemical oxidation (EO) using FeCl₃, Fe₂(SO₄)₃, FeCl₂ and FeSO₄ as precipitants, electrolyte and catalysts. Based on comparison of precipitation efficacy of iron salts, spectroscopic techniques, thermodynamic equilibrium calculations and molecular dynamics (MD) simulations were used to provide insight into the interaction between iron cations and lignin. The results showed that FeCl₃ achieved the highest removal of chemical oxygen demand (COD, 76.05%), UV₂₅₄ (69.21%) and lignin (78.28%). Iron cationic complexation with lignin was identified as the key mechanism in precipitation. Fe³⁺ was more active in binding to organic ligands mainly due to charge effect compared to Fe²⁺. The strong Fe-sulphate coordination affected the complexation with lignin. MD simulations showed the formation of inner sphere complexes of iron cations with deprotonated carboxyl and hydroxyl groups via bidentate and monodentate coordination. The removal efficiency of electrochemical oxidation (EO) as a post-treatment of the precipitation was dependent on iron salts. Removals of COD, UV₂₅₄ and color can achieve 98.88%, 98.9% and 99.97% by FeCl₃ precipitation and EO processes. The effluent reached the primary discharge standard specified in Integrated Wastewater Discharge Standard of China (GB8978-1996). FeCl₃ demonstrated significant advantages in the removal of organic pollutants from cotton pulp black liquor in the combined process of precipitation and electrochemical treatment and may have practical application potential.

Enhanced dewaterability of anaerobically fermented sludge through acid-driven indigenous enzymatic hydrolysis

The poor dewaterability of fermented sludge is an important factor limiting the development of anaerobic fermentation applications. Herein we reported an efficient strategy, i.e., using acidic regulation to stimulate the release of indigenous enzymes, to enhance the hydrolysis and dewatering of fermented sludge. The results showed that after acidic regulation at pH 4.0 for 1 day, the activity of protease and α-glucosidase were improved by 131.4% and 146.0%, while the capillary suction time and specific resistance to filtration were decreased by 93.8% and 69.5%, respectively. Mechanism study revealed that the method firstly destroyed the slime and bound EPS and cells of fermented sludge, causing the release of indigenous enzymes (i.e., protease and α-glucosidase) contained in. Then, the released enzymes directly accelerated the hydrolysis and acidification of fragmentized extracellular polymeric substances, thereby benefited the release of bound water in sludge particles. Finally, such acidic condition decreased the electrostatic repulsive interactions between destroyed sludge particles, further improving their flocculation. The findings not only deepen the understanding of indigenous enzymes contained in fermented sludge affecting sludge dewatering, but also might guide engineers to develop promising strategies to facilitate fermented sludge dewatering and fermentation liquid recovery in the future.

Effects of enzymes on organic matter conversion in anaerobic fermentation of sludge to produce volatile fatty acids

Sludge hydrolysis is a vital step in anaerobic digestion of sludge. This study compared the efficacy of free versus immobilized enzymes at different concentrations in promoting sludge disintegration. Pretreatment with 1,000 mg/L immobilized enzymes was more efficient in promoting sludge disintegration than free enzymes at the same concentration. Under the optimized conditions, volatile fatty acids (VFAs) were produced at 10.6 g/L, accounting for 85 % of total soluble chemical oxygen demand. Improved VFA production was attributed to the release of large amounts of polysaccharides and proteins from the enzymatically pretreated sludge. Released organic matter are the substrates for VFAs generated by the determined microbial community of Firmicutes, Proteobacteria, Bacteroidetes, Actinobacteria, and Chloroflexi. In this study, anaerobic fermentation was used to successfully convert organic matter in sludge into high-value-added VFAs. Therefore, this process can be selected as a strategy to reduce carbon emissions from wastewater treatment plants (WWTPs).

Enhanced nitritation/denitritation and potential mechanism in an electrochemically assisted sequencing batch biofilm reactor treating sludge digester liquor with extremely low C/N ratios

Nitritation/denitritation is a promising strategy to treat sludge digester liquor but would be unstable and inefficient at extremely low C/N ratios. Here, a novel electrochemically assisted sequencing batch biofilm reactor (E-SBBR) was established to treat synthetic/real sludge digester liquor with decreasing C/N ratios. The results showed that the E-SBBR achieved stable nitritation and appreciable TN removal (>70 %) even at C/N < 0.5. The high-strength free ammonium (FA) (91.1-132.8 mg NH₃-N/L) and long inhibition time (>9h) magnified by electrolysis promoted the robustness of nitritation through efficient nitrite-oxidizing bacteria elimination. Meanwhile, mass balance denoted that heterotrophic denitritation dominated in the enhanced TN removal and relied on carbon supplementation from cell apoptosis/lysis stimulated by electrolysis and high-strength FA, further supported by the recovery of heterotrophic denitrifiers, fermentation bacteria, and relevant functional genes at extremely low C/N ratios. This study provides a novel nitrogen removal approach for the sludge digester liquor 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.

Effects of metronidazole on mesophilic and thermophilic fermentation: Biodegradation mechanisms, microbial communities, and reversibility

Metronidazole (MNZ), an antibiotic that is specifically used for the treatment of anaerobic infections, may inhibit anaerobic fermentation. This work was designed to understand the fate and effects of MNZ in mesophilic fermentation (MF) and thermophilic fermentation (TF), respectively. The results showed that the removal of MNZ mainly occurred via biodegradation, rather than adsorption, and that MNZ could be completely degraded by opening the imidazole ring. MFs were more strongly inhibited by MNZ than TFs. MNZ concentration increased from 0 to 25 mg/L, hydrogen yield (HY) decreased from 167.5 to 16.8 mL/g glucose (90.0% decrease), and butyrate yield almost completely disappeared in MFs, whereas in TFs, HY decreased only from 101.1 to 89.3 mL/g glucose (11.7% decrease), and ethanol yield increased by 39.8%. Illumina MiSeq sequencing analysis showed that MNZ reduced the abundance of hydrogen-producing bacteria. Furthermore, the inhibition of MNZ on anaerobic fermentation was reversible.

Sulphonated graphene catalyst incorporation with sludge enhanced the microbial activities for biomethanization of crude rice straw

Biomethanization of crude rice straw (RS) was enhanced by a coupled effectiveness of sulphonated graphene (SGR) with sludge rich anaerobes (SRA). A reduction of 19.2 ± 1.32% for cellulose, 40.8 ± 3.7% for hemicellulose and 30.8 ± 2.4% for lignin was achieved with addition of SRA after fermentation of 60 days. The abundance of hydrolytic microbes in SRA i.e. Acidobacteria, Bacteroidetes, Chloroflexi and Proteobacteria caused RS structure liquefaction and dissolution. The reduction of cellulose, hemicellulose and lignin boosted to 92.3 ± 1.5, 84.9 ± 3.5 and 97.0 ± 1.8% respectively with SGR catalyst addition of 100 mg/gVS. Reducing sugars, phenols and volatile fatty acids (VFAs) were subsequently utilized by bacteria and archaea species of Methanosphaera, Methanocella, Candidatus Methanoregula, Methanolinea and Methanosaeta. The biogas yield was 92 ± 3.1 mL/gRS and methane content amounted to 68 ± 4.6% % at SGR catalyst of 80 mg/gVS. These findings show the potential of using SRA/SGR to improve the RS fermentation with a novel application for biogas productivity.

Sulfadiazine inhibits hydrogen production during sludge anaerobic fermentation by affecting pyruvate decarboxylation

The overuse and random discharge of antibiotics can cause serious environmental pollution. Sludge acts as a repository for antibiotics, its anaerobic fermentation process will inevitably be affected. This study investigated the effects of a typical antibiotic contaminant, sulfadiazine (SDZ), on the anaerobic fermentation of sludge for hydrogen production. Results demonstrated that the production of hydrogen was significantly inhibited by SDZ, and the inhibition was enhanced with increasing SDZ content. Within 5 days, the cumulative amount of hydrogen with 500 mg SDZ/kg dry sludge was 8.5 mL, which was only 32.2% of that in the control (26.4 mL). Mechanistic investigation showed that the reduced hydrogen production when SDZ existed was mainly attributed to the suppression of pyruvate decarboxylation during the hydrogen production stage, and the diversity of microorganisms, especially the abundance of microorganisms and the activities of key enzymes closely related to hydrogen production were inhibited with SDZ, resulting in less hydrogen accumulation.

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.

Mechanism of dielectric barrier discharge plasma technology to improve the quantity of short-chain fatty acids in anaerobic fermentation of waste active sludge

The mechanism of improving the anaerobic fermentation performance of waste active sludge by using dielectric barrier discharge (DBD) plasma pretreatment technology was investigated. The maximum accumulation of short-chain fatty acids (SCFAs) was observed on the 7th day of anaerobic fermentation when the DBD power was 76.50 W, which was 1726.70 mg COD/L, 1.50 times of the control group. The ratio of acetic acid in DBD group was 9.30% higher than that in the control. Further mechanism research indicated that DBD pretreatment can destroy the structure of extracellular polymer substances and release organic substances such as protein and polysaccharide. The dissolved organic matter analysis indicated that the DBD technique could increase the release of biodegradable organics (eg., tyrosine proteins, soluble microbial by-products), thus accelerate the biotransformation of organic substance. Bacterial community structure analysis showed that the increase in the abundance of Firmicutes and Bacteroidetes and the decrease in the abundance of Proteobacteria in DBD group were beneficial to the accumulation of SCFAs. Besides, further archaeal analysis indicated that the decrease of Methanosaeta sp. and Methanosarcina sp. abundance in the DBD group facilitate acetic acid accumulation. This study demonstrated that the DBD technique can be used as an effective and potential pretreatment method to improve sludge anaerobic fermentation performance.

Sulfite-based pretreatment promotes volatile fatty acids production from microalgae: Performance, mechanism, and implication

Volatile fatty acids (VFAs) production from anaerobic fermentation of microalgae is generally constrained by low organics solubilization and poor substrate-availability. In this study, sulfite-based pretreatment was developed to overcome such situation. Experimental results showed that the maximum concentration of VFAs (467.5 mg COD/g VSS) and corresponding acetate proportion (54.5%) was obtained at 200 mg sulfite-S/L with fermentation time of day 8, which was respectively 2.1- and 1.9-fold of control. It was found that after sulfite pretreatment, more and relatively easy biodegradable organics were released into liquid phase, providing available substrate for acid-producing bacteria. The rigid cell wall of microalgae was destroyed, evidenced by the decreased particle size and increased surface area, which made the microalgae more accessible for subsequent hydrolysis and acidification. Meanwhile, the sulfite-induced sulfate-reducing bacteria facilitated the acetate generation pathway. The accelerated activities of β-glucanase, β-glucosidase, and acetate kinase involved in anaerobic fermentation further validated the above results.

Composition Characterization and Transformation Mechanism of Dissolved Organic Matters in a Full-Scale Membrane Bioreactor Treating Co-Digestion Wastewater of Food Waste and Sewage Sludge

The membrane bioreactor (MBR) serves as the most widely used technology in anaerobic digestion wastewater treatment, but the composition and transformation of the dissolved organic matters (DOMs) are vague. This study focused on the composition characterization and transformation mechanism of DOMs in real co-digestion wastewater of food waste and sewage sludge from a full-scale MBR via molecular weight cut-off, 3D-EEM, FT-IR, and SPME-GC/MS. The results indicated that the co-digestion wastewater mainly comprised organics with molecular weight (MW) lower than 1 kDa and dominated by tryptophane-protein-like substances. The hydrolytic/acidogenic process improved the biodegradability with the conversion of high-MW organics into low-MW organics, while the two-stage A/O process possessed the highest contribution to the organic removal with the consumption of most DOMs. However, the deficient removal of refractory organics (MW < 5 kDa) in the ultrafiltration unit led to the residual DOMs in the effluent. The potential functional bacteria in the biological processes have also been identified and were principally affiliated with Proteobacteria and Firmicutes. These findings could help to advance the understanding of the co-digestion wastewater and provide fundamental information for the optimization and development of MBR in anaerobic digestion wastewater treatment.