Hydrolase activity and microbial community dynamic shift related to the lack in multivalent cations during cation exchange resin-enhanced anaerobic fermentation of waste activated sludge

Ionic liquid coupled plasma promotes acetic acid production during anaerobic fermentation of waste activated sludge: Breaking the restrictions of low bioavailable substrates and altering the metabolic activities of anaerobes

This study explored the potential application of plasma coupling ionic liquid on disintegration of waste activated sludge and enhanced production of short-chain fatty acids (SCFAs) in anaerobic fermentation. Under optimal conditions (dosage of ionic liquid [Emim]OTf = 0.1 g/g VSS (volatile suspended solids) and discharge power of dielectric barrier discharge plasma (DBD) = 75.2 W), the [Emim]OTf/DBD pretreatment increased SCFA production by 302 % and acetic acid ratio by 53 % compared to the control. Mechanistic investigations revealed that the [Emim]OTf/DBD combination motivated the generation of various reactive species (such as H2O2, O3, •OH, 1O2, ONOO-, and •O2-) and enhanced the utilization of physical energies (such as heat). The coupling effects of [Emim]OTf/DBD synergistically improved the disintegration of sludge and biodegradability of dissolved organic matter, promoting the sludge anaerobic fermentation process. Moreover, the [Emim]OTf/DBD pretreatment enriched hydrolysis and SCFAs-forming bacteria while inhibiting SCFAs-consuming bacteria. The net effect was pronounced expression of genes encoding key enzymes (such as alpha-glucosidase, endoglucanase, beta-glucosidase, l-lactate/D-lactate dehydrogenase, and butyrate kinase) involved in the SCFA-producing pathway, enhancing the production of SCFAs from sludge anaerobic fermentation. In addition, [Emim]OTf/DBD pretreatment facilitated sludge dewatering and heavy metal removal. Therefore, [Emim]OTf/DBD pretreatment is a promising approach to advancing sludge reduction, recyclability, and valuable resource recovery.

Effects of eutrophication on the horizontal transfer of antibiotic resistance genes in microalgal-bacterial symbiotic systems

Overloading of nutrients such as nitrogen causes eutrophication of freshwater bodies. The spread of antibiotic resistance genes (ARGs) poses a threat to ecosystems. However, studies on the enrichment and spread of ARGs from increased nitrogen loading in algal-bacterial symbiotic systems are limited. In this study, the transfer of extracellular kanamycin resistance (KR) genes from large (RP4) small (pEASY-T1) plasmids into the intracellular and extracellular DNA (iDNA, eDNA) of the inter-algal environment of Chlorella pyrenoidosa was investigated, along with the community structure of free-living (FL) and particle-attached (PA) bacteria under different nitrogen source concentrations (0-2.5 g/L KNO3). The results showed that KR gene abundance in the eDNA adsorbed on solid particles (D-eDNA) increased initially and then decreased with increasing nitrogen concentration, while the opposite was true for the rest of the free eDNA (E-eDNA). Medium nitrogen concentrations promoted the transfer of extracellular KR genes into the iDNA attached to algal microorganisms (A-iDNA), eDNA attached to algae (B-eDNA), and the iDNA of free microorganisms (C-iDNA); high nitrogen contributed to the transfer of KR genes into C-iDNA. The highest percentage of KR genes was found in B-eDNA with RP4 plasmid treatment (66.2%) and in C-iDNA with pEASY-T1 plasmid treatment (86.88%). In addition, dissolved oxygen (DO) significantly affected the bacterial PA and FL community compositions. Nephelometric turbidity units (NTU) reflected the abundance of ARGs in algae. Proteobacteria, Cyanobacteria, Bacteroidota, and Actinobacteriota were the main potential hosts of ARGs. These findings provide new insights into the distribution and dispersal of ARGs in the phytoplankton inter-algal environment.

Innovative cation exchange-driven carbon migration and recovery patterns in anaerobic fermentation of waste activated sludge

Despite numerous treatments have been developed to enhance anaerobic fermentation of waste activated sludge, the innovative cation exchange (CE) approach has been rarely reported, little attempt was conducted to revealcarbon source fate. The interphase carbon balance was illustrated to clarify endogenous carbon dissolution, biotransformation,and recovery patterns. By CE-mediated divalent cation removal, almost 34.72 % of particulate carbon sources were dissolved in 2-day treatment, corresponding to soluble carbon content of 1165.58 mg C/L. Most of the originally dissolved carbon sources (58.01-66.81 %) were bio-transformed to volatile fatty acids with high bioavailability, while the further transformation to biogas was inhibited, contributing to recoverable carbon source accumulation. Overall, 21.38 % of total solid carbon sources were recovered through 8-day fermentation, the carbon extraction was implemented by solid-liquid separation with carbon loss of 14.21-22.91 %, manifesting the valid carbon recovery of 85.05-87.96 mg C/g VSS. Such CE-driven carbon recovery provided negentropy benefits in sustainable cycle economy.

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

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

Endogenous biopolymer hydrolysis for enhancing short-chain fatty acids recovery from excess sludge: Combination of lysozyme-catalyzing and cation exchange resin-mediated metal regulation

In decades, anaerobic fermentation with short-chain fatty acids (SCFAs) recovery from excess sludge have attained rising attention. However, rigid particulate organic matter (POMs) structure with slow hydrolysis limited anaerobic fermentation performance of excess sludge. Remarkable sludge hydrolysis performance was supposed to be achievable by the synchronous EPS repture and microbial cell lysis. This study clarified the improvement of overall anaerobic fermentation performance by combination treatment of lysozyme (Lyso) catalysis and metal regulation (MR). The Lyso + MR treatment triggered EPS rupture by protein structure deflocculation while catalyzing microbial cell lysis, which promoted massive extracellular and intracellular POMs hydrolysis. As a result, a significant amount of SCOD (5646.67 mg/L) was produced. Such endogenous organic matters hydrolysis led to considerable SCFAs accumulation (3651.14 mg COD/L) through 48-h anaerobic fermentation at 1.75 g/g SS cation-exchange resin and Lyso dosage of 10% (w/w), which was 5.945 times higher than that in the control. Additionally, it suggested that most of the recovered SCFAs remained in fermentative liquid after chemical conditioning and mechanical dewatering towards solid-liquid separation, which provided considerable economic benefit of 363.6-1059.1 CNY/ton SS.

Synergistic effects and mechanisms of plasma coupled with peracetic acid in enhancing short-chain fatty acid production from sludge: Motivation of reactive species and metabolic tuning of microbial communities

Suitable waste activated sludge (WAS) pretreatments that boost short-chain fatty acid (SCFA) production from anaerobic fermentation are essential for carbon emission reduction and sludge resource utilization. This study established an efficient WAS pretreatment process combining atmospheric pressure plasma jet (APPJ) with peracetic acid (PAA). The maximum SCFA production (6.5-fold that of the control) largely increased under the optimal conditions (PAA dosage = 25 mg/g VSS (volatile suspended solids), energy consumption = 20.9 kWh/m3). APPJ/PAA pretreatment enhanced the production of multiple reactive species (including OH, CH3C(O)O, 1O2, ONOO-, O2-, and eaq-) and strengthened the effects of H2O2, heat, and light. This synergistically solubilized WAS and released organic substrates for SCFA-producing microbes. In addition, the enrichment of SCFA-producing bacteria and the decrease in SCFA-consuming bacteria favored SCFA accumulation. The key genes encoding for the main substrate metabolism and SCFA production in the metabolic pathway of fermentation were also enhanced.

Feasibility and mechanism of recycling carbon resources from waste cyanobacteria and reducing microcystin toxicity by dielectric barrier discharge plasma

Recycling carbon resources from discarded cyanobacteria is a worthwhile research topic. This study focuses on the use of dielectric barrier discharge (DBD) plasma technology as a pretreatment for anaerobic fermentation of cyanobacteria. The DBD group (58.5 W, 45 min) accumulated the most short chain fatty acids (SCFAs) along with acetate, which were 3.0 and 3.3 times higher than the control. The DBD oxidation system can effectively collapse cyanobacteria extracellular polymer substances and cellular structure, improve the biodegradability of dissolved organic matter, enrich microorganisms produced by hydrolysis and SCFAs, reduce the abundance of SCFAs consumers, thereby promoting the accumulation of SCFAs and accelerating the fermentation process. The microcystin-LR removal rate of 39.8% was obtained in DBD group (58.5 W, 45 min) on day 6 of anaerobic fermentation. The toxicity analysis using the ECOSAR program showed that compared to microcystin-LR, the toxicity of degradation intermediates was reduced. The contribution order of functional active substances to cyanobacteria cracking was obtained as eaq- > •OH > 1O2 > •O2- > ONOO-, while the contribution order to microcystin-LR degradation was eaq- > •OH > •O2- > 1O2 > ONOO-. DBD has the potential to be a revolutionary pretreatment method for cyanobacteria anaerobic fermentation.

Thiosulfate pretreatment enhancing short-chain fatty acids production from anaerobic fermentation of waste activated sludge: Performance, metabolic activity and microbial community

A novel strategy based on thiosulfate pretreatment for enhancing short-chain fatty acids (SCFAs) from anaerobic fermentation (AF) of waste activated sludge (WAS) was proposed in this study. The results showed that the maximal SCFA yield increased from 206.1 ± 4.7 to 1097.9 ± 17.2 mg COD/L with thiosulfate dosage increasing from 0 to 1000 mg S/L, and sulfur species contribution results revealed that thiosulfate was the leading contributor to improve SCFA yield. Mechanism exploration disclosed that thiosulfate addition largely improved WAS disintegration, due to thiosulfate serving as a cation binder for removing organic-binding cations, especially Ca²⁺ and Mg²⁺, dispersing the extracellular polymeric substance (EPS) structure and further entering into the intracellularly by stimulated carrier protein SoxYZ and subsequently caused cell lysis. Typical enzyme activities and related functional gene abundances indicated that both hydrolysis and acidogenesis were remarkably enhanced while methanogenesis was substantially suppressed, which were further strengthened by the enriched hydrolytic bacteria (e.g. C10-SB1A) and acidogenic bacteria (e.g. Aminicenantales) but severely reduced methanogens (e.g. Methanolates and Methanospirillum). Economic analysis confirmed that thiosulfate pretreatment was a cost-effective and efficient strategy. The findings obtained in this work provide a new thought for recovering resource through thiosulfate-assisted WAS AF for sustainable development.

Enhanced acidogenic fermentation from Al-rich waste activated sludge by combining lysozyme and sodium citrate pretreatment: Perspectives of Al stabilization and enzyme activity

The accumulation of poly aluminum chloride (PAC) in dewatered waste activated sludge (WAS) can cause severe Al pollution and significantly reduce the production of volatile fatty acids (VFAs) from anaerobic fermentation. Herein, the combination of lysozyme and sodium citrate pretreatment was applied to stabilize the aluminum and enhance the VFAs production via anaerobic fermentation. The complexation and stabilization of aluminum by the citrate was efficient, which is conducive to relieving the inhibition of aluminum on lysozymes and other extracellular hydrolases. Compared with the control group, the lysozyme, protease and α-glucosidase activities were obtained at 1.86, 1.72, and 1.15 times, respectively, following the pretreatment. 129.71 mg/g volatile suspended solids (VSS) of soluble proteins and 26.3 mg/g VSS of polysaccharides were obtained within 4 h, together with the degradation of 124 % more proteins and 75 % more polysaccharides within three days. This provided a sufficient number of substrates for VFA production. 588.4 mg COD/g VSS of total VFAs were obtained after the six-day anaerobic fermentation from Al-rich WAS following the combination of lysozyme and sodium citrate pretreatment, which was 7.3 times higher than that of the control group. This study presents a novel approach for enhancing VFA production in anaerobic fermentation as well as reducing risk of Al hazards from Al-rich WAS.

Strategy of electron acceptors for ethanol-driven chain elongation from kitchen waste

A two-phase kitchen waste (KW) fermentation was proposed in the current study to enhance medium-chain fatty acids (MCFAs) production from kitchen waste. In particular, effect of acetate to butyrate ratio (ABR) on MCFAs production was investigated which can be regulated by different pH and organic loading during the acidification phase. Medium ABR (1.00) was obtained when pH is 5.5 and organic loading is 20 g VS/L in FW acidification fermentation. Subsequent chain elongation fermentation demonstrated that the highest yield of caproate 9.67 g/L with selectivity of 79 %, and highest ethanol conversion efficiency of 1.11 was achieved in medium ABR system. Microbial community study showed that medium ABR significantly enrich the functional bacteria especially Clostridium kluyveri. The study provides a new method for chain elongation enhancement without addition of other additives in kitchen waste fermentation system and gives a guide for the regulation of the short-chain fatty acids distribution in its acidification phase.

Reconsidering operation pattern for cation-exchange resin assistant anaerobic fermentation of waste activated sludge: Shorting residence period towards dosage-reduction and anti-fouling

Short-chain fatty acids (SCFAs) generation through anaerobic fermentation has been regarded as a promising pathway to achieve carbon recovery and economic benefits in waste activated sludge management. Despite the cation exchange resin (CER) assistant anaerobic fermentation strategy has been previously reported for enhancing anaerobic fermentation, the overlarge CER usage and serious CER pollution have limited its engineering application. This study provided a reconsideration for the operation pattern modification. Through 4-day anaerobic fermentation with CER residence period shrinking to 1 day, 40.9% sludge VSS solubilization and reduction were achieved, triggering a considerable sludge hydrolysis rate of 28.4%. Thereby, SCFAs production was improved to 264.8 mg COD/g VSS. Such performances were approximately 80.2-87.8% of those with conventional CER residence period (8 days). The organic composition distribution and parallel factor analysis demonstrated that similar biodegradability and utilizability of fermentative liquid were achievable with various operation patterns. Compared with the conventional operation pattern, the modified operation pattern with shortened CER residence period (1 day) also displayed satisfying anaerobic fermentation efficiency and numerous engineering bene fits, e.g. decreased CER usage, reduced engineering footprint, relieved CER fouling, and increased operation convenience. The findings might provide sustainable development for CER assistant anaerobic fermentation strategy and enlighten the direction of anaerobic fermentation process.

Synergistic improvement of short-chain fatty acid production from waste activated sludge via anaerobic fermentation by combined plasma-calcium peroxide process

In this study, the combination of dielectric barrier discharge plasma (DBD) with calcium peroxide (CaO₂) achieved significant synergistic effects in promoting hydrolysis of waste activated sludge (WAS) and short-chain fatty acid (SCFA) production during anaerobic fermentation. Compared with the control, DBD/CaO₂ pretreatment increased SCFA production by 116 %, acetic acid ratio by 39 %, and sludge reduction by 30 % under the optimal conditions (discharge power = 76.5 W, CaO₂ dosage = 0.05 g/g VSS). Mechanism investigations elucidated that DBD/CaO₂ enhanced the generation of •OH, ¹O₂, and •O₂^-, synergistically promoted decomposing extracellular polymeric substances (EPS), lysing cells, releasing biodegradable substances, and enhancing acetic acid-enriched SCFA accumulation from fermentation. Meanwhile, Illumina MiSeq sequencing analysis revealed that the enrichment of hydrolytic and SCFAs-forming bacteria and the decrease in SCFAs-consuming bacteria by DBD/CaO₂ treatment also contributed. This work provides an effective method to boost the SCFA production from WAS fermentation.

Effects of different low temperature conditions on anaerobic digestion efficiency of pig manure and composition of archaea community

To explore the effect of low temperature on the anaerobic digestion of pig manure, the anaerobic digestion experiment was carried out under the conditions of inoculum concentration of 30% and TS of 8%. Five low-temperature gradients of 4, 8, 12, 16 and 20 °C were set to study the activities of gas production, pH, solluted chemical oxygen demand (SCOD), volatile fatty acids (VFAs), coenzymes F₄₂₀ and archaea community composition in the digestion process. The results were demonstrated: as the temperature decreased, the more unstable the gas production became, the less gas production produced, and the later the gas peak occurred. There were no significant peaks at either 4 °C or 8 °C, and the SCOD was unstable over time. From 12 °C, the SCOD increased over time, and the higher the temperature, the faster the growth trend. The pH was always greater than 7.6. 8, 12, 16, 20 °C had different degrees of VFAs accumulation at the late digestion stage. The higher the temperature, the greater the amount of volatile acid accumulation. When the VFAs of each reactor reached the maximum, the proportion of acetic acid also reached the highest. The digestion system of the five treatment groups was dominated by hydrogen-nutrient methanogenic pathway. The results could provide a further reference for the mechanism of anaerobic digestion of pig manure at low temperatures.

Gut Microflora Modulates Th17/Treg Cell Differentiation in Experimental Autoimmune Prostatitis via the Short-Chain Fatty Acid Propionate

Chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS) is a very common urological disorder and has been gradually regarded as an immune-mediated disease. Multiple studies have indicated that the gut microflora plays a pivotal part in immune homeostasis and autoimmune disorder development. However, whether the gut microflora affects the CP/CPPS, and the underlying mechanism behind them remain unclear. Here, we built an experimental autoimmune prostatitis (EAP) mouse model by subcutaneous immunity and identified that its Th17/Treg frequency was imbalanced. Using fecal 16s rRNA sequencing and untargeted/targeted metabolomics, we discovered that the diversity and relative abundance of gut microflora and their metabolites were obviously different between the control and the EAP group. Propionic acid, a kind of short-chain fatty acid (SCFA), was decreased in EAP mice compared to that in controls, and supplementation with propionic acid reduced susceptibility to EAP and corrected the imbalance of Th17/Treg cell differentiation in vivo and in vitro. Furthermore, SCFA receptor G-protein-coupled receptor 43 and intracellular histone deacetylase 6 regulated by propionic acid in Th17 and Treg cells were also evaluated. Lastly, we observed that fecal transplantation from EAP mice induced the decrease of Treg cell frequency in recipient mice. Our data showed that gut dysbiosis contributed to a Th17/Treg differentiation imbalance in EAP via the decrease of metabolite propionic acid and provided valuable immunological groundwork for further intervention in immunologic derangement of CP/CPPS by targeting propionic acid.

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

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

Motivation of reactive oxygen and nitrogen species by a novel non-thermal plasma coupled with calcium peroxide system for synergistic removal of sulfamethoxazole in waste activated sludge

Large amounts of antibiotics are concentrated in waste activated sludge (WAS) and released into the environment. It is thus critical to develop advanced sludge treatment technology to remove these antibiotics. Dielectric barrier discharge (DBD) combined with calcium peroxide (CaO₂), as an innovative technology to attenuate sulfamethoxazole (SMX) in sludge, was investigated. Evident synergistic effects between DBD and CaO₂ were observed on the SMX degradation with a synergistic factor of 2.02. Moreover, the energy consumption of DBD/CaO₂ was significantly lower than that of DBD alone. At a typical CaO₂ dosage of 0.1 g/g TS and discharge power of 64.5 W, the highest SMX removal of 96% was achieved within 50 min. The synergistic effects of DBD/CaO₂ could be associated with the base catalysis of H₂O₂ and O₃, UV-base-photolysis, peroxone oxidation, and photocatalytic H₂O₂. DBD/CaO₂ generated various reactive oxygen species (ROS) and nitrogen species (RNS) that participated in SMX removal. The contributions of these reactive species followed the sequence of e^- > •OH > •O₂^- > ¹O₂ > ONOO^-. Based on the detected transformation by-products and their variations during treatment, a plausible SMX degradation pathway in sludge was proposed. Besides, DBD/CaO₂ also promoted sludge disintegration, dewatering, heavy metal removal, sludge reduction, sludge solubilization, and acetate-enriched volatile fatty acid (VFA) production. Therefore, DBD/CaO₂ exhibited great potential for controlling antibiotic, as well as promoting sludge reduction, decontamination, and resourcization.

Activated carbon modified with nano manganese dioxide triggered electron transport pathway changes for boosted anaerobic treatment of dyeing wastewater

Herein, an expanded granular sludge bed (EGSB) reactor with activated carbon (AC)-nano manganese dioxide (MnO₂) added was employed for azo dye wastewater treatment to investigate its effectiveness at decolorizing of azo dyes and removing COD. The results showed that the treatment of azo dye wastewater with the AC-MnO₂ modified EGSB reactor gave an 83% average decolorization efficiency, which was more efficient than the pure AC modified EGSB reactor. Moreover, the COD removal and changes in the intermediate products were controlled by AC-MnO₂. Additionally, there was a sharp increase in the sludge conductivity, while there was a significant decrease in the coenzyme F₄₂₀ concentration with long-term operation. Moreover, electrochemical analysis showed that the addition of AC-MnO₂ can enhance electron transfer in anaerobic system. The AC-MnO₂ can act as redox mediator; in the presence of the Mn⁴⁺/Mn²⁺ cycle, accelerating the electron transfer between the microbial cells and dyes, thereby promoting the decolorization of azo dyes. This caused a decrease in the methanogenic activity. Furthermore, high-throughput sequencing showed that the relative abundances of Pseudomonas and Desulfovibrio were significantly high among the acidogenic bacteria community, while Methanobacterium and Methanosaeta had very low abundances from among the methanogenic archaea community.

Microbial community dynamic shifts associated with sulfamethoxazole degradation in microbial fuel cells

Though sulfamethoxazole (SMX) degradation at the low or medium concentration (SMX< 30 mg/L) has been reported in the microbial fuel cell (MFC), further exploration is still urgently required to investigate how the high concentration of SMX affect the anode biofilm formation. In this study, the degradation mechanism of SMX and the response of microbial community to SMX at different initial concentrations (0, 0.5, 5 and 50 mg/L) were investigated in MFCs. The highest SMX removal efficiency of 98.4% was obtained in MFC (5 mg/L). SMX at optimal concentration (5 mg/L) could serve as substrate accelerating the extracellular electron transfer. However, high concentration of SMX (50 mg/L) conferred significant inhibition on the electron transfer with SMX removal decline to 84.4%. The 16S rRNA high-throughput sequencing revealed the significant shift of the anode biofilms communities with different initial SMX concentrations were observed in MFCs. Thauera and Geobacter were the predominant genus, with relative abundance of 31.9% in MFC (50 mg/L SMX) and 52.7% in MFC (5 mg/L SMX). Methylophilus exhibited a huge increase with the highest percentage of 16.4% in MFC (50 mg/L). Hence, the functional bacteria of Thauera, Geobacter and Methylophilus endowed significant tolerance to the selection pressure from high concentration of SMX in MFCs. Meanwhile, some bacteria including Ornatilinea, Dechloromonas and Longilinea exhibited a decrease or even disappeared in MFCs. Therefore, initial concentrations of SMX played a fundamental role in modifying the relative abundance of predominant populations. This finding would promote theories support for understanding the evolution of anode biofilm formation related to the different initial concentrations of SMX in MFCs.

An innovative cation regulation-based anaerobic fermentation strategy for enhancing short-chain fatty acids production from waste activated sludge: Metal ion removal coupled with Na+-regulation

This study proposed a cation-regulation strategy based on metal ion removal coupled Na⁺-regulation for enhancing anaerobic fermentation of waste activated sludge. The optimal treatment condition was: cation-exchange resin dosage of 1.75 g/g SS for 1-day treatment, followed by Na⁺-enhanced anaerobic fermentation at NaCl concentration of 20 g/L. The CER induced sludge solubilization and the Na⁺-regulation treatment triggered secondary hydrolysis of CER-solubilized sludge, causing remarkable sludge disintegration and extracellular polymeric substance (EPS) disruption. Numerous SCOD of 6588 mg/L (SCOD/TCOD = 40.6%) was released within 2 days, and the short-chain fatty acids (SCFAs) of 439.9 mg COD/g VSS was produced through 4-day anaerobic fermentation. More than 59% of the SCFAs was composed of acetate and propionate. Nitrogen-free organic matters (i.e. SCFAs and carbohydrates) accounted for 77.9% of SCOD, while considerable sludge solid reduction (51.6% of total VSS) was achievable, which was beneficial for fermentative liquid utilization and sludge disposal.

Response of anaerobic digestion of waste activated sludge to residual ferric ions

This study was conducted to investigate the effects of residual ferric ions (FI), released from iron or its oxides for wastewater or waste activated sludge (WAS) treatment, on anaerobic digestion of WAS. Herein it was found that the anaerobic digestion process was greatly affected by FI dosages as well as FI distributions. The responses of performance and microorganism suggested that a low FI (e.g., 0.125 mmol/g volatile suspended solid (VSS)) enhanced methane production by 29.3%, and a medium FI (e.g., 0.3 mmol/g VSS) promoted short chain fatty acids accumulation to reach the maximum of 247 mg chemical oxygen demand /g VSS, conversely, a high FI (e.g., 0.9 mmol/g VSS) led to severe inhibition on acidogenesis and methanogenesis. The findings may provide some new insights for mechanism understanding on anaerobic digestion process influenced by iron or its oxides, as well as the disposal of WAS contained FI.

Using Fe(II)/Fe(III) as catalyst to drive a novel anammox process with no need of anammox bacteria

A novel 'anammox' in the absence of anammox bacteria was confirmed to occur in an anaerobic sludge slurry system, in which Fe(II)/Fe(III) cycle driven by NO₂^--induced Fe(II) oxidation and subsequent NH₄⁺-induced Fe(III) reduction (Feammox) pushed the nitrogen removal. Results showed that Fe(II) contents significantly (p<0.05) decreased and Fe(III) accordingly increased with NO₂^- addition, indicating that Fe(II) was anaerobically oxidized to Fe(III). With depletion of NO₂^-, the Fe(II) content began to increase which was a result of Feammox. Consequently, 96.0% NH₄⁺-N of the NO₂^--added reactor was removed during 18 days operation, while NH₄⁺-N content remained essentially unchanged in the control in which NO₂^- was not added. X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) analysis indicated that FeOOH was produced from chemical Fe(II) oxidation with NO₂^-. During the treatment, anammox bacteria was not detected, but the relative abundance of Geobacter of the NO₂^--added group increased by 13 folds. Isotope experiment in ¹⁵NH₄⁺-containing reactors found that much more ³⁰N₂ and ²⁹N₂ in the ¹⁴NO₂^--added group were produced than those in the control (without ¹⁴NO₂^-), confirming that ¹⁴NO₂^- induced Fe(II) oxidation to participate in Feammox for ¹⁵NH₄⁺ removal. Also, NO₂^- could be produced from partial denitrification of NO₃^-, meaning that NO₃^- as a more common species might substitute NO₂^- to trigger this new anammox process.

Volatile fatty acids production from waste activated sludge during anaerobic fermentation: The effect of superfine sand

Superfine sand in waste activated sludge (WAS) increased the uncertainty of anaerobic fermentation. Experiments showed that VFAs production from WAS was positively affected by superfine sand, with an increase from 2513 mg COD/L in the control (without superfine sand) to 3002 mg COD/L with superfine sand. A mechanism study demonstrated that the main factor responsible for the improved VFAs accumulation in response to superfine sand was acetic acid, which increased by nearly 30%. Further investigation exhibited that the process of solubilization and acidification were facilitated by superfine sand and the abundance of anaerobic functional microorganisms was greatly increased. Moreover, the activities of acetate kinase (AK) as well as the quantity of AK encoding gene were greatly promoted by superfine sand. The heat release during WAS anaerobic fermentation with superfine sand was higher than that without superfine sand (25.8 × 10^-3 versus 24.7 × 10^-3 W·min at about 70 min).

Understanding the fate and impact of capsaicin in anaerobic co-digestion of food waste and waste activated sludge

Anaerobic co-digestion is an attractive option to treat food waste and waste activated sludge, which is increasingly applied in real-world situations. As an active component in Capsicum species being substantially present in food waste in many areas, capsaicin has been recently demonstrated to inhibit the anaerobic co-digestion. However, the interaction between capsaicin and anaerobic co-digestion are still poorly understood. This work therefore aims to deeply understand the fate and impact of capsaicin in the anaerobic co-digestion. Experiment results showed that capsaicin was completely degraded in anaerobic co-digestion by hydroxylation, O-demethylation, dehydrogenation and doubly oxidization, respectively. Although methane was proven to be produced from capsaicin degradation, the increase in capsaicin concentration resulted in decrease in methane yield from the anaerobic co-digestion. With an increase of capsaicin from 2 ± 0.7 to 68 ± 4 mg/g volatile solids (VS), the maximal methane yield decreased from 274.6 ± 9.7 to 188.9 ± 8.4 mL/g VS. The mechanic investigations demonstrated that the presence of capsaicin induced apoptosis, probably by either altering key kinases or decreasing the intracellular NAD⁺/NADH ratio, which led to significant inhibitions to hydrolysis, acidogenesis, and methanogenesis, especially acetotrophic methanogenesis. Illumina Miseq sequencing analysis exhibited that capsaicin promoted the populations of complex organic degradation microbes such as Escherichia-Shigella and Fonticella but decreased the numbers of anaerobes relevant to hydrolysis, acidogenesis, and methanogenesis such as Bacteroide and Methanobacterium.

Effect of NaCl Concentration on Microbiological Properties in NaCl Assistant Anaerobic Fermentation: Hydrolase Activity and Microbial Community Distribution

Previous studies have demonstrated that sludge hydrolysis and short-chain fatty acids (SCFAs) production were improved through NaCl assistant anaerobic fermentation. However, the effect of NaCl concentrations on hydrolase activity and microbial community structure was rarely reported. In this study, it was found that α-glucosidase activity and some carbohydrate-degrading bacteria were inhibited in NaCl tests, owing to their vulnerability to high NaCl concentration. Correspondingly, the microbial community richness and diversity were reduced compared with the control test, while the evenness was not affected by NaCl concentration. By contrast, the protease activity was increased in the presence of NaCl and reached the highest activity at the NaCl concentration of 20 g/L. The protein-degrading and SCFAs-producing bacteria (e.g., Clostridium algidicarnis and Proteiniclasticum) were enriched in the presence of NaCl, which were salt-tolerant.