Roles of quorum-sensing molecules in methane production from anaerobic digestion aided by biochar.

Biochar has been used to enhance methane generation from anaerobic digestion through establishing direct interspecific electron transfer between microorganisms. However, the microbial communication is still inadequate, thereby limiting further methane production improvement contributed by biochar. This study investigated the roles of quorum-sensing molecules, acylated homoserine lactone (AHL), in anaerobic digestion of waste activated sludge aided by biochar. Results showed that the co-addition of separated biochar and AHL achieved best methane production performance, with the maximal methane yield of 154.7 mL/g volatile suspended solids, which increased by 51.9%, 47.2%, 17.9%, and 39.4% respectively compared to that of control, AHL-loaded biochar, sole AHL, and sole biochar groups. The reason was that the co-addition of separated biochar and AHL promoted the stages of hydrolysis and acidification, promoting the conversion of organic matters and short-chain fatty acids, and optimizing the accumulation of acetate acid. Moreover, the methanogenesis stage also performed best among experimental groups. Correspondingly, the highest activities of electron transfer and coenzyme F420 were obtained, with increase ratios of 33.2% and 27.2% respectively compared to that of control. Furthermore, biochar did more significant effects on the evolution of microbial communities than AHL, and the direct interspecific electron transfer between fermentative bacteria and methanogens were possibly promoted.

Construction of an ideotype root system architecture of subsurface flow constructed wetland macrophytes by vertical spatial stress: strengthening of rhizosphere effects and determination of appropriate substrate depth.

Strengthening rhizosphere effects to enhance pollutant removal is a hotspot of constructed wetlands (CWs) research in recent years, and improving the root traits and metabolic capacity of macrophytes is crucial for strengthening rhizosphere effects. In the field experiment, two types of subsurface flow (SSF) CWs (CW10 and CW20, with substrate depths of 10 and 20 cm, respectively) under the vertical spatial stress of roots (VSSR) and two types of non-VSSR SSF CWs (CW40 and CW60) were adopted with Typha orientalis as cultivated plants to investigate the variability of root development, metabolism, and pollutant removal at different substrate depths. VSSR induced substantial redundant root development, which significantly increased root-shoot ratio, fine and lateral root biomass, root porosity, and root activity, with lateral and fine root biomass of CW20 reaching 409.17 and 237.42 g/m2, respectively, which were 3.18 and 5.28 times those of CW60. The radical oxygen loss (ROL) and dissolved organic carbon (DOC) levels of CW20 single plant were 1.36 and 4.57 times higher than those of CW60, respectively, and more types of root exudates were determined (e.g., aldehydes, ketones and amides). More aerobic heterotrophs (e.g., Massilia, Planomicrobium), nitrification bacteria (e.g., Ellin6067, Nitrospira), aerobic denitrification bacteria (e.g., Bacillu, Chryseobacterium, Pseudomonas) and denitrification phosphorus accumulating organisms (e.g., Flavobacterium) were enriched in the rhizosphere of CW20. This changed the main transformation pathways of pollutants and enhanced the removal of pollutants, with the COD, TN and TP average removal rates of CW20 increasing by 9.99%, 13.28% and 8.92%, respectively, compared with CW60. The ideotype root system architecture CW (RSACW; CW20) constructed in this study, which consists of a large number of fine and lateral roots, can stimulate more efficient rhizosphere effects stably and continuously.

Biochar alleviates inhibition effects of humic acid on anaerobic digestion: Insights to performances and mechanisms.

To recover methane from waste activated sludge through anaerobic digestion (AD) is one promising alternative to achieve carbon neutrality for wastewater treatment plants. However, humic acids (HAs) are one of the major compositions in waste activated sludge, and their accumulation performs inhibition effects on AD. This study investigated the potentials of biochar (BC) in alleviating inhibition effects of HAs on AD. Results showed that although the accumulated HAs reduced methane yield by 9.37% compared to control, the highest methane yield, 132.6 mL CH4/g VSS, was obtained after adding BC, which was 45.9% higher than that in HA group. Mechanism analysis showed that BC promoted the activities of hydrolase such as protease and α-glucosidase, which were 69.7% and 29.7% higher than those in HA group, respectively. The conversion of short-chain fatty acids was accelerated. In addition, the evolutions of electroactive microorganisms like Clostridium_sensu_stricto_13 and Methanosaeta were consistent with the activitiies of electron transfer and the contents of cytochrome c. Furthermore, parts of HAs rather than all of them were adsorbed by BC, and the remaining free HAs and BC formed synergistic effects on methanogenesis, then both CO2 reduction and acetoclastic methanogenesis pathways were improved. The findings may provide some solutions to alleviate inhibition effects of HAs on AD.

Effect of interactions between humic acid and cerium oxide nanoparticles on the toxicity to the Chlorella sp.

With the wide application of nanomaterials, the concentration of nanomaterials in natural water continues to increase, which poses a severe threat to the water environment. However, the influence of organic matter and nanomaterials rich in natural water on the toxic effect of algae growth is still unclear. In this study, the effects of humic acid (HA) and nano-cerium oxide (nCeO2) on the physiology and transcriptome of Chlorella sp. were analyzed, and the mechanism of the toxic effect of HA on Chlorella sp. under nCeO2 stress was revealed. Under 20-200 mg/L nCeO2 stress, the growth of Chlorella cells was inhibited and the highest inhibition rate reached 52% within 200 mg/L nCeO2. The Fv/Fm and ETRmax values of Chlorella sp. decreased from 0.490 and 24.45 (20 mg/L nCeO2) to 0.488 and 23.4 (100 mg/L nCeO2), respectively. Under the stimulation of nCeO2, the level of reactive oxygen species in algal cells was increased, accompanied by lipid peroxidation and membrane damage. However, the addition of HA at concentrations of 5-10 mg/L effectively alleviated the toxic effect of nCeO2 on Chlorella sp. Transcriptome analysis showed that 10 mg/L HA could alleviate the cellular stress at 100 mg/L nCeO2 on Chlorella sp. by regulating genes related to photosynthesis and metabolism pathways. Moreover, the downregulation of genes (e.g., Lhca1, Lhcb1, AOC3, and AOC2) indicated that HA reduced the level of oxidative stress in Chlorella sp. These findings offer novel insights of evaluating the ecotoxicity nCeO2 and HA in natural water environment and their impact on Chlorella sp.

Biochar derived from alkali-treated sludge residue regulates anaerobic digestion: Enhancement performance and potential mechanisms.

Biochar produced from bio-wastes has been widely used to promote the performance of anaerobic digestion. Waste activated sludge (WAS) is considered as a kind of popular precursor for biochar preparation, but the abundant resources in WAS were neglected previously. In this study, the roles of biochar prepared from raw, pretreated, and fermented sludge on anaerobic digestion were investigated. That is, parts of carbon sources and nutrients like polysaccharides, proteins, and phosphorus were firstly recovered after sludge pretreatment or fermentation, and then the sludge residuals were used as raw material to prepare biochar. The methane yield improved by 22.1% with adding the biochar (AK-BC) prepared by sludge residual obtained from alkaline pretreatment. Mechanism study suggested that the characteristics of AK-BC like specific surface area and defect levels were updated. Then, the conversion performance of intermediate metabolites and electro-activities of extracellular polymeric substances were up-regulated. As a result, the activity of electron transfer was increased with the presence of AK-BC, with increase ratio of 21.4%. In addition, the electroactive microorganisms like Anaerolineaceae and Methanosaeta were enriched with the presence of AK-BC, and the potential direct interspecies electron transfer was possibly established. Moreover, both aceticlastic and CO2-reducing methanogenesis pathways were improved by up-regulating related enzymes. Therefore, the proposed strategy can not only obtain preferred biochar but also recover abundant resources like carbon source, nutrients, and bioenergy.

Inducing root redundant development to release oxygen: An efficient natural oxygenation approach for subsurface flow constructed wetland.

Dissolved oxygen (DO) is a limiting factor affecting the purification efficiency of subsurface flow (SSF) constructed wetlands (CWs). To clarify the causes of oxygen environments and the response characteristics of plant oxygen release (POR) in SSF CWs, this study set three oxygen source treatments by limiting atmospheric reaeration (AR) and influent oxygen (IO) and compared the differences in plant physiological metabolism, DO distribution characteristics, and the purification effect of the SSF CWs at different depths. The results showed that limiting exogenous oxygen stimulated root redundancy of the wetland plants. The root volume and proportion of fibrous roots of the wetland plants increased significantly (p < 0.05). When only the POR existed, the root zone DO increased significantly to 2.05-4.37 mg/L (p < 0.05), and was positively correlated with the TN and TP removal rates (p < 0.05). Additionally, in the presence of POR only, the average removal rates of TN and TP in the top layer were 86.5% and 76.9%, respectively. The proportion of fibrous roots, root zone DO, and root-shoot ratio were key factors promoting the purification effect of the SSF CWs under limited exogenous oxygen sources. Enhancing POR by inducing root redundancy enhanced nitrification (hao, pmoABC-amoABC), plant absorption, and assimilation-related functional genes (nrtABC, nifKDH), and enriched nitrogen and phosphorus removal bacteria, such as Flavobacterium and Zoogloea. This consequently improved pollutant removal efficiency. Inducing root redundancy to strengthen POR produced an aerobic environment in the SSF CWs. This ensures the efficient and stable operation of the SSF CW and is an effective approach for natural oxygenation.

Contribution identification of hydrolyzed products of potassium ferrate on promoting short-chain fatty acids production from waste activated sludge.

Potassium ferrate (K2FeO4) has been extensively employed to promote short-chain fatty acids (SCFAs) production from anaerobic fermentation of waste activated sludge (WAS) because of its potent oxidizing property and formation of alkaline hydrolyzed products (potassium hydroxide, KOH and ferric hydroxide, Fe(OH)3). However, whether K2FeO4 actually works as dual functions of both an oxidizing agent and an alkalinity enhancer during the anaerobic fermentation process remains uncertain. This study aims to identify the contributions of hydrolyzed products of K2FeO4 on SCFAs production. The results showed that K2FeO4 did not execute dual functions of oxidization and alkalinity in promoting SCFAs production. The accumulation of SCFAs using K2FeO4 treatment (183 mg COD/g volatile suspended solids, VSS) was less than that using either KOH (192 mg COD/g VSS) or KOH & Fe(OH)3 (210 mg COD/g VSS). The mechanism analysis indicated that the synergistic effects caused by oxidization and alkalinity properties of K2FeO4 did not happen on solubilization, hydrolysis, and acidogenesis stages, and the inhibition effect caused by K2FeO4 on methanogenesis stage at the initial phase was more severe than that of its hydrolyzed products. It was also noted that the inhibition effects of K2FeO4 and its hydrolyzed products on the methanogenesis stage could be relieved during a longer sludge retention time, and the final methane yields using KOH or KOH & Fe(OH)3 treatment were higher than that using K2FeO4, further confirming that dual functions of K2FeO4 were not obtained. Therefore, K2FeO4 may not be an alternative strategy for enhancing the production of SCFAs from WAS compared to its alkaline hydrolyzed products. Regarding the strong oxidization property of K2FeO4, more attention could be turned to the fates of refractory organics in the anaerobic fermentation of WAS.

The relationship between mountain wetland health and water quality: A case study of the upper Hanjiang River Basin, China.

This study investigates the degradation process of mountain wetlands in the upper Hanjiang River Basin (HRB) over a 30-year span from 1990 to 2020. In particular, the landscape development intensity (LDI) index was employed to conduct a comprehensive assessment of the wetland health. This was subsequently combined with the spatio-temporal changes of water quality in the basin to explore the potential correlations between the health status of mountain wetlands and the associated watershed water quality. The results show that over the past three decades, wetland ecosystems have shrunk by 18% due to conversion into farmland, grass, construction land and forest land. This was significant between 2010 and 2020, as shown by a land use dynamic index of -1.121% during 2010-2020, which was significantly higher than that in the preceding two decades (0.003%, 0.367%) (p < 0.05). LDI values for individual sub-watersheds across different years ranged from 2.39 to 4.93, demonstrating an increasing trend since 2010. This indicates a heightened level of human interference in mountain wetlands. Although the water quality within the basin generally adhered to the Class II surface water quality standard, total nitrogen (TN) (primarily from farming) was a concern. Areas with relatively more human activity were observed to exhibit increased pollution levels, as demonstrated by a positive correlation between LDI and the concentrations of total phosphorus (TP), ammonium nitrogen (NH4+-N), and chemical oxygen demand (COD) in the basin. The LDI of the mountain wetland exhibited a consistent positive correlation with the water quality comprehensive function, both during the flood (r = 0.77-0.81) and non-flood (r = 0.61-0.70) seasons (p < 0.05). This indicates the significant impact of the wetland landscape structure on the water quality within a 1000 m radius on either side of the river. Special attention should be paid to the management and allocation of wetland landscapes within this 1000 m buffer zone. Furthermore, efforts to control upstream pollutant emission should be strengthened.

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.

Positive responses and mechanisms of nitrifying sludge to carbon quantum dots: reactor performance, microbial behavior, and antioxidant defense.

Extensive application of carbon quantum dots (CQDs) enlarges its concentration in sewage treatment system. The response of nitrifying sludge to CQDs after long-term exposure was investigated. Results showed that CQD concentrations of 0-100 mg/L presented positive effect to enzymes involved in nitrification, accelerating NH4+-N degradation and NO2--N transformation. The oxidation rate of NO2--N was significantly improved from 3.14 to 7.91 mg/(L h) under the stress of 100 mg/L CQDs. Besides, CQDs stimulated the production of sludge biomass and kept the stability of sludge settleability. Additionally, CQDs were mainly captured by loosely bound extracellular polymeric substances, reducing aromatic-like protein. Microbes alleviated CQD stress by secreting tryptophan-like protein and polysaccharides. After few CQDs entered cells, intracellular antioxidant defense was activated. Total antioxidant capacity level was heightened at least 31%. The activities of superoxide dismutase and catalase were enhanced at relatively low and high CQD concentration levels. Hence, microbial metabolic pathways, microbial community, and nitrifying bacteria were not significantly affected by CQDs. The findings of this work provide new insight for understanding the environmental implication of CQDs in the biological treatment system.

Insights into the roles and mechanisms of a green-prepared magnetic biochar in anaerobic digestion of waste activated sludge.

Methane is one of the most promising renewable energies to alleviate energy crisis or replace fossil fuels, which can be recovered from anaerobic digestion of bio-wastes. However, the engineering application of anaerobic digestion is always hindered by low methane yield and production rate. This study revealed the roles and mechanisms of a green-prepared magnetic biochar (MBC) in promoting methane production performance from waste activated sludge. Results showed that the methane yield reached 208.7 mL/g volatile suspended solids with MBC additive dosage of 1 g/L, increasing by 22.1 % compared to that in control. Mechanism analysis demonstrated that MBC could promote hydrolysis, acidification, and methanogenesis stages. This was because the properties of biochar were upgraded by loading nano-magnetite, such as specific surface area, surface active sites, and surface functional groups, which made MBC have greater potential to mediate electron transfer. Correspondingly, the activity of α-glucosidase and protease respectively increased by 41.7 % and 50.0 %, and then the hydrolysis performances of polysaccharides and proteins were improved. Also, MBC improved the secretion of electroactive substances like humic substances and cytochrome C, which could promote extracellular electron transfer. Furthermore, Clostridium and Methanosarcina, as well-known electroactive microbes, were selectively enriched. The direct interspecies electron transfer between them was established via MBC. This study provided some scientific evidences to comprehensively understand the roles of MBC in anaerobic digestion, with important implications for achieving resource recovery and sludge stabilization.

Insights into response mechanism of anaerobic digestion of waste activated sludge to particle sizes of zeolite.

Anaerobic digestion has been proved as one promising strategy to simultaneously achieve resource recovery and environmental pollution control for biosolid treatment, and adding exogenous materials is a potential alternative to promote the above process. This study investigated response mechanisms of anaerobic digestion of waste activated sludge (WAS) to particle sizes of zeolite. Results showed that the methane production reached 186.75 ± 7.62 mL/g volatile suspended solids (VSS) with zeolite of the particle size of 0.2-0.5 mm and the additive dosage of 0.1 g/g VSS, which increased by 22.08% compared to that in control. Mechanism study revealed that zeolite could improve hydrolysis, acidification, and methanogenesis stages. Rapid consumption rates of soluble polysaccharides and proteins were observed, correspondingly, the accumulations of short-chain fatty acids (SCFAs) were enhanced, and the compositions of SCFAs were optimized. Moreover, the activities of F420 increased by 28% with zeolite, and the syntrophic metabolism between bacteria and methanogens were promoted.

Root vertical spatial stress: A method for enhancing rhizosphere effect of plants in subsurface flow constructed wetland.

The depth of the substrate of subsurface flow (SSF) constructed wetlands (CWs) is closely related to their cost and operation stability. To explore the physiological regulation mechanism of wetland plants and pollutant removal potential of SSF CWs under "vertical spatial stress of roots" (by greatly reducing the depth of the substrate in SSF CWs to limit the vertical growth space of roots, VSSR), the physiological response and wetland purification effect of a 0.1 m Canna indica L. CW under VSSR were studied compared with conventional SSF CWs (0.6 m, 1.2 m). The results demonstrated that VSSR significantly enhanced the dissolved oxygen (DO) concentration (p < 0.05) within the SSF CWs, with the DO in 0.1 m CW remaining stable at over 3 mg/L. Under the same hydraulic retention time (HRT), VSSR significantly improved the removal effect of pollutants (p < 0.05). The removal rates of COD, NH4+-N, and total phosphorus (TP) remained above 87%, and the mean removal rates of total nitrogen (TN) reached 91.71%. VSSR promoted the morphological adaptation mechanisms of plants, such as significantly increased root-shoot ratio (p < 0.05), changed biomass allocation. Plants could maintain the stability of the photosynthetic mechanism by changing the distribution of light energy. The results of microbial community function prediction demonstrated that aerobic denitrification was the main mechanism of N transformation in the 0.1 m CW under VSSR. VSSR could induce the high root activity of plants, augment the concentration of root exudates, enhance the redox environment of the plant rhizosphere, further foster the enrichment of aerobic denitrifying bacteria, and strengthen the absorption efficiency of wetland plants and substrate, thus achieving an efficient pollutant removal capacity. Studies showed that VSSR was an effective means to enhance the rhizosphere effect of plants and pollutant removal in SSF CWs.

An overview of bioelectrokinetic and bioelectrochemical remediation of petroleum-contaminated soils.

The global problem of petroleum contamination in soils seriously threatens environmental safety and human health. Current studies have successfully demonstrated the feasibility of bioelectrokinetic and bioelectrochemical remediation of petroleum-contaminated soils due to their easy implementation, environmental benignity, and enhanced removal efficiency compared to bioremediation. This paper reviewed recent progress and development associated with bioelectrokinetic and bioelectrochemical remediation of petroleum-contaminated soils. The working principles, removal efficiencies, affecting factors, and constraints of the two technologies were thoroughly summarized and discussed. The potentials, challenges, and future perspectives were also deliberated to shed light on how to overcome the barriers and realize widespread implementation on large scales of these two technologies.

The nutrient removal and tolerance mechanism of a heterotrophic nitrifying bacterium Pseudomonas putida strain NP5 under metal oxide nanoparticles stress.

The occurrence of metal oxide nanoparticles (NPs) in wastewater treatment plants (WWTPs) has raised great concerns about their adverse impacts on nitrification performance. In this study, a heterotrophic nitrifying bacterium Pseudomonas putida strain NP5 showed strong resistance against TiO2 and NiO NPs. Under 5-50 mg/L NP stress, cell viability was still normal, and the final nutrient removal rates, always higher than 80%, were slightly inhibited. Correspondingly, the PO43--P removal rates were almost the same as those observed in the control test. Although the enzyme assay demonstrated ammonia monooxygenase and hydroxylamine oxidoreductase activities markedly decreased caused by increased reactive oxygen species (ROS) level under 50 mg/L NPs stress. The total antioxidant capability of NP5 could eliminate excess ROS to maintain a balance between oxidants and antioxidants. Besides, in response to the escalating burden of NPs, strain NP5 tended to secrete more extracellular polymeric substances (EPS), which could protect cell from being damaged by binding to ions and coating. Thus, the strong NP resistance of NP5 would help to overcome the vulnerability of the nitrification process in WWTPs.

Roles of zero-valent iron in anaerobic digestion: Mechanisms, advances and perspectives.

With the rapid growth of population and urbanization, more and more bio-wastes have been produced. Considering organics contained in bio-wastes, to recover resource from bio-wastes is of great significance, which can not only achieve the resource recycle, but also protect the environment. Anaerobic digestion (AD) has been proved as one of the most promising strategies to recover bio-energy from bio-wastes, as well as to realize the reduction of bio-wastes. However, the conventional interspecies electron transfer is sensitive to environmental shocks, such as high ammonia, organic pollutants, metal ions, etc., which lead to instability or failure of AD. The recent findings have proved that the introduction of zero-valent iron (ZVI) in AD system can significantly enhance methane production from bio-wastes. This review systematically highlighted the recent advances on the roles of ZVI in AD, including underlying mechanisms of ZVI on AD, performance enhancement of AD contributed by ZVI, and impact factors of AD regulated by ZVI. Furthermore, current limitations and outlooks have been analyzed and concluded. The roles of ZVI on underlying mechanisms in AD include regulating reaction conditions, electron transfer mode and function of microbial communities. The addition of ZVI in AD can not only enhance bio-energy recovery and toxic contaminants removal from bio-wastes, but also have the potential to buffer adverse effect caused by inhibitors. Moreover, the electron transfer modes induced by ZVI include both interspecies hydrogen transfer and direct interspecies electron transfer pathways. How to comprehensively evaluate the effects of ZVI on AD and further improve the roles of ZVI in AD is urgently needed for practical application of ZVI in AD. This review aims to provide some references for the introduction of ZVI in AD for enhancing bio-energy recovery from bio-wastes.

Response of anaerobic digestion of waste activated sludge to types of alkalis: Contribution identification of metal ions.

Alkaline pretreatment is one promising strategy for promoting anaerobic digestion of waste activated sludge (WAS). This study selected three types of alkalis with monovalent (NaOH and KOH), divalent (Ca(OH)2 and Mg(OH)2), and trivalent (Fe(OH)3 and Al(OH)3) cations to reveal the roles of metal ions on short chain fatty acids (SCFAs) production. The enhanced production potentials of SCFAs were reduced by order of alkalis with monovalent, divalent, and trivalent cations. Na+, K+, Ca2+, and Mg2+ did no contributions on SCFAs production, while Fe3+ and Al3+ performed better than control, especially the latter. The mechanism analysis proved that Na+, K+, Ca2+, and Mg2+ did no significant effects on solubilization, hydrolysis, acidification and methanogenesis stages, while the first three stages were improved by Fe3+ and Al3+ and the methanogenesis stage was inhibited. The findings may provide some new insights when using alkalis or residual metal ions to improve anaerobic digestion of WAS.

Role of extracellular polymeric substances in methane production from waste activated sludge induced by conductive materials.

Conductive materials have been widely used to establish direct interspecies electron transfer (DIET) for enhancing methane production potential from anaerobic digestion of waste activated sludge (WAS). However, the roles of extracellular polymeric substances (EPSs) affected by conductive materials on anaerobic digestion have been rarely reported. This study selected four widely used conductive materials, i.e., granular active carbon (GAC), biochar (BC), zero-valent iron (ZVI), and magnetite (Mag), to reveal the roles of EPSs. Results showed that methane production potentials were increased by BC, ZVI and Mag compared to that of control, with increase ratios of 13.4 %, 22.2 %, and 12.2 %, while a decrease was observed by GAC. The contents, components and characteristics of EPSs were all affected by conductive materials. The contents of EPSs were increased by ZVI and Mag, while they were decreased by BC and GAC. The ratios between proteins and polysaccharides (PN/PS) in loosely bound EPSs (LB-EPSs) were reduced in all groups, while they were similar in tightly bound EPSs (TB-EPSs) of ZVI and Mag groups. In addition, the cytochrome C and redox properties were remarkably promoted in suspension rather than in LB- and TB-EPSs. It was found that the correlation relationships between the maximal methane production potential (Pmax) and PN/PS in EPSs were positive, as well as fluorescent substances, especially tyrosine-like and tryptophan-like substances, with R2 of 0.96 and 0.98. Furthermore, the correlation relationships also existed between EPSs and microbial communities. Clostridium and Methanobacterium, potential DIET partners, presented significant positive correlation relationships (P < 0.05) with Pmax, PN/PS and fluorescent substances in EPSs. The findings may provide some new insights for mechanism investigation of anaerobic digestion induced by conductive materials.

Potassium ferrate followed by alkali-stripping treatment to achieve short-chain fatty acids and nitrogen recovery from waste activated sludge.

Waste activated sludge (WAS) is a potential resource to achieve carbon-neutral goal of wastewater treatment plant. However, the solubilization is always the rate-limiting step for resource recovery from anaerobic digestion of WAS. This study reported a novel strategy, i.e., potassium ferrate (PF) followed by alkali-stripping treatment, to achieve short-chain fatty acids (SCFAs) and nitrogen recovery from WAS. Results showed that whether the stripping process was conducted under alkaline condition or not, the SCFAs production potential was increased rather than reduced. The promoted SCFAs production was due to the accelerated solubilization and hydrolysis stages but the inhibited methanogenesis stage. The SCFAs yield reached 258 mg chemical oxygen demand (COD)/g volatile suspended solids (VSS), and the carbon source, including SCFAs, soluble polysaccharides and proteins, reached 384 mg COD/g VSS. The potentially recovered nitrogen was about 8.71 mg NH4+-N/g VSS. This work may provide some new solutions for enhancing resource recovery from WAS.