Accumulation of Long-Lived Photogenerated Holes at Indium Single-Atom Catalysts via Two Coordinate Nitrogen Vacancy Defect Engineering for Enhanced Photocatalytic Oxidation.

Visible-light-driven photocatalytic oxidation by photogenerated holes has immense potential for environmental remediation applications. While the electron-mediated photoreduction reactions are often at the spotlight, active holes possess a remarkable oxidation capacity that can degrade recalcitrant organic pollutants, resulting in nontoxic byproducts. However, the random charge transfer and rapid recombination of electron-hole pairs hinder the accumulation of long-lived holes at the reaction center. Herein, a novel method employing defect-engineered indium (In) single-atom photocatalysts with nitrogen vacancy (Nv) defects, dispersed in carbon nitride foam (In-Nv-CNF), is reported to overcome these challenges and make further advances in photocatalysis. This Nv defect-engineered strategy produces a remarkable extension in the lifetime and an increase in the concentration of photogenerated holes in In-Nv-CNF. Consequently, the optimized In-Nv-CNF demonstrates a remarkable 50-fold increase in photo-oxidative degradation rate compared to pristine CN, effectively breaking down two widely used antibiotics (tetracycline and ciprofloxacin) under visible light. The contaminated water treated by In-Nv-CNF is completely nontoxic based on the growth of Escherichia coli. Structural-performance correlations between defect engineering and long-lived hole accumulation in In-Nv-CNF are established and validated through experimental and theoretical agreement. This work has the potential to elevate the efficiency of overall photocatalytic reactions from a hole-centric standpoint.

Recycling of spent lithium-ion batteries for a sustainable future: recent advancements.

Lithium-ion batteries (LIBs) are widely used as power storage systems in electronic devices and electric vehicles (EVs). Recycling of spent LIBs is of utmost importance from various perspectives including recovery of valuable metals (mostly Co and Li) and mitigation of environmental pollution. Recycling methods such as direct recycling, pyrometallurgy, hydrometallurgy, bio-hydrometallurgy (bioleaching) and electrometallurgy are generally used to resynthesise LIBs. These methods have their own benefits and drawbacks. This manuscript provides a critical review of recent advances in the recycling of spent LIBs, including the development of recycling processes, identification of the products obtained from recycling, and the effects of recycling methods on environmental burdens. Insights into chemical reactions, thermodynamics, kinetics, and the influence of operating parameters of each recycling technology are provided. The sustainability of recycling technologies (e.g., life cycle assessment and life cycle cost analysis) is critically evaluated. Finally, the existing challenges and future prospects are presented for further development of sustainable, highly efficient, and environmentally benign recycling of spent LIBs to contribute to the circular economy.

Recovery of valuable metals from spent lithium-ion batteries using microbial agents for bioleaching: a review.

Spent lithium-ion batteries (LIBs) are increasingly generated due to their widespread use for various energy-related applications. Spent LIBs contain several valuable metals including cobalt (Co) and lithium (Li) whose supply cannot be sustained in the long-term in view of their increased demand. To avoid environmental pollution and recover valuable metals, recycling of spent LIBs is widely explored using different methods. Bioleaching (biohydrometallurgy), an environmentally benign process, is receiving increased attention in recent years since it utilizes suitable microorganisms for selective leaching of Co and Li from spent LIBs and is cost-effective. A comprehensive and critical analysis of recent studies on the performance of various microbial agents for the extraction of Co and Li from the solid matrix of spent LIBs would help for development of novel and practical strategies for effective extraction of precious metals from spent LIBs. Specifically, this review focuses on the current advancements in the application of microbial agents namely bacteria (e.g., Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans) and fungi (e.g., Aspergillus niger) for the recovery of Co and Li from spent LIBs. Both bacterial and fungal leaching are effective for metal dissolution from spent LIBs. Among the two valuable metals, the dissolution rate of Li is higher than Co. The key metabolites which drive the bacterial leaching include sulfuric acid, while citric acid, gluconic acid and oxalic acid are the dominant metabolites in fungal leaching. The bioleaching performance depends on both biotic (microbial agents) and abiotic factors (pH, pulp density, dissolved oxygen level and temperature). The major biochemical mechanisms which contribute to metal dissolution include acidolysis, redoxolysis and complexolysis. In most cases, the shrinking core model is suitable to describe the bioleaching kinetics. Biological-based methods (e.g., bioprecipitation) can be applied for metal recovery from the bioleaching solution. There are several potential operational challenges and knowledge gaps which should be addressed in future studies to scale-up the bioleaching process. Overall, this review is of importance from the perspective of development of highly efficient and sustainable bioleaching processes for optimum resource recovery of Co and Li from spent LIBs, and conservation of natural resources to achieve circular economy.

Insights into interactions of biodegradable and non-biodegradable microplastics with heavy metals.

Biodegradable microplastics (BMPs) are considered to be environmentally friendly compared to non-biodegradable plastics (NMPs). However, BMPs are likely to become toxic during their transport because of the adsorption of pollutants (e.g., heavy metals) onto them. This study investigated the uptake of six heavy metals (Cd2+, Cu2+, Cr3+, Ni2+, Pb2+, and Zn2+) by a common BMPs (polylactic acid (PLA)) and compared their adsorption characteristics to those of three types of NMPs (polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC)) for the first time. The order of heavy metal adsorption capacity among the four MPs was PE > PLA > PVC > PP. The findings suggest that BMPs contained more toxic heavy metals than some NMPs. Among the six heavy metals, Cr3+ showed considerably stronger adsorption than other heavy metals in both BMPS and NMPs. The adsorption of heavy metals on MPs can be well explained using the Langmuir isotherm model, while the adsorption kinetic curves showed the best fit to the pseudo-second-order kinetic equation. Desorption experiments revealed that BMPs released a higher percentage of heavy metals (54.6-62.6%) in the acidic environment in a shorter time (~ 6 h) compared to NMPs. Overall, this study provides insights into interactions of BMPs and NMPs with heavy metals and their removal mechanisms in aquatic environment.

Hydrothermal Treatment of Biomass Feedstocks for Sustainable Production of Chemicals, Fuels, and Materials: Progress and Perspectives.

Hydrothermal process is an emerging technology that contributes to sustainable production of biomass-derived chemicals, fuels, and materials. This technology uses hot compressed water to convert various biomass feedstocks including recalcitrant organic compounds in biowastes into desired solid, liquid, and gaseous products. In recent years, considerable progress has been made in the hydrothermal conversion of lignocellulosic as well as nonlignocellulosic biomass to value-added products and bioenergy to fulfill the principles of circular economy. However, it is important to assess hydrothermal processes in terms of their capabilities and limitations from different sustainability aspects so that further advances can be made toward improvement of their technical maturity and commercialization potential. The key aims of this comprehensive review are to (a) explain the inherent properties of biomass feedstocks and physio-chemical characteristics of their bioproducts, (b) elucidate related transformation pathways, (c) clarify the role of hydrothermal process for biomass conversion, (d) evaluate the capability of hydrothermal treatment coupled with other technologies for producing novel chemicals, fuels and materials, (e) explore different sustainability assessments of hydrothermal processes for potential large-scale applications, and (f) offer our perspectives to facilitate the transition from a primarily petro-based to an alternative biobased society in the context of changing climate.

Silver contamination and its toxicity and risk management in terrestrial and aquatic ecosystems.

Silver (Ag), a naturally occurring, rare and precious metal, is found in major minerals such as cerargyrite (AgCl), pyrargyrite (Ag3SbS3), proustite (Ag3AsS3), and stephanite (Ag5SbS4). From these minerals, Ag is released into soil and water through the weathering of rocks and mining activities. Silver also enters the environment by manufacturing and using Ag compounds in electroplating and photography, catalysts, medical devices, and batteries. With >400 t of Ag NPs produced yearly, Ag NPs have become a rapidly growing source of anthropogenic Ag input in the environment. In soils and natural waters, most Ag is sorbed to soil particles and sediments and precipitated as oxides, carbonates, sulphides, chlorides and hydroxides. Silver and its compounds are toxic, and humans and other animals are exposed to Ag through inhalation of air and the consumption of Ag-contaminated food and drinking water. Remediation of Ag-contaminated soil and water sources can be achieved through immobilization and mobilization processes. Immobilization of Ag in soil and groundwater reduces the bioavailability and mobility of Ag, while mobilization of Ag in the soil can facilitate its removal. This review provides an overview of the current understanding of the sources, geochemistry, health hazards, remediation practices and regulatory mandates of Ag contamination in complex environmental settings, including soil and aquatic ecosystems. Knowledge gaps and future research priorities in the sustainable management of Ag contamination in these settings are also discussed.

Short-chain fatty acid production and phosphorous recovery from waste activated sludge via anaerobic fermentation: A comparison of in-situ and ex-situ thiosulfate-assisted Fe2+/persulfate pretreatment.

Recently, increasing attention is given on the resource and energy recovery (e.g. short-chain fatty acids (SCFAs) and phosphorus (P)) from waste active sludge (WAS) under the "Dual carbon goals". This study compared four thiosulfate-assisted Fe2+/persulfate (TAFP) pretreatments of WAS, i.e. in-situ TAFP pretreatment (R1), ex-situ TAFP pretreatment (R2), in-situ TAFP pretreatment + pH adjustment (R3) and ex-situ TAFP pretreatment + pH adjustment (R4), followed by anaerobic fermentation over 20 days for SCFA production and P recovery. The results showed that the maximal SCFA yields in R1-4 were 730.2 ± 7.0, 1017.4 ± 13.9, 860.1 ± 40.8, and 1072.0 ± 33.2 mg COD/L, respectively, significantly higher than Control (365.2 ± 17.8 mg COD/L). The findings indicated that TAFP pretreatments (particularly ex-situ TAFP pretreatment) enhanced WAS disintegration and provided more soluble organics and subsequently promoted SCFA production. The P fractionation results showed the non-apatite inorganic P increased from 11.6 ± 0.2 mg P/g TSS in Control to 11.8 ± 0.5 (R1), 12.4 ± 0.3 (R2), 13.2 ± 0.7 (R3) and 12.7 ± 0.7 mg P/g TSS (R4), suggesting TAFP pretreatments improved P bioavailability due to formation of Fe-P mineral (Fe(H2PO4)2·2H2O), which could be recycled through magnetic separators. These findings were further strengthened by the analysis of microbial community and related marker genes that fermentative bacteria containing SCFA biosynthesis genes (e.g. pyk, pdhA, accA and accB) and iron-reducing bacteria containing iron-related proteins (e.g. feoA and feoB) were enriched in R1-4 (dominant in ex-situ pretreatment systems, R2 and R4). Economic evaluation further verified ex-situ TAFP pretreatment was cost-effective and a better strategy over other operations to treat WAS for SCFA production and P recovery.

Enhancing anaerobic fermentation of waste activated sludge by investigating multiple electrochemical pretreatment conditions: Performance, modeling and microbial dynamics.

Electrochemical pretreatment (EPT) is an efficient technology to improve volatile fatty acids (VFAs) production during anaerobic fermentation of waste activated sludge (WAS). This study investigated the co-effects of different current intensities, electrolyte NaCl dosage and pretreatment time for promoting VFAs production. The results showed that it was considerably enhanced by 51.6 % when EPT was performed at 1.0 A, 1.0 g/L and 60 min, and response surface methodology strategy adjusted the optimal EPT conditions to 1.0 A, 1.2 g/L and 66 min. The potential mechanisms were proposed that EPT at optimal conditions greatly enhanced solubilization and hydrolysis of WAS and selectively inactivated methanogens, causing the enrichment of acidogenic bacteria (i.e., Lactobacillus, Saccharimonadales, Tetrasphaera and Prevotella) due to generated reactive chlorine species. Finally, the economic analysis indicated the promising application potential with the profit of EPT at optimal conditions increasing by 36.0 %.

Dynamic anaerobic membrane bioreactor coupled with sulfate reduction (SrDMBR) for saline wastewater treatment.

This study investigated organic removal performance, characteristics of the membrane dynamics, membrane fouling and the effects of biological sulfate reduction during high-salinity (1.0%) and high-sulfate (150 mgSO42--S/L) wastewater treatment using a laboratory-scale upflow anaerobic sludge bed reactor integrated with cross-flow dynamic membrane modules. Throughout the operational period, dynamic membrane was formed rapidly (within 5-10 min) following each backwashing cycle (21-16 days), and the permeate turbidity of <5-7 NTU was achieved with relatively high specific organic conversion (70-100 gTOC/kgVSS·d) and specific sulfate reduction (50-70 gSO42--S/kgVSS·d) rates. The sulfide from sulfate reduction can be reused for downstream autotrophic denitrification. 16S rRNA gene amplicon sequencing revealed that the microbial communities enriched in the sludge were different than those accumulated on the dynamic layer. Overall, this study demonstrates that the anaerobic dynamic membrane bioreactor coupled with sulfate reduction (SrDMBR) shows promising applicability in saline wastewater treatment.

Biochar-based bioretention systems for removal of chemical and microbial pollutants from stormwater: A critical review.

Biochar has been increasingly used as a filter medium in engineered low impact development systems (e.g., bioretention systems) for decontamination of urban stormwater and management of hydrology. This review paper critically analyzes the performance of biochar-based biofiltration systems for removal of chemical and microbial pollutants present in urban runoff. Biochar-amended biofiltration systems efficiently remove diverse pollutants such as total nitrogen (32 - 61%), total phosphorus: (45 - 94%), heavy metals (27 - 100%), organics (54 - 100%) and microbial pollutants (log10 removal: 0.78 - 4.23) from urban runoff. The variation of biofiltration performance is due to changes in biochar characteristics, the abundance of dissolved organic matter and/or stormwater chemistry. The dominant mechanisms responsible for removal of chemical pollutants are sorption, ion exchange and/or biotransformation, whereas filtration/straining is the major mechanism for bacteria removal. The pseudo-second order and Langmuir isotherm are the best models that describe the kinetics and chemical equilibrium of pollutants, respectively. This critical review provides the fundamental scientific knowledge for designing highly efficient biochar-based bioretention systems for removal of diverse pollutants from urban stormwater. The key knowledge gaps that should be addressed in future research include long-term field-scale bioretention study, development of novel methods for filter media regeneration/reuse, and dynamics of filter media microbial communities.

Biological nitrogen removal from stormwater in bioretention cells: a critical review.

Excess nitrogen in stormwater degrades surface water quality via eutrophication and related processes. Bioretention has been recognized as a highly effective low-impact development (LID) technology for the management of high runoff volumes and reduction of nitrogen (N) pollutants through various mechanisms. This paper provides a comprehensive and critical review of recent developments on the biological N removal processes occurring in bioretention systems. The key plant- and microbe-mediated N transformation processes include assimilation (N uptake by plants and microbes), nitrification, denitrification, and anammox (anaerobic ammonia oxidation), but denitrification is the major pathway of permanent N removal. Overall, both laboratory- and field-scale bioretention systems have demonstrated promising N removal performance (TN: >70%). The phyla Bacteroidetes and Proteobacteria are the most abundant microbial communities found to be enriched in biofilter media. Furthermore, the denitrifying communities contain several functional genes (e.g., nirK/nirS, and nosZ), and their concentrations increase near the surface of media depth. The N removal effectiveness of bioretention systems is largely impacted by the hydraulics and environmental factors. When a bioretention system operates at: low hydraulic/N loading rate, containing a saturation zone, vegetated with native plants, having deeper and multilayer biofilter media with warm climate temperature and wet storm events periods, the N removal efficiency can be high. This review highlights shortcomings and current knowledge gaps in the area of total nitrogen removal using bioretention systems, as well as identifies future research directions on this topic.

Bioretention systems for stormwater management: Recent advances and future prospects.

Bioretention is a popular stormwater management strategy that is often utilized in urban environments to combat water quality and hydrological impacts of stormwater. This goal is achieved by selective designing of a system, which consists of suitable vegetation at the top planted on an engineered media with drainage system and possible underdrain at the bottom. Bibliometric analysis on bioretention studies indicates that most of the original research contributions are derived from a few countries and selected research groups. Hence, most of the bioretention systems installed in diverse geographical locations are based on guidelines from climatically different countries, which often lead to operational failures. The current review critically analyzes recent research findings from the bioretention literature, provides the authors' perspectives on the current state of knowledge, highlights the key knowledge gaps in bioretention research, and points out future research directions to make further advances in the field. Specifically, the role and desired features of bioretention components, the importance of fundamental investigations in laboratory, field-based studies and modeling efforts, the real-time process control of bioretention cells, bioretention system design considerations, and life cycle assessment of full-scale bioretention systems are discussed. The importance of local conditions in guiding bioretention designs in difference climates is emphasized. At the end of the review, current technical challenges are identified and recommendations to overcome them are provided. This comprehensive review not only offers fundamental insights into bioretention technology, but also provides novel ideas to combat issues related to urban runoff and achieve sustainable stormwater management.

Anaerobic self-forming dynamic membrane bioreactors (AnSFDMBRs) for wastewater treatment - Recent advances, process optimization and perspectives.

Recently, anaerobic self-forming dynamic membrane bioreactors (AnSFDMBRs) have attracted increasing attention, and are considered as an alternative to conventional anaerobic membrane bioreactors (AnMBRs). The key advantages of AnSFDMBRs include high flux, low propensity towards fouling, and low capital and operational costs. Although there have been several reviews on AnMBRs, very few reviews on AnSFDMBR system. Previous AnSFDMBR studies have focused on lab-scale to investigate the long-term flux, methods to improve performance and the associated mechanisms. Microbial analysis showed that the phyla namely Proteobacteria, Bacteroidetes and Firmicutes are dominant in both bulk sludge and cake biofilm, but their abundance is low in biocake. This review critically examines the fundamentals of AnSFDMBRs, operational conditions, process optimization and applications. Moreover, the current knowledge gaps (e.g., dynamic membrane module optimization, membrane surface modification and functional microbes enrichment) that should be studied in future to design an efficient AnSFDMBR system for treatment of diverse wastewaters.

Rational design of sulfidogenic granular sludge reactor with clostridia as dominant bacteria for energy-efficient sulfate-laden wastewater treatment.

The sludge flotation and washout are frequently observed in anaerobic sulfidogenic reactor. This challenge raised the interests of re-thinking/re-designing of a compact and low-flotation bioreactor. The present study investigated to understand the temporal dynamics of microbial community and granular sludge properties in a pneumatic-mixing reactor treating sulfate-laden wastewater. The findings revealed that the reactor performance and sludge properties were dynamically changed and correlated over long-term run. In the bioreactor, a rarer type of sulfate reducing bacteria (genus Clostridium XVIII) was remarkably enriched (~30% abundance). The Clostridium XVIII-mediated COD removal (92.7 ± 3.9%) was further confirmed via mass balance which demonstrated the growth rate of total active biomass and sulfate-reducing active biomass were 19.95 and 6.0 mg-COD/Linfluent respectively. The PICRUSt data suggested that i) high abundance of carbohydrate metabolism and S-reductase enzymes enriched, and ii) energy metabolism enzymes decreased which implies that the new SRB communities are more energy-efficient than conventional ones.

Bioaugmentation of marine anammox bacteria (MAB)-based anaerobic ammonia oxidation by adding Fe(III) in saline wastewater treatment under low temperature.

This work investigated a new method of using Fe(III) to enhance the reactor performance enriched with marine anammox bacteria (MAB). The experiments were conducted in a sequencing batch reactor at low temperature (15 °C), high salinity (35 g/L) and varying Fe(III) concentrations (0-250 mg/l). The results of this study showed that at low Fe(III) (6 mg Fe/L), the rate of ammonium removal, nitrite removal and specific anammox activity remarkably increased to 0.42 kg/(m3·d), 0.53 kg/(m3·d), 0.56 kg/(kg·d), respectively. However, Fe(III) at above 120 mg Fe/L, the reaction time was significantly shortened from 5 to 2 h. MAB-based nitrite removal could be predicated based on the change of pH (ΔpH) and oxidation-reduction potential (ΔORP). Kinetics analysis demonstrated, the "Remodified Logistic Model" could simulate the Fe(III) enhanced anammox process. Overall, this research shed the light of designing a new high-rate anaerobic nitrogen removal technology for carbon insufficient, nitrogen-laden saline wastewater.

Elucidating the effect of mixing technologies on dynamics of microbial communities and their correlations with granular sludge properties in a high-rate sulfidogenic anaerobic bioreactor for saline wastewater treatment.

In this study, three lab-scale anaerobic sulfidogenic bioreactors were operated independently using three different mixing modes (hydraulic, mechanical and pneumatic). One-way ANOVA test indicated various performance parameters (e.g. sulfate reduction and sulfide production) and granular sludge properties (e.g. EPS and particle size) statistically different in three mixing modes. Principal component analysis (PCA) and OTUs-based network demonstrated bacterial composition greatly varied among the three mixing modes. The phylum Proteobacteria was predominant among the bacterial communities, and the genus Desulfobacter (35.1% in hydraulic, 31.1% in mechanical and 27.4% in pneumatic sample) was the most dominant SRB. The PCA/Pearson's correlation analysis confirmed SRB had significant positive relationship with sludge properties (e.g. particle size). PICRUSt data highlighted that bacterial communities contained diverse predicted functions including sulfur metabolism enzymes (sulfite reductase and adenylylsulfate reductase). The findings of this research could be helpful for selection of an appropriate mixing technology for anaerobic sulfidogenic or similar bioprocess.

Sulfidogenic anaerobic digestion of sulfate-laden waste activated sludge: Evaluation on reactor performance and dynamics of microbial community.

This study investigated the impact of sludge retention times (SRTs: 40, 20, 10 and 5 days) on performance of the sulfidogenic anaerobic digestion (SAD) reactor treating sulfate-laden waste activated sludge and dynamics of sulfate reducing bacteria (SRB). The findings showed that sulfide production, volatile sludge removal efficiency, ammonia release and methane yield decreased by 33.7%, 66.4%, 21.3% and 68.7%, respectively when SRT was shortened from 40 to 5 d. Significant enrichment of hydrolyzers/fermenters (genera Mesotoga and Sulfurovum) was observed at longer SRT (40 d), but shorter SRT (5 d) favors enrichment of diverse SRB (genera Desulfomicrobium and Desulfovibrio). PICRUSt data revealed bacterial communities possessed diverse predicted functions including sulfur metabolism enzymes (e.g. sulfate adenylyltransferase), and their abundance was higher at shorter SRT. Statistical analysis (PCA) confirmed positive relationships between SRB and SAD performance. The findings of this research could be useful for design and optimization of sulfidogenic-based anaerobic digestion process.

Investigation on polyphosphate accumulation in the sulfur transformation-centric EBPR (SEBPR) process for treatment of high-temperature saline wastewater.

This study investigated the polyphosphates accumulation rate in a novel sulfur transformation-centric enhanced biological phosphorus removal (SEBPR) process. The SEBPR system was continuously operated over 120 days in a sequencing batch reactor (SBR) that alternated between the anaerobic mode and the anoxic mode of operation (temperature: 30 °C and salinity: 6000 mg/L Cl-). In addition to the SBR, batch experiments were carried out to test the effect of two different sulfate concentrations on the system performance and sulfur-phosphorus transformations. The key intercellular polymers of polyphosphates and polysulfur (poly-S) were identified by employing advanced microscopes. Metagenomic analysis was performed to characterize the diversity of microbes and their functions enriched in the SEBPR system. Finally, several molecular techniques including flow cytometry cell sorting and 16S DNA high-throughput sequencing were applied to identify the phosphorus-accumulating organisms (PAOs). The amounts of P release and P uptake in the SEBPR increased gradually to nearly 18 ±â€¯6.4 mg P/L and 26.5 ±â€¯6.7 mg P/L respectively, yielding a net P removal efficiency of 84 ±â€¯25%. Batch tests indicated no polyhydroxyalkanate (PHA) synthesis, but P uptake was observed and it was correlated with the intracellular poly-S consumption, suggesting that the poly-S could act as an intracellular energy source for P uptake and polyphosphates formation. Moreover, CLSM and TEM micrographs clearly showed the presence of intercellular polyphosphates and poly-S respectively. Metagenomic analysis revealed that Proteobacteria (36.5%), Bacteroidetes (23.3%), Thermotogae (7.1%), Chloroflexi (4.5%) and Firmicutes (2.3%) were the dominant phyla in Bacteria. The conventional PAO of Candidatus Accumulibacter was found at a low abundance of 0.32% only; and an uncultured genus close to Rhodobacteraceae at the family level is speculated to be the putative sulfur PAO (SPAO). Finally, this research suggests that poly-S considerably impacts on polyphosphates accumulation in the SEBPR system when no PHAs are formed.

Electrochemical pretreatment for stabilization of waste activated sludge: Simultaneously enhancing dewaterability, inactivating pathogens and mitigating hydrogen sulfide.

Stabilization of waste activated sludge (WAS) is an essential step for the disposal or reuse. In this study, WAS stabilization via electrochemical pretreatment (EPT) at 0-15V was evaluated for simultaneous dewaterability enhancement, pathogen removal and H2S mitigation. The mechanism underlying EPT was investigated and discussed based on the changes in the physicochemical (e.g., particle size, zeta potential, hydrophobicity and extracellular polymeric substances) and biological characteristics (i.e. cell morphology, and distribution and percentages of live/dead cells) of WAS with different EPT voltages. The results revealed that EPT disintegrated WAS flocs and disrupted the cell walls leading to a reduction in particle size (by up to 50%), increased release of extracellular and intracellular substances (by up to 4 times) to facilitate WAS stabilization. With EPT at 15V, the capillary suction time of WAS decreased by 42%, and the concentrations of E. coli and indicator pathogens (Salmonella spp. and Streptococcus faecalis) fell by nearly 5 log10 reaching U.S. EPA hygienization levels. Furthermore, EPT at 12V or higher suppressed the amounts of dissolved sulfide and H2S(g) produced from the WAS under anaerobic conditions by over 99%. This study demonstrates the feasibility of EPT for simultaneous WAS dewaterability enhancement, pathogen inactivation and H2S mitigation, providing a one-step alternative for sludge stabilization.

Long term performance and dynamics of microbial biofilm communities performing sulfur-oxidizing autotrophic denitrification in a moving-bed biofilm reactor.

Sulfide-oxidizing autotrophic denitrification (SOAD) implemented in a moving-bed biofilm reactor (MBBR) is a promising alternative to conventional heterotrophic denitrification in mainstream biological nitrogen removal. The sulfide-oxidation intermediate - elemental sulfur - is crucial for the kinetic and microbial properties of the sulfur-oxidizing bacterial communities, but its role is yet to be studied in depth. Hence, to investigate the performance and microbial communities of the aforementioned new biosystem, we operated for a long term a laboratory-scale (700 d) SOAD MBBR to treat synthetic saline domestic sewage, with an increase of the surface loading rate from 8 to 50 mg N/(m2·h) achieved by shortening the hydraulic retention time from 12 h to 2 h. The specific reaction rates of the reactor were eventually increased up to 0.37 kg N/(m3·d) and 0.73 kg S/(m3·d) for nitrate reduction and sulfide oxidation with no significant sulfur elemental accumulation. Two sulfur-oxidizing bacterial (SOB) clades, Sox-independent SOB (SOBI) and Sox-dependent SOB (SOBII), were responsible for indirect two-step sulfur oxidation (S2-→S0→SO42-) and direct one-step sulfur oxidation (S2-→SO42-), respectively. The SOBII biomass-specific electron transfer capacity could be around 2.5 times greater than that of SOBI (38 mmol e-/(gSOBII·d) versus 15 mmol e-/(gSOBI·d)), possibly resulting in the selection of SOBII over SOBI under stress conditions (such as a shorter HRT). Further studies on the methods and mechanism of selecting of SOBII over SOBI in biofilm reactors are recommended. Overall, the findings shed light on the design and operation of MBBR-based SOAD processes for mainstream biological denitrification.