Granulation process in an expanded granular sludge blanket (EGSB) reactor for domestic sewage treatment: Impact of extracellular polymeric substances compositions and evolution of microbial population

De novo anaerobic granulation with varying organic substrates: granule growth and microbial community responses

Anaerobic granulation from dispersed inoculum is recognized as a slow process. However, studies under saline conditions have shown that adding complex proteinaceous substrates can accelerate this process. To explore whether this holds true also under non-saline conditions, we conducted a 262-days experiment with four lab-scale upflow anaerobic sludge blanket reactors inoculated with digested sewage sludge. Each reactor received a synthetic feed containing varying amount of carbohydrate/protein substrate: glucose (RGlu), acetate/tryptone (RAc+Try), glucose/tryptone (RGlu+Try), and glucose/starch (RGlu+Sta). Development of granules with different influent composition was monitored with macroscopy, analysis of the extracellular polymeric substances, and microbial diversity. Granulation was faster in reactors RGlu+Try and RGlu+Sta. Increasing granule diameters positively correlated with the occurrence of bacteria from Muribaculaceae and Lachnospiraceae families, suggesting their involvement in de novo granulation. Granules of RGlu+Try also had high relative abundances of both fermenting bacteria (e.g. Lactococcus, Streptococcus, Trichococcus) and bacteria involved in the oxidation of volatile fatty acids (Smithella, Acetobacteroides). The results of this study provide a basis for strategies to enhance the sludge granulation rate in practice when granular inoculum is not available. Specifically, supplementing small amounts of waste protein during reactor start-up can be effective.

Electro-dewatering performance of sewage sludge under interrupted pulsating voltage: A comparison between square shape and half-sine shape waveform

Electro-dewatering of sewage sludge with pulsating voltage was conducted under the two different wave shapes (square wave (SQW) and half-sine wave (HSW)) to investigate the influence of wave shape and duty cycle on sludge dewatering performance. The results indicated that, under the same average voltage, the moisture content of dewatered sludge with HSW was 10.3%-35.4% lower than that with SQW, suggesting the better dewatering performance of HSW. The optimal dewatering performance was achieved at duty cycle of 80% for SQW and 60% for SHW. The chemical oxygen demand of filtrate from HSW could be 13% higher than that from SQW, indicating the higher capacity of HSW in breaking sludge cells/floc structure. The applied voltage during electrochemical treatment promoted the hydrolysis of protein in filtrate, and the main components in the electro-dewatered filtrate were fulvic acid- and humic acid-like substances. The specific energy consumption for sludge electro-dewatering were 0.015-0.269 kWh/(kg removed water), and it was almost in linear relationship with duty cycle. By overall considering the energy consumption and electro-dewatering performance, the condition of 60% duty cycle with HSW was obviously better than other conditions, which provides a meaningful guidance for future application of sludge electro-dewatering technology with pulsating voltage.

Iron cycle-enhanced anaerobic ammonium oxidation in microaerobic granular sludge

Granule-based partial nitritation and anaerobic ammonium oxidation (PN/A) is an energy-efficient approach for treating ammonia wastewater. When treating low-strength ammonia wastewater, the stable synergy between PN and anammox is however difficult to establish due to unstable dissolved oxygen control. Here, we proposed, the PN/A granular sludge formed by a micro-oxygen-driven iron redox cycle with continuous aeration (0.42 ± 0.10 mg-O2/L) as a novel strategy to achieve stable and efficient nitrogen (N) removal. 240-day bioreactor operation showed that the iron-involved reactor had 37 % higher N removal efficiency than the iron-free reactor. Due to the formation of the microaerobic granular sludge (MGS), the bio(chemistry)-driven iron cycle could be formed with the support of anaerobic ammonium oxidation coupled to Fe3+ reduction. Both ammonia-oxidizing bacteria and generated Fe2+ could scavenge the oxygen as a defensive shield for oxygen-sensitive anammox bacteria in the MGS. Moreover, the iron minerals derived from iron oxidation and Fe-P precipitates were also deposited on the MGS surface and/or embedded in the internal channels, thus reducing the size of the channels that could limit oxygen mass transfer inside the MGS. The spatiotemporal assembly of diverse functional microorganisms in the MGS for the realization of stable PN/A could be achieved with the support of the iron redox cycle. In contrast, the iron-free MGS could not optimize oxygen mass transfer, which led to an unstable and inefficient PN/A. This work provides an alternative iron-related autotrophic N removal for low-strength ammonia wastewater.

Enhancing anaerobic digestion performance of oxytetracycline-laden wastewater through micro-nano bubble ozonation pretreatment

This study investigated the potential of micro-nano bubble (MNB) ozonation pretreatment to eliminate oxytetracycline (OTC) from wastewater and improve subsequent anaerobic digestion (AD) performance. The findings revealed that MNB ozonation achieved efficient OTC oxidation (>99 % in 60 min), and significantly enhanced methane production by 51 % compared to conventional ozonation (under 30 min of pretreatment). Additionally, MNB ozonation resulted in a decrease in the soluble chemical oxygen demand and reduced volatile fatty acid accumulation compared to conventional ozonation. Furthermore, the study sheds light on the profound impact of OTC and its oxidation by-products on the sludge microbiome. Exposure to OTC and its oxidation by-products resulted in alterations in extracellular polymeric substances composition and led to significant shifts in microbial community structure. This study highlights the promise of MNB ozonation as an effective approach for pharmaceutical pollutant removal and the optimization of AD performance in wastewater treatment, with implications for improved environmental sustainability.

Unifying concepts in methanogenic, aerobic, and anammox sludge granulation

The retention of dense and well-functioning microbial biomass is crucial for effective pollutant removal in several biological wastewater treatment technologies. High solids retention is often achieved through aggregation of microbial communities into dense, spherical aggregates known as granules, which were initially discovered in the 1980s. These granules have since been widely applied in upflow anaerobic digesters for waste-to-energy conversions. Furthermore, granular biomass has been applied in aerobic wastewater treatment and anaerobic ammonium oxidation (anammox) technologies. The mechanisms underpinning the formation of methanogenic, aerobic, and anammox granules are the subject of ongoing research. Although each granule type has been extensively studied in isolation, there has been a lack of comparative studies among these granulation processes. It is likely that there are some unifying concepts that are shared by all three sludge types. Identifying these unifying concepts could allow a unified theory of granulation to be formed. Here, we review the granulation mechanisms of methanogenic, aerobic, and anammox granular sludge, highlighting several common concepts, such as the role of extracellular polymeric substances, cations, and operational parameters like upflow velocity and shear force. We have then identified some unique features of each granule type, such as different internal structures, microbial compositions, and quorum sensing systems. Finally, we propose that future research should prioritize aspects of microbial ecology, such as community assembly or interspecies interactions in individual granules during their formation and growth.

Three-dimensional graphene aerogel mitigated the toxic impact of chloramphenicol wastewater on microorganisms in an EGSB reactor

Anaerobic treatment of chloramphenicol wastewater holds significant promise due to its potential for bioenergy generation. However, the high concentration of organic matter and residual toxic substances in the wastewater severely inhibit the activity of microorganisms. In this study, a three-dimensional graphene aerogel (GA), as a conductive material with high specific surface area (114.942 m2 g-1) and pore volume (0.352 cm3 g-1), was synthesized and its role in the efficiency and related mechanism for EGSB reactor to treat chloramphenicol wastewater was verified. The results indicated that synergy effects of GA for Chemical Oxygen Demand (COD) removal (increased by 8.17 %), chloramphenicol (CAP) removal (increased by 4.43 %) and methane production (increased by 70.29 %). Furthermore, GA increased the average particle size of anaerobic granular sludge (AGS) and promoted AGS to secrete more redox active substances. Microbial community analysis revealed that GA increased the relative abundance of functional bacteria and archaea, specifically Syntrophomonas, Geobacter, Methanothrix, and Methanolinea. These microbial species can participate in direct interspecific electron transfer (DIET). This research serves as a theoretical foundation for the application of GA in mitigating the toxic impact of refractory organic substances, such as antibiotics, on microorganisms during anaerobic treatment processes.

Evaluating the stability and performance of a novel core-shell ZVI@C-montmorillonite particle for anaerobic treatment of chloramphenicol wastewater

ZVI@C-MP is a novel composite particle consisting of zero-valent iron (ZVI) enclosed within a carbon shell. The purpose of this composite material is to enhance the anaerobic treatment of wastewater containing chloramphenicol (CAP). This approach aims to address the initial challenge of excessive corrosion experienced by ZVI, followed by its subsequent passivation and inactivation. ZVI@C-MP was synthesized through a hydrothermal process and calcination, with montmorillonite as binder, it exhibits stability, iron-carbon microelectrolysis (ICME) properties, and strong adsorption for CAP. Its ICME actions include releasing iron ions (0.70 mg/L) and COD (11.3 mg/L), generating hydrogen (3.82%), and raising the pH from 6.30 to 7.71. With minimal structural changes, it achieved release equilibrium. ZVI@C-MP boasts high removal efficiency of CAP (98.96%) by adsorption, attributed to surface characteristics (surface area: 167.985 m2/g; pore volume: 0.248 cm3/g). The addition of ZVI@C-MP increases COD removal (10.16%), methane production (72.86%), and reduces extracellular polymeric substances (EPS) from 70.58 to 52.72 mg/g MLVSS. It reduces microbial by-products and toxic effects, enhancing CAP biodegradation and microbial metabolic activity. ZVI@C-MP's electrical conductivity and biocompatibility bolster functional flora for interspecies electron transfer. It's a novel approach to antibiotic wastewater treatment.

Unravelling the resilience of magnetite assisted granules to starvation and oxytetracycline stress

Starvation and antibiotics pollution are two frequent perturbations during breeding wastewater treatment process. Supplying magnetite into anaerobic system has been proved efficient to accelerate microbial aggregates and alleviate the adverse effect caused by process disturbance. Nevertheless, whether these magnetite-based granules are still superior over normal granules after a long-term starvation period remains unknown, the responsiveness of these granules to antibiotics stress is also ambiguous. In current study, we investigated the resilience of magnetite-based anaerobic granular sludge (AnGS) to starvation and oxytetracycline (OTC) stress, by unravelling the variations of reactor performance, sludge properties, ARGs dissemination and microbial community. Compared with the AnGS formed without magnetite, the magnetite assisted AnGS appeared more robust defense to starvation and OTC stress. With magnetite supplement, the average methane yield after starvation recovery, 50 mg/L and 200 mg/L OTC stress was enhanced by 48.95%, 115.87% and 488.41%, respectively, accompanied with less VFAs accumulation, improved tetracycline removal rate (76.3-86.6% vs. 51.0-53.5%) and higher ARGs reduction. Meanwhile, magnetite supplement effectively ameliorated the potential sludge breakage by triggering more large granules formation. Trichococcus was considered an important impetus in maintaining the stability of magnetite-based AnGS process. By inducing more syntrophic methanogenesis partnerships, especially for hydrogenotrophic methanogenesis, magnetite ensured the improved reactor performance and stronger resilience at stress conditions.

Conceptual system for sustainable and next-generation wastewater resource recovery facilities

Shifting the concept of municipal wastewater treatment to recover resources is one of the key factors contributing to a sustainable society. A novel concept based on research is proposed to recover four main bio-based products from municipal wastewater while reaching the necessary regulatory standards. The main resource recovery units of the proposed system include upflow anaerobic sludge blanket reactor for the recovery of biogas (as product 1) from mainstream municipal wastewater after primary sedimentation. Sewage sludge is co-fermented with external organic waste such as food waste for volatile fatty acids (VFAs) production as precursors for other bio-based production. A portion of the VFA mixture (product 2) is used as carbon sources in the denitrification step of the nitrification/denitrification process as an alternative for nitrogen removal. The other alternative for nitrogen removal is the partial nitrification/anammx process. The VFA mixture is separated with nanofiltration/reverse osmosis membrane technology into low-carbon VFAs and high-carbon VFAs. Polyhydroxyalkanoate (as product 3) is produced from the low-carbon VFAs. Using membrane contactor-based processes and ion-exchange techniques, high-carbon VFAs are recovered as one-type VFA (pure VFA) and in ester forms (product 4). The nutrient-rich fermented and dewatered biosolid is applied as a fertilizer. The proposed units are seen as individual resource recovery systems as well as a concept of an integrated system. A qualitative environmental assessment of the proposed resource recovery units confirms the positive environmental impacts of the proposed system.

Critical role of extracellular DNA in the establishment and maintenance of anammox biofilms

Anaerobic ammonium oxidation (anammox) has been widely used for the sustainable removal of nitrogen from wastewater. Extracellular DNA (exDNA), as one of the main components of biofilms, not only determines the initial formation process, but also allows the three-dimensional structure to be maintained. Since the effects of exDNA on anammox biofilm formation are still poorly understood, this study elucidated the effects of exDNA on different stages of anammox biofilm establishment and maintenance under static conditions and its mechanism. The results revealed that exDNA mainly affected the maintenance stage of anammox biofilm formation. Compared with the absence of exDNA, nitrogen removal efficiency in the presence of exDNA was 6.17 % higher; the number of bacteria cells attached to the carrier was 2.23 times that in the absence of exDNA. The spatiotemporal distribution of bacteria was revealed by fluorescence in situ hybridization. After 30 days, the relative abundances of anammox in biofilms were 6.19 % and 0.4 % in the presence and absence of exDNA, respectively, indicating its positive role in anammox bacteria (AnAOB) adhesion and biofilm formation. The presence of exDNA in extracellular polymeric substances (EPS) promotes the synthesis of proteins and soluble microbial products. According to the extended Derjaguin-Landau-Verwey-Overbeek (X - DLVO) theory, the presence of exDNA also reduced the Lewis acid-base interaction energy and created favorable thermodynamic conditions for AnAOB adhesion. These findings advance our understanding of the role of exDNA in anammox-mediated biofilm formation and offer insights into the mechanism of exDNA in the establishment and maintenance stages.

Effects of biochar addition towards the treatment of blackwater in anaerobic dynamic membrane bioreactor (AnDMBR): Comparison among room temperature, mesophilic and thermophilic conditions

Effects of biochar addition on the performance of anaerobic dynamic membrane bioreactor (AnDMBR) under different temperatures for blackwater treatment was investigated. When the organic load ratios (OLR) was 1.0 g COD/L·d, the specific methane yield for the three biochar-amended reactors were 125.7, 148.0 and 182.3 mLCH₄/g COD_removed, respectively. Compare to those digesters without biochar participation, the methane production in the thermophilic reactor with biochar increased by 12% while the other two digesters increased by 6-10%. An analysis of membrane filtration resistance showed a reduction in total resistance (R_t) of 6.2 × 10¹¹-7.3 × 10¹¹ m^-1 when biochar was added to the three reactors. The thermophilic reactors with biochar increased the relative abundance of Methanothermobacter and promoted gene expression of metabolic pathways related to hydrolysis, acid production and methane production. Overall, biochar showed great potential as an inexpensive conductive material to increase methane production with reduced membrane fouling in AnDMBR systems.

Synergistic effect of extracellular polymeric substances and carbon layer on electron utilization of Fe@C during anaerobic treatment of refractory wastewater

Nano zero-valent iron (NZVI) has been widely used to improve refractory wastewater treatment. However, the rapid dissolution of NZVI causes a waste of resources and an unstable bioaugmentation. Herein, to verify the essential role of slow release of NZVI on biological systems, a core-shell structured Fe@C composite was developed to demonstrate the long-term feasibility of Fe@C for enhancing azo dye biodegradation in comparison to a mixture of NZVI and carbon powder (Fe+C). The 150 days of long-term reactor operation showed that, although both Fe@C and Fe+C enhanced azo dye degradation, the former achieved a better performance than the latter. The strengthening effect of Fe@C was also more durable and stable than Fe+C. It may be due to the fact that the carbon layer of Fe@C could interact with extracellular polymeric substances (EPS) through physical adsorption and chemical bonding to form a stable buffer to regulate NZVI dissolution. The buffer layer could not only regulate the attack of H⁺ on NZVI to reduce its dissolution rate but also complex released Fe²⁺ and neutralize OH^- to alleviate the passivation layer formed on the NZVI surface. Moreover, microbial community analysis indicated that both Fe@C and Fe+C increased the abundance of fermentative bacteria (e.g., Bacteroidetes_vadinHA17, Propionicicella) and methanogens (e.g., Methanobacterium), but only Fe@C promoted the growth of azo dye degraders (e.g., Clostridium, Geobacter). Metatranscriptomic analysis further revealed that only Fe@C could substantially stimulate the expression of azoreductase and redox mediator (e.g., riboflavin, ubiquinone) biosynthesis involved in the extracellular degradation of azo dye. This work provides novel insights into the bioaugmentation of Fe@C for refractory wastewater treatment.

Monthly dynamics of microbial communities and variation of nitrogen-cycling genes in an industrial-scale expanded granular sludge bed reactor

Introduction

The expanded granular sludge bed (EGSB) is a major form of anaerobic digestion system during wastewater treatment. Yet, the dynamics of microbial and viral communities and members functioning in nitrogen cycling along with monthly changing physicochemical properties have not been well elucidated.

Methods

Here, by collecting the anaerobic activated sludge samples from a continuously operating industrial-scale EGSB reactor, we conducted 16S rRNA gene amplicon sequencing and metagenome sequencing to reveal the microbial community structure and variation with the ever-changing physicochemical properties along within a year.

Results

We observed a clear monthly variation of microbial community structures, while COD, the ratio of volatile suspended solids (VSS) to total suspended solids (TSS) (VSS/TSS ratio), and temperature were predominant factors in shaping community dissimilarities examined by generalized boosted regression modeling (GBM) analysis. Meanwhile, a significant correlation was found between the changing physicochemical properties and microbial communities (p <0.05). The alpha diversity (Chao1 and Shannon) was significantly higher (p <0.05) in both winter (December, January, and February) and autumn (September, October, and November) with higher organic loading rate (OLR), higher VSS/TSS ratio, and lower temperature, resulting higher biogas production and nutrition removal efficiency. Further, 18 key genes covering nitrate reduction, denitrification, nitrification, and nitrogen fixation pathways were discovered, the total abundance of which was significantly associated with the changing environmental factors (p <0.05). Among these pathways, the dissimilatory nitrate reduction to ammonia (DNRA) and denitrification had the higher abundance contributed by the top highly abundant genes narGH, nrfABCDH, and hcp. The COD, OLR, and temperature were primary factors in affecting DNRA and denitrification by GBM evaluation. Moreover, by metagenome binning, we found the DNRA populations mainly belonged to Proteobacteria, Planctomycetota, and Nitrospirae, while the denitrifying bacteria with complete denitrification performance were all Proteobacteria. Besides, we detected 3,360 non-redundant viral sequences with great novelty, in which Siphoviridae, Podoviridae, and Myoviridae were dominant viral families. Interestingly, viral communities likewise depicted clear monthly variation and had significant associations with the recovered populations (p <0.05).

Discussion

Our work highlights the monthly variation of microbial and viral communities during the continuous operation of EGSB affected by the predominant changing COD, OLR, and temperature, while DNRA and denitrification pathways dominated in this anaerobic system. The results also provide a theoretical basis for the optimization of the engineered system.

Strategy to enhance semi-continuous anaerobic digestion of food waste by combined use of calcium peroxide and magnetite

Optimizing methane production from food waste (FW) efficiently is always a hot topic in the field of anaerobic digestion (AD). In this study we aimed to improve the conversion of organics to methane by using CaO₂ and magnetite to enhance the semi-continuous AD of food waste. Under the organic load of 2.5 g VS/L·d^-1, the specific methane yield was increased from 333.9 mL CH₄/g·VS to 423.4 mL CH₄/g·VS by adding 0.01 g/L CaO₂ with 0.4 g/L magnetite, improving the production of methane from FW. We assessed reactor performance, ORP changes, mass balance, enzyme activities and characterized the metagenomic profile of microorganisms involved in digestion. These microorganisms showed rapid conversion of volatile fatty acids and increased expression of genes related to hydrolysis and acid production. Thus, the addition of CaO₂ and magnetite optimized the relationship between fermentation bacteria and methanogenic archaea to enhance the overall production of methane. Microorganisms evolved unique adaptive mechanisms in the co-operative environment of CaO₂ and magnetite, as their energy metabolism patterns combined those controlled by individual CaO₂ and magnetite addition. This method of combining a micro-aeration environment with conductive materials provides a new perspective for optimizing the AD of FW.

The r/K selection theory and its application in biological wastewater treatment processes

Understanding the characteristics of functional organisms is the key to managing and updating biological processes for wastewater treatment. This review, for the first time, systematically characterized two typical types of strategists in wastewater treatment ecosystems via the r/K selection theory and provided novel strategies for selectively enriching microbial community. Functional organisms involved in nitrification (e.g., Nitrosomonas and Nitrosococcus), anammox (Candidatus Brocadia), and methanogenesis (Methanosarcinaceae) are identified as r-strategists with fast growth capacities and low substrate affinities. These r-strategists can achieve high pollutant removal loading rates. On the other hand, other organisms such as Nitrosospira spp., Candidatus Kuenenia, and Methanosaetaceae, are characterized as K-strategists with slow growth rates but high substrate affinities, which can decrease the pollutant concentration to low levels. More importantly, K-strategists may play crucial roles in the biodegradation of recalcitrant organic pollutants. The food-to-microorganism ratio, mass transfer, cell size, and biomass morphology are the key factors determining the selection of r-/K-strategists. These factors can be related with operating parameters (e.g., solids and hydraulic retention time), biomass morphology (biofilm or granules), and operating modes (continuous-flow or sequencing batch), etc., to achieve the efficient acclimation of targeted r-/K-strategists. For practical applications, the concept of substrate flux was put forward to further benefit the selective enrichment of r-/K-strategists, fulfilling effective management and improvement of engineered pollution control bioprocesses. Finally, the future perspectives regarding the development of the r/K selection theory in wastewater treatment processes were discussed.

Coupling microscale zero-valent iron and autotrophic hydrogen-bacteria provides a sustainable remediation solution for trichloroethylene-contaminated groundwater: Mechanisms, regulation, and engineering implications

Coupling microscale zero-valent iron (mZVI) and autotrophic hydrogen bacteria (AHB) has gained increasing attention owing to its potential to improve dechlorination performance by bridging H₂ donors and acceptors. However, few studies have attempted to test its sustainable remediation performance and to comprehensively unveil the governing mechanisms. This study systematically compared the performances of different systems (mZVI, H₂-AHB, and mZVI-AHB) for trichloroethylene (TCE) removal, and further optimized dechlorination and H₂ evolution of mZVI-AHB synchronously by regulating the mZVI particle size and dosage to achieve a win-win remediation solution. The final removal efficiency and removal rate of TCE by mZVI-AHB were 1.67-fold and 5.30-fold of those by mZVI alone respectively, and mZVI-AHB resulted in more complete dechlorination than H₂-AHB alone. Combining H₂ evolution kinetics, material characterization data, and bacterial community analysis results, the improved dechlorination performance of mZVI-AHB was mainly due to the following mechanisms: H₂ generated by mZVI corrosion was efficiently utilized by AHB, lasting corrosion of mZVI was facilitated by AHB, and dechlorination functional bacteria were highly enriched by mZVI. Finally, the remediation performance of mZVI-AHB with different mZVI particle sizes and dosages was evaluated comprehensively in terms of dechlorination reactivity, H₂ utilization efficiency and chemical cost, and suggestions for possible engineering applications are provided.

The Role of Extracellular Polymeric Substances in the Toxicity Response of Anaerobic Granule Sludge to Different Metal Oxide Nanoparticles

Wastewater treatment plants (WWTP) are regarded as the last barriers for the release of incompletely separated and recycled nanoparticles (NPs) into the environment. Despite the importance and ubiquity of microbial extracellular polymeric substances (EPSs) in the complex wastewater matrix, the interaction between NPs and EPSs of anaerobic microflora involved in wastewater treatment and the resultant impact on the biomass metabolomics are unclear. Thus, the impacts of different metal oxide (TiO₂, ZnO, and CuO) NPs on functional bacteria in anaerobic granular sludge (AGS) and the possible toxicity mechanisms were investigated. In particular, the binding quality, enhanced resistance mechanism, and chemical fractional contribution of EPSs from AGS against the nanotoxicity of different NPs was assessed. The results showed that CuO NPs caused the most severe inhibition towards acetoclastic and hydrogenotrophic methanogens, followed by ZnO NPs, whereas TiO₂ NPs caused no inhibition to methanogenesis. Excessive EPS production, especially the protein-like substances, was an effective strategy for reducing certain NPs’ toxicity by immobilizing NPs away from AGS cells, whereas the metabolism restriction on inner microorganisms of AGS induced by CuO NPs can deteriorate the protective role of EPS, indicating that the roles of EPS may not be amenable to generalizations. Further investigations with lactate dehydrogenase (LDH) and reactive oxygen species (ROS) assays indicated that there are greatly essential differences between the toxicity mechanisms of metal NPs to AGS, which varied depending on the NPs’ type and dosage. In addition, dynamic changes in the responses of EPS content to different NPs can result in a significant shift in methanogenic and acidogenic microbial communities. Thus, the production and composition of EPSs will be a key factor in determining the fate and potential effect of NPs in the complex biological matrix. In conclusion, this study broadens the understanding of the inhibition mechanisms of metal oxide NPs on the AGS process, and the influence of EPSs on the fate, behavior, and toxicity of NPs.

Application of Fenton sludge coupled hydrolysis acidification in pretreatment of wastewater containing PVA: Performance and mechanisms

Hydrolysis acidification (HA) is widely used in pretreatment of macromolecular refractory wastewater to improve its biodegradability. However, because the biological activity could be inhibited by macromolecular substances to a certain extent, its application is limited. In this study, polyvinyl alcohol (PVA), as a classic macromolecular pollutant in TPD wastewater, was treated by the Fenton sludge-coupled HA process to investigate the effects of Fenton sludge addition on the HA performance and identify the probable mechanisms behind it. The results showed that approximately 40% of macromolecular PVA was hydrolyzed into small molecular substances with molecular weight (Mw) < 10⁵ in the Fenton sludge-added reactor. Meanwhile, acidification efficiency (AE), volatile fatty acid production increased by 20.8% and 92.05 mg/L with Fenton sludge addition. The values of BOD₅/COD changed from 0.091 of influent to 0.26 and 0.32 of effluent from the simple HA process and Fenton sludge addition HA process, respectively. These results proved that biodegradability was improved by the two processes and the Fenton sludge addition had a positive effect on HA. Further analysis found that 2-lines ferrihydrite involved in Fenton sludge might serve as an electron acceptor to participate in extracellular respiratory. Besides, the Fe²⁺ observed a positive effect of the sludge characteristics in agreement with the higher activity of dehydrogenase and extracellular polymeric substances (EPS) production. This study suggested that Fenton sludge can be recycled and used as an iron source to enhance HA for industrial wastewater pretreatment.

Formation and characterization of conductive magnetite-embedded granules in upflow anaerobic sludge blanket reactor treating dairy wastewater

Promoting direct interspecies electron transfer (DIET) with conductive additives has proved effective in improving anaerobic digestion performance and stability. However, its application is limited by the need to replenish the washout loss of conductive materials. This study reports the formation of conductive magnetite-embedded granular sludge and its long-term influence on the performance of upflow anaerobic sludge blanket reactors treating dairy wastewater. The magnetite-supplemented reactor maintained better performance than the no-magnetite control, with greater sludge settling and electron transport activity, throughout the 192-d experiment at increasing organic loading rates (1.2-8.5 g chemical oxygen demand/L·d). The abundance of electroactive microbes also remained higher in the magnetite-supplemented reactor. The results suggest that DIET-based electric syntrophy was promoted in the magnetite-embedded granules. This study is the first to demonstrate the self-embedment of submicron conductive material into granular sludge and its benefits. These findings offer a new approach to enhancing anaerobic granular sludge systems.

Removal of organics from shale gas fracturing flowback fluid using expanded granular sludge bed and moving bed biofilm reactor

Shale gas fracturing flowback fluid contains various degradation difficulty organic compounds after hydraulic fracturing. A hybrid treatment method was developed for treating flowback and produced water (FPW) using pre-treatment (NaClO) followed by the expanded granular sludge bed (EGSB) and moving bed biofilm reactor (MBBR). Gas chromatography-mass spectrometry (GC-MS) was employed to detect organic composition in the FPW, the pre-treated FPW, EGSB and MBBR effluent. FPW had high chemical oxygen demand (COD) (3278 mg/L) and the majority of organic compounds in the FPW composed of alkanes and heteroatomic compounds with polymers and polarity. 20% COD removal was achieved after adding 5 g/L of NaClO in FPW (pH = 7, stirring for 20 mins) as pre-treatment and > C₃₀ alkanes in FPW were decomposed a lot in the pre-treatment process. The pre-treated FPW was diluted (volumetric ratio of 20%/50%) with synthetic wastewater/pure water. In the final stage of operation, Cl^- and COD concentration of influent to EGSB-MBBR system was around 7000 ± 100 mg/L and 3000 mg/L. EGSB-MBBR system achieved 93.84% COD removal rate, in which EGSB dominated COD removal (>80%). According to the GC-MS results, EGSB had an increase of C₁₁-C₃₀ compounds and a decrease of less C₁-C₁₀ content due to the consumption of  > C₃₀ compounds and low molecular weight (LWM) compounds. Meanwhile, aerobic microorganisms in MBBR metabolized LWM organics which contributed a lot to the COD removal (25.06∼68. 22%). The results indicated that the pre-treatment and biological EGSB-MBBR system could be an efficient option used for FPW treating.

Integration of CW-MFC and anaerobic granular sludge to explore the intensified ammonification-nitrification-denitrification processes for nitrogen removal

The integration of constructed wetland-microbial fuel cell (CW-MFC) and anaerobic granular sludge (AGS) is an important way to promote its ammonification efficiency and decrease the land use scale. This study explored the integration of CW-MFC and AGS for nitrogen removal via the intensified ammonification-nitrification-denitrification processes with initial NH₃-N, NO₃-N, Org-N and total nitrogen (TN) concentrations of 10.5, 13.8, 21.4, and 45.7 mg L^-1 in wastewater. Two reactors with AGS inoculated with a separated area (R1) and directly inoculated into gravel substrate (R2) were designed, respectively. Results showed that chemical oxygen demand (COD) removal efficiency could reach 85% in R1 and 81% in R2, and the conversion of Org-N to NH₃-N and NO₃-N to gaseous nitrogen were 80% and 90%, respectively. Although the conversion efficiency of NH₃-N to NO₂-N/NO₃-N via nitrification process was only 18%, it could reach 45%, 94%, and 98% with the aeration rates of 50-, 100-, and 200-mL min^-1. According to microstructural property and microbial community analyses, the separation gravel substrate and AGS areas in R1 availed for stable particle size of AGS, archaeal diversity, and metabolic activity even with a 1.5 times daily wastewater treatment capacity than that of R2. Overall, although the intensified ammonification-nitrification-denitrification processes for nitrogen removal could be achieved with supplementary aeration, further investigation is still needed to explore other substrate materials and high CW-MFC/AGS volume ratio for intensified nitrification process in CW-MFC associated with AGS.

Core-shell ZVI@carbon composites reduce phosphate inhibition of ZVI dissolution and enhance methane production in an anaerobic sewage treatment

Inhibitory effects of phosphate on zero-valent iron (ZVI) dissolution have been studied mainly focusing on a single chemical system. Little is known about inhibitory effects and the mechanism of phosphate on ZVI dissolution within a bioreactor during long-term operation. This study demonstrates the feasibility of achieving energy recovery from phosphate-containing domestic sewage using an efficient anaerobic reactor with micro-sized or nano-sized ZVI addition. The results indicate that the chemical oxygen demand (COD) removal and methane production are enhanced by ZVI addition. A maximum COD removal efficiency of 89% and methane content of 60% was achieved. However, the strengthening effect of ZVI on methane production is weakened by the presence of phosphate in domestic sewage. Analyzing the variations of Fe²⁺ ions and phosphate concentrations and characterizing the micro-morphology of corroded ZVI proved that the generated Fe²⁺ ions reacts with phosphate and forms a passivation layer on the ZVI surface, inhibiting further dissolution of ZVI. As an improved alternative, we chose the double layered core-shell structured ZVI@carbon composite as an excellent candidate to reduce the inhibitory effects of phosphate on ZVI dissolution. In this way, the direct formation of precipitates on the ZVI surface can be avoided due to the protective carbon layer which adjusts the ion transfer. Adding ZVI@carbon composites accelerate the methane content by 16%. To our knowledge, this is the first report on adding ZVI@carbon composites to promote the anaerobic metabolism in studies, which are focusing on reducing the inhibition of ZVI by phosphate.

Methanogenic granule growth and development is a continual process characterized by distinct morphological features

Up-flow anaerobic bioreactors are widely applied for high-rate digestion of industrial wastewaters and rely on formation, and retention, of methanogenic granules, comprising of dense, fast-settling, microbial aggregates (approx. 0.5-4.0 mm in diameter). Granule formation (granulation) mechanisms have been reasonably well hypothesized and documented. However, this study used laboratory-scale bioreactors, inoculated with size-separated granular sludge to follow new granule formation, maturation, disintegration and re-formation. Temporal size profiles, volatile solids content, settling velocity, and ultrastructure of granules were determined from each of four bioreactors inoculated only with small granules, four with only large granules, and four with a full complement of naturally-size-distributed granules. Constrained granule size profiles shifted toward the natural distribution, which was associated with maximal bioreactor performance. Distinct morphological features characterized different granule sizes and biofilm development stages, including 'young', 'juvenile', 'mature' and 'old'. The findings offer opportunities toward optimizing management of high-rate, anaerobic digesters by shedding light on the rates of granule growth, the role of flocculent sludge in granulation and how shifting size distributions should be considered when setting upflow velocities.

Influence of microbial spatial distribution and activity in an EGSB reactor under high- and low-loading denitrification desulfurization

To characterize the impact of reactor configuration and influent loading on elemental sulphur (S⁰) recovery during denitrification desulfurization, a laboratory-scale expanded granular sludge bed (EGSB) reactor was established under two influent acetate/nitrate/sulphide loadings; the water flow velocity, microbial community, and functional genes at different heights were investigated. There was no S⁰ generated when acetate/nitrate/sulphide loadings were set to 0.95/0.60/1.05 kg/m³.d (low-loading). Furthermore, there were no typical denitrifying sulphide oxidizing bacteria under this condition, and Syntrophobacter, Anaerolineaceae genera were predominant in the reactor. As the influent loading was doubled (high-loading), S⁰ recovery increased to 87%; the bacterial distribution was relatively homogeneous with sulphide oxidation genera (Thauera) being predominant. Neither nirK nor sqr genes were detected in the low-loading sample at a height of 50 cm. The sqr/sox ratios of low-loading stage were 2.50 (10 cm), 0.94 (30 cm), and 0 (50 cm), and the ratios of the high-loading stage were 1.38 (10 cm), 1.33 (30 cm), and 1.08 (50 cm). A hydrodynamics analysis indicated that the water flow velocity was homogenous throughout the reactor. Appropriate reactor configuration and operation parameters play an important role in the efficient regulation of S⁰ recovery during denitrification desulfurization.

Achieving rapid thiosulfate-driven denitrification (TDD) in a granular sludge system

Sulfur-oxidizing bacteria (SOB) can drive a high level of autotrophic denitrification (AD) activity with thiosulfate (S₂O₃^2-) as the electron donor. However, the slow growth of SOB results in a low biomass concentration in the AD reactor and unsatisfactory biological nitrogen removal (BNR). In this study, our goal was to establish a high-rate thiosulfate-driven denitrification (TDD) system via sludge granulation. Granular sludge was successfully cultivated by increasing the nitrogen loading rate stepwise in thiosulfate-oxidizing/nitrate-reducing conditions in an upflow anaerobic blanket reactor. In the mature-granular-sludge reactor, a nitrate removal rate of 280 mg N/L/h was achieved with a nitrate removal efficiency of 97.7%±1.0% at a hydraulic retention time of only 15 minutes, with no nitrite detected in the effluent. Extracellular polymeric substance (EPS) analysis indicated that the proteins in loosely bound and tightly bound EPS were responsible for maintaining the compact structure of the TDD granular sludge. The dynamics of the microbial-community shift were identified by 16S rRNA high-throughput pyrosequencing analysis. The Sulfurimonas genus was found to be enriched at 74.1% of total community and may play the most critical role in the high-rate BNR. The batch assay results reveal that no nitrite accumulation occurred during nitrate reduction because the nitrate reduction rate (75.90±0.67 mg N/g MLVSS/h) was almost equal to the nitrite reduction rate (66.06±1.28 mg N/g MLVSS/h) in the thiosulfate-driven granular sludge reactor. The results of this study provide support for the establishment of a high-rate BNR system that maintains its stability with a low sludge yield.

Extracellular biopolymers recovered as raw biomaterials from waste granular sludge and potential applications: A critical review

Granular sludge (GS) is a special self-aggregation biofilm. Extracellular polymeric substances (EPS) are mainly associated with the architectural structure, rheological behaviour and functional stability of fine granules, given that their significance to the physicochemical features of the biomass catalysing the biological purification process. This review targets the EPS excretion from GS and introduces newly identified EPS components, EPS distribution in different granules, how to effectively extract and recover EPS from granules, key parameters affecting EPS production, and the potential applications of EPS-based biomaterials. GS-based EPS components are highly diverse and a series of new contents are highlighted. Due to high diversity, emerging extraction standards are proposed and recovery process is capturing particular attention. The major components of EPS are found to be polysaccharides and proteins, which manifest a larger diversity of relative abundance, structures, physical and chemical characteristics, leading to the possibility to sustainably recover raw materials. EPS-based biomaterials not only act as alternatives to synthetic polymers in several applications but also figure in innovative industrial/environmental applications, including gel-forming materials for paper industry, biosorbents, cement curing materials, and flame retardant materials. In the upcoming years, it is foreseen that productions of EPS-based biomaterials from renewable origins would make a significant contribution to the advancement of the circular economy.

Performance of DOuble Circulation Anaerobic Sludge bed reactor: Biomass self-balance

The calcification of Anaerobic Granular Sludge is a serious problem in the application of anaerobic methanization biotechnology. Regular replacement of calcified sludge with exogenous sludge is an effective method, but it is costly and troublesome. A new DOuble Circulation Anaerobic Sludge bed reactor was developed for the enhanced production of endogenous sludge to self-balance the discharge of calcified sludge. The sludge washout rate was demonstrated to fall by 45% and the sludge proliferation rate was shown to rise by 230%, offsetting the regular discharge of calcified sludge. The zoogloea in 100 μm dimension was revealed to be the intermediate component of sludge. The sludge proliferation mode was proposed as follows: (i) Growth of sludge; (ii) Self-cracking of sludge to release fragmental sludge; (iii) Migration of fragmental sludge by self-floatation; (iv) Accumulation of suspended sludge in the sedimentation chamber; (v) Re-granulation of suspended sludge with the aid of Venturi effect.

Effect of steel slag in recycling waste activated sludge to produce anaerobic granular sludge

The amount of waste activated sludge (WAS) has grown dramatically in China. WAS is considered as a problematic and hazardous waste, which should be disposed in a safe and sustainable manner. In order to recycle WAS to an anaerobic granular sludge (AnGS) process for anaerobic digestion, Fe powder and steel slags (rusty and clean slags) were used to enhance the granulation process. The results demonstrated that both rusty and clean slags encouraged the development of granular sludge. Adding 10 g/L clean slags could increase AnGS granulation rate by 37%. In the presence of clean slags, extracellular polymeric substances (EPS) concentration in granules increased noticeably to 715 mg/g mixed liquor suspended solids (MLSS). High throughput sequencing analysis exhibited more diversity and higher abundance of functional microbial communities in the batch bottle with 10 g/L clean slags. This study suggested that adding clean slags at 10 g/L dosage was a sustainable and effective method for the sludge granulation.

A full-scale study of high-rate anaerobic bioreactors for whiskey distillery wastewater treatment with size fractionation and metagenomic analysis of granular sludge

Two full-scale high-rate bioreactors, i.e. external circulation sludge bed (ECSB) and expanded granular sludge bed (EGSB), were monitored for three years. Their performances for treating wastewater in a whiskey distillery were compared in terms of COD, pH, alkalinity and VFA. Even though feed flowrate highly fluctuated, COD removals of ECSB and EGSB were both excellent (95.7 ± 1.3% and 94.8 ± 3.0%, respectively). The influent and effluent characteristics of ECSB reactor were profiled and urea and urethane were also detected. High-strength properties of raw spent wash were exhibited in TOC, soluble COD and BOD5,20°C of 13500, 37750, and 1950 mg·L-1, respectively and characterized by GC-MS. Anaerobic granular sludge sampled from different heights of ECSB reactor were fractionated for demonstrating vertical size distributions. Moreover, major species found by next-generation sequencing technique were archaea, i.e. Methanosaeta and Methanolinea, while major bacteria were Bacteroidetes with minor Nitrospiraceae. This metagenomic analysis provided an insight of anaerobic microbial consortium.

Exposure to polyamide 66 microplastic leads to effects performance and microbial community structure of aerobic granular sludge

Microplastic polyamide 66 (PA66) was used to explore its mechanism of influence on the contaminants removal from aerobic granular sludge (AGS) and the corresponding change to the microbial community. Results showed that the removal pollution efficiency of the experimental groups with PA66 were inhibited during the early treatment stage. However, as the experiment progressed, the removal efficiencies of chemical oxygen demand (COD) (92.66%, 93.10%, 93.11%, 93.79%) and ammonia nitrogen (94.25%, 94.58%, 95.61%, 94.73%) were similar in the addition 0 g/L (A), 0.1 g/L (B), 0.2 g/L (C) and 0.5 g/L (D) PA66 beakers at the last 10 days. On the first day, the intensity of fluorescence peaks representing tryptophan protein-like and aromatic protein-like substances of loosely-bound extracellular polymeric substances (LB-EPS) indicated that the PA66 microplastic caused damage to the sludge structure, and the intensity of fluorescence peaks representing fulvic acid-like and humic acid-like substances were stronger than those in the control beaker (A). Microbial community analysis showed that the main phyla were Firmicutes (49.11%, 59.77%, 44.33%, 41.21%), Proteobacteria (26.44%, 11.96%, 31.44%, 19.4%) and Bacteroidetes (9.24%, 13.05%, 11.89%, 14.71%) in the four beakers. According to Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, genes representing [T] Signal transduction mechanisms illustrated that adding PA66 microplastic resulted in more signaling molecules in the AGS.

Expanded granular sludge blanket reactor treatment of food waste at ambient temperature: Analysis of nitrogen compositions and microbial community structure

The influent and effluent nitrogen compositions of an expanded granular sludge blanket (EGSB) reactor employed for treating food waste (FW) operated under ambient temperature was evaluated. Additionally, dynamic changes in the bacterial community structures and its metabolic functions were investigated. Results show that the EGSB reactor had a good effect on FW disposal and well resistance to variations in the organic loading rate. Furthermore, the COD concentration in the influent increased to about 10,000 mg/L and the COD removal rate stabilized at about 95%. The dissolved ammonia nitrogen (d-ammonia) content was the largest, accounting for approximately 70-80% of the dissolved nitrogen in the effluent. The amount of particulate organic nitrogen (PON) decreased by about 25%-33%. Amino acid, carbohydrate and lipid metabolism decreased at high organic loading rate (OLR). Meanwhile, the abundance of Methanothrix increased from 30.82% to 70.25%, whereas Methanobacterium decreased from 66.14% to 14.49%.

The study of structure of anaerobic granules and methane producing pathways of pilot-scale UASB reactors treating municipal wastewater under sub-mesophilic conditions

This study was carried out to investigate the relationship between the methane producing pathways and the characteristics of anaerobic granules treating municipal wastewater. For this purpose, two pilot scale upflow anaerobic sludge blanket reactors with different granule size distribution (1-2 mm and 3-4 mm) were investigated at operating temperatures of 20 °C and 28 °C for 239 days. There was an increased and stable biogas production when temperature was elevated to 28 °C likely due to reduction in methane solubility. Larger granules had multi-layered internal microstructures with higher acetoclastic methanogenic activities (250-437 mL CH₄ g^-1 VS d^-1) than smaller granules (150-260 mL CH₄ g^-1 VS d^-1). The relative abundance of acetoclastic methanogens of larger granules was higher, confirming acetoclastic methane producing pathway was more prominent. However, there was no significant difference in the performance of the two reactors because they were operating below their capacities in terms of organic loading rate to volatile solids ratio.

Comparison of microbial and physicochemical behavior of expanded granular sludge bed system during methylparaben and triclosan removal

Methylparaben and triclosan are antimicrobial agents widely used as preservatives in a variety of personal care and pharmaceutical products. Wastewater is considered the main source of these compounds in the environment. Expanded granular sludge bed (EGSB) reactors are a high rate technology for wastewater treatment based on biological processes and have been shown to be efficient in removing different types of compounds; however, little is known about the effect of contaminants such as methylparaben and triclosan on their behavior and effectiveness. In this study, we evaluate and compare the microbial and physicochemical behavior of EGSB systems during methylparaben and triclosan removal. The presence of different concentrations of pollutants had an influence on the cluster organization of microbial communities, especially bacteria. However, this did not affect the stability and performance of the EGSB systems. The banding patterns of the denaturing gradient gel electrophoresis of archaea demonstrated the constant presence and abundance of Methanosaeta concilii throughout all stages of operation, showing that this microorganism played a fundamental role in the stability of the reactors for the production of methane. The type of compound and its concentration influenced the expression of the mcrA and ACAs genes; however, these changes did not alter the stability and performance of the EGSB systems.

Performance and microbial protein expression during anaerobic treatment of alkali-decrement wastewater using a strengthened circulation anaerobic reactor

Herein, a strengthened circulation anaerobic (SCA) reactor was employed for the treatment of actual alkali-decrement wastewater. The degradation mechanism of polyester oligomers and the relationship between the treatment performance and microbial community structure were systematically investigated using various advanced techniques. Results suggest that the accumulation of volatile fatty acids has an inhibitory effect on methanogenic activity. Molecular weight distributions suggest that only incomplete degradation of oligomers was achieved, due to acetogenic inhibition in the lower part of the SCA reactor. Meta-proteomic approach analysis revealed that the methanogens containing heterodisulfide reductase were the primary species involved in methane metabolism. Based on these findings, a possible degradation mechanism for alkali-decrement wastewater in the SCA reactor is proposed. This high-performance anaerobic reactor could be further scaled-up and optimized to serve as a promising and effective unit for the treatment of other refractory industrial wastewaters.