Anaerobic Wastewater Treatment Research Articles

Deciphering the role of extracellular polymeric substances in the adsorption and biotransformation of organic micropollutants during anaerobic wastewater treatment

Extracellular polymeric substances (EPS) participate in the removal of organic micropollutants (OMPs), but the primary pathways of removal and detailed mechanisms remain elusive. We evaluated the effect of EPS on removal for 16 distinct chemical classes of OMPs during anaerobic digestion (AD). The results showed that hydrophobic OMPs (HBOMPs) could not be removed by EPS, while hydrophilic OMPs (HLOMPs) were amenable to removal via adsorption and biotransformation of EPS. The adsorption and biotransformation of HLOMPs by EPS accounted up to 19.4 ± 0.9 % and 6.0 ± 0.8 % of total removal, respectively. Further investigations into the adsorption and biotransformation mechanisms of HLOMPs by EPS were conducted utilizing spectral, molecular dynamics simulation, and electrochemical analysis. The results suggested that EPS provided abundant binding sites for the adsorption of HLOMPs. The binding of HLOMPs to tryptophan-like proteins in EPS formed nonfluorescent complexes. Hydrogen bonds, hydrophobic interactions and water bridges were key to the binding processes and helped stabilize the complexes. The biotransformation of HLOMPs by EPS may be attributed to the presence of extracellular redox active components (c-type cytochromes (c-Cyts), c-Cyts-bound flavins). This study enhanced the comprehension for the role of EPS on the OMPs removal in anaerobic wastewater treatment.

Application of Pure and Modified Polyvinylidene Fluoride Materials for Wastewater Treatment Using UASB Reactor Technologies: A Review

Wastewater treatment is now required because of the problems caused by water constraints. Wastewater is anaerobically digested to produce biogas, which can be used as a source of energy for things like lighting and heating. The upflow anaerobic sludge blanket (UASB) reactor has been recognized as an important wastewater treatment technology among anaerobic treatment methods. Although their treated effluent typically does not meet most discharge criteria, UASB reactors are generally stated to have a chemical oxygen demand (COD) reduction ranging from 60 to 90% for most types of wastewater. In comparison to traditional anaerobic procedures, anaerobic municipal wastewater treatment using membranes can produce higher effluent quality in terms of COD, suspended solids (SSs) and pathogen counts, as well as a steady treatment performance to fulfill strict discharge regulations. The objective of this review was to perform a literature review on parameters to consider when selecting a membrane to include in a UASB reactor. Membranes that are available in the market were compared in terms of both physical and chemical properties. Polyvinylidene fluoride (PVDF) membranes were found be superior to the others, and their modification also reduced the fouling propensity. When comparing modified PVDF (PVDF/PVDF-g-PEGMA) to pristine PVDF (116 L·m−2 h−1), a higher pure water flux (5170 L·m−2 h−1) was noted. The main drawback of such modifications could significantly increase the final membrane production costs. Research is still lacking when it comes to research on comparing the membranes and PVDF and UASB reactor technology interaction, including effects of its modification as discussed (stability, longevity of improved flux, etc.

Dual-chamber up-flow fluidized hollow-fiber anaerobic membrane bioreactor (UF-HF-AnMBR) treating synthetic purified terephthalic acid wastewater: Granular sludge regranulation and methane production

Up-flow anaerobic sludge blanket is a representative way to treat high-strength industrial wastewater, but suffers from granule collapse and poor long-term performance. In this study, a dual-chamber up-flow fluidized hollow-fiber anaerobic membrane bioreactor (UF-HF-AnMBR) was constructed to enhance sludge regranulation and process stability for purified terephthalic acid (PTA) wastewater treatment. A stable and satisfactory aromatic compound removal (>99 %) was achieved at the organic loading rate of 7.15 ± 0.21 g-COD·Lreactor−1·d−1, with the methane yield of 0.345 ± 0.028 L·g-CODremoved−1. Although the granular sludge collapsed initially induced the pore blocking and membrane fouling, membrane separation promoted the agglomeration of small particles, sludge regranulation and biomass retention. Parallel factor analysis identified that the amino acid tryptophan in extracellular polymeric substances exerted a more critical effect on membrane fouling. The 16S rRNA and network analysis revealed the robust proliferation/enrichment of Methanosaeta, Pelotomaculum, etc., as well as the establishment of mutualistic symbiotic relationship among different species, ensured a stable microbial community and created a favorable circumstance for highly effective anaerobic treatment of PTA wastewater. This study proves that UF-HF-AnMBR can be an efficient and promising technique for sludge regranulation and microbe enrichment, advancing its application in PTA wastewater treatment and bioenergy recovery.

Needle-Felt Coir fibre: A natural substitute for synthetic media in anaerobic fixed film reactors for wastewater treatment

In the past, studies have been conducted to evaluate different materials as filter media during anaerobic treatment of wastewater. The present study aims to assess the viability of using needle-felt coir fibre as a biofilm carrier in an anaerobic baffled reactor for organic-rich wastewater treatment. Real wastewater from a canteen with chemical oxygen demand (COD) - 1400±560 mg/L, and suspended solids (SS) - 397±23 mg/L after stabilization was treated for six months in a pilot scale unit with the carrier medium. The high porosity (90%) and specific surface area (1450±299 m2/m3) of the medium enabled COD (66%) and SS (70.5%) removal along with biogas recovery, at a lower hydraulic retention time of 14 hrs. The metagenomic sequence analysis revealed rich and diverse bacteria and methanogenic archaea in the carrier medium and settled sludge. Compared to the commonly used synthetic media, the needle-felt coir fibre carrier has technical, environmental, and economic advantages. Therefore, needle-felt coir fibre might be a sustainable replacement for conventional synthetic media in anaerobic wastewater treatment systems.

Biomineralization of phosphorus during anaerobic treatment of distillery wastewaters

This study addresses the pressing environmental concerns associated with the rapidly growing distillery industry, which is a significant contributor to wastewater generation. By focusing on the treatment of distillery wastewater using anaerobic digestion, this research explores the potential to convert organic materials into biofuels (methane). Moreover, the study aims to recover both methane and phosphorus from distillery wastewater in a single anaerobic reactor, which represents a novel and unexplored approach. Laboratory-scale experiments were conducted using mesophilic and thermophilic upflow anaerobic sludge blanket reactors. A key aspect of the study involved the implementation of a unique strategy: the mixing of centrate and spent caustic wastewater streams. This approach was intended to enhance treatment performance, manipulate the microbial community structure, and thereby optimizing the overall treatment performance. The integration of the centrate and spent caustic streams yielded remarkable co-benefits, resulting in significant biomethane production and efficient phosphorus precipitation. The study demonstrated a phosphorus removal efficiency of ∼60 % throughout the 130–140 days operation period. The recovery of phosphorus via the reactor sludge offers exciting opportunities for its utilization as a fertilizer or as a raw material within the phosphorus refinery industry. The biomethane produced during the treatment exhibits significant energy potential, estimated at 0.5 GJ/(m3 distillery wastewater).

Electric field coupled with Fe3O4 modified biochar to enhance anaerobic degradation of phenolic compounds in coal gasification wastewater (CGW)

Anaerobic treatment of coal gasification wastewater is usually limited by the inhibition of high concentration of phenolics. In this study, the Fe3O4 loaded biochar (FeBC) coupled with electric field (E-FB) was used to enhance anaerobic phenolics degradation. E-FB significantly accelerated anaerobic removal of phenol, p-cresol and 3,5-xylenol with methane production improved by 84.2 % compared to the control, where FeBC contributed most. E-FB improved anodic oxidation capacity, adsorption-biodegradation and Fe (III) reduction process, which resulted from the synergistic effects between electric field and FeBC. E-FB significantly enhanced the microbial activity and extracellular electron transfer (EET) in the suspended sludge with the maximum electron transfer system activity (132.9 %), cytochrome C (7.2 nmol/g MLSS) and adenosine triphosphate contents (87.8 nmol/ g MLSS) obtained. Compared with FB, E-FB stimulated the secretion of proteins and humic acid-like substances in extracellular polymeric substance, which could contribute to higher EET efficiency. More abundant phenol degrading bacteria, dissimilatory iron reducing bacteria, electroactive bacteria and potential direct interspecies electron transfer (DIET) partners (e.g., Geobacter and Methanobacterium) were enriched in E-FB especially on the anode. The benzoyl-CoA pathway on the anode and CO2-reduction methanogenesis (possibly via DIET) in the suspended sludge and anode were improved in E-FB.

The influence of inoculum source and pretreatment on the biohydrogen production in the dark fermentation process

The production of biohydrogen from food waste (FW) by dark fermentation (DF) is a promising technology for commercialisation, as it is both a clean fuel and a suitable means of sustainable waste management. The described experiments compared the biohydrogen production yields obtained after the use of inoculum from two different sources: digested sludge from the wastewater treatment plant (WWTP) in Lodz and sludge from the anaerobic treatment of dairy industry wastewater (DIW) (unconcentrated and double-concentrated). In addition, the effect of different temperatures (70, 90 and 121°C) of inoculum pretreatment on the biohydrogen production in DF was tested. The process was carried out batchwise at 37°C. The highest yield of hydrogen production was obtained after the inoculum pretreatment at 70°C. In addition, a higher amount of hydrogen could be obtained by using sludge from the WWTP as the inoculum (96 cm3 H2/gTVSFW) than unthickened sludge from the DIW (85 cm 3 H 2/g TVSFW). However, after thickening the sludge from the dairy industry, and at the same time balancing the dry matter of both sludges, the hydrogen production potential was comparable for bothsludges (for the WWTP sludge – 96 and for the DIW sludge – 93 cm 3 H 2/g TVSFW). The kinetics of hydrogen production was described by modified Gompertz equation, which showed a good fit (determination coefficient R2 between 0.909 and 0.999) to the experimental data.

Mechanisms of anaerobic treatment of sulfate-containing organic wastewater mediated by Fe0 under different initial pH values.

The anaerobic treatment of sulfide-containing organic wastewater (SCOW) is significantly affected by pH, causing dramatic decrease of treatment efficiency when pH deviates from its appropriate range. Fe0 has proved as an effective strategy on mitigating the impact of pH. However, systematic analysis of the influence mechanism is still lacking. To fill this gap, the impact of different initial pH values on anaerobic treatment efficiency of SCOW with Fe0 addition, the change of fermentation type and methanogens, and intra-extracellular electron transfer were explored in this study. The results showed that Fe0 addition enhanced the efficacy of anaerobic treatment of SCOW at adjusted initial pH values, especially at pH 6. Mechanism analysis showed that respiratory chain-related enzymes and electron shuttle secretion and resistance reduction were stimulated by soluble iron ions generated by Fe0 at pH 6, which accelerated intra-extracellular electron transfer of microorganisms, and ultimately alleviated the impact of acidic pH on the system. While at pH 8, Fe0 addition increased the acetogenic bacteria abundance, as well as optimized the fermentation type and improved the F420 coenzyme activity, resulting in the enhancement of treatment efficiency in the anaerobic system and remission of the effect of alkaline pH on the system. At the neutral pH, Fe0 addition had both advantages as stimulating the secretion ofrespiratory chainand electron transfer-related enzymes at pH 6 and optimizing the fermentation type pH 8, and thus enhanced the treatment efficacy. This study provides important insights and scientific basis for the application of new SCOW treatment technologies.

Bioponic systems with biochar: Insights into nutrient recovery, heavy metal reduction, and microbial interactions in digestate-based bioponics

Bioponics is a nutrient-recovery technology that transforms nutrient-rich organic waste into plant biomass/bioproducts. Integrating biochar with digestate from anaerobic wastewater treatment process can improve resource recovery while mitigating heavy metal contamination. The overarching goal of this study was to investigate the application of biochar in digestate-based bioponics, focusing on its efficacy in nutrient recovery and heavy metal removal, while also exploring the microbial community dynamics. In this study, biochar was applied at 50 % w/w with 500 g dry weight of digestate during two 28-day crop cycles (uncontrolled pH and pH 5.5) using white stem pak choi (Brassica rapa var. chinensis) as a model crop. The results showed that the digestate provided sufficient phosphorus and nitrogen, supporting plant growth. Biochar amendment improved plant yield and phosphate solubilization and reduced nitrogen loss, especially at the pH 5.5. Furthermore, biochar reduced the heavy metal accumulation in plants, while concentrating these metals in the residual sludge. However, owing to potential non-carcinogenic and carcinogenic health risks, it is still not recommended to directly consume plants cultivated in digestate-based bioponic systems. Additionally, biochar amendment exhibited pronounced impact on the microbial community, promoting microbes responsible for nutrient solubilization and cycling (e.g., Tetrasphaera, Herpetosiphon, Hyphomicrobium, and Pseudorhodoplanes) and heavy metal stabilization (e.g., Leptolinea, Fonticella, Romboutsia, and Desulfurispora) in both the residual sludge and plants. Overall, the addition of biochar enhanced the microbial community and facilitated the metal stabilization and the cycling of nutrients within both residual sludge and root systems, thereby improving the overall efficiency of the bioponics.

Modeling of two-stage anaerobic onsite wastewater sanitation system to predict effluent soluble chemical oxygen demand through machine learning

The present research aims to predict effluent soluble chemical oxygen demand (SCOD) in anaerobic digestion (AD) process using machine-learning based approach. Anaerobic digestion is a highly sensitive process and depends upon several environmental and operational factors, such as temperature, flow, and load. Therefore, predicting output characteristics using modeling is important not only for process monitoring and control, but also to reduce the operating cost of the treatment plant. It is difficult to predict COD in a real time mode, so it is better to use Complex Mathematical Modeling (CMM) for simulating AD process and forecasting output parameters. Therefore, different Machine Learning algorithms, such as Linear Regression, Decision Tree, Random Forest and Artificial Neural Networks, have been used for predicting effluent SCOD using data acquired from in situ anaerobic wastewater treatment system. The result of the predicted data using different algorithms were compared with experimental data of anaerobic system. It was observed that the Artificial Neural Networks is the most effective simulation technique that correlated with the experimental data with the mean absolute percentage error of 10.63 and R2 score of 0.96. This research proposes an efficient and reliable integrated modeling method for early prediction of the water quality in wastewater treatment.

Enhancement of sulfide removal and sulfur recovery in piggery wastewater via lighting-anaerobic digestion with bioaugmentation of phototrophic green sulfur bacteria.

Anaerobic pig wastewater treatment commonly generates high sulfide concentrations in the treated wastewater. This study aims to apply phototrophic green sulfur bacteria (PGB) to promote sulfide removal in lighting-anaerobic digestion (lighting-AD) treating pig wastewater. Initially, batch AD tests of pig wastewater with/without PGB addition were carried out under dark (D) and light (L) conditions. The results showed that the lighting-AD with PGB gave a higher growth rate of PGB (0.056h-1) and the highest COD/sulfide removals as compared to the dark-AD with PGB and lighting-AD solely. More experiments under various light intensities were performed in order to find an optimal intensity for PGB growth concurrently with metagenomic changes concerning treatment performance. It appeared that sulfide removal rates had increased as increasing light intensity up to 473lx by giving the highest rate of 12.5mg L-1 d-1 with the highest sulfur element content in the biomass. Contrastingly, many PGB species disappeared at 1350lx exposure subsequently sharply decreasing the rate of sulfide removal. In sum, the application of low light intensities of 400-500lx with bioaugmented PGB could promote PGB growth and activity in sulfide removal in pig wastewater in the lighting of the AD process.

Design and development of a pilot plant for the treatment of pharmaceutical wastewater containing molasses

Abstract The present work deals with the anaerobic treatment of molasses-bearing pharmaceutical wastewater using a specifically designed downflow anaerobic bioreactor (DFAB). The capacity of the designed DFAB was 1,000 L of adequate volume with necessary operational controls. Two types of anaerobic treatment, i.e. mesophilic and thermophilic treatments, were applied to the anaerobic biotransformation of wastewater under investigation. The efficiency of both treatments was compared in terms of reducing biochemical oxygen demand (BOD) and chemical oxygen demand (COD) load, as well as methane production. The anaerobic digestion process was carried out with the help of bacterial strains. The bacterial strains used in the mesophilic operation were acclimatised and utilised for the thermophilic treatment, which is a cost-effective and sustainable approach. A significant reduction in BOD and COD content of the wastewater was noticed after 10 days of retention time. Nearly 72–83% and 80–90% reduction in BOD values were recorded under mesophilic and thermophilic operations, respectively. Approximately 73–86% and 82–93% of COD content were reduced by mesophilic and thermophilic treatment, correspondingly. Methane production of 67 and 74% was achieved under mesophilic and thermophilic treatment, respectively. The designed DFAB showed remarkable treatment efficiency for wastewater and methane production.

Effects of Fe/Fe–Mn oxides loaded biochar on anaerobic degradation of typical phenolic compounds in coal gasification wastewater: Performance and mechanism

In this study, two kinds of magnetic biochar (BC) were synthesized by loading Fe (FeBC) and Fe–Mn oxides (FMBC) and their effects on anaerobic phenolics degradation were investigated. Compared with BC/FMBC, FeBC addition achieved the superior phenolics biodegradation even for 3,5-xylenol. Compared with control, FeBC addition enhanced CH4 production by 100.1 % with the lag time shortened from 9.5 days to 6.6 days while it increased to 11.2 days with FMBC addition. FeBC addition activated adsorption-biodegradation and Fe (III) reduction with the improved electron transfer activity, adenosine triphosphate and cytochrome C concentrations. Abundant phenol degrading bacteria, electroactive bacteria, syntrophic partners could be enriched by FeBC addition, contributing to the enhanced benzoyl-CoA and methanogenesis pathways. However, this enhancement was inhibited by FMBC addition owing to the accumulation of reactive oxygen species. This study provided novel insights into the application of magnetic BC to enhanced anaerobic treatment of phenolic wastewater.

Inhibitory effects of long chain fatty acids on anaerobic sludge treatment: Biomass adaptation and microbial community assessment

The study investigated the effects of long-chain fatty acids (LCFA) on anaerobic sludge treating lipid-rich wastewater. It involved batch experiments with three sludge samples: two acclimated to lipids and one non-acclimated. The experiments aimed to observe the degradation of LCFA, specifically oleate and palmitate, by dosing them at concentrations ranging from 50 to 600 mg/L. Measurements of the cumulative methane production and the LCFA concentration, quantified as fat, oil, and grease (FOG) were performed. To ensure the sludge was free from other biodegradable substrates, part of the samples was pre-incubated without feed. The tests were conducted with both pre-incubated and non-incubated inoculum sludge. The findings revealed that oleate was degraded more efficiently than palmitate across all sludge samples, with a greater conversion rate to methane. Sludge samples acclimated to lipids showed a superior capacity to degrade LCFA compared to non-acclimated ones. It was noted that at concentrations above 400 mg/L, the conversion of LCFAs to intermediate compounds was inhibited, although this did not affect the subsequent methane production. The study concludes with a recommendation for sludge adaptation strategies to boost the efficiency of anaerobic wastewater treatment systems dealing with lipid-rich waste. The presence of LCFA-degrading bacteria families like Kosmotogaceae, Petrotogaceae, and Synergistaceae in the acclimated sludge samples underscores the adaptation and potential for improved degradation performance.

Oxidation of sulfides following anaerobic municipal wastewater treatment for non-potable reuse applications

Anaerobic secondary biological wastewater treatment could increase energy efficiency by avoiding energy-intensive aeration while producing methane that could be harvested for energy production.

Prediction of the effluent chemical oxygen demand and volatile fatty acids for anaerobic treatment based on different feature selections machine-learning methods from lab-scale to pilot-scale

The effluent chemical oxygen demand (COD) and volatile fatty acids (VFA) are important indicators for measuring wastewater anaerobic treatment systems. Through lab and pilot experiments on the anaerobic treatment process of high-salt wastewater, combined with different prediction requirements, the feature variables are classified. The effluent-COD and VFA prediction models of five different Machine-learning methods (i.e., back propagation neural network (BPNN), Genetic algorithm optimizes BPNN(GA-BP), support vector machine (SVM), Least squares support vector machine (LSSVM), and random forest (RF)) were established. The modeling results on different dataset (i.e., lab dataset, pilot dataset and mixed dataset) showed that the SVM and LSSVM methods based on statistical learning had the best model prediction performance in lab and pilot datasets, and the RF method performed well in mixed dataset. Furthermore, the genetic algorithm was adopted to optimize the RF model. After optimization of mixed dataset, the effluent-COD prediction (i.e., R2 = 0.966, RMSE/MAE are 61.227/48.584 mg/L) and VFA prediction (i.e., R2 = 0.639, RMSE/MAE are 26.838/21.831 mg/L) have been improved compared with the original RF model (i.e., R2 = 0.918, RMSE/MAE are 86.232/65.03 mg/L of effluent-COD, and R2 = 0.539, RMSE/MAE are 35.489/26.726 mg/L of VFA). This research can provide a reference for the prediction of the anaerobic treatment of actual large-scale industrial wastewater.

Inorganic carbon as a key factor governing competition between methanogens and acetogens in high-rate anaerobic treatment of methanol wastewater

The anaerobic treatment of methanolic wastewater has become a crucial issue. Sodium bicarbonate (NaHCO3), as a typical representative of inorganic carbon substances, not only provides alkalinity in the anaerobic treatment of methanol but also governs the competition between methanogens and acetogens. However, a suitable dosage has rarely been mentioned. In this study, we demonstrate the crucial role of NaHCO3 in the anaerobic treatment of methanol using a long-term operated up-flow anaerobic sludge blanket (UASB) reactor and batch tests. The UASB results revealed that by adding 3 to 5 g/L of NaHCO3 to the influent at a high organic loading rate of 50 gCOD/L/d, a significant accumulation of acetic acid was detected, with a maximum value of up to 1400 mg/L. In the context of microbial community evolution, this study profiled substrate competition between the acetogens Acetobacterium and Methanomethylovorans, which directly utilize methanol, as well as the synergistic effects of Acetobacterium and acetoclastic Methanosaeta. Abundance analysis of genes encoding relevant pathway enzymes also revealed that the acetic acid production pathway was significantly enhanced when more NaHCO3 was added. According to the dose–response mechanism in the batch test based on kinetic analysis, an acceptable threshold prediction scheme was beneficial in reasonably adding NaHCO3 to improve acid production capacity and rapid methanol degradation, and ensuring that the generated acetate could be rapidly degraded without inhibition, in the high-rate anaerobic treatment of methanolic wastewater.

Quantitative structure-biotransformation relationships of organic micropollutants in aerobic and anaerobic wastewater treatments

Biotransformation is one of the dominant processes to remove organic micropollutants (OMPs) in wastewater treatment. However, studies on the role of molecular structure in determining the biotransformation rates of OMPs are limited. We evaluated the biotransformation of 14 OMPs belonging to different chemical classes under aerobic and anaerobic conditions, and then explored the quantitative structure-biotransformation relationships (QSBRs) of the OMPs based on biotransformation rates using valid molecular structure descriptors (electrical and physicochemical parameters). Pseudo-first-order kinetic modeling was used to fit the biotransformation rate, and only 2 of the 14 OMPs showed that the biotransformation rate constant (kbio) values were higher under anaerobic conditions than aerobic conditions, indicating that aerobic conditions were more favorable for biotransformation of most OMPs. QSBRs infer that the electrophilicity index (ω) is a reliable predictor for OMPs biotransformation under aerobic conditions. ω corresponds to the interaction between OMPs and microbial enzyme active sites, this process is the rate-limiting step of biotransformation. However, under anaerobic conditions the QSBR based on ω was not significant, indicating that specific functional groups may be more critical than electrophilicity. In conclusion, QSBRs can serve as alternative tools for the prediction of the biotransformation of OMPs and provide further insights into the factors that influence biotransformation.

New insights into antibiotic stimulation of methane production during anaerobic digestion

Residual antibiotics in swine wastewater pose a critical challenge for stable anaerobic digestion (AD). This study offers fresh insights into the anaerobic treatment of swine wastewater. The results showed that the presence of three typical antibiotics (sulfamethoxazole (SMX), oxytetracycline (OTC) and ciprofloxacin (CIP)) in swine wastewater could promote methane production by stimulating the production and conversion of ethanol. Among them, SMX exhibited the strongest methane promotion effect, with the cumulative methane production increasing from 138.47 to 2204.19 mL/g VS. According to the microbial community structure, antibiotics could promote the growth of Corynebacterium, Lutispora and hydrogenotrophic methanogens (Methanosassiliicoccus, Methanobrevibacter, and Methanobacterium), but inhibit the enrichment of acetoclastic methanogen (Methanosaeta). The relative abundance of Methanosaeta decreased from 2.93–19.80% to 0.52–2.58% under antibiotic stress. Furthermore, there were significant differences in the influence of different antibiotic types on methanogenic pathways. Specifically, OTC and CIP promoted the acetoclastic and hydrogenotrophic pathways, respectively, to enhance methane production. However, SMX could promote both acetoclastic and hydrogenotrophic pathways.

Long-term impact of nano zero-valent iron on methanogenic activity, microbial community structure, and transcription activity in anaerobic wastewater treatment system

Nano zero-valent iron (NZVI) is commonly used in industrial wastewater treatment. However, its long-term impact mechanisms of metabolization in anaerobic systems are not well understood. This study investigated the effects of long-term and continuous addition of NZVI on methanogenic activity, microbial community, and transcription activity. The results demonstrated that low levels of NZVI (1000 mg/L) induced inhibition of methanogenesis after 80 days, while high levels of NZVI (5000 mg/L) immediately led to a sharp decrease of cumulative methane production and chemical oxygen demand removal, which arrived at a steady state (14.4 % of control and 17 %) after 30 days. NZVI adversely affected cell viability, adenosine triphosphate production, and fatty acid evolution of cell membranes played a crucial role in resisting chronic NZVI toxicity. Moreover, high NZVI levels hindered the transcription of key enzymes CoM and mcrA, while low NZVI levels maintained its high CoM and mcrA activity, but down-regulated the transcription of cdh and hdr. Besides, amino-utilizing bacteria was reduced under the high NZVI concentration, while low NZVI changed dominant genus with potential protein hydrolysis function from Candidatus Cloacamonas to Sedimentibacter. These results provide a guideline for proper NZVI utilization in wastewater treatment.