A modeling approach to describe ZVI-based anaerobic system

Unveiling the unique role of iron in the metabolism of methanogens: A review

Methanogens are the main participants in the carbon cycle, catalyzing five methanogenic pathways. Methanogens utilize different iron-containing functional enzymes in different methanogenic processes. Iron is a vital element in methanogens, which can serve as a carrier or reactant in electron transfer. Therefore, iron plays an important role in the growth and metabolism of methanogens. In this paper, we cast light on the types and functions of iron-containing functional enzymes involved in different methanogenic pathways, and the roles iron play in energy/substance metabolism of methanogenesis. Furthermore, this review provides certain guiding significance for lowering CH4 emissions, boosting the carbon sink capacity of ecosystems and promoting green and low-carbon development in the future.

Synergistic improvement of nitrogen and phosphorus removal in constructed wetlands by the addition of solid iron substrates and ferrous irons

Sanjiang Plain is intensively used for rice production, and ditch drainage diffuse pollution prevention is crucial. Groundwater, rich in Fe ions, is the main source of irrigation water in this region. In this study, pyrite and zero-valent iron (ZVI) (sponge iron and iron scraps) were used as substrates to identify the synergistic influence of exogenous Fe2+ addition and solid iron substrates on pollutant removal in constructed wetlands. Based on the results, iron substrates hardly improved the ammonia removal, mainly because of the physical structure and oxidation activity. At a hydraulic retention time longer than 8 h, the pollution removal efficiency in the zero-valent iron (ZVI) substrate treatment increased significantly, and the removal of nitrate (NO3 --N) and total phosphorus (TP) in the iron scrap substrate treatment reached about 60% and 70%, respectively. The high-throughput sequencing results showed a significant increase in the abundance of microorganisms involved in denitrification and phosphate accumulation in biofilms on ZVI substrates. The highest diversities of such microorganisms in biofilms on iron scraps were found for denitrifying bacteria (Pseudomonas), nitrate-reducing Fe (II)-oxidizing bacteria (Acidovorax), and Dechloromonas with autotrophic denitrification and phosphate accumulation, with a 43% cumulative abundance. Dechloromonas dominated in the iron sponge substrate treatment. The highest relative abundance of Acidovorax was found in the mixed iron substrate (pyrite, sponge iron, and iron scraps) treatment. The addition of ZVI substrate significantly improved the removal of NO3 --N and TP and reduced the hydraulic retention time through the continuous release of Fe2+ and the promotion of microbial growth. When designing constructed wetlands for treating paddy field drainage, the appropriate addition of iron scrap substrates is recommended to enhance the pollutant removal efficiency and shock load resistance of CWs.

Modifications to the anaerobic digestion model no. 1 (ADM1) for enhanced understanding and application of the anaerobic treatment processes - A comprehensive review

Anaerobic digestion (AD) is a promising method for the recovery of resources and energy from organic wastes. Correspondingly, AD modelling has also been developed in recent years. The International Water Association (IWA) Anaerobic Digestion Model No. 1 (ADM1) is currently the most commonly used structured AD model. However, as substrates become more complex and our understanding of the AD mechanism grows, both systematic and specific modifications have been applied to the ADM1. Modified models have provided a diverse range of application besides AD processes, such as fermentation and biogas upgrading processes. This paper reviews research on the modification of the ADM1, with a particular focus on processes, kinetics, stoichiometry and parameters, which are the major elements of the model. The paper begins with a brief introduction to the ADM1, followed by a summary of modifications, including extensions to the model structure, modifications to kinetics (including inhibition functions) and stoichiometry, as well as simplifications to the model. The paper also covers kinetic parameter estimation and validation of the model, as well as practical applications of the model to a variety of scenarios. The review highlights the need for improvements in simulating AD and biogas upgrading processes, as well as the lack of full-scale applications to other substrates besides sludge (such as food waste and agricultural waste). Future research directions are suggested for model development based on detailed understanding of the anaerobic treatment mechanisms, and the need to recover of valuable products.

Hydrophilic sulfurized nanoscale zero-valent iron for enhancing in situ biocatalytic denitrification: Mechanisms and long-term column studies

The aim of this study was to investigate the effects of hydrophilic sulfur-modified nanoscale zero-valent iron (S-nZVI) as a biocatalyst for denitrification. We found that the denitrifying bacteria Cupriavidus necator (C. necator) promoted Fe corrosion during biocatalytic denitrification, reducing surface passivation and sulfur species leaching from S-nZVI. As a result, S-nZVI exhibited a higher synergistic factor (f_syn = 2.43) for biocatalytic NO₃^- removal than nanoscale zero-valent iron (nZVI, f_syn = 0.65) at an initial nitrate concentration of 25 mg L^-1-N. Based on kinetic profiles, SO₄^2- was the preferred electron acceptor over NO₃^- when using C. necator and S-nZVI for biocatalytic denitrification. Up-flow column experiments demonstrated that biocatalytic denitrification using S-nZVI achieved a total nitrogen removal capacity of up to 2004 mg L^-1 for 127 d. Notably, microbiome taxonomic profiling showed that the addition of S-nZVI to the groundwater promoted the growth of Geobacter, Desulfosporosinus, Streptomyces, and Simplicispira spp in the column experiments. Most of those microbes can reduce sulfate, promote denitrification, and match the batch kinetic profile obtained using C. necator. Our results not only discover the great potential of S-nZVI as a biocatalyst for enhancing denitrification via microbial activation but also provide a deep understanding of the complicated abiotic-biotic interaction.

Assessment of sulfidated nanoscale zerovalent iron for in-situ remediation of cadmium-contaminated acidic groundwater at a zinc smelter

Unprecedented high concentrations of heavy metals have been detected in the groundwater at a zinc smelter in Seokpo, South Korea. The outflow of the contaminated groundwater into the nearby Nakdong River must be prevented by some means such as permeable reactive barrier (PRB). As a reactive material for injection-type PRB, we have tested sulfidated nanoscale zerovalent iron (S-nZVI) to assess its efficacy in remediating the groundwater from the smelter. The S-nZVI efficiently removed Zn, Ni, and Al in the groundwater, and neutralized the groundwater to pH > 6. Sulfidation of nZVI greatly increased the removal of Cd (99.8%) compared to that by nZVI (7.2%). MINEQL+ modeling and particle characterization were performed to elucidate the forms of heavy metals in the solution and on the surface of S-nZVI. Raman and XPS results suggested that FeS on the surface of S-nZVI reacted with Cd(II) and Zn(II), forming more-stable CdS and ZnS. Sequential application of NaHCO₃ after S-nZVI treatment in a column setup was suited for the removal of remaining Zn and Fe as well as the reduction of microbial toxicity. This study guides to use of S-nZVI for in-situ remediation of cadmium-contaminated groundwater with other coexisting heavy metals from a zinc smelter.

Atomic Hydrogen in Electrocatalytic Systems: Generation, Identification, and Environmental Applications

Electrochemical reduction has emerged as a viable technology for the removal of a variety of organic contaminants from water. Atomic hydrogen (H*) is the primary species generated in electrochemical reduction processes. In this work, identification and quantification for H* are reviewed with a focus on methods used to generate H* at different positions. Additionally, we present recently developed proposals for the surface chemistry mechanisms of H* on the most commonly used cathodes as well as the use of H* in standard electrochemical reactors. The proposed reaction pathways in different H* systems for environmental applications are also discussed in detail. As shown in this review, the key hurdles facing H* reduction technologies are related to i) the establishment of systematic and practical synthetic methods; ii) the development of effective identification approaches with high specificity; and, iii) an in-depth exploration of the H* reaction mechanism to better understand the reaction process of H*.

Fe0-Supported Anaerobic Digestion for Organics and Nutrients Removal from Domestic Sewage

Results from different research suggest that metallic iron (Fe0) materials enhance anaerobic digestion (AD) systems to remove organics (chemical oxygen demand (COD)), phosphorus and nitrogen from polluted water. However, the available results are difficult to compare because they are derived from different experimental conditions. This research characterises the effects of Fe0 type and dosage in AD systems to simultaneously remove COD and nutrients (orthophosphate (PO43−), ammonium (NH4+), and nitrate (NO3− Lab-scale reactors containing domestic sewage (DS) were fed with various Fe0 dosages (0 to 30 g/L). Batch AD experiments were operated at 37 ± 0.5 °C for 76 days; the initial pH value was 7.5. Scrap iron (SI) and steel wool (SW) were used as Fe0 sources. Results show that: (i) SW performed better than SI on COD and PO43− removal (ii) optimum dosage for the organics and nutrients removal was 10 g/L SI (iii) (NO3− + NH4+) was the least removed pollutant (iv) maximum observed COD, PO43− and NO3− + NH4+ removal efficiencies were 88.0%, 98.0% and 40.0% for 10 g/L SI, 88.2%, 99.9%, 25.1% for 10 g/L SW, and 68.9%, 7.3% and 0.7% for the reference system. Fe0-supported AD significantly removed the organics and nutrients from DS.

Zero-valent iron mediated alleviation of methanogenesis inhibition induced by organoarsenic roxarsone

Zero-valent iron (ZVI) can enhance anaerobic digestion, and has great potential to alleviate/eliminate methanogenesis inhibition. Little is known about the feasibility of utilizing ZVI to alleviate methanogenesis inhibition that is caused by typical animal feed additive roxarsone in livestock wastewater. In this study, the role of ZVI on alleviating roxarsone-induced methanogenic inhibition and its mechanisms were investigated. With the increase of roxarsone concentration from 5 to 50 mg/L, the inhibition of methanogenesis increased from 3.0% to 65.7%. This inhibition was alleviated by 80.7% and 57.2% when 1.0 and 10.0 g/L ZVI were added, respectively. Due to ZVI addition, an efficient arsenic immobilization onto ZVI (45.4-85.8%) was achieved mainly through the formation of FeAsO₄ precipitate and adsorption by ZVI. Under the function of ZVI, hydrogenotrophic methanogenic activity was obviously restored. The microbial community analysis indicates that the ZVI-regulated alleviation on the methanogenesis inhibition was attributed to the enrichment of Methanobacterium and Methanosarcina. The findings from this study demonstrate that ZVI addition is an effective way for treatment of organoarsenic-contaminated wastewater.

Integrated remediation of sulfate reducing bacteria and nano zero valent iron on cadmium contaminated sediments

Integrated-remediation technologies on heavy metal polluted sediments have received much attention. In this study, Cd contaminated sediments were treated with various conditions: sulfate reducing bacteria (SRB) only and SRB combined with different dosages of nano zero valent iron (nZVI (0.5-10 mg/g)). The immobilization of Cd was found in all remediation treatments according to the decreases of mobile Cd and the increases of more stable Cd compared with control. Five typical SRBs (Desulfobulbaceae, Desulfobacteraceae, Syntrophobacteraceae, Desulfovibrionaceae and Desulfomicrobiaceae) were identified having significant influences on Cd speciation transformation and they could stabilize Cd into sulfide precipitation through dissimilatory sulfate reduction (DSR). The ANOVA results of mobilization index and Cd concentration in overlying water both demonstrated that integrated-remediation systems with 5 mg/g and 10 mg/g of nZVI (Fe5 and Fe10 systems, respectively) presented better immobilization performance than conventional SRB only system (P < 0.05). It is confirmed that nZVI could stimulate the SRB bio-immobilization possibily through providing electrons and enhancing enzyme activities during DSR. The XPS analyses and Pourbaix diagrams revealed that mackinawite may be produced in the Fe10, resulting in the possible formation of Cd-S-Fe. This study indicates that integrated-remediation of SRB and nZVI have great potential in Cd immobilization of sediments, especially with higher addition of nZVI.

Enriched Co-Treatment of Pharmaceutical and Acidic Metal-Containing Wastewater with Nano Zero-Valent Iron

Among traditional hazardous waste sources, pharmaceutical-containing wastewater and acidic mine drainage need treatment to preserve the expected water supply quality. A nano zero-valent iron (nZVI)-enriched treatment of these two streams is evaluated for simultaneous removal of various heavy metal ions, organic pollutants, sulfates, the efficiency of the treatment system, and separation of reaction products in the fluidized-bed reactor. The reactor packed with silica sand was inoculated with sludge from an anaerobic digester, then 1–3 g/L of nZVI slurry added to cotreat a hospital feed and acid mine wastewater at 5:2 v/v. The biotreatment process is monitored through an oxidation–reduction potential (Eh) for 90 days. The removal pathway for the nZVI used co-precipitation, sorption, and reduction. The removal load for Zn and Mn was approximately 198 mg Zn/g Fe and 207 mg Mn/g Fe, correspondingly; achieving sulfate (removal efficiency of 94% and organic matter i.e., chemical oxygen demand (COD), biological oxygen demand (BOD), dissolved organic carbon (DOC), total dissolved nitrogen (TDN) reduced significantly, but ibuprofen and naproxen achieved 31% and 27% removal, respectively. This enriched cotreatment system exhibited a high reducing condition in the reactor, as confirmed by Eh; hence, the nZVI was dosed only a few times in biotreatment duration, demonstrating a cost-effective system.

Performance prediction of ZVI-based anaerobic digestion reactor using machine learning algorithms

The use of zero-valent iron (ZVI) to enhance anaerobic digestion (AD) systems is widely advocated as it improves methane production and system stability. Accurate modeling of ZVI-based AD reactor is conducive to predicting methane production potential, optimizing operational strategy, and gathering reference information for industrial design in place of time-consuming and laborious tests. In this study, three machine learning (ML) algorithms, namely random forest (RF), extreme gradient boosting (XGBoost), and deep learning (DL), were evaluated for their feasibility of predicting the performance of ZVI-based AD reactors based on the operating parameters collected in 9 published articles. XGBoost demonstrated the highest accuracy in predicting total methane production, with a root mean squared error (RMSE) of 21.09, compared to 26.03 and 27.35 of RF and DL, respectively. The accuracy represented by mean absolute percentage error also showed the same trend, with 14.26%, 15.14% and 17.82% for XGBoost, RF and DL, respectively. Through the feature importance generated by XGBoost, the parameters of total solid of feedstock (TS_f), sCOD, ZVI dosage and particle size were identified as the dominant parameters that affect the methane production, with feature importance weights of 0.339, 0.238, 0.158, and 0.116, respectively. The digestion time was further introduced into the above-established model to predict the cumulative methane production. With the expansion of training dataset, DL outperformed XGBoost and RF to show the lowest RMSEs of 11.83 and 5.82 in the control and ZVI-added reactors, respectively. This study demonstrates the potential of using ML algorithms to model ZVI-based AD reactors.

The bio-chemical cycle of iron and the function induced by ZVI addition in anaerobic digestion: A review

Zero-valent iron (ZVI) is known to be an additive in facilitating waste treatment and improving biogas production in anaerobic digestion (AD) systems. This review concentrates on the chemical cycle of iron as well as the function of the iron cycle in the removal of four kinds of pollutants: organic carbon, nitrogen, sulphur and phosphorus, which are commonly encountered in waste treatment. In recent studies, the addition of ZVI to an AD system promoted the in-situ production of CH₄ from CO₂, enabling carbon capture through biotechnology. Additionally, using iron-carbon microbial electrolytic cells in AD systems in order to accelerate electron transport, as well as specific pollutant degradation mechanisms, are illustrated in the present study. Particularly, the main factors affecting the removal efficiency of contaminants in a ZVI-AD system such as pH, VFA/ Alkalinity (ALK), oxidation-reduction potential and particle size are reviewed. According to the above characteristics, combined with technical model and economic analyses, an AD system based on ZVI was considered to be an economical, efficient and carbon-neutral pollutant treatment technology. Accordingly, Iron-based AD is suggested to be a promising and sustainable approach orientated to a circular economy, which may be applied to many waste treatments fields.

Improving CH4 production and energy conversion from CO2 and H2 feedstock gases with mixed methanogenic community over Fe nanoparticles

To achieve methanogenic community optimization and improve the conversion efficiency of CO₂ to CH₄, Fe nanoparticles were used to promote the Methanothermobacter abundance in methanogens, which significantly increased the conversion efficiency of CO₂ and H₂ feedstock gases to CH₄ product. High-throughput 16S rRNA gene sequencing analysis revealed that Methanothermobacter abundance markedly increased from 7 to 16% when the Fe nanoparticles concentration increased from 0 to 1.5 g/L_R (the working volume in the bioreactor). Therefore, the CH₄ yield significantly promoted from 0.105 to 0.186 L/L_R. However, when the Fe nanoparticles concentration was further increased to 2 g/L_R, methanogenesis was inhibited due to toxic effects. The electron transfer constant k_app of anaerobic sludge increased by 32.8-fold to 5.77 × 10^-2 s^-1 when the Fe nanoparticles concentration increased from 0 to 1.5 g/L_R, which significantly promoted carbon conversion efficiency from 52.9 to 92.9% and energy conversion efficiency from 46.3 to 76.9%.

Zero valent iron supported biological denitrification for farmland drainage treatments with low organic carbon: Performance and potential mechanisms

In this work, the feasibility and performance of zero valent iron (ZVI) coupled anaerobic microorganisms in nitrogen removal under low organic carbon condition were investigated, through the comparison of mono-ZVI system and mono-cell system. Coupled system showed the highest total nitrogen (TN) removal efficiency of 67.85% with the addition of 15 g L^-1 iron shavings at pH 7.0, which was higher than 29.62% in the mono-ZVI system and 43.86% in the mono-cell system. Besides, the activities of nitrate reductase (NAR), nitrite reductase (NIR), nitric oxide reductase (NOR) and nitrous oxide reductase (N₂OR) were significantly improved at ZVI dosage of 15 g L^-1 and pH 7.0, which contributed to the higher TN removal efficiency in coupled system. The extent of sludge granulation was greater in the coupled system than mono-cell system, which benefited to the high operational performance and stability of coupled system. The promoted generation of extracellular polymeric substances (EPS) and formation of iron oxides in the coupled system also took advantages on nitrogen removal through adsorption. In addition, ZVI could largely enrich the functional species related to nitrogen removal in the system at phyla and genera level, which could be reasoned for the enhanced nitrogen removal efficiency. In conclusion, this study will improve the understandings of nitrogen removal in the coupled system and be useful to ensure the application of ZVI-supported biological process in the remediation of farmland drainage.

Enhanced mesophilic anaerobic digestion of waste sludge with the iron nanoparticles addition and kinetic analysis

As the functional material, iron nanoparticles effectively promote anaerobic digestion (AD) process, including the hydrolysis-acidification process and the biogas production. In this study, nano zero-valent iron (nZVI) and Fe₃O₄ nanoparticles (Fe₃O₄ NPs) were added to AD reactors respectively. The AD process was evaluated by the reactors performances, including pH, biogas yields and compositions, as well as the removal ratio of total solids (TS), volatile solids (VS) and soluble chemical oxygen demand (sCOD). Three models (first-order kinetic model, transfer function model and Cone model) were used to explore the kinetics of AD biogas production. The results showed that adding appropriate dose of nZVI or Fe₃O₄ NPs enhanced anaerobic digestibility of sludge. The highest cumulative biogas yield of 140.34 L with 0.5 g L^-1 nZVI and 137.13 L with 1 g L^-1 Fe₃O₄ NPs were obtained by the 80 days of mesophilic operation, respectively. Cumulative biogas productions of these two reactors were significantly enhanced up to 15.70% and 13.44%. TS removal rates reached >70% in all AD reactors with iron nanoparticles, and the highest sCOD removal rates of nZVI and Fe₃O₄ NPs digesters on the 80th day were 88.22% and 77.63%, respectively. The results of the three-day fermentation experiment and the kinetic parameters showed that the nZVI or Fe₃O₄ NPs enhanced the hydrolysis-acidification process of the AD, which eventually promoted biogas production. The Cone model was satisfied with the experimental results, which could be used to evaluate the kinetics of AD with iron nanoparticles more reasonably.

Aerobic removal of iodinated contrast medium by nano-sized zero-valent iron: A combination of oxidation and reduction

The removal performance and mechanisms of diatrizoate (DTA), a typical iodinated contrast medium, from water by nano-sized zero-valent iron (nZVI) under aerobic conditions were investigated in this study. Reactive oxygen species (ROS) and transformation products were detected with electron spin resonance and liquid chromatography electrospray ionization tandem mass spectrometry, respectively. Furthermore, the effects of several operational parameters on DTA removal were illustrated. The results showed that nZVI had a much higher DTA removal ability compared to microscale zero-valent iron (mZVI) in the presence of oxygen. Moreover, the detection of ROS and I^- as well as the analysis of intermediate products suggested a combination of oxidation and reduction pathways for DTA removal by nZVI under aerobic conditions. Additionally, a high dosage of nZVI and acidic conditions led to the enhancement of DTA removal, while nZVI aging, as well as chloride and nitrate ions in the solution, had negative effects on the degradation of DTA by nZVI in the presence of oxygen.

Fe0 inhibits bio-foam generating in anaerobic digestion reactor under conditions of organic shock loading and re-startup

Bio-foaming during the anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) is a major cause of unstable reactor operations, especially under high organic loading rates (OLRs). Here, we used two 40-liter anaerobic reactors to deal with the actual organic fraction of municipal solid waste at OLRs of 5 and 7.5 kgVS/m³/d and tried to inhibit bio-foam generation by dosing with Fe⁰. One of reactors had 400 g of Fe⁰ added before operation and additional 200 g of Fe⁰ was added when the operating conditions changed; the other one was the control reactor with no Fe⁰ dosing. The results show that bio-foam was generated in the control reactor at an organic shock loading of 10 kgVS/m³/d and re-startup at 5 kgVS/m³/d, while the reactor containing Fe⁰ operated normally. Due to the surfactant attributes of volatile fatty acids (VFAs), the probability of bio-foam generation was positively correlated with the effluent's degree of surface activity (SA) induced by VFAs, which is a parameter representing the foaming potential. Because VFA biodegradation was enhanced in the reactor with added Fe⁰, at the position of Fe⁰ accumulation, the SA was decreased by 49.7-59.2%. Furthermore, the effect of Fe⁰ dosing on inhibiting bio-foam was evaluated by comparison with traditional commercial anti-foam agents. The results suggest possible reasons for bio-foam generation in OFMSW AD and suggest the reasonable usage of Fe⁰ to inhibit bio-foam formation.

Competitive dynamics of anaerobes during long-term biological sulfate reduction process in a UASB reactor

To reveal the long-term competitive dynamics of anaerobes in anaerobic bioreactors with sulfate reduction, a comprehensive structured mathematical model was designed for an extension of the Anaerobic Digestion Model No. 1 (ADM1). Sulfate reduction bacteria (SRB) were categorized into acetogenic-likewise SRB (ASRB) and methanogenic-likewise SRB (MSRB). Experimental data from 329 days of continuous operation of a laboratory-scale upflow anaerobic sludge bed (UASB) reactor was used for model calibration and validation. Results show that the model has a good agreement with experimental data and that three stages including the MPA dominant, stalemate and SRB dominant stages were clearly appeared throughout the whole competition period. The model was capable of predicting the long-term dynamic competition of sulfidogens and methanogens for electrons. This could explain a long-term of over 200 days needed for the SRB out-competing the MPA, and support speculation that the SRB could finally out-compete both the AcB and the MPA.

Integration of nanoscale zero-valent iron and functional anaerobic bacteria for groundwater remediation: A review

The technology of integrating nanoscale zero-valent iron (nZVI) and functional anaerobic bacteria has broad prospects for groundwater remediation. This review focuses on the interactions between nZVI and three kinds of functional anaerobic bacteria: organohalide-respiring bacteria (OHRB), sulfate reducing bacteria (SRB) and iron reducing bacteria (IRB), which are commonly used in the anaerobic bioremediation. The coupling effects of nZVI and the functional bacteria on the contaminant removal in the integrated system are summarized. Generally, nZVI could create a suitable living condition for the growth and activity of anaerobic bacteria. OHRB and SRB could synergistically degrade organic halides and remove heavy metals with nZVI, and IRB could reactive the passivated nZVI by reducing the iron (hydr)oxides on the surface of nZVI. Moreover, the roles of these anaerobic bacteria in contaminant removal coupling with nZVI and the degradation mechanisms are illustrated. In addition, this review also discusses the main factors influencing the removal efficiency of contaminants in the integrated treatment system, including nZVI species and dosage, inorganic ions, organic matters, pH, type of pollutants, temperature, and carbon/energy sources, etc. Among these factors, the nZVI species and dosage play a fundamental role due to the potential cytotoxicity of nZVI, which might exert a negative impact on the performance of this integrated system. Lastly, the future research needs are proposed to better understand this integrated technology and effectively apply it in groundwater remediation.

Differentiated stimulating effects of activated carbon on methanogenic degradation of acetate, propionate and butyrate

Granular activated carbon (GAC) could promote methane production from organic wastes, but a wide range of dosages has been reported. In present study, different GAC dosages of 0, 0.5, 5 and 25 g/L were supplemented into anaerobic digesters and the methanogenic degradation kinetics of acetate, propionate and butyrate were characterized, respectively. At high organic load of 5 g/L, the degradation rates of propionate and butyrate increased by 1.5-4.7 and 2.5-7.0 times at varied GAC dosages. The methane production rates (R_max) from propionate and butyrate were significantly elevated when increasing GAC dosage up to 5 g/L. However, only a minor increment was found for acetate degradation either at 1 g/L or 5 g/L. The stimulatory mechanism of GAC for accelerated syntrophic degradation of propionate and butyrate can be primarily attributed to the triggering effect on acetogenesis, as evidenced by the enrichment of syntrophic bacteria e.g. Thermovirga, Synergistaceae, and Syntrophomonas etc.

Effect of Fe0 addition on volatile fatty acids evolution on anaerobic digestion at high organic loading rates

Excessive acidification frequently occurs in the anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) at high organic loading rates (OLR), due to the accumulation of non-acetic volatile fatty acids (VFA). In this study, the performance of Fe⁰ in enhancing various VFA production and metabolism was investigated. The butyric acid concentration in a high OLR reactor with Fe⁰ addition decreased from 7200 to 0mg/L after a short lag phase, and the total VFA (TVFA) concentration also decreased substantially. The corresponding dominant acidogenesis type also changed from butyric type to propionic type fermentation. Furthermore, the CH₄ yield of the reactor with added Fe⁰ was approximately 595ml CH₄/g VS_added, which was an increase of 41.7% compared with the biochemical methane potential (BMP) test results in controls without added ZVI. A microbial diversity analysis, using high throughput sequencing, showed that Methanofollis and Methanosarcina were dominant in terms of the archaeal structures of the Fe⁰ reactor at the butyric converting stage; however, Methanosaeta was predominant in the reactor during the control BMP test. These results suggested that Fe⁰ can convert non-acetic VFA to acetic VFA and improve the CH₄ yield by enhancing the activity of methanogens.

Modeling of acetate-type fermentation of sugar-containing wastewater under acidic pH conditions

In this study, a kinetic model was developed based on Anaerobic Digestion Model No. 1 to provide insights into the directed production of acetate and methane from sugar-containing wastewater under low pH conditions. The model sufficiently described the dynamics of liquid-phase and gaseous products in an anaerobic membrane bioreactor by comprehensively considering the syntrophic bioconversion steps of sucrose hydrolysis, acidogenesis, acetogenesis and methanogenesis under acidic pH conditions. The modeling results revealed a significant pH-dependency of hydrogenotrophic methanogenesis and ethanol-producing processes that govern the sucrose fermentative pathway through changing the hydrogen yield. The reaction thermodynamics of such acetate-type fermentation were evaluated, and the implications for process optimization by adjusting the hydraulic retention time were discussed. This work sheds light on the acid-stimulated acetate-type fermentation process and may lay a foundation for optimization of resource-oriented processes for treatment of food wastewater.

Bioassessment of heavy metal toxicity and enhancement of heavy metal removal by sulfate-reducing bacteria in the presence of zero valent iron

A simple and valid toxicity evaluation of Zn²⁺, Mn²⁺ and Cr⁶⁺ on sulfate-reducing bacteria (SRB) and heavy metal removal were investigated using the SRB system and SRB+Fe⁰ system. The heavy metal toxicity coefficient (β) and the heavy metal concentration resulting in 50% inhibition of sulfate reduction (I) from a modeling process were proposed to evaluate the heavy metal toxicity and nonlinear regression was applied to search for evaluation indices β and I. The heavy metal toxicity order was Cr⁶⁺ > Mn²⁺ > Zn²⁺. Compared with the SRB system, the SRB+Fe⁰ system exhibited a better capability for sulfate reduction and heavy metal removal. The heavy metal removal was above 99% in the SRB+Fe⁰ system, except for Mn²⁺. The energy-dispersive spectroscopy (EDS) analysis showed that the precipitates were removed primarily as sulfide for Zn²⁺ and hydroxide for Mn²⁺ and Cr⁶⁺.The method of evaluating the heavy metal toxicity on SRB was of great significance to understand the fundamentals of the heavy metal toxicity and inhibition effects on the microorganism and regulate the process of microbial sulfate reduction.

Effect of microscale ZVI/magnetite on methane production and bioavailability of heavy metals during anaerobic digestion of diluted pig manure

Low methane production and high levels of heavy metal in pig slurries limit the feasibility of anaerobic digestion of pig manure. In this study, changes in the methane production and bioavailability of heavy metals in the anaerobic digestion of diluted pig manure were evaluated using single and combined action of microscale zero-valence iron (ZVI) and magnetite. After 30 days of anaerobic digestion, the methane yield ranged from 246.9 to 334.5 mL/g VS added, which increased by 20-26% in the group added with microscale ZVI and/or magnetite relative to that in the control group. Results of the first-order kinetic model revealed that addition of microscale ZVI and/or magnetite increased the biogas production potential, rather than the biogas production rate constant. These treatments also changed the distribution of chemical fractions for heavy metal. The addition of ZVI decreased the bioavailability of Cu and Zn in the solid digested residues. Moreover, a better performance was observed in the combined action of microscale ZVI and magnetite, and the ZVI anaerobic corrosion end-product, magnetite, might help enhance methane production through direct interspecies electron transfer in ZVI-anaerobic digestion process.

Wastewater sludges pretreated by different oxidation systems at mild conditions to promote the biogas formation in anaerobic processes

The effect of different oxidation processes at mild conditions including the coupled-Fenton (sono-Fenton, photo-Fenton, and sono-photo-Fenton) and their blank systems (ultrasound, ultraviolet, zero valent iron, and Fenton) on anaerobic digestion of the sludge for biogas production was investigated. Ultrasounds led to the highest organic matter solubilization (3.8 up to 5.2 g chemical oxygen demand (COD)/L, for the raw and treated sludge, respectively), while for the rest, organic matter transformation was observed resulting in an almost soluble COD net balance. Results indicated that for the most oxidative processes, the released organic matter was probably mineralized by the hydroxyl radicals produced during the treatments. It is interesting to remark that even if the biochemical methane potential was barely enhanced by the different methods applied, all the methods demonstrated to enhance the overall kinetics of the biomethanation processes, increasing the rapidly biodegradable fraction of the sludge.

Inhibiting excessive acidification using zero-valent iron in anaerobic digestion of food waste at high organic load rates

Excessive acidification occurs frequently in food waste (FW) anaerobic digestion (AD) due to the high carbon-to-nitrogen ratio of FW. In this study, zero-valent iron (ZVI) was applied to prevent the excessive acidification. All of the control groups, without ZVI addition (pH∼5.3), produced little methane (CH4) and had high volatile fatty acids/bicarbonate alkalinity (VFA/ALK). By contrast, at OLR of 42.32gVS/Lreactor, the pH of effluent from the reactors with 0.4g/gVSFWadded of ZVI increased to 7.8-8.2, VFA/ALK decreased to <0.1, and the final CH4 yield was ∼380mL/gVSFWadded, suggesting inhibition of excessive acidification. After adding powdered or scrap metal ZVI to the acidogenic reactors, the fractional content of butyric acid changed from 30-40% to 0%, while, that of acetic acid increased. These results indicate that adding ZVI to FW digestion at high OLRs could eliminate excessive acidification by promoting butyric acid conversion and enhancing methanogen activity.

Modelling phosphorus (P), sulfur (S) and iron (Fe) interactions for dynamic simulations of anaerobic digestion processes

This paper proposes a series of extensions to functionally upgrade the IWA Anaerobic Digestion Model No. 1 (ADM1) to allow for plant-wide phosphorus (P) simulation. The close interplay between the P, sulfur (S) and iron (Fe) cycles requires a substantial (and unavoidable) increase in model complexity due to the involved three-phase physico-chemical and biological transformations. The ADM1 version, implemented in the plant-wide context provided by the Benchmark Simulation Model No. 2 (BSM2), is used as the basic platform (A0). Three different model extensions (A1, A2, A3) are implemented, simulated and evaluated. The first extension (A1) considers P transformations by accounting for the kinetic decay of polyphosphates (XPP) and potential uptake of volatile fatty acids (VFA) to produce polyhydroxyalkanoates (XPHA) by phosphorus accumulating organisms (XPAO). Two variant extensions (A2,1/A2,2) describe biological production of sulfides (SIS) by means of sulfate reducing bacteria (XSRB) utilising hydrogen only (autolithotrophically) or hydrogen plus organic acids (heterorganotrophically) as electron sources, respectively. These two approaches also consider a potential hydrogen sulfide ( [Formula: see text] inhibition effect and stripping to the gas phase ( [Formula: see text] ). The third extension (A3) accounts for chemical iron (III) ( [Formula: see text] ) reduction to iron (II) ( [Formula: see text] ) using hydrogen ( [Formula: see text] ) and sulfides (SIS) as electron donors. A set of pre/post interfaces between the Activated Sludge Model No. 2d (ASM2d) and ADM1 are furthermore proposed in order to allow for plant-wide (model-based) analysis and study of the interactions between the water and sludge lines. Simulation (A1 - A3) results show that the ratio between soluble/particulate P compounds strongly depends on the pH and cationic load, which determines the capacity to form (or not) precipitation products. Implementations A1 and A2,1/A2,2 lead to a reduction in the predicted methane/biogas production (and potential energy recovery) compared to reference ADM1 predictions (A0). This reduction is attributed to two factors: (1) loss of electron equivalents due to sulfate [Formula: see text] reduction by XSRB and storage of XPHA by XPAO; and, (2) decrease of acetoclastic and hydrogenotrophic methanogenesis due to [Formula: see text] inhibition. Model A3 shows the potential for iron to remove free SIS (and consequently inhibition) and instead promote iron sulfide (XFeS) precipitation. It also reduces the quantities of struvite ( [Formula: see text] ) and calcium phosphate ( [Formula: see text] ) that are formed due to its higher affinity for phosphate anions. This study provides a detailed analysis of the different model assumptions, the effect that operational/design conditions have on the model predictions and the practical implications of the proposed model extensions in view of plant-wide modelling/development of resource recovery strategies.

Research advances in deriving renewable energy from biomass in wastewater treatment plants

Anaerobic digestion (AD) can be used to derive renewable energy from biomass in wastewater treatment plants, and the produced biogas represents a valuable end-product that can greatly offset operation costs.

Assessing chromate reduction by dissimilatory iron reducing bacteria using mathematical modeling

Chromate (Cr (VI)) is a ubiquitous contaminant in aquifers and soils, which can be reduced to its trivalent counterpart (Cr (III)), with the hazard being relieved. The coupling microbial and chemical reduction by dissimilatory iron reducing bacteria (IRB) is a promising approach for the reduction of Cr (VI) to Cr (III). In this work, three mathematical models with different Cr (VI) reduction pathways were proposed and compared based on their ability to predict the performance of an IRB-based stirred-flow reactor treating Cr (VI) contaminated medium and to provide insights into the possible chemical or microbial pathways for Cr (VI) reduction in the system. The Cr (VI) reduction was considered as chemical reaction between Fe (II) and Cr (VI), direct microbial reduction by IRB and combined biotic-abiotic reduction in these three models, respectively. Model evaluation results indicated that the model incorporating both chemical and microbial Cr (VI) reductions could well describe the system performance. In contrast, the other two single-pathway models were not capable of predicting the experimental data, suggesting that both chemical and microbial pathways contributed to Cr (VI) reduction by IRB. The validity of the two-pathway model was further confirmed by an independent experimental data set with different conditions. The results further revealed that the organic carbon availability and Cr (VI) loading rates for the IRB in the system determined the relative contributions of chemical and microbial pathways to overall Cr (VI) reduction.

Evaluation on the Nanoscale Zero Valent Iron Based Microbial Denitrification for Nitrate Removal from Groundwater

Nanoscale zero valent iron (NZVI) based microbial denitrification has been demonstrated to be a promising technology for nitrate removal from groundwater. In this work, a mathematical model is developed to evaluate the performance of this new technology and to provide insights into the chemical and microbial interactions in the system in terms of nitrate reduction, ammonium accumulation and hydrogen turnover. The developed model integrates NZVI-based abiotic reduction of nitrate, NZVI corrosion for hydrogen production and hydrogen-based microbial denitrification and satisfactorily describes all of the nitrate and ammonium dynamics from two systems with highly different conditions. The high NZVI corrosion rate revealed by the model indicates the high reaction rate of NZVI with water due to their large specific surface area and high surface reactivity, leading to an effective microbial nitrate reduction by utilizing the produced hydrogen. The simulation results further suggest a NZVI dosing strategy (3–6 mmol/L in temperature range of 30–40 °C, 6–10 mmol/L in temperature range of 15–30 °C and 10–14 mmol/L in temperature range of 5–15 °C) during groundwater remediation to make sure a low ammonium yield and a high nitrogen removal efficiency.

Zero valent iron simultaneously enhances methane production and sulfate reduction in anaerobic granular sludge reactors

Zero valent iron (ZVI) packed anaerobic granular sludge reactors have been developed for improved anaerobic wastewater treatment. In this work, a mathematical model is developed to describe the enhanced methane production and sulfate reduction in anaerobic granular sludge reactors with the addition of ZVI. The model is successfully calibrated and validated using long-term experimental data sets from two independent ZVI-enhanced anaerobic granular sludge reactors with different operational conditions. The model satisfactorily describes the chemical oxygen demand (COD) removal, sulfate reduction and methane production data from both systems. Results show ZVI directly promotes propionate degradation and methanogenesis to enhance methane production. Simultaneously, ZVI alleviates the inhibition of un-dissociated H2S on acetogens, methanogens and sulfate reducing bacteria (SRB) through buffering pH (Fe(0) + 2H(+) = Fe(2+) + H2) and iron sulfide precipitation, which improve the sulfate reduction capacity, especially under deterioration conditions. In addition, the enhancement of ZVI on methane production and sulfate reduction occurs mainly at relatively low COD/ [Formula: see text] ratio (e.g., 2-4.5) rather than high COD/ [Formula: see text] ratio (e.g., 16.7) compared to the reactor without ZVI addition. The model proposed in this work is expected to provide support for further development of a more efficient ZVI-based anaerobic granular system.

Evaluating enhanced sulfate reduction and optimized volatile fatty acids (VFA) composition in anaerobic reactor by Fe (III) addition

Anaerobic reactors with ferric iron addition have been experimentally demonstrated to be able to simultaneously improve sulfate reduction and organic matter degradation during sulfate-containing wastewater treatment. In this work, a mathematical model is developed to evaluate the impact of ferric iron addition on sulfate reduction and organic carbon removal as well as the volatile fatty acids (VFA) composition in anaerobic reactor. The model is successfully calibrated and validated using independent long-term experimental data sets from the anaerobic reactor with Fe (III) addition under different operational conditions. The model satisfactorily describes the sulfate reduction, organic carbon removal and VFA production. Results show Fe (III) addition induces the microbial reduction of Fe (III) by iron reducing bacteria (IRB), which significantly enhances sulfate reduction by sulfate reducing bacteria (SRB) and subsequently changes the VFA composition to acetate-dominating effluent. Simultaneously, the produced Fe (II) from IRB can alleviate the inhibition of undissociated H2S on microorganisms through iron sulfide precipitation, resulting in further improvement of the performance. In addition, the enhancement on reactor performance by Fe (III) is found to be more significantly favored at relatively low organic carbon/SO4(2-) ratio (e.g., 1.0) than at high organic carbon/SO4(2-) ratio (e.g., 4.5). The Fe (III)-based process of this work can be easily integrated with a commonly used strategy for phosphorus recovery, with the produced sulfide being recovered and then deposited into conventional chemical phosphorus removal sludge (FePO4) to achieve FeS precipitation for phosphorus recovery while the required Fe (III) being acquired from the waste ferric sludge of drinking water treatment process, to enable maximum resource recovery/reuse while achieving high-rate sulfate removal.

Enhanced reductive transformation of 2,4-dinitroanisole in a anaerobic system: the key role of zero valent iron

Accelerated reduction of typical multi-substituted nitroaromatic compounds (NACs),i.e., 2,4-dinitroanisole (DNAN), was achieved in an anaerobic system coupled with zero valent iron (ZVI), with the underlying role of ZVI in this process elucidated.