Zero-valent iron enhanced anaerobic digestion of pre-concentrated domestic wastewater for bioenergy recovery: Characteristics and mechanisms

Enhancing anaerobic digestion of sulphate wastewater by adding nano-zero valent iron

In this paper, the effects of nano-zero valent iron (nZVI) on anaerobic digestion of sulphate wastewater with different SO 4 2 - /COD ratios, including the COD removal rate, methane yield, intermediate products and the change of microbial community structure, were investigated. The results showed that nZVI could effectively enhance the treatment efficiency and methane yield. Compared with the control group without nZVI, the methane yield increased from 348.6833 to 1007.05 mL CH4/gCODremoval with 4 g nZVI loading at SO 4 2 - /COD = 0.1. nZVI could make electron flow from sulphate reduction to methane production, which increased methane yield even at high sulphate concentration. The microbial community analysis showed that adding nZVI could increase the abundance of acetoclastic methanogens, which accelerated hydrolysis acidification.

The coupling system of magnetite-enhanced thermophilic hydrolysis-acidification and denitrification for refractory organics removal from anaerobic digestate food waste effluent (ADFE)

The aim of this study was to remove the refractory organics from high-temperature anaerobic digestate food waste effluent by the coupling system of hydrolysis-acidification and denitrification. Iron-based materials (magnetite, zero-valent iron, and iron-carbon) were used to enhance the performance of thermophilic hydrolysis-acidification. Compared with the control group, magnetite had the best strengthening effect, increasing volatile fatty acids concentration and fluorescence intensity of easily biodegradable organics in the effluent by 47.6 % and 108.4 %, respectively. The coupling system of magnetite-enhanced thermophilic hydrolysis-acidification and denitrification achieved a nitrate removal efficiency of 91.2 % (influent NO₃^--N was 150 mg L^-1), and reduced the fluorescence intensity of refractory organics by 33.8 %, compared with influent. Microbiological analysis indicated that magnetite increased the relative abundance of thermophilic hydrolytic acidifying bacteria, and coupling system enriched some genera simultaneously removing nitrate and refractory organics. This study provided fresh information on refractory organics and nitrogen removal of thermophilic wastewater biologically.

Mechanisms, performance, and the impact on microbial structure of direct interspecies electron transfer for enhancing anaerobic digestion-A review

Direct interspecies electron transfer (DIET) has been received tremendous attention, recently, due to the advantages of accelerating methane production via organics reduction during anaerobic digestion (AD) process. DIET-based syntrophic relationships not only occurred with the existence of pili and some proteins in the microorganism, but also can be conducted by conductive materials. Therefore, more researches into understanding and strengthening DIET-based syntrophy have been conducted with the aim of improving methanogenesis kinetics and further enhance methane productivity in AD systems. This study summarized the mechanisms, application and microbial structures of typical conductive materials (carbon-based materials and iron-based materials) during AD reactors operation. Meanwhile, detail analysis of studies on DIET (from substrates, dosage and effectiveness) via conductive materials was also presented in the study. Moreover, the challenges of applying conductive materials in boosting methane production were also proposed, which was supposed to provide a deep insight in DIET for full scale application.

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

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

Process characteristics and energy self-sufficient operation of a low-fouling anaerobic dynamic membrane bioreactor for up-concentrated municipal wastewater treatment

Up-concentration of municipal wastewater using physico-chemical methods can effectively enrich organic matter, facilitating subsequent anaerobic digestion of up-concentrated wastewater for enhanced methanogenesis at reduced energy consumption. An anaerobic dynamic membrane bioreactor (AnDMBR) assisted with biogas-sparging was developed to treat up-concentrated municipal wastewater, focusing on the effects of operating temperature and hydraulic retention time (HRT) as well as COD mass balance and energy balance. The COD removal stabilized at about 98 % over the experimental period, while gaseous and dissolved methane contributed 43-49 % and 2-3 % to the influent COD reducing greenhouse gas emissions. The formed dynamic membrane exists mainly as a heterogeneous cake layer with a uneven distribution feature, ensuring the stable effluent quality. Without adopting any physico-chemical cleaning, the transmembrane pressure (TMP) maintained at a low range (2.7 to 14.67 kPa) with the average TMP increasing rate of 0.089 kPa/d showing a long-term low-fouling operation. Increasing the concentration ratio, the methane production rate decreased from 0.18 to 0.15 L CH₄/gCOD likely due to the accumulation of particulate organics. Microbial community analysis indicated the predominant methanogenic pathway shifted from hydrogenotrophic to acetoclastic methanogenesis in response to the temperature change. Net energy balance (0.003-0.600 kWh/m³) can be achieved only under room temperature (25 °C) rather than mesophilic conditions (36 °C).

Strategies to enhance the reactivity of zero-valent iron for environmental remediation: A review

Application of zero-valent iron (ZVI) has become one of the most promising innovative technologies for the remediation of environmental pollutants. However, ZVI may suffer from the low intrinsic reactivity toward refractory pollutants, which seriously restricts its practical application in fields. Therefore, strategies have been developing to enhance the reactivity of ZVI. Until now, the most commonly used strategies include pretreatment of ZVI, synthesis of highly-active ZVI-based materials and additional auxiliary measures. In this review, a systematic and comprehensive summary of these commonly used strategies has been conducted for the following purposes: (1) to understand the fundamental mechanisms of the selected approaches; (2) to point out their advantages and shortcomings; (3) to illustrate the main problems of their large-scale application; (4) to forecast the future trend of developing ZVI technologies. Overall, this review is devoted to providing a fundamental understanding on the mechanism for enhancing the reactivity of ZVI and facilitating the practical application of ZVI technologies in fields.

A critical review of process parameters influencing the fate of antibiotic resistance genes in the anaerobic digestion of organic waste

The overuse and inappropriate disposal of antibiotics raised severe public health risks worldwide. Specifically, the incomplete antibiotics metabolism in human and animal bodies contributes to the significant release of antibiotics into the natural ecosystems and the proliferation of antibiotic-resistant bacteria carrying antibiotic-resistant genes. Moreover, the organic feedstocks used for anaerobic digestion are often highly-rich in residual antibiotics and antibiotic-resistant genes. Hence, understanding their fate during anaerobic digestion has become a significant research focus recently. Previous studies demonstrated that various process parameters could considerably influence the propagation of the antibiotic-resistant genes during anaerobic digestion and their transmission via land application of digestate. This review article scrutinizes the influences of process parameters on antibiotic-resistant genes propagation in anaerobic digestion and the inherent fundamentals behind their effects. Based on the literature review, critical research gaps and challenges are summarized to guide the prospects for future studies.

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.

Performance and metagenomics analysis of anaerobic digestion of food waste with adding biochar supported nano zero-valent iron under mesophilic and thermophilic condition

A large amount of food waste (FW) brings environmental pollution and sanitation problems. Anaerobic digestion (AD) is an effective technology to treat FW and generate biogas energy. This study investigated the effect of biochar supported nano zero-valent iron (BC-nZVI) on AD performance of FW. Results showed that the cumulative methane yield (CMY) increased by 21.52%-54.90% and the lag time decreased significantly with BC-nZVI. Under mesophilic and thermophilic condition, the peak of CMY was achieved at 178.82 ± 5.27 mL/g VS and 193.01 ± 6.81 mL/g VS with 5 g/L BC-nZVI, respectively. Besides, BC-nZVI stimulated hydrolysis process and reduced the inhibition of NH₄⁺-N and volatile fatty acids accumulation, and it could improve the system stability. Structural equation model analysis indicated that digestion time, BC-nZVI, NH₄⁺-N, temperature and total volatile fatty acid had significant effects on CMY, explaining 92.20% of its total variation. The metagenomic analysis of key microorganisms and related metabolism pathways involved in AD system was further investigated. The results suggested that BC-nZVI contributed to strengthen methanogenesis through enriching the various predominant methanogenic pathways and activating most enzymes related to methane metabolism. BC-nZVI could improve the AD system function and provided a better AD performance by shifting the microbial communities and altering functional genes. This study provided a theoretical basis for BC-nZVI applications and improvements in AD process of FW.

Combined effect of zero valent iron and magnetite on semi-dry anaerobic digestion of swine manure

Combined effect of zero valent iron (ZVI) and magnetite on semi-dry anaerobic digestion of swine manure was studied. Compared with control, the addition of 5 g/L ZVI, magnetite and their mixture (1:1 wt) increased the CH₄ yield by 17.6%, 22.7% and 21.9%, respectively. The three additives improved CH₄ production through altering the metabolism pathways, rather than improving the solid degradation efficiency. ZVI promoted interspecies hydrogen transfer (IHT) by enriching H₂-comsuming Methanolinea and acetate-oxidizing bacteria (Sedimentibacter and Clostridium). Magnetite enriched dissimilatory iron reduction bacteria (Acinetobacter) to accelerate organic hydrolysis and established direct interspecies electron transfer (DIET) by enriching Methanothrix and Methanospirillum. Key microorganisms relative to IHT (Clostridium) and DIET (Methanothrix and Methanospirillum) were simultaneously enriched with ZVI + magnetite, but they only showed an additive effect on methanogenesis, the lack of synergetic effect was attributable to the possible trade-off between IHT and DIET, or the little improvement effect of additives on substrate biodegradability.

Unveiling the mechanisms of a novel polyoxometalates (POMs)-based pretreatment technology for enhancing methane production from waste activated sludge

This study proposed a novel polyoxometalates (POMs)-based pretreatment technology to improve methane production from waste activated sludge (WAS) for the first time. Experimental results indicated methane production from WAS pretreated with 0.25 g POMs/g TSS increased by 43.7%. Mechanism analysis revealed POMs pretreatment promoted WAS disintegration and improved the biodegradability of the released organics. The declined oxidation-reduction potential of digestion system provided a more favorable situation for anaerobes, and hence had positive impacts on the activity of enzymes associated with hydrolysis/acidification/methanogenesis. Model-based analysis elucidated POMs pretreatment remarkably increased both biochemical methane potential and hydrolysis rate. Microbial community analysis showed microbial community was shifted toward increase hydrolytic and acidification-associated microbes and enriched the abundance of Methanosaeta sp. This work is expected to develop an innovative technology that will simultaneously enhance energy production from WAS in the sludge treatment line and improve biological nutrient removal in the wastewater treatment line of WWTPs.

Mini art review for zero valent iron application in anaerobic digestion and technical bottlenecks

Zero valent iron (ZVI) has been used extensively to control environmental pollution owing to its strong reducibility and low cost. Herein, we evaluate the impact of ZVI (iron scrap and ZVI powder with different scales) on anaerobic digestion (AD) reactor performance improvement and syntrophic relationship stimulation among various microbial groups in the methanogenesis process. In recent studies, ZVI addition significantly enhanced methane and volatile fatty acid (VFA) yields and alleviated excessive acidification, ammonia accumulation, and odorous gas production. Further, we reviewed the changes in enzyme activity and microbial metabolism after the addition of ZVI throughout the reaction process. Certain innovative technologies, such as bioelectrochemical system assistance and combined usage of conductive materials, may improve AD performance compared to the use of ZVI alone, the mechanism of which has been discussed from various viewpoints. Furthermore, the primary technical bottlenecks, such as poor mass transfer efficiency in dry AD and high ZVI dosage, have been illustrated, and syntrophic methanogenesis regulated by ZVI addition can be further studied by conducting theoretical research.

Prediction of the Long-Term Effect of Iron on Methane Yield in an Anaerobic Membrane Bioreactor Using Bayesian Network Meta-Analysis

A method for predicting the long-term effects of ferric on methane production was developed in an anaerobic membrane bioreactor treating food processing wastewater to provide management tools for maximizing methane recovery using ferric based on a batch test. The results demonstrated the accuracy of the predictions for both batch and long-term continuous operations using a Bayesian network meta-analysis based on the Gompertz model. The prediction bias of methane production for batch and continuous operations was minimized, from 11~19% to less than 0.5%. A biochemical methane potential-based Bayesian network meta-analysis suggested a maximum 2.55% ± 0.42% enhancement for Fe2.25. An anaerobic membrane bioreactor improved the methane yield by 2.27% and loading rate by 4.57% for Fe2.25, operating in the sequenced batch mode. The method allowed for a predictable methane yield enhancement based on the biochemical methane potential. Ferric enhanced the biochemical methane potential in batch tests and the methane yield in a continuously operated reactor by a maximum of 8.20% and 7.61% for Fe2.25, respectively. Copper demonstrated a higher methane (18.91%) and sludge yield (17.22%) in batch but faded in the continuous operation (0.32% of methane yield). The enhancement was primarily due to changing the kinetic patterns for the last period, i.e., increasing the second methane production peak (k₇₁), bringing forward the second peak (λ₇, λ₈), and prolonging the second period (k₆₂). The dual exponential function demonstrated a better fit in the last three stages (after the first peak), which implied that syntrophic methanogenesis with a ferric shuttle played a primary role in the last three methane production periods, in which long-term effects were sustained, as the Bayesian network meta-analysis predicted.

Zero-valent iron addition in anaerobic dynamic membrane bioreactors for preconcentrated wastewater treatment: Performance and impact

Wastewater preconcentration to capture abundant organics is promising for facilitating subsequent anaerobic digestion (AD) to recover bioenergy, however research efforts are still needed to verify the effectiveness of such an emerging strategy as carbon capture plus AD. Therefore, lab-scale anaerobic dynamic membrane bioreactors (AnDMBRs) without and with the addition of zero-valent iron (ZVI) (i.e., AnDMBR1 versus AnDMBR2) were developed for preconcentrated domestic wastewater (PDW) treatment, and the impact of ZVI addition on process performance and associated mechanisms were investigated. The stepwise addition of ZVI from 2 to 4 to 6 g/L improved the treatment performance as COD removal slightly increased and TP removal and methane production were enhanced by 53.3%-62.9% and 22.6%-31.3%, respectively, in consecutive operational phases. However, the average increasing rate of the transmembrane pressure (TMP) in AnDMBR2 (0.18 kPa/d) was obviously higher than that in AnDMBR1 (0.05 kPa/d), indicating an unfavorable impact of dosing ZVI on the dynamic membrane (DM) filtration performance. ZVI that has transformed to iron ions (mainly Fe²⁺) can behave as a coagulant, electron donor or inorganic foulant, thus enabling the excellent removal of dissolved phosphorous, enhancing the enrichment and activities of specific methanogens and causing the formation of a compact DM layer. Morphological, componential, and microbial community analyses provided new insights into the functional mechanisms of ZVI added to membrane-assisted anaerobic digesters, indicating that ZVI has the potential to improve bioenergy production and resource recovery, while optimizing the ZVI dosage should be considered to alleviate membrane fouling.