Effect of ferrous chloride on biogas production and enzymatic activities during anaerobic fermentation of cow dung and Phragmites straw

Mathematical Model-Based Optimization of Trace Metal Dosage in Anaerobic Batch Bioreactors

Anaerobic digestion (AD) is a promising and yet a complex waste-to-energy technology. To optimize such a process, precise modeling is essential. Developing complex, mechanistically inspired AD models can result in an overwhelming number of parameters that require calibration. This study presents a novel approach that considers the role of trace metals (Ca, K, Mg, Na, Co, Cr, Cu, Fe, Ni, Pb, and Zn) in the modeling, numerical simulation, and optimization of the AD process in a batch bioreactor. In this context, BioModel is enhanced by incorporating the influence of metal activities on chemical, biochemical, and physicochemical processes. Trace metal-related parameters are also included in the calibration of all model parameters. The model's reliability is rigorously validated by comparing simulation results with experimental data. The study reveals that perturbations of 5% in model parameter values significantly increase the discrepancy between simulated and experimental results up to threefold. Additionally, the study highlights how precise optimization of metal additives can enhance both the quantity and quality of biogas production. The optimal concentrations of trace metals increased biogas and CH4 production by 5.4% and 13.5%, respectively, while H2, H2S, and NH3 decreased by 28.2%, 43.6%, and 42.5%, respectively.

Coagulation in combination with anaerobic digestion for enhancement of resource recovery from faecal sludge

Poorly managed faecal sludge (FS) poses significant challenges to public health and the environment. Anaerobic digestion (AD) of FS provides an effective method for energy recovery while reducing FS associated threats. Recognizing the critical role of the dewatering process before AD, this study investigates the synergistic application of chemical coagulation and mesophilic AD for synthetic FS treatment. FeCl3, AlCl3, Fe2(SO4)3, poly ferric sulfate (PFS) and poly aluminium ferric chloride (PAFC) were utilized at varying dosages to examine their impact on FS properties and subsequent biogas production from the dewatered FS. It was found that coagulation enhances sedimentation efficiencies and dewaterability through mechanisms such as charge neutralization, charge patching and bridging, thereby improving the FS feasibility for AD. Notably, polymer coagulant PFS showed good performance in balancing pollutant removal and methane recovery, contributing to facilitating the hydrolysis and acidogenesis microorganisms involved in the AD process. Optimal dosage was identified at 150 mg/g TS (1.7 g/L FS), achieving prominent removal efficiencies for total COD (67%), turbidity (85%), and total phosphorus (60%), while simultaneously enhancing AD performance with specific CH4 production reaching 517 ml CH₄/g VS or 24.8 ml CH₄/g AD wet feedstock compared to 309 ml CH₄/g VS or 2.7 ml CH₄/g AD wet feedstock in untreated FS.

Enhanced anaerobic digestion of human feces by ferrous hydroxyl complex (FHC): Stress factors alleviation and microbial resistance improvement

Anaerobic digestion (AD) offers a reliable strategy for resource recovery from source-separated human feces (HF), but is limited by a disproportionate carbon/nitrogen (C/N) ratio. Ferrous hydroxyl complex (FHC) was first introduced into the HF-AD system to mediate methanogenesis. Mono-digestion of undiluted HF was inhibited by high levels of volatile fatty acids (VFAs), ammonia, and hydrogen sulfide (H2S). FHC addition at optimum dosage (500-1000 mg/L) increased the cumulative methane (CH4) yield by 22.7%, enhanced the peak value of daily CH4 production by 60.5%, and shortened the lag phase by 24.7%. H2S concentration in biogas was also greatly decreased by FHC via precipitation. FHC mainly facilitated the hydrolysis, acidification, and methanogenesis processes. The production and transformation of VFAs were optimized in the presence of FHC, thus relieving acid stress. FHC elevated the activities of alkaline protease, cellulase, and acetate kinase by 32.3%, 18.2%, and 30.3%, respectively. Microbial analysis revealed that hydrogenotrophic methanogens prevailed in mono-digestion at high HF loading but were weakened after FHC addition. FHC also enriched Methanosarcina, thereby expanding the methanogenesis pathway and improving the resistance to ammonia stress. This work would contribute to improving the methanogenic performance and resource utilization for HF anaerobic digestion.

Advancement of biochar-aided with iron chloride for contaminants removal from wastewater and biogas production: A review

The use of fossil fuels, emission of greenhouse gases (GHG) into the atmosphere, and waste pose a problem to the environment and public health that urgently needs to be dealt with. Among numerous chemical activating agents that can be added to anaerobic digestion (AD) to enhance nutrient removal and increase the quality and quantity of biomethane, iron chloride (FeCl₃) is the one that has the lowest cost and is the most environmentally friendly. This state-of-the-art review aims to revise the influence of FeCl₃ on the Brunauer-Emmett-Teller (BET) surface area of biochar and its ability to increase methane (CH₄) yield and remove contaminants from biogas and wastewater. The novelty of the study is that FeCl₃, an activating agent, can increase the BET surface area of biochar, and its efficacy increases when combined with zinc chloride or phosphoric acid. Regarding the removal of contaminants from wastewater and biogas, FeCl₃ has proven to be an effective coagulant, reducing the chemical oxygen demand (COD) of wastewater and hydrogen sulfide in biogas. The performance of FeCl₃ depends on the dosage, pH, and feedstock used. Therefore, FeCl₃ can increase the BET surface area of biochar and CH₄ yield and remove contaminants from wastewater and biogas. More research is needed to investigate the ability of FeCl₃ to remove water vapor and carbon dioxide during biogas production while accounting for a set of other parameters, including FeCl₃ size.

The greenhouse effect of antibiotics: The influence pathways of antibiotics on methane release from freshwater sediment

The impact of antibiotics on methane (CH₄) release from sediment involves both CH₄ production and consumption processes. However, most relevant studies lack a discussion of the pathways by which antibiotics affect CH₄ release and do not highlight the role played by the sediment chemical environment in this influence mechanism. Here, we collected field surface sediments and grouped them with various antibiotic combination concentration gradients (50, 100, 500, 1000 ng g^-1) under a 35-day indoor anaerobic constant temperature incubation. We found that the positive effect of antibiotics on sediment CH₄ release potential appeared later than the positive effect on sediment CH₄ release flux. Still, the positive effect of high-concentration antibiotics (500, 1000 ng g^-1) occurred with a lag in both processes. Also, the positive effect of high-concentration antibiotics was significantly higher than low-concentration antibiotics (50, 100 ng g^-1) in the later incubation period (p < 0.05). We performed a multi-collinearity assessment of sediment biochemical indicators, followed by a generalized linear model with negative binomial regression (GLM-NB) to obtain essential variables. In particular, we conducted the interaction analysis on CH₄ release potential and flux regression for the influence pathways construction. The partial least-squares path modeling (PLS-PM) demonstrated that the positive effect of antibiotics on CH₄ release (Total effect = 0.2579) was primarily attributed to their effect on the sediment chemical environment (Direct effect = 0.5107). These findings greatly expand our understanding of the antibiotic greenhouse effect in freshwater sediment. Further studies should more carefully consider the effects of antibiotics on the sediment chemical environment, and continuously improve the mechanistic studies of antibiotics on sediment CH₄ release.

Fe2(SO4)3-assisted anaerobic digestion of pig manure: the performance of biogas yield and heavy metal passivation

Abstract

The harmless disposal and recycling treatment technology of livestock manure has received increasing attention in recent years. In this study, Fe2(SO4)3 was added during anaerobic digestion (AD) of pig manure (PM) to investigate the effects of different doses of Fe2(SO4)3 on biogas yield and heavy metal passivation. The results showed that the highest biogas yield was observed after adding a moderate dose of Fe2(SO4)3 (3%, based on the total solids), while the elevated result was inhibited as the Fe2(SO4)3 dosage increased. The analysis of solid digestate (solid matter remaining after AD) revealed that AD effectively passivated Cu, Zn, and As, which can be further improved with the addition of Fe2(SO4)3. However, the passivated Cd performance during this process was negligible. Furthermore, seed germination index (GI) trial results indicated that Fe2(SO4)3-assisted AD reduced the toxicity of end products to plants. To summarize, AD assisted by the addition of an appropriate amount of Fe2(SO4)3 is feasible to treat PM, and the addition of Fe2(SO4)3 at 3% was the most economic and environmental-friendly. This work could provide useful methods for the control of heavy metal pollution in the soil.

Article highlights

Consideration of biological and inorganic additives in upgraded anaerobic digestion BioModel

This paper deals with the numerical simulation of biogas production in the anaerobic digestion process of organic waste. Special attention is focused on the modeling of the activities of biological and inorganic additives, which are used to enhance the process and reduce H₂S content in the biogas. For this purpose, an existing BioModel is upgraded with the modified Michaelis-Menten kinetics in order to model the enzymatic hydrolysis and with adequate modeling of physicochemical processes. The upgraded BioModel was calibrated with experimental data obtained from a full-scale biogas plant, used in combination with an active set optimization procedure; the relative agreement indices were 0.9376, 0.9419, 0.7957, and 0.7663 for biogas, CH₄, H₂, and H₂S flow rates, respectively. Statistical efficiency criteria differ up to 5% in model calibration and validation. The obtained results confirm the importance of additives modeling and the usefulness of the proposed model for industrial biogas plants' performance improvement.

Reduction of refractory Maillard reaction products by Fe3+ during thermal hydrolysis pretreatment and enhanced sludge biodegradability

Refractory Maillard reaction products (MRPs) produced during thermal hydrolysis pretreatment (THP) of waste activated sludge (WAS) may negatively impact the performance of downstream anaerobic digestion (AD) and nitrogen removal processes. Operating THP at lower temperature can mitigate the production of MRPs and improve biodegradability of WAS, while solubilization of WAS is reduced. This study intends to develop a method to reduce the refractory MRPs of WAS without compromising on the solubilization. Fe³⁺ was introduced into THP process (165 °C, 30 min) to mitigate Maillard reaction. Effects of Fe³⁺ on solubilization of WAS, reduction of refractory residuals, accumulative methane production, and microbial community shift were studied. Results confirm that solubilization of WAS was improved and refractory residuals were reduced with the amendment of 10 mg-Fe/L FeCl₃. MRPs mitigation mechanisms were investigated and mainly attributed to Fe³⁺-triggered Fenton-like reactions. Methane production was enhanced by 10.4 ± 0.8% and attributed to the improved biodegradability of THP liquor, as well as to the enrichment of protein degradation and methane production related microbial community. This work provides a simple, economical, and safe strategy to reduce refractory residuals discharged from THP-AD system and to enhance methane production for more energy recovery.

The role of iron-based nanoparticles (Fe-NPs) on methanogenesis in anaerobic digestion (AD) performance

Several strategies have been proposed to improve the performance of the anaerobic digestion (AD) process. Among them, the use of various nanoparticles (NPs) (e.g. Fe, Ag, Cu, Mn, and metal oxides) is considered one of the most effective approaches to enhance the methanogenesis stage and biogas yield. Iron-based NPs (zero-valent iron with paramagnetic properties (Fe⁰) and iron oxides with ferromagnetic properties (Fe₃O₄/Fe₂O₃) enhance microbial activity and minimise the inhibition effect in methanogenesis. However, comprehensive and up-to-date knowledge on the function and impact of Fe-NPs on methanogens and methanogenesis stages in AD is frequently required. This review focuses on the applicative role of iron-based NPs (Fe-NPs) in the AD methanogenesis step to provide a comprehensive understanding application of Fe-NPs. In addition, insight into the interactions between methanogens and Fe-NPs (e.g. role of methanogens, microbe interaction and gene transfer with Fe-NPs) beneficial for CH₄ production rate is provided. Microbial activity, inhibition effects and direct interspecies electron transfer through Fe-NPs have been extensively discussed. Finally, further studies towards detecting effective and optimised NPs based methods in the methanogenesis stage are reported.

Modeling the Methane Production Kinetics of Anaerobic Co-Digestion of Agricultural Wastes Using Sigmoidal Functions

The modified sigmoidal bacteria growth functions (the modified Gompertz, logistic, and Richards) were used to evaluate the methane production process kinetics of agricultural wastes. The mesophilic anaerobic co-digestion experiments were conducted with various agricultural wastes as feedstocks, including cow manure, corn straw, grape leaves, vines, wine residue, strawberry leaves, and tomato leaves. The results showed that anaerobic co-digestion of cow manure and other agricultural wastes increased the methane yields while it prolonged the lag phase time. Compared with the modified Gompertz and logistic models, the modified Richards model obtained higher correlation coefficients and was able to fit experimental data better. The results of this study were expected to determine a suitable model to simulate and study the kinetic process of anaerobic co-digestion with mixed agricultural wastes as feedstocks.

Effects of Adding Zero Valent Iron on the Anaerobic Digestion of Cow Manure and Lignocellulose

Previous studies showed that adding zero valent iron (ZVI) can increase the methane production and degradation rate of organic waste by improving the performance of anaerobic digester. However, our study firstly found that ZVI (37 μm, 10 g/L) inhibited the anaerobic digestion (AD) of cow manure and lignocellulose. ZVI significantly increased the methanogenic rate of cow manure in the first 6 days, but decreased the accumulative methane yield and volatile fatty acids yield by 10.3 and 12%, respectively. The effect of ZVI on AD of liquid biomass separated from cow manure was positive, but the effect on solid biomass was negative. These results indicated that ZVI enhanced the AD of easily biodegradable organics but inhibited the biodegradation of refractory organics (lignocellulose). By analyzing the varying effects of ZVI in diverse anaerobic systems, it was found that the effects were influenced by the characteristics of substrate and inoculum-substrate ratio. This study suggested that only proper ZVI addition can improve the AD process depending on the feeding materials.

17 beta-estradiol biodegradation by anaerobic granular sludge: Effect of iron sources

Steroid estrogens, as typical endocrine disrupting chemicals (EDCs), have raised an increasing concern due to their endocrine disrupting effects on aquatic animals and potential hazards on human health. Batch experiments were conducted to study 17 beta-estradiol (E2) removal and Estradiol Equivalent Quantity (EEQ) elimination by anaerobic granular sludge (AnGS) combined with different valence iron sources. Results showed that E2 was effectively biodegraded and transformed into E1 by AnGS. The addition of different valence iron sources all promoted E2 degradation, reduced E2 Equivalent Quotient (EEQ) concentration, and increased methane production in the batch experiments. The enhancement effect of zero-valent iron (ZVI) on E2 removal and EEQ elimination was stronger than that of Fe²⁺ and Fe³⁺ in our experiments. The enhancement effect proportion of ZVI corrosion, Fe²⁺, and Fe³⁺ in the process of E2 degradation by AnGS combined with ZVI were 42.26%, 40.21% and 17.53%, respectively.

Synergistic effects of co-trace elements on anaerobic co-digestion of food waste and sewage sludge at high organic load

Trace elements play an indispensable role in stabilizing the performance of anaerobic co-digestion (Co-AD) of food waste (FW) and sewage sludge (SS) at greater organic load (OL). The results of high organic-loaded reactors showed that the stability of the system failed due to the buildup of volatile fatty acid (VFA) and ammonia. At the OL of 6.5 g/L, the stability of the system failed due to the buildup of propionic acid. The optimum dosage of Fe (5000 mg/L), Ni (200 mg/L), Zn (320 mg/L), and Mo (2.2 mg/L) was experimentally determined and added to reduce the inhibition condition. Consequently, the propionic acid concentration, which was above 1500 mg/L reduced to under 500 mg/L during Co-AD. Hence, higher biogas production, and biodegradability of 236 ± 23 mL/g VS, and 41.75%, respectively, were obtained. Increasing OL (9.5 g/L), the stability of the system was hindered due to only the buildup of ammonia (up to 188 ± 6 NH₃-N mg/L). Therefore, the trace elements of Cu (250 mg/L) and Co (3 mg/L) were experimentally determined and added into the Co-AD to diminish ammonia accumulation and process instability. The experimental results showed that at OL of 14 g/L, biogas production, low ammonia concentration and biodegradability of 332 ± 21 mL/g VS, and 70 NH3-N mg/L, and 57.89%, respectively, were achieved. However, the performance and stability of the system failed at the higher OL due to the more increased ammonia and VFA concentration, and the greater dosages of trace elements did not enhance the process stability.

A Review on the Fate of Nutrients and Enhancement of Energy Recovery from Rice Straw through Anaerobic Digestion

Open field burning and tilling the rice straw (RS) back into the fields causes environmental threats by contributing to the increased greenhouse gas emissions. Energy and nutrient recovery from RS through anaerobic digestion (AD) is an effective solution for its utilization. Although RS has good methane potential, its characteristics make it a difficult substrate for AD. This paper reviews the characteristics of RS, mass balance, and distribution of nutrients into liquid and solid digestate in the AD. The present review also discusses the effect of temperature, co-digestion, mixing, inoculum, organic loading rate, recycling liquid digestate, the addition of trace elements, and their bioavailability on the enhancement of biogas/methane yield in the AD of RS. In addition, the digestion of RS at various scales is also covered in the review.

Enhancement of methane production and antibiotic resistance genes reduction by ferrous chloride during anaerobic digestion of swine manure

In this study, effects of ferrous chloride (FeCl₂) addition on methane production and antibiotic resistance genes (ARGs) reduction were investigated during anaerobic digestion (AD) of swine manure. FeCl₂ could both improve the accumulative methane production and reduce the abundance of total ARGs, i.e., the maximum increase of CH₄ production of 21.5% at FC5, and the maximum ARGs reduction of 33.3% at FC25. The reduction of pathogenic bacteria and metal resistance genes (MRGs) was enhanced. Acetate and propionate utilization were intensified by enhancing H₂ utilization and direct interspecies electron transfer (DIET), where DIET was further enhanced by the reaction of the FeCl₂ and acetic acid. The bacterial community played important role in the evolution of ARGs (68.26%), which were also affected by MRGs, mobile genetic elements (MGEs), and environmental factors. Therefore, FeCl₂-based AD is a feasible and attractive way to improve methane production and ARG reduction.

Enhancing anaerobic digestion of dairy and swine wastewater by adding trace elements: evaluation in batch and continuous experiments

Trace elements play a critical role for microbial activity in anaerobic digestion (AD) but their effects were probably overestimated in batch tests and should be comparably evaluated in continuous systems. In this study, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ were added in different concentrations to manure wastewater, and the effects were compared in both batch and continuous systems. The results were used to demonstrate suitable trace element compositions for AD of dairy and swine wastewater, and to compare the outcomes from batch and continuous systems. Fe²⁺ and Zn²⁺ were identified as being the most efficient stimulant of dairy and swine wastewater respectively. The addition of 5 mg/L Fe²⁺ and 0.4 mg/L Zn²⁺ increased the batch specific methane yield by 62% and 126% for dairy and swine wastewater, respectively. Nevertheless, a lower increment of 2% and 21%, for dairy and swine wastewater was obtained in the 120-day continuously-fed experiments. The 16S rRNA gene sequencing results indicated a relationship between the methanogens population, specific methanogenic activities, propionate, and dissolved hydrogen. Conclusively, the addition of a low dosage of Fe²⁺ and Zn²⁺ is a feasible strategy to enhance the methanogenic metabolism of the AD of dairy and swine wastewater respectively.

Impact of Fe and Ni Addition on the VFAs' Generation and Process Stability of Anaerobic Fermentation Containing Cd

The effects of Cd, Cd + Fe, and Cd + Ni on the thermophilic anaerobic fermentation of corn stover and cow manure were studied in pilot experiments by investigating the biogas properties, process stability, substrate biodegradation, and microbial properties. The results showed that the addition of Fe and Ni into the Cd-containing fermentation system induced higher cumulative biogas yields and NH₄⁺–N concentrations compared with the only Cd-added group. Ni together with Cd improved and brought forward the peak daily biogas yields, and increased the CH₄ contents to 80.76%. Taking the whole fermentation process into consideration, the promoting impact of the Cd + Ni group was mainly attributed to better process stability, a higher average NH₄⁺–N concentration, and increased utilization of acetate. Adding Fe into the Cd-containing fermentation system increased the absolute abundance of Methanobrevibacter on the 13th day, and Methanobrevibacter and Methanobacterium were found to be positively correlated with the daily biogas yield. This research was expected to provide a basis for the reuse of biological wastes contaminated by heavy metals and a reference for further studies on the influence of compound heavy metals on anaerobic fermentation.

Effect of Zn Addition on the Cd-Containing Anaerobic Fermentation Process: Biodegradation and Microbial Communities

Anaerobic fermentation is considered as a cost-effective way of biomass waste disposal. However, the compound heavy metals contained in the biomass may induce complex effects on anaerobic fermentation, which limit the utilization of metal-contaminated biowaste. In this study, the impacts of Cd and Zn addition on biogas properties, process stability, substrate biodegradation, enzyme activity, and microbial properties were studied. The results showed that the addition of Cd together with Zn (Cd+Zn) increased the maximum daily and cumulative biogas yields, and brought forward the gas production peak compared with the Cd-added group. Taking the whole fermentation process into account, the promotion effects of adding Zn into the Cd-containing fermentation system on biogas yields were mainly attributable to better process stability, higher average NH₄⁺-N concentration in the later stage of fermentation, reduced COD (p < 0.05), and increased biodegradability of lignocelluloses (p < 0.01), especially cellulose (p < 0.05) and lignin (p < 0.01). Meanwhile, the addition of Zn promoted the coenzyme M activity (p < 0.05), and increased the absolute abundance of Methanothermobacter. The bacteria communities during the fermentation process were responsible for the degradation of lignocelluloses. The results demonstrated that the addition of appropriate Zn into the Cd-containing fermentation system enhanced the efficiency of anaerobic fermentation and utilization of biowaste.

Process Analysis of Anaerobic Fermentation Exposure to Metal Mixtures

Anaerobic fermentation is a cost-effective biowaste disposal approach. During fermentation, microorganisms require a trace amount of metals for optimal growth and performance. This study investigated the effects of metal mixtures on biogas properties, process stability, substrate degradation, enzyme activity, and microbial communities during anaerobic fermentation. The addition of iron (Fe), nickel (Ni), and zinc (Zn) into a copper (Cu)-stressed fermentation system resulted in higher cumulative biogas yields, ammonia nitrogen (NH₄⁺-N) concentrations and coenzyme F₄₂₀ activities. Ni and Zn addition enhanced process stability and acetate utilization. The addition of these metals also improved and brought forward the peak daily biogas yields as well as increased CH₄ content to 88.94 and 86.58%, respectively. Adding Zn into the Cu-stressed system improved the abundance of Defluviitoga, Fibrobacter and Methanothermobacter, the degradation of cellulose, and the transformation of CO₂ to CH₄. The bacterial and archaeal communities were responsible for the degradation of lignocelluloses and CH₄ production during the fermentation process. This study supports the reutilization of heavy metal-contaminated biowaste and provides references for further research on heavy metals impacted anaerobic fermentation.

Cadmium Addition Effects on Anaerobic Digestion with Elevated Temperatures

Anaerobic fermentation with biogas as an energy source is influenced by the presence of heavy metals. However, the availability of the heavy metals is dependent on the digestion temperature. In this study, the impacts of Cd on the characteristics of biogas, substrate biodegradation, and enzyme activity during anaerobic co-digestion were investigated under varying digestion temperatures. The results showed that 1 mg/L initial Cd concentration improved cumulative biogas yields by 404.96%, 16.93%, and 5.56% at 55 °C, 45 °C, and 35 °C, respectively. In contrast, at low temperatures (25 °C), the yield decreased by 0.77%. In the 55 °C group, Cd addition improved the activity of cellulase (p < 0.05) and coenzyme F420 (p < 0.01). The total chemical oxygen demand (COD) during the peak period and the transformation of hydrolytic organic components into volatile fatty acids (VFAs) influenced the CH4 and biogas yields. There were no significant differences in cellulase, dehydrogenase, and coenzyme F420 activities with or without Cd addition when the digestion temperature was 45 °C, 35 °C, and 25 °C. Therefore, thermophilic digestion is recommended for the efficient degradation of Cd-contaminated biowaste. Moreover, the impact of metals on the performance of anaerobic digestion should be considered together with temperature conditions in future research and practice.

Heavy metals interact with the microbial community and affect biogas production in anaerobic digestion: A review

Heavy metals (HMs), which accumulate in digestion substrates, such as plant residues and livestock manure, can affect biogas yields during anaerobic digestion (AD). Low concentration of Cu²⁺ (0-100 mg/L), Fe²⁺ (50-4000 mg/L), Ni²⁺ (0.8-50 mg/L), Cd²⁺ (0.1-0.3 mg/L), and Zn²⁺ (0-5 mg/kg) promote biogas production, while high concentrations inhibit AD. Trace amounts of HMs are necessary for the activity of some enzymes. For example, Cu²⁺ and Cd²⁺ serve as cofactors in the catalytic center of cellulase and stimulate enzyme activity. High contents of Cd²⁺ and Cu²⁺ inhibit enzyme activity by disrupting protein structures. Trace amounts of HMs stimulate the growth and activity of methanogens, while high levels have toxic effects on methanogens. HMs affect the hydrolysis, acidification, and other biochemical reactions of organics in AD by changing the enzyme structure and they also impact methanogen growth. A better understanding of the impact of HMs on AD can provide valuable insights for improving the digestion of poultry manure and plant residues contaminated with HMs, as well as help mitigate HMs pollution. Although several studies have been conducted in this field, few comprehensive reviews have examined the effect of many common HMs on AD. This review summarizes the effects of HMs on the biogas production efficiency of AD and also discusses the effects of HMs on the activities of enzymes and microbial communities.

Synergistic effect of alkaline pretreatment and magnetite nanoparticle application on biogas production from rice straw

Agricultural residues have high potential for biogas production, complex lignocellulosic structure is however the main hindrance in their bioconversion. This research focuses on combined effect of alkaline pretreatment of rice straw and magnetite (Fe₃O₄) nanoparticle application. Four doses of magnetite nanoparticles viz. 60, 80, 100 and 120 ppm were used in the anaerobic digestion of untreated and 2% NaOH pretreated rice straw. Compared to control, 2% NaOH pretreatment alone increased biogas and methane yield by 57 and 60% respectively. Magnetite nanoparticle (MNP) application alone gave maximum yield at 100 ppm which consisted of 37 and 33% more biogas and methane yield respectively. Combining the effect of 2% NaOH pretreatment and 120 ppm MNPs synergistically increased biogas and methane yield by 100 and 129% as compared to control. In addition, an energy assessment indicated a positive net gain of 3765 kJ for 2% NaOH pretreated rice straw with 120 ppm MNPs.

A Preliminary Study of the Effect of Bioavailable Fe and Co on the Anaerobic Digestion of Rice Straw

Rice straw is an abundant and sustainable substrate for anaerobic digestion (AD), but it is often deficient in essential trace elements (TEs) for proper microbial growth and metabolism. A lack of TEs leads to AD imbalances and suboptimal biogas yields. However, the total TE concentration is not a sufficient indicator of the amount of TEs available to the microorganisms. Therefore, this study investigated the degree of bioavailability of iron (Fe) and cobalt (Co) during the AD of rice straw, and correlated it to the biomethane yields and volatile fatty acids (VFAs) produced. When the two TEs were dosed at 205 µg Fe/g TS and 18 µg Co/g TS of rice straw, the biomethane production was approximately 260 mL CH4/g VS, i.e., similar to that obtained when Fe and Co were not added. Despite an increased bioavailable fraction of 23 and 48% for Fe and Co, respectively, after TEs addition, the AD performance was not enhanced. Moreover, VFAs did not exceed 250 mg HAc/L both in the presence and absence of added TEs, confirming no enhancement of the methanogenesis step. Therefore, the bioavailability of Fe and Co was not a limiting factor for the biomethane production at low total VFAs concentration.

Process analysis of anaerobic fermentation of Phragmites australis straw and cow dung exposing to elevated chromium (VI) concentrations

Anaerobic fermentation is considered as a cost-effective way of biomass waste disposal. Chromium (Cr) is one of the heavy metals that often been blamed for unsatisfactory operation or failure of anaerobic fermentation. The impact of Cr (added as K₂Cr₂O₇) on mesophilic anaerobic fermentation of Phragmites australis straw and cow dung was demonstrated by investigating the biogas properties, process stability, substrate degradation and enzyme activities during the fermentation process. The results showed that 30, 100 and 500 mg/L Cr⁶⁺ addition increased the cumulative biogas yields by up to 19.00%, 14.85% and 7.68% respectively, and brought forward the daily biogas yield peak. Meanwhile, the methane (CH₄) content in the 30 (52.47%) and 100 (40.57%) mg/L Cr⁶⁺-added groups were generally higher than the control group (37.70%). Higher pH values (close to pH 7) and lower oxidation-reduction potential (ORP) values in the Cr⁶⁺-added groups after the 15th day indicated the better process stability compared to the control group. Taking the whole fermentation process into account, the promoting effect of Cr⁶⁺ addition on biogas yields was mainly attributable to better process stability, the enhanced degradation of lignin and hemicellulose, the transformation of intermediates into VFA, the higher coenzyme F₄₂₀ activities and the efficient generation of CH₄. These results demonstrate that an appropriate addition of Cr⁶⁺ could enhance the anaerobic fermentation which support the regulations utilizing of the Cr⁶⁺ contaminated biowaste.

Trace elements dosing and alkaline pretreatment in the anaerobic digestion of rice straw

The effect of trace elements (TEs) addition and NaOH pretreatment on the anaerobic digestion of rice straw was investigated in batch tests. Co, Ni and Se were added to the raw rice straw at different dosages. The NaOH pretreatment was applied to the rice straw both alone and in combination with the addition of TEs, in order to evaluate potential synergistic effects of the pretreatment and the TEs supplementation on the biogas production yields. The results obtained showed that the alkaline pretreatment was more effective than the TEs addition in increasing the cumulative biogas production, causing a 21.4% enhancement of the final biomethane yield, whereas the increase due to TEs dosing was not statistically significant. The analysis of volatile fatty acids (VFAs) confirmed that the NaOH pretreatment resulted in a higher production of VFAs, indicating an increased hydrolysis, while TEs addition did not cause significant changes in the VFA concentrations.