Biogas Production from Organic wastes and Iron as an Additive – A Short Review

The world is facing a serious energy crisis and environmental pollution problems due to a sharp increase in the world population. Bioenergy is an eminent solution to these problems. Anaerobic digestion is a green energy technology used worldwide for the conversion of organic waste to biogas. It is reported that organic wastes are hard to digest and need some technical improvement in the anaerobic digestion process to improve biogas yield. Iron-based additives, due to their electron acceptance and donation capabilities, have been emphasized as being exceptional in improving anaerobic digestion process efficiency amongst all other enhancement options. This study reviews the major available types of iron-based additives, their characteristics, and their preparation methods. The preferred iron-based additive that has a significant effect on the enhancement of biogas yield is also discussed. The use of iron-based additives in the anaerobic digestion process with varying dosages and their impact on the biogas generation rate is also being studied. Substrates, operating parameters, and types of anaerobic digesters used in recent studies while researching the effects of iron-based additives are also part of this review. Lastly, this study also confirms that iron-based additives have a significant effect on the reduction rate of the volatile suspended solids, methane content, biogas yield, and volatile fatty acids.


-INTRODUCTION:-
Due to a significant increase in the world population during the last decade, lots of problems have arisen, including deforestation, increased energy requirements, environmental pollution, and global warming. Around 82% of the world's energy requirements are fulfilled by burning conventional fossil fuels like natural gas, oil, and coal, which are the main causes of environmental pollution and increased GHG emissions in the atmosphere, resulting in global warming (BP, 2019; World Bioenergy Association (WBA), 2019). Bioenergy is a prominent option to mitigate these environmental concerns and to reduce dependency on fossil fuels. It is obtained by the biological conversion of biomass (organic matter) into biofuels (Basu, 2018).

Figure 1: World Energy Mix -World Bioenergy Association (WBA) Report-2019
Biomass is considered a renewable energy resource as green plants receive power from sunlight to initiate chemical reactions that store energy. The consequence of these chemical reactions results in the generation of numerous types of energy-rich resources, including plants and trees, forestry residues, fruits and vegetable residues, and animal wastes (Maletta & Díaz-Ambrona, 2020). Technologies used for biological conversion of organic matter are combustion, gasification, pyrolysis, anaerobic digestion, and microbial fermentation, and the biofuels obtained as a result are syngas, bio-oil, bio-diesel, charcoal, biogas, methanol, and ethanol (Gao et al., 2018). Biogas is one of the cleanest sources to generate electricity and for cooking and heating applications. The preferable technology used for biogas generation is anaerobic digestion (Omer, 2012).
The anaerobic digestion (A.D.) process is an innovative technology for waste management that involves the conversion of organic wastes to biogas in the absence of oxygen. Organic content present in organic waste contains a large amount of lignocellulosic matter, which is hard to digest due to its complex structure. This leads to low substrate biodegradability and, ultimately, longer time required for digestion . Pretreatment of organic wastes has proven to be a feasible option to improve process performance by facilitating lignocellulosic breakdown, enhancing biogas yield, and reducing hydraulic retention time (HRT) (Carlsson, 2015). Pretreatment has a number of drawbacks, including a high energy cost and suppression of the methanogenic process owing to excessive salt production and an extreme pH value (Zhang et al., 2016).
Anaerobic decomposition driven by microbiology is a sophisticated, multi-stage set of biochemical events. Bacteria and Archaea are two species of microorganisms that primarily digest organic material, resulting in biogas production. Because these microorganisms are oxygen-intolerant, the process takes place in an anoxic environment.
The final product contains approximately 60% methane, 30%-40% carbon dioxide, and trace quantities of gaseous water, hydrogen sulfide, and ammonia (Liu et al., 2011). Hydrolysis, acidogenesis, acetogenesis, and methanogenesis are the four steps of the AD process, as shown in Fig. 2.  (Ogejo, 2013) Hydrolysis is the hydrodriven lysis of bulk molecules into soluble monomers. Carbohydrates, polysaccharides, proteins, nucleic acids, lipids, and other insoluble high-molecular-mass components are broken down by enzymes.
Acidogenesis includes acid-forming essential and heterotrophic anaerobes convert the soluble end-products of the hydrolysis. Acetic acid, butyric acid, propionic acid, ethanol, carbon dioxide, and hydrogen are some of the end products. Acetogenesis is the third phase, which consists of anaerobic oxidation processes. Acetate is created from the alcohol and acids released during acidogenesis. Methanogens mediate the creation of methane from acetate (aceticlastic methanogenesis), carbon dioxide, and hydrogen gas during this stage (hydrogenotrophic methanogenesis). Methane-forming bacteria do not use any of the chemicals from previous phases that have not been converted to acetate, thus they accumulate in the digester (Ganzoury & Allam, 2015).
According to previous studies, adding additives to the A.D. process speeds up starting, decreases contaminants, enhances biogas rate of production, lowers HRT, replenishes lacking nutrients, raises methane content, and improves reactor stability (Kim et al., 2017;Ward et al., 2008). Additives used for the A.D. process cover a wide variety of materials and can be chemical, biological, organic, or inorganic, and can also have variability in their size (Abdelwahab et al., 2020). Nickel, Cobalt, Iron, Selenium, Sulfur, and Carbon, etc. can be used as an additive for improvement of the A-D process (Samson Nnaemeka . Iron is one of the cheapest ways to boost methane production from anaerobic digestion since industrial businesses create around 18,895 thousand tons of iron waste each year, with only 8000 thousand tons recycled. The remaining leftover iron scrap is disposed of in landfills (BP, 2019). If iron proves to be a critical answer for enhancing biogas yield, then biogas from the anaerobic digestion process will be a cost-effective solution, as we have approximately 10,000 tonnes of waste iron ready to be used as an addition in the A-D process.
Iron is a vital element for the methanogenesis process as it replicates the DNA structure and provides an environment for the survival of cells. Iron has exclusive ionization ability for Fe 2+ and Fe 3+ and it acts as both a donor and an acceptor of electrons. Iron-based additives have been reported to have unique advantages amongst all, like supplementation of nutrients, improvement of methane yield, improvement of substrate digestibility, control of H2S toxicity, etc. . Iron-based additives can also improve the hydrolysis enzyme activity by reducing oxidative-reductive potential and support the biological growth of enzymes (Feng et al., 2014). This article highlights all of the key advancements in biogas production that have been made by employing iron as an addition for the A-D process. This study also goes into different types of iron that may be used as an additive, substrates that can handle iron additives, optimum operating conditions with iron additives, digester types that can be employed, and the pros and disadvantages of employing iron as an additive. Figure 3 summarized the effects of additives in anaerobic digestion process. In commercial full-scale AD plants, process stability is a critical concern, as inadequate process stability typically leads to uneven methane output. In addition, prolonged periods of instability might lead to process failure. As a result, a number of research projects have been undertaken to address the causes of AD instability. Different forms of inorganic component and packing materials (clay materials and zeolites) are often used to immobilize and retain microbial communities to manage and diminish AD inhibition. Depending on particle size, porosity, surface area, absorbent capacity, and electric conductivity, different materials may influence the abundance and retention of diverse microbial communities at their surface. The AD process's reaction kinetics are considerably improved by the support materials, allowing for higher COD conversion rates even at high OLR and low HRT. By preserving the microbial flora, removing inhibitory elements, and serving as catalysts in metabolism, support materials play a critical role in manipulating the reaction kinetics of even resistant substrates and methane production (Arif et al., 2018).

-BENEFITS OF ADDTIVE MATERIALS: -
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 5 January 2022 Arif et al. 2018(Arif et al., 2018 This research is categorized into five areas, each of which covers a unique prospect of iron-based additions in anaerobic digestion. Before digging into the details of iron-based additions and their effects on the A-D process, an overview of the world population and its consequences, the demand for renewable energy, the importance of biofuels, and their environmental effects were investigated. The A-D process and its enhancement methods were also investigated. Finally, the significance of iron as an addition, as well as its influence on A-D process performance, is discussed. Figure 4 depicts the approach and methods used in conducting this review. The global energy dilemma was initially discussed, followed by biomass energy prospects as a substitute for fossil fuels, biodegradation of biomasses challenges, and prospective intensification approaches for biogas production. On one hand, as shown in Table 1, reviews of the most often used iron-based additives for anaerobic digestion improvement were identified and classified into iron scraps and wastes, iron powders, iron nanoparticles, and the Fenton process. The impact of iron supplementation on biogas generation and methane content, solid reduction, pathogen reduction, and other topics were discussed. Finally, the development of iron precipitates and their fate following digestate disposal were discussed. It was suggested that future research into suitable additives for preventing iron complexes be conducted.

3-IRON AND IRON-BASED ADDITIVES: -
Because of its conductive qualities and inexpensive cost, iron has become one of the most popular additions for improving anaerobic digestion efficacy. The ability of iron to lower the oxidative-reductive potential (ORP) of anaerobic digestion media and thus provide a more favorable environment for anaerobic digestion; and (ii) its role as a cofactor of several key enzymatic activities, such as pyruvate-ferrodoxin oxidoreductase, which contains Fe-S clusters and plays a key role in fermentation Anaerobic digestion has been reported to be stimulated by various iron types. On the one hand, Fe (III) reduction is a good way to oxidize organics directly into simple molecules.
Nonetheless, because Fe (III) reduction is more thermodynamically advantageous than methanogenesis, it can limit the conversion of organics to methane. Due to its ability to operate as an electron donor, Fe 0 (also known as zero valent iron (ZVI)) has been discovered to speed up the hydrolysis and fermentation processes.
Previous studies show that different types of iron additives can be used in the A-D process for efficiency enhancement, nutrients supplement, and to improve substrate digestibility. Iron-based additives can be prepared by adopting different methods before using them into the A-D process. Most commonly used iron-based additives are Waste iron scraps (Wiss), Iron Nanoparticles (Fe NPs), Iron chlorides (FeCl2,FeCl3), Zero Valent Scrap Iron (ZVSI), Iron oxides (Fe2O3,Fe3O4), Iron powder (Fe powder), Zero Valent Iron (ZVI), Iron sulfate (FeSO4) and NanoZero Valent Iron (NZVI). Table.1 depicts various types of iron-based additives reported until now and their preparation method. Fe3O4 additive to the A-D process reported having a significant effect on biogas yield followed by its nanoparticles (Fe3O4 NPs), Iron powder, and Iron nanoparticles. These additives also help to improve substrate digestibility by decomposition of lignocellulosic biomass into a simple structure (Zhao et al., 2017). It is also reported that Fe3O4 NPs produces higher biogas yield as compared to NZVI (Abdelsalam et al., 2017). The highest biogas yield ever reported from the A-D process is also achieved using Fe3O4 NPs additives (Casals et al., 2014). Studies show that

-SUBSTRATES:-
Various substrates can be used in the A-D process for biogas production. Lots of studies have been done that show that iron additives can increase the biogas yield and improve the process stability by utilizing different substrates. Ali et al. (2017) reported the use of Municipal Solid Waste (MSW) as a substrate for the A-D process with Fe3O4 nanoparticles additive. He suggested that 75mg/L concentration of iron-based additive had a 72.09% improvement in methane generation. Abdelsalam et al. (2017) investigated the effect of iron nanoparticles and iron oxide nanoparticles on biogas and methane production using cattle dung slurry and found that Fe3O4 NPs having 20mg/L concentration result in 65.6% increase in biogas production. Wei et al. (2018) reported the use of ZVI to enhance the methane generation using wastewater sludge as a substrate for the A-D process. He found that there was a 26.9% relative increase in methane yield using ZVI as an additive because it improved the hydrolysis rate of reaction. Noonari et al. (2019) tested the use of canola straws and banana plant waste as Co-Digestion with buffalo dung as inoculum with Fe3O4 nanoparticles additive and found that methane generation from canola straw and buffalo dung was increased by 39.1% with 0.81 mg dosage of iron-based additive. Farghali et al. (2020) reported the use of waste iron powder and iron oxide nanoparticles with cattle manure substrate ( F/I ratio 2:1) and found that methane yield from cattle manure increased by 57% and H2S production rate also reduced by 77.2% using waste iron powder additive with a concentration of 1000 mg/L. Cheng et al. (2020) investigated the effect of rusted iron shavings on a mixture of food wastes and municipal sludge and noted that rusted iron shavings increase the removal rate of volatile suspended solids by 18% and methane yield by 64.4% compared with the case in which no iron shavings added. It also produces a reductive environment which leads to an increase in methane yield.
Previous studies have proved that Fe3O4 and its nanoparticles additives have a significant effect in the enhancement of methane and biogas generation using various substrates like cattle dung, municipal solid waste, wastewater sludge, canola straws and banana plant waste with buffalo dung inoculum, food wastes and municipal sludge waste used in different anaerobic digesters and varying operating parameters.

-OPERATING PARAMETERS & DIGESTER TYPE:-
Previous studies have proven to be very effective in improving the methane generation, process stability, and digestion rate. All these improvements are subjected to the optimum selection of the anaerobic digester and operating parameters of the A-D Process. The batch reactor is the preferable mode for the anaerobic digestion process. Table 2 presents suitable operating parameters for the A-D process and their effects on biogas and methane yield for all the previous studies mentioned above.

-BENEFITS OF IRON BASE ADDITIVES:-
Iron can act as an electron acceptor as well as an electron donor in the redox reaction of the A-D process that's why iron-based additives can be used for the improvement of methane generation. Iron-based additives have lots of advantages like improvement of methane yield, control of H2S inhibition, supplementation of nutrients, and much more. This section highlights the impact of iron-based additives on Volatile Fatty Acids (VFAs), reduction of Volatile Suspended Solids (VSS), and biogas content.

5.1-METHANE CONTENT & BIOGAS OUTPUT: -
Iron based additives can promote the total methane generation because iron based additives promote the production of acetate which is an essential input to produce methane through methanogenesis process. Iron based additives can also act as donor of electrons directly, which causes the reduction of carbon dioxide into methane through hydrogenotrophic methanogenesis triggering the enhancement of methane generation (Abdelsalam et al., 2017).
Amen et al. (Amen & Eljamal, 2017) examined the effects of a pure nZVI, nZVI coated (ICZ), and nZVI mixed with zeolite (IMZ) augmented anaerobic response to a control bioreaction. The addition of 1000 mg/L iron-based nanoparticles to ICZ helped achieve the largest cumulative methane volume. The inclusion of nZVI and IMZ, on the other hand, resulted in upward stimulation of greatest methane level of 88 percent and 74 percent, respectively.
Similarly, the excess hydrogen released by nZVI (30 mol/L) oxidation enhanced hydrogenotrophic methanogenesis, resulting in a 30% increase in biogas output (He et al., 2017). The addition of a larger dose of nZVI/CUo (>1500 mg/L) on the other hand, decreased biogas generation (Amen et al., 2018).
Previous studies show that iron-based additives have a very significant impact on enhanced biogas production and the improvement of methane content. Ali et al. (2017) reported 72.1% escalation in methane content, Abdelsalam et al. (2017) reported 65.6 % increase in biogas production, Farghali et al. (2020) reported 56.9% rise in methane generation and Cheng et al. (2020) reported 64.4 % increase in methane generation using various iron base additives.

5.2-AVOIDING VFAS:-
Excess VFAs are produced when substrates are hydrolyzed; this may lead to a lowering of pH and finally inhibition of methanogens process. This problem can be solved using iron-based additives in the A-D process (Zhang et al., 2016). Feng et al. (2014)  acids (VFAs) are critical in the biogas production process during anaerobic digestion. They are carboxylic shortchain fatty acids (C2-C6) generated by syntrophic bacteria following biomass hydrolysis. In an anaerobic digestion process, several parameters like as pH and the inclusion of iron-based supplements might impact VFAs. At a pH below 2, propionic acid and butyric acid, the two primary VFA products in the acidification step, decompose exclusively into acetate. Propionate accumulation and disruption of pH balance between acidogenesis and methanogenesis are caused by the high rate of propionate production and low decomposition rate (much lower than other VFAs), but according to Zhang, Zhang, and Li (2015), the addition of trace iron could stabilise anaerobic digestion processes. In tests done by Hao (Hao et al., 2017), the addition of WIS enhanced the elimination of VSS by 4.5 percent and 14.0 percent of the additional raw substrate VSS, respectively, at both the acidogenic and methanogenic stages of anaerobic digestion. A comparable study found that removing solids with iron shavings resulted in carbohydrate ingestion of 75 to 98 percent. According to Zhang et al. (2014), the iron-enhanced reactors using rusty scraps, Fe powder, and clean scrap had the maximum VSS reduction rate of 48.27 percent. According to Amen et al. (2017), the addition of nZVI, IMZ, and ICZ to the substrate digestion of volatile solid fraction resulted in a volatile solid reduction of up to 23.8 percent, and that for up-scaling, 1 g of volatile solids will require 2.67 g of ICZ particles to achieve maximum solid removal and methane yield (Amen & Eljamal, 2017).

4-CONCLUSIONS:-
The addition of iron-based additives to an anaerobic digester can have significant effects like enhancement of biogas generation, increased VSS rate of reduction, improvement of process stability, and reduction of VFAs. A detailed literature analysis has been conducted for a better understanding of recent advancements in this field. Previous studies related to iron-based additives in the A-D process have been studied and are presented in this study. This study has discussed the following topics in detail: • types of iron-based additives, their source, preparation method, color, and size.
• Substrates reported in previous studies to check the effect of iron-based additives • Optimal operating parameters using iron-based additives • preferred type and mode of anaerobic digester.
• Benefits of using iron-based additives include methane content, VFA limitation, and VSS reduction rate.
This study examined all these issues and concluded that Fe3O4 and Fe3O4 NPs additives had a considerable effect on biogas yield augmentation when compared to other iron-based additives. Most studies choose a mesophilic temperature range for the A-D process, mixing of the substrate at varying rpm to homogenize the substrate and inoculum, batch-mode anaerobic digesters for research reasons, and HRT ranging from 40 to 100 days depending on the nature of the study. Finally, it has been established that iron-based additions considerably improve the rate of biogas generation and increase the rate of VSS reduction. Since no essential research work has been done in these areas, it is recommended that future researchers investigate the effects of variations in organic load rating (OLR) and thermophilic operating temperature on the anaerobic digestion process utilizing iron-based additions.

Acknowledgement:
The author thanks U.S. Pakistan Center for Advanced Studies in Energy (USPCAS-E), National University of Science and Technology (NUST) Islamabad to carry out this research work.

Funding:
No funding source Availability of data and materials: The required supplementary data is available as per requirement.

Ethics:
The authors declare that they have no competing financial and personal interests