Waste-to-energy nexus for circular economy and environmental protection: Recent trends in hydrogen energy

Journal Pre-proof Waste-to-energy nexus for circular economy and environmental protection: Recent trends in hydrogen energy

Surbhi Sharma, Soumen Basu, Nagaraj P. Shetti, Tejraj M. Aminabhavi PII:

S0048-9697(20)30143-1

DOI:

https://doi.org/10.1016/j.scitotenv.2020.136633

Reference:

STOTEN 136633

To appear in:

Science of the Total Environment

Received date:

24 October 2019

Revised date:

20 December 2019

Accepted date:

9 January 2020

Please cite this article as: S. Sharma, S. Basu, N.P. Shetti, et al., Waste-to-energy nexus for circular economy and environmental protection: Recent trends in hydrogen energy, Science of the Total Environment (2020), https://doi.org/10.1016/j.scitotenv.2020.136633

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© 2020 Published by Elsevier.

Journal Pre-proof Waste-to-Energy Nexus for Circular Economy and Environmental Protection: Recent Trends in Hydrogen Energy Surbhi Sharma1, Soumen Basu1*, Nagaraj P. Shetti2*, Tejraj M. Aminabhavi3 1

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School of Chemistry and Biochemistry, Thapar Institute of Engineering and Technology, Patiala-147004, India. 2 Center for Electrochemical Science and Materials, Department of Chemistry, K.L.E. Institute of Technology, Hubballi-580 030, Karnataka, India. 3 Pharmaceutical Engineering, SET’s of Pharmacy, Dharwad 580 002, Karnataka, India.

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Abstract

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The energy demand has increased exponentially worldwide owing to the continuously

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growing population and urbanization. The conventional fossil fuels are unable to satiate this requirement for price inflation and significant environmental damage due to unrestrained

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emission of greenhouse gases. The focus now has shifted towards alternative, economical,

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renewable and green sources of energy such as hydrogen to deal with this bottle-neck.

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Hydrogen is a clean energy-source having high energy content (122 kJ/g). Recently, biological methods for the hydrogen production have attracted much attention because traditional methods are expensive, energy-exhaustive and not eco-friendly. The employment of biological methods promises utilization of waste or low-value materials for producing energy and building waste-to-energy nexus. Around 94% of the waste is discarded precariously in India and waste generation is growing at an alarming rate of 1.3% per year. The “waste-to-energy” techniques follow „Reuse, Reduce, Recycle, Recovery and Reclamation‟ system solving three subjects at once; waste-management, energy-demand and 1

Journal Pre-proof environmental concern. Moreover, these methods have easy operability, cost-effectiveness and they help to shift from linear to circular model of economy for sustainable development. Biological processing of waste materials like agricultural discard (lignocellulosic biomass), food-waste and industrial discharge can be used for biohydrogen production. Dark and photo fermentation are the chief biological processes for the transformation of organic substrates to hydrogen. Dark fermentation is the acidogenic fermentation of

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carbohydrate-rich materials without light and oxygen. Clostridia, Enterobacter and Bacillus

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spp. are appropriate heterotrophic bacteria for dark fermentation. Various pretreatment methods like heat treatment, acid or base treatment, ultrasonication, aeration, electroporation,

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etc., can be applied on inoculums to increase H2 producing bacteria eventually improving the

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hydrogen yield. However, only around 33% of COD in organic materials is transformed to H 2

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by this method. Photofermentation by the photosynthetic non-sulfur bacteria (PNS) converts organic substrate to H2 and CO2 in the presence of nitrogenase enzyme in ammonium-limited

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and anoxygenic conditions. Rhodobacter or Rhodopseudomonas strains have been widely

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examined in this regard. But these methods are only able to produce H2 with a poor yield. Combining dark and photofermentation is a noteworthy alternative for procuring enhanced

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hydrogen yields. Two-stage sequential method utilizes volatile fatty acids accumulated as byproducts after dark ferementation (in the first stage) for photofermentation by suitable bacteria (in the second stage). A proper investigation of the dark fermenter effluents is required before using them as a substrate for photo-fermentation. In a single-stage dark and photofermentation, co-culture of anaerobic and PNS bacteria in a single reactor is carried out for obtaining improved yield. The single stage system is comparatively inexpensive and less laborious; moreover, a limited requirement for an intermediate dilution stage is necessary.

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Journal Pre-proof Economic analysis of hydrogen production showed that H2 production by the present methods, save pyrolysis, is reasonably higher than the conventional approaches of fuel production. Probable routes to make H2 production more cost-effective are reducing the cost of photobioreactor, installing proper storage system, etc. A constructive effort in the area of research and development of biological approaches of H 2 production technologies is vital. The commercial viability of biohydrogen production is imperative for accomplishment of

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circular economy system and sustainable development.

Corresponding author: [email protected] (N. P. Shetti); [email protected]

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*

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Keywords: Energy; hydrogen; waste; biological; fermentation; economical.

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(Soumen Basu)

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Journal Pre-proof 1. Introduction The rapid rise in global-population and the consequent growth in industrialization has led to a tremendous increase in the world energy demand over the recent years. The economic evolution of any country depends on energy. Conventionally, the main resources to satiate the energy demand are fossil fuels viz., coal, petroleum and natural gas. But the non-renewable nature of fossil fuels is worrisome as they are getting exhausted thereby posing grievous

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challenges like fuel demand, climate change and economic concerns (Turhal et al., 2018;

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Mishra et al., 2019a; Mishra et al., 2019b; Mishra et al., 2019c; Mehta et al., 2019). The disproportionate usage of fossil fuels causes the releases of harmful gases like CO2, CH4,

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NOx, SOx, etc, leading to environmental problems like global warming, acid rain, climate

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change, and loss of biodiversity affecting the living beings significantly and jeopardizing the

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environment (Sheth and Babu, 2010). Moreover, uncontrolled CO2 release in the environment is worrying for the living environment as the current level of CO2 is 394.5 ppmv (parts per

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million volume), which is estimated to increase to 500 ppmv by 2050 if the same unrestrained

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emission is continued (Caspeta et al., 2013). Worldwide consumption of 15 TW energy in 2011 is estimated to rise up to 30 TW by 2050 due to escalation in the population (Dincer and

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Acar 2015). To achieve the target of reaching a low-carbon power, proficient technologies for energy production, conversion, storage, and distribution are essential. Economic concerns such as supply risk and high costs of fuels lead to more and more regular financial and economic uncertainties, especially in the underdeveloped nations (Geissdoerfer et al., 2017). The investigations of alternative energy strategies are therefore crucial for future stability of the world. The focus is now moving towards green and clean energy technologies for the environmental protection and sustainable development. Extremely proficient technologies are

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Journal Pre-proof being explored that satiate the energy requirement without destructively affecting the environment. The alternative renewable fuels such as hydro, wind, solar, biomass and hydrogen energy are inexpensive and eco-friendly to increase the energy security (Xu et al., 2019; Kamyab et al., 2019). Hydrogen is a possible alternative substitute for traditional fossil fuels on account of its clean, renewable nature and high energy content (122 kJ/g). It can be obtained from natural

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and bioresources, presumed to be a secondary form of energy. The energy yield in case of H2

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fuel is 2.75-times superior than hydrocarbon fuels. Hydrogen is a clean fuel without emission of CO2 and other toxic gases; water is the chief combustion product when H2 is used, which

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can be recycled to obtain more hydrogen. As a green energy policy issue, it has gained

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prominence as a fuel cell electricity generation, as a fuel in transportation system and

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combustion (Wadjeam et al., 2019). Besides, H2 gas is also utilized for hydrogenation of fats and oils for producing a variety of chemicals in petrochemical industries, desulfurization as

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well as re-formulation of gasoline in refineries, steel processing, etc, (Konkol et al., 2016;

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Abbasov et al., 2017; Koulocheris et al., 2019; Allen and Nelson, 2019). The hydrogen fuel cell (HFC) vehicles are three-times more efficient than the gasoline engines (Momirlan and

and

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Veziroglu, 2005). The hydrogen demand has increased by three-times since the mid seventies is still increasing as is evident

in Fig. 1 as per the website

report

(https://www.iea.org/hydrogen2019/). However, H2 gas is not naturally freely obtainable and conventional techniques for its production are not economical. Presently, most of the hydrogen generation is chiefly done through non-renewable resources (Mohan et al., 2007). Around 98% of H2 generation is by using fossil fuels; ~ 40% from natural gas or steam reforming of hydrocarbons, ~30% by means of oil, ~18% by means of coal, ~ 4% from water electrolysis and ~ 1% from biomass (Hosseini and Wahid, 2016). Some traditional methods 5

Journal Pre-proof for H2 generation are steam reforming of hydrocarbons, water-electrolysis, non-catalytic partial oxidation, autothermal reforming, gasification in addition to thermo-catalytic processes of fossil fuels like natural gas, naphtha, etc (Wu et al., 2015; Yang et al., 2019b; Guiberti et al., 2016; Blumberg et al., 2019; Yuksel et al., 2019; Ngo et al., 2019). These techniques are not only expensive and energy-exhaustive, but also have low-efficiency and are less eco-friendly as they produce CO2 and some other greenhouse gases as the byproduct. Therefore, these methods do not meet the criteria for sustainable development and

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circular economy.

Biological processes and techniques for hydrogen production are gaining attention in

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recent years owing to their comparatively environment-friendly and economical nature.

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Moreover, their less energy-intensive nature, ability to be operated at ambient temperatures

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and pressures is quite appealing (Lin and Lay, 2004). Employment of waste materials for generating H2 is an excellent choice as it encourages circular economy where recycling of

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waste would recycling help to generate energy.

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Wastes can be converted into energy via efficient waste-to-energy technologies.

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However, the mounting waste-generation would result on problems of landfilling and environmental pollution. Waste management is therefore quite challenging, which is being faced across the globe due to speedy and unforeseen urbanization, thereby agitating the municipalities. The generation of municipal solid waste is at a worrying degree that surpasses the ability of municipal authorities to properly collect, dispose and manage (lei Xiang et al., 2019; Singh, 2019). Municipal waste generation depends on economic growth, urbanization as well as industrial development (Adeogba et al., 2019). The growth rate of waste generation in India is 1.3% per year and ~94% of waste is discarded precariously (Shah and Shah, 2019).

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Journal Pre-proof It is estimated that there will be growth in the amount of food waste (FW) in the coming years owing to population growth and economic advancement, especially in Asia region. In about 70 Asian countries, the amount of metropolitan FW might upsurge from 278 to around 416 million tonnes from 2005 to 2025 (Kiran EU et al., 2014). This worrying situation needs appropriate technological administration for choosing ecological waste-to-energy processing techniques. Renewable techniques have already attracted the attention where biomass is used

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requirement, waste management, and global warming.

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as a feedstock for H2 production. This can concurrently resolve issues like energy-

Waste-to-energy is a very latest development, which is quite convincing and more

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importantly, it is a sustainable approach to manage the waste (Klinghoffer and Castaldi,

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2019). Waste can act as a raw material in the waste-to-energy nexus, where three problems

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can be solved at once; waste management, energy generation and greenhouse gas emission. Waste-to-energy is fundamental for establishing a circular economy system (Chen et al.,

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2016). The circular relationship between economy and environment is maintained for facing

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the existing resource deficit and environmental jeopardy. The 5R principles namely, Reduce, Reuse, Recycle, Recovery, and Reclamation are the basis of waste-to-energy nexus for

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circular economy as shown in Fig. 2. In an industrial system, waste recycling and energy generation should be connected. It is a regenerative system aiming to value and utilize the products and materials. (Chen et al., 2016). The conceptual waste-to-energy cycle for achieving a circular economy system is presented in Fig. 3. In the current review, methodologies of producing bio-hydrogen using waste materials are discussed in depth. The types of waste materials that are appropriate to be used as biomass for biological hydrogen production are described. Hydrogen production from dark

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Journal Pre-proof fermentation, photofermentation, sequential and combined dark and photofermentation are explained. The economic analysis is done where the costs of hydrogen production via biological methods can be compared with the traditional methods. 2. Categories of waste materials Various factors like the type and composition of waste and pre-treatment method would influence the H2 generation. The main conditions for choosing waste materials for

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generating H2 are accessibility, cost, carbohydrate-content in addition to biodegradability.

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The potential of H2 production by carbohydrate-based waste was reported to be 20-times

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greater in comparison to fat and protein-based wastes (Lay et al., 2003). The waste materials

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mainly used for H2 production are discussed in this section and displayed in Fig. 4.

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2.1. Agricultural or food wastes

Several agricultural and food wastes consist of carbohydrate-rich starch and cellulose.

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In accordance with the UN description, agricultural waste is created because of several agricultural processes (United N. Glossary of environment statistics, studies in methods.

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United Nations New York, NY. 1997.). Agricultural waste may consist of wastes from

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agricultural farms, poultry farms, slaughter houses, compost; remains of harvest, stalks, bagasse; fertilizer surplus from agriculture-fields, and other lignocellulosic residues that pass into the environment. Food waste has a high energy and is easily decomposable containing 85 to 95% of volatile solids and >80% moisture that helps for the microbial growth (Kim et al., 2004). The processing of solid waste comprising of starch is simple for carbohydrate and H2 production by firstly acid or enzymatic hydrolysis of starch to glucose and maltose, and subsequent transformation of carbohydrates to organic acids and finally to H2. The farming wastes containing cellulosic‟s require pretreatment or de-lignification 8

Journal Pre-proof prior to fermentation. Pretreatment is essentially needed for overpowering the recalcitrance of the lignocellulosic biomass (Tang et al., 2019). Lignocellulose is a poly-carbohydrate complex comprising of lignin, cellulose, and hemicellulose. It has been stated that lignin content is inversely related to efficacy of enzymatic hydrolysis of agricultural wastes. Pretreating of the biomass breaks the lignocellulosic matrix, decreases the cellulose crystallinity, and enhances the fraction of amorphous cellulose and surface area, which are all the fundamental roots of recalcitrance toward hydrolysis (Kumar et al., 2019; Kamani et al.,

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2019).

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2.2. Industrial waste

Industrial waste, which is non-toxic and biodegradable, is a potential raw material

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producing H2. Industrial waste is rich in carbohydrate content e.g., dairy processing waste

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obtained from dairy industry and baker‟s yeast as well as brewery wastewaters. Dairy processing waste has a high chemical oxygen demand (COD) attributed to lactose, fat plus

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protein content. This requires proper treatment as excessive and inappropriate disposal of

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dairy processing wastes can harm the soil structure and contaminate the groundwater (Zhong

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et al., 2015). The pretreatment of this waste can be done for eradicating detrimental or unwanted constituents and for nutritional balancing. 2.3. Wastewater sludge

Wastewater sludge is a side-product of wastewater treatment plants arising from domestic, industrial, commercial and agricultural forces. Waste-water sludge is mainly rich in carbohydrates, fats and proteins that are useful for hydrogen energy production. Wastewater sludge also consists of complex organic materials, heavy metals, and certain pathogenic microorganisms. Owing to these factors, wastewater sludge is not easy to manage by means 9

Journal Pre-proof of traditional discarding approaches such as landfills, composting and incineration. Thus, using the wastewater sludge to produce hydrogen by means of anaerobic fermentation is a promising method for its treatment (Wang et al., 2019). 3. Biological processes for H2 production The predominantly used biological methods utilizing the waste materials as organic substrates for the production of bio-hydrogen are dark and photofermentation, which are

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discussed in this section.

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3.1. Dark fermentation

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The acidogenic fermentation of carbohydrate-rich materials without light and oxygen

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to produce hydrogen is referred to as dark fermentation. The process is complicated, which is

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carried out by various bacteria, including different biochemical reactions comprising glycolysis, pyruvate degradation via pyruvate:ferredoxin oxido reductase pathway or else

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pyruvate:formate lyase pathway and H2 generation via the formate:hydrogen lyase or reduction of protons (Bundhoo, 2019). Complex polymers can be hydrolyzed into monomers

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and subsequently, into the combination of organic acids and alcohols. The bioreactors utilized

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for dark fermentation are simple and easy-to-handle as they do not require solar input for processing. Dark fermentation can be combined with wastewater treatment plants for producing H2 along with purification of water (Dincer and Acar, 2015). Bacteria are classified on the basis of different scales of temperatures that act on them as psychrophilic (0–20 °C), mesophilic (20–42 °C) and thermophilic bacteria (42–75 °C) (Hay et al., 2013). The appropriate heterotrophic bacteria comprise of stern anaerobes (e.g., Clostridia and thermophiles), facultative anaerobes (for e.g., Enterobacter) as well as aerobes (for e.g., Bacillus). Various organisms like Bacillus coagulans (Kotay and Das, 2007), 10

Journal Pre-proof Thermoanaerobacterium spp. (Sompong et al., 2008), Clostridium butyricum (Mishra et al., 2016, Clostridium thermolacticum (Collet et al., 2004), Clostridium pasteurianum (Liu and Shen, 2004), etc., have been identified for this purpose. Clostrida species produce hydrogen during its exponential growth phase. Temperature and pH are vital factors that can regulate the optimum metabolic pathways of H2 production and abating the activity of hydrogen consuming bacteria (Ghimire et al., 2015). The ideal temperature for dark fermentation is dependent on the type of biocatalyst and the substrate (Hosseini et al., 2015). The rate of H2

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production via thermophiles was reported to be greater than that in mesophilic bacteria (Shin

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and Park, 2013). Thermophilic conditions, because of their thermodynamics, have better

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process performance and lesser complications (Hosseini et al., 2015). The shifting of

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metabolism occurs from a hydrogen/acid generation phase to a solvent production phase after

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the stationary growth phase is reached by the population. Lignocellulosic biomass requires high temperature for greater H 2 yield (Guo et al.,

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2010). However, thermophilic temperatures are preferred for dark fermentation of food

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wastes (Shin et al., 2004), whereas easily decomposable substrates favor mesophilic temperatures for an optimum H2 production. The variation in optimal functioning

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temperatures is because of the variance in the proportion of decomposable substances existing in the feed substrate as well as the inocula used (Ghimire et al., 2015). The pH effect is quite crucial for dark fermentation. Acidic pH is favorable as the pH value below 6 hinders methanogenic activity in mesophilic and thermophilic environments. Nonetheless, the repression of H2 consuming homoacetogenic activity can exclusively be realized in thermophilic environments at the original pH of 5.5 (Luo et al., 2011). The optimal pH for food waste varies from 4.5 to 7, and for lignocellulosic waste from 6.5 to 7, while neutral pH is optimum for animal compost. Apart from pH and temperature, the stability and efficacy of 11

Journal Pre-proof hydrogen production also depends on hydraulic retention time (HRT), inoculum source and the sort of substrate (Santiago et al., 2019). Furthermore, H2 production through dark fermentation is reliant on organic loading rate (OLR), which can be calculated by HRT and chemical oxygen demand (COD). A high OLR doesn‟t automatically promise better H 2 yields, while low HRTs abet H2 production due to the washing out of methanogens (Pakarinen et al., 2011).

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In dark fermentation, hydrogen yield is quite poor as hydrogen consuming bacteria

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reduce H2 to CH4 and organic acids. The yield can be improved through the suppression of methanogenic activity by restraining bacteria that consume H2 and increasing the bacteria that

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can produce H2. Thus, pretreatment of inoculums can be done via a suitable methodology to

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repress bacteria that consume H2. Various pretreatment approaches such as heat treatment

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(Turhal et al., 2018), dilute acid treatment (Yang and Wang, 2019), base treatment, aeration, (Yang et al., 2019a), freezing and thawing (Hu and Chen, 2007), ionizing irradiation (Yin et

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al., 2014), load shock, , physical methods such as ultrasonication, microwave (Goud and

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Mohan, 2012), chemical treatment using sodium 2-BESA (2-bromoethanesulfonic acid) (Zhuang et al., 2012), acetylene, fluoroacetate, chloroform and iodopropane (Pachapur et al.,

favorable.

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2019), and a rather new technique called electroporation (Karim et al., 2018) are found to be

Table 1 represents the H2 production by various waste materials via dark fermentation under different operating conditions. Subhash and Mohan (2014) used deoiled algal biomass as the substrate, which was first pretreated by acid-hydrothermal treatment. Batch experiments were conducted using anaerobic mixed consortia as inoculum at 29 o C and pH 6. A cumulative yield of 217 mL H2/g algal biomass at a rate of 0.2 mL/h was obtained in the 12

Journal Pre-proof case of pretreated substrate as compared to 66 mL H2/g algal biomass at a rate of 0.08 mL/h in the case of untreated biomass. Ramprakash and Muthukumar (2015) firstly optimized the pretreatment and hydrolysis conditions. Various pretreatment conditions such as heat treatment, ultrasonication, alkali pretreatment, acid hydrolysis, enzymatic hydrolysis, and combined hydrolysis were applied on rice mill effluent. Enterobacter aerogenes RM 08 was used along with hydrolyzate acquired after the combined acid and enzymatic hydrolysis to

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obtain a maximum yield of 1.97 mol H2/mol of sugar.

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(Turhal et al., 2018)investigated the melon and watermelon mixture as a substrate to produce H2 via dark fermentation with and without adding inoculum. The presence of natural

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microflora in the mixture caused a minimal inoculum requirement. Hydrogen productivity in

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case of natural microflora was 80.62 mLH2/L/h, while in case of external inoculum (heat

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treated anaerobic sludge), the rate of hydrogen production was 351.12 mLH2/L/h. Gonzales et al. (2018) employed empty fruit brunch from palm oil for H2 production using the anaerobic

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sludge as the inoculum source at 37oC and pH 7 as the optimum conditions. Dilute acid

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hydrolysis pretreated substrate generated 275.75 mL H2/g total sugar with a rate of 2061 mLH2/L/d, while sequential dilute acid and enzymatic hydrolysis pretreated substrate yielded

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282.17 mL H2/g total sugar with a rate of 3175 mLH2/L/d. Karim et al. (2018) used citrus wastewater as a substrate pretreated by electroporation technique wherein different treatment intensities of electroporation were observed at 0.5 min 30, 60, 120 kWh/m3 for 0.5, 1 and 2 min, respectively on an anaerobic sludge. However, with no pretreatment, 0.56 mmol H2 /g COD was obtained, while the highest yield of 2.24 mmol H2 /g COD after 1 min of electroporation treatment was produced. In a recent investigation, Wadjeam et al. (2019) co digested cassava starch wastewater

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Journal Pre-proof and buffalo dung without any external inoculum in a continuous stirred tank reactor (CSTR) at 35oC and pH 5.5, which yielded 165 ± 25.16 mL H2/g CODremoved at a rate of 839 mL H2/L/d. Recently, rice straw hydrolysate was also used as a substrate by Dinesh et al. (2019) by inoculating with Bacillus cereus (KR809374) for dark fermentation providing a yield of 1.53 ± 0.04 mol H2/mol glucose. Simple operating technique, fast rate of reaction, low energy requirements, and stable hydrogen production seen to consume complicated waste materials as a substrate. However, the major drawback of dark fermentation is the inefficient utilization

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of the substrate. This operates at low molar yields, whereas the majority of feedstock

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produces organic acids, such as butyric acids, acetic acid, lactic acid, etc., (Abo-Hashesh et

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al., 2011).

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3.2. Photo-fermentation

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As discussed in the previous section, only around 33% of COD of the organic materials is transformed to H2. The highest H2 yield estimated from dark fermentation of

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glycerol is equivalent to 1 mol of H2 /mol glycerol, which could be surpassed by the

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photofermenation as the substrate conversion is brought to completion (Abo-Hashesh et al.,

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2011). The fermentative transformation of organic substrate to produce H2 can be manifested by photosynthetic bacteria following a sequence of reactions. The photosynthetic non-sulfur bacteria (PNS) conducting photo-fermentation are capable of converting organic substrate to H2 and CO2 under light in the absence of oxygen environment. Furthermore, the seized light energy let PNS bacteria to drive substrate from conversion to completion (Elkahlout et al., 2018). Rhodobacter or Rhodopseudomonas strains have been widely investigated for the conversion of wastewater from brewing industry, olive oil mills to H2 because these streams comprise of organic acids as well as simple alcohols. Rhodobacter sphaeroides (Sargsyan, et

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Journal Pre-proof al., 2016), Rhodobacter sulfidophilus (Maeda et al., 2003), Rhodobacter capsulatus (Zhang et al., 2016), Rhodospirillum rubrum (Dadak et al., 2016) and Rhodopseudomonas palustris (Hitit et al., 2017) are the most commonly used PNS bacteria. Similar to dark fermentation, organic substrates having complex carbohydrates require pretreatment prior to photofermentation. Waste effluents could also be tricky to handle as they might comprise certain inhibiting elements needing removal or alteration for

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better hydrogen production efficiency (de Mello et al., 2018). Furthermore, the presence of

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noxious substances in industrial wastewater like heavy metals, phenolics, aromatic hydrocarbons might be problematic and require pre-treatment for hydrogen production.

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Additionally, the utilization of untreated wastewater for photofermentation is challenging

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because of the growth of native fermentative organisms, which commonly grow at quicker

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rates compared to PNS bacteria and hence, they can compete with the latter. Consequently, for allowing the preferred photosynthetic bacterium growth, appropriate sterile conditions are

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essential. Selecting a suitable method for wastewater pretreatment method is imperative as certain sterilization procedures could possibly harm the potential substrate. Thermal

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treatment at 50oC, treating with 1% H2O2, irradiation with UV radiation, and thermal

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sterilization are reported as the best sterilization methods for the pretreatment of dairy wastewater (Keskin et al., 2011). Environmental conditions are to be controlled for H 2 production to take place efficiently by the photofermentation (Ozmihci and Kargi, 2010a). Optimum pH for photofermentation has been reported to be 6.8-7.5, while the optimal temperature range is 31-36 oC with an appropriate wavelength range between 400 and 1000 nm (Basak and Das, 2007). Energy from the sun can be utilized by PNS bacteria to initiate reactions that are

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Journal Pre-proof thermodynamically unfavorable and possibly redirect all the electrons from the organic substrate for producing H2 (Harwood, 2008). The rate of hydrogen generation depends on carbon source, light intensity, and the type of microbial culture employed (Kapdan and Kargi, 2006). Different wastewaters vary in the transparency or turbidity and thus, different irradiation intensities of light are required for optimum hydrogen production. High turbidity decreases light penetration and eventually the light conversion efficiency (Keskin et al., 2011). Certain photoheterotrophic strains could be employed for producing H2 from the

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simple sugars namely, glucose, fructose, or sucrose occurring in these wastewater sources

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from the dairy or sugar industries. The starch contained in wheat is also reported to be an apt

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substrate for a single stage photofermentation (Sağır and Hallenbeck, 2019). Huge volumes

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of crude glycerol are obtained as a waste product of biodiesel manufacture (1 kg of glycerol

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for 10 L of biodiesel).

A preliminary study reported that 6 mol H2/mol glycerol was attained via photo

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fermentation. However, some refinements like decreasing the lag phase by varying the

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concentration of inoculum or media formulation, adjusting the power of irradiation source, adding nitrogen source (since nitrogen concentration in low in waste glycerol) are required to

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improve the H2 yield. Moreover, photofermentation of glycerol must be a continual operation (Sabourin-Provost and Hallenbeck, 2009). PNS bacteria contains both hydrogenase and nitrogenase enzymes, though the latter has a major role in H2 production under anoxygenic conditions. Fe and Mo have been reported to be the vital cofactors necessary for H 2 production by the nitrogenase enzymes (Argun and Kargi, 2011). High chemical energy input is required in the form of ATP with 2 ATP/e -, or 4ATP/H2. The activity of nitrogenase enzyme is repressed 16

Journal Pre-proof when O2 or NH3 are present, or when N/C ratios are high thereby, ammonium-limited and anoxygenic conditions are necessary (Koku et al., 2003; Takabatake et al., 2004). The H2 production was reported to be lesser when ammonia salts were present, whereas proteins like albumin, glutamate besides yeast extract as a source of nitrogen have improved the H2 yield (Takabatake et al., 2004; Oh et al., 2004). At high concentrations of nitrogen, the metabolism shifts towards consuming organic substrate for cell synthesis instead of producing hydrogen, leading to the growth of additional biomass and decline of light diffusion (Oh et al., 2004).

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But, light irradiation needs to be adjusted to greater values in case of wastewater such as

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olive mill and dairy industry discharges when compared to using pure substrates. Flat plate

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reactors may possibly be a convenient way out for reducing the lighting expenses. Although

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flat panel reactors might have greater manufacturing expenses as well as these require extra area in comparison to cylindrical reactors. The costs related to pre-treatment processes are

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also among the economic concerns (Keskin et al., 2011).

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Pintucci et al. (2013) used olive mill wastewater as the feedstock pretreated by using

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dry-Azolla and granular active carbon and Rhodopseudomonas palustris 42OL inoculum source yielding 0.37 mL H2/ mL of wastewater. In another study, dry-Azolla and granular

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active carbon pretreated olive mill wastewater in the presence of Rhodopseudomonas palustris 6A generated 0.37 mL H2/ mL wastewater (Pintucci et al., 2015). Budiman and Wu, (2016) used a mixture of palm oil and pulp and paper mill effluents for photofermentative H2 production in the presence of Rhodobacter sphaeroides NCIMB8253. A total yield of 8.72 mL H2/ mL of wastewater was obtained after the ultrasonication pretreatment, which was 86.7% higher in comparison to the substrate without pretreatment. In another study, ultrasonication of both the substrate (palm oil and pulp and paper mill effluents) and inoculum (Rhodobacter sphaeroides NCIMB8253) was done to achieve 14.43 mL H2/ mL of 17

Journal Pre-proof wastewater (Budiman et al., 2017). Al-Mohammedawi et al. (2019) used the brewery wastewater pretreated with banana peels particles

for hydrogen production by

photofermentation using Rhodobacter sphaeroides DSM 158, which yielded 0.41 mL H2/ mL of wastewater. Of late, Dinesh et al., (2019) utilized rice straw hydrolysate as a feedstock for photofermentation via Rhodopseudomonas rutila yielding of 0.25 ± 0.04 mol H2/mol glucose at pH 7 and mesophilic system temperature (37 oC).

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The advantages of photofermentation are that formation of hydrogen catalyzed by

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nitrogenase enzyme is an irreversible reaction. Photofermentation can be used in combination with dark fermentation for better output. However, the disadvantages of photofermentation

-p

are below-par light-conversion efficacy, which requires H2 impermeable transparent

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photobioreactors as well as the requirement of excess energy by nitrogenase enzyme (Sağır

lP

and Hallenbeck, 2019).

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3.3. Two stage sequential dark and photo-fermentation Single dark and photofermentation are the significant techniques for H2 production,

ur

but after a poor yield because of the amassing of organic acids, high production cost and low

Jo

energy recovery, which are the huge drawbacks. Thus, coupling of dark and photofermentation comes across as a good method for overcoming this shortcoming. In a two stage sequential dark and photo fermentation, the discarded material obtained after dark fermentation is utilized as the substrate for photofermentation (Takabatake et al., 2004). Theoretical yield of hydrogen in this case is 12 moles after complete oxidation of 1 mole of glucose (Equation 1) (Sağır and Hallenbeck, 2019; Keskin et al., 2011). Complete oxidation: C6H12O6 + 6 H2O → 12 H2 + 6 CO2 18

(1)

Journal Pre-proof Dark fermentation: C6H12O6 + 2 H2O → 4 H2 + 6 CO2 + 2CH3COOH

(2)

Photo-fermentation: 2CH3COOH + 4 H2O + light energy → 8 H2 + 4 CO2

(3)

In dark fermentation, many by products are formed as discussed in previous section,

of

but the maximum yield can be 4 mol H2 /mol glucose if acetic acid is the only side product (Equation 2). In the sequential dark and photofermentation scheme, 4 moles of H2 are

ro

obtained via dark fermentation (Equation 2) and the produced acetic acid is utilized as the

-p

starting material for photofermentation ensuing 8 moles of H2 (Equation 3) with a total of 12

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moles of H2 per mole of glucose theoretically. Although practically, the highest H2 yield per

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mole of glucose by means of sequential arrangement is lesser when compared to the theoretical yield as some part of the material is consumed for growing the microbes as well as

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their metabolism (Seifert et al., 2018).

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In the two-stage sequential arrangement, the first step is economical and easy-tooperate dark fermentation via anaerobic bacteria such as Clostridium species producing

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hydrogen in poor yields accompanied by the concurrent generation of small chain organic acids (lactic acid, butyric acid, etc) and alcohols. The second step is the photofermentation with PNS bacteria (Rhodobacter or Rhodopseudomonas) converting the short chain organic acids to H2. This combination is a promising and useful approach for the profitable H2 production (Seifert et al., 2018). Figure 5 exhibits the system for two-stage sequential fermentation process for H2 production. Innumerable studies regarding the two-stage fermentation have been reported with pure 19

Journal Pre-proof cultures carrying out the dark fermentation and subsequent photofermentation step. The consequential H2 yields by the two-stage sequential arrangements are greater compared to single systems. Enterobacter aerogenes HO-39 and Clostridium butyricum were used as inoculums for dark-fermentation of sweet potato starch producing H2 and volatile fatty acids, which were converted to H2 via photo-fermentation by Rhodobacter sphaeroides (Yokoi et al., 2002). Table 2 represents the H2 production by various waste materials via sequential fermentation. Su et al. (2009) used raw cassava starch as the feedstock inoculated with the

of

preheated activated sludge for dark fermentation producing 240.4 ml H2/g starch. The

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byproducts in dark fermentation were then utilized as the substrates in photofermentation

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using Rhodopseudomonas palustris as inoculum giving 131.9 ml H2/g starch. A cumulative

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yield of 402.3 mL H2/g starch was obtained in the sequential process. Özgür et al. (2010) used sugar beet molasses as feedstock for dark fermentation via Caldicellulosiruptor followed

by

photofermentation

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saccharolyticus

by

Rhodobacter

capsulatus wild

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type, R. capsulatus hup− mutant, and Rhodopseudomonas palustris. The cumulative hydrogen

1.37 mmol H2/L/h.

ur

yield of 13.7 mol H2/mol sucrose was achieved by R. capsulatus hup− mutant with a rate of

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Rai et al. (2012) utilized cheese whey for obtaining hydrogen production in batch experiments. Initially, dark fermentation by immobilized Enterobacter aerogenes MTCC 2822 and subsequent photofermentation of byproducts i.e., volatile fatty (acetic acid, butyric acid, lactic acid) by free Rhodopseudomonas BHU 01 yielded 3.40±0.21 mol/mol lactose, while photofermentation by immobilized Rhodopseudomonas BHU 01 yielded 5.88±0.14 mol/mol lactose. Mishra et al. (2016) investigated hydrogen production by using palm oil mill effluent POME as the feedstock in a sequential fermentation. Dark fermentation via Clostridium butyricum LS2 yielded 0.784 ml H2/mL POME. The diluted dark fermentation 20

Journal Pre-proof discharge was then inoculated with Rhodopseudomonas palustris providing a cumulative yield of 3.064 ml H2/mol POME after two stages. In another investigation, diluted solid wastes from chewing gum manufacture were used as a substrate for two-stage fermentation. The solid was inoculated with digested sludge for dark fermentation to produce a maximum of 0.36 L H2 /L of the medium. The discharge of first step consisted of volatile fatty acids, which were then used for photofermentation providing 0.80 L H2/L of diluted (1:8) discharge. The cumulative yield in the optimized conditions was 6.7 L H2/L of the non-diluted waste

of

(Seifert et al., 2018).

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In a recent investigation (Dinesh et al., 2019), rice straw hydrolysate and rice husk

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hydrolysate were used as the feedstock at pH 7.0 using Bacillus cereus (KR809374) for dark

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fermentation and Rhodopseudomonas rutila for subsequent photofermentation in batch experiments to yield 1.82 ± 0.04 and 1.73 mol H2/mol glucose, respectively. The latest

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investigation by Elsharkawy et al. (2020) made use of a paperboard mill wastewater to

na

produce hydrogen using up-flow intermitted stirring tank reactors via sequential dark and photofermentation. The light penetration efficiency was upgraded by using inner and outer

ur

light-emitting diodes lamps. The study was conducted at varying HRTs without the use of

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any external chemical. HRT of 24 h showed the maximum hydrogen yield 788.6 ± 238.4 mL H2/g COD, while HRT of 12 h showed the highest overall productivity of 1.39 ± 0.07 L H2/L/d with a yield of 618.9 ± 77.9 mL H2/g COD. A complete investigation of the dark fermenter effluents should be done prior to using them as a substrate for photo-fermentation. This is imperative for checking the availability of significant nutrients such as Fe and Mo essential for nitrogenase and the optimum concentration of NH4+ as high concentrations could inhibit the activity of nitrogenase and decrease the yield of H2. Dilution of the discharged solution with lactic or malic acid could 21

Journal Pre-proof ameliorate the H2 generation potential. The utilization of diverse renewable sources along with a substrate rich in certain vital nutrients is a possible solution, which would care of the environmental and the economical factors for a proficient H2 production (Sağır and Hallenbeck, 2019; Keskin et al., 2011). 3.4. Single stage combined dark and photo-fermentation Integrating dark and photofermentation in a single step co-culture aims towards the

of

reduction of side products such as volatile fatty acids of fermentation route, thereby

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enhancing the H2 production. The combined process is simpler and advantageous compared

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to the sequential approach, since it excludes certain manipulations. The acidification because of the dark fermentation reactions could be well-adjusted via alkalinization during the

re

photofermentation, which eliminated the necessity for external pH correction. Further, a

lP

limited requirement for an intermediate dilution stage is present in the combined fermentation, which is requisite in sequential fermentation because the starting material for

na

photofermentation are produced insitu through dark fermentation. The single stage system is

ur

comparatively inexpensive and less strenuous (Keskin et al., 2011; Hitit et al., 2017). Figure

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6 exhibits the system for single-stage combined fermentation process for H2 production. There are some literature reports available concerning the single stage dark and photofermentation using the waste materials when compared to the two stage process. The mixed culture of anaerobic and PNS bacteria in a single stage is investigated by using pure substrates (like glucose) and quite varying yields are reported, ranging from 0.9 to 7.1 mol H2/mole glucose (Abo-Hashesh et al., 2011). In another report, cheese whey wastewater (rich in carbohydrate and micronutrients) discharged from the cheese processing industry was employed for biological H2 production via combined thermophilic dark fermentation and 22

Journal Pre-proof photofermentation using Rhodobacter palustris (DSM 127) (Azbar and Dokgoz, 2010). Literature reports on H2 production via combined dark and photofermentation are compiled in Table 3. Argun et al. (2009) used wheat starch as a substrate to investigate the single stage combined dark and photofermentation via anaerobic sludge with a mixture of Rhodobacter

sphaeroid

NRRL

B-1727)

Rhodobacter

sphaeroid

DSMZ-158)

Rhodopseudomonas palustris (DSMZ-127) Rhodobacter sphaeroid RV and obtained 1.16

of

mol H2/mol glucose. Ozmihci and Kargi (2010b) used wheat powder starch as the starting

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material using a combination of heat-treated anaerobic sludge and pure culture of Clostridium beijerinckii (DSMZ 791T) plus PNS bacteria Rhodobacter sphaeroides-NRRL and

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Rhodobacter sphaeroides-RV as inoculums. The H2 yield was higher with the combination of

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heat-treated anaerobic sludge and Rhodobacter sphaeroides-NRRL in comparison to the yield

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with the pure culture of Clostridium beijerinckii giving a yield of 2.6±0.5 mol H2/mol hexose. Single stage combined dark and photofermentation of ground wheat starch was conducted by

na

Argun and Kargi (2010) in batch experiments using varying light intensities and light sources

ur

like sunlight, infrared, tungsten, fluorescent, tungsten + infrared and halogen lamp. The system was inoculated with heat-treated anaerobic sludge and Rhodobacter sphaeroides-RV

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giving the maximum yield of 1.45 mol H2/mol glucose when halogen lamp was used at the light intensity of 270 W/m2.

Laurinavichene et al. (2014) assimilated Rhodobacter sphaeroides N7 into heterotrophic starch-hydrolyzing consortium comprising of Clostridium butyricum. Hitit et al. (2017) employed the response surface methodology (RSM) using Box-Behnken design for optimizing microorganism ratio and concentrations of feedstock and buffer and their eventual influence on H2 production. Clostridium butyricum and Rhodopseudomonas palustris for 23

Journal Pre-proof dark and photofermentation were grown respectively on potato starch. A light intensity of 40 W/m2 was constantly irradiated at a constant temperature of 36 oC. The maximum yield was 6.4 ± 1.3 mol H2/mol glucose at the optimum conditions (substrate concentration = 15 g/L; buffer concentration = 50 mM; microorganism ratio = 3). Zagrodnik and Łaniecki (2017a) utilized corn starch as the substrate for H2 production via co-culture hybrid dark fermentation by Clostridium acetobutylicum and photofermentation by Rhodobacter sphaeroides in a repeated fed-batch reactor at pH 7. The effect of various organic loading rates was monitored.

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A maximum H2 yield of 2.62 mol H2/mol hexose was produced at the OLR of 0.375 g

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starch/L/day. Moreover, pH 6.5 had a negative effect on H2 production, leading to the

-p

formation of fatty acids instead of H2. In another study, Zagrodnik and Łaniecki (2017b)

re

studied the used starch as feedstock along with the co-culture of Clostridium acetobutylicum and Rhodobacter sphaeroides (for dark and photo fermentation) at the system temperature of

lP

32oC. The stable bacteria co-culture led to complete disintegration of starch with no amassing

na

of the volatile fatty acids. The effect of pH was examined and the pH control at 7.0 was

ur

discovered to be optimal at which a total yield of 5.11mol H2/mol hexose was obtained. There are quite a few hurdles in single stage combined fermentation methods such as

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potential differences in the rates of organic acid generation and depletion, which decline in the light penetration capacity because of opacity due to suspended solids. Anyway, it is evident that sequential/combined fermentations are superior methodologies for enhancement in H2 yields. It was been predicted that for procuring H2 cost-effectively, 8 mol H2/mol glucose yield is adequate (Equation 3) and in certain cases, it was also reported that combined/sequential fermentations has produced around 7 mol H2/mol of glucose. This implies that these systems hold a strong case and may be even more meritorious with more better advances to overcome the shortcuts. 24

Journal Pre-proof 4. Economic analysis of hydrogen production The rising energy demand owing to the industrial revolution has caused the high usage of fossil fuels that are limited and getting exhausted at a rapid rate. Along with the depletion of fossil fuels, environmental issues are another challenge posed by the traditional fuels. The focus is now shifting towards renewable sources and waste-utilization for sustainable development and circular economy. The cost of H2 production by various

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methodologies and energy sources depends on the cost of energy-conversion system and H2

ro

generation plants.

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Generating H2 from waste materials is an ecological route, but the economic viability of the technique is also pivotal. As already discussed, the commercial production of H2 is

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predominantly done by means of natural gas by steam reforming process and coal

lP

gasification in addition to water electrolysis. However, the biological methods are also investigated widely. A technoeconomic study of H2 production provided an assessment of the

na

biological production pathways for producing 10 000 kg H2/d (James et al., 2009). It was

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suggested that the price of H2 generation via a photofermentation chemostat system by PNS

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bacteria (3.5% photochemical efficacy; rate = 244 mL H2/L/h) was about 10.36 USD/kg H2. This is higher than in the case of a similar biophotolysis chemostat system (2% photochemical efficacy; rate=124 mL H2/L/h) producing H2 at a price of 8.15 USD/kg H2, which can be reduced to 2.99 USD/kg H2 if the photochemical efficacy is 9.2%. The high feedstock costs for PNS bacteria and high capital costs for the separation of H2 assuming that biogas contains 95% H2. The cost of producing H2 by fermentation of lignocellulosic biomass was reported to be 4.33 USD/kg H2 (counting pretreating lignocellulosic material), functioning at 940 mL H2/L/h. The cost of delivery of feedstock (corn stover), pretreatment 25

Journal Pre-proof techniques and gas separation were considered during this analysis. Czernik et al. (2004) reported that 0.5 million tons of H2 production is probable using the trap grease assembled in the United States of America. The value of dewatered trap grease was expected to be 0.04–0.08 USD /kg. The prices of supplies were projected to be around 0.13–0.26 USD /kg H2 as over 30 kg of H2 could be attained by 100 kg of grease. Sathyaprakasan and Kannan (2015) examined the economics of several H2 production

of

methods. This comparison was centered on factors such as type and the amount of culture,

ro

design of the reactor, capital costs, and operating costs for power, labor, water, as well as

-p

general supplies.

The economical comparison between dark and photofermentation is compiled in

re

Table 4 as adopted adopted from Sharma and Kaushik, 2017. Table 5 presents the economic

lP

evaluation for H2 production through dark fermentation from food wastes reproduced from Yun et al., 2018. The economic evaluation covers the capital cost (including construction

na

cost, land use, and set up costs), yearly operating cost (electricity, water, labor, chemicals),

ur

and profit earnings. The profit was determined at a rate of 10 USD/ton food waste as in dark

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fermentation and the COD removal was restricted to 10%. Pressure swing adsorption (PSA) was used to remove CO2 (released by the produced biogas) and this purification was included.

The total capital costs are 1,636,560 USD/year and yearly operating costs are 548,568 USD/year. A profit of 360,000 USD could be benefited by food waste treatment. After the overall assessment, the production cost of H2 is projected to be 3.2 USD/kg H2, which is nevertheless greater than the present selling price (0.5-3.2 USD/kg H2) (Yun et al., 2018). A comparison of cost of H2 production from different biological processes to the production of 26

Journal Pre-proof other fuels like gasoline, ethanol and natural gas was done by Karthic and Joseph (2012). This comparison is tabulated in Table 6 (reproduced from Sharma and Kaushik (2017). It is clear from the table that H2 production by many existing methods, excluding pyrolysis is relatively higher when compared to the traditional approaches for fuel production. Therefore, there is still an essential need for putting more sincere emphasis in research and development of biological approaches of H2 production technologies in order to make it commercially

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viable.

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Conclusions and future outlook

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The global energy demand has increased exponentially over the years and is still rising continuously. The conventional fossil fuels such as coal and petroleum fail to satiate

re

the energy requirement because of their non-renewable nature. Moreover, mounting waste-

lP

generation leads to landfilling and environmental pollution. The growing global warming and climate change due to the carelessness of burning of fossil fuels and unrestrained CO2

na

emission is a severe environmental threat. Hydrogen is an exceptional substitute to tackle this

ur

problem since it is a source of green and clean energy having high energy content and

energy-intensive.

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renewability, but the traditional approaches to obtain hydrogen are also expensive and

Hydrogen production from the waste materials via biological approaches is an excellent alternative. This waste-to-energy nexus can simultaneously resolve issues such as energy-requirement, waste management, and global warming. Generating energy and managing waste by this waste-to-energy nexus should be feasible to shift towards circular economy. Biological approaches such as dark and photofermentation use waste materials like agricultural waste (lignocellulosic biomass), sewage, dairy waste, poultry farm waste and 27

Journal Pre-proof industrial waste for producing H2. Fermentative processes have the distinct benefits over chemical processes. However, poor H2 yield and low production rates are some of the shortcomings of these techniques. In this review, dark fermentation, photofermentation, sequential and combined dark and photofermentation have been explicated, which might help to enhance the hydrogen production. The major problem with dark fermentation by anaerobic bacteria is that only around

of

33% of COD in organic materials is converted to H2, and organic acids, like acetic acid, lactic

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acid, etc., that are produced as side products. Photofermentation can help to increase the yield as the photosynthetic non-sulfur bacteria are capable of converting organic substrate to

-p

hydrogen and carbon dioxide in light anoxygenic conditions. However, single dark and

re

photo- fermentation techniques give poor yield because of the amassing of organic acids.

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Coupling of dark and photofermentation may help to tackle this dilemma at an economical level of 8 mol H2 per mole of glucose can be achieved. In two-stage sequential

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dark and photofermentation, the waste material acquired after dark fermentation is employed

ur

as the starting material for the photofermentation. Pretreatment of the dark fermentation

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effluent, adjustment of concentration of NH4+ and volatile fatty acids, pH, nutrient composition, and concentration are essential prior to photofermentation. Large volumes of fermenter unit and separation/pretreatment units are required amidst the two steps of sequential dark and photo fermentation. Single-stage dark and photofermentation system conducts fermentation via co-culture at the same time in the same reactor. No external pH adjustment is required as acidification caused by dark fermentation reactions can be well-adjusted by the alkalinization caused by photofermentation and also no dilution step is needed. The smaller reactor volumes, simple 28

Journal Pre-proof technique and economical nature are some of the merits of combined fermentation. Development of efficient systems, set-ups and optimizing the best conditions can be beneficial for improving the yields and rates of H2 production. The economic analysis revealed that H2 production by the present methods except for pyrolysis is comparatively higher than the traditional approaches of fuel production. The production cost of H2 from food waste is higher than the current selling price. Cutting down

of

the cost of photobioreactor and installing proper storage system are the possible solutions to

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make H2 production more economical. An earnest emphasis in the area of research and development of biological approaches of H2 production technologies is pivotal to make it

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commercially feasible. Ultimately, the society needs benign environment for the benefit of

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better life and living.

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Conflict of interest

Acknowledgments

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All the authors declare that we have not any conflict of interests.

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fellowship.

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Ms. Surbhi Sharma wishes to express her gratitude to UGC, New Delhi, India for a research

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methane production from dairy processing waste using a two-stage digestion in induced bed reactors (IBR). International Journal of Hydrogen Energy 40(45), 15470-15476. Zhuang, L., Chen, Q., Zhou, S., Yuan, Y., Yuan, H., 2012. Methanogenesis control using 2bromoethanesulfonate for enhanced power recovery from sewage sludge in air-cathode microbial fuel cells. International Journal of Electrochemical Science 7, 6512-6523.

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Figures and Tables

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Figures

Fig. 1. Graphical presentation of global rise in H2 demand (reproduced from https://www.iea.org/hydrogen2019/).

44

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REDUCE

5R’s of Sustainable Development

REUSE

ro

of

RECLAMATION

RECYCLE

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RECOVERY

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Fig. 2. The 5R‟s of sustainable development.

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Fig. 3. The conceptual cycle exhibiting circular economy system.

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ro

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Waste materials

lP

Dairy industry, baker’s yeast and brewery wastewaters, plastic industry

Jo

ur

na

Animal compost, waste from farms, poultry farms, slaughterhouses, harvest remains, wheat straw, stalks, bagasse; fertilizer surplus from agriculturefields

Industrial waste

re

Agricultural waste

Wastewater sludge

Wastewater sludge from domestic/ industrial/ commercial/ agricultural wastewater treatment plants

Fig. 4. Different kinds of waste materials exploited as biomass for H 2 generation.

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Fig. 5. H2 production via two-stage sequential dark and photofermentation process.

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Fig. 6. H2 production via single-stage combined dark and photofermentation process.

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Tables

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Table 1

H2 production by various waste materials via dark fermentation.

Cheese whey

Adapted anaerobic sludge

Physico-chemical treated plastic industry Herbal wastewater

Rice mill wastewater pretreated by combined acid and enzymatic

ur

Batch

Anaerobic digester sludge

Batch

Anaerobic sludge

Batch

Slaughter house sludge

Batch

Enterobacter aerogens RM 08

Batch

Jo

Acid pretreated deoiled algae cake

Reactor

Conditions {Temperature, pH}

Max. H2 yield

Max. H2 production rate

Reference

55 oC ;

111 mL H2/g total sugar

3.46 mL H2/L/h

Kargi et al. (2012)

6 (initial)

217 mL H2/g algal biomass

0.2 L H2/L/d

Subhash and Mohan (2014)

36 oC ;

-

0.10 L H2/L/d

MorenoAndrade et al. (2014)

165 mL H2/g COD

-

Sivaramakris hna et al. (2014)

1.97 mL H2/g total sugar

-

Ramprakash and Muthukumar (2015)

lP

Inoculum source

na

Substrate

7 29 oC ;

5.5 36 oC ; 4.5-7.5 35oC ; 7

48

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Leaching bed reactor

37 oC ;

Melon and watermelon mixture

Natural microflora

Leaching bed reactor

37 oC ;

Dilute acid hydrolysis pretreated oil palm empty fruit bunch

Anaerobic sludge

Batch

37oC ;

Sequential dilute acid and enzymatic hydrolysis pretreated oil palm empty fruit bunch

Anaerobic sludge

Batch

Citrus wastewater after electroporation pretreatment

Anaerobic sludge

Batch

Cassava starch wastewater and Buffalo dung

None

CSTR

Rice straw hydrolysate

Bacillus cereus (KR809374)

-

351.12 mL H2/L/ h

Turhal et al. (2018)

-

80.62 mL H2/L/ h

Turhal et al. (2018)

275.75 mL H2/g total sugar

2061 mL H2/L/d

Gonzales et al. (2018)

282.17 mL H2/g total sugar

3175 mL H2/L/d

Gonzales et al. (2018)

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Melon and watermelon mixture

2.24 mmol H2/g COD

-

Karim et al. (2018)

30 oC ;

165 ± 25.16 mL H2/g CODremoved

839 mL H2/L/d

Wadjeam et al. (2019)

1.53 ± 0.04 mol H2/mol sugar

-

Dinesh et al. (2019)

-

-

37oC ;

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7

of

7

36oC ;

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6

37 oC ; 7

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Batch

5.5

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Table 2

Inoculum

(dark fermentation)

(photofermentation)

Cassava starch

Preheated activated sludge

lP

Sugar beet molasses

Caldicellulosiruptor saccharolyticus

Ground wheat

Anaerobic sludge

Cassava starch

re

Inoculum

na

Rhodopseudomonas palustris

Rhodopseudomonas capsulatus hup- mutant

ur

Substrate

-p

H2 production from different waste materials via two-stage sequential dark and photofermentation.

Reactor

Overall H2 yield (sequential fermentation)

Reference

Batch

402.3 mL H2/g starch

Su et al. (2009)

Batch

13.8 mol Özgür et al. H2/mol sucrose (2010)

Continuous

0.45 c mol H2/mol acetate

Ozmihci and Kargi (2010a)

Activated sludge immobilized

Rhodopseudomonas palustris

Batch

6.07 mol H2/mol hexose

Cheng et al. (2011)

Cheese whey

Immobilized Enterobacter aerogenes MTCC 2822

Free Rhodopseudomonas BHU 01

Batch

3.40±0.21 mol H2/mol lactose

Rai et al. (2012)

Cheese whey

Immobilized Enterobacter aerogenes MTCC 2822

Immobilized Rhodopseudomonas BHU 01

Batch

5.88±0.14 mol H2/mol lactose

Rai et al. (2012)

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Rhodobacter sphaeroides (NRRLB1727)

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Journal Pre-proof Clostridium butyricum LS2

Rhodopseudomonas palustris

Batch

3.064 mL H2/mL POME

Mishra et al. (2016)

Waste from chewing gum production

Mixed bacterial culture

Rhodobacter sphaeroides O.U.001

-

6.7 L H2/L non-diluted effluent

Seifert et al. (2018)

Rice straw hydrolysate

Bacillus cereus (KR809374)

Rhodobacter sphaeroides

Batch

1.82 mol H2/mol glucose

Dinesh et al. (2019)

Rice husk hydrolysate

Bacillus cereus (KR809374)

Rhodobacter sphaeroides

Batch

1.73 mol H2/mol glucose

Dinesh et al. (2019)

Paperboard mill wastewater

Anaerobic sludge

Purple non-sulfur bacteria

788.6 ± 238.4 mL H2/g COD

Elsharkawy et al. (2020)

ro

UISTR

lP

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UISTR: Up-flow intermitted stirring tank reactors

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Palm oil mill effluent (POME)

na

Table 3

Wheat starch

Inoculums

Jo

Substrate

ur

The H2 production from different waste materials via single-stage combined dark and photofermentation.

Inoculums

(dark fermentation)

(photo fermentation)

Anaerobic sludge

mixture of Rhodobacter sphaeroid NRRL B-1727) Rhodobacter sphaeroid DSMZ-158) Rhodopseudomonas palustris (DSMZ-127) Rhodobacter sphaeroid RV

51

Reactor

Total H2 yield via combined dark-photo fermentation

Reference

Batch

1.16 mol H2/mol glucose

Argun et al. (2009)

Activated sludge

immobilized Rhodobacter sphaeroid

Batch

0.36 mol H2/mol glucose

Ozmihci and Kargi (2010b)

Ground wheat starch

Activated sludge

Rhodobacter sphaeroid RV

Batch

1.45 mol H2/mol glucose

Argun and Kargi (2010)

Starch

Clostridium butyricum

Rhodobacter sphaeroides N7

-

2.6±0.5 mol H2/mol hexose

Laurinavi chene et al. (2014)

Potato starch, Glucose

Clostridium butyricum

Rhodopseudomonas palustris

-

6.4±1.3 mol H2/mol glucose

Hitit et al. (2017)

Corn starch

Clostridium acetobutylicum

Rhodobacter sphaeroides

repeated fed-batch reactor

2.62 mol H2/mol hexose

Zagrodni k and Łaniecki (2017a)

Starch

Clostridium acetobutylicum

Rhodobacter sphaeroides

-

5.11 mol H2/mol hexose

Zagrodni k and Łaniecki (2017b)

na

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Ground wheat starch

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Table 4

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Comparison of hydrogen production via dark and photofermentation (reproduced from Sharma and Kaushik, 2017).

Method

Dark fermentation

Photo-fermentation

Power cost

2.50

2.50

Water cost

0.03

0.0

Labor cost

17.83

23.03

Costs in million USD

52

Journal Pre-proof 2.70

3.51

Culture production cost

2.01

2.70

Glucose substrate

867.18

144.19

Gas separation and handling cost

0.05

0.14

Subtotal operating cost

892.31

176.10

Contingency 10 %

89.22

17.62

Overall operating cost

981.53

193.69

Cost/GJ of H2

155.79

30.74

Cost/kg of H2

18.70

3.70

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General supply cost

Table 5

Construction

Additional

ur

Capital cost

Cost (USD, USD/y)

Total

1,636,560

Storage tank (25% of fermenter cost)

89,000

H2 fermenter

356,000

Food waste grinding, Heat exchanger

258,000

Purification (PSA/Fermenter ratio: 1.09)

388,040

Land use, set up (50% of construction cost)

545,520

Jo

Item

na

Economic evaluation for H2 production from food waste through dark fermentation (reproduced from Yun et al., 2018).

53

Journal Pre-proof Operation cost

Total

548,568

Materials

Chemicals

48,000

Maintenance

6% of capital cost

98,194

Annual capital cost

19% of capital cost

310,946

Others

Labor cost (20% of total operating cost)

91,428

Profit

360,000 10% of treatment cost (100 USD/ton waste)

360,000

Production cost

Annual cost (Capital cost/20y + operating cost)/Annual H2 production

3.2 USD/kg H2

-p

ro

of

Waste treatment

re

Table 6

lP

The comparison of unit costs of H2 production processes with traditional processes (reproduced from Sharma and Kaushik, 2017).

H2O, organic acids

142

0.0106

-

0.0106

Fermentative H2

Molasses

Jo

ur

Photobiological H2

Feedstock

Unit cost of energy content of the fuel USD/GJ

na

Process

Energy content of fuel (MJ/kg)

Pyrolysis for H2 production

Coal , biomass

-

0.00424

Water electrolysis for H2 production

H2O

-

0.01166

H2 from nuclear energy

Electrolysis and water splitting

-

0.01272-0.02014

H2 from Biomass gasification

Biomass

-

0.04664-0.08692

H2 from wind energy

Wind mill

-

0.03604

H2 from photovoltaic power station

Solar energy

-

0.04452

54

Journal Pre-proof H2O

-

0.01378

H2 from photochemical

Organic acids

-

0.02226

Gasoline

Crude petroleum

43.1

0.00636

Fermentative ethanol

Molasses

26.9

0.03339

Biodiesel

Jatropha seed

37.0

0.00424

Natural gas

Raw natural gas

33-50

0.0106

of

H2 from thermal decomposition of steam

lP

re

-p

ro

Graphical abstract

Recycle

ur

Use

Jo

Municipal Solid Waste

Make

Circular Economy & Sustainable Development

na

Food Waste

Waste collection and processing

Pretreatment Electricity FERMENTATION (dark) or (photo) or (sequential dark and photo) or (Combined dark and photo)

Transportation

Bio-hydrogen (renewable source of energy)

55

Gas/ Heat

Journal Pre-proof

Highlights

Bio-hydrogen production from waste materials is discussed in depth.



Industrial waste water, sludge, farming or food waste can act as raw materials.



Dark, photo, sequential and combined fermentation methods are reviewed.



Economic aspects of bio-hydrogen production are critically analyzed.

Jo

ur

na

lP

re

-p

ro

of



56

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6