Hydrogen gas production by electrohydrolysis of cheese whey using photovoltaic cells (PVC)

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DOKUZ EYLUL UNIVERSITY

GRADUATE SCHOOL OF NATURAL AND APPLIED

SCIENCES

HYDROGEN GAS PRODUCTION BY

ELECTROHYDROLYSIS OF CHEESE WHEY

USING PHOTOVOLTAIC CELLS (PVC)

by

Sinan UZUNÇAR

March, 2012 İZMİR

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HYDROGEN GAS PRODUCTION BY

ELECTROHYDROLYSIS OF CHEESE WHEY

USING PHOTOVOLTAIC CELLS (PVC)

A Thesis Submitted to the

Graduate School of Natural and Applied Sciences of Dokuz Eylül University In Partial Fulfillment of the Requirements for the Degree of Master of

Science in Environmental Engineering Program

by

Sinan UZUNÇAR

March, 2012 İZMİR

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ACKNOWLEDGMENTS

I would like to express my appreciation to my advisor Prof. Dr. Fikret KARGI for his advice, guidance and encouragement during my Master Degree studies.

I wish to thank Prof. Dr. İlgi K. KAPDAN, Asst. Prof. Serkan EKER for their contribution, guidance and support.

Finally, my deepest gratitude to my lovely family. Sinan UZUNÇAR

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HYDROGEN GAS PRODUCTION BY ELECTROHYDROLYSIS OF CHEESE WHEY USING PHOTOVOLTAIC CELLS (PVC)

ABSTRACT

Cheese manufacturing industry generates large amounts of high strength wastewater characterized by high chemical oxygen demand (COD) concentration and its disposal constitutes a serious environmental problem with total sugar being mainly responsible for its high COD contents. In this respect the production of hydrogen gas from cheese whey using electrohydrolysis method with simultaneous COD removal presents a promising and novel approach.

In this study, hydrogen gas production with simultaneous COD removal was observed by application of different DC voltages to cheese whey wastewater with DC power suppliers connected to power grid and also with electrical power generated by a photovoltaic cell (PVC). Hydrogen production yields, hydrogen production rates, energy conversion efficiencies, COD removals were considered as criteria for performance comparison.

Keywords: Electricity, electrohydrolysis, hydrogen gas, photovoltaic cells (PVC), wastewater, cheese whey

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PEYNİRALTI SUYUNDAN ELEKTROHİDROLİZ YÖNTEMİYLE FOTOVOLTAİK GÜNEŞ PANELİ (PVC) KULLANARAK HİDROJEN GAZI

ÜRETİMİ ÖZ

Peynir endüstrisi her yıl yüksek şeker içeriğinden dolayı yüksek KOI konsantrasyonu ile karakterize edilen çok miktarda atık su üretmekte ve bu atıkların dış ortamlara deşarjı ciddi bir çevre sorunu teşkil etmektedir. Bu bağlamda peyniraltı suyundan elektrolizle eşzamanlı KOI giderimi yoluyla hidrojen gazı üretimi yeni ve gelecek vadeden bir yaklaşım sunmaktadır.

Bu çalışmada hem şehir şebekesine bağlı DC güç kaynaklarıyla, hem de fotovoltaik panelle (PVC) üretilen elektrik gücüyle peyniraltı suyu farklı voltajlarda elektrik akımına tabii tutulmuş ve KOİ giderimiyle eş zamanlı hidrojen gazı üretimi incelenmiştir. Performans kıyaslama kriterleri olarak hidrojen üretim verimi v e hızı, enerji dönüşüm verimi ve KOI giderim verimi dikkate alınmıştır.

Anahtar kelimeler: Elektrik, elektrohidroliz, hidrojen gazı, fotovoltaik panel (PVC), atıksu, peyniraltı suyu

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vi CONTENTS

Page

THESIS EXAMINATION RESULT FORM ... ii

ACKNOWLEDGEMENTS ... iii

ABSTRACT ... iv

ÖZ ... v

CHAPTER ONE – INTRODUCTION ... 1

1.1 Hydrogen Gas Production Methods ... 1

1.1.1 Reforming of Hydrocarbons ... 1

1.1.2 Electrolysis of Water ... 4

1.1.3 Photolysis of Water and Fermentation of Carbohydrates ... 7

1.1.4 Microbial Electrolysis ... 10

1.1.5 Electrohydrolysis of Organic Wastes ... 12

1.2 Literature Review ... 13

1.3 Objectives and the Scope ... 16

CHAPTER TWO – MATERIALS AND METHODS ... 17

2.1 Bench Scale Experiments with DC Electrical Power ... 17

2.1.1 Effects of Applied DC voltage ... 17

2.1.1.1 Experimental Set Up and Procedure ... 17

2.1.1.2 Analytical Methods ... 19

2.1.2 Effects of Initial COD Concentration ... 20

2.1.2.1 Experimental Set Up and Procedure ... 20

2.2 Solar-Powered Experiments ... 22

2.2.1 Solar-Powered Experiments with Stainless Steel Reactor ... 22

2.2.1.1 Experimental Set up and Procedure ... 22

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2.2.2 Solar-Powered Experiments with Plastic (poly vinyl chloride) Reactor . 24

2.2.2.1 Experimental Set up and Procedure ... 24

2.3 Calculation Methods ... 25

2.3.1 Calculations for Electrohydrolysis Experiments ... 25

CHAPTER THREE –RESULTS AND DISCUSSION ... 27

3.1 Bench scale experiments with DC electrical power ... 27

3.1.1 Effects of Applied DC voltage ... 27

3.1.1.1 Hydrogen Gas Evolution ... 27

3.1.1.2 COD Removals ... 30

3.1.1.3 pH, ORP and Conductivity Changes ... 33

3.1.1.4 Energy Efficiency ... 38

3.1.2 Effetcs of Initial COD Concentration ... 42

3.2 Solar-Powered Experiments ... 59

3.2.1 Solar-Powered Experiment with Stainless Steel Reactor ... 59

3.2.2 Solar-Powered Experiment with Plastic (poly vinyl chloride) Reactor... 74

3.2.2.1 Hydrogen Gas Evolution... 75

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CHAPTER FOUR – CONCLUSIONS ... 96

REFERENCES ... 99

APPENDICES – RAW EXPERIMENTAL DATA AND FIGURES ... 108

A.1 Bench scale experiments with DC electrical power ... 108

A.1.1 Effects of Applied DC voltage... 108

A.1.2 Effects of Initial COD Concentration ... 123

A.2 Raw Data for Solar-Powered Experiments ... 130

A.2.1 Solar-Powered Experiments with Stainless Steel Reactor ... 130

A.2.2 Solar-Powered Experiment with Plastic (poly vinyl chloride) Reactor 140 B.1 Nomenclature ... 149

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CHAPTER ONE INTRODUCTION 1.1 Hydrogen Production Methods

There has been interest in developing alternative clean fuels to replace fossil fuels due to existing environmental problems. Coal and petroleum-based fuels are responsible for significant amount of carbon dioxide, volatile organic compound (VOC), carbon monoxide (CO) and nitrogen oxide (NOx) emissions. To deal with these issues, there has been an effort to obtain our energy supply from renewable and non-polluting energy sources and increase energy and economic security. It is widely believed that hydrogen is an attractive energy carrier of the future with high energy content of 122 kJ g-1 which is about 2.75 times greater than fossil fuels (Aoutiounian et. al; 2005; Das & Veziroglu, 2001; Han & Shin, 2004; Kapdan & Kargi, 2006; Kotay & Das, 2008; Piel 2001 Zhang & Shen, 2005). Hydrogen gas is a clean fuel producing only water vapor with no COx , SOx and NOx emissions. Almost all hydrogen production methods require expensive technologies and are based on utilization of fossil fuels, which are associated with release of large quantities of greenhouse gases. Therefore, current hydrogen production processes need to be replaced with a renewable and environmentally harmless processes (Kapdan & Kargi, 2006).

The purpose of this chapter is to provide a brief summary of current and developing hydrogen production technologies. The areas to be examined include: reforming of hydrocarbons, electrolysis of water, fermentation of carbohydrates, microbial electrolysis, and electrohydrolysis of organic wastes.

1.1.1 Reforming of Hyrocarbons

Fuel processing technologies convert hydrogen containing materials such as methanol, ammonia or gasoline into a gas stream composed primarily of hydrogen, carbon monoxide and carbon dioxide. There are three primary techniques

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commercialized for hydrogen production from hydrocarbon fuels: steam reforming, partial oxidation (POX), and autothermal reforming (ATR).

Steam reforming has a lower operating temperature (>180 °C for methanol and other oxygenated hydrocarbons that can be readily activated, and >500 °C for most conventional hydrocarbons) than POX and ATR, and does not require oxygen. It produces gas stream with a ratio of H2/CO (~3:1) which is considered beneficial for hydrogen production. However, it does operate with emissions of the three process. (Farrauto et al, 2003; Pietrogrande, Bezzeccheri, in: L.J.M.J. Blomen, M.N. Mugerwa, 1993; Sørensen, 2005; Wilhelm, Simbeck, Karp &. Dickenson, 2001). Two types of catalysts are used, non-precious metal (typically nickel based) and precious metals from Group VIII elements (typically platinum or rhodium based). In general, the noble Group VIII metals, particularly Rh are preferred since they show much higher specific activities than nickel catalysts. However, the high cost of Rh is driving some researchers to develop alternative catalysts (such as Co-based catalysts) (Song, Zhang, Watson, Braden & Ozkan, 2007). Due to severe mass and heat transfer limitations, process results in low energy conversion efficiencies taking account of heating values. Thermal efficiencies of these reactors can be up to approximately 85% based on the higher heating values (Sørensen, 2005).

Partial oxidation or catalytic partial oxidation (CPOX) converts hydrocarbons to

hydrogen by partially oxidizing (controlled combustion) hydrocarbon compounds with oxygen. It does not require a catalyst for operation and is more sulfur tolerant than steam reforming and ATR. The non-catalytic process occurs at high temperatures (1300-1500 °C) with a H2/CO ratio from 1:1 to 2:1. In this case, some soot formation can be observed. Catalysts can be added to the partial oxidation system to decrease operation temperatures. For natural gas conversion, the catalysts are typically based on Ni or Rh. However, coke formation can be observed when these catalyst are used (Farrauto et al, 2003; Sørensen, 2005). Typical thermal energy efficiencies of POX reactors with methane fuel are 60-70%, based on heating values (Sørensen, 2005).

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Autothermal reforming is typically conducted at a lower pressure than POX.

Since POX is exothermic and ATR incorporates POX, these processes do not need an external heat source for the reactor. However, combination of these systems have expensive and complex oxygen separation unit. (Farrauto et al, 2003; D.J Wilhelm et all, 2001) A significant advantage of this process over steam reforming is that it can be stopped and started very rapidly while producing a larger amount of hydrogen than POX alone owing to different thermal and catalytic zone formation in ATR. For methane reforming the thermal efficiency is comparable to that of 60-70%, based on higher heating values, and less than steam reforming (Krumpelt, Krause, Carter, Kopasz, & Ahmed, 2002; Sørensen, 2005).

The Reforming reactions of hydrocarbon fuels can be generalized as follows :

Steam reforming

CmHn + 𝑚H2O = 𝑚CO + (𝑚 + 1 2𝑛) H2

∆H = hydrocarbon dependent, endothermic Eqn 1.1

Partial oxidation CmHn + 1 2𝑚 O2 = 𝑚CO + 1 2 H2

∆H = hydrocarbon dependent, exothermic Eqn 1.2

Autothermal reforming CmHn + 1 2𝑚H2O + 1 4𝑚O2 = 𝑚CO + ( 1 2𝑚 + 1 2𝑛)H2

∆H = hydrocarbon dependent, thermally neutral Eqn 1.3

Most hydrocarbons contain at least some amount of sulfur which is detrimental for the process catalyst. In addition to energy input, this issue is probably the biggest challenge to reforming technology.

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1.1.2 Electrolysis of Water

The interest in extracting hydrogen from water is fueled by the need to find a renewable, sustainable and environmentally safe alternative energy source. Electrolysis is the conversion of electrical energy to chemical energy in the form of hydrogen and oxygen as by-product by splitting water. There are three main type of electrolysis processes, alkaline electrolyzers, proton exchange membrane (PEM) and solid oxide electrolysis cells (SOEC). In addition to these technologies, photoelectrochemical (PEC) water splitting is relatively new developing technology.

Alkaline electrolyzers are generally composed of an aqueous alkaline electrolyte

of approximately 30 % NaOH, electrodes and a microporous separator. The most common cathode material is nickel with a catalytic coating, such as platinum. For the anode, nickel or copper metals coated with metal oxides, such as manganese, ruthenium is used. Due to corrosion of electrodes and correspondingly, other related system losses, the liquid electrolyte must be replenished. In an alkaline cell the water is decomposed into OH- and H+. The OH- travels through the electrolytic material to the anode where O2 is formed. The hydrogen is left in the alkaline solution and then separated from the water in a gas liquid separations unit outside of the reactor (B. Sørensen, 2005) Typical current density is 100-300 mA cm-2

and efficiencies are 50-60% based on lower heating value of hydrogen (Turner, Sverdrup, Mann, Maness, Kroposki, Ghirardi, Evans & Blake, 2008).

PEM electrolyzers typically use platinium black, iridium, ruthenium and rhodium

for electrode catalysts and a nafion membrane which separates the electrodes and acts as gas separator. Therefore, there is no need for a separation unit. In PEM, the water is decomposed into proton and OH- at the anode, and then proton travels through the electrolytic material to the cathode where hydrogen is formed. The oxygen gas remains behind with the unreacted water. PEM electrolyzers have low ionic resistances and therefore high current densities (>1.6 A cm-2) can be achieved while the efficiencies vary between 55 and 70% (Sørensen, 2005; Turner et al, 2008).

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SOECs are essentially solid oxide fuel cells operating in reverse. SOECs operate

similar to the alkaline system in that oxygen ions travel through the electrolyte leaving the hydrogen in unreacted steam stream. In SOEC thermal and electrical energy are utilized to split water. The higher temperatures increase the electrolyzer efficiency by decreasing the demand of electrical energy and the anode/cathode over potentials which cause power loss in electrolysis (Utgikar & Thiesen, 2006). High temperature electrolysis is dependent on the temperature and the utilized thermal source. The efficiency as a function of electrical input alone can be as high as 85-90% (National academy of science, Washington DC, 2004). However, when the thermal source (obtained from nuclear energy or combustion of fuels) is included the efficiencies can slightly drop. Therefore, suitable solar energy systems (such as electricity production with photoconverter) are under development to increase total energy efficiency of high temperature electrolyzers. One advantage with PV (photovoltaic) technology is that PVs do not emit green house gases during the operation.

SOEC electrolyzers are the most energy efficient. However, SOEC technology has challenges with corrosion, thermal cycling, and sealing. PEM electrolyzers operate more efficiently than alkaline electrolyzers and, do not have corrosion and sealing issues, but operation and material costs are higher than alkaline systems. Besides, alkaline electrolyzers are the most experienced and lowest in capital cost. They have the lowest efficiency output having the highest electrical energy input.

The overall reactions for alkaline PEM, SOEC systems at anode and cathode are:

Alkaline electrolyzer

Anode: 4OH- → O2 + 2H2O Eqn. 1.4

Cathode: 2H2O + 2e- → H2 + 2OH- Eqn. 1.5

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PEM electrolyzer

Anode: 2H2O → O2 + 4H+ 4e- Eqn. 1.7 Cathode: 4H+ + 4e- → 2H2 Eqn. 1.8 Overall: H2O → H2 + 2OH- ∆H = -288 kj mol-1 Eqn. 1.9

SOEC Anode: H2O + 2e- → H2 + O2- Eqn. 1.7 Cathode: O2- → 1 2 O2 + 2e Eqn. 1.8 Overall: H2O → H2 + 1 2 O2 Eqn. 1.9

Photoelectrochemical (PEC) electrolysis or photoelectrolysis uses sunlight to

directly decompose water into hydrogen and oxygen, and uses semiconductor materials similar to those used in photovoltaics. In photovoltaics, two doped semiconductor materials, a p-type and n-type, are brought together forming a p-n junction (Norbeck, Heffel, Durbin, Tabbara, Bowden & Montani, 1996). At the junction, a permanent electric field is formed when the charges in the p and n-type of material rearrange. When a photon with energy greater than the semiconductor material’s bandgap is adsorbed at the junction, an electron is released and a hole is formed. Since an electric field is present, the hole and electron are forced to move in opposite directions which, if an external load is also connected, will create an electric current (Khaselev, Bansal, & Turner, 2001). This type of situation occurs in photoelectrolysis when a photocathode, p-type material with excess holes, or a photoanode, n-type of material with excess electrons, is immersed in an aqueous electrolyte, but instead of generating an electric current, water is split to form hydrogen and oxygen. Various materials have been investigated for use in photo electrodes such as thin-film WO3, Fe2O3 and TiO2 as well as n-GaAs, n-GaN, CdS, and ZnS for photoanode; and ClGS/Pt, p-lnP/Pt, and p-SiC/Pt for the photocathodes (Arriaga & Fernandez, 2002; Mor, Varghese, Paulose, Shankar & Grimes, 2006) The hydrogen production efficiency is generally low and is limited by characteristics of photoelectrodes, resistance of the system and the sunlight intensity. The interested readers is referred to the following reference for more details for the photoelectrode

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configurations and the possible photocell and photoreactor design for hydrogen production by PEC water splitting (Lorna, Wan & Mohammed, 2011 ).

1.1.3 Photolysis of Water and Fermentation of Carbohydrates

A great deal of research on bio-hydrogen production was realized over the last thirty years due to increased attention to sustainable development and waste minimization (Carrieri, Kolling, Ananyev, Dismukes, 2006). The main biological methods used for bio-hydrogen production include: photolysis of water by green algae or cyanobacteria (direct photolysis), dark fermentative hydrogen production during acidogenic phase of anaerobic digestion of organic material, photo-fermentative process, sequential or combined dark/light fermentation.

Direct photolysis uses solar energy to convert water to H2 and O2 (Eqn. 1.10). The advantage of this technology is that the primary feed is water, which is inexpensive and available almost everywhere (Kapdan & Kargi, 2006). The use of simple sugars as supplement was reported to promote hydrogen evolution (Shah, Garg & Madamwar, 2001). But, this process requires a large surface area to collect sufficient light for avoiding the occurring turbidity due to microbial growth. Unfortunately, these microorganism in addition to producing hydrogen, they also produce oxygen, which inhibits the organisms to cease hydrogen production. Strong inhibition effect of generated oxygen on hydrogenase enzyme is the major limitation for the process (Kapdan & Kargi, 2006). Therefore studies are performed to either, identify or engineer less oxygen sensitive organism, separate the hydrogen and oxygen cycles, and/or change the ratio of oxygen production during photosynthesis to oxygen consumption during respiration in order to prevent oxygen augmentation. Besides, existence of hydrogen and oxygen in the produced gas stream causes additional gas separation unit for purification of hydrogen gas. Recent studies are concentrated on development of hydrogenase and bi-directional hydrogenase deficient mutant of

Anabaena sp. in order to increase the rate of hydrogen production (Kapdan & Kargi,

2006). At the present time the rate of algal hydrogen production is considerably lower than that obtained by dark or photo-fermentations. Consequently, this

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technology has significant promise, but also tremendous challenges for its commercial application. Low hydrogen production potential and no waste utilization are the other disadvantages of hydrogen production by algae. Generalized reaction of photolysis of water, production of O2 and H2 can be represented by the following equations:

2 H2O + light energy → 2 H2 + O2 Eqn.1.10 6 H2O + 6 CO2 + light energy → C6H12O6 + 6 O2 Eqn 1.11 C6H12O6 + 6 H2O + light energy → 12 H2 + 6 CO2 Eqn 1.12

Dark fermentation is a promising way of using inexpensive feedstock, from several organic wastes as a substrate for hydrogen production. Usually, for fermentation processes monosaccharides are the preferred carbon source which can be produced by acidic or enzymatic hydrolysis of polysaccharides (starch, cellulose) (Kapdan & Kargi, 2006). The pathways are dependent on the type of bacteria used. Standard fermentative pathway has a theoretical maximum production of 4 moles of hydrogen per mole of glucose when acetic acid is the only product. However, lower yields are obtained in practice since part of the glucose is used for microbial growth and maintenance (Argun, Kargı & Kapdan, 2009). Butyric acid formation is accompanied with formation of 2 mol of H2 per mole of glucose and propionic acid formation consumes 1 mol of H2 per mole of propionic acid (Kim, Han, Kim & Shin, 2006; Luo, Xie, Zou, Wang , Zhou & Shim, 2010). Lactic acid and ethanol fermentations do not result in H2 formation or consumption. When both acetic and butyric acids are produced in dark fermentation of glucose, theoretically, 2.5 mol H2 is formed per mole glucose (Krup, Widmann, 2008). In dark fermentation, hydrogen gas evolution reactions are shown in Eqn. 1.13 and Eqn. 1.14 when acetic acid or butyric acid is the only end- product, respectively. The gas produced is a mixture of hydrogen, carbon dioxide, methane, carbon monoxide, and some hydrogen sulfide. Therefore gas purification is required to produce high purity hydrogen. For dark fermentative process, the partial pressure of hydrogen is a important factor too; as the hydrogen pressure increases the hydrogen production decreases (Levin, Pitt & Love,

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2004). The only solution to this limitation is to remove the hydrogen as it is produced. Dark fermentation technologies require further improvements for large-scale applications and commercialization.

C6H12O6 + 2H2O → 2CH3COOH + 2CO2 + 4H2 ∆Go = −206 kJ Eqn 1.13 C6H12O6 + 2H2O → CH2CH2CH2COOH + 2CO2 + 2H2 Eqn 1.14

Photo-fermentation is also a promising method of hydrogen gas production.

Photosynthetic bacteria (also known purple non-sulphur bacteria (PNS) utilize a wide variety of volatile fatty acids (VFA) as electron donors and light as energy source to produce hydrogen in the presence of nitrogenase enzyme. In this process light harvesting pigments such as carotenoids, chlorophylls, and phycobilins absorb light energy which is transferred to membrane reaction centers similar to those in photolytic organisms (algae). Sunlight converts water into electrons, protons, and O2 (Sørensen, 2005). The nitrogenise catalyst is used to react the protons and electrons with nitrogen to produce ammonia, hydrogen and ADP. Since oxygen inhibits the nitrogenase enzyme, cyanobacteria separate nitrogen fixation and oxygen generation either spatially or temporally. In nature the bacteria use the hydrogen by-product to power other energy requiring processes via the uptake hydrogenase enzyme (Tamahnini, Troshina, Oxelfelt, Selma, Lindblad, 1997). Therefore, researchers are trying to genetically modify the bacteria to suppress this enzyme. H2 production performance of PNS bacteria is evaluated on the basis of the H2 yield and the light efficiency. Depending on the carbon source, H2 formation yields up to %80 of the theoretical yield were reported in literature (Akkerman, Janssen, Rocha & Wijffels, 2002; Basak & Das, 2009; Fascetti, Todini, 1995; Brentner, Peccia & Zimmerman, 2010; Kargi & Argun, 2011, Kargi & Sagnak, 2011; Sagnak, Kapdan & Kargi, 2011). Design of photo-bioreactors enabling efficient H2 production is still a challenge (Berberoglu, Pilon, 2010). Operating parameters also affect the photo-fermentation process efficiently. Development of new technologies for more effective illumination and light distribution, elimination of substrate and product inhibition by slow addition of substrate (VFA) and continuous removal of products (H2 gas), development of new strains with improved metabolic capabilities to tolerate

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high VFA and ammonium concentrations may improve the hydrogen yield and production rate and make this method more feasible and profitable for commercial applications (Kargi & Argun, 2011).

As shown in Eqn. 1.15 the maximum theoretical yield is 4 mol of hydrogen per mol of acetic acid in light fermentation.

CH3COOH + 2 H2O + light energy → 4 H2 + 2 CO2 Eqn 1.15

Dark and photo-fermentations can be used in sequential (consecutive operation)

or combined (simultaneous operation) modes. Most of the reported studies are focused on sequential batch fermentations of dark fermentation effluent after some pre-treatment for bio-hydrogen production by photo-fermentation. Combined dark-light fermentations for bio-hydrogen production have significant advantages over sequential fermentation owing to reduced fermentation time and high hydrogen yields. The major problem in the combined fermentation is the lower hydrogen formation rates, once PNS bacteria are adapted to carbohydrate utilization first then VFA fermentation takes place after a long lag time (Argun & Kargi, 2010a, 2010b, Ozmihci & Kargi, 2010, Kargi & Sagnak, 2011). Theoretically, as shown in Eqn.1.16, in sequential or combined photo-fermentation 1 mol of glucose can be converted to 12 mol of hydrogen if the only VFA utilized is acetic acid. However, real achievable yields are much lower due to formation of a mixture of VFAs and utilization of part of the substrate for growth and maintenance (Kargi & Argun, 2011, 2012).

C6H12O6 + 6 H2O + light energy  12 H2 + 6 CO2 Egn.1.16

1.1.4 Microbial Electrolysis

Microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) are bioelectrochemical systems (BESs). MFC technology is a galvanic process in which a biological anode where anaerobic organisms (exoelectrogens or anode respiring organisms) degrades organic compounds (non-fermentable compounds such as acetate) in the feed generating electrons and protons in the anodic chamber .

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The released protons and electrons are transferred to the cathode and combine with oxygen to form water at cathode (in MEC a biocathode can be used as a modification). In MEC, the only difference is reduction of proton at cathode to produce H2 gas with some energy input (Kargi & Eker, 2009; Logan, Hamelers, Rozendal, Schroder, Keller, Freguia, Aelterman,, Verstraete & Rabaey, 2006; Rozendal, PhD thesis, 2007).

MEC operates in anaerobic mode and an external voltage is applied to the cell to form H2 gas. The added energy is required since acetate (or non-fermentable substrate) decomposition is not spontaneous under standard conditions (Call & Logan, 2008; Ditzig, Liu & Logan, 2007). The theoretical potential for hydrogen production in neutral (pH = 7) is -0.61 V, Vcat vs. Ag/AgCl (Ditzig, Liu, & Logan, 2007). Exoelectrogens generate an anode potential of approximately Van= - 0.5 V. Therefore, the minimum applied potential (Vapp = Van - Vcat) is 0.11 V (Ditzig, Liu, & Logan, 2007). For acetate, the actual applied voltage is bigger than 0.3 V due to electrode overpotentials and ohmic resistance. Optionally, membrane (anion exchange membrane AEM, cation cellulose-ester membrane CEM, bipolar BPM etc.) can be utilized to separate anode and cathode chamber. But, there are many challenges for the use of ion exchange membranes in BESs using wastewater. And also these problems arise when membranes other than cation exchange membranes are used; such as BPM, UFM (ultra filtration membranes, AEM etc.). Recent studies recommend membranless cell system due to aforementioned reason and ohmic voltage losses. Although ohmic losses are independent of current according to Ohm’s law, ohmic voltage loss occurred by resistance of electron flow through electrical conductors (external circuitry) and resistance of ion flow through liquid electrolyte. Therefore, membrane decreases the system performance in real applications of MEC. In some instance, at the anode, methanogenesis could be faced. To control the methanogenesis in these systems, intermittent draining and air exposure or situ air-sparging (Ahn, & Logan. (2012), controlling the pH (Logan, Oh, Kim & Ginkel, 2002; Kim, Hwang, Jang, Hyun, Lee, 2004), removing hydrogen as it produced (Mizuno, Dinsdale, Hawkes, Noike, 2000) and applying relatively low (3-4.5 V) DC voltage (Dictor, Jolulian, Touzé, Ignatiadis & Guyonnet, 2010; Roychowdhury,

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2006) have been proposed. However, these strategies may result in more complex system with maintenance requirements, converting into more expensive system. The generation of hydrogen gas with MEC system is a field that is experiencing continued growth and development.

1.1.5 Electrohydrolysis of Organic Wastes

Hydrogen gas production by electrohyrolysis of organic compounds is relatively new and developing method. The most important advantage of this developing method is the wide type of industrial organic waste utilization capability. In contrast to MFC and MEC system, this system operates in a single chamber under anaerobic conditions and does not need membrane to separate the anode and cathode electrodes. Hydrogen generation occurs where the electron and the proton meets. In electrohydrolysis cell, organic compounds in wastewater are dissociated and decomposed releasing VFAs and protons into the media by the natural digestion (Eqn. 1.17). Protons released by natural digestion of organic compounds combine with the electrons released from electrodes (Eqn. 1.18) to form hydrogen gas (Kargi, 2011)

The level of applied voltage affects the rate and extent of H2 gas formation. Depending on the proton-electron balance and changes in the system resistance during the operation time, energy conversion efficiency (produced hydrogen gas/electrical energy input) of the system varies. The more proton by the natural digestion releases, the more hydrogen gas production depending upon applied DC voltage level can be achieved. Part of the hydrogen gas is produced from degradation of VFAs by DC voltage resulting in significant COD removal in some instance. Only drawback of this method is corrosion of the utilized electrodes and metal salt formation in the cell medium. Maybe, another source of additional electrode efficiency loss is the precipitation of carbonates and biomass on the anode and cathode, which reduce the conductivity and so, electrical current for a given applied voltage to the organic waste. In addition, formation of metal salts may cause inhibition affect on natural digestion which directly reduces the hydrogen gas evolution. Gradual dosage of a precipitant into the cell and using more resistant

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electrodes may solve these problems. Hydrogen production from organic wastes by electrohydrolysis has a great potential for commercial applications due to its simplicity and low voltage requirements. Moreover, this process becomes more attractive since waste materials are used as substrates, such as dark fermentation effluent (Tuna, Kargi & Argun, 2009), industrial wastewaters (Eker & Kargı, 2009), waste anaerobic sludge (Kargi, Catalkaya & Uzuncar, 2011), olive mill wastewater and landfill leachate ( Kargi & Catalkaya, 2011a, 2011b).

Organic Waste Natural Digestion  Decomposed Organics-

... + mCO2 + nH+ Eqn.1.17

nH+ + ne-  𝑛

2 H2 (g) Egn.1.18 1.2 Literature Review

Tuna, Kargi & Argun (2009) published the first report of hydrogen gas production by electrohydrolysis method. This study constitutes the first comprehensive study on hydrogen gas production by DC power application over dark fermentation effluent to replace light fermentation. They conducted dark fermentation effluent of wheat powder solution containing VFAs (1-5 g L-1) with a low voltage DC (1-3 V) current using copper and graphite electrodes at different pHs (pH = 2-7) to investigate the cumulative hydrogen gas production from VFA. The highest hydrogen production rate was obtained with 2 V applied voltage at pH = 2 and 10.85 g TVFA L-1 as 1.25 m3 H2 d-1 (10.4 ml h-1 0.2 L solution). In the light of these results, the effects of applied DC voltages (0.5-5 V) on the rate and extent of hydrogen gas production was investigated using different organic wastewaters with and without photovoltaic panel (PVC).

Waste anaerobic sludge was subjected to different DC voltages (0.5-5 V) using aluminum electrodes with a DC power supply (Kargi, Catalkaya & Uzuncar, 2011). The highest cumulative hydrogen production (2775 mL), daily hydrogen formation (686.7 mL d-1), hydrogen yield (96 mL H2 g-1 COD) and percent hydrogen (94.3%) in the gas phase were obtained with 2 V DC voltage. The highest energy conversion

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efficiency (74%) was obtained with 2 V. In this study, COD concentrations decreased from initial 70 ± 3 g L-1

to nearly 12 ± 3 g L-1 almost in all experiments no matter what the applied voltage was.

Diluted olive mill wastewater was subjected to 0.5-4.0 V DC voltages with DC power supplier using aluminum electrodes (Kargi & Catalkaya, 2011a). The highest cumulative hydrogen production (3020 mL) and hydrogen yield (2500 mL H2 g-1 COD) were obtained with 3 V DC voltage while the highest percent hydrogen (95%) in the gas phase, hydrogen gas formation rate (614 mL d-1), percent COD removal (44%) and energy conversion efficiency (95%) were obtained with 2 V.

Landfill leachate was also used to produce hydrogen gas by electrohydrolysis method with aluminum electrodes and DC power suppliers at DC voltages, 0.5-5.0 V (Kargi & Catalkaya, 2011b). The highest cumulative hydrogen production (5000 mL) hydrogen yield (2400 ml H2 g-1 COD), daily hydrogen formation (1277 mL d-1), and percent hydrogen in the gas phase (99%) were obtained with 4 V DC voltage. The highest energy conversion efficiency (80.6%) was obtained with 1 V DC voltage. The highest COD removal (77%) was also obtained with 4 V DC voltage.

Also, hydrogen gas production from industrial wastewater was investigated with a mechanically mixed and sealed stainless-steel reactor using photovoltaic cell (PVC) (Kargi, 2011). As a raw material an industrial wastewater was utilized. The system consisted of a PVC panel with 80 cm x 120 cm dimensions, a voltage regulator (to adjust the voltage to the desired level of 13.5 V), a battery (to store electrical energy generated by the PVC panel) and a stainless steel reactor with dimensions of Do = 21 cm, H = 48 cm and volume of 16.8 L. Three different electrodes, graphite, stainless steel and aluminum electrodes were used for comparison (Do = 0.9 cm, L = 49.5 cm). Within 432 h (Nov 10-28, 2008), 9.4 L hydrogen gas produced from water with the hydrogen gas formation rate of 0.522 L d-1 using stainless-steel electrodes while 95 L hydrogen gas formed within 528 h with the hydrogen gas formation rate of 4.32 L d-1 when the same electrical power was applied to the industrial wastewater using PVC cells (Dec 18, 08-Jan 9, 2009). Percentage of hydrogen gas in the head space varied

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between 90 and 98%. TOC content of wastewater decreased from 4500 to 4200 mg L-1. Energy conversion efficiency with the stainless-steel electrode was found to be 42% (Eker & Kargi, 2009).

Within 264 h, only 3.22 L hydrogen gas produced yielding hydrogen gas production rate 0.29 L d-1, percent hydrogen (85-90%) while 3.81 L H2 was produced in 192 h with H2 production rate (0.476 L d-1) with graphite electrodes at the same applied PVC power (June 24-July 2, 2009). Percentage of hydrogen reached 95% in then head space with wastewater. TOC content decreased from 2726 to 2404 mg L-1. Energy conversion efficiency with the graphite electrodes was calculated as 0.92% using PVC cells (Eker & Kargi, 2009).

Hydrogen gas production from water was 41.4 L within 144 h (July 4-10, 2009) yielding hydrogen formation rate of 6.85 L d-1 with the utilization of aluminum electrodes using PVC cells. Percent H2 in the head space was nearly 98%. When wastewater was used in the reactor, 98 L H2 was produced within 144 h (July 14-20, 2009) with approximately 99% H2 in the gas phase. Hydrogen formation rate from wastewater was 16.33 L d-1. TOC decreased from 2350 to 1640 mg L-1 at the end of 144 h. Hydrogen yield of 10.45 L H2 g-1 TOC was higher than that obtained from stainless-steel electrode. In this case, energy conversion efficiency reached 55% (Eker & Kargi, 2009).

There are limited fermentative hydrogen gas production studies using cheese whey wastewater and powder (Antonopoulou et al, 2008; Azbar, Dokgoz, Keskin, Korkmaz. & Syed, 2009; Azbar, Dokgoz & Peker , 2009; Castello, Santosa, Iglesiasb, Paolinob, Wenzel, Borzacconi & Etchebehere, 2009; Davila, Alatriste-Mondrago´n, Rodriguez & Razo-Flores, 2008; Ferchichi, Crabbe, Gil, Hintz & Almadid, 2005; Kargi, Eren & Ozmihci, 2012; Yang, Zhang, McGarvey & Benemann, 2007). But there is no hydrogen gas production from cheese whey wastewater by electrohydrolysis method with and without PVC cells reported on literature.

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1.3 Objectives and the Scope

The major objective of this study is to investigate hydrogen gas production from cheese whey wastewater by electrohydrolysis.

Detailed objectives of the study can be summarized as follows:

 To investigate the effects of applied DC voltage on hydrogen gas production rate and yield with simultaneous COD removal.

 To investigate the effects of initial cheese whey concentration (COD) on the rate and yield of H2 gas formation and COD removal.

To investigate the effects of initial cheese whey concentration (COD) on hydrogen gas production rate and yield using photovoltaic panel (PVC).

 To determine the most suitable operating conditions maximizing hydrogen gas yield and formation rate.

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17

CHAPTER TWO

MATERIALS AND METHODS 2.1 Bench Scale Experiments with DC Electrical Power

2.1.1 Effects of Applied DC Voltage

2.1.1.1 Experimental Set Up and Procedure

The experimental set up was consisted of DC power suppliers (TT-T-ECHNI-C MCH-305T and MCH-305D-II, China), two aluminum electrodes for every bottle and 1 L serum bottles containing 500 ml cheese whey (CW). Aluminum electrodes (in all experiments L= 24.5 cm, Dcat=8 mm, Dan= 6 mm) were immersed inside the bottles containing the CW and connected to the DC power supply by wires. The bottles were closed tightly by silicone rubber stoppers and screw caps and were sealed carefully using silicone to avoid any gas leakage. All experiments were done at room temperature (25 °C). Applied voltages were varied between 0.5- 5 V DC voltage. The CW wastewater was obtained from Pınar Sut Company, Pinarbasi, Izmir. Composition of CW is summarized in Table 2.1.

Table 2.1 Initial characteristics of raw cheese whey wastewater.

Parameters

Total Chemical Oxygen Demand (TCOD, mg L-1₎ ₈₆₈₃₀

Total Sugar (TSg, mg L-1₎ ₆₂₃₂₀

Total Volatile Fatty Acids (TVFA, mg L-1₎ ₅₄₅₀

Suspended Solids (SS, g L-1₎ _4.95 Total Solids (TS, g L-1₎ _54.12 Total N (mg L-1₎ ₈₁₂ Total P (mg L-1₎ ₂₃₀ NH4-N (mg L-1) 150.8 pH 4.59 ORP (mV) 154.5 Conductivity (mS cm-1₎ _6.52

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The initial COD, pH, ORP and conductivity of the experimental CW are presented in Table 2.2.

The bottles were mixed manually several times a day and the experiments were done in duplicates. The applied DC voltages (V) and the amperes (A) were monitored during the course of experiments. The current intensities (mA) varied during the course of experiments due to changes in the system’s resistance as a result of corrosion on the electrodes and changes in the CW wastewater composition. Current intensities were recorded by power supply unit and were also measured by using a multimeter (T-TTECHIC-MY64, China) connected to the system in series. Produced hydrogen gas was collected in the head space of the serum bottles.

Table 2.2 Initial characteristics of diluted CW wastewater. DC Voltage (V) Total COD (mgL-1₎ Total Sugar (mg L-1₎ TVFA (mg L-1₎ pH ORP (mV) Conductivity (mS/cm) SS (gL-1₎ TS (gL-1₎ 0.5 32061 23457 2020 4.7 135.4 4.31 2.6 19.5 1 32061 23457 2020 4.7 135.4 4.31 2.6 19.5 2 32061 23457 2020 4.7 135.4 4.31 2.6 19.5 3 31837 13449 1948 4.83 133.7 4.29 1.79 19.48 4 31837 13449 1948 4.83 133.7 4.29 1.79 19.48 5 32098 25601 1960 4.72 137.2 4.17 1.96 22.12

Two control experiments were performed for every applied DC voltage. In water control, the same voltages were applied to water to determine H2 production by electrolysis of water. CW control was used to determine H2 production by bacterial decomposition of CW with no DC power application. Aluminum anode released Al(III) ions and electrons into the solution upon application of DC voltage according to the following reaction.

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Anode: Al0 → Al(III) + 3e- Eqn. 2.1 The released electrons combined with the protons released from decomposition of organic compounds present in the CW.

nH+ + ne- → n/2 H2(g) Eqn. 2.2 Organic compounds in CW are mainly volatile fatty acids (VFAs), carbohydrates, proteins and NH4+-N. Al(III) ions were released into the CW forming Al salts such as Al(OH)3.

Anaerobic conditions were maintained by passing argon gas from the head space of the bottles for 15 minutes at the beginning of the experiments. In all experiments, the initial oxidation reduction potentials (ORP) were around 135 ± 2 mV which decreased under −100 ± 40 mV at the end of the experiment.

All CW wastewater containing bottles were operated at and interval of 0.5-5 V DC voltage. Samples were removed from the bottles and every day for COD, pH and oxidation-reduction potential (ORP) measurements after gas analysis. Total sugar (TSg), total volatile fatty acids (TVFA), suspended solids (SS), total solids (TS) were also measured at the beginning and the end of the experiments. The initial and final weight of the electrodes were measured to determine Al(III) loss to the solution.

2.1.1.2 Analytical Methods

CW samples were removed from the bottles everyday and COD analyses were done according to Standard Methods using the closed reflux method (Greenberg et al., 2005). pH and oxidation-reduction potential (ORP) measurements were done by using pH and ORP meters with the relevant probes (WTW Scientific, Germany). Conductivities of water and CW were determined using Hach & Lange conductometer (Model 58258-00, Germany). Total sugar concentrations were determined by the acid-phenol spectrometric method (Dubois et al., 1956). TVFA analyses were carried out by using analytical kits (Spectroquant, 1.01763. 0001, Merck, Darmstadt, Germany) and a PC spectrometer (WTW Photolab S12).

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Hydrogen gas was sampled from the head space of the bottles by using gas-tight glass syringes. Hydrogen gas concentration in the gas phase was measured by using a gas chromatograph (HP Agilent 6890). The column was Alltech, Hayesep D 80/100 6” × 1/8” × 085”. Nitrogen gas was used as carrier with a flow rate of 30 ml min−1 and the head pressure was 22 psi. Temperatures of the oven, injection, detector, and filament were 35 oC, 120 oC, 120 oC, 140 oC, respectively.

The amount of total gas produced was determined by water displacement method everyday using sulfuric acid (2%) and NaCl (10%) containing solution. The cumulative hydrogen gas production was determined by using the following equation (Logan et al., 2002):

VH2, i = VH2, i -1 + VW CH2,i + VG,i CH2,i – VG,i-1 CH2, i-1 Eqn 2.3 where VH2, i and VH2, i -1 are the volumes of cumulative hydrogen (mL) calculated after the ith and the previous measurement; VW is the total gas volume measured by the water displacement method (mL); CH2,i is the concentration of H2 gas in the total gas measured by the water displacement method (%); VG,i and VG,i-1 are the volumes of the gas in the head space of the bottle for the ith and the previous measurement (mL); CH2,i and CH2, i-1 are the percent H2 in the head space of the bottle for the ith and the previous measurement. The amount of released hydrogen gas and in the head space of the bottle were measured independently and added up to determine cumulative H2 formation for every period of sampling.

2.1.2 Effects of Initial COD Concentration

Experimental set up and procedures were the same as in section 2.1.1.1. The only difference was the initial COD concentrations and the other characteristics as presented in Table 2.3. Serial dilutions were made from original CW to obtain desired COD concentrations. The applied voltage was adjusted to 3V. Also, one experiment for hydrogen gas evolution from water at 3V applied DC voltage and

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another experiment with no voltage application were performed as control experiments. Diluted CW concentrations were varied between 4851-25025 mg L-1. Table 2.3 Initial characteristics of diluted CW wastewater after serial dilutions.

DC Voltage (V) Total COD (mgL-1₎ Total Sugar (mg L-1₎ TVFA (mg L-1₎ pH ORP (mV) Conductivity (mS/cm) SS (gL-1₎ TS (gL-1₎ 3 4852 3790 387 6.44 92.3 0.922 0.37 4.18 3 11530 8535 715 5.06 110.4 1.581 0.77 8.27 3 16295 12648 1010 5.09 108.5 2.16 1.13 12.83 3 21438 16942 1308 4.91 119.0 2.69 1.47 16.51 3 25025 19093 1449 4.8 124.8 3.22 1.72 18.81

Anaerobic conditions were maintained by passing argon gas from the head space of the bottles for 15 minutes at the beginning of the experiments. The initial oxidation reduction potentials (ORP) were around 110 ± 20 mV which decreased under -50 mV at the end of the experiment.

All diluted CW containing bottles were operated at 3V DC voltage. Samples were removed from the bottles every day for COD, pH and oxidation-reduction potential (ORP) measurements after gas analysis. Total sugar (TSg), total volatile fatty acids (TVFA), suspended solids (SS), total suspended solids (SS), and the initial and final weight of the electrodes were also measured to determine Al(III) loss to the solution.

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2.2 Solar-Powered Experiments

2.2.1 Solar-Powered Experiment with Stainless Steel Reactor

The experimental system consisted of a PVC panel with 80 cm x 120 cm dimensions, a voltage regulator, a battery and a stainless-steel sealed reactor with dimensions of Do = 21 cm, H = 48 cm and volume of 16.8 L as showed in Figure 2.1. Wastewater volume in the reactor was 13.5 L with a headspace of 3.3 L. The PVC panel was manufactured at the Solar Energy Res. Institute, Ege University, Izmir, Turkey and contained 32 cells with a total power supply of 115 W (i.e., 3.6 W for each cell) providing 18 V voltage with 6 A current. A voltage regulator was used to adjust the voltage to desired level of 13.5 V in our system. A battery was used to store electrical energy generated by the PVC panel and to provide constant current to the reactor when the sun light was not available. Mechanically mixed and sealed stainless steel reactor contained diluted CW, aluminum anode/ cathode and pressure gauge. Aluminum electrodes were used to transmit the electrical current (electrons) generated by the PVC to the aqueous medium in the reactor. Dimensions of the electrodes were L = 49.5 cm and Do = 0.9 cm which were mounted on the head plate of the reactor and completely immersed in aqueous organic waste inside the reactor.

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Serial dilutions were made from original CW (Table 2.4) to obtain desired COD concentrations and experiments were started by filling the reactor with diluted wastewater, closing the head plate tightly and connecting the PVC to the electrodes through a voltage regulator and battery. Control experiments were performed with tap water to determine hydrogen gas production by water electrolysis. Control experiments with no voltage application to CW were done for comparison. In all experiments the liquid and head space volumes in reactor were 13.5 L, 3.3 L, respectively. Table 2.4 presents the initial wastewater characteristics in this set of experiment. This set of experiments was operated during summer 2011 at 30-35 °C. Table 2.4 Initial characteristics of diluted CW wastewater for solar-powered stainless-steel reactor.

Voltage (V) Total COD (mgL-1₎ Total Sugar (mg L-1₎ TVFA (mg L-1₎ pH ORP (mV) Conductivity (mS/cm) TS (gL-1₎ SS (gL-1₎ 13.63 9859 6446 617 6.53 222.4 1.03 9.9 0.9 13.44 15658 11902 917 5.15 196.2 1.62 14.03 1.44

During the course of the experiments, the voltages applied to the reactor were almost constant. However, the current intensities (A) varied due to changes in the resistance of the system and in sun-light intensity.

Voltage and electrical current supplied to the reactor were measured using a voltage regulator. Voltage from the regulator was nearly 13.5 V and current varied between 81 mA and 351.75 mA depending on the availability of sunlight. Ampere x hour (I*t) was recorded by the voltage regulator everyday and used for energy calculations.

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2.2.2 Solar-Powered Experiment with Plastic (poly vinyl chloride) Reactor

In this experiment, we utilized the same reactor with a PVC (poly vinyl chloride, plastic) reactor. This container includes stainless steel upper, lower lids and plastic perimeter to avoid the corrosion of the inner surface of the reactor. Dimensions are Do = 21 cm, H = 48 cm with a volume of 16.8 L as shown in Figure 2.1.

Reactor was filled with diluted CW (13.5 L) and connected the PVC to the electrodes through a voltage regulator and battery. Control experiments were conducted with tap water to determine hydrogen gas production by water electrolysis along with control experiments without voltage application. Table 2.5 depicts all initial wastewater characteristics and applied average voltages in this set of experiment. This experiment was conducted in winter and temperature was lower than 10 °C which directly affected the bacterial decomposition of organic waste in some cases.

Table 2.5 Initial characteristics of diluted CW wastewater for solar-powered plastic reactor

Voltage (V) Total COD (mgL-1₎ Total Sugar (mg L-1₎ TVFA (mg L-1₎ pH ORP (mV) Conductivity (mS/cm) SS (gL-1₎ TS (gL-1₎ 11.84 20415 15672 1182 5.34 106.7 2.48 1.44 15.24 11.88 25175 20160 1457 _5.22 _117.8 _3.14 2.05 22.16 11.90 31781 24248 1840 _5.16 _117.6 _3.94 2.11 23.90 11.65 35172 26543 2142 _5.02 _98.20 _4.37 2.40 26.50

.Applied voltage and current intensities, energy input (Ah) were calculated as explained in part 2.2.1.2.

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2.3. Calculation Methods

2.3.1 Calculations for Electrohydrolysis Experiments

The electrical energy supplied to the system was calculated using the following equation.

Ee = V  I d t = V  Iiti Eqn 2.4 Where Ee is the electrical energy supplied to the system by the DC power supply (J); V is the applied DC voltage (V); Ii is the average current (A) measured by a multimeter within a time period of ti (h). Electrical energy input was calculated by determining the area under the curve of current (I) versus time (t).

The amount of hydrogen gas was calculated by using the ideal gas law.

P *VH2 = (m/M) *R *T Eqn 2.5 Where P is the pressure (1 atm); VH2 is the volume of the cumulative hydrogen gas; m is the mass of the cumulative hydrogen (g); M is the molar mass of hydrogen (2 g mol-1); R is the gas constant (0.082 L atm mol-1 K-1), T is the absolute temperature (K).

The energy content of the produced hydrogen was calculated using the following equation.

EH2 = m *(122 kJ g-1) Eqn 2.6 Where m is the mass of the cumulative hydrogen produced within a specified time period.

Energy efficiency of the system varied during the course of experiment due to daily variations in current intensity and hydrogen gas production. The highest energy efficiencies were reported for every experiment for comparison. Energy efficiency of the system was calculated by using the equation 2.4.

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 = EH2/ Ee Eqn 2.7

The yield of hydrogen gas formation based on COD removal (mL H2 g-1 COD) was calculated by using the following equation, where VH2 is the cumulative hydrogen gas volume at the end of the operation time, V is the volume of the cheese whey in the reactor, So and S are the initial and the final CODs (g L-1).

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27

CHAPTER THREE RESULTS AND DISCUSSION 3.1 Bench Scale Experiments with DC Electrical Power

3.1.1 Effects of Applied DC Voltages

Two sets of experiments were performed. The effects of applied DC voltage on the rate and yield of hydrogen gas production and COD removal from the CW were investigated. The first set of experiments include 6 identical experimental bottles containing the CW with different applied DC voltages and a CW control bottle with no DC voltage. The second set of experiments included experimental bottles containing water (control water) with different applied DC voltages to determine H2 production by electrolysis of water. No DC power was applied to the CW control bottle to measure hydrogen gas production and COD removal by bacterial decomposition.

3.1.1.1 Hydrogen Gas Evolution

Figure 3.1 depicts time course of variation of cumulative H2 gas formation (CHF) at different applied DC voltages (0.5 V- 5V) along with H2 gas production from electrolysis of water and bacterial decomposition of CW. Only 60.8 mL hydrogen gas was produced within 158 h with 0.5V DC voltage application. Hydrogen gas from the water and CW controls was comparable with the experimental bottle indicating no significant H2 gas formation by electrohydrolysis of CW organics with 0.5 V DC voltage. When 1 V DC voltage was applied to the CW, nearly 711.2 ml H2 gas was produced within 158 h with only 52.9 and 22.4 ml H2 productions from water and CW controls, respectively.

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Figure 3.1 Variations of cumulative hydrogen gas formation with time for different applied DC voltages. Initial chemical oxygen demand: CODo = 31.9 ± 0.2 g L−1.

H2 gas evolution from bacterial decomposition of CW and electrolysis of water were negligible with 1 V DC voltage application. Further increases in DC voltages resulted in 3021.3, 4808.2 and 4958.7 ml H2 gas production within 158 h at 2 V, 3 V, 4V DC voltages, respectively. Again, H2 gas formation from water electrolysis and natural decomposition of the CW was low. Only 191.8, 242.2, 331.3 ml H2 were obtained from water control with 2, 3 and 4 V DC voltages. Hydrogen gas production from CW control was less than 60 ml. When 5 V DC voltage was applied to the CW, cumulative hydrogen gas volume increased to 5551 mL at the end of 158 h with negligible H2 production in control bottles. The most suitable voltage yielding

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the highest cumulative H2 gas formation (CHF) is 5 V. Voltages higher than 5V should be applied to determine the optimum voltage yielding the highest CHF.

Figure 3.2 Variations of cumulative hydrogen gas volume and H2 gas percentage with the applied DC voltage after 158 h operation.

Figure 3.2 depicts variations of final CHF and H2 percentage in the gas phase with the applied DC voltage after 158 h of operation. CHF increased with increased DC voltage up to 5 V and yielded the highest CHF 5551 mL) at 5 V DC voltage. Gas phase was mainly composed of H2 and CO2 without methane. Hydrogen percentage in the gas phase also increased in accordance with applied DC voltage and reached the highest level (99%) at 3, 4 and 5 V DC voltages.

Variation of H2 gas formation rate (HFR, mL d-1) with the applied DC voltage is shown in Figure 3.3. HFR also increased with increased DC voltage almost linearly between 1 and 3 V. Further increase in DC voltage from 3 V to 5 V resulted in hydrogen formations with higher rates and reached the highest level at 5 V.

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Figure 3.3 Variation of hydrogen gas formation rate with the applied DC voltage.

The most suitable DC voltage yielding the highest cumulative hydrogen gas volume (5551 mL), H2 production rate (913 mL d-1) and hydrogen percentage (99%) in the gas phase was 5 V. Voltages higher than 5V should be tested to determine the optimum voltage.

At DC voltages below 2 V hydrogen gas evolution was limited by the available electrons yielding low volumes of H2 gas. The released electrons at low voltages were not sufficient for high volumes of H2 gas formation. At higher voltages above 3 V, large amounts of Al(III) were released from electrodes and Al(III) ions competed with H+ ions for the electrons provided by DC current. Part of the electrons were combined with Al(III) ions and part of them produced hydrogen gas by combining with protons (H+). That may be the reason for lower rate of increase in hydrogen gas formation rate and cumulative hydrogen gas volume at high DC voltages above 3 V.

3.1.1.2 COD Removals

Hydrogen gas formation by electrohydrolysis of cheese whey organics resulted in simultaneous COD removal. COD removals from the CW were quantified at

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different DC voltages between 0.5 and 5 V and also in the CW control. Figure 3.4 depicts time course of variations of COD content of the experimental and the control cheese whey for different voltages.

Figure 3.4 Time course of variations of COD concentration at different DC voltages.

COD content of the cheese whey decreased with time in both the control and the experimental bottles indicating bacterial decomposition of organic compounds to VFAs and CO2 under anaerobic conditions in the absence of DC voltage application. Bacterial degradation of organic compounds and therefore COD removals were almost completed within 96 hours. COD removals in the experimental bottles were somewhat higher than those obtained from the CW controls indicating decomposition of some organic compounds by DC voltage application. COD content

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of the cheese whey decreased from an initial level of 31967.9 ± 0.1 g L-1 to 26895, 26847, 26338, 24801, 24919, 26602 mg L-1 when 0.5, 1, 2, 3, 4 and 5 V DC voltages were applied while the final COD content of the CW control was 26931.5 ± 0.5 g L-1. Approximately 15.2% of COD was removed by bacterial decomposition, 2.7% removed by electrohydrolysis. Since no COD removal was realized after 96 h of bacterial decomposition no CO2 release was observed and the gas phase was consisted of 98-99% H2 gas at the end of 158 h operation.

Organic compounds were mainly degraded to volatile fatty acids (VFAs) and CO2 by anaerobic bacteria resulting in COD removal. H2 gas formation took place by combination of electrons provided by DC current and protons released from VFAs by dissociation and decomposition.

Figure 3.5 depicts variation of percent COD removal and yield of hydrogen gas formation (mL H2 g-1 COD) with the applied DC voltage at the end of 158 h of operation. The hydrogen yield was calculated by using the Eqn 2.8. COD removal percentage was around 20% as compared to previously reported studies. Three CW control bottles were operated without DC voltage and percent COD removals were nearly 15.2 % for all control experiments. COD removal was mainly realized by bacterial decomposition of organics. However, hydrogen gas yield (HY) increased with increasing DC voltage and reached the highest level (1708.9 mL g-1 COD) at 5 V DC voltage. Percent COD removal also increased when the applied voltage was increased from 0.5 to 3 V. The highest COD removal percentage (22.10%) was obtained at 3 V. Further increase in DC voltage resulted in lower COD removals. At 3V and 4V DC voltages, hydrogen yields were 1366.5 and 1433.4 mL g-1 COD, respectively.

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Figure 3.5 Variation of percent COD removal and hydrogen gas yield with DC voltage after 158 h operation.

3.1.1.3 pH, ORP and Conductivity Changes

ORP, pH and conductivity of the cheese whey were determined during the course of all experiments. Figure 3.6 and 3.7 depicts variation of pH and conductivity with the applied DC voltage within 158 h, respectively.

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Figure 3.6 Time course of variations of pH in the experimental and control bottles at different applied DC voltages.

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Figure 3.7 Time course of variations of conductivities in the experimental and control bottles at different DC voltages.

Initial conductivity of the CW was 4.22 ± 0.11 mS cm-1. The final conductivities of the CW decreased with the increased applied DC voltage due to high levels of (H+) ion removal from the solution at high DC voltages. Almost no change in the final conductivity was observed at 0.5 V DC voltage due to low levels of (H+) ion removal from the medium for H2 gas formation. However, the conductivity decreased to 3.62, 2.56, 2.55, and 2.92 mS cm-1 at 2, 3, 4, and 5 V DC voltages due

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to high volumes of H2 gas formation at high DC voltages. pH and the conductivity of the cheese whey were inversely related when CW was exposed to voltage. High (H+) ion concentration in the CW yielded low pH and high conductivity. The initial pH of the CW was 4.72 which decreased to 4.35 in the CW control due to VFA formation by natural bacterial decomposition of the organics with no DC application. The final pH of the CW increased with increasing DC voltage application due to (H+) ion removal from the solution for H2 gas formation and varied between 8.5-9.5 for 2 to 5 V DC voltages. pH also increased with time during the course of every experiment due to removal of (H+) ions from the solution. Figure 3.8 depicts variation of final pH and conductivities with the applied DC voltage.

Figure 3.8 Variations of final pH and conductivities with the applied DC voltage.

Oxidation-reduction potentials (ORP) were also inversely related to pH and decreased during the course of experiments from the initial ORP 135.2 ± 2.2 mV. The final ORPs at the end of 158 h operation also decreased with increasing DC voltages. The final ORP decreased from 135.2 ± 2.2 mV to -141.4 mV when DC voltage was increased from 0.5 to 5 V.

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Figure 3.7 Time course of variations of oxidation-reduction potentials (ORP) for the experimental and control bottles at different DC voltages.

The final pHs were 4.06, 4.35, 8.87, 9.32, 9.36, 9.46 and the final ORPs were 156.2, -108.1, -133.8, -137.1, -141.4 mV for the applied DC voltages of 0.5, 1,2,3,4 and 5V.

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3.1.1.4 Energy Efficiency

Figure 3.8 presents time course of variation of the current intensities and conductivities for all DC voltages.

Figure 3.8 Time course of variations of current intensities and conductivities for different applied DC voltages.

At the beginning of the 0.5 V experiment, (H+) ion concentration was not sufficient for current formation between the electrodes. But after 90 h, due to VFA formation as a result of decomposition of CW organics by microbial activity, conductivity of CW increased, and current became measurable with a low amount of

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hydrogen gas formation in the head space of the bottle. Hydrogen gas accumulation was not observed in the control CW during the same period. Although cumulative hydrogen volume was lower than those of higher voltages, nearly 60 mL H2 gas was produced at the end of the 0.5 V experiment, as compared to the control CW where only 19 mL H2 was produced. Current intensity did not decrease with time until 114 h and on the contrary, current increased from 22 to 23.5 mA. Afterwards, throughout the remaining experimental period, current intensity decreased from the maximum level of 23.5 to 16.5 mA due to decreasing amount of hydrogen ion in the medium and changing environmental conditions around the electrode surface submerged into the solution. For other experimental cheese whey bottles, current was measurable from the beginning to the end of the experiment and decreased with time due to changes in environmental conditions and electrode surface properties.

Figure 3.9 Variations of current intensities with time for different applied DC voltages.

Initial current intensities increased with increases in DC voltage (Figure 3.9). Current intensity decreased from 42 mA to 29 mA, and from 47 to 28 mA at the end

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of 158 h operation for 2 V and 3 V DC voltages, respectively. High current intensities were obtained at high DC voltages such as 4 V and 5 V. Current intensity decreased from 54 mA to 18.5 mA and from 69.5 mA to 29.5 mA at the end of the operation for 4 V and 5 V DC voltages. High current intensities at high DC voltages provided large number of electrons to the CW yielding high volumes of H2 gas production, but affected the energy efficiency of the system due to competition of H+ and Al(III) ions for the electrons.

Energy efficiencies for hydrogen gas production at different voltages were calculated using eqn 2.7. Energy efficiencies varied with time due to variations in current intensities (I, amperes) as a result of electrode corrosion and changes in the cheese whey (CW) composition. Variations of current intensities with time for different DC voltages are shown in Figure 3.9. At low voltage (0.5 V) the current was lover than 1 mA yielding low levels of H2 gas production. In general, the current intensities decreased with time for every DC voltage due to increased resistance of the CW as a result of decreased conductivity.