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Pretreatment of cellulosic waste and high-rate biogas production

Solmaz Aslanzadeh

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Copyright © Solmaz Aslanzadeh School of Engineering

University of Borås SE-501 90 Borås (Sweden)

Handle-ID http://hdl.handle.net/2320/12853 ISBN 978-91-87525-10-0 (Printed) ISBN 978-91-87525-11-7 (pdf)

ISSN 0280-381X, Skrifter från Högskolan i Borås, nr. 47 Printed in Sweden by Ineko AB

Borås 2014

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Abstract

The application of anaerobic digestion technology is growing worldwide, mainly because of its environmental benefits. Nevertheless, anaerobic degradation is a rather slow and sensitive process.

One of the reasons is the recalcitrance nature of certain fractions of the substrate (e.g., lignocelluloses) used for microbial degradation; thus, the hydrolysis becomes the rate-limiting step.

The other reason is that the degradation of organic matter is based on a highly dynamic, multi-step process of physicochemical and biochemical reactions. The reactions take place in a sequential and parallel way under symbiotic interrelation of a variety of anaerobic microorganisms, which all together make the process sensitive. The first stage of the decomposition of the organic matter is performed by fast growing (hydrolytic and acid forming) microorganisms, while in the second stage the organic acids produced are metabolized by the slow growing methanogens, which are more sensitive than the acidogens; thus, methanogenesis becomes the rate-limiting step.

The first part of this work evaluates the effects of a pretreatment using an organic solvent, N- methylmorpholine-N-oxide (NMMO), on cellulose-based materials in order to overcome the challenge of biomass recalcitrance and to increase the rate of the hydrolysis. NMMO-pretreatment of straw separated from the cattle and horse manure resulted in increased methane yields, by 53%

and 51%, respectively, in batch digestion tests. The same kind of pretreatment of the forest residues led to an increase by 141% in the methane production during the following batch digestion assays.

The second part of this work evaluates the efficacy of a two-stage process to overcome the second challenge with methanogenesis as the rate-limiting step, by using CSTR (continuous stirred tank reactors) and UASB (up flow anaerobic sludge blanket) on a wide variety of different waste fractions in order to decrease the time needed for the digestion process. In the two-stage semi- continuous process, the NMMO-pretreatment of jeans increased the biogas yield due to a more efficient hydrolysis compared to that of the untreated jeans. The results indicated that a higher organic loading rate (OLR) and a lower retention time could be achieved if the material was easily degradable. Comparing the two-stage and the single-stage process, treating the municipal solid waste (MSW) and waste from several food processing industries (FPW), showed that the OLR could be increased from 2 gVS/l/d to 10 gVS/l /d, and at the same time the HRT could be decreased from 10 to 3 days, which is a significant improvement that could be beneficial from an industrial point of view. The conventional single stage, on the other hand, could only handle an OLR of 3 gVS/l/d and HRT of 7 days.

Keywords: Biogas, Two-stage anaerobic digestion, N-methylmorpholine-N-oxide (NMMO) pretreatment, Lignocelluloses, Textile waste

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List of Publications

This thesis is mainly based on the results presented in the following articles:

I. Aslanzadeh S, Taherzadeh MJ and Sárvári Horváth I. (2011): Pretreatment of straw fraction of manure for improved biogas production. Bioresources 6: 5193-5205.

II. Aslanzadeh S, Berg A, Taherzadeh MJ and Sárvári Horváth I. (2014): Biogas production from N-Methylmorpholine-N-oxide (NMMO) pretreated forest residues. Applied Biochemistry and Biotechnology, in press.

III. Jeihanipour A, Aslanzadeh S, Rajendran K, Balasubramanian G and Taherzadeh MJ.

(2013): High-rate biogas production from waste textiles using a two-stage process.

Renewable Energy 52: 128-135.

IV. Aslanzadeh S, Rajendran K, Jeihanipour A and Taherzadeh MJ. (2013): The Effect of Effluent Recirculation in a Semi-Continuous Two-Stage Anaerobic Digestion System.

Energies 6: 2966-2981

V. Aslanzadeh S, Rajendran K and Taherzadeh MJ. A comparative study between conventional and two-stage anaerobic processes: Effect of organic loading rate and hydraulic retention time (Submitted).

Statement of Contribution

Paper I: Performed the experimental work of the pretreatments and anaerobic digestion assays and responsible for the data analyses and manuscript writing.

Paper II: Responsible for parts of the experimental work and data analyses. Active participant in the preparation and organization of the manuscript.

Paper III: Responsible for parts of the experimental work and involved in the manuscript preparation and its revision.

Paper IV: Responsible for parts of the experimental work and for the manuscript preparation.

Paper V: Responsible for major part of the experimental work and data analyses as well as the manuscript preparation.

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List of Publications not included in this thesis

Articles:

I. Rajendran K., Aslanzadeh S., Taherzadeh M.J. (2012): Household Biogas Digesters—A Review. Energies 5, 2911-2942.

II. Rajendran K., Aslanzadeh S., Johansson F., Taherzadeh M.J. (2013): Experimental and Economical Evaluation of a Novel Biogas Digester. Energy Conversion & Management 74:

183-191.

Book chapters:

I. Aslanzadeh S, Ishola MM, Richards T, Taherzadeh,MJ, (2014): An Overview of Existing Individual Unit Operations in Biological and Thermal platforms of Biorefineries, In: N.

Qureshi, D. Hodge & A.V. Vertes (Eds): Biorefineries: Integrated Biochemical Processes for Liquid Biofuels (Ethanol and Butanol), Elsevier, Chapter 1, in press

II. Aslanzadeh S, Rajendran K, Taherzadeh MJ. (2013): Pretreatment of Lignocelluloses for Biogas and Ethanol Processes, In: Ram Sarup Singh, Ashok Pandey and Christian Larroche (Eds): Advances in Industrial Biotechnology, Asiatech Publishers Inc, New Delhi, India, Chapter 8, Pages 125-150.

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Table of content

Abstract ... iii

List of Publications ... v

List of Publications not included in this thesis ... vi

Chapter 1. Introduction ... 1

Chapter 2. Anaerobic Digestion ... 5

2.1. Biogas industry: current status and challenges ... 5

2.2. The AD process and its complexities ... 7

2.2.1. Factors influencing the AD process ... 10

2.3. Bottlenecks of anaerobic digestion ... 12

2.3.1. Organic loading rate ... 12

2.3.2. Retention time ... 13

2.4. Phase separation ... 14

Chapter 3. Substrates for biogas production ... 17

3.1. Substrate composition and its effect on AD ... 17

3.1.1. Lignocellulosics-structural carbohydrates ... 18

3.1.2. Textile waste-cellulose and synthetic fibers ... 20

3.1.3. Starch-non structural carbohydrates ... 21

3.1.4. Organic fraction of municipal solid waste ... 22

3.2. Remarks on theoretical and experimental methods for determination of biogas potential ... 23

3.2.1. Theoretical methods ... 23

3.2.2. Experimental methods ... 25

Chapter 4. Approaching the challenge of biomass recalcitrance ... 27

4.1. Definition of substrate biodegradability ... 27

4.2. Challenges with lignocellulosic recalcitrance ... 27

4.3. Microbial strategy for lignocellulose recalcitrance: Cellulosome ... 29

4.4. Goal of pretreatment ... 30

4.5. Effect of pretreatment on biogas production ... 30

4.6. Pretreatment technologies ... 31

4.6.1. Physical pretreatment ... 31

4.6.2. Physiochemical pretreatments ... 31

4.6.3. Biological pretreatment ... 32

4.6.4. Chemical pretreatments ... 33

Chapter 5. High-rate anaerobic treatment systems ... 39

5.1. Background and Status ... 39

5.2. Upflow anaerobic sludge blanket reactor ... 40

5.2.1. Biogranulation of microorganisms ... 42

5.2.2. Factors influencing anaerobic granulation... 43

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5.2.3. Characteristics of anaerobic granules ... 46

5.1. Two-stage process for high-rate methane production ... 47

5.1.1. Batch process- single vs. two-stage ... 48

5.1.2. Two-stage semi-continuous process ... 51

5.1.3. Two-stage- open system vs. closed system ... 54

5.1.4. Semi-continuous process- Single vs. two stage ... 56

Concluding Remarks ... 59

Future work ... 61

Nomenclature ... 64

Acknowledgments ... 65

References ... 68

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Chapter 1.

Introduction

The ultimate aspiration of energy conversion systems is to achieve steady energy output at the maximum possible conversion rate. Actually, in reality this is easier said than done because of the recalcitrance nature of the substrate, in addition to the complexity of the anaerobic digestion (AD) process. The rate of the biogas production is a function of the biochemical processes [1]. The presence of difficult to degrade material fractions slows down the hydrolysis rate, which in turn limits the rate of the overall anaerobic digestion process. However, for the easily degradable materials the methanogenesis is considered as being the rate-limiting step due to the slow growth rate of methanogens [2, 3].

Attaining the maximum biogas yield, by complete degradation of the substrate, would require a long retention time of the substrate inside the digester and an equally large digester size. Putting this into practice, the choice of a system design or of an applicable retention time is often based on a compromise between receiving the highest achievable biogas yield and having a reasonable plant economy [4]. In this regard, the organic load is a significant operational parameter, which indicates how much organic dry matter can be fed into the digester, per volume and time unit. Today, the total degradation time of the solid organic waste is normally about 30 days for the biogas process.

Nevertheless, it can be even longer depending on the specific substrate and the operational temperature [4-6]. At lower HRTs (hydraulic retention times), the risk for a washout of certain microorganisms is high. This makes it difficult to preserve the effective number of useful microorganisms in the system. To maintain the population of anaerobes, large reactor volumes or higher retention times is essential [7]. Today, this problem has been solved in the wastewater treatment systems due to the introduction of the modern ―high-rate‖ reactors, in which the HRT is decreased dramatically, usually to less than 1 day [8, 9]. Biomass immobilization is the key factor for the successful applications of the high-rate anaerobic systems in wastewater treatment processes [8]. However, the drawback of this technique is that it cannot handle a higher total solid content;

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hence, only dissolved or soluble materials can be used as feed, which is the reason why this technology has been successful in the wastewater treatment processes [10, 11]. On the other hand, for the utilization of substrates with a high solid content, it is mandatory to divide the process into two stages in order to take advantage of the high-rate reactors. Two-stage processes are divided into two steps in order to optimize the conditions for different groups of microorganisms that are active in the digestion process. The first step is a hydrolysis reactor, and the conditions are optimized there to get the solid matter to be solubilized, while in the second step a high-rate reactor is used to convert the solubilized material into biogas [11]. However, it should be mentioned that regardless of the process configuration used, the rate of the biogas production will largely depend on the composition of the substrate, and particularly, on its biodegradability. The hydrolysis of difficult to degrade substrate fractions is one of the challenges the biogas industry is facing today. Although materials, such as lignocelluloses, are available in large amounts and receive special attention for utilization in the biogas production, they are prone to slow degradation; hence, they require some kind of pretreatment to increase their degradation rate.

In this thesis, the potential of using an organic solvent N-methylmorpholine-N-oxide (NMMO) for the pretreatment is studied in order to deal with the reluctant nature of lignocellulose- and cellulose- based substrates and to increase the rate of hydrolysis during the following anaerobic digestion process. The second part of this thesis focuses on a two-stage process and investigates the performance at various organic loading rates and hydraulic retention times. For this propose, a wide variety of substrates with a high total solid content and different degradability was used. The following studies were performed:

o The effects of an organic solvent called N-methylmorpholine-N-oxide (NMMO) used for the pretreatment of the straw fraction from the manure and forest residues were evaluated by measuring the biogas potential during the following anaerobic digestion process (paper I and II).

o The long-term effects of the best pretreatment conditions used for the forest residues determined by batch digestion assays were also examined in a semi-continuous anaerobic digestion system (paper II).

o Application of the NMMO–pretreatment on cellulose-based textile waste and their subsequent digestion in a high-rate two-stage anaerobic digestion process was examined at various organic loading rates and hydraulic retention times (paper III).

o The effect of effluent recirculation in a two-stage anaerobic process using carbohydrate-based starch and cotton as the substrate at various organic loading rates and hydraulic retention times was evaluated (paper IV).

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o The effect of an organic loading rate and hydraulic retention time comparing single stage and two stage processes using municipal solid waste and food processing waste was evaluated (paper V).

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Chapter 2.

Anaerobic Digestion

2.1. Biogas industry: current status and challenges

There is a variety of waste produced by human activities, and the amount of waste generated is on the rise [12]. Anaerobic digestion of organic waste is of increasing interest as it offers an opportunity to deal with some of the problems regarding the reduction of the amount of organic waste, while diminishing the environmental impact and facilitating a sustainable development of the energy supply [3, 13]. Long-term successful practice and understanding have made anaerobic digestion to be one of the favorite treatment technologies for the organic fraction of MSW, applying a range of technological approaches and systems [14].

Anaerobic digestion technology has been developed in the last 20 years. With a total of 244 plants and a capacity of nearly 8 million tons of organic treatment capacity, anaerobic digestion is already taking care of about 25% of the biological treatment in Europe [14]. By the year 2015, the Netherlands and Belgium are expected to convert 80% of the composting plants into anaerobic digestion as the primary treatment technology [14].

In comparison to other biofuels, in biogas production a wide range of substrates can be utilized as long as they are biodegradable, which is one of the great advantages [13]. AD systems are employed in a wide variety of wastewater treatment plants for sludge degradation and stabilization, and are used in highly engineered anaerobic digesters to treat high-strength industrial and food processing wastewaters before discharge. In addition, there are many cases of AD systems applied in the agricultural sector at animal feeding operations and dairies to alleviate some of the impacts of manure and for energy production [15]. The majority of these AD systems in operation are single stage. The European market has shown a large inclination toward single-stage over two-stage digesters [15]. The number of plants treating MSW using two-phase digestion has continued to

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decline since the beginning of the 90s. It is predicted that no change is expected in this trend, mainly due to the higher investment and operating costs of running two-stage processes [14]. There are studies arguing that two-stage anaerobic digestion could provide great advantages over the single-stage digestion due to a more rapid and more stable treatment achieved [16]. In practice, however, it is argued that the two-stage digestion has not been able to validate its claimed advantages in the market, and the added benefits in increasing the rate of hydrolysis and methanization have not been confirmed [17]. Industrial applications, therefore, have displayed little acceptance for the two-stage systems so far [18].

Anaerobic digestion systems are often appropriate for all wastewater treatment systems, given that the solids can be introduced to the system at an acceptable concentration, which includes new installations as well as retrofits. In fact, a great deal of the existing research on anaerobic digestion is aimed at retrofitting multi-stage systems into facilities where single-stage processes are already present. The most important factor in determining whether a multi-stage anaerobic digestion process is achievable for a system is the concentration of the feed solids. Given that a multi-stage process could be sensitive to variation in the feed solids, it might not be practicable if the characteristics of the feed solids concentrations fluctuate extensively [19].

Figure 1. Outline of the development and ratio of 1-phase and 2-phase digestion capacity in Europe. Adapted from [20, 21]

Figure 1 illustrates an overview of the development and the ratio of the one-phase and two-phase digestion capacity in Europe, respectively. As noticeable, the vast majority is one-phase system [14, 22].

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 1000 2000 3000 4000 5000 6000

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Cumulative (Kton/year)

One cumulative Two cumulative Two stage % Single stage %

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In order to get an overview of the status of anaerobic digestion of the organic fraction of MSW in Europe, taking into account a wide variety of criteria, a quantitative analysis was performed on the installed annual capacity up to the year 2014 [14]. It was estimated that the cumulative percentage of the one-phase processes would add up to 93%, with only 7% remaining for the two-phase capacity installed in 2014 [14].

Nonetheless, the future role of biogas in Europe is based on the availability of the substrates. The technology development concerning biofuel production has opened up a larger substrate supply base. On the other hand, for the same substrate, it generates more rivalry with the other related technologies [23]. There is an abundant availability of cellulose-based waste, which could be appropriate for biogas production e.g., lignocelluloses and waste textiles. These materials are carbohydrate-rich and could be used as a substrate for biogas production. However, the reluctant nature of these substrates makes them very difficult to digest, as their structure opposes microbial hydrolysis in biogas production [12, 24]. Today, the application of lignocellulosic materials in biogas production is limited and for waste textiles, it is nonexistent [12, 24].

The main goal of this thesis is to increase the rate of the biogas production as well as to investigate the possibilities of difficult-to-degrade cellulose-based materials, utilized as a substrate for the biogas production. In order to achieve this goal, one must first overcome the difficulties of the degradation by using a pretreatment to make the material available for the following microbial degradation, which was the focus in the first part of this thesis. Furthermore, the extent of the increase in the organic loading and the decrease in the retention time while developing a two-stage process, utilizing different waste fractions including cellulose-based materials, was evaluated in the second part of this work.

2.2. The AD process and its complexities

Anaerobic digestion is often considered to be a complex process. The digestion itself is based on a reduction process consisting of a number of biochemical reactions taking place under anoxic conditions. By the actions of a variety of anaerobic and facultative anaerobic microorganisms, multi molecular organic substances are degraded into simpler, chemically stabilized compounds, and the final products are primarily methane and carbon dioxide and some smaller amounts of other gases, such as hydrogen sulfide, hydrogen, carbon monoxide, nitrogen, ammonia NH3, and water [25, 26].

These reactions can be divided into four phases of degradation: hydrolysis, acidogenesis,

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acetogenesis, and methanogenesis [25]. The AD process involves four fundamental steps, as outlined in Figure 2. The individual phases are carried out in parallel; however, in each phase different groups of microorganisms are involved, which partially stand in a syntrophic relation to each other, with dissimilar requirements on the environment. Normally, the first and the second phase are closely linked to each other while the third phase is closely connected to the fourth phase [27].

Due to the small amount of energy available in methanogenic conversion, the microorganisms are compelled to be part of a very complex, well-organized and efficient cooperation, which could be the primary reason that this step is the final step to occur in the anaerobic digestion process [28].

The mutual reliance of the partner bacteria regarding energy limitations can go so far that neither group of microorganisms can function without the other and that together they show a metabolic activity that neither group could carry out on its own. This type of cooperation is called syntrophic relationship [28]. Syntrophism is a special case of symbiotic collaboration between two metabolically different types of microorganisms relying on each other, often for energetic reasons in order to degrade a certain substrate. The term was created to express the close interrelation of fatty acid oxidizing, fermenting bacteria with hydrogen oxidizing methanogens [28].

Hydrolysis / acidogenesis process

Undissolved compounds like carbohydrates, proteins, and fats are degraded into monomers, which usually are water-soluble fragments, by exoenzymes. The microorganisms involved are facultative and obligatorily anaerobic bacteria. In this phase, the covalent bonds are broken down with water in a chemical reaction. The monomers produced in the hydrolytic phase are taken up by different facultative and obligatorily anaerobic bacteria and are degraded further into short-chain organic acids, such as butyric acid, propionic acid, acetic acid, alcohols, hydrogen, and carbon dioxide. The concentration of the hydrogen formed as an intermediate product in this stage influences the type of final products produced during the fermentation process. For example, if the partial pressure of the hydrogen were too high, it would decrease the amount of reduced compounds (e.g., acetate). In general, during this phase, simple sugars, fatty acids, and amino acids are converted into organic acids and alcohols [29].

Acetogenesis /Methanogenesis

The products produced in the acidogenic phase are consumed as substrates for the other microorganisms, active in the third phase. In the third phase, also called acetogenic phase, anaerobic

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oxidations are performed. It is important that the organisms, which carry out the anaerobic oxidation reactions, collaborate with the next group, the methane forming microorganisms. This collaboration depends on the partial pressure of the hydrogen present in the system. Under anaerobic oxidation, protons are used as the final electron acceptors, which lead to the production of H2. However, these oxidation reactions can only occur if the partial pressure of H2 is low, which explains why the collaboration with the methanogens is very important, since they will continuously consume the H2 to produce methane. Hence, during this symbiotic relationship ―inter- species hydrogen transfer‖ occurs [5, 27, 28, 30].

In the fourth phase, or the methanogenic phase, the methane is formed under strict anaerobic conditions. These reactions are exergonic. The most important substrates for these microorganisms are H2, CO2, and acetic acid. The methanogenic microorganisms can be divided into three main groups:

(1) Acetoclastic methanogenesis Acetate → CH4 + CO2

(2) Hydogenotrophic methanogenesis H2 + CO2 → CH4

(3) Methylotrophic methanogenesis Methanol→ CH4 + H2O

Acetogenesis

Complex organic matter

Biodegradable organic matter Carbohydrates

Proteins Lipids

Amino acids,

Sugars Fatty Acids

5% 21%

21% 40%

100%

0%

20%

Intermediate products Propionate, Butyrate, etc.

Acetate H2, CO2

46% 34%

35% 11%

Methane, CO2 70% 30%

20% 8%

12%

11% 23%

Hydrolysis

Acidogenesis

Methanogenesis Rate-limiting step

Figure 2. Schematic diagram of the anaerobic digestion process. Adapted from [31]

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2.2.1. Factors influencing the AD process

As is the case for all biological processes, the steadiness of the living conditions is of great importance. Factors that affect the anaerobic digestion could be physical, chemical, or biological.

An alteration in the temperature, the composition, and/or amounts of substrates can have fatal consequences for the gas production. The microbial metabolism processes are reliant on many parameters. In order to achieve optimal conditions for the degradation process, apart from the organic loading rate and the hydraulic retention time, various other parameters ought to be considered and controlled. Given that the environmental requirements of the fermentative bacteria vary from those of the methane forming microorganisms, the only way that the optimum environmental conditions for all microorganisms involved can be achieved is in a two-stage system, i.e., one stage for hydrolysis/acidification and one stage for acetogenesis/methanogenesis [27].

However, if the complete degradation process has to happen in the same reaction system (one- phase), the requirements for methanogenesis must be prioritized, if not, it would be tough for the methanogens to continue to be active within the mixed culture, due to their lower growth rate and higher sensitivity to environmental factors.

Temperature

The time-span of the fermentation period is dependent on the temperature. The temperature of the digester, even a few degrees, has an effect on nearly all the biological activities, especially on the methane-forming archaea. The majority of the methane formers are active at two temperature ranges: mesophilic range (30–35 °C) and the thermophilic range (50–60 °C) [27].

The methanogens are very responsive to thermal fluctuations. Thus, any rapid alterations in the operating temperature should be avoided. In comparison to the psychrophilic and mesophilic ranges, the thermophilic operation offers a shorter degradation time, better pathogens reduction, higher gas production, and enhanced sludge separation. The drawback is that it is more difficult to control the process [29]. The experiments in papers I and II were performed both at thermophilic and mesophilic conditions, respectively. The operational temperature in the two-stage continuous process, investigated in papers III, IV, and V, was in the thermophilic range in the first stage while the second phase was under mesophilic conditions.

pH

pH is an important parameter in the AD process. It has an extensive influence on the performance and growth of the various microorganisms involved in the different stages of the process [32, 33].

The pH of the digester can be maintained at a desired range (7.0–8.5) by feeding the system at an

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optimal organic loading rate (OLR). A pH outside this range could cause disturbances to the system by affecting most of the microorganisms including the methanogens. The pH of the system relies on the rate of the intermediates formed (e.g., volatile fatty acids) during fermentation. Upon starting up a biogas process, the pH in the digester can drop below 6.0 due to the production of volatile acids during the first degradation steps. However, as methane-forming microorganisms consume the volatile acids, the pH of the digester increases and then stabilizes [5, 34].

Volatile fatty acid

Volatile fatty acids (VFAs) are important intermediates of the anaerobic digestion process. They exist in two forms: undissociated and dissociated. The dissociated form takes over at a high pH level, whereas the undissociated fraction dominates at a lower pH [27]. An increase in the VFAs leads to a drop in the pH; hence, the undissociated form of VFAs (free fatty acids) will dominate, which in turn will inhibit the methanogenesis [27, 35]. Apart from the pH-value, the amount of VFAs therefore is commonly used as an indicator of the performance of anaerobic digesters. It should be noted that the level of inhibition of total VFA and individual VFAs differ from each other [36, 37]. In order to monitor the stability of the process in papers II, III, IV, and V, the total volatile fatty acid concentration was monitored. In paper V, the effect of the individual acid was analyzed as well.

C/N ratio and ammonia

A C/N ratio in the range of 20 to 30 is considered to be an optimum level for anaerobic digestion [32]. If the C/N ratio is too high, microorganisms will quickly consume the nitrogen in order to meet their protein requirements and will no longer take care of the available carbon content of the material, which would accordingly decrease the gas production. Conversely, if the C/N ratio is too low, due to the degradation of the proteins and other nitrogenous materials, nitrogen will be released and build up in the form of ammonium ion (NH4+) or ammonia (NH3) in the system [30, 38]. The chemical equilibrium between the ammonium and the ammonia is controlled by the temperature and the pH. An increase in the temperature or the pH would shift this equilibrium more toward NH3. The free ammonia could be a source of inhibition as it is capable of diffusing into the cell, causing proton imbalance or leading to a potassium loss [39]. Moreover, it should be noted that microorganisms are capable of adapting to higher levels [27]. The C/N ratio can be adjusted by feeding the digester with a proper substrate mixture [30, 38]. In papers III and IV, NH4Cl was added as an ammonium supplement to keep the C/N ratio at 25. In papers III, IV, and V the concentration of ammonium was monitored.

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12 Substrate

The biogas yield and composition are directly affected by the composition of the feed materials with respect to carbohydrate, fat, and protein contents [5]. Moreover, physical and chemical characteristics of the substrate used such as pH, moisture content, total and volatile solids (VS), particle size, and biodegradability play a considerable role in the anaerobic digestion process.

2.3. Bottlenecks of anaerobic digestion

Controlling the anaerobic digestion process is a complicated task. Because of the complex mixed microbial and substrate spectrum, advanced studies and development are necessary to eliminate various bottlenecks in the degradation chain. However, practical experience shows that there are several factors that can be attributed to the process failures in anaerobic digestion. These factors include: microbiological limitations, affecting automatically the microbial community (e.g., ammonia inhibition, trace element insufficiency, etc.) or technical weaknesses of the equipment, such as insufficient mixing caused by the inappropriate particle size or rheological limitations [40].

For a balanced and stable process, the reaction rate in both stages must be equivalent. If the rate of the degradation in the first stage is too fast, the concentration of the acids increases, causing an inhibition of methanogenic microorganisms in the second phase. On the other hand, if the second phase runs too fast, the production rate of methane becomes limited by the hydrolytic stage [41].

Other bottlenecks related to the process performance include extended reactor start-up times and process instability, as a result of the slow growth rates and sensitivity to changes in the environmental conditions of the microorganisms involved in the process. Hence, monitoring the process by measurements aiming to attain and maintain effective and robust microbial communities are considered necessary to guarantee stable performance with high efficiencies [42]

2.3.1. Organic loading rate

Organic loading rate (OLR) is defined as the amount of substrate expressed as e.g., total solids (TS), volatile solids (VS) or chemical oxygen demand (COD) fed to the system per unit volume per unit time. It is a helpful criterion used for measuring the biological performance of the AD system [18], since it is very sensitive to the organic loading rate (OLR) and the waste composition [43, 44].

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It is well-known that easily degradable substrates can be quickly converted into volatile fatty acids (VFA), which can cause the inhibition of methanogenesis as a consequence of the rapid hydrolysis rate and accumulation of VFAs. At high OLRs, there is a risk for overloading the system/reactor, particularly during the period of reactor start-up. In such cases, the feeding rate to the system should be reduced [45, 46]. Higher OLRs can permit smaller reactor volumes, thus, reducing the capital cost.

2.3.2. Retention time

Another parameter that basically controls the rate of the substrate conversion into biogas is the retention time [18]. It is an important parameter in terms of evaluating the conversion efficiency in the process. Normally, shorter retention times are desired in order to reduce the system costs [30].

The retention time is usually expressed as: the hydraulic retention time (HRT), which states the approximate time that the liquid sludge remains in the digester, and the solid retention time (SRT), which is the time that the microorganisms /solids spend in the digester [47]. In general, HRT is more important if the substrate is complex and slowly degradable, whereas SRT is significant for easily degradable biomass [13]. In addition, at high OLRs, the retention times should be long enough for the microorganisms to be able to utilize the substrate. Thus, there is a balance between the OLR and HRT that must be determined in order to optimize the digestion efficiency and reactor volume [48].

The different steps in the digestion process are directly connected to the SRT. Reducing the SRT would decrease the extent of the reactions and vice versa. Whenever the sludge (mixture of biomass solids and water) is removed from the digester, a portion of the bacterial population is also removed [49]. Since methanogenic microorganisms have a significantly longer generation time compared to hydrolytic and acid forming microorganisms, shorter HRTs would cause a washout of slow growing biomass from the system, which would ultimately jeopardize the process stability and decrease the conversion efficiency of the process [30]. Therefore, in order to avoid process failure and keep a steady state condition, the rate of the cell growth must at least compensate the rate of the cell removal [47, 49]. In general, the hydraulic retention times must be at least 10 – 15 days to avoid washout from the system [27]. However, the risk for a washout of the microorganisms from the system can be prevented with phase separation. In order to reach high cell densities of the slow growing methanogenic microorganisms, the hydraulic and solid retention time should be uncoupled in the second stage, and it is necessary to raise the solid content in the methanogenic reactor [18]. In this way, the digestion rate can be increased for a given substrate and reactor volume, and the

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conversion to methane can be achieved at shorter HRTs. Consequently, a greater amount of substrate can be converted into methane in a given period of time, thus, increasing the productivity [30].

2.4. Phase separation

Generally, in an anaerobic digestion process, the rate-limiting step can be defined as the step that causes process failure under imposed kinetic stress. In other words, in a context of a continuous culture, kinetic stress is defined as the imposition of a constantly reducing value of the SRT until it is lower than its limiting value; hence, it will result in a washout of the microorganism [50].

The AD process can be divided into two phases as illustrated in Figure 3. The microorganisms carrying out the degradation reactions in each of these phases differ widely regarding physiology, nutritional needs, growth kinetics, and sensitivity to environment. Very often, it is difficult to keep a delicate balance between these two groups: the acid forming and the methane forming microorganisms, which lead to reactor instability and consequently low methane yield [51]. Poland and Gosh [17] were the first to propose that two main groups of microorganisms could physically be separated with the intention of making use of the difference in their growth kinetics. In order to accomplish phase separation, several techniques have been employed such as membrane separation, kinetic control, and pH control [52-56].

Liquification Acidification Methane Formation

Suspended Solids

Dissolved Solids

Organic

Acids Acetate Methane

Gas (Methane) Phase Acid phase

Acetification

Figure 3. Phase separation of the anaerobic digestion system. Adapted from [19]

A two-phase process allows for the selection and enrichment of the microorganisms corresponding to each of the phases independently from each other. Thus, the first phase can operate at optimal conditions for the growth of hydrolytic and acidogenic microorganisms, while the second phase can be optimized for the acetate and methane formation [57].

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The two-phase process has numerous potential advantages. First of all, it allows for a decrease in the total reactor volume. Another advantage is the appropriate control of the acidification, which improves the stability owing to the more heterogeneous bacterial population. The process would tolerate organic and hydraulic overloading and fluctuations, as the first-phase will function as a metabolic buffer. Toxic materials and substances that can affect the more sensitive methanogenic microorganisms will possibly also be eliminated in the first phase [58]. Moreover, fast growing acidogenic microorganisms may be disposed of, thus, avoiding the loss /washout of the methanogens.

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Chapter 3.

Substrates for biogas production

3.1. Substrate composition and its effect on AD

The substrate composition is extremely important for the microorganisms in the AD process, as it affects the process stability, gas production, and composition. The substrate should meet the nutritional requirements of the microorganisms, regarding the energy sources and various components, vital for building new cells. The substrate should also include a wide variety of components necessary for the activity of microbial enzyme systems, such as trace elements and vitamins. When it comes to the decomposition of organic material in the AD process, the ratio of carbon to nitrogen (C/N ratio) is also regarded to be of great importance [59]. Therefore, the performance of the AD process is shown to be enhanced by using substrates from different sources and with the right proportions. Investigations show that co-digestion of substrates from different sources produce more gas than predicted compared to gas production from the individual substrates [33, 60, 61]. Substrates that are complex and not too homogeneous encourage the growth of the numerous types of microorganisms in the digester. A continuous process that is fed with a uniform composition of substrate, for instance, carbohydrates, for a longer period will lead to a buildup of a consortium of microorganisms, which will find it difficult to digest the proteins and fat, since most of the organisms that had the ability to break down the fat and proteins have been washed out from the process. Therefore, feeding the reactor with a diverse substrate is advantageous, as it amplifies the build-up of diverse microbiological composition, hence, resulting in the possibility of a stable and robust process [5].

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3.1.1. Lignocellulosics-structural carbohydrates

Lignocellulose is the most abundant renewable biomass worldwide [62] with an estimated annual production between 10–50 billion tons [63]. Since both the cellulose and hemicelluloses are polymers of sugars, they are potential sources of fermentable sugars. While the hemicelluloses can be readily hydrolyzed, the cellulose fraction is more unwilling toward the hydrolysis due to the presence of a lignin shield as well as its crystallinity. A more rigorous pretreatment, therefore, is required to access the sugars [62]. Consequently, various pretreatment methods have been used to improve the rate of the hydrolysis of lignocelluloses toward biogas production [24].

Lignocellulosic waste is produced by several sectors including industries, forestry, agriculture, and municipalities [64]. A large fraction of animal manures consist of straw, which is used as bedding material in animal cultivation. Straw is a lignocellulosic material; therefore, it makes these kinds of manure fractions difficult to degrade. A pretreatment is needed to improve the rate as well as the degree of enzymatic hydrolysis during the degradation process.

Forest residues, another example of lignocellulosic waste, have a potential for energy production.

Forest residues are the biomass material remaining in the forests that have been harvested for timber, and are more or less identical in composition to forest thinning [65]. Forest residuals consist of tops and branches, needles, bark, roots, logging residues, etc. It is estimated that in Sweden forest residues have the energy potential of between 49–59 TWh/year [24]. Today, a major part of these residues are not used for biogas production due to a high lignin content, which makes it hard to digest.

In this thesis the effect of the NMMO-pretreatment on the straw fraction of manure (paper I) and forest residues (paper II) and its subsequent effect on the hydrolysis and biogas production were investigated.

Cellulose

Cellulose is the main structural component of plant cell walls. Typically, a plant cell wall consists of up to 35 to 50% cellulose [66], which is a linear polysaccharide polymer of glucose molecule linked together through β-1,4 glucosidic bonds. The character of the β-1, 4 glucosidic bonds allows the polymer to build long and straight chains. The level of polymerization of the cellulose, which refers to the number of glucose units making up one polymer molecule, can range from 800–10,000 units [67]. Cellulose occur in two forms: an unorganized amorphous form and an organized crystalline form. Within the cell wall, however, the crystalline form of cellulose dominates, which are less vulnerable to enzymatic degradation than the amorphous cellulose [30]. In crystalline

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that are removed in order to achieve a high quality fiber for textile manufacturing, which means that the cotton fibers in the waste textile can be considered to be more or less pure cellulose [12]. Still, after extensive research, the use of cellulose as a platform for industrial production for different by products has failed. The challenge of using cellulose is that it is highly crystalline, which opposes microbial degradation, and a cost effective pretreatment method to overcome the crystallinity has up to now been elusive. Jeihanipour et al. [81] showed the possibility for hydrolyzing the cotton using enzymes or acids to achieve glucose and subsequently utilize it as a carbon source in the ethanol fermentation.

Cotton based blue jeans is a textile waste, basically made of pure cellulose. In this thesis, the possibility of using pure cotton and blue jeans with and without pretreatment as a substrate in two- stage semi-continuous high-rate biogas production was investigated in papers III and IV.

3.1.3. Starch-non structural carbohydrates

Apart from sugars, starch is one of the most commonly found non-structural carbohydrates in anaerobic digesters. Starch is present in food, coming mainly from grains, such as corn and wheat, and tubers, such as yam and cassava. Starch comprises of two primary biopolymers: amylose, which is a linear chain of α- 1,4-linked D-glucose units, and amylopectin, which is a chain of α-1,4- linked D-glucose with branches of α-1,6-linked D-glucose [82]. Starch, which is partially water soluble, is the primary polysaccharide for storing energy in higher plants. Some forms of starches are insoluble and resistant to degradation (e.g., wheat breads), whereas others are partially bioavailable [83]. In this thesis, pure starch was used as an easily degradable substrate to compare with cotton in order to evaluate the semi-continuous two-stage process (paper IV).

Given that carbohydrates vary in their nature, they are degraded at different rates in the AD process.

Simple sugars and disaccharides are broken down easily and rapidly; this might appear advantageous, but it can cause instability problems as a result of the accumulation of fatty acids as intermediary degradation products [84-86]. In addition, carbohydrate-rich materials used to have poor buffering capacity, and there is a risk of process instability due to a decrease in the alkalinity of the system [51].

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3.1.4. Organic fraction of municipal solid waste

Municipal solid waste (MSW) is the waste generated from residential sources, for instance, households and from institutional and commercial sources such as offices, schools, hotels, and other sources. The main components of MSW are food, garden waste, paper, board, plastic, textile, metal, and glass waste [87]. The global production of municipal solid waste (MSW) reached 1.3 billion tons /year in 2010, and it is predicted to increase to more than 2 billion tons/ year by 2025 [88, 89]. The disposal of this increasing volume of waste in a sustainable manner is a major challenge. It is estimated that the major fraction of the global municipal solid waste consists of food waste [88]. The application of the anaerobic digestion for the treatment of the organic fraction of municipal solid waste (OFMSW) has been of interest because of its high content of fats /lipids and proteins. The main obstacle in the treatment of this type of organic waste is its conversion, due to the complexity of the organic material [90, 91].

Fats are a major part of the OFMSW and food processing waste (FPW). There are numerous different lipids (fats, oils, greases), with a varying composition depending on their origin. Lipids are distinguished by the length of their fatty acid chain, extent of chemical saturation, which refers to the number of double bonds, and also their physical state, i.e., liquid or solid. Fats are classified as saturated (found in meat and dairy products), monounsaturated (in vegetable oils and nuts), or polyunsaturated fats (in fish and corn oil). Saturated fats are less biodegradable than unsaturated fats. Triglycerides, the most common type of fat, are primarily hydrolyzed into glycerol and long chain fatty acids (LCFAs) in the AD process[5, 92]. The degradation of fats is generally both easy and fast [93]. However, while glycerol is rapidly converted into acetate by acidogenesis, the degradation of LCFA is more complicated. The inhibitory effect of fats is usually connected to the LCFAs [93, 94]. Fats are a very promising substrate for anaerobic digestion, since high methane yields can be achieved.

Proteins are present in many organic materials such as OFMSW and FPW, which are rich in energy and produce a relatively high amount of methane in the AD process. Proteins are linear polymers, consisting of a string of subunits called amino acids. Proteins are primarily hydrolyzed into individual amino acids or peptides by the action of an extracellular enzyme called protease [50].

Amino acids are then further broken down to amine groups while releasing ammonia (NH3) or ammonium (NH4+

) in the process. Ammonia and ammonium are in balance with each other, and the form that would be present in the AD process is dependent on the pH and the temperature. At high concentrations, ammonia (NH3) could cause inhibition in the AD process, as it can be lethal to many microorganisms. Methane-producing archaea is the first to become inhibited, as the

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concentration of ammonia begins to increase [86, 95, 96]. How this inhibition happens is not completely understood yet. There are hypotheses that ammonia, as an uncharged compound, is capable of entering the cell and changing the pH inside the cell leading to cell disruption [95]. The rate and extent of protein degradation is dependent on many factors such as solubility, the category of end group, pH, and tertiary structure. In general, the rate of protein hydrolysis under an anaerobic environment is slower than the hydrolysis rate of carbohydrates [50, 97]. In this thesis, OFMSW and waste from the FPW have been used as a substrate in a high-rate two-stage biogas production system. Rapid processing was achieved by increasing the loading rate and decreasing the digestion time (paper V).

3.2. Remarks on theoretical and experimental methods for determination of biogas potential

3.2.1. Theoretical methods

Biogas production from the organic substrates engages internal redox reactions that convert organic molecules into CH4 and CO2. The fraction of these two gases are defined by the composition as well as the biodegradability of the substrates [98].

During the conversion of the carbohydrates, such as sugars, starch, or cellulose, an equal amount of CH4 and CO2 is produced [27]:

C6H12O6  3 CH4+ 3 CO2 (1)

For proteins, the process can be described as follows:

C13H25O7N3S+ 6 H2O  6.5 CH4 + 6.5 CO2 + 3 NH3+ H2S (2) The degradation of fats and vegetable oils (triglycerides) can be summarized by the following equation:

C12H24O6 + 3 H2O  7.5 CH4 + 4.5 CO2 (3)

The ideal stoichiometry, for a two-phase digestion, considering the simple case of carbohydrate degradation, can theoretically be defined as follows:

First stage: C6H12O6 + 2 H2O  4 H2 + 2 C2H4O2 (acetic acid) + 2 CO2 (4)

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Second stage: 2 C2H4O2  2 CH4 + 2 CO2 (5)

4 H2+CO2  CH4 + 2 H2O (6)

With the remaining sugars present in the substrate being converted into a more reduced form of products such as propionic acid, butyric acid, ethanol, etc.:

First stage: C6H12O6  C4H8O2 (butyric acid) + 2 CO2 + 2 H2 (7) Second stage: C4H8O2 + H2O  2.5 CH4 + 1.5 CO2 (8) These simplified examples can vary according to the effects of numerous factors [27, 98]. For instance, the reactions are often not complete e.g., up to half of the cellulose is refractory to microbial degradation, and lignin is entirely inert. Part of the substrates is utilized by the microorganisms for growth; consequently, there is also some biomass produced.

The theoretical methane potential in practice can be determined, for instance, by using the elemental composition (C,H,O,S,N) and the Buswell formula (papers III and IV):

CcHhOoNnSs+ yH2O xCH4 + nNH3 + sH2S+ (c-x) CO2 (9)

Where: x= 1/8 (4c+h-2o-3n-2s)

The component composition e.g., carbohydrate, fat, and protein content of the substrate (papers I and V) can also be used for the calculations according to the data presented in Table 1 below:

Table .1 Buswell’s formula for theoretical methane potential

Component Chemical formula Theoretical methane yield (m3CH4 /kg VS)

Carbohydrates C6H10O5 0.42

Lipids/fats C57H104O6 0.50

Proteins C5H7O2N 1.01

For liquid substrates, such as wastewater, with low particulate organic content, the chemical oxygen demand (COD) is followed by:

CH4 + 2 O2 CO2 + H2O (10)

References

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