Biogas from marine macroalgal waste

J ACOBS U NIVERSITY B REMEN

Biogas from marine macroalgal waste Supervisor: Prof. Dr. h.c. Roland B ENZ Author:

Reviewer:

Yann Nicolas B ARBOT

Prof. Dr. Laurenz T HOMSEN Reviewer: Dr. Florian K UHNEN

A

THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE

ACADEMIC TITLE

P HILOSOPHIAE D OCTOR (P H D)

IN

B IOCHEMICAL E NGINEERING

E NGINEERING AND S CIENCE J ACOBS U NIVERSITY B REMEN B REMEN , G ERMANY DATE

OF DEFENSE :

14.11.2014

Summary The scarcity of fossil combustibles and the environmental problems involved with their use has led to considerable interest in energy from renewable sources such as solar, wind, tides and biomass. Biogas belongs to the biomass-based renewable energy family and biogas plants are nowadays largely operated on terrestrial energy crops. Their growth requires fertile farmland, fresh water and fertilizer and faces direct competition with food crop production which is currently leading to serious environmental, social and ethical conflicts. It is hoped that a new (3rd ) generation of biofuels derived from marine biomass will relieve the pressures of the present situation. At the same time, extensive (macro)algal blooms and hypertrophication are events repeatedly reported in coastal regions all over the world, causing serious harm to the marine ecosystem and impairing local tourism. This otherwise unwanted biomass presents an interesting substrate candidate for use in biogas plants, offering the benefits of both the disposal of algae waste and the provision of alternative biogas substrate to diminish the massive demands for energy crops. In this study the biomethanation potentials of three types of macroalgal biomass were investigated, namely Baltic Sea brown seaweed Fucus vesiculosus (Fv), Baltic Sea beach macroalgae blend Rügen-Mix (RM) and a waste residue material from Laminaria japonica processing industries (Lj). All candidates were representatives for disposable material and either involved in eutrophication events and/or present marginal biomass waste. The degradation studies were carried out in batch and continuous systems using mesophilic and thermophilic process temperatures. Mild thermo-acidic hydrolysis and enzymatic (alginate lyase) pretreatment were applied to the biomaterial to solubilize the organic matter, increase its degradability and boost the biomethane yield. Acid hydrolysis on Fv and RM was successfully triggered at 80◦ C in technical acid media (HCl, H3 PO4 ) as well as in flue gas condensate (FGC), a liquid acid waste accumulating in power plants. Enzymatic pretreatment using alginate lyase was successfully applied on Lj biomass with moderate effect on boosting

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Summary the methane recovery. The increase of biomethane yield in batch ranged between +12% and +140% compared to untreated biomass and showed final total methane yields of 116 mL (Fv) to 214 mL (Lj) per g of volatile solids. Co-digestion of macroalgae with maize silage and comparison of mesophilic to thermophilic anaerobic digestion did not lead to any significant benefits in methane yield. However, acid hydrolysis pretreatment and thermophilic anaerobic digestion led to a considerable acceleration in methane formation. Continuous anaerobic digestion of the respective single macroalgae (monofermentation) was successfully conducted throughout several hydraulic residences and yielded between 66 mL (Fv) and 189 mL·g-1 VS (Lj) in mesophilic and thermophilic mode. The trials showed biochemical stability and steady specific CH4 production at organic loading rates between 2.0–3.0 g·L-1 ·d-1 and hydraulic retention times between 15–40 days. The suitability of fermentation residue to serve as biofertilizer was generally possible but exhibited contrasting abilities regarding macronutrient (nitrogen, phosphorous, sulfur, potassium) and trace element (iron) concentrations. The pollution of macroalgae digestate with heavy metals was acceptable for all tested biomass. Continuous pilot-scale trials with native Lj demonstrated the feasibility of upscaling the overall process with convergence to industrial process conditions. The comparison showed similar methane production in laboratory- (174 mL·g-1 VS) and pilot-scale (189 mL·g-1 VS) trials. Some practical experience was gathered regarding the substrate preparation and removal of interfering substances prior to biomethanation to prevent the wearing out of plant devices and plant-related mechanical interferences. Finally, it has been shown that bioreactors can be operated with single Fv, RM or Lj biomass in continuous long-term anaerobic digestion, generating steady methane production and steady biochemical conditions in the digester. It has also been proven that acid hydrolysis with technical acids or flue gas condensate and pretreatment with alginate lyase can successfully increase the methane potential of the tested substrates.

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Statutory declaration I, Yann Nicolas Barbot born on July 10th , 1984, hereby declare that my PhD thesis entitled “Biogas from marine macroalgal waste” is the result of my own work. I did not receive any help or support from commercial consultants. All sources and/or materials applied are listed and specified in the thesis. Furthermore, I verify that this thesis has not yet been submitted as part of another examination process neither in identical nor in similar form.

December 1, 2014 Yann B ARBOT

PhD committee 1. Reviewer / Supervisor: Roland Benz / Professor of Biotechnology (Chair)

2. Reviewer / Supervisor: Laurenz Thomsen / Professor of Geophysics

3. Reviewer / Supervisor: Dr. Florian Kuhnen / University Lecturer in Chemistry

Day of the defense (for PhD): 14.11.2014

Signature from head of PhD committee:

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"Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better."

– Samuel Beckett –

Thank you for reading my PhD thesis ...

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Table of Contents Summary

i

List of Abbreviations

xv

1 Introduction

1

1.1 Global energy supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

1.2 Time for a change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

1.3 Bioenergy from biomass . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

1.4 Promoting renewable energy – EEG in Germany . . . . . . . . . . . . .

7

1.5 Biogas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

1.5.1 History

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

1.5.2 Design of a biogas plant . . . . . . . . . . . . . . . . . . . . . . . 10 1.5.3 Biochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.5.3.1

Hydrolysis – Cleavage of substrate polymers . . . . . . 13

1.5.3.2

Acidogenesis – Organic acid-forming step . . . . . . . 13

1.5.3.3

Acetogenesis – Acetic acid formation . . . . . . . . . . 15

1.5.3.4

Methanogenesis – Methane formation . . . . . . . . . . 15

1.5.4 Sources of substrate for biogas production . . . . . . . . . . . . 16 1.5.5 Degradability of biomass . . . . . . . . . . . . . . . . . . . . . . 17 1.5.6 Process parameters . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.5.6.1

Temperature . . . . . . . . . . . . . . . . . . . . . . . . 19

1.5.6.2

Methane flow rate, methane production, degradation rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.5.6.3 1.5.6.4

pH value . . . . . . . . . . . . . . . . . . . . . . . . . . 20 ⇀ NH3 . . . . . . . . . 21 Dissociation equilibrium of NH4 + ↽

1.5.6.5

Buffer capacity of hydrocarbonate (HCO3 - ) . . . . . . . 21

1.5.6.6

Hydrogen partial pressure (pH2 ) . . . . . . . . . . . . . 22

1.5.6.7

Volatile fatty acids (VFAs) . . . . . . . . . . . . . . . . . 22

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Table of Contents 1.5.6.8

Organic loading rate (OLR) . . . . . . . . . . . . . . . . 23

1.5.6.9

Hydraulic retention time (HRT) . . . . . . . . . . . . . . 23

1.5.6.10 Measurement and control technology – The black box system . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 1.6 Actual problems linked to biofuel and biogas production . . . . . . . . . 25 1.7 Macroalgae biomass – A prospective future energy source . . . . . . . 28 1.8 Biomass growth and cultivation potential . . . . . . . . . . . . . . . . . . 28 1.9 Eutrophication, green tides and beach algae (Treibsel) . . . . . . . . . . 31 1.10 Degradability of marine macroalgae . . . . . . . . . . . . . . . . . . . . 34 1.10.1 Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 1.10.2 Experimental BMP . . . . . . . . . . . . . . . . . . . . . . . . . . 36 1.10.3 Potential bottlenecks in AD of macroalgae . . . . . . . . . . . . . 37 1.10.4 Improving degradability . . . . . . . . . . . . . . . . . . . . . . . 37 1.11 Use of waste materials for AD . . . . . . . . . . . . . . . . . . . . . . . . 38 2 Motivation and Scope

41

3 Material & Methods

45

3.1 Sludge inocula, maize silage and macroalgae . . . . . . . . . . . . . . . 45 3.1.1 Fucus vesiculosus (Fv) – Beach macroalgae . . . . . . . . . . . 45 3.1.2 Rügen-Mix (RM) – Beach macroalgae mix . . . . . . . . . . . . . 47 3.1.3 Laminaria japonica (Lj) – Macroalgae waste from industry . . . . 49 3.1.4 Maize silage (MS) . . . . . . . . . . . . . . . . . . . . . . . . . . 50 3.1.5 Substrate pretreatment . . . . . . . . . . . . . . . . . . . . . . . 50 3.1.6 Inoculum sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.2 Flue gas condensate (FGC) . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.3 Reactor systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.3.1 Batch setup BatchLab . . . . . . . . . . . . . . . . . . . . . . . . 52 3.3.2 Biomethane potential tests (BMP) . . . . . . . . . . . . . . . . . 53 3.3.3 Continuous stirred tank reactor (CSTR) . . . . . . . . . . . . . . 54 3.3.4 Pilot system Algas . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.4 Gas counter gasUino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3.5 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3.5.1 Measurement of volatile solid (VS) and total solid (TS) content . 60

x

Table of Contents 3.5.2 High performance liquid chromatography (HPLC) . . . . . . . . . 60 3.6 Data treatment from methane production recordings . . . . . . . . . . . 63 4 Results & Discussion

65

4.1 Methanation of Fucus vesiculosus (Fv) . . . . . . . . . . . . . . . . . . . 65 4.1.1 Elementary composition and theoretical methane potential . . . 65 4.1.2 Acid hydrolysis PT of Fv biomass and BMP . . . . . . . . . . . . 67 4.1.2.1

Effect of PT acidity and PT reaction temperature . . . . 67

4.1.2.2

Effect of PT reaction time . . . . . . . . . . . . . . . . . 70

4.1.2.3

Effect of FGC RWE and 0.2M HCl media on AD . . . . 70

4.1.3 Effect of PT on degradation dynamics of Fv . . . . . . . . . . . . 73 4.1.4 Discussion – BMP from Fv in batch trials . . . . . . . . . . . . . 76 4.1.5 Effect of FGC RWE PT on Fv in continuous AD . . . . . . . . . . 80 4.1.5.1

PT of Fv with FGC RWE . . . . . . . . . . . . . . . . . 80

4.1.5.2

Untreated Fv (1) . . . . . . . . . . . . . . . . . . . . . . 84

4.1.6 Effect of HCl PT on Fv in continuous AD . . . . . . . . . . . . . . 89 4.1.6.1

PT of Fv with 0.2M HCl . . . . . . . . . . . . . . . . . . 89

4.1.6.2

Untreated Fv (2) . . . . . . . . . . . . . . . . . . . . . . 92

4.1.6.3

Analysis of fermentation residue . . . . . . . . . . . . . 94

4.1.7 Discussion – Continuous AD of Fv . . . . . . . . . . . . . . . . . 96 4.2 Methanation of Rügen-Mix (RM) . . . . . . . . . . . . . . . . . . . . . . 100 4.2.1 Elementary composition and theoretical methane potential . . . 100 4.2.2 Acid hydrolysis PT of RM biomass and BMP . . . . . . . . . . . 102 4.2.2.1

Effect of media acidity . . . . . . . . . . . . . . . . . . . 102

4.2.2.2

Effect of PT reaction time . . . . . . . . . . . . . . . . . 105

4.2.2.3

Replacing technical HCl by FGC swb . . . . . . . . . . 107

4.2.3 Co-digestion of RM with maize silage (MS) . . . . . . . . . . . . 109 4.2.4 Adaptation of digestion sludge to RM and thermophilic AD . . . . 111 4.2.5 Effect of PT on degradation dynamics of RM . . . . . . . . . . . 113 4.2.6 Discussion – BMP from RM in batch trials . . . . . . . . . . . . . 116 4.2.7 Continuous AD of RM . . . . . . . . . . . . . . . . . . . . . . . . 120 4.2.7.1

Mesophilic AD of RM . . . . . . . . . . . . . . . . . . . 120

4.2.7.2

Analysis of fermentation residue . . . . . . . . . . . . . 122

4.2.7.3

Thermophilic AD of RM . . . . . . . . . . . . . . . . . . 124

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Table of Contents 4.2.8 Discussion – Continuous AD of RM . . . . . . . . . . . . . . . . 126 4.3 Methanation of Laminaria japonica (Lj) . . . . . . . . . . . . . . . . . . . 130 4.3.1 Elementary composition and theoretical methane potential . . . 130 4.3.2 Acid hydrolysis PT of Lj biomass and BMP . . . . . . . . . . . . 132 4.3.2.1

Effect of PT acidity and PT reaction temperature . . . . 132

4.3.2.2

Using technical acids as media for PT . . . . . . . . . . 134

4.3.2.3

Effect of FGC swb PT and increased PT temperature . 134

4.3.3 Co-digestion of Lj with maize silage . . . . . . . . . . . . . . . . 137 4.3.4 Enzyme PT of Lj . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 4.3.5 Adaptation of digestion sludge to Lj and thermophilic AD . . . . . 141 4.3.6 Effect of PT on degradation dynamics of Lj . . . . . . . . . . . . 143 4.3.7 Discussion – BMP from Lj in batch trials . . . . . . . . . . . . . . 144 4.3.8 Continuous AD of Lj . . . . . . . . . . . . . . . . . . . . . . . . . 148 4.3.8.1

Mesophilic AD of Lj . . . . . . . . . . . . . . . . . . . . 149

4.3.8.2

Analysis of fermentation residue . . . . . . . . . . . . . 153

4.3.8.3

Thermophilic AD of Lj . . . . . . . . . . . . . . . . . . . 154

4.3.8.4

AD of Lj in pilot-scale . . . . . . . . . . . . . . . . . . . 157

4.3.9 Discussion – Continuous AD of Lj . . . . . . . . . . . . . . . . . 160 4.4 Application scheme for AD of macroalgae . . . . . . . . . . . . . . . . . 165 4.4.1 Process chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 4.4.2 Operation of a 75 kWel. biogas plant with macroalgae . . . . . . 168 5 Conclusion and Outlook

171

5.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 5.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 List of Figures

177

List of Tables

181

References

183

Appendix A – Publications

207

Appendix B – Construction pilot plant system Algas

231

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Table of Contents Acknowledgement

245

xiii

List of Abbreviations µl ... Microliter AD ... Anaerobic digestion AFEX ... Ammonia fibre expansion AL ... Alginate lyase BioAbfV ... Bioabfallverordnung BMP ... Biomethane potential or biochemical methane potential CHP ... Combined heat and power plant CSTR ... Continuous stirred tank reactor d ... Days E ... Experiment EEG ... Erneuerbare-Energien-Gesetz FAO ... Food and Agricultural Organization of the United Nations FGC RWE ... Flue gas condensate from a RWE coal power plant FGC swb ... Flue gas condensate from a swb middle-calorific power plant FNR ... Fachagentur Nachwachsende Rohstoffe e.V. Fv ... Fucus vesiculosus test biomass FvR ... Fucus vesiculosus fermentation residue g ... Gram

xv

Table of Contents GHG ... Greenhouse gas h ... Hour ha ... Hectar HAB ... Harmful algae blooms HRT ... Hydraulic retention time HPLC ... High performance liquid chromatography kWel. ... 1 kW electrical energy kWtherm. ... 1 kW thermal energy kWh ... Kilowatt hour LCA ... Life cycle assessment LHW ... Liquid hot water pretreatment Lj ... Laminaria japonica waste test biomass LjR ... Laminaria japonica waste fermentation residue MAPT ... Mild acid-temperature pretreatment MAS ... Mesophilic adapted sludge mcc ... Microcrystalline cellulose MD ... Methane digester MF ... Methane fermenter MIS ... Mesophilic inoculum sludge mL ... Milliliter mM ... Millimolar MS ... Maize silage

xvi

Table of Contents MBS ... Mesophilic blending sludge MO ... Microorganism NASA ... National Aeronautics and Space Administration OLR ... Organic loading rate PCU ... Process control unit PT ... Pretreatment rpm ... Rounds per minute RM ... Rügen-Mix test biomass RMR ... Rügen-Mix fermentation residue RT ... Fermentation residue tank SB ... Sensor block SD ... Standard deviation t ... Time TAS ... Thermophilic adapted sludge TIS ... Thermophilic inoculum sludge TS ... Total solids TWh ... Terawatt hour U.V.-VIS ... Ultraviolet-Visible Detector VDI ... Verein Deutscher Ingenieure e.V. VFA ... Volatile fatty acid VIS ... Visible light VS ... Volatile solids yr ... Year

xvii

1 Introduction 1.1 Global energy supply Energy always the impetus of technological innovation, was first provided by human or animal brawn and later, from the industrial revolution onwards, by fossil fuels. Combustion of coal, oil, gas and their derivatives allowed technological development as we encounter it today. However, fossil fuels are finite and their ultimate depletion is a fact and must be considered for technical development and commodity production in the future. Global energy consumption has lately shown a steady and fast increase, particularly in industrialized and emerging nations [5]. Most of these national economies are largely based on crude oil and its scarcity would greatly affect their domestic industries [70]. It is estimated that oil supplies will be completly depleted by the middle of this century according to the current trend of consumption [3]. The rapid growth of the world population acts as a catalyst to the depletion process and enhances geopolitical conflicts over the last remaining reserves [160]. Besides the multiple environmental impacts of the extraction of crude oil and coal, combustion of carbonaceous fuels causes greenhouse gas (GHG) emissions resulting in global warming and climate change [138]. The concentration of CO2 in the Earth’s atmosphere has increased by 30% compared to the pre-industrialized period before 1800 [237, 175]. Growing concerns about the negative consequences of global warming on the environment, societies and economies have led to various climate summits over the last two decades. Decisions were taken to reduce domestic CO2 emissions to abate the global warming process [2]. Besides fossil energy sources, nuclear-based energy is used to provide electricity. The great hope of the 1970s was to solve the problem of global energy supply. Although nuclear power presents a CO2 -neutral energy source regarding emissions, its byproducts include radioactive waste which is complex to dispose of [70]. Also, the recent nuclear disaster in Fukushima has led decision-makers to reconsider the safety of nuclear energy technology [243]. A fundamental change in global energy generation and distribu-

1

1 Introduction

Figure 1.1: The world’s total primary energy supply from 1971 to 2011 displayed by energy sources. To date around 80–90% of global energy needs are met by fossil fuels [240]. However, the renewable energy sector is expected to play a significant role in the world energy supply of the future [225, 199]. Online Data Service: Total primary energy supply WORLD © OECD/IEA, 2013. tion shifting from non-renewable fuels to sustainable renewable energy is therefore inevitable for long-term energy security.

1.2 Time for a change The global oil crisis in 1973 has revealed the vulnerability of industrialized economies dependent on crude oil [13]. Many armed conflicts have hence been initiated to secure a steady oil supply to consumer nations [160]. The volatility of the global oil price urged governments to think of alternative ways to provide energy safety and initiated a cascade of innovations which continue to the present date. Also, with in-

2

1.3 Bioenergy from biomass creasing resource limitation the importance of providing energy locally rather than relying on supplies from abroad is generally recognized [107]. Renewable energy sources such as water, wind, sun, tidal, geo-thermal and biomass-based energy are generated locally and have been promoted intensively for technological development within the last decade [121]. Countries have the choice of different technologies depending on their climatic and geographical location [70]. Finding a suitable renewable energy mix according to the country’s needs and resources could help to balance out dependencies and secure a constant and sustainable supply. The potential of renewable energy is enormous and far outcompetes that of fossil fuels (see Figure 1.2). It is expected that the global energy demand will increase two or threefold [240, 32] and that by the year 2060 about half of these needs will be covered by renewable energy sources [199]. A changing mindset in the provision of energy can be discerned in all sectors of society. Governments worldwide have been recently subsidized renewable energy technology, realizing the long-term benefits of such development [121, 127, 46, 150].

1.3 Bioenergy from biomass Throughout human history, biomass has been one of the most important energy sources e.g. the burning of wood. Even today about 15% of global energy production is based on biomass, especially in developing countries [104]. Sustainable energy production is directly or indirectly related to solar energy. Biomass is stored solar energy and if a closed cycle of CO2 release and capture is maintained, a net increase in global CO2 concentration will be avoided. Biomass energy exhibits the advantage of storage and can be used on demand. It provides thermal, electrical and fuel energy covering the major energetic needs of a society (see Figure 1.4). Differentiations can be made between the various categories of fuel generation. First-generation biofuels are fuels based on easily extractable sugar and vegetable oil from rape seed, sugar cane or palm trees. Experiencing strong promotion in the 2000s, these high quality fuel products, however, led to a variety of problems such as increasing freshwater needs for irrigation, stress on food commodities and large-area fire clearance [195, 98]. Many other environmental and social issues are related to the cultivation of first-generation biofuel crops and therefore efforts have been initiated to change this focus [205]. With almost 50 billion liters annual production, first-generation bio-

3

1 Introduction

Figure 1.2: Comparison of finite and renewable planetary energy reserves (terawatt/year). Finite resources show the total recoverable planetary reserves. Data taken from Perez et al. [179].

fuels are still largely produced in the U.S., Brazil and Asia [168, 48]. In the form of bio-ethanol or biodiesel they are often blended with conventional fuels and used for transportation [168]. Second-generation biofuels produced from ‘plant biomass’ refer largely to lignocellulose materials, agricultural waste and manure. As these products compose the majority of the cheap and abundant non-food materials available from plants, they are considered expendable and find better acceptance in usage than first-generation biofuels [247]. However, the extraction of the required fuel is more elaborate due to technical barriers or diminished microbial degradability [79]. At present, the production of such fuels is often not cost- and energy-effective, but its potential seems promising [79, 22]. Third-generation biofuels refer to fuels avoiding the issues met with first- and second-generation biofuels. They circumvent the food-

4

1.3 Bioenergy from biomass

Figure 1.3: Overview of different processing strategies for energetic conversion of biomass. Outcome products are ethanol, methanol, methane and oils or energy in form of heat or pressure [210]. fuel competition, prevent land-use change, are less difficult to convert to biofuel and are thus considered a viable alternative energy resource with better environmental, social and economical sustainability [207]. Third-generation biofuels mostly refer to products derived from algae biomass [144], with the majority of research focussed on microalgae [77, 181, 14, 206, 99, 140].

5

1 Introduction

6 Figure 1.4: Various options for using biogas are available. Electricity, heat or transport fuel is provided by either the combustion of biogas or by upgrading to biomethane. Storage capacities in tanks, caverns or the domestic natural gas grid offer the possibility of efficient use on demand [93]. (Source: Specialist agency renewable resources e.V. (FNR)).

1.4 Promoting renewable energy – EEG in Germany

1.4 Promoting renewable energy – EEG in Germany The German Erneuerbare-Energien-Gesetz (EEG) is a renewable energy source act created to promote green energy generation in Germany. It was designed for the development of a sustainable energy supply, considering the reduction of long-term economic costs, the protection of the environment and the reduction of conflicts over fossil energy resources [39]. After its creation in 2000, it was subsequently amended and updated in 2004, 2009, 2012 and 2014. It mainly relies on a feed-in tariff, which the grid operator has to pay to the producer of electricity or bioenergy from renewable sources. An equalization scheme allows grid operators and utilities to pass on the additional costs arising from producing electricity from renewable energy sources to the final customers. In short, the costs for changing the energy infrastructure to renewable energy are borne by all electricity consumers in the country. Other European, Asian and American countries have designed similar compensation schemes or have created governmental subsidy programs to promote the development of their domestic renewable energy sector [121, 215, 177].

Figure 1.5: Number of biogas plants with their total installed electrical capacity in Germany [92]. (Source: Specialist agency renewable resources e.V. (FNR)).

7

1 Introduction In Germany the number of biogas plants has increased from a few hundred in 1999 to over 7,500 in 2012 with a total installed electrical capacity of about 3.500 MWel (see Figure 1.5). This makes Germany one of the world leading countries in the export of biogas technology. The popularity and success of biogas technology in Germany is closely linked to its promotion via the EEG. However, the utilization of biogas as an energy source has also been established in developing countries such as China and India. Thousands of mini-scale biogas plants of various capacities supply a great number of households with lighting and gas for cooking purposes [104]. The compensation system for biogas-based energy relies on the amount of the fermentable fraction and the methane potential of a certain feedstock. To counteract the extensive usage of corn, the use of corn silage as feedstock in biogas plants was limited to only 60% by weight of input material. Small biogas plants up to 75 kWel. using municipal organic waste or animal manure as feedstock have an enforced compensation with up to 25 cent per kWh. Generally, the basic amount of compensation was increased compared to EEG 2009, implying the usage of 60% waste heat as a requisite for all new constructions to ensure the optimal use of available energy. EEG 2012 was to create new dynamics on the biogas feedstock markets by extending the portfolio of possible substrates eligible for subsidization. The amendments of EEG 2014 regarding biogas aim to reduce statutory tariffs, enhance direct marketing of electricity to lower energy prices for consumers and increase flexibility of electricity feed-in into the public power grid. One of the ideas of EEG 2014 is also to reinforce the support for the development of small-scale biogas plants using manure or biowaste as substrate feedstock. Figure 1.6 presents an overview of the EEG 2012 feed-in tariff. The latest remuneration values of the compensation system EEG 2014 slightly vary therefrom.

1.5 Biogas Anaerobic digestion is a biological process consisting of many sub-reactions in which organic matter is degraded to biogas. In the absence of oxygen a symbiosis of various microorganism species decomposes feedstock biomass to energetically relevant methane, the desired end product. Biogas is mainly composed of methane (50–75%) and carbon dioxide (25–45%) with low quantities of trace gases such as oxygen (< 2%), nitrogen (< 2%), hydrogen sulfide (< 1%) or ammonia (< 1%) [93].

8

1.5 Biogas

Figure 1.6: Overview of feed-in tariff system in EEG 2012 [38].

1.5.1 History

The first scientific examinations of biogas were performed by Alessandro Volta in 1770 when analyzing the combustibility of swamp gas. Subsequently, Michael Faraday found its flammable fraction to be hydrocarbon. Only in 1821 Amedeo Avogadro discovered the carbonaceous chemical structure to be methane [70]. Towards the end of the 19th century, sewage sludge from waste water treatment plants was used to generate gas for public street lighting in European cities [70]. By World War II, biogas, then known as sewage gas, had become a popular domestic fuel used for transportation, heating and lighting [70]. However, the interest in sewage gas gradually faded due to easy access to crude oil, its versatile applicability in the energy sector and its outstanding energetic features. It was only after the 1973 oil crisis that biogas technology in Germany experienced a strong renaissance with concerns regarding domestic energy safety and reduced external dependency on fuel imports [70]. Since its comeback, the biogas sector has grown and prospered and energy from biogas has found its place among today’s renewable energy sources. Besides

9

1 Introduction industrialized nations, many developing countries such as China, India and Nepal are utilizing biogas as an energy source. [70, 116, 185].

1.5.2 Design of a biogas plant In the course of the development of biogas technology many different construction methods and construction designs for biogas plants have been created. According to the feedstock in use, its consistency and availability a plant operator decides upon the plant design, plant size and process conditions [71]. The choice between dry or wet digestion process conditions is dependent on the dry matter content of the feed. The substrate feedstock will also determine whether the type of feed is intermittent, quasi-continuous or full-continuous and whether a single or two-stage system will be used [91]. The one-stage system consists of only one digester tank. All fermentation sub-processes are located within the same repository although optimal conditions for each type of participating microorganism can never be reached due to their different ideal growth and metabolic requirements. Neither spatial nor temporal sub-process separation can be found in this type of operating system. In two-stage systems a spatial separation of the fermentation sub-processes is performed. In addition to a methane digester, there is a pre-acidification stage in a separate container in which the substrate is initially solubilized at low pH by hydrolytic microorganisms [91]. The pre-digested substrate is then transferred to the methane fermenter where the main biogas generating sub-processes take place. This spatial separation therefore allows process optimization. The most important anaerobic digestion systems are described as: • Batch system Intermittent batch feeding comprises the complete filling of the digester with fresh substrate and inoculum. Hermetically sealed, the feedstock remains inside the digester tank until the selected hydraulic retention time (HRT) elapses. During this time no substrate is added or removed to or from the system. Subsequently, the digester is emptied and refilled with a fresh batch of feedstock. A small proportion of the digestate is used as seed material to inoculate the fresh substrate batch. • Semi-continuous system A semi-continuous system (quasi-continuous) entails the addition of unfermented substrate to the digester at least once per

10

1.5 Biogas working day. Therefore the system falls between a batch and a continuous process mode. • Continuous system The continuous fermentation system provides a uniform gas production and is seen as a stable and suitable process for most commercially operated biogas plants. Continuous processes are characterized by regular feeding intervals of the digester through the day. A quantity of fresh substrate equivalent to the one added to the fermenter is expelled or removed into a downstream fermenter (post-digestion tank). This results in fairly regular gas – and therefore – energy production. Most biogas plants are operated in continuous mode. Besides the methane digester tank (MF) where anaerobic digestion takes place and biogas is generated a biogas plant system consists of further working units (see Figure 1.7). • Post-digestion tank The secondary digester stores the fermentation residue expelled from the MF. The remaining organic matter is used as valuable organic fertilizer and can be applied to agricultural areas. • Gas storage To store and collect the generated biogas, methane and secondary fermenter are covered by gas-tight, enclosed and expandable roofs which serve as gas storage. Peaks in gas production can thus be recovered. • Desulfurization During the biological degradation of sulfur-containing biomass hydrogen sulfide (H2 S) is produced. H2 S is considered toxic to living beings (also for microorganisms) even in low quantities and harmful to engine systems due to corrosion effects. It is biologically or chemically discharged from the gas before further application. • Combined Heat and Power Unit (CHP) The generated biogas is primarily combusted in a gas engine and electrical power is generated which is fed into the public power grid. In addition to a combustion engine generator the CHP module is equipped with a heat exchange system for recovering the heat produced by the exhaust, cooling water and lubricating oil system.

11

1 Introduction

12 Figure 1.7: Scheme of a typical agricultural biogas plant using animal manure and/or energy crops as feedstock substrate. Biogas is upgraded or combusted to generate heat, electricity or biomethane [93]. (Source: Specialist agency renewable resources e.V. (FNR)).

1.5 Biogas 1.5.3 Biochemistry The process of biogas formation is divided into four main stages which are subsequently specified. With every sub-reaction the microorganisms gain energy to maintain their metabolism and to allow proliferation. The respective phases are carried out by different groups of microorganisms co-existing in a syntrophic cooperation. All preceding and subsequent phases are closely interconnected and can strongly affect each other. Figure 1.8 shows an overview flow chart of organic matter depletion during the anaerobic digestion process.

1.5.3.1 Hydrolysis – Cleavage of substrate polymers In this first step, an organic substrate is disassembled by extracellular bacterial enzymes into oligomers and monomers; this is performed by bacterial species such as Clostridium spp., Bacillus spp. and Pseudomonas spp.. Carbohydrate cleavage results in simple or oligo-sugars, proteins are degraded to single amino acids and (oligo)-peptides and energy-rich fat is decomposed to glycerol and fatty acids. This crucial step is necessary in all bacterial processes due to the fact that bacteria cannot uptake large molecules exceeding a certain size. For substrates which are particularly slowly biodegraded, such as lignin, hydrolysis is the velocity-limiting step of decomposition in the anaerobic digestion process. The facultative anaerobic microorganisms use the remaining oxygen dissolved in the water and optimize the redox potential for subsequent obligatorily anaerobic microorganisms [69, 37].

1.5.3.2 Acidogenesis – Organic acid-forming step In this step acid-forming bacteria such as Bidobacterium spp., Selenomonas spp. and Flavobacterium spp. perform a series of bio-chemical reactions where hydrolyzed substrates are decomposed to short-chain organic acids such as C1 -C5 molecules (acetic acid, propionic acid, butyric acid ...), lactic acid, alcohols, CO2 and hydrogen (H2 ). The pH optimum of these first two fermentation sub-steps, hydrolysis and acidogenesis, is at around pH 4–5. Depending on parameter values such as pH, H2 concentration or substrate loading rate different degradation reactions take place preferentially. High organic loading rate (OLR) enhances lactic acid formation, low

13

1 Introduction

Figure 1.8: Simplified diagram of how organic matter is degraded in four main steps during anaerobic digestion. In the hydrolysis phase (1) substrate polymers are cleaved into oligo- and monomers. These mono- and oligomers are furthermore degraded in the acidogenesis (2) to short chain organic acids and alcohols and in the acetogenesis (3) to acetic acid, CO2 and H2 . The last stage, the methanogenesis (4) releases CH4 , H2 and CO2 as final products [90]. (Source: Specialist agency renewable resources e.V. (FNR)).

14

1.5 Biogas pH value favors ethanol production and high pH value leads to preferential volatile fatty acid (VFA) formation [40].

1.5.3.3 Acetogenesis – Acetic acid formation In this step long-chain fatty acids are reduced to acetic acid (C2 ) and elementary H2 due to the action of acid-forming bacteria such as Acetobacterium spp., Sporomusa spp. and Ruminococcus spp.. The acetogenic bacteria have to coexist syntrophically with methanogenic bacteria since ones products are the other’s direct energy sources. A low H2 partial pressure preferentially shifts the thermodynamical equilibrium towards a CH4 final product (∆ G0 ’ < 0) and allows the overall reaction process to run smoothly and coherently [216]. Propionic acid (C3 ) is mainly oxidized through the methlyl-malonyl-CoA pathway in which C2 , H2 and CO2 products are generated. Butyric acid (C4 ), iso-butyric acid (iC4 ) and valeric acid (C5 ) are degraded to C2 and C3 . Valeric acid is, like iso-valeric acid, reduced to C3 and C2 by a ß-oxidation. In the degradation reaction of iso-valeric acid (iC5 ) to C2 and H2 , CO2 serves as a co-substrate [71].

1.5.3.4 Methanogenesis – Methane formation The capability to produce CH4 from H2 and CO2 is restricted to a very few organisms of the Archaea family, such as Methanococci spp., Methanobacteria spp. and Methanomicrobia spp.. The methanogenesis is a central step in anaerobic digestion. Methanogens are very sensitive to all environmental variations, especially pHfluctuations and the presence of oxygen. Due to their very slow growth rate, the functional replacement of methanogen biomass after removal or flush out is also diminished. In methanogenesis the organic carbon undergoes a disproportional reaction while being transformed into its most oxidized form (degree of oxidation +IV) as the CO2 molecule and into its most reduced form as the CH4 molecule (degree of oxidation -IV). The formation of CH4 can be divided into three major types of reaction according to the feeding substrate: • CO2 type: CO2 , HCOO• Methyl-type: CH3 OH, CH3 NH3 , (CH3 )2 NH2 + , (CH3 )3 NH+ , CH3 SH, (CH3 )2 S

15

1 Introduction • Acetate type: CH3 COOWhen methane formation is hindered or for some reason inhibited, this results in the accumulation of acidic intermediate products and thus overacidification in the digester [71]

1.5.4 Sources of substrate for biogas production Many types of organic matter can be used for biogas production. However, different considerations have to be taken into account when selecting the potential feedstock biomass. The nutritional value and content of organic matter in the biomass should be appropriately high since this part alone contains the final methane potential. Harmful substances should only be present in low concentration to avoid inhibition of AD. This is particularly true for disinfectants, detergents, antibiotics, solvents, herbicides, salts or heavy metals. Even smaller quantities thereof can inhibit the fermentation process drastically [93]. Also, the composition of the generated biogas and the composition of the fermentation residue should be such that the materials can be used in further applications. The quality of fermentation residue will determine whether it represents a valuable organic fertilizer or a undesirable waste product. Biomass which is suitable for AD is termed "substrate" [71]. Agriculturally operating plants use animal manure (e.g. bovine and swine manure) as basic substrate feedstock. Due to its accumulation in farming, manure presents an undesired product which is productively used as feeding material for AD. Industrially operating plants normally use energy crops such as maize, rye, wheat or their silage equivalents as main feedstock biomass. Due to their high biogas yields and their good degradability, these crop plants find application in large-size industrial plants which focus on high gas production performance [91]. Other organic materials e.g. fatty material from grease traps in slaughterhouses can also be fermented and are often added in smaller quantities to the digester to increase the biogas production or the methane ratio in the gas (co-digestion). Along with renewable raw materials, non-agricultural substrates are also suitable for producing biogas, such as residues from the food industry, e.g. distiller’s wash, vegetable waste water residues, vegetable waste from wholesale markets, organic domestic waste or grass clippings [91]. The AD of residue material allows the closing of sustainable cycles where waste is properly degraded in a low-emission and hygienic way. A long-proven feed-

16

1.5 Biogas

Figure 1.9: Utilizable energy potential of biogas in Germany (Hartmann/Kaltsschmitt, 2002, reworked by FNR) [93]. (Source: Specialist agency renewable resources e.V. (FNR)). stock for biogas production is sewage sludge accumulating in waste water treatment plants. This biomass, formed during the biological purification step, is anaerobically digested in a digester tower on site. Direct combustion of the generated biogas provides electricity and process heat for the water treatment facility thus improving overall process efficiency [71]. Figure 1.9 shows the potential of different types of feedstock for biogas production in Germany in 2009 and their potential contribution to the total amount of substrate used.

1.5.5 Degradability of biomass In principle, any type of organic matter could serve as substrate feedstock in anaerobic digestion. AD is a natural decomposition process used for the remineralization of complex structures of organic matter similar to the aerobic composting process. The key solution to high performance biogas production remains in the optimal, fast and reliable degradation of these complex biomass structures by microorganisms. This can be achieved by the pretreatment of substrate feedstock biomass or/and by sup-

17

1 Introduction porting the microbiological and bio-chemical degradation processes in the digester. The quality of degradability of biomass can strongly vary according to the material in use [71]. Energy crops such as maize or forage beets contain large amounts of soluble carbohydrates and little quantities of ligno-fibrous material. They are therefore easy to degrade and accessible to the microorganisms, which is necessary to attain high biogas yields [43]. These crops can be stored by ensiling, a bio-chemical method of biomass conservation and pretreatment. Part of the carbohydrate content of the biomass is converted by microorganisms to organic acids such as lactic acid, acetate and propionate. These intermediate products lower the pH to values between 3 and 4 and cleave the structural polysaccharides of plant material resistant to AD. Ensiling is therefore also considered a pretreatment process [240]. Fibre- and lignocellulosic-rich substrates such as grass clippings, rye straw or forestry and industrial waste often cause problems in the degradation process due to their extremely slow decomposition. Due to their robust polymeric structure, these substrates are difficult to solubilize for hydrolytic bacteria. They must be disintegrated enzymatically, thermally and/or chemically before application in the biodigester and pretreatment is performed to improve their digestibility [109]. To allow the utilization of fibre- and lignocellulosic-containing substrates and to improve accessibility for hydrolytic enzymes, different pretreatment strategies have been considered to achieve a higher substrate solubilization [222]. Mechanical treatment such as milling and chopping is generally applied to increase the effective surface of the biomass. Physical and chemical pretreatment such as dilutedand concentrated-acid hydrolysis [221, 222], alkaline hydrolysis [222], liquid hot water pretreatment (LHW) [133, 169, 72], ammonia fibre expansion (AFEX) treatment [133, 72], steam explosion [222, 169] and wet oxidation [222] are some of the methods which have shown successful results in cleaving the macropolymers.

1.5.6 Process parameters Bio-chemical processes, their reaction velocity and product outcome are highly dependent on the environmental conditions at hand. Based on the previous description of AD steps and their individual sub-phase reactions, a quantitative and qualitative equilibrium between the different types of microorganisms involved in AD needs to

18

1.5 Biogas be developed to maintain a consistently running process. The following listed factors are known as process-influencing parameters in the AD process.

1.5.6.1 Temperature For every biological process, the temperature is one of the most important factors and determines the destiny of its reaction product in terms of quality and quantity. Dependent on the microorganisms involved in the monitored procedure different temperature optima are set for the fermentation process. There are three modes of operation temperature to run the methane digester (MD): • psychrophilic at a temperature range between 15◦ C and 25◦ C • mesophilic at a temperature range between 25◦ C and 45◦ C • thermophilic at a temperature range between 45◦ C and 55◦ C At elevated temperature, the biogas yield rate is superior (thermophilic microorganisms), but the process susceptibility is also increased. If the process temperature is low, the biogas production rate will be lower. Most biogas plants will therefore operate at mesophilic temperature range with temperatures around 37◦ C to 40◦ C to ensure a balanced compromise between biogas production rate and process stability. In two-phase systems the pre-acidification and the methane digester can be operated at different temperatures, although this varies individually depending on the substrate feed. Digesters need to be externally heated since the bio-chemical reactions produce very little of their own reaction heat [91].

1.5.6.2 Methane flow rate, methane production, degradation rate The specific CH4 flow rate or CH4 productivity and the CH4 yield are some of the important parameters to describe the performance of a biogas plant and output yield from a certain substrate type. The CH4 yield is stated in relation to the amount of substrate fed and the active digester volume of the plant. The CH4 yield describes the quotient of the volume of methane produced and the amount of organic matter added and is given by V˙ CH4 3 −1 [m · t V S] ACH4 = m ˙ VS

19

1 Introduction ˙ CH4 = methane production [m3 ·d-1 ] and where A(CH4 ) = methane yield [m3 ·t-1 VS], V ˙ VS = added volatile solids [t·d-1 ]. m The formula for specific CH4 flow rate is

PCH4 =

V˙ CH4 3 −1 −1 [m · t · d ] mV S

˙ = methane production [m3 ·d-1 ] and mVS = where P = CH4 productivity [m3 ·t-1 ·d-1 ], V feedload input of VS [t]. The degree of degradation (η VS ) provides information on the degradation efficiency of used substrates. It can be calculated using the values for VS concentration or the chemical oxygen demand (COD). Due to common practice analysis of organic matter content the utilization of VS values is recommended for this calculation. The formula for degree of degradation is ηV S =

V Ssub · min − V Sdig · mout · 100[%] V Ssub · min

where VSsub = share of organic matter in the substrate [g·kg-1 FM], min = mass of added substrate fresh mass [kg], VSdig = share of organic matter in digestate [g·kg-1 FM] and mout = mass of removed digestate [kg].

1.5.6.3 pH value The bacteria of the various fermenting stages have different optima in pH value. Thus, the respective optima of the hydrolytic and acidogenic bacteria are situated in the range between pH 4.5 to 6.3 [40]. However, these bacteria exhibit only marginally reduced activity at slightly higher pH. Acetogenic and methanogenic bacteria require a slightly alkaline pH range (around pH 7.5) for activity and proliferation and are more susceptible to pH fluctuations [42]. A stable methanogenic process is required in any case since methane as a key product is generated only in this last step of the process. Therefore, kinetically faster running acid-producing steps would only result in an acid accumulation followed by a pH-drop causing the inhibition of the methane production. A single-stage system must hence be optimized and adjusted to the pH of the methanogenic organisms to ensure a continuity in methane production.

20

1.5 Biogas

Table 1.1: Environmental requirements in anaerobic digestion [69]. Parameter Hydrolysis/acidogenesis Methane formation Temperature 25–35◦ C Mesophilic: 32–42◦ C Thermophilic: 50–58◦ C pH value 5.2–6.3 6.7–7.5 C:N ratio 10–45 20–30 VS content pH 7.25 and reaching pH 6.9. In both cases the values recovered and balanced out in spite of maintaining the same operating conditions. It seemed that MOs in the digester sludge found a way to tackle the problem of coordinated degradation of RM after an adjustment period. The visual and olfactory analysis of digester sludge showed a quite viscous and black-colored slurry emitting a strong and distinct sulfurous odor. Comparing the CH4 yields of RM obtained in this study to the ones given in the literature, the methane production was poorer but feasible with RM alone. The decreased CH4 conversion performance was likely to originate from RM composition and presence of process disturbing substances and substance ratios. Generally, AD of RM at mesophilic and thermophilic process temperature exhibited stability for

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4 Results & Discussion the measured process parameters after an (initial) period of adjustment. The main difference between mesophilic and thermophilic process operation, apart from different process temperature, consisted in shortened HRT (15 days vs. 30–40 days) and higher OLR (3.0 g vs 2.5 g·L-1 ·d-1 ) in the thermophilic mode. Though, no gains in methane recovery could be observed choosing either mode of operation, a fact which matched the findings from Hansson (1983) in AD of green macroalgae from southern Sweden [106].

Process parameters The results suggested a bioreactor to be operated at OLR 2.5–3.0 g·L-1 ·d-1 and depending on the operational mode chosen the HRT can vary between 30–40 days (mesophilic) and 15 days (thermophilic). Habig & Ryther noted the most productive CH4 conversion rate at HRT 50 d for mesophilic AD of Ulva spp. and Gracilaria spp. [103]. It might be possible to operate bioreactors at higher OLR but this was not tested in this study. In any case, a slow increase of OLR is advised, especially during thermophilic process operation. The pH during the AD of RM in both experiments approached the value 7.0 when reaching the steadystate condition suggesting the value to be the valid benchmark. Conductivity was between 10–20 mS·cm-1 thus in standard range [241] and did not presume a large accumulation of salts dissolved in the bioreactor. VS and TS concentration were 4.5% and 7.5% for the mesophilic and 3.2% and 4.7% for the thermophilic approach coinciding with the applied OLRs and HRTs in the respective experiments. The ratios of VS/TS were 60% (mesophilic) and 68% (thermophilic) reflecting the differences in degree of degradation stated in the previous paragraph. Both values, but especially the thermophilic ratio, were close to the one in the source substrate also confirming handicapped biomass conversion.

Quality of fermentation residue RMR The potential interest for further use of RMR was already described in section 4.1.7. The concentration of macronutrients in RM and RMR were similar except for K and C which showed reduced (-50%) and elevated (42% from TS) quantities, respectively. C/N ratio and total C, N and Ca concentration were comparable to cattle manure but P, K and Mg showed poorer results. As anticipated from the source substrate analysis the S quantity in RMR was 4-times higher than in the references. Among micronutrients, only Fe showed a more favorable concentration range (85-fold the declaration limit).

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4.2 Methanation of Rügen-Mix (RM) Concentration of heavy metals were generally above those of common digestate and cattle manure but below the legal limitation values. This was true except for Cd which’s concentration exceeded by 2-fold the limiting concentration for fertilizer application. Elevated Cd concentration was detected in digestate from AD of Baltic Sea macroalgae originating from Trelleborg in Sweden which was designed for biofertilization [31]. Comparing heavy metal concentration in RM and RMR one could detect a slight concentration increase in the digestate. Methanogenic MOs are generally sensitive to heavy metals but the intensity can differ according to the microbial pool composition and the presence of metal-tolerant or meta-resistant species. Macroalgae and seaweed can act as absorption structure for heavy metals through mitigation of environmental concentration and increased uptake concentration in the algal biomass [161]. This is specifically the case in industrialized coastal areas exhibiting increased heavy metal concentration and low distribution through tidal activity as for instance the enclosed Baltic Sea. Using RMR as fertilizer was generally possible but this is dependent on the nutriental needs of the soil. RMR was rich in N and S, two of the essential nutrients important for fertilizer application. A problem might cause the increased concentration of heavy metals, specifically Cd. Filipkowska et al. (2008) noted that beached macroalgae from the Sopot area for use as agricultural biofertilizer is not recommended due to enhanced stress from contaminants [85]. However, there are attempts to develop new strategies for heavy metals detoxification from fermentation residue to allow further usage in agriculture [31, 172].

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4 Results & Discussion

4.3 Methanation of Laminaria japonica (Lj) Laminaria japonica is a typical macrophyta of the brown algae family largely cultivated in aquatic farms in Eastern Asia. In this study the Lj biomass originated from L. japonica processing industries and acts as an representative biomass for algal organic waste from macroalgae production.

4.3.1 Elementary composition and theoretical methane potential Background An elementary analysis of the L. japonica sample was performed to check the quality and composition of the biomass prior to experimentation. Macroand micronutrient content and quantity were defined and heavy metal concentrations were analyzed according to the German Waste Act (Bioabfallverordnung, BioAbfV). The results of the analysis are displayed in Table 4.13. To obtain a reliable benchmark reference concentrations for heavy metal, macro- and micronutrient content, the results of Lj were compared to those of maize56 . To estimate the possible methane output from Lj biomass in AD a theoretical calculation has been performed in the same manner as it was done in section 4.1.1 and 4.2.1 for Fv and RM, respectively. Lj biomass was analyzed and percentile shares of carbohydrate, protein and lipids with their respective values for theoretical methane potential were determined and are shown in Table 4.14. Observations and findings The elementary analysis of L. japonica waste showed that its general composition was comparable to the one of maize with higher macronutrient concentrations for potassium (K) (5-fold), magnesium (Mg) (2.5-fold), calcium (Ca) (3-fold) and sulfur (S) (3-fold) and micronutrient concentrations for molybdenum (Mo) (3-fold), iron (Fe) (19-fold) and manganese (Mn) (5-fold). S may inhibit the process due to formation of H2 S [69] but the overall concentration was much lower if compared with Fv or RM biomass. The C/N ratio of 10.5:1 was situated in poorer range as the optimal value favorable to AD [166]. The high Mo, Mn and Fe concentrations may attract interest due to their importance in microbial degradation performance [68]. The composition analysis showed that Lj consisted mainly 5 6

Fachagentur für nachwachsende Rohstoffe (FNR) – Handreichung, 2006, Nehring, 1970; "Leitfaden – Erfolgreicher Maisanbau, Pickert, Bayer CS 2003

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4.3 Methanation of Laminaria japonica (Lj)

Table 4.13: Elementary analysis of Laminaria japonica waste. Element L. japonica Maize Unit BioAbfV mg·kg-1 mg·kg-1 mg·kg-1 mg·kg-1 mg·kg-1 mg·kg-1 mg·kg-1

TS TS TS TS TS TS TS

2,200 17,800 2,700 4,500 2,700 ∼30:1 43 14,000

mg·kg-1 mg·kg-1 mg·kg-1 mg·kg-1 mg·kg-1

TS TS TS TS TS

0.3 184 65

mg·kg-1 mg·kg-1 mg·kg-1 mg·kg-1 mg·kg-1

Lead (Pb) Cadmium (Cd) Chromium (Cr) Copper (Cu) Nickel (Ni) Mercury (Hg) Zinc (Zn)

3 0.5 14 5 3