I EXAMENSARBETE INOM TEKNIK,
GRUNDNIVÅ, 15 HP
STOCKHOLM, SVERIGE 2020
The feasibility of using
macroalgae from anaerobic
digestion as fertilizer in Grenada
A literature study of the potential use of residue as fertilizer in Grenada, and a complementary laboratory study to evaluate the biogas
potential.
ANNA STERLEY
DANIEL THÖRNQVIST
KTH
SKOLAN FÖR ARKITEKTUR OCH SAMHÄLLSBYGGNAD
II
Acknowledgments
This bachelor thesis was conducted at the programme Energy and Environment at the Royal Institute of Technology during the spring of 2020. The project was initially supported and given a grant by SIDA as a minor field study in Grenada, but due to the situation caused by COVID- 19, the original work was unable to be executed. Though, the SIDA introductory course has given us insights on both sustainability aspects and international competence that has been useful for the thesis, thank you.
We are most grateful for our supervisor Fredrik Gröndahl, who helped form the structure of this work and introduced us to the astonishing importance of algae in the scientific world. We appreciate your infectious enthusiasm and curiosity about the topic, your reassurance of the flexibility of this work when we hit dead ends, as well as you generously sharing your connections and guiding us to relevant sources of information.
Next, we would like to thank Johan Andersson from RISE and Anna Schnürer from the Swedish University of Agricultural Sciences for generously sharing their knowledge of biogas with interest and encouragement, which enabled us to perform a laboratory study at home. Thank you RISE for providing us with quality equipment. Also, we thank Francesco Ometto from Scandinavian Biogas, who arranged for us to get samples of digestate from their plant, and shared relevant studies. The same goes to Martin Sterner, postdoctoral at the Royal Institute of Technology who provided us with macroalgae and also suggested relevant methods for pretreatment. We would like to thank Magdalena Sterley and Mikael Sterley for enabling the laboratory study to be practically executed and monitored in your home. Thank you Mikael for making supervision of the experiment possible.
We are very thankful for Erik Östling and Benjamin Nestorovic, the founders of AlgaeFuel, who introduced us to the topic, offered a wide range of projects to perform on the field, and eagerly and devotedly helped us with any obstacles and issues we had on the way. They also gave massive support when the initial project had to be cancelled. We hope that this study will be of use for any future AlgaeFuel projects.
Last but certainly not least we would like to thank our collaborating bachelor thesis group, consisting of Matilda Carlsson and Ingeborg Myhrum Sletmoen. Thank you for your countless hours of discussion and boundless information exchange leading to an enjoyable, fruitful and effective co-operation. We are also endlessly grateful for your infinite love and support.
Stockholm May 2020
Daniel Thörnqvist & Anna Sterley
III
Abstract
Coastal areas in Grenada and the Caribbean are experiencing an abundance of stranded macroalgae. Climate change and eutrophication are probable causes of this inconvenience. This leads to logistic and economic dilemmas for the Caribbean societies. Research of methods to benefit from the algal bloom is therefore valuable for a sustainable future in these countries.
Studies of biogas and fertilizer production are initiated around the world, but a large scale production is absent. Therefore, this thesis scrutinize the requirements for producing biofertilizer from biogas by examine the content of macroalgae and the conditions in Grenada.
To achieve this, a literature study and a miniature biogas experiment were conducted.
Grenada would presumably benefit from substituting synthetic fertilizer with biofertilizer from macroalgae utilized in biogas production. The positive aspects includes the recirculation of nutrients, development of renewable energy and autonomous fertilizer production. Further research of the definite macroalgae content is essential to determine the exact extent and conditions of the fertilizer utilization.
IV
Keywords
BMP - Biochemical Methane Potential CH4 - Methane
CO2 - Carbon dioxide HM - Heavy metals
NaOH - Sodium hydroxide Nm3 - Normal cubic meter RE - Renewable energy
S. latissima - Saccharina latissima SIDS - Small Island Developing States TS - Total solids (dry substance)
VS - Volatile substance (organic material) WW - Wet weight (fresh material)
V
Table of content
Acknowledgments ... I Abstract ... III Keywords ... IV
1. Introduction ... 1
2.Background ... 2
3. Aim and goal ... 5
4. Method ... 5
4.1 Literature study ... 5
4.2 Laboratory study ... 6
5. Theory to the laboratory study ... 7
5.1. Inoculum ... 7
5.2 Substrate ... 7
5.2.1 Food waste ... 8
5.2.2 Algae ... 8
5.3. Anaerobic digestion and biogas... 9
5.4 Calculations of produced biogas ... 9
5.4.1 Carbon dioxide ... 9
5.4.2 Methane gas ... 9
5.5 Pretreatment... 9
5.5.1 Pretreatment of algae ... 10
5.5.2 Pretreatment of food waste ... 10
5.5.3 Pretreatment of inoculum ... 10
5.6 Set-up of the experiment ... 10
5.7 Calculations of content ... 11
6.Results ... 12
6.1 Algae in the Caribbean ... 12
6.2 Heavy metals ... 13
6.2.1 In the Caribbean ... 13
6.2.2 In Algae ... 14
6.2.3 Removal of HM ... 15
6.3 Biofertilizer ... 16
6.3.1 Allowed contents of nutrients and HM in biofertilizer ... 16
6.3.2 Grenada’s current use of fertilizer ... 17
6.3.3 Recirculation ... 18
7. Results from laboratory study ... 19
VI
7.1 Biogas potential ... 19
7.2 Temperature in greenhouse ... 24
7.3 Calculations of theoretical waste content ... 26
8. Discussion ... 27
8.1 Biofertilizer; content and Grenada scenario ... 27
8.2 Heavy metals ... 28
8.3 Future scenario in Grenada ... 29
8.4 Error sources and delimitations in literature study ... 30
8.5 Laboratory study ... 31
8.6 Error sources and delimitations in the laboratory study ... 31
9. Conclusion ... 34
References ... 35
Appendix/Calculations ... 39
1. Calculations of TS in algae... 39
2. Calculations of inoculum, food waste and sodium hydroxide proportions ... 40
3. Content of inoculum ... 42
4. Data of food waste ... 43
5. Imports of fertilizer 2014-2016 in Grenada ... 45
6. Analytical report: Eurofins Environment Testing Sweden AB ... 46
7. Data of feedstock for biogas production ... 47
8. The allowed content of HM in fertilizer ... 48
9. Instructions: Building a biogas plant in a mini format ... 49
1
1. Introduction
This project is a bachelor thesis concerning the contents of the waste from anaerobic digestion of macroalgae. An overgrowth of algae causes great stranding events around the coasts of Grenada, and are spread out in the Caribbean. Two students from the Royal Institute of Technology, Benjamin and Erik, have a start-up business called AlgaeFuel. Their goal is to investigate the possibilities to build a large scale biogas plant in the Caribbean, which was the source of inspiration for this study. An important part of AlgaeFuel’s project is to contribute to sustainability in terms of creating a better environment and creating a circular economy of the biogas production in the Caribbean areas. Therefore, this study will contribute as a feasibility study about the fertilizer from a biogas production of macroalgae in Grenada.
The waste from anaerobic digestion of organic materials contains nutritions, and it is of interest from a sustainable and economic aspect to see if it can be used as fertilizer. The algae bloom has increased in the last century in Grenada and affect the ecosystems and tourism in the country. A solution to the problem is to use the macroalgae as a resource and find ways to meet the demand for energy and sufficient fertilizer. A sustainable use of natural resources will lead to a healthier ocean, a better economy for the country and could decrease the use of diesel as energy source in Grenada.
A laboratory study was conducted as a complement to the literary findings of this thesis. It consisted of a small scale biogas experiment with different mixtures of food waste and macroalgae. The data found of biogas efficiency and residue content was compared to previous studies. The combined results of the literature study and the experiment contributed to a more extensive analyze of the conditions for biofertilizer in Grenada.
The supervisor of this project was Fredrik Gröndahl, head of the department for Sustainable Development Environmental Science and Engineering (SEED). Gröndahl’s assistance catalyzed a successful project outcome in terms of the social, economical and environmental aspects. He guided the project to fulfill the expectations of a bachelor thesis. Gröndahl is additionally the project manager for Seafarm which is a research project about macroalgae and how they can be cultured for a wide range of applications.
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2.Background
The macroalgae problem
According to the United States Environmental Protection Agency, there are several climate change effects that might affect the extent of algae blooms in the oceans. This includes warming water temperature, changes in salinity, higher carbon dioxide levels, rainfall changes and sea- level rise (US EPA, n.d.). Together with the overuse of nutrients, climate change entails a major algal bloom in the oceans. The macroalgae are washed up on the beaches all around the Caribbean and is causing an inconvenience (Barnes et.al., 2019; Worley, 2018; Milledge and Harvey, 2016). Grenada is only one of the islands that is affected by this problem. This includes an evident impact on their tourism since many hotels are located on the coast. Tourism is important for the labor market in the Caribbean countries and accounts for about 80% of the GDP. At the moment, hotel employees empties the beaches on their own and deposits the macroalgae in piles. This emits methane gas and noxious hydrogen sulfide and causes greenhouse gas emissions (Thomson et al., 2020). Also, the ministry of the Environment in Grenada have created an infrastructure around cleaning the beaches from macroalgae, with workers helping to do this when needed (NOW Grenada, 2020).
A natural bloom of algae in the Caribbean is known from recorded history. However, in the past two decades there has been an increase of intensity, frequency and geographical distribution. From being beneficial for the aquaculture, the algal bloom presently causes negative effects for the environment, human health, tourism operations and recreational activities (Anderson et.al., 2004). Further issues with the overgrowth of macroalgae regards coral reefs. Algae block the sunlight for photosynthesis and uses space in the oceans where corals might propagate. It might also threaten the biodiversity and the functioning of the ecosystem (Nimrod et al., 2013).
Biogas
There is no current large scale production of biogas from macroalgae globally, and more research is required in the field. The essential focal points include pretreatment options, values of content in macroalgae, co-digestion options, harvest methods, and many more aspects (Vivekanand et al., 2012; Hansson, 2012; Ganesh Saratale et al., 2018). Generally, macroalgae has a great potential of being used for biogas from anaerobic digestion and further research is necessary to find the most economic methods in relation to the conditions in Grenada.
Macroalgae can be digested separate or used as co-digester, with the latter giving a potentially higher biogas yield. Substrates with more complex characteristics, achieved by co-digestion, has demonstrated an efficient production of biogas (Mata-Alvarez et al., 2014;). For instance, food waste has a great nutrition content and biomethane potential (Banks et.al., 2018; Xu et.al, 2018). A recent study in Barbados estimated the energy recovery by fivefold when using organic food waste as co-digestion with Sargassum (Thompson et al., 2020).
Energy conditions in Grenada
To evaluate the possible development of biogas as an energy solution in Grenada, the possibilities of interacting biogas with the current energy supply must be investigated. The year 1994 Grenada’s government signed a contract with the electricity company Grenlec, which gave them a private license for supplying electricity to the citizens of Grenada. The contract was signed until the year 2073 as legislation of The Electricity Supply Act of 1994 which makes it impossible for other energy producers to establish competitive alternatives (Grenlec, 2018).
3 However, with new climate policies and pressure from the government there has been a change of that since 2016 but the changes are going really slow and the relationship between Grenlec and the government is still unsure.
The energy supply in Grenada is mostly imported oil products, which makes them dependent of the oil market to provide energy for the population. 99 % of the energy for electricity is sourced from diesel (Espinasa et.al., 2015). The installed RE capacity covers for 1,51 % of the total electricity supply 2018 (Grenlec, 2018). In addition, the aim in Grenada’s INDC (Intended nationally determined contributions), is to reduce the CO2 equivalent emissions by 30 percent the year 2025, counting from 2010 and a reduction of 40 % the year 2030 (IMF, 2019).
Biofertilizer
To achieve a circular economy for the biogas production using macroalgae it is crucial to establish management of the residue. Therefore, an investigation of the contents in the waste is of importance to determine if it can be used as organic fertilizer in Grenada, instead of synthesized fertilizer. The nutrients and the heavy metals in macroalgae are generally conserved in anaerobic digestion. Hence, analyzing the feedstock enable to predict the content of the waste. Both nutrient and heavy metal content in macroalgae and food waste can vary a lot in the world depending on many variables like harvest method, emissions in the vicinity, and fertilizer use (Banks et al., 2018).
When using fertilizer of any kind it is important to consider the heavy metal content to prevent any soil contamination. Pollution of heavy metals are common in the Caribbean due to the mishandling of technical products and irresponsible handling of waste from industrial sites into water bodies. The Caribbean Environmental Program (CEP) has addressed both the severity and lack of legislation and regulations around this issue and implemented National Programmes of Action (NPA’s) to prevent the ongoing pollution (CEP, 2015). Avfall Sverige (2019) has a certification called SPCR 120 with the limit values listed of heavy metals in biofertilizer that can be used for agricultural food production purposes.
Sustainable Development Goals
This study could promote to achieve the Sustainable Development Goals (SDG) on many levels.
For example, a milestone in #14: life below water, is to increase the economic benefit of sustainable management of marine resources (Goal 14: UNDP, n.d). By seeing the macroalgae as a resource instead of a problem, farmers can use the waste as locally produced fertilizer instead of importing it. This will build stronger economic growth and make Grenada less dependent on other countries. This also connects with goal #7: affordable and clean energy. The lifecycle of biogas plants need to be taken into account to make it sustainable, and this can not be done without waste management (Goal 7: UNDP, n.d). Biofertilizer promotes a healthy soil ecosystem and improves soil texture unlike synthetic (Niu and Kozai, 2016; Montingelli et al., 2015). The contents of organic fertilizer consist of nutrients that circulates in the ecosystems and therefore this will contribute to goal #12: responsible consumption and production
(Goal 12: UNDP, n.d).
By contributing to Algae Fuels feasible study of their planned project, this thesis can lead to improvements in Grenada in several ways. For instance, strengthen the economy of Grenada that is depending on tourism and marine recreation, give local farmers cheaper and more environmentally friendly fertilizer and provide the population with cleaner cooking methods.
With this, the linked climate changes, the SDG’s and biofertilizer in mind, research like this has a great potential of contributing to sustainable development.
4
Laboratory study adequacy
An experimental biogas plant was built at home as a collaboration with another bachelor thesis group. This laboratory study contribute as compliment to the literature study. The aim of the experiment was to examine the amount of produced biogas from different mixtures of macroalgae and food waste, and draw conclusion about their co-digestion potential. This was compared with nutrient and HM values of the macroalgae and the food waste in the discussion of biogas and biofertilizer potential.
5
3. Aim and goal
The aim of this project was to investigate if the waste from a biogas plant that digest macroalgae can be used as a fertilizer. The goal was to evaluate the conditions in Grenada and analyze the results from Grenada’s perspective. Thereby examine the potential for using the ongoing algae bloom as a resource. Additionally, a goal was to investigate the food waste potential as co- digester, and how it affects the feasibility of the residue to be used as fertilizer.
4. Method
Figure 1. Model of the method. The dotted lines demonstrates the system boundaries for this report. The green boxes illustrates the in- and output.
4.1 Literature study
• To get information about Grenada’s conditions, energy use, algal bloom, biofertilizer, eutrophication, biogas with anomalous materials, heavy metals in algae, the limit values of nutrient and heavy metal content in biofertilizer.
• The sources of the literature study are mostly find by searching on Google Scholar, KTH Primo, DIVA, and by contact with the Ministry of Agriculture of Grenada. Search terms:
“Anaerobic digestion macroalgae”, “Algae bloom Caribbean”, “Biogas from macroalgae”, “Biogas from food waste”, “Biofertilizer macroalgae”, “Organic fertilizer macroalgae”, “Heavy metal biofertilizer”, “Heavy metal macroalgae”, “Co-digestion macroalgae”, “Pretreatment anaerobic digestion”, “Allowed content fertilizer” and
“Recirculation of nutrients”.
• In this study all, “algae” refers to macroalgae if nothing else is stated.
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4.2 Laboratory study
• A small scale biogas plant was assembled at home with four different batches. The trials consisted of anaerobic digestion of algae and food waste. The results includes calculations of the methane gas potential and a theoretical estimation of total phosphorus, total nitrogen, potassium, lead, cadmium and mercury in the residue.
The laboratory study was conducted with guidance from a lab instruction from the Swedish University of Agricultural Sciences. It will be designed in consultation with Anna Schnürer who wrote the lab instruction. The lab instruction for this study is placed in appendix 9.
Figure 2. Model of the laboratory set-up
Picture 1. The complete set-up with all 4 systems in a small greenhouse (private picture).
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5. Theory to the laboratory study
The assumptions made in the laboratory study was chosen in consultation with Anna Schnürer, who assisted with the experiment.
5.1. Inoculum
A test evaluating biochemical methane potential (BMP) of substrates on a small scale requires the presence of active microorganisms to start the digestion. The inoculum contains the anaerobic microorganisms breaks down the organic materials and is a significant parameter for the outcome. In order to get satisfying results from the test, the inoculum should contain a broad spectrum of different microorganisms with a similar composition to the substrate investigated.
Usage of inocula from a co-digestion plant, a digester on a treatment plant or a mixture of both, are sufficient. Alternatively cow manure might be used, but often has lower microbial activity (Schnürer and Carlsson, 2011). Different outcomes of the BMP-test could be attributed to differences in methanogenic abundance, and activity in the inoculum (Vrieze et al. 2015).
Digestate from Scandinavian biogas food waste digestion plant in Huddinge was used as inoculum for this laboratory study. The condition in their reactors is mesophilic (37°C) and to ensure maximum activity from the microorganisms the experiment was conducted at the same temperature (Schnürer and Carlsson, 2011).
According to Schnürer’s and Carlsson’s (2011) handbook of methane potential a quantification of the BMP-test with regard to the inoculum and substrates ratio of organic material or more exactly VS ratio is required. A ratio suggestion of inoculum and substrate is 2:1 (Schnürer and Carlsson, 2011). For this study it was assumed that the inoculum had 12% VS of WW (Appendix 2.2). The inoculum was then diluted with water according to Appendix 2.2 to attain the optimal concentration organic matter for the process (Schnürer and Carlsson, 2011).
The inoculum contains anaerobic microorganisms and some organic material as well that will contribute to the production of biogas. Therefore, the inoculum should be degassed before a BMP-test (Schnürer and Carlsson, 2011). This induce the risk to lose activity, and for this experiment a reference test with only inoculum was conducted for comparison instead (Schnürer, 2007).
5.2 Substrate
The substrate in this experiment is algae and food waste. There are several aspects that needs to be analyzed before deciding if a substrate is suitable to use as biogas feedstock (Avfall Sverige, 2009).
These are among others:
• Total solid and volatile solid percentage
• Nutrition values
• Biogas yield
• The need of pretreatment
With this in mind, it is important to find ways to assess new substrates in general and also evaluate how they work as co-substrates. A previous study about co-digestion a Sargassum specie and food waste showed that the productivity increased with bigger ratio of food waste (Morrison and Gray, 2017). Though, other research about co-digestion of macroalgae and food waste was not found.
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5.2.1 Food waste
Since only a small amount of substrate was used in this experiment (3g), it was not possible to get a mixture of a wide range of food waste. According to the website of Home Biogas, substrate as bread, corn and peas, have a high biogas yield and therefore these were used in this experiment (Farhi, n.d). General nutrition values of food waste used in this report, is presented in appendix 4.
According to several sources, food waste alone has a potential of producing around 0,37-0.53 Nm3 CH4 / kg VS (Banks et.al., 2018; Nordberg et.al., 2019; Dow and Kuo, 2017; Ge et.al., 2018). The BMP of food waste is around 450 m3 CH4 / kg VS (Banks et.al., 2018)
To measure how much food waste that is reasonable to put into the reactor container, the amount of organic waste in food need to be determined. The ratio of inoculum and substrate was assumed to be optimal at 2:1 and was determined to regulate how many microorganisms that were available to break down the substrate. This was especially important so that the system avoids getting overloaded (Schnürer and Carlsson, 2011). An overloaded reactor bottle is caused by an underestimation of the organic material in the substrate, in relation to the amount of microorganisms. An average value of VS of food waste in a few countries around the world according to IEA is around 23 % of the WW (Banks et.al., 2018). To make sure the system will not get overloaded, an overestimation was assumed that the VS was 30 % of WW.
TS (average) [% of WW]
VS (average) [% of WW]
VS (average) [% of TS]
VS (used in experiment) [% of WW]
Food waste
25.48* 22.82* 90.30* 30.00
Table 1. TS and VS of algae
*See appendix 4.2, Banks et.al., 2018
5.2.2 Algae
Existing research about algae as biogas feedstock was found, but it is not used industrially at the moment. The algae used in this experiment is a brown macroalgae, Saccharina latissima harvested on Västkusten, Sweden 2018. It was frozen directly after being collected and cut to smaller pieces (0,5-1m). This specie have been utilized in several projects of production of biogas with varying results (Ometto et al., 2018a,b; Vivekanand et al., 2012).
The methane potential of the specie S. latissima is 0.22 Nm3 CH4 / kg VS (Vivekanand et al., 2012). The TS and VS of the algae are shown in Table 2. Dry algae was assumed to contain 100% VS of TS since there was no possibility to heat the algae to 550 °C.
TS [% of WW] TS [% of WW] VS [% of TS] VS (used in experiment) [% of WW]
Algae 12.1* 11.6** 95** 12
Table 2. TS and VS of algae
* See appendix 1
** See appendix 6
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5.3. Anaerobic digestion and biogas
Anaerobic digestion could summarily be explained as microorganisms digesting a substrate in an anaerobic environment, which results in biogas (a majority of CH4 and CO2) and residue. The major part of the energy transformed during an aerobic digestion is bound in the methane, containing 35 MJ/m3. Heat needs to be applied for the anaerobic digestion to work optimally.
The process is divided into four steps with different microorganisms functioning at each step.
Correct composition of each type of microorganisms in relation to another is needed for maximum outcome (Karlson, 2016). The steps are:
• Hydrolysis
• Fermentation
• Anaerobic oxidation
• Methane step (Karlson, 2016)
In the methane step, methanogens use acetan, hydrogen and carbon dioxide to produce methane.
The methane concentration of the produced biogas is a measurement of the efficiency of the last step (Karlson, 2016). Methane has a calorific value of around 10 kWh/m3, while carbon dioxide has zero. Hence methane concentration correlates directly to the energy content and efficiency of the biogas (SGC, 2012).
5.4 Calculations of produced biogas
5.4.1 Carbon dioxide
The sodium hydroxide (NaOH) solution in bottle 2 function as an absorber of the produced CO2. The NaOH solution will firstly form carbonic acid. Further, the carbonic acid reacts with hydroxide ions and form bicarbonate ions. NaOH in chemical reaction with CO2 is a complex reaction flow which was not analyzed in this report. The net reaction, after it has reached equilibrium, can be described as:
CO2(g) + NaOH(aq)⟶NaHCO3(aq) (Yoo et al., 2012)
The pH of the sodium hydroxide solution was measured, in the beginning and in the end of the experiment. If the pH has decreased in the end, it is possible that CO2 has been produced from the reactor bottle, but these calculations is not a part of this report. A conclusion can be drawn whether CO2 has been produced or not.
5.4.2 Methane gas
Bottle 3 consisted of water which was displaced into bottle 4 by the produced methane gas. The volume of water in bottle 4 was assumed equal to the volume of produced CH4 gas.
5.5 Pretreatment
To make sure that an anaerobic digestion is functioning optimally, a pretreatment of the substrate is essential. The purpose of pretreatments is to decompose the material so that it can be digested by the microorganism, within a reasonable time. Pretreatment methods includes milling, separation and dilution (Avfall Sverige, 2009). Other common pretreatments are pre- heating, chemical treatments, steam explosion, and downsizing which can improve the effectiveness of the anaerobic digestion (Mc Kennedy and Sherlock, 2015; Thomson et al.,
10 2020). The food waste contains small amount of lignin and a lot of the fibres have already been milled when produced in industries, which is positive for the anaerobic digestion (Banks et al., 2018). The pretreatments used in this experiment are described below in 5.5.1 and 5.5.2.
5.5.1 Pretreatment of algae
In the manual of methane potential (Schnürer and Carlsson, 2011), a benchmark for VS substrate in similar biogas experiments is 0,5-3g/L liquid volume. To get VS, the TS substrate needs to be heated at 550 °C for 24 h. This step was not available for this experiment.
Assumption: VS = 100% of TS
The first pretreatment for the algae was to heat it in an oven of 105 °C for 24 hours, a day before starting the experiment, to get the TS substrate. It is probable that dry algae absorb moisture from the air, therefore they were put in a sealed box afterwards. Two samples of 140 g algae were compared since the VS of algae can differ considerably.
Sample 1: Dried without being decomposed.
Sample 2: Decomposed in food processor and then dried.
(See appendix 1)
Decomposed algae will have a bigger hit surface in the biogas reactor, and this will increase the speed of methane gas production (Arkelius, 2015). In Grenada, an important pretreatment to consider is washing of algae since high salinity values can cause osmotic stress in plants if the waste later is used as fertilizer (Thomson et al., 2020).
5.5.2 Pretreatment of food waste
The food waste used in this experiment are corn, white bread and peas, since these products have a high biogas yield (Farhi, n.d). An equal mass of each type of food was put in a food processor and milled into a thick paste. The purpose for this was to get a bigger hit surface and a faster reaction with the inoculum.
5.5.3 Pretreatment of inoculum
The liquid inoculum was collected at Scandinavian Biogas plant in Huddinge the same day as the experiment was started so that most of the microorganisms activity was conserved. Ometto (2020) suggested a pretreatment of sieving to remove large particles and plastics. The resulting inoculum was therefore homogenized (Schnürer and Carlsson, 2011)
5.6 Set-up of the experiment
A figure of the set-up was placed in “method” as figure 2.
Bottle 1: Containing inoculum and substrate with the ratio 2:1, and water as dilution.
Bottle 2: Aqueous solution of sodium hydroxide that captures produced CO2(g).
Bottle 3: Only water and caramel color which later will be pushed into bottle 4 by the produced CH4(g).
Bottle 4: Empty bottle for calculations of produced CH4(g).
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5.7 Calculations of content
The contents of the waste that was calculated are:
Nutrients: Phosphor, nitrogen and potassium.
Heavy metals: Cadmium, mercury, lead.
Since both the nutrients and the heavy metals are generally conserved during anaerobic digestion, the content of the feedstock can be assumed equal to the content of waste (Banks et al., 2018).
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6.Results
The results of this thesis consist of an investigation of the current algae growth and existence in the Caribbean area, as well as the heavy metal content of algae, general HM pollution in the Caribbean. Followed by, the allowed content of biofertilizer regarding nutrients and HM, biofertilizer use, recirculation of nutrients and Grenada’s current use of fertilizer.
6.1 Algae in the Caribbean
The algae bloom in the Caribbean has increased since 2011 and the reason for this is still not fully understood (Wang et al., 2017). A study from 2013 in Grenada explored nutrient and sediment inputs in a river on the island and the results showed that overfishing and an overuse of fertilizer on land caused a significant increase of macroalgae covering the coral reefs.
Overfishing leads to a reduced amount of organisms that consume plants in the ocean. In the study they pointed out that there are probably several factors contributing to an overgrowth of algae. These factors include the ones mentioned above. Climate change can serve as an additional factor (Nimrod et al., 2013).
The reasons for an overgrowth of algae are probably many and can vary geographically. A study made in Florida, analyzed different species of algae to determine their source. The main issue was to find evidence if the stranded algae occurred because of fragmentation, transport and growth or other sources like urbanization and agriculture (Milbrandt et.al., 2018). Further investigations about fragmentation, transport and growth would be interesting to investigate since most sources mention leakage of nutrients from agriculture practices as the cause.
The Optical Oceanography Laboratory on the University of South Florida have a routinely forecast of algae bloom in the Caribbean by looking at satellite maps (2020). Pictures from April 2020 show a decrease comparing to April 2019. Another analyze of satellite pictures show large algae belts, stretching from West Africa to Central America (Wang et.al., 2019). Like earlier works about eutrophication, this report support the hypothesis that nutrient enrichment leads to larger and faster growth rate of algae as a result of increased deforestation and fertilizer use. Deforestation causes erosion and sedimentation, which in turn leads to an increase of nutrients leaking into soils and watercourses (Garzón-Ferreira and Diáz, 2003).
There are many aspects to consider in the techno-economical perspective of using algae as biomass in the Caribbean. One of them are methods to predict the growth. There are several formulas to use when calculating specific growth rate and daily growth rate etc. of algae.
According to Yong et al. (2013), the most commonly used formulas are containing the variables W0 = initial wet weight, Wt = final wet weight, where t is days since the start of cultivation. The most accurate formula according to Yong et al (2013) with lowest degree of error is the formula below [% / day]:
Other formulas depending on temperature are presented in a case study made in Trelleborg by Risén et al (2013) where factors like available light and nutrients were also considered. The conclusions from the report by Yong et al (2013) are that the algae show a high initial growth which decreases due to self-shading that reduce the possibility of photosynthesis. Another important factor to consider when calculating the possibility for algae as biomass and fertilizer are to analyze the seasonal variation of growth. This is important in both cases of overestimation and underestimation of algae growth (Montingelli et al., 2015).
13 The two most common species of macroalgae in the Caribbean are Sargassum fluitans and Sargassum natans (WAB, 2007). Dried Sargassum has been utilized historically as fertilizer all around the world because of the high nutrient values (Thomson et al., 2020). The nutrient values however, can vary a lot during different months, at different growth depths, within species, between species and geographically. The TS, and VS values of algae can also differ under these circumstances (Ometto et al., 2018a; Ometto et al., 2018b; Montingelli, 2015). Biogas from algae does not compete with the food market and the algae grow fast and contain low amount of lignin, which is positive for an adequate biogas production (Vivekanand et al., 2012).
6.2 Heavy metals
One reason for examining the heavy metal concentration in the waste from digestion of algae and food waste is because the produced fertilizer can toxify the soils. A contaminated soil affects the crops, and when later consumed, this can lead to further health problems. The microbial activity and metabolic activities can also be affected by HM’s which further affect the biological availability negatively. Heavy metals are found in soils due to natural reasons or because of anthropogenic effects. There are many ways for HM to transport into soils. In the agriculture industry, the reasons are often the use of phosphatic fertilizers, sewage sludge and pesticide treatment (Oves et.al., 2012). The chain of HM accumulation is demonstrated in
Figure 3.
Figure 3. Text from Figure 1.1 by Oves et.al., 2012.
When the HM’s reach the soil or watercourses, the effects are several and hard to track. The consequences include negative crop productivity, health problems for animals and plants and biodegradation. Sewage sludge could be a good alternative to feed nutrients to the soil but the problem is that it also can contain large amounts of heavy metals Oves et.al. (2012).
6.2.1 In the Caribbean
The land in the Caribbean are part of a volcanic arch and have a high volcanic activity, this leads to a natural source of mercury in the soil and ground. Mercury is also produced as a by- product in many industries in the region but is not allowed in insecticides and fungicides. Lead
14 emissions in the Caribbean are often caused through the use of lead-acid batteries in industries and vehicles. Regional lead pollution is also traced from open burning of waste products and illegal dump sites (UN, n.d). Many studies confirm that the Central American coastal areas have experienced higher metal pollutions recently, both natural and anthropogenic. This is due to sewage discharges, the use of chemicals and fertilizers in the agriculture, and oil spills (Guzmán and Jiménez, 1992). A study about metal pollution in the Wider Caribbean Region, identified the sources as pesticide usage in the agriculture, sewage input and mineral extraction (Fernandez et al., 2007). The conclusion from this report was that small amounts of metals could be traced all over the region and concentrations were found on places that is far away from obvious pollution sources. This could give a hint that long-range transports of pollutants occur and the environmental effects of this could be important to investigate (Fernandez et al., 2007). Many projects held by the Caribbean Environment Programme (CEP) aims to reduce the pollution of HM’s. The concentration of the pollution is measured in several affected areas (UN, n.d). There is also a suspicion that the public awareness is low about this problems and the waste management in industries are limited in the small developing island states (SIDS). A comparison between North America and Caribbean countries, of the amount of Hg and Pb in the blood of pregnant woman also showed that the Caribbean countries are exposed to this pollutants at a higher level. A reason for this could be that countries in the Caribbean consume more ocean fish and shellfish that can contain high amount of HM (Forde et al., 2014).
6.2.2 In Algae
It is clear that HM’s pollution in the ocean, soils and watercourses occur because of several anthropogenic effects. Algae have been used in several research projects to investigate their ability of uptake of HM’s with a target to treat polluted water, and especially wastewater from industries and agriculture activities (Filip et al., 1979; Yu et al., 1999). According to Shamsad et al (2015), macroalgae often show good capability to take in toxic elements which is good for water treatment purposes, but for fertilizer purposes it is negative. A laboratory study showed that macroalgae absorbed 70–90% of the cadmium from the wastewater media (Filip et al., 1979). Another study explained that the pH can affect the ability of the specific algae (Oedogonium westti) to absorb HM’s. The Cd removal efficiency at pH 4 and 4.5 was 95%
respectively 89% and the increase of pH gave a removal efficiency of 55% (Shamshad et al., 2015).
It is important to analyze the heavy metal content in algae before it is used as fertilizer so that no contamination occur into farming soils, watercourses and into the ocean. It is also important to understand that the composition of algae can fluctuate, geographically, seasonally and between different species (Milledge and Harvey, 2016). This also makes it more complicated to draw conclusions about if algae generally is a good substance in fertilizer (Mc Kennedy and Sherlock, 2015; Ometto et al., 2018a). It has also shown that the metal content in algae can vary even within the same area and also that lower amount of salinity in the ocean increases the bioavailability of Cd (WAB, 2007).
The discovered data in this thesis of the HM content of algae, is generally scant. The identified studies were mostly applied on the specie S. latissima in Sweden. Table 3 display a comparison of HM content of two analyzes of S. latissima and further another one of Sargassum fluitans in the Caribbean.
15 Amount of HM in TS [mg/kg]
H M
Västkusten (SE)
1
Kristiansun d (NO)2
Kristiansun d
(NO)3
Clacha n Sound (UK)4
The British Virgin Island5
Vanvika n (NO)6
Vanvika n (NO)7
Cd 0.38 2 1 - N/A 4.6 0.76
Pb 1.7 <1 <1 2 0.32 <0.48 <0.49
Hg N/A - - - <0.00
5
<0.05 <0.05
1 S. latissima harvested June/July, see appendix 6.
2 S. latissima harvested in June (Ometto et al., 2018b)
3 S. latissima harvested in July (Ometto et al., 2018b)
4 S. latissima harvested in May (Ometto et al., 2018b)
5 Sargassum fluitans (Morrison and Gray, 2017)
6 S. latissima harvested in October (Ometto et al., 2018a)
7 S. latissima harvested in May (Ometto et al., 2018a)
“-” = data below detectable limit.
N/A = no data available
Table 3. Amount of Cd, Pb and Hg in macroalgae [mg/kg TS]
This results show no or very small amounts of mercury in the algae and also a big variation of both lead and cadmium in different countries and harvest month. In a report where samples of algae where collected on different beaches in Trelleborg, Sweden, the analysis showed that there were great variations within the municipality. The cadmium values varied between 0.525- 1.71 mg/kg TS, the lead values varied between 2.25-5.2 mg/kg TS and the mercury values varied between 0.0208-0.0506 mg/kg TS (WAB, 2007).
6.2.3 Removal of HM
A different technology for biogas production with algae was used during the Wetland, Algae, Biogas (WAB) project in Trelleborg municipality. The plant contained a 2 stage hydrolysis/fermentation instead of the more commonly used continuously stirred tank reactor (Hansson, 2012). The plant was at a pilot scale and consisted more specifically of hydrolysis beds containing algae and straw with water passing through and collected in a well. The fluid is then led through filters to a reactor for the fermentation and methane step. A biological filter and a ammonium strip are in place to reduce the relatively high content of cadmium in the fluid.
Cadmium reduction of 50% was achieved with this method, along with nitrogen precipitated to solid ammonium phosphate as well as potassium leached out and solidified. Thus the nutrients could be collected without higher concentrations of cadmium and used directly as fertilizer (Hansson, 2012).
Using above mentioned technology with hydrolysis in a biogas plant, electrospun filters and absorbents could be another relevant method for HM removal in algae. The filtration of HM by electrospun filters is mainly due to the adsorption process between the HM ions and the electrospun fibres. The organic/inorganic composite nanofibers has high potential to remove heavy metal from wastewater and for different variations of these fibers the adsorption capacity were up to 83.9% for cadmium ions and 57.9% for lead ions. Though for a realistic commercial
16 use it has to improve in areas such as mechanical strength as well as lower cost and reusability (Zhao et al., 2018).
It is shown that macroalgae can contain too high amounts of HM to be used as fertilizer and filtration may not be cost-effective in certain scenarios. Another alternative for removal of HM is to use phytoremediation in the soils were the residue has been used. Phytoremediation could be implemented with various processes and mechanisms such as phytoextraction (absorption and accumulation) or phytostabilization (adsorption and precipitation of less soluble forms).
These methods are eco-friendly, cost- effective and risk-free ways to depollute soils or water from HM (Haq et al., 2020).
6.3 Biofertilizer
The digestate from biogas plants that digest waste, like sorted food waste, waste from the agriculture or food industry is called biofertilizer. Unlike waste from sewage systems, these are relatively clean (Schnürer and Jarvis, 2017). According to Nappa et al. (2015), the potential of using algae biomass as fertilizer is good since it is has a great capacity of binding water and it improves the mineral composition in the soil. The Swedish certification of biofertilizer (SPRC 120), as well as Grenada’s use of fertilizer will be described below.
6.3.1 Allowed contents of nutrients and HM in biofertilizer
The Swedish Waste Management Association has determined certification rules - SPRC 120 for the quality of digestate from biodegradable waste, used as biofertilizer. For any waste from a biogas plant to be certified as biofertilizer in Sweden certain criterias must be fulfilled regarding the HM content. The HM content must not exceed the limit values per kg TS as well as not exceeding the limit values for input on arable land area per year (Avfall Sverige, 2020).
The Swedish Environmental protection agency (The Swedish EPA) suggested in 2013 that measuring the fertilizers amount of HM in relation to the phosphorus amount, would be a more suitable parameter. The limit values decrease every five years (Tekniska verken, 2013). This is demonstrated in Table 4.
Metal Maximum amount in TS [mg/kg]*
Maximum amount per year [g/ha]*
Maximum amount [mg/kg P]
year 0,5,10 **
Cd 1 0,75 40, 35, 30
Pb 100 25 1600, 1450, 900
Hg 1 1,5 40, 30, 20
Table 4. Maximum amount of Cd, Pb and Hg in fertilizer. Values from Avfall Sverige (2020) and Tekniska verken (2013)
* See appendix 8.1
**See appendix 8.2
Comparing The Swedish EPA’s suggestion of the new limit values of fertilizer, as mg metal per kg phosphor, with HM data of S. latissima, it is clear that the HM content versus phosphor content in S. latissima exceeds the limit values shown in Table 5. The values varies a lot within the specie, between different harvest periods, and location.
17 HM June/July1
[mg/kg P]
Aug2 [mg/kg P]
Oct3 [mg/kg P]
Feb4 [mg/kg P]
May5 [mg/kg P]
Cd 418 546 1533 836 1634
Pb 1868 208 160 87 690
Hg - 21 17 9 70
1 Harvested in June/July, Västkusten, Sweden, see appendix 6
2 Harvested in August, Vanvikan, Norway (Ometto et al., 2018b)
3 Harvested in October, Vanvikan, Norway (Ometto et al., 2018b)
4 Harvested in February, Vanvikan, Norway (Ometto et al., 2018b)
5 Harvested in May, Vanvikan, Norway (Ometto et al., 2018b)
Table 5. Data of metal content per kg phosphor, in S. latissima.
In Sweden, the maximum dose of applied fertilizer needs to be determined with regard to the nutrient values, according to the Swedish Board of Agriculture’s regulations. Calculation of nutrient demand, as N, P, K, should be estimated by the expected harvest level for each field.
The dose of fertilizer should also be limited by the maximum amount of HM allowed. The limiting parameter determines the maximum dose that can be used of the specific fertilizer (Avfall Sverige, 2020). A nutrient balance before and during appliance of biofertilizer could be of advantage. The purpose is to ensure minimum leakage of nutrients and to determine if any nutrient has a deficit (RVF, 2005). Different crops demand different compositions of nutrients in fertilizer (see Table 6). The potential to be a suitable fertilizer to a certain cultivation increases with more similar composition of nutrient demand to the fertilizer composition (WAB, 2007). Table 6. show some crops different nutrient demand as quotas in relation to nitrogen, thus values of phosphorus and potassium is shown as mass proportions of nitrogen.
N P K
Cereals 1 0,16 0,17 Oil seed 1 0,17 1,5 Sugar beet 1 0,17 1 Peas 1 0,11 0,28
Table 6. Some crops that need fertilizer as quotas in relation to nitrogen (WAB, 2007).
6.3.2 Grenada’s current use of fertilizer
A large part of Grenada's income consists of massive exports of agricultural products. 27 % of their total exports are nutmeg and other important exports are fresh fish, wheat flours, cocoa beans and other fruits. Most of the products are exported to the United States, Netherlands, France, and Germany (OEC d, 2017). This means that it is important that both the marine life and the growth on land is not damaged and works efficiently. Good fertilizer and clean oceans plays a big role in this. Import of fertilizers in Grenada varies over the years and it is hard to find updated data. Data from the Land use officer of Ministry of Agriculture in Grenada (appendix 4), shows that the total import of fertilizer 2016 was US$ 758 669 which is equal to 349 006,97 kg of fertilizer. Compared to 2015, the imports of fertilizer was bought for US$ 1 598 005,73 wich is equal to 930 080,16 kg. In other words, The purchase amount decreased from 2015 to 2016, and the amount of imported fertilizer decreased with 75 %. This shows that
18 the prices had increased between 2015 and 2016 and Grenada imported lower amounts of fertilizer during 2016.
Data from OEC year 2016 clearly shows that there are several import origins for Grenada’s fertilizer around the world. The dominated import origins are, China, France, USA, Dominican Republic, Morocco, Barbados (OEC a, 2016;OEC b, 2016;OEC c, 2016). Another important factor of Grenada’s import dependency in the future is the phosphorus security when the access to high quality phosphate rocks may decrease and the price goes up (Cordell and Neset, 2011).
An initiative was taken in 2016, by the government of Grenada together with the German Agency for International Cooperation (GIZ), to make a reform in the electricity sector and support climate policies. By building small biogas systems, this initiative could increase the use of renewable energy and reduce CO2 emissions, and they were able to use the waste as biofertilizer. The aim was to make Grenada less import dependent on oil and fertilizer. Since the waste from the agriculture sector is currently dumped in water courses or burnt, this new waste management will also protect the groundwater and the ecosystems around the coast (GIZ, 2018). In a press release in December 2018 it was announced that this pilot project was finished with great results but no future plans of expanding were announced (NOW Grenada, 2018).
Further on, according to a report handed by the Government of Grenada (2017), the use of synthetic fertilizers is one of the causes of land degradation in Grenada. Due to increased availability of products and increased economic growth, the amount waste sent to landfills have increased. A solution to this, according to the report, is to recycle more and if possible, the waste should be used as waste-to-energy. This could be a indicator for a future possibility to use digestion of food waste for biogas and fertilizer in Grenada.
6.3.3 Recirculation
Removal of algae from water areas is not only essential for the tourist industry but also has an important benefit for nutrient removal. IVL - the swedish environmental institute estimated that removal of algae from the eutrophic Baltic Sea could result in major environmental and social economical benefits. Currently 2000m3 is collected from the Baltic Sea, but it is estimated that an additional 8000m3 is possible to extract. The marginal benefits for removal of nitrogen and phosphorus in 10 000m3 algae is estimated to 1,600,000 - 8,430,000 SEK/year (Olsson et al., 2013).
A study investigating the possibility of the green macroalgae Oedogonium sp. as treatment of municipal waste showed promising results for nutrient removal. A culture of Oedogonium (0.25–1.5 g/L fresh weight) in a pond yielded nutrient removal of 0.50g nitrogen/m2/day and 0.11 g phosphorus/m2/per day. Nitrogen were reduced by 62% and phosphorus by 75% in the treated water (Neveux et al., 2016).
The nutrient content of a substrate used for biogas production is almost the same before and after digestion. Parts of the organically bound nitrogen is mineralised to ammonium during digestion which is favorably for fertilizer purpose since it enables faster nutrient release in soils (Avfall Sverige, 2009).
19
7. Results from laboratory study
The laboratory study results consist of measurements and calculations of biogas production and waste content.
7.1 Biogas potential
Methane gas
After 7 days, all collecting containers contained only small amount of water. This amount could not be exactly measured but was estimated to 1-2 ml. The following days, the production stopped or progressed very slow.
After 24 days of anaerobic digestion the experiment was dismantled and the amount of water in bottle 4 was measured. The amount is described in Table 7.
Control1 S.L2 FW3 FW + S.L
Volume water in bottle 4 [ml] 13 11 3 15
1. Control sample containing only inoculum 2. S. latissima
3. Food waste (peas, corn and white bread)
Table 7. Amount of water in bottle 4 after 24 days of anaerobic digestion.
The volume of water in bottle 4 was considered equal to the amount produced methane gas.
The results showed that the different digestions produced 0,0001-0,005 Nm3 CH4 / kg VS.
Compared to the results from the literature study, this methane potential is around 100-1000 times lower than expected and indicated on a failed experiment. The mono-digestion of food waste produced the lowest volume of CH4, and the co-digestion of FW and S. latissima produced the greatest volume of CH4. Note, if the water ended up in bottle 4 for other unknown reasons, no biogas was produced during the experiment.
Picture 2. Stratification in the reactor bottle of digestion of food waste, after 1 week of the experiment (private picture).
20 Carbon dioxide absorption in sodium hydroxide
Sample pH
NaOH before it is dissolved in water: ~14
NaOH in aqueous solution: ~14
NaOH solution after 24 days of anaerobic digestion: ~14
Table 8. pH measurements of sodium hydroxide
The measurements showed no decrease in pH, therefore it was not possible to estimate the amount of produced CO2.
After 24 days of anaerobic digestion
1. Control experiment containing only inoculum
Picture 3. Bottle 1-4 (from the left) after digestion (private picture).
If the water in bottle 4 was considered equal to the volume of produced CH4, the control experiment containing only inoculum had similar efficiency as the mono digestion of
S.latissima and the co-digestion of FW and S. latissima.
21 2. Anaerobic digestion of S. latissima
Picture 4. Bottle 1-4 (from the left) after digestion (private picture).
The volume of liquid in bottle 2.1 (reactor bottle) had increased from 700ml to approximately 900 ml during the experiment (24 days). The sodium solution had most likely been transferred from bottle 2.2. The color of the liquid in bottle 2.1, was darker than bottle 1.1 and 4.1, in the other digestion trials (control experiment plus FW + S.latissima), where sodium hydroxide had not been displaced.
22 3. Anaerobic digestion of food waste
Picture 5. Bottle 1-4 (from the left) after digestion (private picture).
The volume of liquid in bottle 3.1 had increased from 700ml to approximately 800 ml. The sodium solution had most likely been transferred from bottle 3.2. The color of the liquid in bottle 3.1, was darker than bottle 1.1 and 4.1 (control experiment and FW + S.latissima), where sodium hydroxide had not been displaced. The color was slightly lighter than in the mono digestion of S. latissima. Other observations was that the tube between bottle 3.1 and 3.2 was filled with sodium hydroxide solution.
23 4. Anaerobic co-digestion of food waste and S. latissima
Picture 6. Bottle 1-4 (from the left) after digestion (private picture).
The color of the liquid in bottle 4.1 had changed notably, and was the only one of the 4 different reactor containers that changed into a yellow color. The sodium hydroxide solution had not dispersed into the reactor bottle.
24
7.2 Temperature in greenhouse
The temperature in the greenhouse was controlled by a “greenhouse guard” that assisted to keep the temperature stable at a set temperature. The maximum setting was 35℃ and this was chosen to satisfy the requested conditions for mesophilic bacterias, in the inoculum. The temperature was observed with a food thermometer during the first 6 days, and by daily check-ups, it was noted that the temperature varied during the day, and also on different positions in the green house. The temperature varied between 33-40℃. On the 7th day, a digital thermometer was installed, that stored the values on a computer. This graph is shown below as figure 4 and stored the temperature values from the 7th day to day 24, the end of the experiment.
Figure 4. Graph of temperature fluctuation in the greenhouse during the 7th-24th days of the experiment
As the graph displays, the temperature fluctuated mostly between 33-37℃. On the 7th, 9th and 17th day of the experiment, the temperature decreased remarkably when the greenhouse was open a longer period of time. The radiator was installed to decrease the generated temperature if it exceeded the set temperature of 35℃, and increase the generated temperature if it fell below. A closer look of the temperature fluctuations during one day are shown as an example in figure 5.
25 Figure 5. Example of daily temperature fluctuation in the greenhouse during the 11th day of the experiment.
26
7.3 Calculations of theoretical waste content
The calculations were based on data of the algae used in this experiment and general data of food waste, see appendix 7. The aim was to get a theoretical value of the waste from the 4 laboratory trials made in this report. The results are shown in table 9.
The HM content of the waste in relation to TS [mg/kg TS] was acceptable to be use as fertilizer.
When using the parameter mg/kg P the limit value was exceeded in all cases and especially for the cadmium content. The limit values can be found in Table 2.
Table 9. Calculations of HM and nutrient content in waste from different digestion mixtures.
The calculations in table 10 are based on data from appendix 7.
N P K 50/50 algae, food 1 0,13 1,66
Algae 1 0,05 3,65 Food 1 0,18 0,42
Table 10. Nutrient content as quotas in relation to nitrogen
27
8. Discussion
The aim of this project was to investigate Grenada´s potential of using the waste from algae- based biogas as environmentally friendly fertilizer. The prerequisite for producing biofertilizer goes hand in hand with Grenada’s energy demand since the biogas need to be requested for biofertilizer production to be substantial. Biogas as an energy source has the benefits of having a number of application areas, as cooking, electricity production and fuel.
With this said, there is a potential, an aim, and a demand for renewable energy in Grenada but the development is going slow. The biggest motivation for increased RE in Grenada is to decrease the oil import dependency, be resilient to changes in oil prices, improve competitiveness in the tourism sector, strengthen the economy, create more jobs and contribute to reach the climate goals (Singh et.al., 2012). The report from the Government of Grenada (2017), presented that an increase of biogas production can increase the economic competitiveness. What kind of RE that is most suitable for Grenada is hard to tell and the answer is probably that there is a need for a stable mix of different RE’s. There is a great potential for wind and solar power in Grenada but they require a big land use and on the relatively small island, the land is mostly used for agriculture and tourism (Singh et.al., 2012). Biomass from algae and food waste as an energy source could, therefore, be a good complement to get rid of the algae-problem and since agriculture is producing a lot of waste. Additionally this could also benefit agriculture if the biogas waste could be used as fertilizer.
8.1 Biofertilizer; content and Grenada scenario
Moreover, the discussion about algae in general and more specifically digestate of algae as biofertilizer is relevant. From the literature study result is shown that Grenada import large quantities of fertilizer for agriculture. The need for fertilizer are thereby indisputable and no reports were found on why it could not be replaced by biofertilizer. There is however an increased interest and willingness for usage of biofertilizer in Grenada. If the biofertilizer could replace the existing fertilizer it would likely be a vital part of the solution to the macroalgae abundance problem. It would also be beneficial in an environmental aspect.
Different crops has different need for amounts of nutrients and composition of nutrients to yield maximum harvest. This implies that a closer investigation must be executed regarding which crops Grenada cultivate and their individual nutrient demand. As seen from the laboratory results the nutrient composition of the digestate could vary a lot with different mixture of food waste. Thus could the nutrient composition of the digestate theoretically be controlled with the ratio of food waste input. Although that could lead to more insufficient biogas outcome. In that scenario or if the composition of nutrients is inadequate independent of the mixture ratio, the biofertilizer could instead be considered to be used together with a complimentary synthetic fertilizer. From the Swedish regulations regarding biofertilizer it seems though that the HM content is a more substantial limiting aspect. Interviews carried out by RVF (2005) reveal that Swedish cultivators using biofertilizer had in most cases replaced synthetic fertilizer entirely and found cultivation advantages regarding growth speed with biofertilizer.
As a complement to the production of fertilizer from anaerobic digestion of algae, dried algae can be used directly as fertilizer. This is already a common method used by small scale farmers around the Caribbean. The benefit of drying the algae after collection is that the algae will not decompose and release methane gas and hydrogen sulfide into the atmosphere. Despite this fact, it is more efficient to first digest the algae for biogas production and later use the waste as
