IN
DEGREE PROJECT ENVIRONMENTAL ENGINEERING, SECOND CYCLE, 30 CREDITS
STOCKHOLM SWEDEN 2017,
Harvesting and utilizing beach cast on Gotland
A study of the benefits, challenges and
opportunities of turning a waste into a resource FILIP DESSLE
KTH ROYAL INSTITUTE OF TECHNOLOGY
SCHOOL OF ARCHITECTURE AND THE BUILT ENVIRONMENT
TRITA IM-EX 2017:16
www.kth.se
Abstract
Accumulation of beach cast biomass on coastal zones around Gotland is an increasing problem that reduce the recreational value of beaches and cause environmental degradation of coastal
environments. Beach cast was once regularly harvested on Gotland, as it was considered a valued biofertilizer, but as it seized to be used in combination with the eutrophication of the Baltic sea, beach cast accumulation has increased in scale. The use of beach cast as a resource for bioenergy and agriculture does not only provide important services that can replace the use of greenhouse gases and finite resources but can also mitigate eutrophication and increase the quality of coastal zones as nutrients are retrieved from the water. Macroalgae and seaweeds are hyperaccumulators of heavy metals which pose a challenge for the utilization of beach cast as a fertilizer. Especially
cadmium, a heavy metal that is harmful for human consumption, is strictly regulated and limits the amount of beach cast that can be used for agriculture.
This thesis has analysed the potential benefit of beach cast harvesting and three potential utilization strategies of beach cast; fertilizer to food crops, fertilizer to fast growing energy forest (Salix) and biogas production with digestate utilization. The result indicate that all strategies are possible and viable utilization strategies under certain conditions. Because of the complex and site specific factors that affect beach cast utilization, adapting strategies depending on the conditions and needs of the local coastal zones is required. Cadmium uptake is affected by a range of factors that can be
controlled so that beach cast can provide nutrients and improve soil structure without contaminating the soil. Especially promising is the prospect of fertilizing Salix with beach cast as the cadmium absorption and growth rate of Salix enables large amounts of beach cast to be spread without risk for it accumulating in the soil. The cadmium contained within the Salix can later be removed from the environment entirely as it is collected from the ash when Salix is converted to bioenergy. Recent studies also indicate that food crops can be safely fertilized with beach cast without cadmium being transferred if specific crops are chosen. Cadmium uptake to crops can also be limited if specific beach cast with low cadmium content are used and if the beach cast is pre-composted with other
substrates. Both for Salix and food crops its instead legal restraints on cadmium spreading that limits the use of beach cast. When abiding by the set cadmium restrictions, beach cast can only marginally supply the macronutrient requirement of the average food crop on Gotland. Biogas production and digestate utilization from beach cast provides many environmental benefits as clean renewable energy is generated that can replace fossil fuels and the nutrients contained in the digestate can be spread on arable land. From the conducted energy balance of the system on Gotland it was found that beach cast has theoretically good conditions to ferment beach cast. However, beach cast isn’t practically viable on Gotland because the substrate can’t compete financially with other available substrates and it requires costly pre-treatments. Regardless of which beach cast utilization strategy chosen, harvesting is concluded to have a positive effect on mitigating coastal eutrophication and beach and water quality. Although it cannot on its own mitigate a net nutrient loading to coastal zones in Gotland, harvesting easily available beach cast can reduce the nutrient loading to coastal zones on Gotland with up to 27 % and 4,5 % phosphorus and nitrogen respectively.
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Keywords
Beach cast, macroalgae, bioenergy, biofertilizer, eutrophication, cadmium
Foreword
This master thesis is the culmination of my master program, Sustainable Technology at KTH in Stockholm. Choosing a project with a focus on the environmental restoration of the Baltic sea was natural for me as I have been spending every summer since I was a child in the archipelago and seen first-hand the effect of eutrophication. I also have a great interest for smart and sustainable solutions that combine ecology and technology. The topic of beach cast harvesting and utilization is a great example of “closing the loop” between production and waste management which requires a
multidisciplinary approach which my master program has taught me. To focus specifically on Gotland for the study was great as the people I met and interviewed has been very knowledgeable and accommodating and apart from providing valuable input to the thesis also made me realize the importance and urgency of tackling this problem.
I want to thank my supervisor Daniel Franzén for great inspiration, supervision and support. I also want to send lots of gratitude to all the people I met on Gotland and that has contributed to this report. Finally, I want to thank the staff at Grå Gåsen in Burgsviken for letting me stay there during my stay at Gotland.
Stockholm, June 2017 Filip Dessle
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Legend
TS (Total solids) – DW (dry weight) - Dry matter content, quantity of matter remaining after drying VS (Volatile solids) – The quantity of the dry matter that is combustible. Measure of organic content.
Substrate – Digestible matter.
Digestate – fermented residue from the anaerobic process that can be used for agriculture.
Normal cubic meters = 𝑁𝑚3 = Standard unit for gas production. 1 𝑚3of gas with 0 degrees Celsius and 1,013 bars.
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Table of content
Keywords ... 1
Foreword ... 1
Legend ... 2
List of tables ... 6
1. Introduction ... 7
2. Background ... 9
2.1 Beach cast macroalgae and seaweed ... 9
2.2 Beach cast utilization methods ... 9
2.3 Ecological and social effects of beach cast harvesting ... 10
2.4 Legal obstacles regarding the harvesting of beach cast ... 11
2.5 Cadmium a restriction for beach cast utilization ... 11
3. Aims and objectives ... 13
4. Method ... 14
Result division... 15
5. Results ... 19
Literature study ... 19
5.1 Cadmium concentration in beach cast ... 19
5.2 Biogas production from beach cast biomass... 20
5.3 Beach cast as biofertilizer ... 24
Estimation ... 26
5.4 Beach cast availability, composition and occurrence ... 26
5.5 Macronutrient and cadmium content in beach cast ... 29
Evaluation ... 31
5.6 Eutrophication mitigation potential from beach cast harvest ... 31
5.7 Strategy A - Algae to biomethane and digestate utilization ... 32
5.7.1 Energy analysis, anaerobic digestion of beach cast ... 32
5.7.2 Biogas strategy, source: Rikard Hansson, COO Bro Biogas ... 35
5.8 Strategy B - Biofertilizer to cropland ... 36
5.8.1 Heavy metal uptake by plants from algae and soil ... 36
5.8.2 Macronutrient benefit of beach cast to cropland ... 37
5.8.3 Local attitudes and challenges - source: Tönu Saartok, president Lau beach association & Henrik Ahlsten, conservative farmer ... 39
5.9 Strategy C - Biofertilizer to Salix plantation ... 40
4.9.1 Macronutrient potential and land requirement ... 41
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5.9.2 Local attitudes and challenges - source: Torgny Hellström, Salix owner ... 42
SWOT analysis ... 44
5.10.1 Strategy A - Biomethane production and digestate utilization ... 44
5.10.2 Strategy B - Fertilizer food crops ... 45
5.10.3 Strategy C - Fertilizer Salix plantation ... 46
6. Discussion ... 47
Reliability and validity of the study ... 48
7. Conclusion ... 49
8. Reference list ... 50
Appendix 1 Surveys ... 55
Hur mycket av kustremsan rensas idag? Hur ofta? ... 55
Uppskattning av mängden Släke (ton/kubikmeter eller ton/meter strandkant eller annan uppskattning) och artsammansättning (procentuell fördelning av röd, grön, brun (Blåstång) samt Ålgräs) som spolas upp varje år på lokal vik. ... 58
Hur sker nuvarande hantering av Släke, hur mycket tid och pengar läggs ned för hantering av Släke idag? vilka resurser finns tillgängliga. ... 61
Appendix 2 Interviews ... 64
Intervju Torgny Hellström, Salix ägare ... 64
Intervju Peter Landergren, Länsstyrelsen Gotland... 67
Intervju Rikard Hansson, Bro biogas ... 68
Appendix 3: Raw data and calculations ... 69
Parameters for energy calculation ... 69
Parameters for transport calculation ... 69
Energy calculations: Energy production ... 70
Energy calculations: Energy balance ... 70
Energy calculations: car supporting potential ... 70
Raw data nutrient and cadmium content beach cast samples ... 71
Nutrient emissions from various location in Gotland. Source: SMHI http://vattenweb.smhi.se/kustzonanalys/ ... 72
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List of figures
Figure 1 Conceptual model ... 17
Figure 2 Seasonal accumulation of stranded beach cast………29
Figure 3 Macronutrient values from beach cast samples….. ... 30
Figure 4 Cadmium values to the beach cast samples ... 30
Figure 5 Phosphorus loading and retrieval to/from coastal water……….31
Figure 6 Nitrogen loading and retrieval to/from coastal water ... 31
Figure 7 Bioenergy system ... 32
Figure 8 Macronutrient requirement of crops. ... 38
Figure 9 Beach cast required ... 39
Figure 10 Aannual macronutrient requirement of Salix ... 42
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List of tables
Table 1 Cadmium restrictions for fertilization ... 12
Table 2 Average cadmium concentration of beach cast. ... 19
Table 3 Methane yield potential ... 23
Table 4 Biomass availability. ... 28
Table 5 Index beach cast samples ... 29
Table 6 Primary energy data for the process steps... 33
Table 7 Energy balance... 35
Table 8 Annual energy production and number of cars that beach cast can supply. ... 35
Table 9 Macronutrient removal from harvests ... 37
Table 10 nutrient mitigation and land requirement ... 42
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1. Introduction
The Baltic sea is under severe ecological stress due to a combination of anthropogenic pressures and environmental characteristics that makes the area unique and fragile to disturbances (Andersen et al., 2009). It is the world largest brackish sea but has a very small water volume and limited water exchange in relation to its large catchment area that covers 14 countries and roughly 90 million people (Nekoro, 2013). Eutrophication due to excessive nutrient enrichment from anthropogenic activities is today one of the most serious threats to the Baltic sea and its coastal areas (Nekoro, 2013; Andersen et al., 2009). Effect of eutrophication include; reduced water transparency leading to light deprivation for ecologically important macroalgae and seaweed species, oxygen depletion leading to increased mortality of benthic fish and animal species and accumulation of beach cast mats at shores (Andersen et al., 2009). Together with other pressures such as overfishing, invasive species and climate change human activities are threatening to halt the ecosystem services and goods that people receive from the Baltic Sea every year (Nekoro, 2013). Beach cast accumulation is a today generally viewed as a nuisance in the Baltic sea that reduce the ecological and recreational value of beaches, both for residents and tourists (Risén, 2014). A local newspaper in Gotland reports of popular swimming beaches in Gotland being increasingly scarce of people during peak season due to excessive beach cast that cause foul odours and inaccessible water for swimmers (Olofsson, 2013).
Beach cast has a long history of being used as a valuable resource by farmers and landowners on Gotland before industrial farming methods where introduced during the end of the world war 2 that replaced beach cast as a biofertilizer for an industrial fertilizing system (Niklasson, 2017).
Today, 5000-10 000 tonnes of beach cast are cleaned each year on swimming beaches around Gotland, financed by the Gotland country administrative board aimed at improving the quality of beaches (Landergren, 2017). When beach cast is harvested from beaches, the smell, access and water quality are improved which increase the value and wellbeing for residents and tourists (Risén et al., 2016). Most of the beach cast that is harvested in Gotland is utilized for soil improvement but in many locations around the Baltic sea the beach cast is only marginally utilized after harvest and often piled in the close vicinity of the beach only to be put back into the water at the end of the summer season (Niklasson, 2017). But beach cast is growing in recognition for its potential to be used as a bioresource and several projects around the Baltic sea, aimed at utilizing beach cast has been started. For example, utilizing beach cast as biofuel for biogas production can potentially reduce the reliance on fossil fuels and mitigate global warming while producing high value renewable energy (Kaspersen et al., 2016). Production of biogas from beach cast has been conducted both in Trelleborg and Solrød on Denmark, while it has been used as fertilizer and soil improver in several parts of Skåne and Öland (Risén, 2017). Systems for utilizing beach cast on arable land as fertilizers has the potential to recirculate vital and increasingly scarce nutrients and thus creating circular flows of nutrients instead of linear ones that are responsible of creating eutrophication in the first place. But there are several uncertainties and challenges that hinder large scale utilization of beach cast on Gotland. Those are primarily for agricultural purposes uncertainties regarding the benefit of beach cast as a fertilizer and risks for cadmium contamination of plants and soil (Greger et al., 2006). For biogas production, the crucial factor is the energy potential of beach cast as a substrate (Bucefalos, 2016; Risén et al., 2014).
In a report by Nissling, (2016) that assessed the ecological status of coastal waters around Gotland found that the goal of achieving good ecological status was not achieved on any location. The status, that was based on the water content of phosphorus and nitrogen, concluded that moderate status was achieved on open coastal areas with large amount of water exchange. On more isolated bays with less water exchange, the ecological status was concluded to be moderate to poor. Sweden has
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in compliance with the Baltic Sea Action Plan agreed to reduce the annual emissions of nutrients to the Baltic sea by 9240 tonnes of nitrogen and 530 tonnes of phosphorus in relation to the reference period 1997-2003 (HELICOM, 2015). However, Gotland country administrative board is arguing that Gotland and Sweden will not fulfil those set goals with the current measures that are considered positive but not sufficient (Miljömål, 2017). In 2014, Sweden emitted to the oceans 114600 tonnes of nitrogen and 3340 tonnes of phosphorus about of which 50 % is derived from natural emissions from forest and soil (Jordbruksverket, 2017). About 50 % of the emissions to the Baltic Proper, which is the part of the Baltic sea where Gotland is located, derives from agriculture (Jordbruksverket, 2017) but also point emissions from sewage plants and industries play an important role, especially from phosphorus emissions (Jordbruksverket, 2017). Actions to address the eutrophicated
conditions on Gotland has mainly been focused at reducing nutrient leaching from agriculture by support from Greppa Näringen, actions to improve water quality trough a national program aimed at mitigating eutrophication, called LOVA (Länsstyrelsen Gotland, 2011) and improving the purification of sewage water (Naturvårdsverket, 2017). Beach cast harvesting and utilization could prove to be an important step in not only mitigating problems of eutrophication but also stimulating the local economy and provide job opportunities.
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2. Background
2.1 Beach cast macroalgae and seaweed
Macroalgae or seaweed are macroscopic, multicellular aquatic plants that can live and thrive in a range of different types of environments all over the world (Blidberg & Gröndal, 2012). Depending on their pigments used in photosynthesis they are divided into three categories: red, green and brown which is also the colour they show physically. Macroalgae have an important ecological role as primary producers of oxygen and food for other species such as fish and invertebrates (Blidberg &
Gröndal, 2012). They also act as shelters for juvenile organisms. Macroalgae don’t have roots but stick to substrates with certain sticking organs. Perennial macroalgae can live for up to 20 years but lose their upper flowering parts each year in late summer which can be observed on coastlines (Berglund, 2010). Filamentous macroalgae are short-lived opportunistic species that benefits from the elevated nutrient levels in the Baltic sea and have in many areas therefore outcompeted perennial species in terms of substrates to connect to and sunlight exposure (Berglund, 2010). The eutrophicated conditions in the Baltic sea cause an overproduction of filamentous algae that detach and drift which results in major ecological, economic and social problems such as anoxic coastal zones and inaccessible coast lines (Risén, 2014). Eelgrass is a vascular plant that thrives in sandy coastal zones were the light conditions are favourable (Nekoro, 2013). When the conditions are favourable, the plant commonly establishes meadows that serves as important benthic habitats for various species of animals and fish (Nekoro, 2013). Light deprivation caused by eutrophicated conditions negatively affect eelgrass meadows and leads to its habitat being outcompeted in favour of short lived filamentous species (Nekoro, 2013).
2.2 Beach cast utilization methods
Beach cast on Gotland is today generally considered a waste that is either left on beaches to
decompose or harvested where the main purpose is increase the recreational value of public beaches (Weber-Qvarfort, 2016). When harvested, the beach cast is analysed for cadmium content and either used as a soil improver or land filled if the cadmium content is to high (Weber-Qvarfort, 2016).
Depending on how the beach cast is treated, it is legally defined as either a waste or a resource (Landergren, 2016). If the beach cast is to be used as a soil improver for agriculture it is usually composted until the next season where it is transported and spread to arable land. But if the sole purpose of the cleaning is to get rid of the beach cast, it is defined as a waste that cannot legally be landfilled without special permit (Landergren, 2016). A common strategy used in for example
Trelleborg to circumvent that is to accumulate beach cast in piles and dump it back into the sea once the summer season is over. This strategy has limited social and environmental effect, as the nutrients retrieved are released back into the sea, the smell of rotting algae is not necessarily avoided and considerable amounts of fossil fuels is used from harvesting (Risén et al., 2017). Methane is also emitted from the rotting piles during anaerobic digestion (Risén et al., 2017). The harvesting process is also very costly, on the German Baltic coastline were regular beach cleaning is conducted by seaside resorts mechanically by hand, the cost is up to 38 euro/meter beach line to remove an average of 269 tonnes/km beach cast (Mossbauer et al., 2011). Apart from LOVA support, beach cleaning in Gotland is in part financed by beach association that pay a member fee to cover beach cast harvest. Käldhagens beach association located in the southeast coast of Gotland has for example an annual member fee of 300 kroners which cover the machine cost used for harvest (Niklasson, 2017).
On a global scale, the commercial large and small-scale use of algae has increased a lot in recent years and is today a valuable commodity for a range of different areas such as: human consumption,
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animal feed, fertilizer, biochemical production and gelling substances (Blidberg & Gröndal, 2012). The global production of aquatic plants for commercial use was 2010 roughly 19,8 million tonnes
(Blidberg & Gröndal, 2012). The clear majority of that (95%) came from cultivation with a value of roughly 4,3 million euros (Blidberg & Gröndal, 2012). In Europe, the utilization of aquatic biomass is much lower in magnitude with a production of roughly 82 000 tonnes (Blidberg & Gröndal, 2012).
Cultivation of macroalgae in the Baltic sea with its brackish water is however different from other oceans as the growth rate is considerably lower and with a different species composition (Risén, 2012). How beach cast can be utilized and marketed depends on the species and quality. For high value products, such as human food, biochemical products and gelling substances, the biomass need to be of high quality; that is fresh and not degraded and not mixed with different species of algae or contaminated with sand and stones (Blidberg & Gröndal, 2012). High value products are thus generally derived from macroalgae cultivation where the quality and homogenous composition can be controlled (Blidberg & Gröndal, 2012). Low value, algae derived products and are on the other hand products that has a lower market value but doesn’t require the same quality and homogenous quality of the resource. Such end-uses are biofertilizer, bioenergy production, soil improver, land restoration and animal feed (Blidberg & Gröndal, 2012). How the algae are utilized depends both on the locally available resources and the local market.
2.3 Ecological and social effects of beach cast harvesting
Ecological effects of beach cast harvesting
The impact of beach cast harvesting was investigated in a report by Malm et al., (2004) where three types of shorelines were compared: uncleaned, moderately cleaned and intensively cleaned beaches.
The result showed that water clarity was significantly increased from the intensely cleaned beach while the moderate cleaned beach showed no difference. The organic content on the sand was lower for both cleaned beaches compared with the uncleaned one. The biodiversity was also highly
affected by the removal of beach cast, resulting in less animal biomass for the intensively cleaned beach compared to the uncleaned one. Especially planktivory species such as the opossum shrimp and the microbial biodiversity was much higher on the uncleaned beach because of the food-web being stimulated by the decomposing algal mats (Malm et al., 2004). On the macro fauna
biodiversity, the study detected no significant different between the cleaned and uncleaned beaches.
Martinsson (2016) studied the ecological effects of beach cast cleaning on Gotland in a study from 2014-2016. The study analysed improvements in organic content and turbidity at the coastal waters that were cleaned. It was concluded that no effects on turbidity or organic content could be detected on exposed beaches, but on isolated bays there was an improvement as the organic content had declined (Martinsson, 2016). The study confirms that beach cast harvesting influences the water quality on isolated bays that are worst affected by eutrophication.
Even though most ecological consequences of beach cast removal are positive, certain species and ecosystems can also be negatively affected by beach cast removal (Blidberg & Gröndal, 2012).
Whether the effect is positive or negative depends on the abundance of beach cast and algal mats in the shallow water and the local ecological conditions of species composition and ecosystem
dynamics (Blidberg & Gröndal, 2012). Beach cast removal could for example lead to coastal erosion when the coastal zones are exposed to waves and birds could have their habitats disturbed. Due to the complexity and knowledge gaps, locations for harvest should be carefully chosen (Blidberg &
Gröndal, 2012).
Socio-economic benefits of beach cast harvesting
Apart from the ecological effects of beach cast harvesting and utilization, removal also increases the recreational value of beaches by improving the access and removing foul odours from decomposing
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biomass. This does not only lead to a higher well-being for residents but could also generate more income to the region from tourism (Malm et al., 2004). In a recent study by Risén et al., (2017) the non-market benefits from beach cast cleaning and utilization was analysed by assessing local resident’s willingness to pay (WTP) for such programs. The inclusion of nonmarket values can be crucial for environmental programs aimed at harvesting and utilizing beach cast as the market value can otherwise be too low to be viable (Risén, 2017). This was the case for the pilot biogas plant in Trelleborg which was concluded to be financially non-viable because of low methane yield and high cadmium content in the algae unless nonmarket values were included (Bucefalos, 2016). From the study by Risén et al., (2017) it was concluded that the will to pay for beach cast utilization
programmes was high, approximately 1,3 million EURO in total for all taxpayers in Trelleborg. The study is site specific and depends on many factors but gives an indication of what the WTP might be for Gotland with a similar problem situation as Trelleborg.
2.4 Legal obstacles regarding the harvesting of beach cast
There are no requirements for permits to harvest beach cast in Gotland on public swimming beaches if the person who is conducting the harvest have a right to the land. If a beach is covered by
vegetation, a permit is needed unless the beach has previously been harvested for beach cast.
Beaches that are part of a nature reserve or Natura 2000 requires permits in almost all cases (Landergren, 2016). When beach cast is harvested for beach cleaning but not utilization, its considered a waste. By Swedish law beach cast cannot be landfilled since it is considered organic waste, a common solution that is allowed is to put the beach cast in piles in close vicinity to the beach (Landergren, 2016). Before a beach can be harvested on Gotland the land owners must agree, and that is not always practically simple since beaches are often owned by large collective commons where all members must agree on harvesting. It is however generally not a problem, since decaying beach cast is generally considered a nuisance (Niklasson, 2017).
2.5 Cadmium a restriction for beach cast utilization
Central for the viability of the different strategies is the cadmium content in the beach cast, that restrict its usage as a biofertilizer on arable land (Karlsson, 2008; Kaspersen, 2016; Avfallsverige, 2012). Cadmium is a heavy metal that accumulates in the body with a risk for a range of negative health consequences such as; cancer (Hartwig, 2013), kidney failure, diabetes, disruption of the body’s ability to use calcium (Naturvårdverket, 2017). Humans are exposed to cadmium primarily from smoking and through consumption of cadmium rich foods (Naturvårdsverket A, 2017). The recommended intake of cadmium per month is ranging from 10-30 micro gram per kg of bodyweight (Eriksson, 2016). Eriksson conclude that there are no margins to increase cadmium content in food due to the health risks. Cadmium is spread to agricultural lands either by fertilizers or atmospheric deposition from the burning of fossil fuels and garbage (Naturvårdsverket A, 2017).
Macroalgae and seaweeds are natural heavy metal accumulators as it binds in the polysaccharides in the cell wall of the algae (Greger et al., 2006). The degree of accumulation depends on the
surrounding concentration and the availability which increase as the salinity get lower further north in the Baltic sea (Greger et al., 2006; Blidberg & Gröndal, 2012). For example, the brown perennial macroalgae Fucus vesiculosis, growing in the North Sea with a salinity of roughly 3 % has 10 times lower cadmium levels compared with the same species growing in the central Baltic Sea with a salinity of around 0,7 % (Greger et al., 2006). It has been concluded in experiments examining heavy metal contamination from algae to soil that cadmium is the heavy metal that have transgressed the natural level of the soil, while other potentially harmful heavy metals have been below that level (Karlsson, 2008; Greger et al., 2006). Cadmium is generally not advisable on land for food production as it remains available in the soil for crops for a long time where it accumulates (Karlsson, 2008). It is
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also stated in the Swedish environmental goal of a toxic free environment that cadmium is to be phased out from production and production processes so that more cadmium does not leak out into the environment (Avfallsverige, 2017). The lime rich bedrock on Gotland doesn’t contain high levels of cadmium but the concertation on arable soil varies depending on how the soil has been fertilized with beach cast and mineral fertilizer in the past (Weber-Qvarfort, 2016). By Swedish regulation it is not allowed to fertilize arable land with a cadmium concentration of 0,4 mg/kg (Avfallsverige, 2017).
Currently there are no EU law regulating cadmium levels in fertilizers whereas the national variations of cadmium content in fertilizers are great (Blidberg & Gröndal, 2012). However, the European union is planning to introduce regulations on the use of cadmium in organic and waste-basted fertilizers as a part of the circular economy package to increase recycling of nutrients (European commission, 2017). The regulation would include a limit of cadmium in biofertilizers to ultimately 46 mg/kg phosphorus in 12 years (European commission, 2017). In Sweden, there are certification systems for biofertilizers that aims to limit the spreading of heavy metals. To be certified as a biofertilizer per the SPCR 120 guidelines the cadmium content in the fertilizer cannot transgress 1 mg/kg TS
(Avfallsverige, 2017), that is 1 mg cadmium / the total dry weight. It is voluntary to be certified per these rules, but being certified generates a trust towards the product and the options of available land to dispose onto increases (Avfallsverige, 2017). When using digestate from biogas production the fertilizer comes under the same laws as those of sewage sludge, were the limit is 2 mg/kg TS (Tideström, 2008).
Apart from the cadmium content an important factor for the value of the algae as a biofertilizer is the content of nutrients, especially phosphorus as it is a non-renewable resource that is mined from phosphorus rock (Avfallsverige, 2012). Like the diminishing resources of oil, society is facing peak- phosphorus where the resource is becoming increasingly difficult and expensive to mine (Beardsley, 2011). A circular management of phosphorus is therefore important, not only to mitigate
eutrophication but also to secure future availability. Consequently, a cadmium-phosphorus quota is commonly used (Avfallsverige, 2012). There is not yet any legal binding limit for the
cadmium/phosphorus quota but for mineral fertilizer the national set limit is 100 mg Cd/kg P
(Avfallsverige, 2012). Average national Cd/P levels in mineral fertilizer is roughly around 6 mg Cd/kg P (Avfallsverige, 2012) and 8-16 in animal manure (Eriksson, 2016)
Table 1 cadmium restrictions for fertilization Legal limit for
Cd in sewage sludge for fertilizer (biogas digestate)
SPCR 120 Cd limit content in
biofertilizer.
Maximum amount of Cd to arable lands per KRAV
Maximum amount of Cd to arable lands
Maximum amount Cd per kg P on arable land in mineral fertilizer.
Current regulations
2 mg/kg TS (Avfallsverige, 2017)
1 mg/kg TS (Avfallsverige, 2017)
0,45 g/Ha (Avfallsverige, 2017)
0,75 g/Ha (Avfallsverige, 2017)
100 mg/kg P (Avfallsverige, 2017)
Proposed regulation
46 mg/kg P (Riksdagen, 2015)
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3. Aims and objectives
The aim of the thesis can be divided into two separate aims that concerns the management of stranded beach cast on Gotland. The first aim is to evaluate the potential effect of beach cast harvesting to mitigate coastal eutrophication on Gotland. Secondly, the aim is to evaluate the strengths, weaknesses, opportunities and threats of three potential strategies to utilize beach cast biomass as a resource once the beach cast has been harvested. The specific objectives are:
1 Describe the technical, practical and environmental challenges and opportunities to utilize beach cast as a resource.
2 Asses the annual biomass availability and species distribution of beach cast on Gotland.
3 Quantify the content of cadmium and nutrients in the beach cast on Gotland 4 Quantify the eutrophication mitigation potential form beach cast harvesting.
5 Quantify the theoretical energy and nutrient benefit of beach cast under the various scenarios.
6 Make a SWOT-analysis over the strategies to utilize beach cast.
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4. Method
The management of beach cast on Gotland has been studied from a macro perspective where the main factors influencing the harvest and utilization of beach cast has been analysed from a systems perspective. Apart from quantifying the potential effect of beach cast harvesting the utilization of beach cast has been analysed with a special focus on 4 different key factors that argued to be crucial for the viability of the strategy. These key factors were decided based on literature and input from stakeholders on Gotland and are the following:
1) Energy cost and benefit, 2) fertilizer potential, 3) cadmium uptake, and 4) local opportunities and challenges
The background of the thesis contains a description of the potential utilization methods of beach cast biomass and the social and ecological effect of beach cast harvesting. It also describes how heavy metals, in particular cadmium affect the utilization strategies. The result of the study is presented in 4 different parts that is explained below: literature study, estimation, evaluation and SWOT-analysis, using 4 different methods: literature study, interviews, surveys and calculations. This methodology was chosen because of the wide scope of the study and the complex factors that were assessed, requiring different methods.
Methods used Literature study
The literature study reviewed relevant scientific literature to get a general picture of the main benefits and challenges of beach cast harvesting and utilization. Information was largely gathered from experimental studies of beach cast utilization projects around the Baltic sea but also from theoretical reports on the usage of beach cast a resource for biogas production and fertilization.
From these studies the cadmium and methane potential from various species of macroalgae and seaweed could be gathered. Also, the viability of beach cast as a fertilizer and biogas substrate is described with a technical focus on the biogas system.
Interviews
The local opportunities and challenges of each beach cast utilization strategy was partly evaluated through three interviews with stakeholders directly involved with each strategy. For strategy A, that is biogas production with digestate utilization, the COO of Bro biogas was interviewed. For strategy B, that is food crop fertilization, a conservation farmer in the south part of Gotland was interviewed.
For strategy C, that is Salix fertilisation, a Salix farmer was interview. The interviewees were selected based on their involvement and knowledge of using beach cast for the respective purpose and are argued to hold representable views on the matter for Gotland as a whole. Two additional interviews were also conducted, one with Ulf Smedberg, a local entrepreneur who are responsible for the majority of the harvesting on the island and one with Peter Landergren who works with marine environmental questions for the county administrative board. The interviews were conducted to acquire site specific data and information regarding harvesting practices, amounts and species composition. The interviews were conducted in semi-structured manner and face-to-face with all interviewees except for one that was held over the phone with Peter Landergren. The semi-
structured interviewing method was chosen because it enabled the flexibility of using a template of predetermined question that could be modified and developed depending on the situation. Since the strategies were technically different the template was also adopted accordingly. The result of the interviews was interpreted and put in the evaluation section of the report. The input of the
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interviews provides reliability to the study and a practical dimension that is otherwise hard to get.
The interpretation of the interviews can be viewed in appendix 2.
Survey
A survey was sent out to members of the Havsmiljöföreningen Gotland, a non-profit organization aimed at improving the condition of the coastal environments around Gotland. The organization represents 7 different bays around the east and south coast of Gotland whereas different conditions and perspectives were included. The questions in the survey was concentrated around the local attitudes of beach cast and how it was perceived. The surveys were also important to get first-hand information on the biomass availability and species composition and how it differed between different coastal areas. The result of the survey can be viewed in appendix 1
Calculations
Calculation was conducted for the eutrophication mitigation potential and each beach cast utilization strategy. How the calculations were done, what data they build upon and how it is presented is described in the evaluation section 4.3 for each respective part.
Result division
This section of the method aims to clarify the structure of the results and describes which objectives that are met in each part.
4.1 Literature study Objective met:
Describe the technical, practical and environmental challenges and opportunities to utilize beach cast as a resource.
Parts included:
Cadmium concentration in beach cast, biogas potential of beach cast, and fertilizer potential of beach cast.
4.2 Estimation on beach cast availability, cadmium content and macronutrient content Objectives met:
Asses the annual biomass availability and species distribution of beach cast on Gotland.
Quantify the content of cadmium and nutrients in the beach cast on Gotland Parts included:
Beach cast availability and species distribution
Beach cast concentration of nutrient and cadmium
Beach cast biomass availability and content of macronutrients and cadmium was assessed in the estimation part which was the basis for the calculations made to evaluate the eutrophication mitigation potential and strategies. Beach cast availability was assessed by approximating the average biomass availability on beaches per meter and then multiplying it with the length from various harvesting scenarios. The different harvesting scenarios was defined as: implementation potential, technical potential and theoretical potential based on how much biomass that are harvested. Since there have been no systematic, large scale measurements on the amount of beach cast on Gotland, adoptions and assumptions from other locations were made to complement the assessment.
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Data on beach cast availability was acquired by adopting and interpreting data from: literature reports from Gotland country administrative board who collect data from beach cleaning projects, surveys from local stakeholders and relevant scientific reports. Data on the macronutrient content and cadmium content in the beach cast that is specific for Gotland was taken from a recent report by Gotland country administrative board and from an unpublished scientific report.
4.3 Evaluation
Eutrophication mitigation potential Objectives met:
Quantify the eutrophication mitigation potential form beach cast harvesting.
Quantify the theoretical energy and nutrient benefit of beach cast under the various scenarios.
Parts included:
Eutrophication mitigation potential from beach cast harvest
Beach cast utilization, strategy evaluation
The potential eutrophication mitigation effect under various harvesting scenarios was evaluated by comparing and quantifying the emissions of nutrients from Gotland and comparing that to the theoretical uptake of nutrients from beach cast harvesting. Data on nutrient emissions was adopted from the SMHI tool Vattenwebb (SMHI, 2017). Data on the theoretical nutrient uptake from
harvesting was calculated based upon the nutrient content of beach cast multiplied by each
harvesting scenario. The result of the theoretical coastal eutrophication benefit is displayed in figure 5 and 6 as loading and retrieval of phosphorus and nitrogen to the coastal areas around Gotland. The raw data input on nutrient loading can be viewed in appendix 3.
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The utilization of beach cast after harvest was studied for three chosen strategies that is argued to be viable end uses of beach cast based on literature and surveys. Figure 1 illustrates the various
pathways of beach cast, from harvest to end use.
A: Anaerobic digestion with digestate utilization B: Food crop fertilizer
C: Salix Fertilizer
Figure 1 Conceptual model illustrating the main steps for the three strategies analysed Strategy A: Anaerobic digestion with digestate utilization
Key factor: 1) Energy cost and benefit, 4) local opportunities and challenges
The Energy cost and benefit was analysed by conducting an energy balance for the system, from harvest to digestate spreading. The conceptual model for the biogas system was adopted from Risén et al., (2014) who studied the Trelleborg pilot plant. Data input values were based upon the Gotland system which were gathered from various scientific reports and own calculations, see appendix 3.
Specific values on the energy requirement of biogas production and upgrading was also adopted from the Trelleborg case study while the methane yield of beach cast was approximated based on the average methane yield of macroalgae and seaweed species and compared to the species distribution and availability on Gotland. The energy balance is displayed in table 7 as inputs and outputs of energy per wet tonnes of beach cast from the process steps. From the input and output data the two indicators Energy Return On Investment (EROI) and Net Energy Value (NEV) could also be calculated. From data on the theoretical energy production from beach cast the number of biogas vehicles that could be supplied was also calculated. The raw data which is the basis for the
calculations can be viewed in appendix 3.
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Key factor: 2) fertilizer potential, 3) heavy metal uptake 4) local opportunities and challenges
The fertilizer potential of beach cast to food crops was analysed by calculating to which degree beach cast could supply the macronutrient requirement of food crops without transgressing the legal cadmium restrictions. Data on nutrient requirement of food crops was taken from a report by Jordbruksverket. The land required to spread the beach cast without transgressing the legal limit was also calculated from data on nutrient and cadmium content of beach cast. Data on heavy metal uptake by food crops was taken from experimental reports on Gotland and Öland which analysed the heavy metal transfer from beach cast to food crops. The heavy metal uptake by plants was
qualitatively assessed using literature and not quantified.
Strategy C: Salix fertilizer
Key factor: 2) fertilizer potential, 3) heavy metal uptake 4) local opportunities and challenges The macronutrient potential of beach cast to supply the macronutrient requirement of Salix was calculated based on the amount of beach cast that could be applied per hectare without cadmium being accumulated in the soil. The number of hectares of Salix plantations required to safely spread beach cast without cadmium being accumulated in the soil under the varying harvesting scenarios was also calculated. Data on nutrient requirement of Salix, cadmium phytoextraction capacity and growth rate was taken from various scientific reports.
4.4 SWOT-analysis Objectives met:
Make a SWOT-analysis over the strategies to utilize beach cast.
Parts included:
SWOT-analysis
SWOT is an analytic tool used to present strengths, weaknesses, opportunities and threats for a studied project. The SWOT-analysis conducted in the thesis provide a comprehensive summary over the results that was reached from the different methods regarding the utilization of beach cast on Gotland. The SWOT-analysis contains both specific results for Gotland that is derived from calculation and literature based on site specific data and general result that is derived from theoretical studies and reviews on the subject. Interviews and surveys conducted on Gotland also contributed to the SWOT-analysis regarding local opportunities and challenges. Which method that was used to reach each result is also presented in the result.
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5. Results Literature study
Although very few studies have been conducted on Gotland regarding cadmium concentration in beach cast and the suitability of beach cast as a biogas substrate and fertilizer, there have been studies made in other parts of Sweden and around the Baltic sea. The broad scope of this section reviews relevant literature from experimental and theoretical reports and provides the general benefits and challenges of beach cast as a bioresource.
5.1 Cadmium concentration in beach cast
There have been several studies conducted in the Baltic sea measuring the cadmium content in beach cast with very varying results. Beach cast collected for an experiment in eastern Öland that consisting of roughly 85 % red algal species contained cadmium levels ranging between 1,1-4,7 mg/kg TS (Greger et al., 2006). During the biogas pilot plant in Trelleborg the beach cast was dominated by red algal species, with cadmium concentration ranging between 0,4-2,4 mg/kg TS (Bucefalos, 2016). Gotland seems to exhibit lower cadmium concentration in the beach cast compared to other parts of Sweden, see table 2. In an unpublished report by Franzén et al., (2017) only eelgrass contained higher concentrations of cadmium than the limit for biofertilizers of 1,0 mg/kg TS, compared to other species of macroalgae. Tests were conducted in several parts around the southern parts of Gotland with low concentrations of cadmium in all species except eelgrass (Franzén et al., 2017). The study also made clear that various locations that are very close to each other can have a very different rate of cadmium concentration in the beach cast. Beach cast in Burgsvik, which is an isolated bay surrounded by mostly farmland and settlements, contained cadmium contents up to 2,2 mg/kg TS on a spot close to the harbour, while roughly 5 km away, on the inlet to the bay, samples contained only 0,13 mg/kg TS (Franzén et al., 2017). The coast of Poland for example exhibits low levels of cadmium in the beach cast whereas it is commonly used in
agriculture (Risén, 2014). The full reason for the varying levels of cadmium concentration in macroalgae and seaweed, both for various species and different location isn’t known and more research in undoubtedly needed.
Table 2 table shows the average cadmium concentration of beach cast samples taken from various locations on Gotland, Trelleborg and Öland.
Beach cast species Cd mg/kg TS Location Source
Brown algae
Fucus vesiculosis 0,5 Burgsvik, Gotland Franzén et al.,
2017 Red algae
Furcellaria lumbricalis Furcellaria lumbricalis
0,9 - 4,0 0,1
Böda, Öland Burgsvik, Gotland
Greger, 2006 Franzén et al., 2017
Green algae
Patomogeton pectinatus 0,8 Burgsvik, Gotland Franzén et al., 2017
Seagrass
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Zostera marina 1,5 Burgsvik, Gotland Franzén et al.,
2017 Mix
Mix of red filamentous macroalgae Mix of red filamentous macroalgae
0,4–2,4 0,45–0,7
Trelleborg
Various locations, Gotland
Karlsson, 2008 Larsson, 2016
Mix, various algae 0,98 Various locations, Gotland Landergren, 2016
5.2 Biogas production from beach cast biomass
Dependency of depleting fossil fuel reserves combined with the high environmental footprint resulting from its combustion has promoted a search for novel alternative energy fuels that are environmentally viable and renewable (Singh, 2015). The first-generation biofuels are fuels that derive from food crops such as bioethanol from corn and biodiesel from soybeans. Although
succeeding to mitigate carbon dioxide emissions when replacing fossil fuels, the technology has been unsuccessful in terms of having a low net energy balance, having limited biomass resources,
competing and driving up food prices etc. (Singh, 2015; Sirajunnisa & Surendhiran, 2016). The second-generation biofuels, that is non-food crops and food crops residuals such as leaves and stems, led to a small increase of the net energy balance as well as avoiding using food crops in favour of crops like straws, grass etc. But these biofuels still require an agricultural system with arable land, fertilizer, irrigation etc. (Singh, 2015). Also, the processing of the fuels was complicated and led to high cost and low biogas yields (Singh, 2015).
The third-generation biofuels consisting of algae and seaweeds is now gaining attention for their biogas potential and ability to avoid the problems associated with 1st and 2nd generation fuels (Singh, 2015). Algal biofuels are now seen as one of the main solutions towards mitigating climate change and transitioning from fossil fuels (Montingelli, 2014). The Swedish EPA state that biogas production has a prominent role in Sweden to fulfil its environmental goals of reducing the use of fossil fuels and increase energy independence (Jarvis, 2012). The main advantages for using algae as biofuel is the high growth rate which often is higher than that of terrestrial plants (Milledge et al., 2014), no competition with food crops on arable lands and low amount of lignin which reduces the need for energy intensive pre-treatment. However, there are several drawbacks that limits the viability of algae as biofuel that needs to be met for it to become economically feasible and commercial used on a large scale. The main ones are: its high water content, salt content and content of polyphenols and sulphated polysaccharides that can that can inhibit anaerobic digestion (Montingelli, 2014; Blidberg
& Gröndal, 2012). For beach cast in the Baltic there is also the added problem of elevated cadmium concentrations that hinders efficient recirculation of the nutrients back to agriculture. Using beach cast for biogas generation there are additional challenges which include; sand and debris
contamination and the rate of decomposition which adds to the cost and labour of the beach cast management (Blidberg & Gröndal, 2012; Aldentun, 2013). Resource availability is also a problem.
Compared with cultivated algae, that completely dominates the commercial utilization of algae, the beach cast biomass and composition of species are difficult to estimate due to seasonal, annual and geographical variations and lack of systemic monitoring on shores (Blidberg & Gröndal, 2012).
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There are several ways in which algal biomass can be converted to biofuel and biogas involving different methods and techniques that poses different challenges that needs to be solved.
Economically viable commercialization of algal biofuels is yet to be proven, however most research has focused on producing fuels from microalgae whereas the potential of macro algae remains to be seen. Some of the ways biofuels can be extracted from macro algae includes combustion, pyrolysis, gasification, alcoholic fermentation and anaerobic digestion (Milledge et,al. 2014). Gotland has a two-step anaerobic digestion chamber located 8 km from Visby, which therefore will be the only viable option to produce biogas and focused on in this thesis, although macroalgae is for example considered an ideal candidate for bioethanol production with its high content of natural sugars and carbohydrates which makes them suitable to be fermented to produce alcohol-based fuels
(Sirajunnisa & Surendhiran, 2016). When algal biogas is used to replace diesel and petrol vehicles the environmental benefits are great. It has even been concluded that biogas system utilizing macro algae can be carbon negative (Risén, 2014).
Anaerobic digestion for biogas production
Anaerobic digestion (AD) is the process of using anaerobic bacteria to break down organic material in the absence of oxygen (Davidsson, 2008) and is considered the most direct and easy way of
producing biofuel from macroalgae and seaweed (Montingelli, 2014). The process can tolerate the high moisture content of macroalgae and seaweed and extract the entire carbon content out of the substrate making it the most suitable biogas production technology (Milledge et al., 2014). It is a highly complex microbiological process that is constantly under development and research concerning for example new mixes of substrates which requires a different anaerobic digestion handling (Jarvis, 2012). The end products are methane, carbon dioxide and a digestate rest product that is rich in nutrients and can be used as a biofertilizer (Jarvis, 2012). Methane is highly diverse and energy rich gas which can be used for electricity and heat or as a high value environmentally friendly transport fuel after it is upgraded (Jarvis, 2012). Upgrading biogas requires the removal of corrosive components, particles and water and an increase of the energy content by removing carbon dioxide (Davidsson, 2008). For the biogas to be used as a transport fuel it needs to consist of at least 97 % methane (Jarvis, 2012). Methane also has multiple uses in various industrial sectors as a fuel or raw material (Jarvis, 2012).
Anaerobic digestion doesn’t require beach cast to be dried, instead the high water content (around 80-90 %) make it suitable for wet anaerobic digestion (Blidberg & Gröndal, 2012). The high water is also a challenge when using beach cast in anaerobic digestion processes as the organic loading rate (OLR) can get too high, leading to a stop of bacterial activity (Montingelli, 2014). The high water content also makes storage a complication. Another challenge for the use of beach cast in anaerobic digestion processes in the varying nutrient levels which varies from different algal species and seasons. For example, brown algae don’t have easily fermentable sugars in contrast to red and green algal species (Montingelli, 2014). This makes regular anaerobic digestion treatment non-viable and thus requires a pre-treatment were the polysaccharides are broken down into monomers before the hydrolysis step. Green and red algal species contain high levels of easily fermented sugars which increases the anaerobic digestion processes (Montingelli, 2014). The ash-content or non-degradable matter, which ranges from 10-40 % in macroalgae and seaweed highly affect the anaerobic digestion process. A low ash-content is correlated with a high carbohydrate content which occur when the photosynthesis activity is at its peak in the summer months and increase the methane yield as more organic material is available for digestion. The carbon to nitrogen ratio, (C/N ratio) is another important factor during aerobic digestion. Optimally the C/N ratio should be around 20-30,
macroalgae have a C/N ratio of around 10 which require the addition of other substrates to reach a
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suitable ratio (Montigelli, 2014). The amount of time that beach require to be properly broken down in an anaerobic digestion process is different depending on the species distribution. The
characteristics of different types of macroalgae and seaweed require a different type of treatment for anaerobic digestion (Montingelli, 2014). The formation of sulphuric acid during the anaerobic process is also a risk when using algae as a substrate for biogas production. The acid competes with the methane production and causes damage to equipment and may cause hearth issues when being exposed to it (Mentingelli, 2014).
Important for the overall financial and environmental performance of a biogas system is that it has a positive energy balance (Risén, 2014), that is that the total energy output is larger than the energy input. The largest energy input generally consists of electricity and heating inputs to support the system (Risén, 2014). Therefore, the efficiency of the biogas facility is highly important for the overall performance and especially the steps were methane is collected from the digestate storage tanks and when the methane is upgraded as leakages can easily occur there (Risén, 2014). From the biogas pilot plant in Trelleborg it was concluded that the digestate needs to be utilized as a biofertilizer for the system to have a positive energy, environmental and financial balance as the digestate otherwise had to be disposed of and its nutrients not recirculated (Bucefalos, 2016). The cadmium content in the digestate forbids landfilling per Swedish law and requires additional cost of incineration and management (Bucefalos, 2016).
Biogas potential of macro algae and seaweeds
For biogas production, it is important that the beach cast collected is as fresh as possible and clear of debris such as sand and stone (Blidberg & Gröndal, 2012; Bucefalos, 2016; Davidsson, 2008). Dry, partially decomposed beach cast used in anaerobic digestion require longer residence time to break down and provide lower yields of methane (Davidsson, 2008). As anaerobic digestion is a very complex biological process were substrates can enhance or inhibit the methane production a thorough characterization of the beach cast is needed to properly asses their potential and compatibility with other substrates there to be co-digested with (Davidsson, 2008). To achieve a positive energy balance for the whole biogas system the methane yield potential for algae is vital (Risén, 2014). The methane potential of beach cast species has been proven to be similar or higher than that of terrestrial biomass when taking growth rate into account (Bruhn et al., 2011). In the biogas pilot plant in Trelleborg the methane yield fluctuated between 80-200 𝑁𝑚3/𝑡𝑜𝑛𝑛𝑒𝑠. The dominant algal species used was red filamentous algae, Polysiphonia Fucoides who thrives in highly eutrophicated waters (Bucefalos, 2016). The methane yield from the red filamentous algae used in Trelleborg was concluded to be low compared with other species of macroalgae (Risén, 2014). The wide range of different properties in beach cast depending on species and geographical locations stress the need to create a specific system design depending on the locally available substrates (Risén, 2014).
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Table 3 methane yield potential from various species of macroalgae and seaweed Organic substrates Methane yield (𝑁𝑚3/𝑡𝑜𝑛𝑛𝑒𝑠
VS)
Source
Brown algae
Brown macroalgae 140-410 Blidberg & Gröndal, 2012
Laminaria spp. 260-280 Bird et al., 1990 from
Risén et al., 2014
Sargassum spp. 120-190 Bird et al., 1990 from
Risén et al., 2014 Red algae
Mixture of red filamentous beach cast algae
130-200 Blidberg & Gröndal, 2012
Mix of Polysiphonia fucoides and other filamentous red algae
80-200 Bucefalos, 2016
Red filamentous algae 210 Gregeby & Welander,
2012 Green algae
Ulva lactura 162-271 Bruhn et al., 2014
Seagrass
Zostera marina 150 Kaspersen et al., 2016
Terrestrial biomass
Tops of sugar beets, maize, timothy clover forage
270-370 Blidberg & Gröndal, 2012
Pig manure 310 Blidberg & Gröndal, 2012
Co-digestion with other substrates
One of the main benefits of biogas production is that the system can be used locally without required transports of substrates (Jarvis, 2012). Locally available organic materials can be used as substrate with often added benefits such a cost-effective way of disposing waste. As previously stated, one of the main advantages of beach cast as biogas substrate is its high moisture content that enables them to be mixed and co-digested with dryer materials and that it doesn’t contain hard materials that are difficult for microorganisms to break down (Blidberg & Gröndal, 2012). Finding suitable substrates that are locally available in large quantities all year around is a necessary step to get a suitable mix that produce high quality biogas (Kaspersen, 2016). Co-digestion often generates higher methane yields than if the substrates were to be digested separately (Jarvis, 2012; Davidsson, 2008). The biogas plant located on the Danish coast of Solrød uses 4 locally available substrates, among them beach cast that consist mostly of eelgrass (Kaspersen, 2016). The initial batch tests have proven that a mixture of pectin (a food additive which can be extracted from citrus peels), carrageenan which is extracted from locally cultivated red macroalgae, pig-manure and beach cast eelgrass has an average methane production of 312 𝑁𝑚3/𝑡𝑜𝑛𝑛𝑒𝑠 VS (Kaspersen, 2016). Beach cast consisted only of a small portion of the mixture, around 3 %, and would therefore not affect the methane yield or cadmium content in the digestate considerably, even if the methane yield from the beach cast would be lower or the cadmium content higher in relation to the other substrates (Kaspelsen, 2016). It has also been shown in batch tests conducted by Davidsson et al., (2008) that algae co-digested with sewage sludge
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and food waste generates higher methane yields than if the algae would be digested on its own, indicating that the nutrient content in the substrates could be complementing each other (Davidsson, 2008).
Anaerobic digestion residues as fertilizer for commercial use
Most of the organic material that is processed in an anaerobic digestion plant ends up as methane and carbon dioxide, but some that isn’t fully digested ends up as digestate of considerably smaller volume, consisting of solid materials (Jarvis, 2012). The digestate contains water, organic materials, nutrients, and microorganisms that makes it suitable as a biofertilizer (Jarvis, 2012). Heavy metals present in the substrate doesn’t break down either, which causes the digestate to increase in heavy metal concentration when the organic matter is broken down (Davidsson, 2008). For the biogas projects to be financially viable it is important that the digestate can be used as certified biofertilizer which in turn depends on the level of cadmium in the digestate (Davidsson, 2008). When algae are co-digested with other substrates with low amounts of heavy metals the digestate is diluted which results in potentially low heavy metals contents (Kaspersen, 2016). During the fermentation process no losses of macronutrients occur, but the effect on nutrient availability to plants and growth are not conclusive (Risberg, 2015; Möller, 2012). It has been shown in small scale pot experiments that anaerobic digested residues increase the plant availability of nitrogen, but the effect was not replicated in field experiments (Möller, 2012).
Both the case pilot biogas plant in Trelleborg and the under-construction pilot plant in Solrød have threshold for the limit value of cadmium that is allowed in the algae to use the residue as a fertilizer.
The limit was set to 1 mg/kg TS cadmium in beach cast used as substrate which was transgressed several times for the beach cast collected in Trelleborg and ultimately caused the pilot plant to be financially non-viable (Bucefalos, 2016). Other challenges for digestate utilization is the high sulphur content in the macroalgae and seaweed which can cause the digestate to be ill-advised for crops whereas a sulphur stripper might be needed if dilution with other substrates isn’t enough (Risén, 2014).
Cadmium cleaning methods
When the cadmium content in beach cast is considered too high for fertilization on arable land or when stricter laws could make beach cast non-viable, cadmium separation methods could be a way to circumvent that (Davidsson, 2008). For the pilot plant in Trelleborg it was concluded that cadmium separation methods were economically non-viable on a commercial scale as of now, but new
technology has the potential to lower the cost and thereby prove to be viable method in the future (Bucefalos, 2014). Cadmium separation can be done both before and after digestion but most practically before (Davidsson, 2008). Separation is done by extracting the cadmium from the solid materials through acidification and oxygen supply and thereafter removing it with suitable strategy which could be chemical precipitation, ion exchange and absorption (Jogbratt, 2011).
5.3 Beach cast as biofertilizer
Using beach cast as fertilizer is an old tradition used in coastal areas all over the world, Carl Von Linné reports of farmers on Gotland using Fucus Vesicolusis in the 1800th century (Greger et al., 2006). In the 1900s the use of beach cast was so popular that authorities had to regulate the usage.
There are many benefits of using macro algae as fertilizers as they provide the vital nutrients
phosphorous, nitrogen and potassium and important vitamins, trace elements and growth hormones (Blidberg & Gröndal, 2012). It has been shown that the growth of food crops fertilized with
macroalgae is greater than what could be expected from the nutrients that was supplied (Greger et
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al., 2006). This is believed to be explained by the growth hormones within macroalgae (Greger et al., 2006). When applied, algal fertilisers release its nutrients slowly while condition and aerate the soil, (Chojnacka, 2012). Especially nitrogen from beach cast have been shown to be responsible for the high growth rate of plants grown in its substrate. This was shown in an experimental study by (Greger et al., 2006) where edible crops grown in various substrates was compared. It was shown that crops have a considerably higher growth rate in beach cast compost compared to manure except for beans which produced lower biomass in beach cast in comparison with manure. The reason being is
thought to be that beans can fix nitrogen from the air. Beach cast also improves soil structure by increasing the content of humus (Weber-Qvartfort, 2016). Daniel Ahlsten, a conservation farmer on Gotland is primarily interested in beach cast for its soil improving capabilities that increases the water and nutrient holding capabilities. Low humus content in the soil can negatively affect the harvest yield and leads to increased runoff of nitrogen and phosphorus (Bertilsson, 2010). As more coal is bound to the soil, increasing the humus content is also a way to decrease CO2 levels in the atmosphere and mitigate climate change (Bertilsson, 2010).
Stranded beach cast can either be applied directly on arable land as a fertilizer, with or without pre- composting it, and/or be dried and macerated to create an algal meal (Chojnacka, 2012). The most common practice on Gotland is to compost the biomass in piles for one year and spread it the coming season (Landergren, 2017), but applying the biomass directly without pre-treatment by pressing it down into the soil also occurs (Larsson, 2017). The main benefit of pre-composting beach cast is the biomass stabilization which can improve land application strategies (Han et al., 2014). But using beach cast for composting also has several challenges that can hinder large scale application if not overcome. The major ones being low C/N ratio that can result in losses of nitrogen in the form of ammonia and nitrous oxide, low aeration due to low porosity and high water content (Han et al., 2014). Most macroalgae has a C/N ratio of about 8 to 11 while the recommended ratio for
composting is about 30, which means that macroalgae should be co-composted with biomass of high C/N ratios, such as animal manure and straw, to avoid wasting valuable nutrients along with smelling odours (Han et al., 2014). The salt content of algae can also inhibit the usefulness of algae as a fertilizer as some crops have a low salt tolerance (Karlsson, 2008). Fertilising crops with a low salt tolerance can therefore be avoided for that reason. Fertilization with beach cast is primarily valuable for ecological farming since the cost for conventional KRAV-approved fertilizers is high unless high quality and cheap animal manure is available (Harlén & Zackrisson, 2001).
