Energy Production from Biodegradable Waste as an Example of the Circular Economy

: A growing population, technological progress and economic development result in a constant increase in energy demand. Energy is mostly obtained from fossil energy resources such as coal, natural gas, and crude oil. Burning them leads to air pollution with greenhouse gases (CO 2 , CH 4 , NH 3 and N 2 O) and dust (PM 2.5 and PM 10 ). They are recognized as the cause of global warming and air pollution. Wind, water, solar and biomass energy are used to eliminate harmful emissions. The latter may come from special plant crops or from biodegradable waste from farming, animal husbandry, the agrifood industry and households. These wastes are transformed into biogas in biogas plants, the basic ingredient of which is methane. Most often, biogas is burned in a cogeneration process, providing electricity and heat. After puriﬁcation of admixtures, it can be injected into the high-methane gas network or converted into hydrogen in the steam reforming process. In this way, environmentally harmful waste becomes a raw material for energy production, which is an example of a circular economy. The article discusses the functioning of biogas plants in selected EU countries. The current biogas production in Poland was assessed and compared with the production potential of dairy farms. The aim of this article was to show that the production of biogas reduces the emission of greenhouse gases into the atmosphere and the electricity produced from it is not burdened with the cost of purchasing CO 2 emission allowances applicable in the EU.


Introduction
Energy is required to make the products people need. The growing population, technological progress, and economic development result in a constant increase in energy demand. The energy sector produces almost 90% of CO 2 emissions and 75% of total greenhouse gas (GHG) emissions in developing countries. About half of these emissions arise from the combustion of fossil fuels in power plants and refineries [1]. In developed countries, energy producers and agriculture constitute the main emitters of greenhouse gases, generating 25.9% and 13.5% of annual global emissions respectively [2]. Obtaining energy almost exclusively from fossil energy resources pollutes the atmosphere with greenhouse gases (CO 2 , CH 4 , NH 3, and N x O) and dust (PM 2.5 and PM 10 ). Greenhouse gases increase the temperature of the atmosphere and oceans. The rapid increase in CO 2 content is causing climate change and sea level rise, which endanger people living in coastal regions and island countries [3][4][5][6].
Manufacturing of products and their use as well as meeting human existential needs (food, clothing, heating) generate waste. The increase in the amount of waste and the difficulties connected with its management are becoming a problem that can be effectively solved in accordance with the principles of the circular economy. In this system, waste can become a useful resource [7]. The production of electricity from biodegradable waste is an example of the benefits of the circular economy. Biogas plants are also a part of the concept of sustainable development, bioeconomy, and green economy [8]. H 2 S mg/m 3 100-900 1000-4000 3000-10,000 100 NH 3 --50-100 400 Source: Own study based on [11].
The biogas yield and its composition are also influenced by the fermentation temperature, retention time, decomposition technology, and raw material pretreatment [12,13]. In order to counteract the further increase of greenhouse gases in the Earth's atmosphere, intensive development of technologies enabling the use of renewable energy sources (RES) has started. In recent years, the greatest progress was made in the production of electricity from wind and the sun. However, due to the variability of weather conditions, energy systems cannot rely solely on these sources. It is, therefore, necessary to build energy storages and increase the possibilities of the production of electricity from biogas [14]. Biogas plants, unlike other RES, are multi-energy systems as these provide biomethane that can be used to generate energy in various processes. Currently, it is most often burned in cogeneration units producing electricity and heat. After cleaning, it can be injected into natural gas networks or, in a compressed form, used as fuel in cars with CNG installations [6,12,13,[15][16][17]. After steam reforming, it can become a source of biohydrogen used as fuel in fuel cells. Due to the development of renewable energy sources (including biogas plants), electricity can be generated close to end users. This, combined with the possibility of storing energy, reduces the losses associated with its transmission, diversifies energy sources, and reduces dependence on fossil energy resources [12]. Japan can serve as an example of this approach. A self-sufficient energy farm project was created there, in which electricity and heat will be produced, as well as fuel that will fully meet the farm's own needs [18].
Many developing countries comprise predominantly municipal landfills where unsorted waste is collected. A large part of that waste is biodegradable. Such a method of collection is very burdensome due to the limited availability of land, limited resource management, and the need to reduce greenhouse gas emissions to the atmosphere. Alternative methods of processing biodegradable municipal waste can be applied, such as [19,20]: pre-composting before landfilling, • biogas production.
Composting and pre-composting produce compost and liquid fertilizer used in agriculture. Electricity is generated during landfill biogas capture and biogas production in biogas plants and then sold to a power grid [19,21,22].
A study by the World Bank showed that about 50% of the organic waste produced could be used for biogas production. Based on the level of income of the population, it is estimated that in 2025 half of household waste will contain 50% of biodegradable waste [23].
Biogas plants are the best way to manage organic waste generated in the agri-food and municipal sectors because these reduce the amount of greenhouse gases emitted into the atmosphere by burning fossil fuels. Therefore, they are a part of the circular economy. This concept organizes agricultural production according to the following principle: resourcesagricultural products-renewable energy sources, focusing on recycling and re-use of waste and by-products instead of traditional and extensive production. Circular agriculture requires the implementation of several changes, namely the reduction of the resource and energy consumption, maximization of their use in production and consumption systems, and the reduction of waste and pollutant emissions. Integrated production systems are created, such as: "grain production-pig breeding-fish breeding", "cattle breeding-biogas plant-vegetable growing", or "pig breeding-biogas plant-fish farming" [24][25][26][27].
In a biogas plant, the fermentation process utilizes waste generated during agricultural production, including plant cultivation, horticulture, and livestock breeding. All these release large amounts of GHG into the atmosphere. On livestock farms, due to the regulations on nitrates, slurry is stored through the winter. Only in spring is it used as a fertilizer on arable fields [2]. Livestock farms generate large amounts of waste in the form of animal manure, which is a source of greenhouse gas emissions [28]. Traditionally, manure is used directly for natural fertilization. Chicken manure can serve as an example, as it is spread as fertilizer on farmland or grassland directly, without additional preparatory steps. The disadvantage of such use of chicken manure is the emission of NH 3 and N 2 O to the atmosphere. An alternative is to use it as a substrate in a biogas plant. During methane fermentation, manure is converted into biogas and into a by-product of the process, i.e., digestate. Research by Meyer et al. proved that converting manure into biogas is associated with reduced greenhouse gas emissions reaching the atmosphere [29]. Kreidenweis et al., also proved that greenhouse gas emissions are reduced when broiler manure is used to produce biogas. Methane emissions are lower because the manure is neither stored in the air nor composted, resulting in higher emissions than is the case with the storage of untreated chicken manure [6]. This method of producing renewable energy from biogas is promoted in European countries such as Germany, Switzerland, and Austria [2].
Biogas can also be produced from organic waste from grocery stores: from fruits, vegetables, flowers, dairy products, meat waste, and confectionery. Kitchen waste and other municipal biodegradable waste can be used to produce biogas as well. Research has shown that they can be used to obtain biogas containing from 44.3% to 75.4% methane. The temperature that should be taken into account when fermenting food waste is also an important factor in the process [23].
The aim of this article is to demonstrate that the production of biogas from biodegradable waste and the reduction of the emission of greenhouse gases into the atmosphere allow us to reduce the costs of electricity generation which stem from the system of purchasing CO 2 emission allowances in force in the EU. This goal will be achieved through the analysis of the operation of biogas plants in the EU countries based on literature. Based on data from the Central Statistics Office and Energy Regulatory Office, the current biogas production in Poland was described. For dairy farms, an attempt was made to estimate the potential of biogas production. A possible reduction from the lack of burdening it with the costs of purchasing CO 2 emission allowances.

Emission Allowances Prices in the EU and Their Impact on the Price of Electricity
The total amount of greenhouse gases that can be emitted by industry, power plants, and other fixed installations covered by the EU Emissions Trading Scheme (EU ETS) is limited by the cap on emission allowances. Each allowance gives its holder the right to emit [30]: • one ton of carbon dioxide (CO 2 ), the main greenhouse gas; or • the equivalent amount of two more powerful greenhouse gases: nitrous oxide (NO 2 ) and perfluorocarbon (PFC).
Each year, the allowance ceilings for fixed installations are reduced. In 2013, the ceiling was 2,084,301,856 tons of CO 2 . In the third phase of the EU ETS (2013-2020), its level was reduced each year through the application of a linear reduction factor of 1.74% of the average total number of allowances issued annually in 2008-2012. This resulted in a reduction of 38,264,246 tons of allowances per year, i.e., the number of allowances that can be used in fixed installations was 21% lower than it was in 2005. To achieve the target of reducing emissions in the European Union by 40% by 2030 compared to the year 1990, from 2021, the level of allowances is reduced by 2.2% each year. This will reduce emissions from fixed installations by approximately 32% by 2030 as compared to 2005 levels. By 2050, emissions should be reduced by around 90% from 2005 levels [31]. Auctioning is the primary allocation method under the EU Emissions Trading System (EU ETS). Companies wishing to use solid fuels have to buy an increasing percentage of emission allowances at auctions. In the opinion of the European Commission, this is the clearest method of allocating emission allowances, as it follows the "polluter pays" principle. Pursuant to Art. 10 of the Emissions Trading Scheme Directive, in 2021-2030, 90% of emission allowances for auctioning will be allocated to the EU Member States based on their verified emissions and 10% of allowances will go to poorer EU countries to foster economic growth and cohesion [31]. Figure 1 shows the price of CO 2 emission allowances, which stood at EUR 25.31 on 1 January 2020 and at EUR 82.76 on 12 December 2021, i.e., it increased by 227.98%. During this period, the price of allowances for Polish companies increased from 107.77 zł to 381.78 zł (an increase of 254.25%). One of the largest investment banks, Goldman Sachs, predicts that carbon prices will grow by around 7% annually in the coming years. More pessimistic scenarios assume that prices could rise to EUR 100. The prices of CO 2 emissions significantly affect the prices of electricity [33][34][35]. Figure 2 shows how the average prices of 1 MWh changed in Poland between 2010-2021. In 2010, the price amounted to 45.95 EUR/MWh and increased to 55.93 EUR/MWh in 2021. The price of electricity increased to a lesser extent than the price of CO 2 emission allowances, which, as shown in Figure 3, is the result of almost unchanged coal prices between 2007-2020.   In the EU countries, the price of electricity becomes more dependent on the price of CO 2 emission allowances than on the prices of energy resources such as coal and natural gas. The prices of allowances will increase because the climate package of the European Commission "Fit for 55" assumes that in 2030 CO 2 emissions will be reduced by 55% compared to 1990. In order for energy prices not to grow at such a rapid pace, all renewable energy sources should be used, with particular emphasis on biogas plants. They transform biodegradable waste, which is burdensome for people and the environment, into energy that does not increase the level of carbon dioxide in the atmosphere.

Biogas Plants in Europe
The biggest number of biogas plants can be found in Asia [9,17]. The plants there are usually small installations that provide an alternative source of energy for cooking and lighting in rural areas [40]. According to Pathak, a farm in India with four cows produces 2300 m 3 of biogas, which replaces 316 L of kerosene and 5535 kg of hardwood, whose global warming potential (GWP) is 762 and 10,571 kg of CO 2 , respectively [41]. In Poland, as in other EU countries, biogas is mainly used to produce electricity, which must have parameters that allow it to power professional energy networks. Therefore, the further part of the study will only consider the operating conditions of biogas plants in Poland and other EU countries.
The majority of biogas is produced in biogas plants from a variety of substrates. It is also collected in landfills. Biogas is a renewable energy source. It can be used in various ways, as shown in Figure 5. It is most often burned in cogeneration units with 35% electrical efficiency and 55% thermal efficiency. In the absence of heat recipients, a better way to use biogas could be its purification to the high-methane gas parameters specified in the standard [42] and injection into a gas network, compression allowing for the use in vehicle propulsion, or the production of hydrogen after reforming [43][44][45][46][47]. • agricultural-using biomass from target crops, manure from farm animals, • waste from the agri-food industry, • in sewage treatment plants-using organic waste separated from municipal sewage, • on landfills-collecting biogas generated in landfills, • municipal-processing selectively collected biodegradable municipal waste, • mixed (other).
At the end of 2019, there were 18,943 biogas plants in Europe, which generated 167 TWh of electricity from 15.8 billion m 3 of biogas [48]. Figure 6 shows that the majority of new biogas plants were built in 2010-4281, and 2015-2319. The largest biogas producer is Germany, where 11,454 biogas plants of all types were operating in 2018 (see Figure 7). The second country in terms of the number of biogas plants is Italy with 1620 installations. In other EU countries, there were fewer than 1000 biogas plants. In 2020, biogas plants in Germany had a total capacity of 9.301 MW [52]. For the production of biogas in this country, very large amounts of maize silage are used, which makes it possible to obtain large amounts of ethane biomass per mass unit of this substrate. This resulted in a reduction in the production of plants for animal feed and food for humans. As a result, food had to be imported from neighboring countries such as Poland. From the beginning of the 90s, mainly high-power biogas plants were established in Germany, which also caused soil sterilization and degradation of the natural environment. Since 2009, subsidies for small biogas plants have increased. New forms of bonuses for biogas plants producing biogas from manure were also introduced. As a result of this tactic, the deliberate cultivation of maize was limited and the variety of crops intended for energy purposes increased [53,54].
In Austria, the climatic conditions are similar to those in Germany. 427 biogas plants were operating there in 2019, including 190 agricultural plants, 100 at sewage treatment plants, and 15 landfills. The total reported energy production was 1.487 GWh. Since 2003, thanks to the Green Electricity Act, there has been an increase in biogas production on the Austrian market. Many installations were built between 2003 and 2005 and subsidies for their operation were granted for a period of 13 years [48].
In Italy, 2.5 billion m 3 of biogas is produced in nearly 1600 biogas plants, which use waste and side products from the agri-food sector as substrates [55]. The oldest biogas plants in Italy are high power (over 5 MW). They are usually located in landfills and generate landfill gas. Most of them are agricultural biogas plants with a capacity of up to 1 MW. There is also a new type of installation powered by biodegradable waste, which most often has a capacity of up to 0.3 MW [56]. In Italy, 67% of biogas is produced from animal excrement and agricultural and forestry activities, the rest is biogas produced from sewage sludge. Most installations produce biogas which is burned in cogenerators. There are also drying and CO 2 biogas purification installations which produce biomethane with a composition similar to that of natural gas. The upgraded biogas is then injected into a natural gas grid or sent to a standalone compressed natural gas (CNG) distribution system. Italy is one of the few European countries where biomethane can be injected into the public gas grid (Green Gas Grids). According to Gatti et al., such a solution is more economically viable than the production of electricity and heat in the cogeneration process [57]. However, despite political incentives to promote the production of biomethane, such a solution is not very popular.

Biogas Plants in Poland
Poland is a European country with a high potential for biogas production, where three types of biogas plants operate: agricultural biogas plants, biogas plants in sewage treatment plants, and biogas plants in landfills. Almost all biogas produced is burned in cogeneration units producing electricity and heat. Electricity is sold to a power grid, while heat is used for fermentation processes, and the surplus is sold to consumers.
According to the data of the Energy Regulatory Office in Table 2 In these biogas plants, the utilization rate of the installed capacity decreased from 23.57% in 2018 to 8.31% in 2020. This decrease can be associated with the effect of the obligation to collect biodegradable municipal waste in force since 2017 [58]. Biogas plants in wastewater treatment plants maintained fairly stable electricity production levels and the installed capacity utilization ratio oscillated around 40%. In the period discussed, the largest increase (25) occurred in agricultural biogas plants. The energy production in 2018 and 2019 was similar and exceeded 0.56 GWh. However, in 2020, it dropped to the level of 0.508 GWh.
Also, the power utilization ratio, which in agricultural biogas plants in 2018 and 2019 exceeded 64%, in 2020 decreased to 49.19% [59]. The difference in the amount of electricity produced may be related to the quantitative changes in the substrates used, as shown in Table 3. Differences in consumption may result from a different structure of crops and their different productivity. The reason for this may be that some substrates can be used more efficiently for other purposes (e.g., beet pulp can become feed for cattle).   Table 3 shows that Polish agricultural biogas plants mainly process waste and maize silage from special purpose crops and it is about 10% of the mass of all substrates. The consumption of maize silage in 2018-2020 was almost unchanged as compared to 2016 when it stood at 439 tons [61]. In the discussed period, the share of out-of-date food in agricultural biogas plants increased by 62.8%, fat by 46.6%, and the amount of processed technological waste from the agri-food industry by 23%.
In Polish biogas plants, biogas is almost exclusively burned in cogeneration units, which produce electricity and heat. Between 2011-2015, agricultural biogas producers were required to provide the National Center for Agricultural Support with information on the amount of heat generated. This obligation was lifted by the provision of the Act of February 20th, 2015 on renewable energy sources [62]. The amounts of energy produced from biogas are very small. It is estimated that about 5 billion m 3 /year of biogas with the parameters of high-methane natural gas can be produced from waste generated in agriculture and the agri-food industry [63]. The number of launched biogas plants is small in relation to the assumptions adopted in 2010 in the document of the Council of Ministers entitled "Direction of Development of Agricultural Biogas Plants in Poland in 2010-2020" [64].
The document states that by the end of 2020 there will be an average of one biogas plant in each commune. The program failed as there are approximately 2200 municipalities in Poland. This situation in the development of agricultural biogas plants is caused by the low profitability of investments and their long cycle. Low profitability results, among other factors, from the adopted definition of agricultural biogas, which makes it difficult to use municipal biodegradable waste. The long cycle of investments is related to the problems of locating large biogas plants in provinces with a high percentage of small farms [63]. Changing the definition of agricultural biogas and increasing the number of small and agricultural micro-biogas plants is necessary to increase the production of biogas and to obtain biomethane from it. It would reduce the costs of electricity production caused by the need to purchase CO 2 emission rights.4. The potential of biogas production from farm waste.
In 2020, 508,381 MWh of electricity was obtained from biogas produced in agricultural biogas plants from 4,409,055 tons of substrate mass, as shown in Table 2. The substrates included 759,774 tons of slurry and 91,681.445 tons of manure. There is no information in Table 3 whether it was cattle or pig faeces. This is an important piece of information because Table 4 shows that the yield of biogas from 1 tons of pig manure is over 25% higher than that of 1 tons of cattle manure. Assuming that animal faeces used in 2020 were 50% from cattle and 50% from pigs, they allowed for the production of 32,165,138 m 3 of biogas containing 19,299,083 m 3 of biomethane. Combustion of this amount of gas in cogeneration units allowed for the production of 65,600,578 MWh of electricity, i.e., 12.9% of electricity generated in biogas plants.
The share of animal excrements in the total mass of substrates was very small because in 2020 the livestock bred in Poland counted 6,278,896 heads for cattle and 11,727,410 for pigs. It is not possible to estimate the amount of manure and slurry produced precisely because there are different types of barns and pigsties, and some cattle are grazed on pastures in summer. Furthermore, it is not possible to use 100% of the manure and slurry produced. On farms with several cows or several dozen porkers, the construction of a biogas plant would be an economic absurdity. The transport of manure from small farms to biogas plants installed on larger farms would be uneconomical due to transport costs and burdensome for the environment due to odors.
In order to estimate the supply of cattle manure that can be used in agricultural biogas plants, the population of dairy cows was analyzed. Due to the feeding method (feeding with Total Mix Ration mixtures) and milking technology, the herds of such cows spend most of their time in barns. In 2020, there were 2,125,694 such cows. In 2020, there were 2,125,694 such cowsheds with more than 20 heads house 1,482,747 cows, i.e., 69.75% of the dairy cows' population. Cowsheds of this size have a litter-free system that allows them to collect the slurry. Using the data contained in Table 4, the potential possibilities of biogas, biomethane, and energy production from slurry from barns with more than 20 dairy cows were assessed. The results of the calculations are summarized in Table 5. It shows that only the slurry produced in cowsheds with more than 20 dairy cows had the potential to produce 874,078 MWh of electricity. Such an amount is much more than what the agricultural biogas plants produced in 2020 from all substrates. By burning biogas in cogeneration units, it is also possible to obtain a net 961,436 MWh of thermal energy. In total, the slurry from these barns can be used to obtain 1,835,514 MWh of electricity and heat. The production of biogas (and thus electricity and heat) may be higher because the barns, apart from dairy cows, also house heifers and calves which are not included in a dairy cow herd.   [66]. For the calculations, the price of CO 2 emission allowances was adopted, which on 12 December 2021 amounted to PLN 381.78/ton (82.76 EU/ton). Table 6 presents the calculated amount of reduction of the production costs of such amount of electricity and heat for sale that can be obtained from biogas produced from slurry from the analyzed dairy herds (see Table 5). Table 6. Reduction of the costs of electricity and heat production from biogas in relation to fossil fuels.

Description
Unit Coal Lignite Natural Gas The reduction of electricity production costs shown in Table 6 is possible under the current legal status. The reduction of heat production costs will be possible after the obligation to provide the National Agricultural Advisory Center with information on the amount of heat produced, which was abolished in 2015, is restored [63]. The level of thermal energy cost reduction will also depend on whether all the heat generated is used. Due to the fact that all types of biogas plants are located away from human settlements, there are problems with using all thermal energy from cogeneration.

Conclusions
The role of renewable energy sources in the energy mix must increase. The growing demand for electricity and heat cannot be met only by fossil fuels, the combustion of which increases the amount of CO 2 in the atmosphere. Renewable energy sources such as solar and wind have enormous potential, but due to their volatility, they must be supplemented by energy storage and more stable sources. Biogas plants, in which the biogas produced can be burned on an ongoing basis in the cogeneration process, but also stored, purified to the parameters of natural gas, or converted into hydrogen (biohydrogen) in reforming processes, are considered to be a stable source. Purified biogas can be compressed and used in combustion engines of agricultural vehicles and agricultural machines. Biohydrogen could be used for the same purposes as well as in zero emission fuel cells.
In Polish conditions, agricultural biogas plants located on farms engaged in animal husbandry have the greatest potential to increase biogas production. In such biogas plants, manure is transported directly from the barns to the installation, which reduces the costs of waste transport and odor emissions. The production of electricity and heat in combination with the production of electricity from photovoltaic panels can make a livestock farm selfsufficient in terms of energy and even bring additional income from their sale. Operating and servicing a biogas plant is also an opportunity to increase non-agricultural employment in rural areas.
The possibility of further development of biogas plants and the increase of the production of biogas depend largely on the change of regulations. Currently, they do not allow agricultural biogas plants to use municipal kitchen waste as substrates. Their addition to the slurry would increase the biogas yield (see Table 4). This solution is used, for example, in Denmark in Central Anaerobic Digestion (CAD) biogas plants.
The production of biogas, a renewable energy carrier, from biodegradable waste can also solve the problem of waste management, becoming the best example of a circular economy.