## 1 Introduction
For example, in 2017, 113 out of 240 landfills used biogas. It was found that approximately 47% of the recovered energy is converted into electricity, 43% into heat and almost 10% into biomethane. The raw materials used to produce biogas in France are residues from sewage treatment plants, bio-waste, agricultural and industrial waste and landfills [18]. In Austria and Sweden, almost 300 biogas plants operated in 2020 [21; 22]. While in Austria, most of the raw material used consists of manure and organic waste, in Sweden, the focus is on the utilization of sewage sludge, which constitutes almost 50% of the substrate used. Additionally, bio-waste, agricultural and industrial waste and landfills are used [22]. Comparing the amount of energy produced annually, in Sweden, the production efficiency was 3.4 times higher than in Austria. Austrian regulations require that at least 30% of the substrate used for biogas production be manure in order to obtain the feed-in tariff. If organic waste is used, this tariff may be reduced by 20% [21]. In 2018, the Netherlands had 250 reported biogas plants. Although this number remained almost the same (252) until 2019, the energy efficiency of this installation was truly impressive. There were only 95 co-substrate installations registered in the report dated 2018, while the annual energy production amounted to 3720.83 GWh/year [23], which is a number similar to the result achieved by over 600 French biogas plants. Most Dutch digesesters use local agricultural and animal waste as raw material for biogas production. Many facilities are technologically adapted to refine biogas into biomethane, which can be used as a substitute for natural gas and can be directly injected into the natural gas network. In Denmark, the number of biogas plants (which in 2020 was 166) is over 100 fewer than in Sweden and Austria, and the amount of energy produced is very comparable to that achieved by its Scandinavian neighbor and much higher than in Austria. There is a strong initiative to fully utilize digestate from agricultural plants, which is further used as fertilizer for crops. In Denmark, by 2020, the use of no more than 12% of energy crops for the production of biogas as an AD raw material is also limited. The new regulation was intended to further reduce this limit [24]. There were 116 biogas plants operating in Poland in 2020 [25], which is a significant increase since 2014 when there were 45 operating installations [26]. The raw materials used for production are mainly agricultural waste, manure and energy plants. In recent years, there has been a systematic increase in the consumption of waste raw materials used for the production of agricultural biogas. The percentage of substrates such as corn silage, cereals or fruit and vegetables used in Polish biogas plants in 2019 was only 10.6%, which is the lowest result since the beginning of the development of biogas plants in this country. The energy generated this year from biogas plants amounted to 306,396 million m\({}^{3}\). The lowest number of biogas plants are located in Norway. In this Scandinavian country, there were 46 biogas plants operating in 2020, of which 6 are farm-scale, 13 are modernized to produce transport gas and 2 are connected to the local gas network. About 40% of the extracted gas is used for transport purposes, and the total production in 2019 was 922 GWh. The dominant substrate used for the operation of biogas plants in Norway is sewage sludge and food waste [27]. The development of the biogas industry in Europe is visible, although the dynamics differ depending on the country. Various models were presented, which are based on different types of raw materials used for biogas production and the proportions of substrates used in co-fermenters. Of course, Germany is the undisputed pioneer in the field of this renewable energy source.