## 1 Introduction
Rapid VFA production, if not properly managed, can lead to accumulation in the AD reactor and digestate toxicity. Acetogenic bacteria are responsible for the next stage, called acetogenesis. In this step, acetate, CO\({}_{2}\) and H\({}_{2}\) are generated from the products from the previous step and some of the long-chain fatty acids from the first step. In this process, excessive H\({}_{2}\) concentration can slow down the AD process, but methanogenic bacteria consume the hydrogen produced. This is an example of the need for cooperation between various types of microorganisms needed for the effectiveness of the AD process, activating at various operational stages. The final stage is called methanogenesis, where methane is produced from all the intermediate products present in the digester. Acetate and hydrogen are converted to CO\({}_{2}\) and CH\({}_{4}\). This process is strictly anaerobic because the acetophilic and hydrogenophilic bacteria responsible for this reaction cannot survive in the presence of oxygen. For an efficient AD process, it is crucial to optimize several process conditions because AD bacteria are sensitive to any changes occurring in them. Some critical parameters are temperature, pH, organic loading ratio (OLR), hydraulic retention time (HRT) or carbon-to-nitrogen ratio. OLR is a parameter that refers to the input rate of organic raw material per unit volume of the digester. It varies depending on the type of substrate used for biogas production and is determined experimentally. HRT refers to the average time that the raw material remains in the digester. Optimally, the substrates should remain in the digester for a long enough time to allow for the greatest possible breakdown of organic matter into biogas. Another parameter that strongly influences the AD process is the carbon-to-nitrogen ratio in the substrate. Microorganisms use carbon as an energy source, and nitrogen is necessary for their growth and metabolism. A low C/N ratio can lead to overproduction of ammonia, which inhibits the process and thus reduces the efficiency of biogas production. On the other hand, too high a C/N ratio may result in insufficient nitrogen concentration for bacterial metabolism and growth. A very difficult parameter to define is pH because different types of bacteria present in the digester require or prefer different pH for optimal performance. Maintaining the optimal pH for all microorganisms present in the same reactor is a difficult task and even more complicated if the substrate has a different nutrient composition [16]. Without a doubt, the outstanding leader in terms of biogas production and the number of biogas plants in Europe is Germany, which already had over 10,000 functioning biogas plants in 2020. In Germany, 52,158 GWh of energy was produced annually, and in 2018, almost 13% of renewable electricity was produced from biogas and was mainly used to produce electricity and heat. The German model is based on corn silage as the main substrate for biogas production. There is significant potential for the use of small AD plants fed mainly with slurry; however, plants using energy plants as a substrate have a higher electrical potential [17]. There are over 600 biogas plants in operation in Great Britain, Switzerland and France [18; 19; 20]. In the UK, most of the substrate used in biogas plants is sewage due to regulations, and according to these regulations, no more than 50% of the raw materials used can be energy-producing plants. There are also significant limitations regarding the type of installation that can be used to produce biogas and the place where it is produced [19]. In Switzerland, the substrates are similar, and although the number of operating power plants is similar, the reported energy production was almost 3 times lower [20] compared to the energy generated in France. In France, for several years now, there has been a strong development of biogas plants on farms and centralized ones, the purpose of which is to recover biogas to produce electricity for self-service.