## 3 Results and Discussion
### The Topicality of the Problem and the Scope of Possible Solutions  
Biogas is a renewable fuel with favorable properties and parameters, enabling its wide use in virtually every economic area. Biogas is a fuel produced locally, most often from raw materials obtained nearby, and is an energy carrier that leaves a relatively small carbon footprint [28]. The agricultural industry's interest in small-scale biogas production allowed design companies to develop a prosumer concept for mobile installations, often with a simple container structure, ready to be placed in the economic infrastructure. Designing a specific micro-biogas plant is basically carried out individually for each farm. The biogas plants implemented so far are characterized by great diversity--both in terms of the equipment installed, the amount of biogas produced and the energy obtained from it. The main idea of the circular economy is to separate economic growth from resource consumption and environmental impact. The characteristic circular economy approach assumes minimization of the amount of waste generated at the project level and normally includes innovations throughout the entire value chain. Adequate waste management is essential not only to ensure healthy living conditions but also to mitigate climate change. Accordingly, the research on developing strategies to boost the circularity of waste management systems is ongoing. In this context, two waste streams are concurrently managed to recover energy and materials in the present study. Specifically, real leachate collected from a full-scale mature landfill site was preliminarily treated through active filtration to remove inhibitory substances partially and then tested, at the laboratory scale, as a nutrient solution for semi-continuous anaerobic digestion of a carbonaceous substrate represented by market waste. The results demonstrate that, at an organic loading rate of 1.0 gVS-L\({}^{-1}\).d\({}^{-1}\), the process was impossible without using the nutrient solution, while the nitrogen present in the pretreated leachate could balance the carbon content of the market waste and provide the system with the necessary buffering capacity, ensuring process stability. The average methane yield (approximately 0.29 NL\(\cdot\)gVS\({}^{-1}\)) was satisfactory and consistent with the literature. Despite the increases in both the organic loading rate (up to 1.5 gVS-L\({}^{-1}\).d\({}^{-1}\)) and volume of added pretreated leachate (up to 100% of the dilution medium), the process remained stable with a slightly lower methane yield of 0.21 NL\(\cdot\)gVS\({}^{-1}\), thanks to nitrogen supplementation. The potential use of produced methane as a renewable energy source and residual digestate as fertilizer would close the loop of managing these waste streams [40]. From an agricultural point of view, agricultural biogas plants are particularly important because they enable the management of all waste biomass and its conversion into useful energy and agricultural fertilizer. Target group, i.e., a group with access to the substrate (pig slurry) and possibly problems with its disposal for an optimal amount of livestock of 1100-2400 piglets or fattening pigs for one farm. Of course, the technology can be adapted to an inventory of less than 1100 piglets or pigs for one farm. The required technical and technological criteria for the production and processing of biogas were indicated. An example of pilot production of biogas for mesophilic fermentation using pig slurry is presented. The technology and a pilot installation adapted to a farm were presented. The course of changes in the volume flow of biogas for the average daily production of agricultural biogas and the qualitative composition of agricultural biogas produced from pig slurry indicated 80% CH\({}_{4}\). In this research, direct measurements were taken, and for this purpose, instruments for measuring gas flow, pressure (pressure difference) and temperature were used.