# Digestate Quality Originating from Kitchen Waste
## 1 Introduction  
Digestate is a stabilised residue and by-product from the methane fermentation process--i.e., the process of microbiological decomposition of organic waste, which is one of the methods of neutralising biowaste in anaerobic conditions with simultaneous production of biogas. Biogas plants process food waste from the agricultural industry or organic fraction of municipal solid waste (OFMSW), preferably selectively collected kitchen waste. The OFMSW is a heterogeneous mixture of an organic fraction contained in biowaste that includes food waste and kitchen waste, as well as brushes, leaves, grass, and other season-dependent yard waste [1]. It is derived from municipal waste and is most often selected at the source. In turn, kitchen waste is generally defined as leftovers from kitchen activities, such as peels, rinds, and other scraps from food processing produced in households. The morphology of kitchen waste is more homogeneous and less seasonal than OFMSW, making it a suitable substrate for anaerobic digestion [2], with high certainty that there will be no shortage of feedstock. After the anaerobic digestion (AD) process, the nutrients present in the reactor feedstock are converted into more easily assimilable forms for plants [3]. The resulting digestate is, therefore, a material rich in organic matter, as well as mineral compounds, including so-called 'primary macronutrients': nitrogen, phosphorus, and potassium, and'secondary macronutrients': calcium, magnesium, sodium, and sulphur [4]. A valued raw material for agriculture is created as a by-product [5], which after further processing can be used for nutrient enrichment and as a soil texture improper or fertiliser. According to the currently promoted European Union (EU) policy, organic fertilisers, which include digestate, have the potential to initiate changes in the modern agricultural and farming sector, as they are a good and sustainable alternative to traditional mineral fertilisers. Fertilisers from digestate provide nutrients to plants or fungi. Their application is also in line with the assumptions of the circular economy since digestate may enrich the soil on which new food products will grow. These will eventually end up as substrates for anaerobic digestion, and hence nutrients will be allowed to be recycled and the loop will be closed. Digestate usage provides both economic and environmental added value. The treatment of selectively collected organic fractions in the anaerobic digestion process reduces the amount going to landfill. This is relevant concerning current EU objectives, which state that by the year 2023, only 10% of all municipal waste produced can be disposed of in landfills (EC, 1999), 65% of MSW must be recycled, and biowaste will have to be collected separately [6]. Additionally, the EU Fertiliser Product Regulation, which entered into force in 2022, creates conditions conducive to the use of digestate [4]. The management of digestate is important not only in the environmental dimension but also from the economic and social point of view. The recently observed economic slowdown, as well as the increase in gas prices, which translated into an increase in fertiliser prices, and their limited or even suspended production, which temporarily took place in 2022, further support the need for the widespread use of digestate as an alternative source of fertiliser. It is estimated that over 500 kg [7] of municipal waste per person is produced annually in the EU, of which approximately 34% is the so-called biowaste, including waste from gardens and parks, as well as kitchen waste from individuals, but also from offices, restaurants, wholesales, canteens, and retail premises [8]. In the European Union area, up to 60% of the overall municipal biowaste is comprised of food waste from households, general kitchen waste, and that from the retail sector [9]. In 2020, the European Union inhabitants produced on average up to 70 kg of food waste in their households [10].