# Digestate Quality Originating from Kitchen Waste
## 2 The Quality of Digestate  
The quality of feedstock determines the quality of a digestate. It should be properly collected and sorted to ensure a safe and usable composition. Studies confirm that the quality and safety of digestate are closely related to the quality of the material directed to the methane fermentation process, which determines the applicability and profitability of digestate [12; 13; 14; 15]. It is advantageous when the digestate comes from biogas plants that operate based on material from the agri-food industry, to which the feedstock is supplied by specific companies, in the form of, e.g., residues from filtration and extraction processes, such as whey or fruit pulp. The feedstock from such plants is most often better than non-homogeneous feed from selected municipal waste because it is known, repeatable, and free of undesirable contaminants. Thus, the biogas plant can achieve higher and predicted efficiency of methane production, and the waste itself in the form of digestate will more reliably meet the standards and legal requirements in terms of safety of further use. The downside limiting the use of raw materials from the agri-food industry, especially in the case of fruit and vegetables, is their seasonality. In turn, the selectively collected at-source kitchen waste is not subjected to seasonal fluctuations and also has a homogeneous composition. Those properties make feedstock pretreatment not a critical issue, and kitchen waste is an attractive feedstock for large and regional biogas plants. Undesirable substances negatively affecting quality should be considered both in terms of presence in the process input and unwanted residue in the post-fermentation mass. They might reduce the efficiency of the methane fermentation process and biogas production, or not react and end up in the post-fermentation material, limiting the possibility of digestate further use as a valuable material in fertilisation or making it unsuitable for further use at all. On the one hand, these will be substances that are toxic to microorganisms involved in the AD process, e.g., antibiotics or other toxins, and on the other, substances that pose a threat if further used in the environment, i.e., pathogens, heavy metals, plastics, and other foreign solids such as metals or glass particles [16; 17; 18]. In the case of delivering food waste to a biogas plant, an effective automated de-packaging system is needed in order to ensure that pieces from packaging will not contaminate feedstock. In the case of kitchen waste, the situation is simpler, because the waste collected from households most often is contained in one package. In addition, depending on the regulations that are in force in each country or region, waste can be collected into paper bags or so-called crafted bags instead of traditional plastic ones. Crafted bags are gaining in popularity, especially in regions where it is estimated that kitchen waste will end up in industrial composting plants. Examples are Mater-Bi(r) (Novamont S.P.A., Novara, Italy) or Ecopond(r) (Kingfa Sci. and Tech. Co., Ltd., Guangdong, China) bioplastic materials, which according to the information on the packaging are 'commercially compostable'. Generally, it could be assumed that the digestate from the anaerobic digestion process, in which an organic fraction selected from municipal waste is used as a reactor feedstock, will require additional processing to meet the quality requirements for further use in agriculture. The main limitation may be the presence of undesirable substances and those whose fate in the process is not fully investigated. In this regard, one of the solutions is to study how to increase and control the degradation efficiency of biodegradable plastics. A team from the University of California, Berkeley, and Lawrence Berkeley National Laboratory is working on a solution to prevent the forming of microplastics from biodegradable plastics by adding small amounts of appropriate enzymes and components to polylactic acid and polycaprolactone [19; 20].