# Digestate Management and Processing Practices: A Review
## 4 Digestate
Authors concluded that spore-forming bacteria, as well as _Listeria monocytogenes_, _Salmonella_ spp., and enterococci, can persist during AD, but the concentration of the pathogens was similar or lower in DGs than in liquid manures. Therefore, further treatment of the DG is recommended to obtain a more efficient reduction of pathogens. Since the DG is often stored before being spread on agricultural land, the potential regrowth of pathogens during the storage may also occur. Therefore, [81] attempted to find a technical solution to either eliminate or reduce the content of spore-forming pathogen _Clostridium_ spp. in the stored solid DG so that it could be safely used as fertilizer or amendment. They converted the solid part of DG into pellets and after microbiological analysis, the results showed complete elimination of _Clostridium_ spp. in pelleted DG samples. HM contents in DG can vary widely (Table 4). These variations are related to the input materials, which are usually classified as green waste (organic waste from private gardens and public green areas) and bio-waste (green waste and food and kitchen waste from households, restaurants, caterers, retail premises, and waste from food processing plants). In addition, the origin of the input material could be important because the aerial deposition of HMs is usually higher in urban environments than in rural regions, while the season of input material collection might play a role, as the deposition of HMs tends to peak in winter [82]. In recent times, the DG composition has been increasingly studied at the molecular level. Such data may be useful in identifying structural changes during the mineralization of OM [83]. For these purposes spectroscopy techniques like nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) have been proposed and most commonly used. These methods can selectively identify different types of C and H and therefore enable the comparison of different concentrations of main functional groups [84]. According to [85], during AD most OM is converted into biogas, while the biological stability of remaining OM increases during AD with the increase of more recalcitrant molecules such as lignin, cutin, humic acids, steroids, and complex proteins which are possible humus precursors with high biological stability. This was also confirmed by other studies. [PERSON] et al. [86] conducted research in which they analyzed twelve DGs sampled from a full-scale biogas plant co-digesting swine manure, various energy crops, organic residues, and the OFMSW. The authors conducted the spectroscopic analysis of DGs. The spectra obtained by NMR showed four regions representing four different types of organic molecules such as carboxyl-C, aromatic-C, O-alkyl-C, and the aliphatic-C. Comparing the spectra of one of the considered anaerobic substrates and its relative DG, the more degradable molecules (the region of the O-alkyl-C) resulted in a net decrease in the DG, while the more recalcitrant OM fraction (other three types of organic molecules) resulted in a relative net increase in the DG. Therefore, the authors confirmed the relative accumulation of molecules containing carboxyl-C, aromatic-C, and aliphatic-C in more recalcitrant fractions such as lignin, cutin, humic acids, steroids, and complex proteins. [PERSON] et al. [87] co-digested CM with bedding material (straw) for a prolonged period to evaluate the changes in OM content. Degradation of the OM was evaluated using thermal analysis and NMR. Stabilization through AD increased the quality of the OM (characterized by enrichment in thermostable compounds), and an accumulation of long-chain aliphatic materials. AD feedstocks (mainly sewage sludge, domestic wastewaters, industrial organic waste, household waste, and even food waste) can contain various amounts of unwanted chemical matter of which some are persistent organic pollutants.