# Digestate Management and Processing Practices: A Review
## 4 Digestate
Pathogenic microbes (_Escherichia coli_, _Salmonella_ spp., _Listeria monocytogenes_, _Listeria perfringens_, _Campylobacter jejuni_, _Cryptosporidium parvum_, _Giardia intestinalis_, _Clostridium botulinum_) which are not a major problem in AD of various animal wastes (particularly cattle, pig, poultry and sheep manure) [71], can survive the digestion process and remain in the DG [69]. Since DG can play a role in transporting pathogens from agricultural land through the food chain to humans, proper sanitation is essential. Proper sanitation of DG also depends on the quality of the substrates fed to the reactor, as well as the reactor performance, digestion temperature, slurry retention time, pH, and NH\({}_{3}\) concentration. In addition, different pretreatment methods can be used to reduce the number of pathogens in the final DG, such as pasteurization [72], treatment with chlorine, UV-light, ozone, and high internal pressure in the vessel [73]. However, animal by-products that originate from the food industry (various waste from livestock production, slaughterhouse, and food processing) and are not suitable for human usage, that are used for AD, require pasteurization i.e., hygienization before AD. This process is carried out at 70 \({}^{\circ}\)C for at least 60 min whereas the maximum size of particles that enter hygienizers should not exceed 12 mm. The objective is to reduce the concentration of _Enterococcus faecalis_ or _Salmonella sentfenberg_ by a factor of 10\({}^{5}\) and thermal-resistant virus by 10\({}^{3}\). _Enterococcus faecalis_ (Gram-positive bacteria) and _Escherichia coli_ (Gram-negative bacteria) are two microorganisms that often serve as indicators characterizing the performance of the hygienization process. _Enterococcus faecalis_ is also chosen by EU regulation No. 142/2011 as indicator bacteria [74]. Although the thermal inactivation of pathogens has been widely studied on a laboratory scale, consideration should be paid when applying the results to large-scale systems. Exposure time that is required for the inactivation of pathogens may be affected by uneven heating, fluctuating temperatures, or shielding properties of solids. Therefore, the transfer of the laboratory results to full-scale systems requires further investigation [75]. Besides conventional thermal pasteurization technologies, alternative thermal pasteurization technologies such as electro-technology, microwave, pressurization, ultrasound, and chemical treatments, are capable of considerably reducing the number of bacteria and at the same time increasing the CH\({}_{4}\) yield. However, their performance varies greatly and depends on the type of bio-waste, the operational parameters, and energy input [76]. Yet, several spore-formers and therefore less heat-sensitive bacteria (e.g., _Clostridium_ spp. and _Bacillus_ spp.) cannot be reduced [69; 77]. This was confirmed with a study in which the microbiological quality of the DG from three different biogas plants in Sweden was studied. Control of spore-forming species and presence of antibiotic resistance were conducted, and the results confirmed that all formal criteria were met, the amounts of _Escherichia coli_, enterococci, and _Salmonella_ were all below the maximum accepted levels at all three locations. However, DGs from all plants had unsanitary levels of the food-borne spore-forming pathogen _Bacillus cereus_[78]. The results are in agreement with a previous study [79] conducted on the hygiene aspects of DGs where high levels of _Bacillus_ spp. were detected, and it was concluded that neither the hygienization treatment nor the subsequent AD affected the number of _Bacillus_ spp. In another study by [PERSON] et al. [80] liquid manures and DGs from five biogas plants in France were examined to assess the contamination by sporulating (_Clostridium perfringens_, _Clostridioides difficile_, and _Clostridium botulinum_) and nonsopvulating (_Escherichia coli_ and enterococci, _Salmonella_ spp., _Campylobacter_ and _Listeria monocytogenes_) bacterial species.