Common pathogens of digestate are microorganisms like Pseudomonas, Klebsiella, Samonella, Enterococci, Escherichia, Bacillus, Penicillum, Shigella,
Bacteriodes, Aspergillus, and Clostridium (Owamah et al. 2014; Alfa et al. 2014).
During MAD, elevated temperature together with high concentration of ammonia
has a significant impact on pathogen removal (Ottoson et al. 2008; Orzi et al. 2015).
Ammonia is released during MAD process due to protein decomposition, but
additional amounts of ammonia may also be supplemented to the initial digestate
mixture for optimal conditions adjustment (Su et al. 2015). However, decreased
abundance of some pathogen species does not always ensure full sanitation of the
final product. Orzi et al. (2015) and Duarte et al. (1992) observed a decrease in fecal
Streptococci after MAD treatment of agricultural wastes. In the study of Orzi et al.
(2015), Escherichia coli and Salmonella were not removed from all of the samples
after MAD. Presence of Salmonella after MAD treatment was also confirmed by
Sahlström et al. (2004). In contrast, Ottoson et al. (2008) observed that at high
ammonia concentration, the removal of Salmonella and Enterococci was significant.
Nevertheless, the sanitation effect was not confirmed for other pathogens as Yersinia
enterocolitica and Listeria monocytogenes as well as Helminth eggs (Orzi et al.
2015). There is also no unambiguous conclusion about Clostridium spp. content
reduction during MAD. Bagge et al. (2005) concluded that agricultural wastes
processed during anaerobic digestion caused less risk in Clostridium spreading
after field application in comparison to unprocessed products. Ottoson et al. (2008)
observed that pathogenic clostridia might be present in digestate posing a risk while
applied on field. Orzi et al. (2015) detected Clostridium removal in some samples but
it was not a rule. Eventually, most of the abovementioned studies concluded a
general decrease in pathogen content, nevertheless the potential pathogen presence
in digestate should not be omitted.
Owamah et al. (2014) observed a decrease in microbial count after anaerobic
digestion of a feedstock consisting of food waste and human excreta. The mean
microbial count in the feedstock before anaerobic digestion was 2.4 Â 10
10 CFU/
100 mL for coliform, 2.0 Â 10
12 CFU/100 mL for total aerobic plate, and
1.9 Â 10
8 CFU/100 mL for fungi. After AD, the value had dropped to
2.0 Â 10
8 CFU/100 mL for coliform, 1.0 Â 10
4 CFU/100 mL for total aerobic
plate, and 2.0 Â 10
3 CFU/100 mL for fungi (Owamah et al. 2014). Shu-Hsien et al.
(2007) have observed that microbial population decreased within the first 7 days of
anaerobic digestion due to acidic environment and remained constant in digestate.
However, the residual total coliform content of 2.1 Â 10
8 CFU/100 mL observed by
Owamah et al. (2014) is still above tolerable limits for direct land application of
digestate (Yun et al. 2000). Retention time of 90 day for MAD and retention time of
30 day for thermophilic anaerobic digestion are recommended for a better quality of
digestate in terms of pathogen content (Owamah et al. 2014).
Efficient posttreatment of digestate should ensure the sanitation of the final
product. Composting in last years was evaluated in terms of usefulness in removing
pathogens from digestate. The efficiency of pathogen inactivation during
composting is favored by elevated temperature such as 50–55
C and increases
with temperature rise (Vinnerås et al. 2010). During the maturation process of
5 Potential Use of Waste-to-Bioenergy By-Products in Bioremediation of Total. . .
263
Bacteriodes, Aspergillus, and Clostridium (Owamah et al. 2014; Alfa et al. 2014).
During MAD, elevated temperature together with high concentration of ammonia
has a significant impact on pathogen removal (Ottoson et al. 2008; Orzi et al. 2015).
Ammonia is released during MAD process due to protein decomposition, but
additional amounts of ammonia may also be supplemented to the initial digestate
mixture for optimal conditions adjustment (Su et al. 2015). However, decreased
abundance of some pathogen species does not always ensure full sanitation of the
final product. Orzi et al. (2015) and Duarte et al. (1992) observed a decrease in fecal
Streptococci after MAD treatment of agricultural wastes. In the study of Orzi et al.
(2015), Escherichia coli and Salmonella were not removed from all of the samples
after MAD. Presence of Salmonella after MAD treatment was also confirmed by
Sahlström et al. (2004). In contrast, Ottoson et al. (2008) observed that at high
ammonia concentration, the removal of Salmonella and Enterococci was significant.
Nevertheless, the sanitation effect was not confirmed for other pathogens as Yersinia
enterocolitica and Listeria monocytogenes as well as Helminth eggs (Orzi et al.
2015). There is also no unambiguous conclusion about Clostridium spp. content
reduction during MAD. Bagge et al. (2005) concluded that agricultural wastes
processed during anaerobic digestion caused less risk in Clostridium spreading
after field application in comparison to unprocessed products. Ottoson et al. (2008)
observed that pathogenic clostridia might be present in digestate posing a risk while
applied on field. Orzi et al. (2015) detected Clostridium removal in some samples but
it was not a rule. Eventually, most of the abovementioned studies concluded a
general decrease in pathogen content, nevertheless the potential pathogen presence
in digestate should not be omitted.
Owamah et al. (2014) observed a decrease in microbial count after anaerobic
digestion of a feedstock consisting of food waste and human excreta. The mean
microbial count in the feedstock before anaerobic digestion was 2.4 Â 10
10 CFU/
100 mL for coliform, 2.0 Â 10
12 CFU/100 mL for total aerobic plate, and
1.9 Â 10
8 CFU/100 mL for fungi. After AD, the value had dropped to
2.0 Â 10
8 CFU/100 mL for coliform, 1.0 Â 10
4 CFU/100 mL for total aerobic
plate, and 2.0 Â 10
3 CFU/100 mL for fungi (Owamah et al. 2014). Shu-Hsien et al.
(2007) have observed that microbial population decreased within the first 7 days of
anaerobic digestion due to acidic environment and remained constant in digestate.
However, the residual total coliform content of 2.1 Â 10
8 CFU/100 mL observed by
Owamah et al. (2014) is still above tolerable limits for direct land application of
digestate (Yun et al. 2000). Retention time of 90 day for MAD and retention time of
30 day for thermophilic anaerobic digestion are recommended for a better quality of
digestate in terms of pathogen content (Owamah et al. 2014).
Efficient posttreatment of digestate should ensure the sanitation of the final
product. Composting in last years was evaluated in terms of usefulness in removing
pathogens from digestate. The efficiency of pathogen inactivation during
composting is favored by elevated temperature such as 50–55
C and increases
with temperature rise (Vinnerås et al. 2010). During the maturation process of
5 Potential Use of Waste-to-Bioenergy By-Products in Bioremediation of Total. . .
263