dry anaerobic digestion of municipal solid waste and digestate ...

if surface specific loads (loads per hectare) were considered, source-separated compost and
digestate were the most important inputs by more than a factor of 25 and 20 for PCBs and
PAHs. Total PAH loads accounted for 33% of the input from aerial deposition. Loads of
PCBs were low and are even expected to decrease over the next decades due to the banning
of PCBs.
2.9.4 Presence of heavy metals
Digestates are so-called ‘recycling fertilizers’ or ‘low price fertilizers’ which are used in
agricluture because these can provide plant nutrients and organic matter to improve soil.
They however, contain some pollutants in the form of heavy metals such as Cd, Cr, Cu,
Hg, Ni, Pb, Zn, etc., that can contaminate soil and food. The most important pathways of
heavy metals in the digestate are deposition from air and direct application of pesticides to
the material of origin. The heavy metal contents in digestates are influenced principally by
the materials of origin (Fuchs et al., 2008). The contents of heavy metals determined
mostly are low except for Cu, Zn as shown in Table 2.10.
2.9.5 GHG emission potential of digestate
The treatment process of anaerobic digestion removes certain amount of carbon from the
waste while the remaining carbon still remains in the digested residues (digestate). Thus
the digestate can act as source of GHGs if not managed properly. Rico et al., ( 2011)
collected four anaerobic effluents from the digester (digesting dairy manure) at different
HRTs and analyzed to measure their residual methane potentials. They reported residual
methane potential of digestates in the range of 12.7 to 102.4 L/g VS. These methane
potentials were highly influenced by the feed quality and HRT of the previous CSTR
anaerobic digestion process. Menardo et al., (2011) also reported that the residual methane
potential of digestate (taken from digesters with feed of animal manure, energy crops and
food industry waste) was very variable (2.88-37.63 NL/kgVS) and depended on the OLR,
HRT and feedstock quality during digestion. Mumme et al., (2010 ) also reported similar
results.
Table 2.10 Heavy Metal Content in Different Types of Digestates (mg/kg DM)
Digestate Type Cd Cr Cu Hg Ni Pb Zn Reference
OFMSW 0.64 13.88 55 0.035 12.16 0 105
Eliyan, 2007
Cattle manure - 8.05 128 - - - 555
Composted
digestate
Biowaste
digestate
Animal
digestate
0.36 23.00 72 0.097 11.50 26.8 179
38
Sanchez et al.,
2008
Riedel and Marb,
2008
0.28 12.00 40 0.100 10.00 7.0 160 Persson, 2008
0.30 9.30 113 0.080 9.70 4.1 375 Palm, 2008
Similar to the digestion of original waste, specific methanogenic activity (SMA) of
digestate from MSW digester is also linearly linked to moisture content. Hyaric et al.,
(2011) found that low moisture content is detrimental to SMA of digestate. The SMA test
for digestate was performed at mesophilic temperature at four different moisture contents
(65-82%) and it was found that SMA is highest at 82% moisture content.