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

For instance, Mumme et al., (2010) reported an increase in TS concentration from 15.8 to
22.1% continuously since day 39 (during feeding steps with higher OLR of 12.7 and 17 kg
VS/m 3 d). Moreover, compaction of solid-state bed was also observed during the same
time. The increase in TS content of reactor medium has been found to decrease VS
removal of the digester as proved in section 2.4.2. Similarly, increase in TS of digestate
also increases remaining methane formation potential of digestate. Both the TKN and
NH4–N accumulate in the solid residues of a continuously operated digester, which is just
similar to that in liquid digestate.
Fuchs et al., (2008) analyzed one hundred compost and digestate samples representative of
the different composting systems and qualities and concluded that the respiration rate,
enzymatic activities and phytotoxicity varied greatly which depend on maturity and
management process. Mature composts showed less respiration rates and enzymatic
activity. In Europe humic substances are described as parameters for quality of composts in
addition to low contents of pollutants (Binner et al., 2008). Unhurried degradation with
long lasting biological reactivity of the feedstock is important for high compost quality that
leads to the formation of more humic acids. Anaerobic pretreatment seems to be positive
for the development of humic acids during the following rotting period. In contrast,
intensive supply of oxygen mineralizes the metabolic products quickly and completely and
discharges into air. Pure sewage sludge or sewage sludge with low amount of yard wastes
lead to a low development of humic acids.
Tambone et al., (2010) studied a total of 23 samples of digestates, ingestates, composts and
digested sludge to assess their amendment and fertilizing properties. The results shown that
digestates differed from ingestates and also from compost, although the starting organic
mix influenced the digestate final characteristics. In amendment properties, compost and
digestate were better than digested sludge and all of these were better than ingestate.
As to fertilizer properties, digestion produced digestate with very good fertilizing
properties because of the high nutrient (N, P, K) content in available form. Thus, the
digestate appears to be a good candidate to replace inorganic fertilizers, also contributing,
to the short-term soil organic matter turnover.
2.9.2 Characteristics of liquid digestates
It has been found that the liquid digestate resulting from fermentation of farm manures,
agricultural biomass, OFMSW and their combination from biogas plants provide liquid
fertilizer. Typically, the liquid digestates resulting from liquid-solid separation of digested
residues contain low total solids content (3-6 % of farm manure). Moreover, they are rich
in nutrients such as N (> 6% of TS), NH 4-N (> 2% of TS), K (> 4% of TS), Ca and Mg.
Because of high N content of liquid digestates, their C/N ratio is very low (< 6), which
makes them suitable for direct application to the field. The required C/N ratio for an
organic fertilizer (or amendment) to be applied on soil is < 20 (Wood, 2008). However,
high pH (about 8.5) of liquid digestates makes them more prone to be lost to atmosphere
through volatilization, when applied in the field especially at high ambient temperature.
Thus special soil management practices need to be adopted to avoid this loss of N. The
characteristics of liquid digestates from different digesters as investigated by different
environmentalists have been shown in Table 2.8.
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