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

2.9 Characteristics of Digestates
Digestate is the liquid-solid suspension that is produced by anaerobic digestion of food
waste, OFMSW, manures and wastewater. It is combination of solid part and liquid phase
respectively called solid digestate or fiber and liquid digestate or liquor. The proportion of
solid and liquid part depends upon the type and nature of feedstock used for digestion as
well as type of digestion process (dry/wet). It has been found to contain certain proportion
of organic matter and plant nutrients like N, P, K, Ca, Mg, etc. that make it suitable to be
used as soil amendment or fertilizer. Liquid part contains high N percentage and solid part
contains high P content. The presence of heavy metals ( Cd, Cr, Pb, Ni, Hg, Cu, Zn) and
organic pollutants on the other hand decreases the possibility of digestate to be used in
agriculture. Some other parameters are also used for characterization of digestates such as
moisture content, total solids and volatile solids content, calorific value of dry fiber, pH,
etc. Physical characteristics of digestates are described by such parameters as bulk density,
cellulose structure/grain size, or presence of plastic, rubber, metals, glass, ceramics, sand
and stone. Biological parameters to describe the digestate are presence of pathogens or
weed seeds and biological stability of the digestate.
2.9.1 Characteristics of solid digestates
In case of dry anaerobic digestion, most of the liquid digestate is recycled to inoculate and
moisten the fresh waste. Thus solid fraction of digestate is the major part of digestate that
needs to be managed in dry anaerobic digestion. Characteristics of solid digestate in dry
anaerobic digestion systems have been shown in Table 2.7. Total solids, volatile solids, salt
content, pH, nutrients (N, P, K, Ca, Mg, etc.) and C/N ratio are assessed to describe the
characteristics of solid digestate. Sometimes, density, pore volume and water holding
capacity are also discussed. The pH of the solid digestate varies between 7.5 and 8.5. Salt
contents vary greatly in composts and digestates.
Through a more consistent choice of the materials of origin, the compost producers can
obtain a more constant salt content in the final product, because this is also influenced by
material of origin. The salinity of biosolids from an animal slurry and cattle manure
digester has been reported as 0.0469 and 2 dS/cm respectively. Organic matter is
principally influenced by the maturity of the products (Fuchs et al., 2008). Similarly, other
characteristics also vary depending on the type of feedstock.
Nutrient contents in the digestates are influenced principally by the materials of origin. The
results of analysis of 100 samples of digestates and composts representative of different
systems and taken from Swiss market show that the nutrient content of the digestates vary
greatly among the digestates. The median values of total N, P, K, Ca, Mg, Fe and Na have
been found to be 1.53%, 0.36%, 1.25%, 4.66%, 0.68%, 0.89% and 0.13% of dry matter
respectively (Fuchs et al., 2008) in this research, while the contents of total N, P and K in
the anaerobic digestion digestate of OFMSW have also been reported as 1.09%, 0.65% and
0.65% respectively (Eliyan, 2007). These differences in nutrient content of different
digestates depend upon the materials of origin.
In a continuously fed dry anaerobic digester, if the moisture content is not maintained by
addition of lost water through digestate and biogas removal, the TS content of the reactor
material (digestate) increases. This increase in TS content of digestate is more rapid, if the
OLR is increased as compared to a constant OLR.
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