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plant growth (N, P, K) and can be used in agriculture due to its excellent qualities
for soil fertilization. Digestate proves improved fertilizer efficiency compared to
raw animal manure, because it has a more homogeneous texture and has a better
ability to release nutrients, a better carbon/nitrogen ratio and much lower odours
[133]. Other more in-depth studies that have compared the fertilizing performance
of digestate and some types of manure have shown that digestate is better suited for
clay soils and high levels of organic matter, while pig and cattle manure are more
suitable for sandy soils with lower content of organic compounds [134]. It can be
estimated that for every 1 tonne of organic waste treated by anaerobic digestion,
around 400 kg of liquid digestate and 300–400 kg of fibre digestate can be generated [77]. Digestate management is a complex task that involves adequate solutions
for storage, processing, transportation and utilization and should be economical and
in compliance with environmental protection measures [135].
Despite the many advantages that this by-product can bring to agriculture or
other sectors through bioprocessing, there are also many challenges that, if not well
managed, can make the investment in the biogas plant unprofitable. Since digestate
contains >90% water, it requires large space for storage, and such auxiliary facilities
significantly augment the cost of the anaerobic digestion plant. Digestate processing
to volume reduction and nutrient concentration involves drying, evaporation and
retention of other pollutants (ammonia) or contaminants (heavy metals, organic pollutants, pathogens, etc.), but some of these techniques require high energy or reagent
consumption, some generate residual products, etc. [135]. If used directly as a soil
fertilizer, the digestate is discharged from the digester and transported outside the
facility by gas pipelines or with the help of special tanks [133], but the cost of transportation is an important issue to be assessed in planning a viable biogas plant.
Before making the decision on the best management option for recovering the
digestate effectively and economically, it is mandatory to know its physico- chemical
characteristics as the quality of the digestate is taken into account in the decision of
supply to end users [135]. The application of digestate to arable lands is limited on
the one hand by the maximum level of nitrogen allowed to be supplied to the land,
which, in most of the EU member states, is 170 kg nitrogen/hectare/year, as stipulated by Directive 91/676/EEC [136]. On the other hand, many countries have
adopted national and local regulations regarding the quality parameters of the digestate resulting from the anaerobic digestion of the sewage sludge which is known to
contain heavy metals. Regulations encourage decentralized anaerobic digestion and
allow land application of the digestate only for on-farm product [137].
Use of digestate in agricultural farms for soil fertilization cannot cover the need
to manage the entire amount of digestate generated in huge quantities from tens of
thousands of large-scale biogas plants operating worldwide. The biggest biogas
plants of a size greater than 20 MW being constructed at the moment generate over
400,000 t/year digestate [138].
Although valuable, such increasingly generated anaerobic digestion by-product
raises concerns about transportation costs, emissions of pollutant gases over longterm storage but also in terms of the large amount of nitrogen content that limits the
digestate use exclusively for agricultural crops [139, 140].
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…
plant growth (N, P, K) and can be used in agriculture due to its excellent qualities
for soil fertilization. Digestate proves improved fertilizer efficiency compared to
raw animal manure, because it has a more homogeneous texture and has a better
ability to release nutrients, a better carbon/nitrogen ratio and much lower odours
[133]. Other more in-depth studies that have compared the fertilizing performance
of digestate and some types of manure have shown that digestate is better suited for
clay soils and high levels of organic matter, while pig and cattle manure are more
suitable for sandy soils with lower content of organic compounds [134]. It can be
estimated that for every 1 tonne of organic waste treated by anaerobic digestion,
around 400 kg of liquid digestate and 300–400 kg of fibre digestate can be generated [77]. Digestate management is a complex task that involves adequate solutions
for storage, processing, transportation and utilization and should be economical and
in compliance with environmental protection measures [135].
Despite the many advantages that this by-product can bring to agriculture or
other sectors through bioprocessing, there are also many challenges that, if not well
managed, can make the investment in the biogas plant unprofitable. Since digestate
contains >90% water, it requires large space for storage, and such auxiliary facilities
significantly augment the cost of the anaerobic digestion plant. Digestate processing
to volume reduction and nutrient concentration involves drying, evaporation and
retention of other pollutants (ammonia) or contaminants (heavy metals, organic pollutants, pathogens, etc.), but some of these techniques require high energy or reagent
consumption, some generate residual products, etc. [135]. If used directly as a soil
fertilizer, the digestate is discharged from the digester and transported outside the
facility by gas pipelines or with the help of special tanks [133], but the cost of transportation is an important issue to be assessed in planning a viable biogas plant.
Before making the decision on the best management option for recovering the
digestate effectively and economically, it is mandatory to know its physico- chemical
characteristics as the quality of the digestate is taken into account in the decision of
supply to end users [135]. The application of digestate to arable lands is limited on
the one hand by the maximum level of nitrogen allowed to be supplied to the land,
which, in most of the EU member states, is 170 kg nitrogen/hectare/year, as stipulated by Directive 91/676/EEC [136]. On the other hand, many countries have
adopted national and local regulations regarding the quality parameters of the digestate resulting from the anaerobic digestion of the sewage sludge which is known to
contain heavy metals. Regulations encourage decentralized anaerobic digestion and
allow land application of the digestate only for on-farm product [137].
Use of digestate in agricultural farms for soil fertilization cannot cover the need
to manage the entire amount of digestate generated in huge quantities from tens of
thousands of large-scale biogas plants operating worldwide. The biggest biogas
plants of a size greater than 20 MW being constructed at the moment generate over
400,000 t/year digestate [138].
Although valuable, such increasingly generated anaerobic digestion by-product
raises concerns about transportation costs, emissions of pollutant gases over longterm storage but also in terms of the large amount of nitrogen content that limits the
digestate use exclusively for agricultural crops [139, 140].
Biochemical Conversion of Residual Biomass: An Approach to Fuel Gas and Green…