74
More recent research has been directed towards testing and implementing alternative ways of digestate recovery to minimize the impact on the environment and
add value to the technological solution of the residual biomass energy recovery.
Also, digestate management aims to open new business opportunities for the manufacture and sale of products based on digestate and thus reduce the operating costs
of biogas plants [137].
The recovery of valuable components from the digestate can be achieved by
processing it through various mechanical, physical, chemical, biological or combined techniques [141]. Digestate management strategies are expected to be focused
on biorefinery processes to produce marketable materials. Thus, as alternatives for
digestate recovery, some scientists have developed optimized procedures to transform solid digestate into high-quality compost [142, 143]. Use of solid digestate for
vermiculture is another option for its efficient capitalization [144]. Other studies
tested and implemented methods for making solid fuels after drying and pelletizing
solid fraction of the digestate [145], or it can be just dried and incinerated for energy
recovery [137]. Incineration is a proper management way for reducing the volume
and the polluting organic matter content in digestate; some elements (P, K, Ca)
which are collected in the bottom ash could be recovered to be used for soil fertilization [146]. Biochar that can be obtained from pyrolysis of digestate is not only a
valuable adsorbent material; some researchers proved that biochar can enhance the
anaerobic digestion performance, since it can act as an adsorbent for the process
inhibitory compounds and stand for microbial growth or electric conductor allowing electron transfer. Also, they proved that biochar is an effective adsorbent to be
used for upgrading biogas [147]. Other studies regarding digestate processing aimed
at producing bio-oil to be used as fuel for engines [148, 149]. Digestate can be used
for industrial purposes to produce composite materials and as building material [138].
The liquid phase resulted after filtration of the digestate is rich in ammonium
nitrogen, carrying ca. 70–80% of the total NH 4
+
-N [146]. Therefore, the liquid
digestate may have various beneficiaries to exploit its nutrient-rich composition. It
is mainly used for agriculture or can be recirculated to the digester instead of process water for diluting the biomass feedstock and supplying fermentative microbiota. Another option for liquid digestate management is to utilize nutrients, such as
nitrogen and phosphorus, and organic matter for cultivating mixotrophic microalgae, thus enhancing the biomass productivity in bioreactors and supply feedstock
for the biorefinery industries [137, 150]. Digestate use as a carbon source in microbial fuel cell reactors has gained popularity in the last decade [151, 152]. Figure 7
displays the main management options of the advanced recovery of the by-products
resulting from the biomass residues’ anaerobic bioconversion within the concept of
circular bioeconomy.
Catalytical conversion of anaerobically digested biomass takes place in catalytic
reactors that produce pyrolysis gaseous mixtures which could be used as feedstocks
in the production of chemicals/source of hydrogen/running of gas turbines for generation of power. Advanced techniques of catalytical biomass conversion through
supercritical water gasification has been investigated by Güngören Madenoğlu for
C. Mateescu and A.-D. Dima
More recent research has been directed towards testing and implementing alternative ways of digestate recovery to minimize the impact on the environment and
add value to the technological solution of the residual biomass energy recovery.
Also, digestate management aims to open new business opportunities for the manufacture and sale of products based on digestate and thus reduce the operating costs
of biogas plants [137].
The recovery of valuable components from the digestate can be achieved by
processing it through various mechanical, physical, chemical, biological or combined techniques [141]. Digestate management strategies are expected to be focused
on biorefinery processes to produce marketable materials. Thus, as alternatives for
digestate recovery, some scientists have developed optimized procedures to transform solid digestate into high-quality compost [142, 143]. Use of solid digestate for
vermiculture is another option for its efficient capitalization [144]. Other studies
tested and implemented methods for making solid fuels after drying and pelletizing
solid fraction of the digestate [145], or it can be just dried and incinerated for energy
recovery [137]. Incineration is a proper management way for reducing the volume
and the polluting organic matter content in digestate; some elements (P, K, Ca)
which are collected in the bottom ash could be recovered to be used for soil fertilization [146]. Biochar that can be obtained from pyrolysis of digestate is not only a
valuable adsorbent material; some researchers proved that biochar can enhance the
anaerobic digestion performance, since it can act as an adsorbent for the process
inhibitory compounds and stand for microbial growth or electric conductor allowing electron transfer. Also, they proved that biochar is an effective adsorbent to be
used for upgrading biogas [147]. Other studies regarding digestate processing aimed
at producing bio-oil to be used as fuel for engines [148, 149]. Digestate can be used
for industrial purposes to produce composite materials and as building material [138].
The liquid phase resulted after filtration of the digestate is rich in ammonium
nitrogen, carrying ca. 70–80% of the total NH 4
+
-N [146]. Therefore, the liquid
digestate may have various beneficiaries to exploit its nutrient-rich composition. It
is mainly used for agriculture or can be recirculated to the digester instead of process water for diluting the biomass feedstock and supplying fermentative microbiota. Another option for liquid digestate management is to utilize nutrients, such as
nitrogen and phosphorus, and organic matter for cultivating mixotrophic microalgae, thus enhancing the biomass productivity in bioreactors and supply feedstock
for the biorefinery industries [137, 150]. Digestate use as a carbon source in microbial fuel cell reactors has gained popularity in the last decade [151, 152]. Figure 7
displays the main management options of the advanced recovery of the by-products
resulting from the biomass residues’ anaerobic bioconversion within the concept of
circular bioeconomy.
Catalytical conversion of anaerobically digested biomass takes place in catalytic
reactors that produce pyrolysis gaseous mixtures which could be used as feedstocks
in the production of chemicals/source of hydrogen/running of gas turbines for generation of power. Advanced techniques of catalytical biomass conversion through
supercritical water gasification has been investigated by Güngören Madenoğlu for
C. Mateescu and A.-D. Dima