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is similar to compost in chemical
and physical composition, but it
is produced in an anaerobic envi-
ronment rather than the oxygen rich
environment needed for compost.
Digestate typically has more nu-
trients and less carbon compared to
compost and digestate has a higher
moisture content than compost. Like
compost, digestate can be used as a
plant nutrient source in lieu of dama-
9
ging synthetic fertilizers.
The digestate is approximately 85-
90% of the input biomass and is
sent to a separation process to re-
move the liquid portion from the solid
portion. There are different post tre-
atment processes that will produce
fertilizer in powder or pellets, ammo-
nium sulphate, compost, pure water.
In that way we have a smooth disposal of the According to the Food and Agricultural Orga-
digestate with additional revenues and an incre- nization (FAO) of the United Nations, emissions
asing of the circular economy from synthetic fertilizers grew at an average rate
of 3.9% annually from 1961 to 2010, with abso-
All of the nutrients contained with the feedstock lute values increasing more than nine-fold, from
of AD, including nitrogen, phosphorus, and po- 0.07 to 0.68 Gt CO equivalents annually.
2
tassium (collectively known as NPK), are embo- By reducing our usage of these synthetic ferti-
died within digestate, so it can be used as a soil lizers, and instead using digestate, we are not
amendment in agriculture, landscaping, storm only curtailing GHG emissions, but also redu-
water management and consumer-based horti- cing our dependence on conventional natural
culture. gas. N
By using digestate, we can effectively reduce Moreover, introducing digestate into the soil is
our dependence on synthetic fertilizers, which an important part of the carbon sequestration
relies on the Haber-Bosch process to fix nitro- mechanism as the carbon within the feedstock
gen. The Haber-Bosch process fixes most of of AD is embodied in the digestate.
the world’s nitrogen and accounts for 1-2% of Thus, adding digestate to soil effectively puts
the world’s total energy consumption and 3-5% the carbon within plants back into the soil,
of the world’s total natural gas consumption. which closes the carbon cycle.

Impiantistica Italiana - Maggio-Giugno 2021 93 93