Added value of manure (co-)digestion in light of GHG emission abatement: moving beyond COP21

In the EU alone already approximately 1,27 billion tons of manure are produced annually. Not surprisingly, this type of agro side stream has been recognized for its considerable potential for producing renewable energy, with anaerobic digestion often as the preferred pathway. Nevertheless, fossil energy replacement is not the only added value that can be associated to energetic conversion of manure. When manure is frequently and freshly digested rather than stored for prolonged times, the avoided emissions of methane from storage also significantly contribute to reducing GHG as methane itself is 21 times more harmful than CO2 itself towards global warming. Better to recover and turn into energy than allow slow release in the air, as is usually the case in current practices in animal husbandry. In addition, digestion will effectively mineralize organic nitrogen and phosphorus rendering them more available for plant uptake. Novel pathways for processing digestate can therefore result in effective substitution of mineral fertilizers derived from fossil resources, again resulting in reduced primary energy use. Finally, the recalcitrant (humus) fractions of organic matter present in digested manure tend to remain stable in soil for considerable periods of time. Current proposals to the COP21 carbon accounting include carbon sequestration of such recalcitrant organic matter as a carbon sink. Simultaneously soil fertility and soil quality themselves are improved by improving soil structure, soil cation exchange capacity (CEC) and adhesion surface for soil microbial life. The sum of all before mentioned qualities make manure (co-)digestion an prime strategy when aiming towards GHG emission abatement beyond the scope of mere substitution of fossil energy by renewable energy. Because of this, the CO2 eq. reduction per MWh of energy produced in systems making use of anaerobic digestion of manure exceeds that of any other type of renewable energy. Regional/national implementation plans beyond COP21 agreements should take this into account when developing sustainable policy roadmaps. This study focuses on the qualitative description of the merits associated to manure digestion as well their quantification.

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