Evaluation of batch mesophilic anaerobic digestion of waste Euro banknotes for methane Production: Preliminary studies and kinetic approach.

The European Central Bank is striving to find environmentally friendly alternative methods of waste disposal. In 2020, it decided to end the disposal of Waste Euro Banknotes (W€B) in landfills and planned to use them for recycling and/or energy recovery. Despite being recognized as an effective tool in a circular economy model, there are no reported studies on the use of W€B as a substrate in anaerobic digestion (AD). Thus, the aim of this research was to assess the viability of W€B to be converted into high-value-added energy products (mainly methane) through AD. W€B (€10 and €20 denominations) provided by the Bank of Spain were used. Biochemical methane potential (BMP) tests of pre-treated (grinded) and untreated W€B were assessed at mesophilic temperature. The ultimate methane yield was considerably higher for pre-treated W€B (334 ± 23 NmL CH ₄ g VS ^-1 _added ) than for untreated W€B (297 ± 14 NmL CH ₄ g VS ^-1 _added ). The Logistic or Sigmoidal kinetic model adequately fit the experimental data and allowed for obtaining the kinetic parameters of each case studied. In this sense, an increase of 22.4 % in the maximum methane production rate (R _max ) was observed for the pre-treated W€B (52.5 ± 0.9 mL CH ₄ (g VS·d) ^-1 compared to the untreated W€B (16.2 ± 1.8 mL CH ₄ (g VS·d) ^-1 ). According to the obtained results, AD may be a good alternative for the energetic valorization and recycling of W€B.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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