Effect of Ink and Pretreatment Conditions on Bioethanol and Biomethane Yields from Waste Banknote Paper

Waste banknote paper is a residue from the banking industry that cannot be recycled due to the presence of ink, microbial load and special coating that provides protection against humidity. As a result, waste banknote paper ends up being burned or buried, which brings environmental impacts, mainly caused by the presence of heavy metals in its composition. To minimize the environmental impacts that come from the disposal of waste banknote paper, this study proposes to produce value-added products (bioethanol and biogas) from waste banknote paper. For this, the effect of ink and pretreatment conditions on bioethanol and biomethane yields were analyzed. Waste banknote paper provided by the Central Bank of Iran was used. The raw material with ink (WPB) and without ink (WPD) was pretreated using sulfuric acid at different concentrations (1%, 2%, 3%, and 4%) and the nitrogen explosive decompression (NED) at different temperatures (150 °C, 170 °C, 190 °C, and 200 °C). The results show that the use of NED pretreatment in WPD resulted in the highest glucose concentration of all studies (13 ± 0.19 g/L). The acid pretreatment for WPB showed a correlation with the acid concentration. The highest ethanol concentration was obtained from the fermentation using WPD pretreated with NED (6.36 ± 0.72 g/L). The maximum methane yields varied between 136 ± 5 mol/kg TS (2% acid WPB) and 294 ± 4 mol/kg TS (3% acid WPD). Our results show that the presence of ink reduces bioethanol and biogas yields and that the chemical-free NED pretreatment is more advantageous for bioethanol and biogas production than the acid pretreatment method. Waste banknote paper without ink is a suitable feedstock for sustainable biorefinery processes.