Anaerobic co-digestion of expired alcohol-based hand sanitizer with synthetic wastewater for biogas production

Abstract

In this work, anaerobic co-digestion was demonstrated as a viable method for utilizing​ expired alcohol-based hand sanitizer from COVID-19 response for additional biogas production. Co-digestion experiments were conducted using three parallel continuous flow anaerobic digesters. The results highlight the importance of acclimatisation to avoid process instability. Process instability was observed when co-digesting ethanol-based sanitizer (at 0.3% v/v) with sewage sludge without acclimatisation. However, the digester was fully recoverable and a gradual increase in the addition of ethanol-based sanitizer showed stable and good performance even at 0.6% v/v. The specific biogas production was in the range of 295–304 mL/gCOD.day and the COD removal efficiency was 76%. When isopropanol-based sanitizer was used as a co-substrate, digester previously acclimatised with ethanol-based sanitizer showed 20% higher biogas production than another digester that has not been acclimatised. These results highlight the need for an acclimatisation period so that the microbial community can adjust to the new substrate. Furthermore, due to the easily degradable nature of alcohol-based sanitizer and synthetic wastewater in this study, there was a threshold of organic loading rate of 3.5–4 gCOD/L.day. This threshold can be explained by the accumulation of volatile fatty acids that could inhibit the methanogenesis process. Results in this study demonstrate anaerobic co-digestion as a sustainable option for valorizing expired alcohol-based hand sanitizer.