Impact of impeller design on anaerobic digestion: Assessment of mixing dynamics, methane yield, microbial communities and digestate dewaterability

[Display omitted] •Helical impeller improves mixing & reduces stratification vs. pitched-blade impeller.•Pitched-blade impeller diminishes bacterial diversity, no effect on methanogens.•Pitched-blade impeller enhances digestate dewaterability in lab reactors.•Impeller geometry – a new perspective to enhance digestate dewaterability. The efficiency of anaerobic digestion (AD) processes is intricately tied to mixing quality. This research investigates the influence of two impeller types, namely a helical ribbon impeller (HRI) and a pitched-blade impeller (PBI), on key aspects of AD. The investigation encompassed mixing dynamics, methane production, microbial communities, and the previously unexplored impact on digestate dewaterability. Results show that agitation with the PBI exhibited stratification, with bottom layer total solids (TS) values of 3.1% for the PBI and 2.6% for the HRI. Nevertheless, methane yield remained unchanged, averaging 286 LN/kg volatile solids (VS)added. Slower mixing with the HRI achieved more uniform mixing and reduced energy requirements. Additionally, impeller type significantly affected digestate dewaterability, leading to a 3.8% increase in TS of the dewatered sludge when using the PBI. These findings highlight the importance of considering mixing not only for methane production and reduced maintenance but also for achieving optimal digestate dewaterability.