Near-ideal plug-flow enables stage separation during dry anaerobic digestion of biowaste in a rotating drum fermenter

Abstract

Conventional designs of anaerobic plug-flow reactors frequently exhibit stagnant zones and pronounced short-circuiting, undermining plug-flow behavior and reliable biowaste pasteurization. Ideal plug-flow conditions could, in contrast, enable a spatial separation of metabolic stages and thus combine the benefits of two- or multi-stage anaerobic digestion within a single reactor, while ensuring pasteurization. This study demonstrates the technical feasibility of achieving stage separation in a single-reactor setup by employing a rotating drum fermenter. The experimental scope comprises an analysis of the residence time distribution and a 150-day operation of 105 L rotating drum fermenter treating source-separated organic municipal solid waste at 34.9% total solids content. The reactor was operated at a target organic loading rate of 8 kg VS/m 3 /d, a recirculation ratio of 2:1 (substrate to as-extracted digestate), and a hydraulic retention time of 21.1 days. Tracer breakthrough showed near-ideal plug-flow behavior. Four sampling ports along the longitudinal axis revealed pronounced axial gradients of pH and volatile fatty acids, indicating effective stage separation. Median pH progressively increased along the axis from 7.35 to 8.28, which generally followed the sequential order in pH preferences of the metabolic stages. Total volatile fatty acids concentrations mirrored this gradient, decreasing from a median 14,000 mg/L to a median 176 mg/L and a narrow interquartile range, suggesting reliable volatile fatty acid mitigation. Rotating drum fermenters thus represent a proper plug-flow system distinct from conventional anaerobic plug-flow reactors. The average methane yield of 222 m 3 CH 4 /Mg VS and median volatile solids reduction of 34.4% hold potential for optimization.