Liquid mixing and solid segregation in high-solid anaerobic digesters.

Area of Science:

• Biochemical Engineering
• Waste Management
• Environmental Science

Background:

• Dry anaerobic digestion (DAD) is a key technology for biogas production from solid organic waste.
• Understanding macro-mixing is crucial for optimizing DAD reactor performance and efficiency.
• Previous studies have highlighted challenges in solid phase mixing and particle segregation within DAD systems.

Purpose of the Study:

• To characterize the macro-mixing behavior of both liquid and solid phases in a laboratory-scale DAD.
• To investigate the influence of waste origin and total solids (TS) content on mixing and particle segregation.
• To identify key parameters governing particle segregation mechanisms in DAD.

Main Methods:

• Utilized the Residence Time Distribution (RTD) technique with appropriate tracers to analyze macro-mixing.
• Experimentally varied waste origin and total solids (TS) content (22% to 30% w/w).
• Investigated the segregation of particles with different densities in the solid phase RTD.

Main Results:

• Increased TS content from 22% to 30% (w/w) promoted a macro-mixing mode closer to a Plug Flow Reactor (PFR).
• Particle segregation, particularly of dense particles, was observed at lower TS content.
• Yield stress was identified as a critical parameter influencing particle segregation, with high yield stress stabilizing dense particles and preventing settling.

Conclusions:

• Operating DAD at higher TS content enhances macro-mixing and approximates plug flow behavior.
• Elevated TS content and corresponding high yield stress effectively prevent the sedimentation of denser particles.
• Optimizing TS content is a viable strategy to improve solid phase mixing and mitigate segregation issues in DAD.