Transport and discharge of solid waste by measuring static angles

Abstract

This article presents an experimental study of the static angle (θs), defined as the inclination at which 50 % of a municipal solid waste (MSW) sample begins to slide. This angle marks the transition from static to dynamic friction and serves as a measurable indicator of surface–material interaction and flow resistance. The study examines θs for different MSW components (wood, metal, plastic, pruning waste, and glass) tested on two different surfaces: acrylic, used as a reference, and rubber, commonly found in waste sorting equipment. Both individual and collective tests were performed, with special attention to the effects of added water mass (AWM).

A custom tilting-platform setup combined with image processing was employed to monitor particle motion and determine θs for each material-surface combination. The method implemented quantifies changes in θs as a function of moisture content, providing a proxy for blockage potential in waste-handling systems. Results indicate a consistent increase in θs with moisture content; for example, on acrylic, the average θs for wood rose from 26.2° to 46.2° as AWM increased from 0 % to 2.5 %. By comparison, θs for glass rose from 13.8° to 42.0° as AWM increased from 0 % to 5.0 %. Higher AWM levels led to greater material adhesion: in the case of wood on acrylic at 5.0 % AWM, 100 % of the sample remained immobile, preventing further testing. On rubber, a fraction of wood samples (3.0 %) remained adhered at a low 2.5 % AWM.

These findings provide quantitative evidence of how surface type and moisture affect waste mobility, supporting the design of discharge slopes and transport systems in MSW facilities. The approach can guide practical strategies to optimize material flow and reduce blockages under different operating and environmental conditions.