Characterising faecal particle settling hydrodynamics to reenvision solid-liquid separation in decentralised...

Area of Science:

• Environmental Engineering
• Fluid Dynamics
• Sanitation Science

Background:

• Current sanitation literature uses the Bristol Stool Scale (BSS) as a proxy for faecal structural characteristics impacting solid-liquid separation.
• Empirical data on faecal particle settling velocity and its correlation with physical attributes is scarce.

Purpose of the Study:

• To quantify fresh faecal particle settling velocity and establish explicit correlations with physical attributes (size, shape) and implicit links to morphology via BSS.
• To develop an improved model for particle settling in fresh faeces across different flow regimes for better sanitation design.

Main Methods:

• Quantified settling velocity of 677 individual fresh faecal particles.
• Correlated settling velocity with particle size, shape, and BSS classification.
• Developed and refined particle drag models considering irregular shapes, rotation, and fractal character.

Main Results:

• Existing models adequately described settling for particles <3000 µm, but inertial forces and aspect ratio dominated for larger particles.
• Particle shape irregularity and fractal character significantly influence drag and settling velocity.
• A revised irregular solid particle model improved settling velocity estimation across Stokes, intermediate, and turbulent flow regimes.

Conclusions:

• Faecal particle morphology, particularly shape and fractal nature, is crucial for effective solid-liquid separation in sanitation.
• The revised particle settling model enables improved design of pit latrines for faecal sludge management.
• Optimized solid-liquid separation can significantly reduce sludge transportation costs, a major barrier to affordable sanitation.