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

Given the scarcity of empirical data, current literature employs the Bristol Stool Scale (BSS) as a surrogate for evaluating the role of faecal structural characteristics in solid-liquid separation efficiency. To establish a more definitive relationship, this study quantified the settling velocity of 677 individual fresh faecal particles, enabling explicit correlations between particle drag and physical attributes (size and shape), and implicit associations with morphological features via BSS classification. For faecal particles below 3000 µm, settling velocity could be relatively well described by existing models that assume viscous forces dominate drag. However, the particle population largely exceeded this threshold size, where the role of particle aspect ratio and inertial forces were evidenced to be of primary importance to solid-liquid separation. Modelling of particle drag was undertaken to account for particle shape irregularity and particle rotation across the particle size range but led to an overestimation of settling velocity due to its derivation being based on the hydrodynamic behaviour of solid particles. The deviation was accounted for by the diversity in fractal character amongst the particle population, which can enhance drag; a contribution which can be inferred from morphological classification provided by the BSS. The constants associated with the irregular solid particle model were therefore interrogated to improve estimation of particle settling for fresh faeces across the three flow regimes (namely Stokes, intermediate and turbulent). Current sanitation practice does not take advantage of solid-liquid separation in existing pit latrine design. The revised particle model permits solid-liquid separation design for fresh faecal sludge, where the volume reduction provided by fast effective segregation of high solids concentration particles can transform sludge transportation costs, which remains the critical economic barrier to achieving safe and affordable sanitation.