Towards a Solid Particle Hydrodynamic (SPH)-Based Solids Transport Model Applied to Ultra-Low Water Usage Sanitation in Developing Countries

Distribution of toilet facilities and low-cost small-bore simplified sewerage systems (SS) in peri-urban areas provide opportunities to improve public health, provide safely managed sanitation, and protect the environment in low-to-middle income countries. Smoothed particle hydrodynamics (SPH) offers opportunities for optimisation of ultra-low water usage systems, but not without computational challenges. Results from SPH modelling of low cost, low water usage sanitary appliances were compared to a validated 1D finite difference model (DRAINET) for evaluation and calibration. An evaluation of system performance linked solid transport capabilities to physical geometries. The SPH model was developed for a pour-flush toilet pan connected to a 100 mm diameter pipe. The scheme utilized a free surface turbulent model to evaluate solid (faecal) transport efficacy. Performance was greatly influenced by the artificial viscosity factor, ViscoBoundFactor, within SPH, relating to the interaction of fluid and fluid particles and fluid and boundary particles. Results indicate that an increase in this factor leads to a reduction in fluid velocity with an attendant reduction in solid transportation distance, leading to inaccuracies. Other issues such as the use of density and mass in the definition of solid characteristics made it less predictable than the established 1D model for predicting solid transport. Overall, SPH was found to be useful for characterising the geometry of the pour flush pan but not for whole system assessment. A hybrid method is thus recommended whereby the design and performance characteristics for the input stage can be modelling in SPH but the whole system pipe network evaluation is better suited to the 1D DRAINET model.