Sediment-laden wastewater jet under the effect of linear regular waves

Abstract

Sedimentation of particles from wastewater disposal is a major issue in marine and coastal ecosystems due to the potential for the particles to accumulate and persist over time, resulting in lasting ecological and health consequences. In this study, the settling mechanism of sediment from marine outfalls is investigated to quantify its deposition profile through physical experiments, numerical modelling, and dimensional analysis for various source and surface wave conditions. The results show that the sediment undergoes an up-and-down oscillatory motion caused by the orbital motion of water molecules in waves, causing the sediment concentration at the nozzle to vary at different wave phases. Unlike the jet in still water, the wave induces additional turbulence together with the jet flow turbulence which exerts a substantial impact on the average speed of sediment movement, which ultimately determines the pattern of sediment accumulation on the bottom. The sediment turns to settle more slowly under stronger wave momentum intensity (i.e., smaller jet-to-wave velocity ratio R_w). To complement the experimental findings, a 3D Computational Fluid Dynamics (CFD) model was developed using ANSYS Fluent, incorporating a realizable k-ε turbulence model for the fluid phase and a Lagrangian Discrete Phase Model (DPM) for sediment tracking. The simulations validated the observed wave-modulated sedimentation patterns, including transverse spreading of deposition, which revealed that wave orbital velocities exceeding 0.03 m/s generate sufficient shear to significantly enhance lateral particle dispersion. New parameter formulations with wave effects are proposed, including the sedimentation length scale l_mw, the location where the sedimentation begins $x_1$, the maximum deposition rate F_smax, and its corresponding location x_max. The study demonstrates the necessity of accounting for surface waves, including their role in enhancing transverse sediment dispersion, when designing actual wastewater discharge systems.