Sediment Resuspension in Water Distribution Storage Tanks

Animated simulation of particle suspension in a water distribution storage tank. 3 million gallon tank, 10,000 gpm flow rate. Particles colored by particle diameter (red=1 mm, green=0.1 mm, blue=0.01 mm)

Particle Resuspension Calculator – Shields, Paphitis, Beheshti models

Executive Summary

Sediments in storage tanks are known to pose human health risks because of the potential for accumulation of pathogens, metals, and other hazardous materials. This report addresses sediments in storage tanks and the potential for these sediments to be transported back into water distribution systems. Computational fluid dynamics (CFD) models were developed and three simulation studies conducted to provide insights into sediment resuspension processes in tanks. In addition, a pilot-scale experiment was conducted to validate the model predictions. The results of this study highlight tank operating conditions which might reduce resuspension and removal of sediments from tanks.

The simulation studies used a cylindrical, ground level, 11,000 m3 (3 million gallon) tank with a single inlet/outlet as its model domain. Sediment was assumed to be distributed on the tank bottom and made up 0.01, 0.1, or 1 mm diameter particles. CFD models were used to calculate shear stresses on the tank bottom to predict if the particles would be resuspended from the tank bottom and then entrained in the fluid flow, removed from the tank through the drain, or deposited back on the tank bottom. Flow rates, the inlet/outlet location and diameter, filling or draining cycles were varied in order to understand the effects of these key parameters on the resulting shear stress and the potential for particle resuspension. Key results of the simulation studies were as follows:

  • Resuspension occurred under all operating conditions tested; however less than 25% of particles were resuspended and less than 1% were drained
  • Particle resuspension only occurred within a short distance of the inlet/outlet
  • Particle resuspension generally occurred immediately following the start of either the filling or draining processes
  • Particle size, flow rate, and inlet/outlet location were found to be important factors for particle resuspension
    • Smaller particles were more susceptible to resuspension
    • Higher flow rates yielded greater resuspension
    • Inlet/outlets located near the side wall yielded greater resuspension
    • Draining yielded greater resuspension of particles than filling
  • Particle resuspension was directly correlated to the amount of momentum flow, or jet effect, through the inlet/outlet. Reducing the flow rate or increasing the diameter of the inlet/outlet reduces the momentum flow and the potential for particle resuspension.
  • A raised inlet pipe extending 15 cm (6 inches) above the tank bottom substantially reduced the number of particles resuspended during filling, while a pipe height extending 30 – 61 cm (12 – 24 inches) reduced the number of particles drained from the tank

Small-scale experimental tests were performed to validate the resuspension model. Glass beads and sand particles were placed in a 1.2 m (4 ft) diameter water-filled tank with a 2.0 cm (0.8 in) diameter inlet/outlet to study resuspension and removal. Photos and videos were recorded before and after each filling and draining event to determine where particles were resuspended from the tank bottom. Tracer tests were also performed to characterize the flow patterns and velocity fields. Finally, mitigation measures were investigated by raising the pipe inlet, which was normally flush with the tank bottom, a short distance above the tank bottom. Key results were as follows:

  • Measured and simulated velocities along tank bottom matched well in the region where particles were resuspended
  • Resuspension of particles was only observed within ~1 cm from the inlet/outlet during filling and draining cycles for the flow rates used in the study
  • Smaller particles and less dense particles yielded a greater resuspension
  • Model predictions of resuspension generally matched experimental data for glass beads, and generally over predicted the amount of resuspension for silica sand. The non-spherical shape of the sand may have reduced the amount of resuspension in the tests.
  • A raised inlet height of 3-8% of the head of the water above the tank bottom was able to completely mitigate particle movement near the inlet/outlet

Based on the simulation and experimental studies conducted for this report, several strategies for reducing resuspension and removal of sediments from tank bottoms were identified:

  • constructing a raised inlet pipe 30 – 60 cm (12 – 24 inches) above the tank bottom;
  • placing the inlet/outlet near the center of the tank rather than near the side wall;
  • reducing the inlet/outlet flow rates; and
  • increasing the diameter of the inlet/outlet pipes.