Fluid flow pattern and water residence time in waste stabilisation ponds

摘要

As treatment processes are kinetic-dependent, a consistent description of water residence timesnis essential to the prediction of waste stabilization ponds performance. A physically-based 3Dntransient CFD model simulating the water velocity, temperature and concentration fields as anfunction of all influent meteorological factors – wind speed and direction, solar radiation, airntemperature and relative humidity – was used to identify the relationships between thenmeteorological conditions and the hydrodynamic patterns and water residence times distributionsnin a polishing pond. The required meteorological data were recorded on site and waterntemperatures recorded at 10 sampling sites for 141 days. Stratification events appear on veryncalm days for wind speeds lower than 3ms21nand on sunny days for wind speeds lower thann5ms21n. De-stratification is related to two mixing processes: nightly convection cells and globalnmixing patterns. Numerical tracer experiments show that the results of the flow patterns can benevaluated using the dispersed flow regime approximation and, for wind speeds exceedingn6ms21n, the completely stirred tank reactor assumption.