Simultaneous In Situ Characterization of Turbulent Flocculation and Reactor Mixing Using Image Analysis and Particle Image Velocimetry in Unison

Abstract

Floc characteristics, including their size distribution, mean size, and fractal dimension, are impacted by mixing intensity and duration, coagulant dosing method, dose, type, and pH. Herein, we directly employ flocs inherently generated during treatment to determine instantaneous velocity fields, which were further utilized to estimate local velocity gradients and turbulent kinetic energy dissipation (TKED) rates. This novel non-intrusive methodology, which combines particle image velocimetry (PIV) and an imagery-based sizing scheme, was examined through the characteristics of reactor mixing and flocculation from in situ measurements for conventional FeCl3 chemical coagulation and iron electrocoagulation (EC). Our first-of-its-kind procedure avoids the need to externally add artificial seeding particles for PIV analysis, automatically reducing the number of experiments, and simultaneously improves both accuracy and precision by linking fluid dynamics and particle characteristics with only a single experiment. The TKED rates estimated using flocs were largely similar with conventional artificial seeding at the early stage of flocculation when flocs were smaller. However, the new method is expected to estimate turbulence more accurately in the flocs’ microenvironment during the later stages of tapered flocculation when particle sizes are similar to dissipation eddy dimensions, especially at high particle concentrations (i.e., highly turbid waters). Measured size distributions, mean sizes, and fractal dimensions confirm the robustness of the present imagery-based sizing scheme and showed that EC produced numerous and more compact flocs than conventional coagulation.