Characterization, fate and transport of floc aggregates in full-scale flocculation tanks

Abstract

Floc size distribution is of high operational importance as it governs the transport and removal of contaminants from drinking water. The complex nature of turbulent flow in treatment plants has limited the research to quantify the dynamics of flocculation. This paper describes the use of a submersible digital in-line holographic microscopy (DIHM) technique for the measurement of the spatial distribution of floc sizes in a direct filtration treatment facility. The DIHM tool was positioned at thirteen different locations in a flocculation tank and holograms were recorded for at least 10 minutes at each location. The acquisition of morphological details of flocs (e.g., floc counts, size distributions and floc velocities relative to the fluid motion) from the reconstructed DIHM images is discussed. The results of the spatial distribution of floc sizes indicated that the volume equivalent floc diameter measured in the flocculation tank was mostly of large-sized aggregates greater than 100 μm, which can have adverse impacts on the performance of a direct filtration process. The relative motion of flocs calculated from the DIHM analysis ranged from 0.002 to 0.008 m s⁻¹. Results showed that the investigated DIHM technique could be used as an operational tool to evaluate flocculation performance in terms of floc sizes, which is otherwise difficult to characterize in most treatment plants. The information acquired from this tool is important to understand the fate and transport of flocs during flocculation for process optimization that can lead to minimize chemical and energy usage in treatment plants.