Graduate Thesis Or Dissertation
 

The effect of coagulation conditions on floc size distribution and morphology : implications for downstream treatment processes

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  • Coagulation and flocculation are commonly used in drinking water treatment as a means of destabilizing and aggregating particles and promoting adsorption of dissolved organic carbon (DOC). In practice, jar tests are used to determine optimal conditions for the removal of turbidity and DOC. However, beyond the inferences that can be made with respect to turbidity removal during the sedimentation (and/or coarse filtration) step, little attention is paid to details of the particle size distribution (PSD) and the morphology of the aggregates that are formed. These physical properties can strongly influence the efficiency of downstream processes (e.g., sedimentation, granular media filtration, and membrane filtration). A greater quantitative understanding of how coagulation conditions influence the size distribution and morphology of floc formed during water treatment is necessary. For example, the optimal PSD for efficient removal in sedimentation is likely very different than the desired PSD for direct filtration using membranes. Experiments were conducted using monodisperse polystyrene microspheres, polydisperse synthetic water (kaolin clay and humic acid), and natural waters. The focus the experimentation was to develop a new methodology for characterizing aggregate morphology and to apply that method to evaluate the influence of physical and chemical conditions on the resultant PSD, fractal dimension, and removal of DOC during coagulation. Samples were collected and analyzed using a Coulter Counter Multisizer 3 and a Brightwell Technology DPA 4100-D digital particle analyzer. PSDs based on solid volume (Coulter Counter) and maximum Feret's diameter (DPA 4100) allowed the determination of the fractal dimension using a modification of the two-slope method developed by Jiang and Logan (1996). Results from experiments conducted using latex microsphere destabilized in 0.6M NaCl show that distinguishable changes in the PSD and floc morphology due to changes in the mixing conditions can be observed using the methodology described in the following chapters. Microsphere coagulation was conducted at three distinct mixing conditions: flocculation, breakup, and regrowth. Flocculation mixing resulted in a dramatic reduction of primary particles as large porous floc were formed. These floc were characterized by a low three dimensional fractal dimension, D₃, implying that the floc were loosely packed and had a low density. Restructuring was observed during breakup mixing that saw large floc broken up into multiple smaller and more compact floc, as characterized by an increase in D₃. Regrowth allowed the floc size to recover from the restructuring of the breakup mixing phase, with the resultant floc being characterized by a D₃ between that observed for the flocculation and breakup phases. Enhanced alum coagulation of synthetic and natural waters produced fragile floc which were difficult to characterize in terms of the fractal dimension as measured using the methodology validated using the latex microspheres. The difficulty in analyzing the large alum floc was likely caused by breakup during measurement. The results show that coagulant pretreatment significantly alters the size distribution of the feed water. Optimizing the coagulation conditions with respect to specific downstream processes has the potential to increase efficiency and decrease operational and maintenance costs.
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