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Characterization of ballasted flocs in water treatment using microscopy

Lapointe, Mathieu; Barbeau, Benoit

Water research. Volume 90 (2016); pp 119-127 -- Elsevier

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  • Title:
    Characterization of ballasted flocs in water treatment using microscopy
  • Author: Lapointe, Mathieu;
    Barbeau, Benoit
  • Found In: Water research. Volume 90 (2016); pp 119-127
  • Journal Title: Water research
  • Subjects: Water--Pollution--Research--Periodicals; Ballasted flocculation--Floc density--Floc size--Floc shape--Optical microscopy--Settling velocity; Dewey: 363.7394
  • Rights: legaldeposit
  • Publication Details: Elsevier
  • Abstract: Abstract:

    Ballasted flocculation is widely used in the water industry for drinking water, municipal wastewater, storm water and industrial water treatment. This gravity-based physicochemical separation process involves the injection of a ballasting agent, typically microsand, to increase the floc density and size. However, the physical characteristics of the final ballasted flocs are still ill-defined. A microscopic method was specifically developed to characterize floc 1) density, 2) size and 3) shape factor. Using this information, probability density functions (PDFs) of the floc settling velocity were calculated. The impacts of the mixing intensity, polymer dosage, microsand size and contact time during the floc maturation phase were assessed. No correlation was identified between the floc diameter, form and density PDFs. The floc equivalent diameter mainly controls the settling velocity ( r  = 0.94), with the floc density ( r  = 0.26) and shape factor ( r  = 0.25) having lower impacts. A velocity gradient of 165 s −1 was optimal to maintain the microsand in suspension while simultaneously maximizing the floc diameter. An anionic high molecular weight polyacrylamide formed 1.5-fold larger aggregates compared with the starch-based polymer tested, but both polymers produced flocs of similar density (relative density = 1.53 ± 0.03). Generally, the floc mean settling velocity is a good predictor of the turbidity removal. An in-depth analysis of the floc characteristics indicates a correlation between the floc size and the largest microsand grain potentially embeddable in the floc structure.

    Graphical abstract:

    Highlights:

    The ballasted floc equivalent diameter has a higher impact on the settling velocity than the floc density or shape.

    The turbidity removal is positively correlated with to the mean floc settling velocity.

    The shape factor of the ballasted flocs has a negligible impact on the calculated settling velocity.

    A flocculation of 165 s −1 was established as an optimal shear rate when microsand is employed as the ballasting agent.


  • Identifier: System Number: LDEAvdc_100030668343.0x000001; Journal ISSN: 0043-1354; 10.1016/j.watres.2015.12.018
  • Publication Date: 2016
  • Physical Description: Electronic
  • Shelfmark(s): ELD Digital store

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