Flocculation

Flocculation is the process by which small primary particles in an aqueous suspension join to form a larger secondary, or composite, particle. Although the term composite particle encompasses all multigrain suspended sediment particles, the term flocculation refers to all processes by which suspended particles are brought together in water to create larger units termed flocs. This distinguishes flocs from aggregates, which are composite particles formed in a nonaqueous medium. In engineered systems, flocculation has been referred to variously as aggregation, agglomeration, and coagulation, but the result is still the formation of larger composite particles. In natural systems, flocculation is known best in estuaries and open-ocean environments but has also been documented in freshwater fluvial and lacustrine environments. Flocculation is an important aspect of water quality in both natural and engineered systems, as the floc settling velocity must exceed the settling velocity of the constituent particles due to an increase in mass and as flocs are more easily filtered from water than are primary particles.

Flocculation is a dynamic process controlled by a variety of physical, chemical, and biological factors. The two necessary conditions for flocculation are collision of primary particles and subsequent cohesion. The mechanisms responsible for collision include turbulence within the suspending liquid and differential settling of the particles by size, shape, or [Page 1128]density. Following collision, cohesion can be attributed to electrochemical attraction or the presence of “sticky” organic substances. Larger flocs exhibit increasingly complex arrangements of primary particles, and water or organic material occupies the void space between primary particles. The floc density must therefore be lower than the density of the primary particles (typically considered equal to the density of quartz, 2.65 g/cm3 [grams per cubic centimeter]) and greater than the density of water (1.0 g/cm3 at 4 °C). Smaller flocs show strong cohesion between closely arranged primary particles and are thus less fragile than larger flocs, which exhibit a more complex geometry, fewer opportunities for cohesion between primary particles, and larger void spaces. Flocs are difficult to sample without disturbance, and floc density is often estimated using measurements of the settling velocity and the diameter and a subsequent rearrangement of Stokes's law to solve for the density. During laboratory analysis of sediments, samples are often deflocculated using chemical and/or mechanical means. Assuming complete deflocculation, the resulting grain size distribution is representative of the primary particles alone. Analysis of secondary particle size often requires in situ measurements (e.g., laser diffraction), and any difference between in situ and ex situ size distributions can be used as an index of flocculation. Although a positive relationship between floc settling velocity and size is well documented, there is typically a high degree of scatter in this relationship because flocs of similar diameter often exhibit unique geometry, void space volume, or composition of either primary particles or organic substances. Flocs of identical diameter can show settling velocities that range over three orders of magnitude, and scatter typically increases with floc diameter.