Flocculation in natural freshwater systems has been suggested and inferred by many researchers,1-4 and was explicitly investigated by Droppo and Ongley.5 These studies and others6-9 have concluded that in addition to the electrochemical processes, the bacterial processes also play a role in the formation of freshwater flocs. It is believed (Ongley et al.10) that the biological processes contribute for flocculation in two different ways: bacterial bonding and bacterial "glue." Marshall6 had shown that the

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bacteria have a high affinity for fine grained sediment particles and thereby promotes flocculation by increasing the surface area and bonding two or more mineral particles together. Biddanda11 and Muschenheim et al.8 had shown that secretion of extracellular polymeric exudates by certain bacteria provide the necessary bonding material (glue) to hold particles together.

Modeling of the flocculation process in freshwater system has been attempted by several investigators.12-15 The approach used by these investigators is based on the premise that the freshwater flocculation is a two step process in which the particles are first brought into contact by collision mechanisms such as Brownian motion, laminar and turbulent fluid shear, inertia, and differential settling, and subsequently, a certain amount of such collisions result in the formation of flocs because of the electrochemical and bacterial bonding and bacterial "glue." While our knowledge on the collision mechanisms and the collision frequencies is reasonably well established, the same cannot be said for the actual mechanism of flocculation (i.e., how collided particles bind and form flocs). The approach used in the existing models is to introduce a collision efficiency parameter that is a measure of the probability of successful collisions, and to determine the value of this parameter as part of the calibration process of the model. A flocculation modeling approach proposed by Krishnappan and Marsalek15 for a stormwater detention pond is reviewed here to highlight the current state of knowledge in the area of modeling of freshwater flocculation.

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