TABLE OF CONTENTS Coagulation and flocculation techniques are used to separate suspended and colloidal matter from water when the natural sedimentation rate of the suspended matter is too low to provide effective clarification. Most wastewater contains both settleable solids and non-settleable solids. A large portion of the latter may be colloidal in nature; i.e., each particle is stabilized by negative electrical charges on its surface and surrounding it. These small, negatively charged colloids repel one another, much as do the negative poles on magnets, and this repulsion prevents the particles from clumping together to form larger, more settleable solids.
Coagulation is the destabilization of these colloids by neutralizing the negative charges on the colloidal surfaces that keep the particles apart. This is accomplished by adding chemical coagulants and mixing the solution together so that the coagulants can neutralize the colloidal particle surfaces. Salts of aluminum or iron and polyelectrolytes are used for colloidal charge neutralization.
Flocculation is a different mechanism that is used to promote sedimentation of suspended matter, although the terms are sometimes used interchangeably. After chemical coagulation, the colloidal particles tend to clump together, but they often do not clump together in sufficient numbers to settle rapidly enough. Chemical flocculants are added to a coagulated wastewater to tie the colloidal clumps together in a process termed " bridging" to increase the floc size.
The mechanisms of coagulation and flocculation are depicted in Figure 6.9. On the left side of the figure we have...
