In processing biological solids such as municipal and industrial
wastewater sludges, the solids are very fine, from micron to submicron ranges, and the density difference between the solids
and the suspending medium is very small, both of which result
in difficult separation. Coagulants, such as aluminum sulfate, ferric
chloride and ferric sulfate, are used to neutralize the charges (typically
negative) left on the particles that keep them in dispersion in
a suspension by electrical repulsion. Upon being neutralized, the
particles can agglomerate to larger particles by the Van der Waals’
attractive force. Another effective solution to separate these fine
particles is to flocculate the suspension with polymers which can
extend their molecular chains in solution attracting and wrapping
up the smaller particles by ionic or non-ionic interactions (Van der
Waals’ force) such that much larger, stable flocculated particles
form that can settle readily under centrifugation [1–3]. The polymer
(also known as polyelectrolytes) used for flocculation can be
anionic, or cationic, to neutralize the charges left on these fine particles
in the flocculation process to form much larger flocculated
solids (abbreviated hereafter as floc). Both coagulation and flocculation
can work independently, or in combination, to achieve the
most desirable results.