86Destabilization Mechanisms
There are two primary destabilization mechanisms in drinking water treatment: charge
neutralization and sweep flocculation. The mechanism is dependent upon the coagulant dose. Most
drinking water treatment plants operate using sweep flocculation, which requires a higher coagulant
dose, rather than charge neutralization. In charge neutralization, the positively charged metal
coagulant is attracted to the negatively charged colloids via electrostatic interaction.
Flocs start to form during the neutralization step as particle collisions occur. Adding excess
coagulant beyond charge-neutralization results in the formation of metal coagulant precipitates.
These metal hydroxide compounds (e.g., Al(OH)3 or Fe(OH)3) are heavy, sticky and larger in
particle size. Sweep flocculation occurs when colloidal contaminants are entrained or swept down
by the precipitates as they settle in the suspension.
This course contains EPA’s federal rule requirements. Please be aware that each state
implements drinking water regulations that may be more stringent than EPA’s
regulations. Check with your state environmental agency for more information.