The Colloidal Problem
Natural water contains billions of colloidal particles per liter — clay, organic matter, bacteria, and viruses ranging from 1 nm to 10 μm. These particles carry negative surface charges from ionized surface groups and adsorbed anions. The resulting electrostatic repulsion (quantified by zeta potential) keeps them suspended indefinitely. Without chemical intervention, gravity settling and filtration cannot adequately remove them.
Charge Neutralization and Sweep Floc
Coagulation works through two primary mechanisms. Charge neutralization occurs when positively charged metal hydrolysis products adsorb onto negative particle surfaces, reducing zeta potential toward zero. Sweep flocculation occurs at higher doses when amorphous metal hydroxide precipitates form and physically enmesh particles as they settle. The simulation models the dose-dependent transition between these mechanisms.
The Jar Test: Empirical Optimization
Despite advances in modeling, the jar test remains the gold standard for coagulant optimization. Six beakers with different doses undergo rapid mix (1-2 minutes at G = 100-300 s⁻¹), slow mix/flocculation (15-30 minutes at G = 20-50 s⁻¹), and settling (30-60 minutes). The dose producing lowest settled turbidity at acceptable pH is selected. Plants run jar tests daily to track changing source water quality.
Mixing Energy and Floc Growth
The velocity gradient G (s⁻¹) controls both the rate of particle collisions and the shear forces on growing floc. Rapid mixing at high G distributes coagulant uniformly. Slow mixing at lower G promotes gentle collisions that build floc without breaking it. The Camp number Gt (typically 10,000-100,000) represents total mixing energy. The simulation shows how G affects floc size — too high shears floc apart, too low produces small, slowly settling aggregates.