Coagulation
Coagulation destabilizes particles so they can collide and stick. Understand colloidal charge (clay and organics carry negative surface charge), charge neutralization, sweep floc with metal salts, and the role of pH in determining which hydrolysis species dominate. Know the common coagulants-aluminum sulfate (alum), polyaluminum chloride (PACl), ferric chloride, ferric sulfate-and their optimum pH windows (alum roughly 5.5–7.5; iron salts effective over a wider, often lower, range). Recognize natural organic matter (NOM) as the target for DBP precursor removal and as a consumer of coagulant.
Traps: assuming more coagulant is always better (overdosing can restabilize particles and reverse charge, raising turbidity); ignoring pH drift from coagulant acidity; confusing coagulant dose with feed rate or with concentration of the stock solution; forgetting that cold water slows hydrolysis and often requires higher dose or a different coagulant. Watch questions that give jar-test results and ask you to pick the best dose-read the settled turbidity and pH, not just the flash-mix appearance.
Rule of thumb: optimize on settled turbidity and filtered turbidity together, and confirm pH is inside the coagulant's window before chasing dose.
Worked example: A jar test shows 10 mg/L alum gives 1.5 NTU settled turbidity at pH 6.8, while 20 mg/L gives 3.0 NTU at pH 6.2. The higher dose both overdosed and dropped pH out of range; 10 mg/L is the better choice, but verify filtered turbidity before committing.