Water Treatment Processes
Think of treatment as a multiple-barrier train: source protection, coagulation/flocculation, sedimentation, filtration, disinfection, and sometimes softening or special processes. Coagulation destabilizes particles by neutralizing negative surface charge (with alum, ferric salts, or polymers); flocculation is gentle mixing that grows floc; sedimentation lets floc settle; filtration polishes remaining particles. Know jar testing for dose selection, optimum pH, zeta potential in concept, overflow rate for clarifiers, and filter basics: media (sand, anthracite, mixed), backwash, head loss, turbidity, and filter-to-waste.
Disinfection: understand CT (concentration × contact time), log inactivation, and how pH, temperature, and turbidity affect chlorine effectiveness. Know that turbidity protects microbes, so filtration and disinfection are linked.
Traps: confusing coagulant dose with pH adjustment; assuming more coagulant always helps (overdosing can restabilize or drop pH); mixing sedimentation with flocculation; and forgetting that CT depends on actual contact time and residual, not just dose.
Rule of thumb: lower turbidity before disinfection, then verify CT. Worked example: if free chlorine residual is 1.0 mg/L and effective contact time is 30 minutes, CT = 30 mg·min/L; compare that to the required CT for the target log inactivation at the measured pH and temperature before declaring compliance.