Drinking Water Treatment: Core Concepts and Exam Traps
What You Must Understand
Drinking water treatment is a sequence of barriers, each targeting a different contaminant. Know the order and purpose: source intake → coagulation → flocculation → sedimentation → filtration → disinfection → storage/distribution. Coagulation (rapid mix) destabilizes particles by neutralizing surface charge; flocculation (slow mix) gently stirs so particles collide and grow into settleable floc; sedimentation removes floc by gravity; filtration polishes residual turbidity; disinfection inactivates pathogens last so nothing downstream re-contaminates the water.
Key quantitative relationships: CT (disinfectant concentration × contact time) governs pathogen inactivation; higher turbidity shields microbes and consumes disinfectant, so filtration performance directly affects disinfection. Log removal/inactivation targets (e.g., 3-log Giardia, 4-log virus) are cumulative across treatment barriers. Know common coagulants (alum, ferric chloride), the meaning of NTU (turbidity), and Langelier Saturation Index for corrosion control.
Traps to Watch
- Order questions: coagulation before flocculation, disinfection after filtration. Mixing these up is the most common error.
- CT vs. concentration alone: doubling chlorine dose does not double CT if contact time is fixed-CT is a product.
- Turbidity and disinfection: lower turbidity improves disinfection efficiency; questions often invert this.
- pH effects: coagulation works in a narrow pH band; chlorine speciation (HOCl vs. OCl⁻) shifts with pH, and HOCl is the stronger disinfectant.
Rule of Thumb
For CT problems, compute CT = residual concentration (mg/L) × contact time (minutes) and compare to the required CT for your target pathogen, temperature, and pH. Example: 1.0 mg/L residual × 30 min = CT of 30 mg·min/L. If the table requires 25 for your conditions, you meet it; if it requires 40, you do not-extend contact time or raise residual.
Study Focus
Practice reading CT tables, tracing treatment train order, and explaining why each unit process exists.