Water Treatment Processes and Unit Operations
What the candidate must know
Conventional surface water treatment follows: coagulation, flocculation, sedimentation, filtration, and disinfection. Know the purpose of each step and the key operating parameters. Coagulation destabilizes particles; flocculation gently mixes to grow floc; sedimentation removes settleable floc; filtration polishes; disinfection inactivates pathogens.
Concepts to master
- Jar testing to determine coagulant dose and pH.
- The importance of pH and alkalinity in coagulation (alum consumes alkalinity).
- Filter operation: rate, headloss, backwash triggers, and turbidity limits.
- The concept of CT (concentration × time) for disinfection and its relationship to treatment technique requirements.
- Membrane and alternative processes at a high level.
- Corrosion control and the Lead and Copper Rule basics.
Traps in questions
Questions often test whether you know that coagulant overdose can increase turbidity, or that filter breakthrough is signaled by rising effluent turbidity. Another trap is confusing sedimentation with filtration, or assuming more coagulant is always better.
Worked example / rule of thumb
Rule of thumb: if raw turbidity rises, expect to adjust coagulant dose and possibly pH; monitor filter effluent turbidity continuously. For CT, higher disinfectant residual or longer contact time both increase CT - but CT requirements depend on pH, temperature, and the target pathogen.