Water Treatment
Core treatment train
A surface water plant typically runs coagulation, flocculation, sedimentation, filtration, and disinfection. Know what each step removes and what upsets it. Coagulant dose is set by jar test and raw-water quality; pH and alkalinity control how well the coagulant works. Flocculation is gentle mixing to grow floc; sedimentation removes settleable solids; filters polish. Filtration performance is judged by filtered-water turbidity and filter run time; backwash when head loss or turbidity limits are reached, not just on a clock.
Disinfection and CT
Disinfection depends on concentration, contact time, temperature, pH, and the organism. CT is residual concentration (mg/L) times contact time (min); required CT rises as water gets colder and as pH moves away from the optimum for chlorine. Free chlorine is a stronger primary disinfectant than chloramine, but chloramine lasts longer in the distribution system. Watch DBP precursors: high TOC and bromide can push THM and HAA formation.
Traps
Questions often swap dose and residual, or ask for contact time when you need CT. “More coagulant” is not always better - overdosing can restabilize particles and raise filter loading. Do not confuse turbidity, a physical measure, with disinfectant residual.
Worked example
If the required CT for a target inactivation is 100 mg/L·min and the free chlorine residual at the point of measurement is 1.0 mg/L, you need 100 minutes of effective contact time. At 2.0 mg/L, the same target needs 50 minutes, assuming the same temperature and pH basis.