Disinfection and Chemical Feed
Killing pathogens safely
Disinfection questions test your grasp of chlorine chemistry, CT, and safe chemical feed. Know the forms of chlorine: free chlorine (hypochlorous acid and hypochlorite ion), combined chlorine (chloramines), and the difference in strength and persistence. Understand the breakpoint curve: as chlorine is added, it reacts with ammonia and organics, forms chloramines, then breaks through to free chlorine. Hypochlorous acid (HOCl) is the dominant disinfecting species; its fraction depends on pH (more effective at lower pH).
Concepts to master
- CT = concentration × contact time, adjusted for pH and temperature; higher CT means more inactivation credit.
- Chlorine demand and residual; dose must exceed demand to leave a residual.
- Dechlorination for discharges and spills (sulfur dioxide, sodium bisulfite).
- UV and ozone as alternatives; UV needs low turbidity, ozone produces bromate.
- Chemical feed systems: metering pumps, day tanks, and secondary containment.
- DBP control: manage TOC and water age.
Traps
A common trap: assuming more chlorine always means better disinfection, when high pH or high demand can reduce effectiveness. Another: confusing demand with residual. Also watch for questions that ignore pH effects on HOCl vs. OCl⁻.
Worked example
If free chlorine residual is 1.0 mg/L after 30 minutes contact, CT ≈ 30 mg·min/L before corrections. If the required CT is higher, increase contact time or residual, not just the dose.