Disinfection and Oxidation
What you must understand
Disinfection inactivates pathogens; oxidation transforms contaminants. Know the common disinfectants-free chlorine, chloramines, chlorine dioxide, ozone, UV-and their strengths and limits. Key concept: CT (disinfectant concentration × contact time) and how pH, temperature, and demand affect it. Free chlorine is more effective at lower pH because hypochlorous acid (HOCl) dominates over hypochlorite ion (OCl⁻) below about pH 7.5.
Understand chlorine demand and breakpoint chlorination: initial demand is satisfied, combined chloramines form, and beyond the breakpoint free chlorine appears. Know DBP formation (trihalomethanes, haloacetic acids) from chlorine reacting with natural organic matter, and why utilities switch to chloramines or ozone. Recognize UV as a physical disinfectant (no residual; depends on dose and transmittance) and ozone as a powerful oxidant with no lasting residual and bromate risk.
Traps
A common trap: assuming chloramines are stronger disinfectants than free chlorine-they are weaker but more stable, giving better distribution residual and lower DBP formation. Another: forgetting that UV and ozone provide no residual. Watch for confusion between contact time and CT, and between primary and secondary disinfection goals.
Rule of thumb
At pH ~7, roughly half or more of free chlorine is HOCl; at pH 8.5, most is OCl⁻ and disinfection is slower. So a pH rise of one unit can meaningfully increase required CT. Always confirm the required CT from tables using measured pH, temperature, and residual.