Disinfection and Residual Management
CT, and what chlorine actually is
Disinfection is judged by CT - disinfectant residual (mg/L) times effective contact time (minutes) - compared with tables of required CT for a target log inactivation of Giardia, viruses, or Cryptosporidium. Candidates must know that T is the baffled, effective time, not theoretical detention: compute theoretical time, then apply a baffling factor (a well-baffled 2.0–4.0 contactor is roughly 0.5–0.7). CT must be met at peak hourly flow and worst-case conditions.
Each disinfectant has its own table and weaknesses: free chlorine is strong against viruses but poor against Crypto; ozone and chlorine dioxide are powerful oxidants that can form bromate and chlorite; UV provides no residual and is dosed in mJ/cm².
Know the chemistry: free chlorine speciation depends on pH - HOCl is far more effective than OCl⁻ (pKa ≈ 7.5), so raising pH weakens disinfection. Chloramines are weaker but longer-lasting and prone to nitrification in the distribution system. Chlorine demand consumes dose before any residual remains. DBP limits are TTHM 80 µg/L and HAA5 60 µg/L as locational running annual averages. Groundwater systems must meet a 4-log virus standard; distribution residuals are commonly held at ≥0.2 mg/L entering the system.
Worked example: theoretical detention 40 min × 0.6 baffling = 24 min effective; outlet residual 1.2 mg/L → CT = 1.2 × 24 = 28.8 mg·min/L, then compared against the required CT for that pH and temperature.
Traps: using theoretical rather than baffled time; sampling residual at the wrong point; applying CT to UV; forgetting that cold water raises the required C·T.