Drinking Water Treatment
Core process train
A conventional surface-water plant runs coagulation, flocculation, sedimentation, filtration, and disinfection. Coagulant dose (alum, ferric salt, polymer) is set by jar test, and optimum pH and alkalinity matter because alum consumes alkalinity. Flocculation is slow, gentle mixing; sedimentation is quiescent detention; filtration is the main barrier for Giardia and Cryptosporidium; disinfection mainly targets bacteria and viruses.
CT and disinfection
CT = residual (mg/L) x contact time (min), in mg-min/L. Required CT depends on the disinfectant, target log inactivation, pH, and temperature. Free chlorine is weaker at high pH and low temperature, so required CT is larger. Traps: using the dose instead of the measured residual; ignoring the baffling factor (effective time = nominal time x baffling factor); assuming chloramines behave like free chlorine, when they need far more CT.
Worked example
Residual 1.2 mg/L free chlorine, clearwell nominal time 25 min, baffling factor 0.7. Effective time = 17.5 min; CT = 1.2 x 17.5 = 21 mg-min/L. If the table requires 18 mg-min/L for 3-log Giardia, you meet it; log credit = 3 x (21/18), capped at the target.
Traps
Combined filter effluent turbidity must be 0.3 NTU or less in at least 95% of monthly readings and never above 1 NTU; individual filters also have a 1 NTU limit. 5 NTU is not the treatment limit. Residual disinfectant must be measured before the first customer. Backwash rates are set by filter media expansion, not by turbidity.