Drinking Water Treatment: Process Train, CT, and Compliance Traps
Follow the train, not the vocabulary list
Treatment questions reward knowing why each unit exists. The conventional train is coagulation (alum, ferric chloride, or a polymer destabilizes particles), flocculation (slow, gentle mixing grows floc), sedimentation (settleable floc drops out), filtration (polishing, usually anthracite over sand), disinfection, and clearwell contact time. A jar test is the bench tool for choosing coagulant dose and coagulation pH; if finished-water turbidity climbs, the answer is rarely "add more chlorine."
Traps that show up repeatedly
- Free chlorine is the disinfectant; total chlorine includes chloramines and is what chloraminated systems track for compliance.
- CT = disinfectant residual (mg/L) × actual contact time (minutes), reported in mg/L·min. Use detention time at peak flow through baffled volume, not basin volume divided by average flow.
- Cryptosporidium is chlorine-tolerant; UV or ozone provides the credit. Giardia and viruses are the usual chlorine log-credit targets.
- Conventional filtration turbidity limits: 0.3 NTU or less in at least 95% of monthly samples, and never above 1 NTU.
- Cold water slows both coagulation and disinfection, so required CT rises in winter.
Worked example
A clearwell gives a measured free chlorine residual of 1.5 mg/L and an actual contact time of 20 minutes at peak flow: CT = 1.5 × 20 = 30 mg/L·min. If the required CT for the intended log inactivation at that pH and temperature is 24 mg/L·min, the ratio 30 ÷ 24 = 1.25 exceeds 1.0, so the requirement is satisfied. Always pull the required value for the correct pH, temperature, and target organism; using the wrong table row is the classic error.