Drinking Water Treatment - Ohio Operator Study Guide
Drinking Water Treatment is the Ohio operator exam domain that covers converting raw surface or ground water into finished water that meets primary standards. Expect questions on coagulation, flocculation, sedimentation, filtration, disinfection, CT, turbidity, and chemical feed-plus how Ohio EPA's primary drinking water rules translate into operating decisions.
Treatment barriers
- Coagulation neutralizes particle charge; flocculation builds settleable floc. Jar tests set coagulant dose and pH; overdosing can restabilize particles or raise residuals.
- Sedimentation removes floc by overflow rate; short-circuiting and sludge blankets reduce performance.
- Rapid-rate filtration must produce combined filter effluent turbidity ≤0.3 NTU in at least 95% of monthly measurements and never >1 NTU for conventional plants. Watch individual filter effluent and backwash performance.
- Disinfection is quantified by CT = C × T10, where C is residual disinfectant concentration and T10 is the time for 10% of tracer to pass through. Required CT depends on disinfectant, temperature, pH, and target log inactivation.
- TOC removal and DBP control begin upstream; chlorine dose and contact time affect both inactivation and THM/HAA formation.
Worked example
A clearwell provides T10 = 30 min at peak flow. Free chlorine residual is 1.5 mg/L, so CT = 1.5 × 30 = 45 mg·min/L. If the required CT for 3-log Giardia at the measured temperature and pH is 60 mg·min/L, the plant is short. Raising the residual to 2.0 mg/L gives CT = 60 mg·min/L, but only if peak flow and T10 stay the same. The reasoning: use T10, not theoretical detention time, and confirm the CT table value for actual pH and temperature.
Traps
Using theoretical detention time instead of T10; treating 1 NTU as the monthly filter limit instead of the never-exceed value; ignoring chlorine demand; mixing turbidity (NTU) with chlorine residual (mg/L); and forgetting that pH and temperature change required CT.