Drinking Water Treatment
Core concepts
A treatment train is a series of barriers: coagulation, flocculation, sedimentation, filtration, and disinfection. Know what each unit removes and what it cannot. Coagulation neutralizes particle charge so flocs can form; flocculation is gentle mixing that grows the floc; sedimentation removes settleable solids; filtration polishes remaining particles; disinfection inactivates what is left.
Most of the math lives in disinfection and loading. CT = disinfectant residual (mg/L) × contact time (min), and the required CT depends on the target organism, the log credit claimed, pH, and temperature. Cold water and higher pH raise the required CT for free chlorine. Also know coagulant dose from jar tests, filter loading rate (gpm/ft²), backwash triggers, and the conventional-filtration turbidity limits: at or below 0.3 NTU in at least 95% of monthly readings and never above 1 NTU.
Traps Questions Love
Dose is not residual. Theoretical detention time is not contact time - baffling factors shrink it. Chlorinating before turbidity is controlled wastes chlorine because particles shield pathogens. Alkalinity is not hardness; alkalinity buffers coagulation and pH. And a low pH reading is not automatically a corrosion problem if the water is naturally low in buffering capacity.
Worked Example
Free chlorine residual is 1.0 mg/L and contact time is 30 minutes: CT = 1.0 × 30 = 30 mg·min/L. If the required CT for 3-log Giardia inactivation at that pH and temperature is 60, you have only half the credit needed - raise the residual, add contact volume, or lower pH.