Source Water and Water Treatment
What a candidate must know
Start with source characterization: groundwater versus surface water, aquifer types, and the contaminants each tends to carry (iron and manganese in groundwater; turbidity, color, and pathogens in surface water). Know the treatment train and the purpose of each step: screening and presedimentation, coagulation and flocculation, sedimentation, filtration (rapid sand, slow sand, membrane), and disinfection. Understand coagulant chemistry in practical terms (alum lowers pH and consumes alkalinity), and be able to explain why flocculation is gentle mixing while rapid mix is violent. Learn filter operation: backwash triggers (head loss, turbidity breakthrough), filter-to-waste, and the difference between surface loading rate and run length. Disinfection concepts include CT (concentration x contact time), demand, decay, and the tradeoff between pathogen inactivation and disinfection byproduct formation. Also know corrosion control basics and the role of pH and alkalinity.
Traps
Questions love to swap the order of unit processes, or to ask what happens to pH when alum is added (it drops). Another common trap confuses sedimentation (gravity settling of floc) with filtration (removal of remaining particles). Watch for CT questions that ignore pH or temperature corrections, and for claims that higher chlorine dose always means proportionally higher CT.
Rule of thumb
If turbidity rises sharply right after a backwash, suspect a ripening filter; return it to service slowly or filter to waste. For a worked example: a clearwell gives 30 minutes of contact time at a measured residual of 1.0 mg/L; CT is about 30 mg-min/L before applying the pH and temperature correction factors from the CT tables.