Water Treatment
Water treatment questions test whether you can link a treatment goal to the unit process that achieves it, then prove with arithmetic that the process is really working.
The process chain
Know the conventional chain and the control variable for each step: coagulation destabilizes particles (alum, ferric salts, polymers); flocculation grows floc with gentle, tapering mixing; sedimentation removes floc by gravity; filtration polishes; disinfection inactivates pathogens. Control variables: jar test and pH for coagulation, weir and surface loading for clarifiers, head loss and effluent turbidity for filters, and CT for disinfection.
CT is the big trap
CT equals disinfectant residual concentration (mg/L) multiplied by contact time (minutes). Required CT comes from tables keyed to pH, temperature, and the target log inactivation. The classic error is treating CT as a fixed pass/fail number. Cold water and higher pH raise the required CT sharply, so a plant that earned full credit in July may fail in January. Also know that CT credit depends on the residual actually measured at the end of the contact basin, not the dose applied at the start.
Other frequent traps: the combined filter effluent standard (≤0.3 NTU in at least 95% of monthly measurements, and never above 1 NTU); confusing mg/L with lb/day; and forgetting that coagulant chemicals consume alkalinity and depress pH.
Worked example
Raw alkalinity is 80 mg/L and the alum dose is 40 mg/L. Alum consumes roughly 0.45 mg/L alkalinity per mg/L of alum, so 40 × 0.45 = 18 mg/L is used, leaving 62 mg/L. If pH begins to fall as alkalinity approaches zero, add lime or switch coagulants before the floc turns pin-floc and shears through the filters.
Use lb/day = mg/L × MGD × 8.34 for every chemical feed calculation.