Water Treatment
Treatment train and the concepts tested
A surface-water plant typically strings together coagulation, flocculation, sedimentation, filtration, and disinfection, and exam items probe both the purpose of each step and the arithmetic behind it. Know that coagulation destabilizes particles (charge neutralization), flocculation builds settleable floc with gentle mixing, sedimentation removes it by gravity, filtration polishes, and disinfection inactivates pathogens. Understand jar testing as the bench-scale tool for choosing coagulant dose and pH.
Numbers to own
Removal efficiency = (influent - effluent) / influent x 100, always in the same units. Filter hydraulic loading = flow (gpm) ÷ filter area (ft²), typically about 2-6 gpm/ft² for conventional gravity filters; backwash rates run much higher. Disinfection credit uses CT = disinfectant concentration (mg/L) x contact time (min), with T10 rather than theoretical detention, and at the peak flow of record. Chlorine demand = dose - residual.
Traps
Mixing mg/L with pounds or gpm; forgetting that a coagulant dose may be stated as product rather than active ingredient; applying a percent-removal answer to a concentration question; and assuming more chlorine always means more CT credit, when pH, temperature, and turbidity all matter. Turbidity limits apply to combined filter effluent, not raw water.
Worked example
A filter receives 1,200 gpm over 300 ft² of surface area. Loading = 1,200 ÷ 300 = 4 gpm/ft², a normal conventional range. If that filter's effluent must average under 0.3 NTU, a result of 0.45 NTU signals a coagulation, media, or backwash problem, not a reason to add chlorine.