Water Treatment Processes
Core treatment train
Most Florida systems treat groundwater, but the exam covers both groundwater and surface water trains. Know the sequence: source → pretreatment (aeration, oxidation, softening, coagulation/flocculation) → sedimentation → filtration → disinfection → storage/distribution. Groundwater plants commonly remove iron, manganese, and hydrogen sulfide by oxidation (chlorine, ozone, or permanganate) followed by filtration; lime softening removes hardness and raises pH.
Key calculations and concepts
- Chemical dose: lb/day = MGD × mg/L × 8.34
- Detention time: DT (hr) = tank volume (gal) ÷ flow (gpd) × 24
- Surface loading rate = flow ÷ surface area (gpm/ft²)
- CT = disinfectant residual (mg/L) × contact time (min); compare to required CT for pH, temperature, and log credit.
Traps
Watch unit mismatches (gpm vs gpd vs MGD), and remember that dose is applied, demand consumes some, and only residual remains. Filter backwash rates and run times are common trick items. Also, CT must use peak hourly flow, not average day, and the lowest residual in the contact basin.
Worked example
A plant treats 1.5 MGD at a chlorine dose of 2.5 mg/L. lb/day = 1.5 × 2.5 × 8.34 = 31.3 lb/day. If the residual after demand is 1.0 mg/L and contact time is 20 min, CT = 20 mg·min/L - compare to the required table value.
Rule of thumb
One mg/L in one MGD equals 8.34 lb/day. Memorize it; it solves most feed problems.