Treatment Processes and Process Control
What you must know
Match treatment to source quality. Conventional surface treatment is coagulation, flocculation, sedimentation, filtration and disinfection. Also understand direct filtration, membranes (MF/UF for particles and pathogens, NF/RO for dissolved contaminants), greensand for iron, manganese and arsenic, granular activated carbon and ion exchange for organics, nitrate, radium and PFAS, lime softening versus ion-exchange softening, aeration and sequestration, fluoridation, and corrosion-control inhibitors such as zinc orthophosphate or silicates.
Process control means watching the right numbers: coagulant dose and pH, alkalinity, filtered-water turbidity plus early-warning tools (particle counts, streaming current), disinfectant residual, temperature, and CT. Backwash on head-loss limit or turbidity breakthrough, with enough bed expansion (roughly 15 to 25 percent) to fluidize the media.
Traps
- Confusing theoretical detention time with T10 - CT calculations use T10.
- Forgetting CT requirements rise in colder water and at higher pH, and that chloramines need far more CT than free chlorine.
- Reading a log credit as removal rather than inactivation: 2-log is 99 percent, 3-log is 99.9 percent, 4-log is 99.99 percent.
- Mixing dose (mg/L) with mass feed (lb/day).
Worked example
A plant treats 2.5 MGD and doses 1.5 mg/L of coagulant. Mass feed = mg/L x MGD x 8.34 = 1.5 x 2.5 x 8.34, about 31.3 lb/day. For disinfection, 1.2 mg/L free chlorine times 30 min T10 gives CT = 36 mg-min/L; if the required table value for your pH and temperature is 40, you are short and must raise residual or contact time.