Water Treatment Processes
Treatment questions ask you to place each process in the train, state its performance standard, and troubleshoot when a limit is missed. Massachusetts enforces filtration and disinfection for surface water under 310 CMR 22.20.
The conventional train
Coagulation (alum, ferric chloride, polyaluminum chloride) destabilizes particles; flocculation builds floc; sedimentation drops it out; filtration polishes; disinfection provides the barrier; corrosion-control chemicals (pH adjustment, phosphate) protect the pipe network. Alternatives include direct filtration, slow sand, diatomaceous earth, membranes (MF, UF, RO), and packaged plants. Iron and manganese are removed by oxidation (aeration, chlorine, permanganate) plus filtration, or controlled by sequestration. Granular or powdered activated carbon handles taste, odor, and organics; greensand handles iron, manganese, and arsenic.
Performance you must know
- Combined filter effluent: turbidity ≤0.3 NTU in at least 95 percent of monthly measurements, never above 1 NTU.
- Individual filter effluent: ≤0.15 NTU in 95 percent of measurements; investigate any reading above 0.5 NTU.
- CT = disinfectant residual (mg/L) × T10 (minutes), expressed as mg·min/L. Required CT rises as water gets colder and, for free chlorine, as pH rises. T10 is baffled time to 10 percent of the tracer, not theoretical detention time.
Worked example
One 250 ft² filter treats 1.0 MGD. Loading rate = 1,000,000 gal/day ÷ 250 ft² = 4,000 gpd/ft². Divide by 1,440 min/day: 2.78 gpm/ft², inside the usual 2–4 gpm/ft² design range for rapid sand filters.
Traps
Reporting average turbidity instead of the percentage of readings. Calling 0.3 NTU an absolute ceiling when 1 NTU is. Using CT without the temperature and pH adjustment.