Water Treatment: Unit Processes, Dosing Math, and Finished-Water Quality
What this domain covers
The Water Treatment domain tests the unit processes that turn a raw surface-water supply into safe finished water, and the operator math and monitoring that go with each. In the District, the Potomac River is the source: the Washington Aqueduct (U.S. Army Corps of Engineers) operates the Dalecarlia and McMillan treatment plants, and DC Water distributes that finished water. Examinations therefore combine general treatment theory with distribution-relevant finished-water quality.
The process train
- Coagulation destabilizes particles with a metal salt (alum, ferric chloride, PACl); jar tests set the dose as raw water turbidity, pH, alkalinity, and temperature change.
- Flocculation gently stirs (tapered energy) to grow floc that sedimentation can remove; short-circuiting and floc shear are the enemies.
- Sedimentation removes settleable solids; tube or plate settlers raise effective area and improve performance.
- Filtration (rapid gravity, dual-media anthracite/sand) removes remaining particles and most protozoa. Combined-filter effluent must stay below 0.3 NTU in 95% of monthly samples and never exceed 1 NTU; individual filters are watched for breakthrough (rising turbidity) and air binding.
- Disinfection by chlorine or chloramine is judged by CT (residual mg/L × contact time, min), corrected for temperature, pH, and disinfectant.
Worked example: coagulant feed
An 18 MGD plant doses 20 mg/L of liquid alum. Convert concentration and flow to mass: 20 mg/L × 18 MGD × 8.34 = 3,002 lb/day of alum product. The 8.34 factor converts mg/L × MGD to pounds per day because one gallon of water weighs 8.34 lb. If the dose were quoted as dry alum and the product were 48% strength, you would divide by 0.48 to find the liquid feed - the most common arithmetic error in dosing questions.
Traps that cost marks
- Reporting percent removal when the question asks for log removal (log10 of influent/effluent).
- Forgetting that CT requirements tighten as temperature falls and pH rises.
- Assuming free chlorine when the system uses chloramine: monochloramine reads as total chlorine, and free chlorine will be near zero.
- Confusing filter-to-waste with backwash, or the 0.3 NTU combined-filter limit with the 1 NTU maximum.
- Ignoring that coagulant demand rises with turbidity while alkalinity consumption drives pH down and can hurt floc.