Water Treatment: Processes, Performance, and Calculations
The treatment train
Most surface-water plants follow the same sequence: coagulation, flocculation, sedimentation or clarification, filtration, and disinfection, with corrosion control and fluoridation added as required. Know the purpose of each step and how one step's performance shifts the load on the next. Coagulation destabilizes particles so they can stick together; flocculation builds settleable floc; sedimentation removes the easy solids; filters polish the remainder; disinfection supplies the required CT (residual concentration multiplied by contact time) for pathogen inactivation. Groundwater plants often need only disinfection, plus oxidation and filtration for iron and manganese or softening for hardness.
Traps that show up in questions
Terminology and units are the usual tripwires. Dose is what you feed, demand is what the water consumes, and residual is what is left: residual = dose − demand. Convert volumes carefully (1 MGD ≈ 694 gpm; 1 cfs ≈ 448.8 gpm). Filter and surface loading rates are gpm/ft², not gpm; CT is mg/L·min. A jar test optimizes coagulant dose and pH, not disinfectant dose. Percent removal is (in − out) ÷ in × 100, so 5 NTU dropping to 0.25 NTU is 95 percent, and it is easy to report the complement by mistake. Filter effluent is judged by turbidity: for conventional or direct filtration, at least 95 percent of monthly combined filter effluent readings must be ≤0.3 NTU, and no reading may exceed 1 NTU.
Worked example
A plant feeds 3.0 mg/L chlorine and measures 0.8 mg/L free residual. Demand = 3.0 − 0.8 = 2.2 mg/L. At 2.0 MGD, chlorine fed = mg/L × MGD × 8.34 = 3.0 × 2.0 × 8.34 ≈ 50 lb/day. To raise the residual to 1.0 mg/L, add 0.2 mg/L to the dose, not 1.0 mg/L. Always verify state-specific limits with your primacy agency.