Water and Wastewater Engineering: Treatment and Process Design
Core ideas
Water treatment. A conventional train is coagulation, flocculation, sedimentation, filtration, and disinfection. Sedimentation design uses surface overflow rate (Q/A), which must be less than the settling velocity of the design particle. Filtration is rated in gpm/ft². Disinfection uses CT = disinfectant concentration × contact time, compared with a required CT that depends on temperature, pH, and target log inactivation. Softening removes hardness expressed as CaCO3.
Wastewater. BOD5 measures biodegradable organics over 5 days at 20 °C; ultimate BOD is higher. With k = 0.23/day, BOD5/BODu ≈ 0.68. Activated sludge design uses SRT (MCRT) = solids in the system ÷ solids leaving per day, and F/M = QS0/(VX). Clarifier performance depends on surface overflow rate and solids loading rate. Nutrient removal uses nitrification/denitrification and biological or chemical phosphorus removal.
Rules of thumb and traps
Mass loading: lb/day = 8.34 × mg/L × MGD. Also 1 MGD = 1.547 cfs = 694 gpm. Common traps: using influent concentration where effluent is required; ignoring recycle flows in SRT; confusing BOD5 with ultimate BOD; and mixing gallons with MGD.
Worked example
Flow = 2.5 MGD, influent BOD5 = 220 mg/L. Mass = 8.34 × 220 × 2.5 = 4,587 lb/day. With k = 0.23/day, BODu = 220/(1 − e^(−0.23×5)) = 220/0.683 ≈ 322 mg/L, showing why BOD5 understates oxygen demand.