Water and Wastewater Engineering
Water treatment
A conventional surface-water plant is a chain: coagulation (often alum or ferric salts) destabilizes particles, flocculation grows floc, sedimentation removes it by overflow rate (Q/A), filtration polishes, and disinfection provides the pathogen barrier. Disinfection is judged by CT = disinfectant concentration × contact time, adjusted by a baffling factor; detention time is t = V/Q. Distribution design balances pressure, storage, and fire flow.
Wastewater treatment
Influent is characterized by flow, BOD5, and TSS. Primary clarifiers remove settlable solids; activated sludge removes dissolved organics. Key controls are solids retention time (SRT = solids in the aeration basin divided by solids wasted per day), food-to-microorganism ratio, recycle ratio, and dissolved oxygen. Secondary clarifiers must separate floc without denitrifying and floating sludge. Sludge is thickened, digested, and dewatered.
Traps
The mass-loading conversion is the most common stumble: lb/day = concentration (mg/L) × flow (MGD) × 8.34. Do not substitute gpm or cfs without converting. BOD5 is a five-day demand, not ultimate BOD. SRT (days) is not the same as hydraulic detention time (hours). Percent removal uses influent and effluent concentrations on the same basis.
Worked example
A plant receives 2.5 MGD at 220 mg/L BOD5. Load = 220 × 2.5 × 8.34 = 4,587 lb/day. If effluent is 20 mg/L, removal = (220 − 20)/220 = 91%.