Physical Treatment Processes
Core Concepts
Physical treatment removes contaminants without changing their chemical form. Key processes include screening, grit removal, sedimentation, flotation, filtration, and adsorption. Sedimentation (clarification) relies on Stokes' Law, where settling velocity depends on particle size, density difference, and viscosity. Type I settling is discrete particle settling; Type II is flocculant settling; Type III is hindered settling; Type IV is compression. Dissolved air flotation (DAF) is effective for low-density particles and oils. Filtration (granular media, membrane) removes suspended solids; adsorption (activated carbon) removes dissolved organics. Equalization balances flow and load. Traps include confusing primary vs. secondary clarifiers (primary before biological treatment, secondary after), misapplying Stokes' Law to flocculant settling, and overlooking hydraulic loading rate (HLR) and solids loading rate (SLR) in clarifier design. Also, surface overflow rate is a critical design parameter. Another trap: assuming DAF works well for dense, rapidly settling solids-it is for buoyant particles. Membrane fouling and carbon exhaustion are operational concerns.
Worked Example
A clarifier has a surface area of 500 m² and receives a flow of 10,000 m³/d. The surface overflow rate (SOR) is 10,000/500 = 20 m³/(m²·d). If the design SOR for the expected particle size is 15 m³/(m²·d), the clarifier is overloaded. Rule of thumb: SOR must be less than the settling velocity of the slowest particle to be removed. This example shows the trap of ignoring units and the relationship between flow, area, and settling.