Sedimentation Sludge Zone Inlet Zone Outlet Zone V o = Q/A s The overflow rate H

Sedimentation All particles with a settling velocity greater than or equal to the overflow rate will be 100% removed. Particles with a lesser settling velocity will be removed to a fractional extent. Those that happen to enter the settling zone near the bottom will be completely removed and those that enter near the top will not. Typical design overflow rates for settling tanks are: gpd/ft 2

Sedimentation All particles with a settling velocity greater than or equal to the overflow rate will be 100% removed. Particles with a lesser settling velocity will be removed to a fractional extent. Those that happen to enter the settling zone near the bottom will be completely removed and those that enter near the top will not. Typical design overflow rates for settling tanks are: gpd/ft 2

Sedimentation Tank Configurations Rectangular Clarifiers –most common Circular clarifiers –Center feed –Peripheral feed Flocculator-Clarifiers

Sedimentation Tank Design Side water depth: about 12 ft. settling velocities: 2-6 ft/hr retention time: 4-8 hr overflow rates: gpd/ft 2 or gpm/ft 2 linear velocities: less than 0.5 ft/min

Typical rectangular clarifier

Sedimentation Tank Design Side water depth: about 12 ft. settling velocities: 2-6 ft/hr retention time: 4-8 hr overflow rates: gpd/ft 2 or gpm/ft 2 linear velocities: less than 0.5 ft/min

Sedimentation Tank Configurations Rectangular Clarifiers –most common Circular clarifiers –Center feed –Peripheral feed Flocculator-Clarifiers

Filtration A “polishing” solid/liquid separation step Intended to remove particles Other impacts –biodegradation –organics adsorption (especially to GAC) –Mn and Fe adsorption

Types of Filtration Granular media filters –slow sand filters –rapid sand filters –high-rate granular media filters Membrane filters –microfiltration, ultrafiltration, nanofiltration Cake filtration –diatomaceous earth

Filtration: Mechanisms Interception –lines of flow strike media sedimentation diffusion straining –too large to fit between spaces flocculation –promoted by increased turbulence

Deposition in a Filter

Filter Design 4.9 m/hr = 2 gpm/ft 2 From Tobiason, 1995

Filter Operation