Sedimentation

Sludge Zone

Inle

t Zon

e

Out

let Z

one

Vo = Q/As

vH

t

H Q Q

Aod

VQ

VH s

The overflow rateThe 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 Typical design overflow rates for settling tanks are: 500-1000 gpd/ftsettling tanks are: 500-1000 gpd/ft22

Fraction Removed = vQAs

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 Typical design overflow rates for settling tanks are: 500-1000 gpd/ftsettling tanks are: 500-1000 gpd/ft22

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: 500-1000 gpd/ft2 or 0.3-0.7 gpm/ft2

• 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: 500-1000 gpd/ft2 or 0.3-0.7 gpm/ft2

• 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

Type Filtr. Rate(m/hr)

Media Size(mm)

Bed Depth(m)

Slow Sand 0.04-0.4 0.25-0.35 1.0

RapidSand

5-10 0.45-1.0 0.5-1.0

High-rate 10-35 0.8-2.0 1.2-2.5

Diatom.Earth

2.5-7.5 0.01-0.05

4.9 m/hr = 2 gpm/ft4.9 m/hr = 2 gpm/ft22From Tobiason, 1995

Filter Operation