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SEDIMENTATION-1 TEKNOLOGI PENGOLAHAN LIMBAH

Definition separation of unstable and destabilized

suspended solids from a suspension by the

force of gravity

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Applications in Wastewater

Treatment

grit removal

suspended solids removal in primary

clarifier

biological floc removal in activated

sludge (secondary clarifier)

Settling of particles from

suspension depends on:

1. Characteristics of the Particles

Discrete Particle: particles whose size, shape and specific gravity do not change with time.

Flocculating particles: particles whose surface properties are such that they aggregate upon contact. Thus, changing in size, shape, and perhaps specific gravity with each contact

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Settling of particles from

suspension depends on:

2. Concentration of Particles in Suspension:

Dilute suspension: suspensions in which the

conc. of particles is not sufficient to cause

significant displacement of water as they settle

or in which the particles will not be close

enough for velocity field interference to occur

Concentrated Suspension: suspensions in which

the conc. of particles is too great to meet the

conditions mentioned for dilute suspensions

Basic Theory of Settling

pwpG gVF

2

pwpd

D

vACF

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Velocity of Settling

18

)( 2

pwp

p

dgv

Coefficient of drag as a

function of Reynolds Number

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SETTING VELOCITY VS PARTICLE DIAMETER

Settling in a Tank

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Discrete Settling in 3 types of Basin

GRIT REMOVAL

Sandy material, mostly inorganic, is grit

Mostly between 0.05 and 1 mm in diameter

Considered to be “non-sticky” particles, but are mostly “sticky” particles. Coated with fats, oils and grease in the sewers, increasing buoyancy (upward force)

Need to know true grit size to determine the settling velocity –wet sieving versus dry sieving

If not removed in grit tanks, ends up in digesters

Grit characteristics change with flow rates to the plant

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HORIZONTAL-FLOW GRIT CHAMBER

The flow passes through the chamber in a

horizontal direction and the straight-line

velocity

Flow is controlled by the dimensions of the

unit, an influent distribution gate, and a

weir at the effluent end.

Horizontal-flow

Rectangular Square

HORIZONTAL-FLOW GRIT CHAMBER

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Typical design information for

horizontal-flow grit chamber

Unit Range Typical

Detention Time s 45-90 60

Horizontal Velocity m/s 0.25-0.4 0.3

Settling velocity for removal of

0.21 mm material m/min 1.0-1.3 1.15

0.15 mm material m/min 0.6-0.9 0.75

Headloss in control section as percent of depth in channel % 30-40 36

Added length allowance for inlet and outlet turbulence % 25-50 30

Aerated Grit Chamber

Nominally designed to remove 0.21-mm-diameter or larger, with 2-to-5-minute detention periods at the peak hourly rate of flow.

Air is introduced along one side of a rectangular tank to create a spiral flow pattern perpendicular to the flow through the tank.

The heavier grit particles that have higher settling velocities settle to the bottom of the tank.

Lighter, principally organic, particles remain in suspension and pass through the tank.

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Typical Section Through on Aerated Grit Chamber

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Helical Flow Pattern in An Aerated Grit Chamber

Aerated grit chamber with traveling-bridge type grit removal system

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Typical design information for aerated grit chamber

Unit Range Typical Detention time at peak flow rate min 2-5 3

Dimensions:

Depth min 2-5

Length min 7.5-20

Width min 2.5-7

Width-depth ratio ratio 1:1 -5:1 1.5:1 Length-width ratio ratio 3:1 - 5:1 4:1 Air supply per unit of length m3/m.min 0.2-0.5

Grit quantities m3/103m3 0.004-0.29 0.0015

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VORTEX GRIT CHAMBER

Used in small plants and these require lesser area

Grit is removed with vortex flow pattern

Due to inertia the grit particle will remain in the

chamber and liquid free from grit will only

escape

1. The rotating turbine maintains constant flow velocity, and its adjustable pitch blades promote separation of organics from the grit.

2. The action of the propeller produces a toroidal flow path for grit particles.

3. The grit settles by gravity into the hopper in one revolution of the basin's contents.

4. Solids are removed from the hopper by a grit pump or an airlift pump.

5. Grit removed by a grit pump can be discharged to a hydro-clone for removal of the remaining organic material.

6. Grit removed by an airlift may be dewatered on a wedge wire screen.

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Typical design for Vortex grit

chamber

Unit Range Typical

Detention time of average flow rate s 20-30 30

Diameter

Upper chamber m 1.2-7.2

Lower chamber m 0.9-1.8

Height m 2.7-4.8

Removal rates

0.3 mm % 92-98 95

0.24 mm % 80-90 85

0.15 mm % 60-70 65

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Next week………

Primary sedimentation tanks

Design calculation for all

sedimentation tanks………….