Chapt4 Flocculation

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cmutsvangwa: Water Quality & Treatment: Dept. of Civil & Water, NUST 10/10/2006 4.1 Chapter 4 FLOCCULATION Flocculation It is the process of gentle and continuous agitation, during which suspended particles in water coalesce into larger masses so that they may be removed in subsequent treatment processes (growth of flocs). Flocculation and sedimentation basins shall be as close together as possible. When a colloidal suspension has been destabilised, primary floc particles are formed and grow in size through contact with other particles as a result of Brownian motion (bombardment of colloidal particles by molecules). As the particle grows in size, the influence of Brownian effects diminish and the rate of particle aggregation reduces. The process of utilising Brownian motion in flocculation is called perikinetic flocculation. To accelerate the rate of particle collision, velocity gradients are created within the body of dispersing fluid and the use of controlled velocity gradient to promote flocculation is called orthokinetic flocculation. Rapid mix The desired reactions between the coagulant and water are irreversible and take place in fraction of a second. Therefore mixing must be fast to avoid formation of undesired products. The design parameters of rapid mix units are: mixing time, t volume occupied by the particles velocity gradient, G Velocity Gradient Difference in velocities of two particles, divided by the distance between their paths. It is a measure of relative velocity of two particles of fluid at a distance. L V V G A B = , s -1 A 1 Fig. 4.1 L B R B R A = radius of particle A R A R B = radius of particle B Chapter 6 Flocculation

Transcript of Chapt4 Flocculation

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Chapter 4

FLOCCULATION Flocculation It is the process of gentle and continuous agitation, during which suspended particles in water coalesce into larger masses so that they may be removed in subsequent treatment processes (growth of flocs). Flocculation and sedimentation basins shall be as close together as possible. When a colloidal suspension has been destabilised, primary floc particles are formed and grow in size through contact with other particles as a result of Brownian motion (bombardment of colloidal particles by molecules). As the particle grows in size, the influence of Brownian effects diminish and the rate of particle aggregation reduces. The process of utilising Brownian motion in flocculation is called perikinetic flocculation. To accelerate the rate of particle collision, velocity gradients are created within the body of dispersing fluid and the use of controlled velocity gradient to promote flocculation is called orthokinetic flocculation. Rapid mix The desired reactions between the coagulant and water are irreversible and take place in fraction of a second. Therefore mixing must be fast to avoid formation of undesired products. The design parameters of rapid mix units are:

• mixing time, t • volume occupied by the particles • velocity gradient, G

Velocity Gradient Difference in velocities of two particles, divided by the distance between their paths. It is a measure of relative velocity of two particles of fluid at a distance.

LVVG AB −= , s-1

A

1

Fig. 4.1

L

B

RB

RA= radius of particle A RA RB= radius of particle B

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Attraction occurs when: . BA RRL +<Consider a small cubical element of fluid with flow in x-direction and velocity gradient in the z-direction (Fig. 4.2):

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Fig. 4.2

Velocity gradient dz

vvdzdv 12 −=

Newton’s law of viscosity for one dimensional flow:

dzdvµτ =

Where; τ =shear stress per unit area or momentum transferred in the

direction µ =dynamic viscosity v =velocity

=planetoponforceShear ( ) areasurfacestressshearyx ×=∆∆ .τ

But dzdvµτ =

⇒ ( )yxdzdvFforceShear ∆∆= .µ

gradientvelocitycedisforcePPower ××= tan,

dzdvzFP ∆×=

( )dzdvzyx

dzdvP ∆∆⋅∆= µ

( )zyxdzdvP ∆⋅∆⋅∆⎟

⎠⎞

⎜⎝⎛=

2

µ

x∆

z∆

τ y∆ dzdvzv ∆+

v

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)And ( zyx ∆⋅∆⋅∆ is equal to the volume, V

VdzdvPPower

2

⎟⎠⎞

⎜⎝⎛== µ

2

⎟⎠⎞

⎜⎝⎛==

dzdv

VP

VolumePower µ

gradientvelocitydzdvG =⎟

⎠⎞

⎜⎝⎛=

2G

VP µ=

Therefore the velocity gradient can be expresses in terms of power as:

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⎟⎟⎠

⎞⎜⎜⎝

⎛=

µVPG

Where: G =velocity gradient (20 to 100s-1)

P =power input, W or N.m/s (in baffle flocculators, QgHP ρ= ) V =volume of water in flocculator, m3

µ =dynamic viscosity, N.s/m2 or kg/m.s g =9.81m/s2

High velocity gradient promotes rapid flocculation, and too velocity gradients can result in fragile flocs, which get torn apart. Turbulence promotes collisions and the Camp Number is analogous to the number of collisions experienced by an element of water as it moves through the flocculator. The Camp Number is defined as the product of the velocity gradient and the retention time.

tGNumberCamp ×= Where: t =retention time (10 to 30 mins) Mechanical flocculators, with rotating impellers are prone to short circuiting because of the mixing effect of the impeller. To counteract several flocculators in series are installed. Following the rapid mix when the coagulants are added, a more gentle agitation is required to establish velocity gradients of magnitude suitable for flocculation. This can be induced by:

Chapter 6 Flocculation

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• mechanical flocculators • hydraulic flocculators

Hydraulic flocculators They are:

• horizontal flow basins with fixed baffles and mixing is accomplished by reversing the flow of water through channels.

• upward flow sludge blanket clarifiers • use of hydraulic structures like weirs.

Hydraulic structures Hydraulic mixing The degree of mixing is lower at low flow rates in the case of weir. So the likely variation of flow rate should be taken into account at the design stage. The coagulant should be injected just upstream for the maximum turbulence. Types of hydraulic mixers (a) Rectangular weir: coagulant should be injected at several points across

the width of channel

Fig. 4.3

(b) Vee notch: A plate just downstream helps to spread the coagulant across the width.

Fig. 4.4

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(c) Hydraulic jump: The jump must be in the same position whatever the flow rate. This can be achieved by using a flume such as the Parshall flume (Fig. 4.5). At low flows much of the may by-pass the turbulent “roller”.

Fig. 4.5

(c) Pipe lines an orifices: In a pipeline an orifice or an in-line mixer may

be used.

Fig. 4.6 Baffled flocculators Disadvantages of baffled flocculators

• energy is not uniformly distributed • being excessive at bends and inadequate in the straights (Fig. 4.10) • not flexible in operation i.e. they respond poorly to changes in raw water

quality • head loss is appreciable • cleaning may be difficult • limited to relatively large treatment plants, >10 000m3/day

There are two types of baffled flocculators (Fig. 4.7 and 4.8):

• Horizontal flow • Vertical flow

Horizontal flow flocculators preferred over the vertical flow, because they are easier to drain and clean, but the horizontal flow flocculators needed more land.

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Fig. 4.7 Horizontal flow baffled channel flocculator Source: Surface water treatment, Okun et al (1992)

Fig. 4.8 Vertical flow baffled channel flocculator Source: Surface water treatment, Okun et al (1992)

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Tappered flocculation Velocity gradients in a flocculation basin can be tappered to be high at the inlet and low at the outlet ends to achieve more efficient mixing and agglomeration of the floc particles (Fig. 4.9). Such a design can reduce the magnitude of shearing forces on the flocs as they agglomerate, and thereby reduce the chances of floc break up.

Fig. 4.9: Tappered flocculator Source: Surface water treatment, Okun et al (1992)

Fig. 4.10: Head loss diagram Source: Surface water treatment, Okun et al (1992)

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Design of hydraulic flocculators The velocity gradient is given as:

21

⎟⎟⎠

⎞⎜⎜⎝

⎛=

µVPG or

21

⎟⎟⎠

⎞⎜⎜⎝

⎛=

tghGµρ or

21

⎟⎟⎠

⎞⎜⎜⎝

⎛=

µρV

ghQG

Where: G =velocity gradient (20 to 100s-1)

P =power input, kg.m2/s3 (in baffle flocculators, QghP ρ= ) V =volume of water in flocculator, m3

µ =dynamic viscosity, kg/m.s h =head loss, m

t =retention/detention time, s (VQt = )

Q =Inflow to flocculator, m3/s g =9.81m/s2

tGNumberCamp ×= (104 to 105) Where: t =retention time (10 to 30 mins) Number of baffles for the horizontal flow flocculator The number of baffles needed to achieve a desired velocity gradient (Okun et al, 1992):

( )

31

2

44.12

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎥⎦

⎤⎢⎣

⎡⎥⎦

⎤⎢⎣

⎡+

=Q

HlGf

tnρ

µ

Number of baffles for the vertical flow flocculator The number of baffles needed to achieve a desired velocity gradient (Okun et al, 1992):

( )

31

2

44.12

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎥⎦

⎤⎢⎣

⎡⎥⎦

⎤⎢⎣

⎡+

=Q

wlGf

tnρ

µ

Where: n =number of baffles H =depth of water in basin, m l =length of basin, m G =velocity gradient, s-1

Q =flow, m3/s t =time of flocculation, s (15-30min) µ =dynamic viscosity, kg/m.s f =coefficient of friction of baffles, f=3 fof timber w =width of basin, m Chapter 6 Flocculation

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The design guidelines are given are as follows: Around the end for horizontal flow

• Distance between baffles should not be less than 45cm to permit cleaning

• Clear distance between the end of each baffle and the wall is about 1.5 time the distance between the baffles should not be less than 60 cm.

• Depth of water should not be less than 1m • Decay-resistant timber should be used for baffles, wood construction

is preffered over metal parts • Avoid using asbestos-cement baffles because they corrode at the pH

of alum coagulation.

(b) Over and under (vertical flow) • Distance between the baffles should be less than 45cm • Depth should be 2 to 3 times the distance between gabbles • Clear space between the upper edge of a baffle and the water

surface or the lower edge of a baffle and the basin should be about 1.5 time the distance between the baffles.

• Material for baffles is the same as in around the end units • Weep holes should be provided for drainage •

Example 4.1 Design a horizontal flow baffle channel flocculator for a treatment plant of 10000m3/day. The flocculation basin is to be divided into three sections of equal volume, each section having constant velocity gradients of 50, 35, and 25 s-1, respectively. The total flocculation time is to be 21min and the water temperature is 15oc. The timber baffles have a roughness coefficient of 0.3. A common wall is shared between the flocculator and the sedimentation tanks; hence the length of the flocculator is fixed at 10m. A depth of 1m is considered reasonable for horizontal flow flocculator. Solution Design the first flocculator section with a velocity gradient of 50s-1 and detention time of 7min. Total volume of flocculation basin

Total width of flocculator:

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Chapter 6 Flocculation

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Fig. 5 Schematic illustration of the design Source: Surface water treatment, Okun et al (1992)

Mechanical flocculators

• They are available in a variety of designs to suit any plant. Principal elements are agitator impellers, drive motors, speed controllers, reducers etc (Fig. 6 and 7).

• They can be adjusted to suit variations in flow, temperature, or raw water quality

• The flocculation is independent of flow through the units • Expensive to operate • Prone to short circuiting because of the mixing effect of the impeller • To counteract this problem, several flocculators are installed in series • Coagulant should be injected at the eye of the impeller for maximum

turbulence • Head losses are minimal • The flow of the coagulant should be visible so that it can be checked • Motor failures can affect the operation of the whole plant and also has

high maintenance costs

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Fig. 4.12 Schematic illustration of mechanical flocculators Design equations

21

221

2

22 ⎥⎦

⎤⎢⎣

⎡=⎥

⎤⎢⎣

⎡=

VvAvC

VvAC

G rdrd

γµρ

Where; Cd =drag coefficient for blades (1.8 for blades) A =area of blade (15 to 25% of tank cross-section) v =mean blade velocity (0.2 to 0.6 m/s) vr =velocity of blade relative to the fluid V =volume of flocculator t =retention time (20 to 40 min) (Casey, )

vvr 75.0=

2

2vvACP rd ρ=

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ExamA watestinoptim

• •

• •

Chapte

Fig.41

ple ter treatment plant is being designed to treat 50000m3/day of water. Jar g indicate the alum dose of 40mg/l with flocculation at Gt =40x10-4 produce um results at 15oC. Determine:

Monthly alum requirement Flocculation basin dimensions if three cross flow paddle horizontal paddles are to be used. The flocculator should be a maximum 12m wide and 5m deep in order to be connected appropriately with the settling basin. Power requirement Paddle configuration

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Chapter 6 Flocculation

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Chapter 6 Flocculation

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References

1. Casey T. J, Unit treatment processes in water and wastewater engineering

2. Schulz and Okun (1992), Surface water treatment for communities in developing countries, John Wiley and Sons, UK

3. Peavy H. S., Rowe D. R., and Tchobanoglous G., (1985), Environmental Engineering, McGraw Hill, New York, USA

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