W6-L1-2-Flocculation Process PDF

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WATER TREATMENT ENGINEERING

PART 2- SURFACE WATER TREATMENT FLOCCULATION PROCESS

Lectures Notes Environmental Engineering Program Zewail City Of Science and Technology Associate Prof. Emad S. Elmolla

CONTENTS      

Flocculation process definition Types of flocculators Flocculation process key parameters Example Chemical building Example

Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

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SCHEMATIC FOR SURFACE WATER WORKS

Raw water Abstraction/ Screening sludge Alum Coagulation Polymers

Filtration sludge Cl2

Flocculation Sedimentation Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

Disinfection Storage

sludge

Distribution 3

FLOCCULATION PROCESS DEFINITION 

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Introduction Coagulation Process (unit process) is the addition and rapid mixing of coagulant resulting in destabilization of the colloidal particles and formation of pin-head floc. Flocculation Process (unit operation) : is the gentle agitation(physical conditioning) of the coagulated solution for aggregation of the destabilized colloidal particles to form rapid settling flocs. Typical floc size that can be removed by sedimentation and filtration is in the range of 0.1-2.0 mm. Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

The purpose of flocculation is to cause particles contact, while not creating sufficient turbulence to break up the flocs already formed. 4

TYPES OF FLOCCULATORS Types of Flocculators  Flocculation units are often divided into two general groups: (1) Hydraulic flocculators, and (2) Mechanical flocculators. The design objective is to achieve gentle, uniform mixing that will not shear the floc. Hydraulic flocculators 1. The hydraulic flocculators simply utilize cross-flow baffles or 180 degree turns to produce the required turbulence. 2. Hydraulic flocculators are used in small scale water treatment plants and rarely used in medium- and large-sized water treatment plants, because of their sensitivity to flow changes Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

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TYPES OF FLOCCULATORS Hydraulic flocculators 

Horizontally baffled tank: The water  Vertical baffled tank: The water flows flows horizontally and the baffle walls vertically. The baffle walls help to create help to create turbulence and thus turbulence and thus facilitate mixing. facilitate mixing.

http://www.environmental-center.com/magazine/iwa/jws/art4.pdf Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

Section

http://www.environmental-center.com/magazine/iwa/jws/art4.pdf

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TYPES OF FLOCCULATORS  

Hydraulic flocculators Pipe flocculators: The water flows through bended pipes that help to create turbulence and thus facilitate mixing. It is common to be used in the small capacity flocculators (less than 1000m3/day)

Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

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TYPES OF FLOCCULATORS Mechanical flocculators 

The rotating unit consists of a series of paddles mounted on a vertical or horizontal shaft; or vertical impeller . System is driven by a reduction gear mechanism that may or may not include a variable-speed drive.

Horizontal Paddle type flocculators (blades are perpendicular to flow)

Vertical impeller type flocculators

Vertical Paddle type flocculators Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

8

FLOCCULATION PROCESS KEY PARAMETERS   

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1- Agitation Requirements The degree of agitation employed in flocculators is much less than that used for rapid mixing. Typical velocity gradients (G) for flocculators range from 15 to 60 s-1. Flocculation basins are normally designed with multiple mixing compartments in a series, with velocity gradients successively lower in each compartment. This type of design is called tapered flocculation and has been found to produce a uniform and tough floc that will settle readily.

Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

9

FLOCCULATION PROCESS KEY PARAMETERS   

1. Agitation Requirements The higher G value in the first compartment causes a rapid transformation of the primary particles into higher-density floc; The lower G values in subsequent compartments cause the buildup of progressively larger-size floc, for better settling. The velocity gradient for mechanical mixers can be calculated as follows:

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G =velocity gradient, s-1 (G = 15 to 60 s-1) P =power imparted to the water, N-m/s or Watt,V =volume of the basin, m3 μ = absolute (dynamic) viscosity of the fluid, N-s/m2 V=Agitated Volume, m3

Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

10

FLOCCULATION PROCESS KEY PARAMETERS 

1. Agitation Requirements-paddle-wheel mixers In the case of paddle-wheel mixers, the water power is given by the following Equation: 3 P=(C d *Apaddle *ρ*Vr )/2

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P = power imparted to water, Watt CD = coefficient of drag, which varies with the length-to-width ratio (L/W) of the paddle blades. CD 'for different L/W ratios are summarized in the following Table:

A = area of paddles, m2 Vr = relative velocity of the paddle to the water, m/s Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

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FLOCCULATION PROCESS KEY PARAMETERS 1. Agitation Requirements  The paddle velocity relative to the water (Vr) is:  Vr =(1-k)* V  K coefficient=0.25-0.50  The average absolute peripheral velocity of the paddle, V = 2π*r* n Where:  

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r: distance from the center of the paddle to center of the shaft n : rotational speed of the shaft

The peripheral velocity of a paddle-wheel mixer should be limited to 0.40 m/s for alum coagulation Mechanical mixers should have variable- speed drives, to enable the operator to vary the velocity gradients to match variations in the raw water conditions. Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

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FLOCCULATION PROCESS KEY PARAMETERS   

2. Detention Time in Flocculation Basin The detention time in flocculation basins is much higher than that in rapid-mix basins. Detention times from 20 to 60 minutes are recommended. The average detention time is determined by the following equation: t=V/Q or V=Q*t Where: t =average detention time, sec Q =flow rate, m3/sec. V =volume of the tank, m3

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The key design factor in a flocculation basin is the value of Gt (velocity gradient x detention time), because the number of particle collisions within the basin is directly proportional to the value of Gt. Typical Gt values range from 10,000-150,000. Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

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FLOCCULATION PROCESS KEY PARAMETERS 

3. Geometry of Flocculation Basin Rectangular, square or circular basin tanks could be used.

Rectangular and square flocculator basin  The basin should be divided to number of compartments to minimized the short-circuiting in the flocculation basin.  Three or four compartments could be enough to minimize the short circuiting.  Each compartment should be separated by a baffle wall. 

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For Vertical-shaft flocculators: Square compartments with maximum Compartments for vertical mixers dimensions of 6 m * 6 m and a depth of 3 to 5 m are usually used for vertical flocculators. For Horizontal-shaft reel or paddle flocculators: the compartments are typically 6 to 30 m long and 3 to 5 m wide. (length of compartment is equal to the tank width)

Compartments for horizontal mixers Assoc. Prof. Emad S. Elmolla ‘s Lectures Notes

Water Treatment Engineering

14

FLOCCULATION PROCESS KEY PARAMETERS 

3. Geometry of Flocculation Basin Circular flocculator basin 

It is usually located in the center of the clari-flocculator tank Water depth 3-4 m

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Diameter...