Mixing , coagulation

3/26/2017Chemist/Ahmed Hasham
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Mixing, coagulation , and flocculation
By/ Ahmed Mohamed Hasham

Mixing, coagulation , and flocculation
3/26/2017
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https://eg.linkedin.com/in/ahmed-hasham-mmba-01024b27
Chemist / Ahmed Hasham (Hesham) ahmedhasham83@gmail.com

About the presenter
 Member of the Board scientists - Egypt.
 Member of Scientific Professions Syndicate.
 Member of the Arab Society for experts and Safety Professionals.
 Member of the International Association of Engineers.
 Expert of water and waste water treatment technologies.
 Certified trainer in water treatment field .
 Certified trainer in Quality Management Systems field.
 https://eg.linkedin.com/in/ahmed-hasham-mmba-01024b27
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3/26/2017
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Chemist/Ahmed Hasham
Contents
1. Introduction
2. Coagulation and flocculation
3. Particulates in source waters
4. Rapid mixing process
5. Enhanced coagulation
6. Coagulant chemicals
7. Flocculation
8. Flocculation aids
9. Factors Which Affect the Coagulant Work
10.Process Control
11.Coagulation/Flocculation Facilities
12.Types of Rapid Mixing Systems
13.Types of Flocculators

Coagulation and flocculation
 May be described as chemical and physical processes that mix
coagulating chemicals and flocculation aids with water.
 The overall purpose is to form particles large enough to be
removed by the subsequent settling or filtration processes.
 colloids, suspended material, bacteria, and other organisms an
be removed by coagulation, flocculation , sedimentation, and
filtration.
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Coagulation
 Coagulation is the process in which chemicals are added to
water, causing a reduction of the forces tending to keep
particles apart.
 Particles in source water are in a stable condition.
 The purpose of coagulation is to destabilize particles and
enable them to become attached to other particles so that
they may be removed in subsequent processes.
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Particulates in source waters
 Particulates in source waters that contribute to color
and turbidity are mainly clays , silts, viruses, bacteria,
fulvic and humic acids, minerals (including asbestos,
silicates , silica, and radioactive particles), and organic
particulates.
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Rapid mixing process
 At pH levels above 4.0, particles or molecules are generally negatively charged.
 The coagulation process physically occurs in a rapid mixing process.
 Mixing is commonly referred to as flash mixing, rapid mixing, or initial mixing.
 The purpose of rapid mixing is to provide a uniform dispersion of coagulant
chemical throughout the influent water.
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Enhanced coagulation
 Enhanced coagulation (removal of TOC) can be achieved in most cases
by either:
1. Increasing coagulant chemical dosage.
2. Adjusting the pH during the coagulation reaction.
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Coagulant chemicals
 Coagulant chemicals are inorganic or organic chemicals that, when
added to water at an optimum dosage, cause particle destabilization.
 Most coagulants are cationic when dissolved in water and include
chemicals such as alum, ferric salts, lime, and cationic organic polymers.
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Coagulant chemicals
 The most commonly used coagulants are :
1. Alum (aluminum sulfate), A12(SO4)3 • 14H20. The most common coagulant in the United
States, it is often used in conjunction with cationic polymers.
2. Polyaluminum chloride, AI(OH)x(Cl)y. This is efficient in some waters, requiring less pH
adjustment and producing less sludge.
3. Ferric chloride, FeCl3. This may be more effective than alum in some applications.
4. Ferric sulfate, Fe2(SO4)3. It is effective in some waters and more economical in some
locations.
5. Cationic polymers can be used alone as the primary coagulant or in conjunction with
aluminum or iron coagulants.
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Flocculation
 Flocculation is the agglomeration of small particles and colloids to
form settleable or filterable particles (flocs).
 A separate flocculation process is most often included in the
treatment train to enhance contact of destabilized particles and to
build floc particles of optimum size, density, and strength.
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Flocculation aids
 Flocculation aids are chemicals used to assist in forming larger,
denser particles that can be more easily removed by sedimentation
or filtration. Cationic, anionic, or non-ionic polymers are most often
used in dosages of less than 1.0 mg/L.
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Floc formation with polymers
12/3/2017water treatment
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Flocculation Aids
 Floc formed in many waters with alum is light and fragile and somewhat
difficult to settle. Polymers and other additives can often help form a floc
that is more efficiently removed by settling and filtration. Typical additives
used for flocculation aids are :
1. High-molecular-weight anionic or nonionic polymers.
2. Activated silica.
3. Bentonite
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19 Factors Which Affect the Coagulant Work
1. Mixing Conditions
2. pH
3. Alkalinity
4. Water Temperature
5. Turbidity
 If the alkalinity concentration in the water is not high enough, and
effective floc will not form when either alum or ferric sulfate is used.
 Metal salts (alum, ferric sulfate, ferric chloride) consume natural alkalinity.

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Factors Which Affect How Well a Coagulant Work
 Each mg/L of alum will consume 0.5 mg/l total alkalinity (as CaCO3).
 Each mg/L ferric sulfate will consume 0.75 mg/L total alkalinity (as CaCO3).
 Each mg/L ferric chloride will consume 0.92 mg/L total alkalinity (as CaCO3).
 It may be necessary to add alkalinity to the water (lime, soda ash, caustic soda) to the
water in order for the metal salts to work properly. The doses should be confirmed with
jar testing.

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Process Control
 These raw water characteristics should be monitored in order to do a
thorough job of chemical selection.
1. Temperature
 Low water temperatures slow chemical reactions, causing decreased
efficiency and slow floc formation.
 Higher coagulant doses may be required to maintain acceptable results.
2. pH
 Extremes can interfere with the coagulation/flocculation process.
 The optimum pH depends on the specific coagulant.

22 Process Control
3. Alkalinity
 Low alkalinity causes poor coagulation.
 May be necessary to add alkalinity (lime, caustic soda, soda ash).
4. Turbidity
 Difficult to form floc with low turbidity water, may need to add weighting
agents.
5. Color
 Indicates presence of organic chemicals which can react with the
coagulant, and with chlorine to form disinfection byproducts.

Jar test
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24 Coagulation/Flocculation Facilities
 Flash Mix - purpose is to distribute the coagulant rapidly and evenly throughout the
water.
 Water should be stirred violently for a brief time to encourage the greatest number
of collisions between particles as possible.
 Types of Mixers: Mechanical - Pumps and Conduits
 Detention time should be 30 seconds or less (Design Criteria).
 Flocculation - provides for gentle mixing to encourage floc formation.
 Detention time of at least 30 minutes, with a detention time of 45 minutes preferred.

Adjustment of pH
Control of pH and alkalinity is an essential aspect of coagulation. The optimum pH
for coagulation varies but is generally within the following ranges for turbidity
removal:
 Alum: pH 5.5 to 7.5; typical pH 7.0
 Ferric salts: pH 5.0 to 8.5; typical pH 7.5
 It can be necessary to adjust the pH of some source waters to achieve optimum
coagulation.
 The pH is often lowered by adding carbon dioxide or an acid.
 Alum and ferric chloride consume alkalinity and can lower pH
 it may be necessary to add caustic soda or lime to raise pH and to offset the
acidity of metal-ion coagulants, even in an enhanced coagulation mode of
operation.
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optimum pH for organics removal
 Typically, the optimum pH for organics removal with alum is between 6.0
and 6.5, and between 5.5 and 6.0 for ferric coagulants. Often,
polyaluminum chloride can provide organic removal without as significant
a decrease in pH.
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Types of Rapid Mixing Systems
 Coagulant chemicals can be mixed by several methods, including :
1. Mechanical devices in a dedicated basin
2. In-line blenders
3. Hydraulic methods
4. Air mixing
5. Induction mixing
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Types of Flocculators
 Mechanical Flocculators. Mechanical flocculators are preferred by most
design engineers because of their greater flexibility in varying G values and
because they have low head loss. Typical arrangements for horizontal
shaft, reel-type flocculators, and vertical paddle units
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Types of Flocculation Tanks
Mechanical Flocculators
Paddle wheel Type (vertical and Horizontal Types)
Foil Type Mixing Blade

31 Types of Flocculation Tanks
Hydraulic Flocculators
 The axial flow flocculators are typically used because they impart a nearly
constant gradient in each compartment.
 Flocculators are designed to have a minimum of three compartments to
provide for tapered (to make smaller gradually) mixing.
 The velocity gradient, G is tapered so that it is larger in the first compartment
and less is the other compartments as the floc grows.

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Pipe Flocculators system
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Vertical Shaft Flocculators

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References
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• water treatment plant design , american water works association edward e, baruth
• http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/wasteWater/Domestic
water treat.htm
• http://resources.jorum.ac.uk/xmlui/handle/123456789/1015

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 00201159465989
 00201146139692
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https://eg.linkedin.com/in/ahmed-hasham-mmba-01024b27

Mixing , coagulation

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