Sanitary engineering

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Sanitary Engineering
Coagulation and Flocculation
Week 3

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Coagulation and Flocculation
Colloidal particles are too small to be removed by sedimentation or by sand
filtration processes.
Coagulation: Destabilization colloids by
Flocculation:Aggregation of destabilized colloids
To turn the small particles into larger particles removed in subsequent
processes such as settling or filtration.
When combined with subsequent physical removal، it achieves Removal of
turbidity
• Remove infectious agents,
• Remove toxic compounds that have adsorbed to the surface of particles,
• Remove precursors to the formation of disinfection byproducts, and
• Make the water palatable.

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Characteristics of Particles
• Electrical Properties: surface charge .. causes the particles to remain in suspension without
aggregating for long periods of time. Particle suspensions are thermodynamically unstable
and. aggregation process is very slow, the particles cannot be removed by sedimentation in
a reasonable amount of time. Negative charge arises in four principal ways (Ionization,
Adsorption, Isomorphous replacement and Structural imperfections)
• Electrical Double Layer: In natural waters, the processes described above nearly always
result in a negative surface charge on particles. Negatively charged particles accumulate
positive counterions on and near the particle’s surface As shown on (Fig.1) a layer of cations
will bind negatively charged particle to form a fixed adsorption layer known as the
Helmholtz layer or Stern layer. The layer of cations and anions that extends from the
Helmholtz layer to the bulk solution where the charge is zero and electroneutrality is
satisfied is known as the diffuse layer. Taken together the adsorbed Stern diffuse layer are
known as the electric double layer.

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Fig.1

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• Zeta Potential: When a charged particle is placed in an electric field, it will migrate to the
pole of opposite charge. This movement is called electrophoresis. As the particle moves, a
portion of the water near the surface moves with it. This movement displaces the ion cloud
and gives it the shape shown in (Fig.2). The electric potential between the shear plane and
the bulk solution is called the zeta potential.
Fig.2

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• Particle Stability: Particles in natural waters remain stable when there is a balance between
the electrostatic force of the charged particles and attractive forces known as van der Waals
forces. Because the particles have a net negative charge, the principal mechanism
controlling stability is electrostatic repulsion. Van der Waals forces arise from magnetic and
electronic resonance when two particles approach one another. Because the double layer
extends further into solution than the van der Waals forces, an energy barrier is formed
that prevents particles from aggregating.
Coagulation
Coagulants :
Inorganic coagulants used for the treatment of potable water exhibit the following
characteristics
• They are nontoxic at the working dosage.
• They have a high charge density.
• They are insoluble in the neutral pH range.

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Physics of Coagulation:
There are four mechanisms employed to destabilize natural water suspensions:
• Compression of the Double Layer: If the electric double layer is compressed, the repulsive
force is reduced and particles will come together as a result of Brownian motion and remain
attached due to van der Waals forces of attraction. Both the ionic strength and the charge
of counterions are important in the compression of the double layer.

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• Adsorption and Charge Neutralization: Hydrolyzed metal salts, prehydrolyzed metal salts,
and cationic polymers have a positive charge. They destabilize particles through charge
neutralization.
• Enmeshment in a Precipitate. With doses exceeding saturation for the metal hydroxide,
aluminum and iron salts form insoluble precipitates and particulate matter is entrapped in
the precipitate. This type of destabilization has been described as sweep coagulation
Chemistry of Coagulation:
• Aluminum: Aluminum can be purchased as either dry or liquid alum [Al2 (SO4 ) 3 · 14H2O].
When alum is added to water and aluminum hydroxide precipitates, the overall reaction
is:
• Iron. Iron can be purchased as either the sulfate salt (Fe 2 (SO4)3 · x H2O) or the chloride salt
(FeCl3 · x H2O). It is available in various forms, Dry and liquid forms are available. the overall
precipitation reactions for ferric sulfate and ferric chloride are as follows.

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Ferric sulfate:
Ferric chloride:
Flocculation
The basis for describing the mechanisms of flocculation is that stirring water containing
particles created velocity gradients that brought about particle collisions.
• Microscale Flocculation: The floccula on of small par cles (less than 0.1 μm in diameter)
is caused by diffusion. The rate of flocculation is relative to the rate at which the particles
diffuse. Thus, the primary mechanism of aggregation is through Brownian motion. After a
period of seconds, the microflocs range in size from 1 to about 100 μm in diameter.
• Macroscale Flocculation: Mixing is the major flocculation mechanism for particles greater
Mechanical mixing causes unequal shearing forces on the floc, and some of the floc are

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broken up. After some period of mixing, a steady state distribution of floc sizes is achieved
and formation and breakup become nearly equal.
• Differential Settling: Because the floc particles are of different size, they settle at different
rates. Differences in the settling velocities cause the particles to collide and flocculate.
• Chemical Sequence: The addition of multiple chemicals to improve flocculation is common
practice. The order of addition is important to achieve optimum results at minimum cost.
Typically, the addition of a polymer after the addition of hydrolyzing metal salts is most
effective. Ideally, the polymer addi on should be made 5 to 10 minutes a er the addi on
of the hydrolyzing metal salt. This allows for the formation of pinpoint floc that is then
“bridged” by polymer. In conventional water treatment plant design this is rarely possible
because of space limitations.