chapt5 coagulation

8/4/2019 Chapt5 Coagulation

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cmutsvangwa: Water Quality and Treatment: Dept. of Civil & Water Eng., NUST 06/10/2011

COAGULATION

Sedimentation alone will not be effective for small suspended particles less than

50µm. Coagulation and flocculation will result in bigger particles with higher settling velocities, which can settle down due to gravity in sedimentation tanks.

SuspensionDispersion of solid particles in liquid

ColloidsSmall particle 1 to 200x10-9m and are called stable if they do not join together and hence form stable suspension. Colloids do not join together because:

• molecular arrangement within crystals

• loss of atoms due to abrasion of surfaces• they are small and therefore have a large surface to volume ration

• because of the large surface area, colloidal particles a accumulate a netnegative charge at the surface-water interface and hence colloidalparticles are negatively charged in water

• the net negative electrical charges at the particle surface result inpreferential adsorption of ions. The first layer of cations (+ve) is attractedto the negatively charged surface to form the Stern layer and will travelwith it (Fig. 1, 2 and 3). The Stern layer produces a rapid drop inpotential. The net charge (+ve) is strong at the bound layer (Stern layer)

and decreases exponentially with distance from the colloid (Fig. 2).• the surface charge will contribute to stability and there will be no

coalescence due to the repelling by the like charge.

Fig.1 Guoy-Stern colloidal model

Chapter 5 Coagulation

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Source:Water Supply, Terence McGee

FlocCollection of colloidal particles held together and has density closer to water

CoagulationA process of making conditions favourable for colloidal particles to join together to form flocs. It is a process directed towards destabilisation of the colloidalparticles.

FlocculationThe growth of flocs

Fig.2: Charge system in colloidal suspensionSource: Environmental Engineering, Peavy (1998)

CoagulationWhen two colloids come in close proximity, thee are two forces acting on them:

• electrostatic potential

• Van der Waals force

Electrostatic potential

• created by counter ions surrounding each colloid.

• It is a repelling force and hence colloids will repel each other.

Van der Waals force

• It supports contact and is an attraction force.

• It is inversely proportional to the 6th power of the distance between

particles.


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• It decreases exponentially with distance (Fig. 3).

• It decreases more rapidly than the electrostatic potential (Fig. 3).

• It is more attractive force at close distances and repulsive at greater distances

•It become attractive only after passing through a maximum net repulsiveforce called the energy barrier at some distance between the colloids.

• Once the forces become attractive, contact between the particles takesplace.

A means of overcoming the energy barrier must be available beforeagglomeration of particles can occur. Some of the means of overcoming thisenergy barrier are outlined hereafter:

• Brownian movement random movement of smaller colloids becauseof molecular bombardment may produce enough momentum for particles

to overcome the energy barrier for the colloidal particles to collide.• Mechanical agitation mechanical agitation of the water may impart

enough momentum to larger particle to move them across the energybarrier

The above processes are too slow to be efficient in water treatment andtherefore other means of agglomeration must be used. Generally this isaccomplished by chemical coagulation with aluminium sulphate (Al2(SO4)3 andferric chloride (FeCl3).


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Fig. 3: Force fields between colloids of like chargeSource: Environmental Engineering, Peavy (1998)

Mechanisms of coagulation

• ionic layer compression

• adsorption and charge neutralisation

• sweep coagulation

• inter-particle bridging

Ionic layer compressionA higher negative charge concentration compresses layers composed of +veions towards the surface of colloid and if this layer is sufficiently compressed,then Van der Waals force will become predominant producing a net attractiveforce and removing the energy barrier.

Adsorption and charge neutralisationIt is through the use of aluminium sulphate and ionisation of the aluminiumsulphate in water produces sulphate anions and aluminium cations:

( ) +−

+→32

434200 Al S S Al


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+3 Al react immediately with water to form a variety of aquometallic ions and

hydrogen, a process called hydrolysis

Hydrolysis

+++

+→+ H AlOH O H Al 2

2

3

( ) +++

+→+ H OH Al O H Al 2222

3

( ) +++

+→+ H OH Al O H Al 171774

1772

3

( ) ++

+→+ H OH Al O H Al 3332

3

aquometallic ions

• the aquometallic ions become part of the ionic cloud surrounding thecolloid i.e. large, insoluble positively charged particles

• they are adsorbed onto the surface of the colloid where they neutralisethe surface of the charge

• once the surface charge has been neutralised, the ionic cloud dissipatesand the electrostatic potential disappears and contact occurs

• overdosing with a coagulant can result in restabilising the suspension.

Sweep coagulation

From the above equations, the last product formed in the hydrolysis of Al (SO4)3

is aluminium hydroxide (Al (OH)3.

• The aluminium hydroxide forms gelatinous flocs that are heavier thanwater and settle by gravity.

• The colloids may become entrapped in a floc as it is formed or maybecome enmeshed by its sticky surface as the flocs settle.

• The process by which colloids are swept from the suspension in thismanner is known as sweep coagulation.

Inter-particle bridging

Synthetic polymers may be used in addition to the stated coagulants. Suchpolymers are linear or branched and a surface which is highly reactive. Theyhave adsorption forces and the colloids may get attached to these surfaces toform polymer-colloid, and the several polymer-colloids may become enmeshedand grow into bigger particles, which can settle (Fig. 4).

Also synthetic polymers may carry a negative, positive charge or can be neutral,metallic polymers from the aluminium and ferric carry positive charge. Hencethe negative synthetic polymers will help in the attraction of colloids.


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Fig. 4 Inter-particle bridging by polymersSource: Environmental Engineering, Peavy (1998)

Conditions

• for optimum formulation of flocs, the pH should be between 6-8.

• an optimum dosage of coagulant is required and is determined by the jar test

• too high or too low dosages are ineffective• acid is formed when alum reacts with water, so t maintain the desired ph

range, or to achieve a satisfactory final pH, it’s usually necessary to addan alkali and rapid mixing will be important.

• hydrates lime is often added for pH correction on coat grounds

Disadvantage of lime

• low solubility i.e. needs constant agitation

• blocks pipes

As an alternative sodium carbonate (washing soda) is used or sodium hydroxide(caustic soda). But caustic soda is more expensive and dangerous


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Coagulant aidsPoly-ectrolytes (polymers); They make flocs larger, tougher anddenser. They have longer molecules with charged site which kink smallflocs together (inter-particle bridging). There are three groups:

cationic, anionic and non ionic, ampholytic

Activated silica It is prepared from sodium silicate. It has thefollowing benefits:

• increases the rate of coagulation and flocculation

• reduces coagulant dosage

• broaden ph range for effective coagulation

• produces large and tougher floc particles

• increases removal of both colour and colloidal particle

Purified claysMay be added to water with low turbidity to add weight to the flocs andreduce coagulant requirement e.g. bentonite clay

Jar testSelection of an appropriate coagulant is through experiments and taking intoconsideration costs, sludge management and the Jar test is mostly used.Overdosing may lead to filter clogging.

The required dose cannot be calculated because water chemistry is toocomplex. The right dose is found by comparing the effects of different doses of coagulant on water, and choosing the lowest dose that gives a satisfactory result(Jar test). If too low a dose is used, little treatment is achieved and money iswasted, and a high residual goes into supply. If too much is dosed, the turbidityremoval is below the optimum. This result in the filters clogging quickly and ahigh residual goes into supply and this is a health hazard.

Procedure

• normally six jars are used one for control (Fig. 5)

• the other five jars are dosed with different doses of aluminium sulphate

• initial turbidity, pH and alkalinity of sample should be known

• one minute flash mix at 200 rpm• this is followed by 10 to 30 min flocculation period at between 20 to 70

rpm (i.e. water is being allowed to settle).

• the time of appearance of the flocs, the size of floc and the turbidities of the treated water should be noted

• the jar with the minimum dose but with the lowest final turbidity gives theoptimum dose

• a jar test is performed every shit


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Fig. 5: Schematic illustration of a jar-test equipment

Chemo-feedersCoagulant can be dosed by:

• gravity when power is not available

• Dosing pumps

References


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