Coagulation. CE 547. Overview. Turbidity in surface waters is caused by colloidal clay particles. Color in water is caused by colloidal forms of Fe, Mn, or organic compounds. Colloidal Particles Difficult to settle Pass through small pores of conventional filters
Turbidity in surface waters is caused by colloidal clay particles. Color in water is caused by colloidal forms of Fe, Mn, or organic compounds.
How to remove colloidal particles?
Why aggregation is difficult?
How to aggregate?
It is the process of destroying the stabilizing forces and causing aggregation
Aggregation occurs in two steps:
1. reduction or elimination of the inter-particle forces responsible for stability (by addition of chemicals)
2. collision due to
After destabilization, gentle mixing is provided to increase the rate of particle collision without breaking the aggregates or flocs (this process is called flocculation)
Colloidal particulates remain in suspension for very long periods due to their stability (is it possible to give such particles sufficient time to settle?)
Particles larger than 5 nm are in suspension
2. Nature of solid-water interface
The main mechanism of particle stability is the electrical repulsion
Presence of adsorbed water molecules (this will provide barrier to successful collision)
How electrical charges exist at particle surfaces? There are three principle ways:
1. Crystal Imperfections
Silicon atoms in crystals can be replaced by atoms with lower valence (such as Al) giving excess negative charge to the crystal material.
2. Preferential Adsorption of Specific Ions
When particles are dispersed in water, soluble polyelectrolytes of natural origin may adsorb on particle surfaces (for example, fulvi acid, -vely charged, can be adsorbed on CaCO3, +vely charged)
3. Specific Chemical Reactions of Inorganic Groups on Particulate Surfaces
many particulate surfaces contain inorganic groups such as hydroxyl or carboxyl functional groups which dissociate in water producing a surface electrical charge (that depends on the pH of the solution)
When particles are dispersed in water, ions with opposite charge to the particle surface accumulate closer to the particle to produce electro-neutrality.
This accumulation is opposed by the tendency of ions to diffuse in the direction of decreasing concentration
The result is a diffuse cloud of ions surrounding the particle, which is known as the electrical double layer (Fig 12.3)
As ionic strength (TDS) increases, this will compress the diffuse layer.
Removal of colloidal and suspended particulates depends on reduction in particulate stability.
Destabilization can be achieved through:
Increasing the ionic strength will compress the double layer, causing a decrease in its thickness. This will result in decreasing the Zeta Potential
The amount of dissolved ions that produce rapid coagulation is defined as the Critical Coagulation Concentration (CCC), which depends on:
for hydrophobic particles, CCC is inversely proportional to the sixth power of the charge on the ion. So, for mono-, di-, and trivalent ions, the CCC values are in the ratio:
As an Example: 3000 mg/l NaCl is equivalent to 44 mg/l CaCl2
This occurs when surfaces are oppositely charged, which is promoted by the adsorption of specific ions on the surface of the particle. Particles in natural waters exhibit both +ve and –ve charges based on the pH of the water.
Zero Point of Charge (ZPC): is the pH corresponding to a surface charge of zero.
Reduction of surface charge can be achieved by:
Long-chain polymers carrying –ve charges can form bridges between particle, thus destabilizing the suspension. This mechanism was shown to be the major mechanism controlling the aggregation of bacterial and alga suspensions.
This mechanism is predominant in water treatment where pH values are between 6 and 8 and Al or Fe salts are used at concentrations exceeding saturation with respect to amorphous metal hydroxide solid that is formed. In this mechanism, finely divided particles are entrapped in the amorphous precipitate formed.
Functions of Coagulants
Selection of Coagulants
Selection of Type and Dose of Coagulants Depends on:
Jar test is used to investigate:
In selection of coagulants, cost and quantity and dewaterability of produced solids (sludge) should be taken into consideration.
Full-scale testing is necessary to determine:
The rate of reaction of coagulants with water depends on:
At pH values less than 6:
Similar to Aluminum, but at pH below 4.
Above pH 6 for Aluminum and pH 4 for ferric, formation of amorphous precipitates occurs reapidly causing entrapment of particles “sweep floc”. This sweep floc mechanism requires greater quantity of coagulant than charge neutralization which will result in producing more sludge.
Organic polymers are used as coagulants and are termed as “ployelectrlytes”. They are used as:
The use of polymers is restricted in water treatment due to:
natural (sodium alginate and chitosan; very high cost)
synthetic (predominant in water treatment)
Functions of Polymers