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
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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
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
The main mechanism of particle stability is the electrical repulsion
Presence of adsorbed water molecules (this will provide barrier to successful collision)
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)
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)
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)
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:
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.
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
Jar test is used to investigate:
Full-scale testing is necessary to determine:
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