The DLVO Theory (named after Derjaguin, Landau, Verwery and Overbeek) is the classic explanation of how particles interact. It looks at the balance between two opposing forces - electrostatic repulsion and van der Waals attraction - to explain why some colloids agglomerate and flocculate while others will not.Repulsion: Electrostatic repulsion becomes significant when two particles approach each other and their electrical double layers begin to overlap. Energy is required to overcome this repulsion and force the particles together. The level of energy required increases dramatically as the particles are driven closer and closer together.Attraction: Van der Waals attraction between two colloids is actually the result of forces between individual molecules in each colloid. The effect is additive; that is, one molecule of the first colloid has a van der Waals attraction to each molecule in the second colloid. This is repeated for each molecule in the first colloid and the total force is the sum of all of these.
The DLVO theory combines the van der Waals attraction curve and the electrostatic repulsion curve to explain the tendency of colloids to either remain discrete or to flocculate. The combined curve is called the net interaction energy. At each distance, the smaller energy is subtracted from the larger to get the net interaction energy. The net value is then plotted - above if repulsive, below if attractive – and the curve is formed. The net interaction curve can shift from attraction to repulsion and back to attraction with increasing distance between particles. If there is a repulsive section, then this region is called the energy barrier and its maximum height indicates how resistant the system is to effective coagulation. In order to agglomerate, two particles on a collision course must have sufficient kinetic energy (due to their speed and mass) to jump over this barrier. Once the energy barrier is cleared, the net interaction energy is all attractive. No further repulsive areas are encountered and as a result the particles agglomerate. This attractive region is often referred to as an energy trap since the colloids can be considered to be trapped together by the van der Waals forces.
The results obtained through this study suggests that the Lake beside the Cricket field has the lowest requirement for alum dosage, followed closely by the Lake behind the Library and Lake Serpentine. However, the River Brahmaputra has a higher requirement for alum dosage. This may be explained by the fact that the river Brahmaputra carries a significant amount of turbidity owing to its high debris carrying capacity. This is exacerbated by the anthropogenic sources of pollution in and around the vicinity of the source of collection of sample. The lakes require a lesser amount of alum dose as the level of pollution within the IIT Guwahati campus is very low. Another reason for the low alum requirement may be that the samples were collected from near the surface of the lakes; most turbidity settles down in such still lakes and thus do not manifest themselves in the sample taken.
Alum requirement for surface water
STUDY OF ALUM DOSAGE REQUIREMENTFOR VARIOUS SURFACE WATER SOURCES IN AND AROUND IIT GUWAHATI CAMPUS Ankur Bansal (09010408) Doshi Param (09010415) Gagandeep Singh (09010417) Md. Qamaruddin Khan (09010433) Minakhi Prasad Misra (09010435)
CoagulationandFlocculationDestabilisation and Agglomeration of Colloids
Ionic Layer CompressionAddition of ionic Salts bringsabout an ionic compression.Ionic compression squeezes therepulsive energy curve reducingits influence. Furthercompression would completelyeliminate the energy barrier.
Charge NeutralisationCoagulant addition lowers thesurface charge and drops therepulsive energy curve. Morecoagulant can be added tocompletely eliminate the energybarrier.
Inter-particle BridgingBridging occurs when acoagulant forms threads orfibers which attach to severalcolloids, capturing and bindingthem together.Inorganic primary coagulantsand organic polyelectrolytesboth have the capability ofbridging.
Colloid EntrapmentColloid entrapment involvesadding relatively large doses ofcoagulants. Some chargeneutralization may occur butmost of the colloids are literallyswept from the bulk of thewater by becoming enmeshedin the settling hydrous oxidefloc. This mechanism is oftencalled sweep floc.
Alum – How it works.Role of Alum in Coagulation-Flocculation
Addition in waterWhen aluminum sulfate is added to water, hydrous oxides ofaluminum are formed.The simplest of these is aluminum hydroxide (Al (OH)3), whichis an insoluble precipitate.But several, more complex, positively charged soluble ions arealso formed, including:
Alum based Coagulation and FlocculationThe mechanism of coagulation by alum includes both chargeneutralization and sweep floc.One or the other may predominate, but each is always acting tosome degree. It is probable that charge neutralization takesplace immediately after addition of alum to water.Simultaneously, aluminium hydroxide precipitates will form. Theprecipitate grows independently of the colloidpopulation, enmeshing colloids in the sweep floc mode.
ExperimentationDetermination of Alum Requirement
Standard Solution usedAl2(SO4)3.16H2O10 mg per ml of solution in water
ResultsDiscussions on Observations and Conclusions
Source 2: Lake Behind the Library 9 8 7Turbidity (in NTU) 6 5 4 3 2 1 0 0 1 2 3 4 5 6 7 8 9 10 11 Alum Dose (in mg/l)