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COAGULATION CHEMISTRYParticle sizes and nomenclatureHow a coagulant worksCharacteristics of typical coagulantsFitting the right coagulant to a sourcewater

Basketball ~0.3mGolf ball ~ 0.03mGravel Particle ~ 0.003mSand Particle ~ 0.0003m or .3mmColiform Bacteria ~ 10-6m or 1mColloids ~ 10-8 Atoms ~1 to 5*10-10m (1 angstrom = 10-10m

0.3m .00000001 .0000001 .000001 .00001 .0001 .001 .01 .1 110-810-710-610-510-410-310-210-1100 m mm cm mmicrometer millimetercentimetermeter0.03m.003mLog Scale of Distance (meters)bacteriaatomssandcolloidsNaked eyeOpt scopeE scope

Easier to think from 1 to a million, than it is to think from 1 to one millioneth.2Charge Neutralization---------------------Negative chargedStable Colloid+Al+ + +Al+ + +Al+ + +Al+ + +Al+ + +Al+ + +Al+ + +Al+ + +---------------------Positively Charged AluminumLarger flocCharge-alanced+High Surface area on colloid, usually negatively charged (water applications)Not stoichiometrictoo variable in composition and charge characteristicsCant be balancedFeedrates determined by jar tests and particle charge characteristics (Zeta Potential)

In coagulation, the valence state electrons are important in compressing the repulsive forces between particles. For negatively-charged particles, Aluminum and Iron are particularly effective in their 3+ valence state.

Valence states are important for coagulation. Repulsive layers are compressed, more collisions happen, and a larger particle results. Iron and aluminum are plus-3 positive4Flocculation-----------------------------------Al+ + +Al+ + +Al+ + +Al+ + +-----------------------------------Al+ + +Al+ + +Al+ + +Al+ + +-----------------------------------Al+ + +Al+ + +Al+ + +Al+ + +-----------------------------------Al+ + +Al+ + +Al+ + +Al+ + +Destabilized particlesMixingenergyLarge particlesdense enough to sink

Coagulation TheoriesChemical charge neutralizationCharges on the colloid are neutralized to allow particles to aggregatePhysical charge-layer compressionrepulsive forces are complexAdsorption and charge neutralization are activebridging of coagulant complexes occurenmeshment processes can be significantHigher valence state chemicals are very effective in compressing the repulsive forces

Coagulation/Flocculation Effective in removing: Bacteriasoil particles,color, organic material that react with chlorine to form DBPsArsenic

Aluminum CoagulantsAlum Al2(SO4)3 . 14 H2O

Minimum solubility pH 5.5Precipitates as Al(OH) 3 ~(10-8 M)Process works best at lower pH (6-7) Al2(SO4)3 + 14H20 + 3Ca(OH)2 2Al(OH)3+3Ca(SO)4+14H2O+6CO2Each mg/l alum consumes 0.5 mg/l alkalinity

SOOOOSOOOOSOOOOAlAlAlOHOHOH

Polyaluminum Chloride (PACl)Al(OH)x(Cl)yPolymerized, long chain chemicalEffective in bridging

Ferric CoagulantsFerric Chloride FeCl3.6H2OFerric Sulfate Fe2(SO4)3 . 9H2OMinimum Solubility ~pH 8 10-8MWorks in a wider pH range , better than alum at pH 8Fe(H2O)6+3 +H2O = Fe(H2O)5(OH)+2 + H3O+

PolymersLong-chain moleculesIf charged, referred to as polyelectrolytesAnionic polymersCationic polymersNon-ionic polymers (no net charge)Cationic (positive charge) works well on clay particles (negative charge) through bridgingOverfeeding can be a problemDont affect pHwork well in low-alkalinity water with high turbidity

Sowhats important?Alum works best at lower pH valuesThe reaction consumes alkalinityIf you have a low alkalinity water (less than 60 to 80 mg/l as CaCO3), you may have to add alkalinity to get efficient coagulationLime or soda ash are the most commonly used chemicals to increase alkalinityIf you overfeed alum, you can get re-stabilization of the floc (and poor treatment performance)

more important stuffIron coagulants work across a wider pH range than alum (more forgiving of pH changes)Iron flocs are heavier than alum floc, and thus iron works better in cold water conditions than alum (check the periodic table)Polymers require far less quantity fed, but cost much more that alum and ferric.Optimization is required to select best coagulant and mixand this can change from season to season!