1、Chapter 2Four Ways to FlocculateCoagulate, Then FlocculateIn water clarification, the terms coagulationand flocculation are sometimes used inter-changeably and ambiguously, but it isbetter to separate the two in terms offunction.Coagulation takes place when the DLVOenergy barrier is effectively elim
2、inated; thislowering of the energy barrier is also re-ferred to as destabilization.Flocculation refers to the successfulcollisions that occur when the destabilizedparticles are driven toward each other bythe hydraulic shear forces in the rapid mixand flocculation basins. Agglomerates of afew colloid
3、s then quickly bridge together toform microflocs which in turn gather intovisible floc masses.Reality is somewhere in between. The linebetween coagulation and flocculation isoften a somewhat blurry one. Most coagu-lants can perform both functions at once.Their primary job is charge neutralizationbut
4、 they often adsorb onto more than onecolloid, forming a bridge between them andhelping them to flocculate.Coagulation and flocculation can be causedby any of the following: double layer compression charge neutralization bridging colloid entrapmentIn the pages that follow, each of these fourtools is
5、discussed separately, but thesolution to any specific coagulation-floccu-lation problem will almost always involvethe simultaneous use of more than one ofthese. Use these as a check list whenplanning a testing program to select anefficient and economical coagulant system.9Chapter 2Four Ways to Flocc
6、ulateDouble Layer CompressionDouble layer compression involves theaddition of large quantities of an indifferentelectrolyte (e.g., sodium chloride). Theindifference refers to the fact that the ionretains its identity and does not adsorb tothe colloid. This change in ionic concentra-tion compresses t
7、he double layer aroundthe colloid and is often called salting out.The DLVO theory indicates that this resultsin a lowering or elimination of the repulsiveenergy barrier. It is important to realizethat salting out just compresses thecolloids sphere of influence and does notnecessarily reduce its char
8、ge.In general, double layer compression is nota practical coagulation technique for watertreatment but it can have application inindustrial wastewater treatment if wastestreams with divalent or trivalent counter-ions happen to be available.CompressionFlocculation by double layercompression is unusua
9、l, buthas some application inindustrial wastewaters.Compare this figure to theone on page 2.Highly NegativeColloidStern LayerDiffuse LayerIons In EquilibriumWith Solution10Charge NeutralizationInorganic coagulants (such as alum) andcationic polymers often work throughcharge neutralization. It is a p
10、ractical wayto lower the DLVO energy barrier and formstable flocs. Charge neutralization involvesadsorption of a positively charged coagulanton the surface of the colloid. This chargedsurface coating neutralizes the negativecharge of the colloid, resulting in a nearzero net charge. Neutralization is
11、 the key tooptimizing treatment before sedimentation,granular media filtration or air flotation.Charge neutralization alone will not neces-sarily produce dramatic macroflocs (flocsthat can be seen with the naked eye). Thisis demonstrated by charge neutralizing withcationic polyelectrolytes in the 50
12、,000-200,000 molecular weight range. Microflocs(which are too small to be seen) may formbut will not aggregate quickly into visibleflocs.Charge neutralization is easily monitoredand controlled using zeta potential. This isimportant because overdosing can reversethe charge on the colloid, and redispe
13、rse itas a positive colloid. The result is a poorlyflocculated system. The detrimental effectof overdoing is especially noticeable withvery low molecular weight cationic polymersthat are ineffective at bridging.Charge ReductionLowering the surface chargedrops the repulsive energycurve and allows van
14、 derWaals forces to reduce theenergy barrier. Comparethis figure with that on theopposite page and the oneon page 2.Slightly NegativeColloidStern LayerDiffuse LayerIons In EquilibriumWith Solution11Chapter 2Four Ways to FlocculateBridgingBridging occurs when a coagulant formsthreads or fibers which
15、attach to severalcolloids, capturing and binding themtogether. Inorganic primary coagulants andorganic polyelectrolytes both have thecapability of bridging. Higher molecularweights mean longer molecules and moreeffective bridging.Bridging is often used in conjunction withcharge neutralization to grow fast settlingand/or shear resistant flocs. For instance,alum or a low molecular weight cationicpolymer is first added under rapid mixingconditions to lower the charge and allowmicrofloc
