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3BASICPHYSICAL-CHEMICALPROCESSESINWATERTREATMENT1.COAGULATION-FLOCCULATION1.1.GENERAL1.1.1.Suspendedsolidsandcolloids1.1.1.1.DefinitionsWatercontainsmanycompounds,whichcanbeclassifiedinthreecategories(seepage5)..SuspendedsolidsTheseproductsmaybemineralinorigin(sand,silt,clays,etc.)ororganic(productsresultingfromthedecompositionofplantoranimalmatter,humicorfulvicacids,forexample).Addedtothesecompoundsaremicroorganismssuchasbacteria,plankton,algaeandviruses.Thesesubstances,inparticular,areresponsibleforturbidityandcolour..Colloidalparticles(lessthan1micron)TheseareSSofthesameoriginastheabovebutofsmallersizeandwithasettlingratethatisextremelyslow.Theyarealsoresponsibleforturbidityandcolour..Dissolvedsubstances(lessthanseveralnanometres)Theseareusuallycationsoranions.Partoftheorganicmatterisalsodissolved.Gasesarealsopresent(O2,CO2,H2S,etc.).1.1.1.2.Theroleofcoagulation-flocculationThecoagulation-flocculationprocessesfacilitatetheremovalofSSandcolloids.Thisoccursinafinalstageofsolids-liquidseparation:settling,flotationorfiltration(sub-chapters3,4and5).Inordertoremovedissolvedsubstances,eachtypeofsubstancerequiresaspecifictreatment,whichmayormaynotbeprecededbycoagulation-flocculation,orevenasolids-liquidseparationprocess.Chap.3:Basicphysical-chemicalprocessinwatertreatment1.1.2.Colloidalsuspensions1.1.2.1.Stabilityofcolloidalsuspensions-ThenecessityforcoagulationTable35listsanumberofmaterialsandorganismswiththeirsizeandanindicationofthetimeneededfortheseparticlestosettleverticallythroughonemetreofwater,at20°C,undertheinfluenceofgravityalone.Table35.Settlingtimeforvariousparticles.(AccordingtoStokes'law)ParticlediameterTypeofSettlingSpecificmmµmÅparticletimethrough1mofwaterAream2.m-310104108Gravel1second6.102110310'Sand10seconds6.10310-1102106Finesand2minutes6.10410-210105Clay2hours6.10510-31104Bacteria8days6.10610-410-1103Colloid2years6.10710-510-2102Colloid20years6.10810-610-310Colloid200years6.109Thetablealsoshowsthatthesmallertheparticle,thelargeritsspecificarea.Thuscolloidsareparticlesthatcannotsettlenaturallyandforwhichsurfaceareafactorsaremostimportant.Thesefactorsdeterminethestabilityofcolloidalsuspensions.Infact,colloidsaresubjecttotwomajorforces:-VanderWaalsattraction,whichrelatestothestructureandformofcolloidsaswellastothetypeofmedium(EA),-theelectrostaticrepulsiveforce,whichrelatestothesurfacechargesofthecolloids(EB).Thestabilityofacolloidalsuspensiondependsonthebalancebetweentheforcesofattractionandrepulsion,theenergylevelofwhichis:E=EA+EBThisrelationisshowninthediagraminfigure32:1.Coagulation-FlocculationInordertodestabilizethesuspension,itisnecessarytoovercometheenergybarrierEs.Toaccomplishthisand,thereby,promotetheagglomerationofthecolloids,itisnecessarytoreducetheelectrostaticrepulsiveforces.Thisdestabilizationisbroughtaboutbycoagulation1.1.2.2.ThedoublelayertheoryInrawwater,colloidsinvariablycarryanegativecharge(imperfectionsinthecrystallinestructure,ionizationofperipheralchemicalgroups,etc.).Inordertoneutralizethisnegativesurfacecharge,positiveions,whicharepresentintherawwaterorareintroducedintoit,cometogethertoformalayeraroundthecolloid.Varioustheorieshavebeenputforward(Figure33):.TheHelmholtztheory:Alayerofpositiveionscoverstheentiresurfaceofthecolloidandensurestheneutralityoftheentiremass(boundlayer)..TheGouy-Chapmantheory:Thelayerofpositiveionsisspacedunevenlyaroundthecolloid;neutralityisobtainedatagreaterdistance(diffuselayer)..TheSterntheorybringstogetherthetwoprecedingtheoriesandintroducestheideaofadoublelayer.Thefirstlayer,whichisattachedtothecolloid,rapidlylosesitspotential.Thesecondlayer,whichismorediffuse,undergoesaslowerlossofpotential.1.1.2.3.ThezetapotentialThecolloidmoveswithpartofitsdoublelayer.Thislayer,whichisboundtothecolloid,correspondstothestationarylayerintheSterntheory.Inthiscase,thecolloidhastwopotentials(Figure33):-E:Thesurfacepotentialofthecolloidorthethermodynamicpotential(Nernst,par.8.1.1).-Z:Thepotentialattheshearsurfaceortheelectrokineticpotential.Figure33.Thedoublelayertheory.ThisZpotential,calledthezetapotential,determinesthemovingofthecolloidsandtheirmutualinteraction.Itcanbedefinedthroughelectrophoresis:whenaparticleissubjectedtoanelectricalfield,italmostinstantlyattainsavelocitysothatthereisabalancebetweentheelectricalforceofattractionandthefrictionduetotheviscosityofthemedium.Thefollowingrelationshipbetweenthezetapotentialandtheelectrophoreticmobilityisobtainedbycalculation:me:Electrophoreticmobilityε:DielectricconstantofthemediumηDynamicviscosityk:4or6,dependingonthehypothesisThoseparticlesthathavethesameelectrokineticzetapotentialpossessthesameelectrophoreticmobilityregardlessoftheirdiameter.Chap.3:Basicphysical-chemicalprocessinwatertreatmentInthedoublelayertheory,coagulationnullifiesthezetapotential.Theequipmentusedformeasuringtheelectrokineticpotentialisthezetameter(seepage351).1.1.2.4.ThechemicaltheorySincethecovalentforcesofattractionare20to50timesgreaterthantheelectrostaticforces,achemicaltheoryhasbeenintroducedtointerpretthedestabilizationofcolloidals