Coagulation and Flocculation Purpose of Coagulation and ...

WTRG16 Coagulation and Flocculation 7/1/13Coagulation and Flocculation• Coagulation is a physical and chemical reaction occurringbetween the “AKALINITY” of the water and the coagulantadded to the water which results in the formation ofinsoluble flocs (floc that will not dissolve).• The reaction is very complex and is effected by a variety offactors including :– Temperature–pH– Alkalinity– Zeta Potential1Purpose of Coagulationand FlocculationRemove turbidity fromthe water• Turbidity is the cloudyappearance of watercaused by smallsuspended particles• Commonly referred toas non-settleable orcolloidal solids21

WTRG16 Coagulation and Flocculation 7/1/13Particles in Water• Chemicals in solution (have beencompletely dissolved in the water).• Colloidal solids, also known asnonsettleable solids (do not dissolve inwater although they are electricallycharged).• Suspended, or settleable solids (willsettle out of water over time).3ConventionalCoagulation/Flocculation Process• Pretreatment• Chemical Feed• Flash Mixing• Coagulation& Flocculation• Sedimentation• Filtration• Disinfection42

WTRG16 Coagulation and Flocculation 7/1/13Pretreatment Considerations in theCoagulation/Flocculation ProcessFactors that May Dictate Pretreatment– Leaves, plant material and silt– Seasonal raw water qualitychanges caused by drought, highwater or temperature changes– Potential upstream pollution– Wind conditions– Algae blooms– Bacterial problems5Particle Removal by CoagulationNegatively chargedparticles repel each otherdue to electricity.Positively charged coagulantsattract to negatively chargedparticles due to electricity.• Coagulants neutralize negative charged particles• Allows particles to come closer and form larger clumps• Provides “agglomeration sites” for larger floc63

WTRG16 Coagulation and Flocculation 7/1/13Factors Affecting the Effectivenessof the Coagulation Process• Best pH (pH Range: Al, 5 – 7 ; Fe, 5 – 8)• Alkalinity of water (> 30 PPM residual)• Concentration of Salts (affect efficiency)• Turbidity (constituents and concentration)• Type of Coagulant used (Al and Fe salts)• Temperature (colder requires more mixing)• Adequacy of mixing (dispersion of chemical)7Chemical Coagulants used inWater TreatmentChemical Name Chemical Formula PrimaryCoagulantAluminum Sulfate Al 2 (SO 4 ) 3 XFerrous Sulfate FeSO 4 XFerric Sulfate Fe(SO 4 ) 3 XFerric Chloride FeCl 3 XCationic Polymer Various X XCalcium Hydroxide Ca(OH) 2 X a XCalcium Oxide CaO X a XSodium Aluminate Na 2 Al 2 O 4 X a XBentonite Clay XCalcium Carbonate CaCO 3 XSodium Silicate NaSiO 3 XAnionic Polymer Various XNonionic Polymer Various Xa- Used as primary coagulant in water softening processes.CoagulantAid84

WTRG16 Coagulation and Flocculation 7/1/13Primary Coagulants• Primary coagulants are always used in thecoagulation/flocculation process. Alum and ferricsulfate are most commonly used.• Different sources of water need different coagulantsto react over a wider pH range.• Primary coagulants have multivalent charges (+2,+3, +4) which allow them to react with thenegatively charged colloidal materials in the water.• All of the coagulant chemicals will remove alkalinityfrom the water.9Coagulant Aids• Add density to slow-settling floc and addtoughness to the flocs so that they will notbreak up during mixing and settlingprocesses.• Not always required and are generallyused to reduce flocculation time.• Effective at extending pH ranges forprimary coagulant performance.• Most are expensive.105

WTRG16 Coagulation and Flocculation 7/1/1311Use of Alum as aCoagulant• Earliest and most widely used coagulant• Effective range pH 5.0 to 7.0; (6.5optimal)• Effective range for color is ~ 5.5• Reacts with alkalinity to form floc -results in drop in pH• For every 2 mg/l Alum; 1 mg/l Lime isadded to replace alkalinity126

WTRG16 Coagulation and Flocculation 7/1/13Floc Formation• The positively charged aluminum ions (Al +3 ) attract thenegatively charged particles that cause color and turbidity, thusforming microfloc.Al +3 Al +3 Al +3 Al +3• The positive charged microfloc particles begin to attract andhold more negatively charged particles in the water.• The microfloc grows into large settable floc due to adsorptionand collison of other floc particles13Use of Ferrous Sulfate (Copperas)and Lime for Coagulation• Combination produces Ferric Hydroxide• pH 8.4 range to 9.0• Oxygen must added by aeration orchemically such as chlorine• Very Effective for turbid water• Care must be taken because color notremoved at high pH147

WTRG16 Coagulation and Flocculation 7/1/13Use of Ferric Chloride asa Coagulant• Has wider pH range than FerrousSulfate• Typically used where color removal isalso desirable.• Does not require oxygen supplement15Use of Ferric Sulfateas a Coagulant• Does not require oxygen supplement• Effective over wider pH ranges• Effective within all temperature ranges• Eliminates odors from existingHydrogen Sulfide• Lower doses required than FerrousSulfate168

WTRG16 Coagulation and Flocculation 7/1/13Use of Coagulant Aids• Lime• Weighting Agents• Polymers– Cationic +– Anionic -– Nonionic17Coagulant Mixing andFlash MixersPurpose of the flash mix process is to rapidlymix and equally distribute the coagulantchemical throughout the water.• Coagulant must make contact with all of thesuspended particles.• Process occurs in seconds.• First results are formation of very small particles.• Detention time and the speed of the mixer shouldbe sufficient to thoroughly mix all the chemical withno breakup of floc particles.189

WTRG16 Coagulation and Flocculation 7/1/13Mixing Coagulantsand Coagulant Aids• Rapid/Flash Mixtypically occurs in atank• Mixing velocity 5 – 7fps• Coagulant andcoagulant aidsadded in firstchamber19Static Mixer for Polymers• Used prior toCoagulation• Addition of oxidizingagents, GAC andpolymers• Used as far aspossible upstream tomaximize contacttime2010

WTRG16 Coagulation and Flocculation 7/1/13Flocculation Process• Flocculation is a slow stirringprocess that causes thegathering together of small,coagulated particles intolarger, settleable particles.• Controlled by the rate ofcollisions between particlesand the effectivenesspromoting attachment.• Changes in temperature cannecessitate longer detentiontimes.21Flocculation Followed bySedimentation• Instantaneous mix of coagulants• Neutralizes the negative charges on colloids• During the gentle mixing of flocculation, allowsparticles to agglomerate into larger particles• Heavier particles are removed by settling2211

WTRG16 Coagulation and Flocculation 7/1/13Importance of Flocculator Speed• If the speed of the stirring process is to greatthen the floc particles will be “sheared” orbroken apart causing an increase in turbidity.• If flocculator speed is to slow then “shortcircuiting”may occur.• Purpose is to create a floc of good size (0.1to 3mm), density, and toughness for laterremoval in the sedimentation and filtrationprocesses.23What is Short-Circuiting?“A condition that occurs intanks or basins when some ofthe water travels faster thanthe rest of the flowing water.”May result in shorter contact, reaction,or settling times.2412

WTRG16 Coagulation and Flocculation 7/1/13Desirable Floc Quality• A popcorn flake is a desirable floc appearance.• Smooth circular particles tend to settle quickerwhile irregular shaped particles settle slower.• Tiny alum floc may be an indication that thechemical dosage is too low.• If the water has a milky appearance or a bluishtint, the alum dose is probably too high.• Should be increasing in size as it moves throughthe basins25Performance Monitoring• Mixing needs to be adequate• Monitor pH for optimum conditionsWhat adjustment shouldbe made in the speed offlocculators when thewater temperaturedrops?Increase Speed• Flow measurement is important to accuratelyestablish chemical feed rates, wash waterrates, and unit loadings• Chemical feed systems need dosage control• Jar Test at the Start of Every Shift or more!2613

WTRG16 Coagulation and Flocculation 7/1/13Performance MeasurementUsing the Jar Test• A jar test is a laboratory procedure where varyingdosages of coagulant are tested in a series of glassor plastic jars under identical conditions.• The jars are injected with coagulant dosages andgently paddled or flocculated to match fieldconditions as closely as possible.• After a set of time to simulate field conditions thejars are observed to determine which dosageproduces the largest, strongest floc or which dosageproduces the floc that settles the fastest.• Other laboratory tests sometimes include a jar test todetermine the optimal pH or determine the turbidityof the settled water and its filterability. 27Jar Test Apparatus andProcedures• Six paddles• One container iscontrol• RPM gauge allowsfor mixing speedto match plantFlash Mix: 1 Minute at 80 RPM conditionsFlocculation: 30 Minutes at 20 RPMSettling: 30 Minutes2814

WTRG16 Coagulation and Flocculation 7/1/13Jar Test Plot for Low AlkalinityWater• Alum initially reactswith low alkalinity• With Ferric Chloriderequires chemical toreach optimal pHbefore reacting• Adding too muchcoagulant increasesturbidity29Jar Test Plot for HigherAlkalinity Water• Higher Coagulantdoses are needed forhigh alkalinity waters• Ferric Chloriderequired morechemical but reachedlower turbidity• Since Alum did notproduce water < 1NTU a coagulant aid isnecessary3015

WTRG16 Coagulation and Flocculation 7/1/13Evaluation of Jar Test Results• Rate of Floc Formation (15 min.)• Type of Floc Particles (circular or popcorn)• Clarity of the Water between the Floc• Size of the Floc (.1 to 3 mm)• Amount of Floc (too little may need agent)• Clarity of Water above Settled Floc (clear)• Volume of Floc31Coagulation-Flocculation ProcessMonitoring and Sampling PointsSourceRaw WaterTU, TEMP, ALKColor, pHCoagulationEffluentpH, VISFlocculationEffluentTU, ALK,pH, VISRaw Water∞Flash MixKEY:(CoagulationTU – TurbidityTEMP – TemperatureALK – AlkalinityVIS – Visual ObservationHighSpeed∞ ∞ ∞MediumSpeedFlocculationBasinSlowSpeedSedimentationBasin3216