[LECTURE] Coagulation and Flocculation

8/7/2019 [LECTURE] Coagulation and Flocculation

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Coagulation and FlocculationCoagulation and Flocculation

NCWOA Spring School

Jason GreenWater Treatment SuperintendentCity of Morganton


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4.04.0 Nature of Particulate ImpuritiesNature of Particulate Impurities ininWaterWater

Particulate impurities in water result from land erosion, pickup ofminerals, and the decay of plant material. Additional impurities are addedby airborne contamination, industrial discharges, pollution, are likely tocontain suspended and dissolved organic and inorganic materials, andbiological forms.

These contaminates have a broad size range. Slowing the flow of waterwill remove larger sized particles, such as sand. These particles are calledSettleable Solids. These solids will settle naturally if stored for asufficient amount of time.

Smaller sized particles, such as bacteria, fine clays, and silts, do not

settle readily and treatment is required to produce larger settleablesolids. These smaller particles are often called NONSETTLEABLESOLIDS or COLLODIAL MATTER.


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4.14.1 Need for Coagulation andNeed for Coagulation andFlocculationFlocculation

The primary purpose of coagulation and flocculation is to removeparticulate impurities, especially nonsettleable solids, and color. Theseparticles are removed by the use of Coagulating chemicals. Thesechemicals cause to colloidal mater to clump together to form floc. Thefloc clumps together with other floc particles to for heavier particles thatwill settle out.

In the COAGULATION PROCESS, chemicals are added to destabilize theparticles and cause them to clump together. These particles gathertogether to form larger particles in the FLOCCULATION PROCESS.


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4.24.2 CoagulationCoagulation

4.20 Process DescriptionThe term coagulation describes the effect produced when certainchemicals are added to raw water containing slowly settling ornonsettleable particles. The small particles begin to form larger orheavier floc. The mixing of the coagulant chemical and the raw water tobe treated is commonly referred to as FLASH MIXING. The purpose ofthe flash mix is to rapidly mix and equally distribute the coagulantthroughout the water.

4.21 CoagulantsPrimary coagulates neutralizes the charges of the particles, causingclumping. It adds density and toughness to floc to that the floc will settleand not break apart.

Metallic salts (Aluminum Sulfate, Ferric Sulfate, Ferrous Sulfate) andsynthetic organic polymers (Cationic, Anionic, Nonionic) are commonlyused coagulation chemicals.

The coagulant then reacts with the naturally, or added, alkalinity in thewater to form the insoluble floc.


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4.24.2 CoagulationCoagulation

4.21 PolymersPolymers with positively charged groups on the monomeric units arereferred to as Cationic polyelectrolyte, while polymers with negativelycharged groups are calledAnionic polyelectrolyte. Polymers withoutionizable groups are referred to as Nonionic polymers.


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4.224.22 Basic Coagulant ChemistryBasic Coagulant Chemistry

Coagulation is a physical and chemical reaction occurring between theAlkalinity of the water and the coagulant added to water, which forms aninsoluble floc.

The best pH for coagulation usually falls in the range of a pH between 5and 7. The proper pH must be maintained to ensure proper coagulation.Alkalinity in the water serves a Buffer (prevents pH from changing) thesystem and aids in the complete precipitation of the coagulant chemicals.

If natural alkalinity is low, the Plant will have to feed Soda Ash or Lime tohave an adequate alkalinity dose. Operators have little control of thesource pH and Alkalinity. Careful evaluation of these factors determinethe proper coagulant chemical type and dosage.

Overdosing and underdosing will lead to reduced solids removal

efficiency. The Operator will have to perform Jar Tests to ensure proper

Typically it takes 0.5 mg/L Alkalinity + 1Typically it takes 0.5 mg/L Alkalinity + 1m /L of Coa ulate to form Floc.mg/L of Coagulate to form Floc.


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4.2314.231 Types of MixersTypes of Mixers

Flash mixing is used to thoroughly mix particulate matter and coagulantchemicals ensure proper coagulation and flocculation.

Mixing can be achieved by hydraulic (baffles and throttling valves),mechanical (paddles, turbines, and propellers), diffusers (perforatedtubes or nozzles), and pumped blenders (chemical added directly in theline through a diffuser).

4.2324.232 Coagulation BasinsCoagulation Basins

Mixing of coagulants can also be achieved by a special rectangular tank

with mixing devices. Mixing can also occur in the influent channel or apipeline to the flocculation basin if the flow velocity is high enough.


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Methods of Flash MixingMethods of Flash Mixing


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4.34.3 FlocculationFlocculation

4.30 Process DescriptionFlocculation is a slow stirring process that causes the gathering together ofsmall, coagulated particles into larger, settleable particles. The flocculation

process provides contact between particles to promote the clumping of floc.

4.31 Floc FormationFormation is controlled by the rate at which collisions occur betweenparticles and the effectiveness of these collisions. The best floc size rangesfrom 0.1 mm to about 3 mm.

4.32 Process Performance ConsiderationsAn efficient process involves the selection of the right stirring time(detention time), proper stirring intensity, properly shaped basin, andmechanical equipment to facilitate mixing. Excessive speed will cause thefloc to shear.

4.320 Detention TimeDetention time is not a critical factor in the Coagulation/Flocculationprocess if the chemicals are satisfactorily dispersed. The minimumdetention time for flocculator basins ranges from 5 to 20 minutes for directfiltration to 30 minutes for Conventional Treatment Plants.


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4.3214.321 Types of FlocculatorsTypes of Flocculators

The two most common types offlocculators are the horizontal paddle

wheel type and vertical flocculator.Flocculation can also occur fromturbulence from the roughness in theconduits or channels, or by thedissipated energy of head lossesassociated with weirs, baffles, andorifices.

4.322 Flocculation BasinsActual shape of the Flocculation Basin isdetermined partially by the flocculatorselected and for compatibility with theSedimentation Basins. Floc Basins areequipped with baffles to prevent Short-Circuiting of the water being treated.

Turbulence can also be reduced by usingtapered-energy mixing. By reducing thespeed of the stirrers is to preventbreaking apart of the floc particles.


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Typical Flocculation BasinTypical Flocculation Basin


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4.44.4 Interaction with other TreatmentInteraction with other Treatment

Most particles are too light to settle out and will not be large enough to betrapped during filtration. Disinfection of the water can also be affected by

poor coagulation-flocculation performance. Bacteria and other disease-causing organisms can be bound up in suspended particles and therebyshielded from disinfection. Effective coagulation-flocculation also removesnatural organic matter that reduces the formation of disinfection by-products.

4.54.5 Process ControlProcess Control

The Operator must be able to verify the coagulation-flocculation process toensure proper coagulation and flocculation. The most approved method is aJar Test. Jar Tests should be done at least once daily and whenever a changein raw water or dosage requirements occurs.


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4.64.6 Operating ProceduresOperating Procedures

4.60 Indicators of NormalOperating Conditions

The measurement of the filteredwater turbidity is verified by grabsample or via an online Turbidimeter.However, you CANNOT rely solely onfiltered turbidimeter to gauge

process performance. Depending onthe flow, any coagulant change couldtake several hours to show up in thefiltered water turbidity results.Turbidity, pH, temperature, chlorinedemand, and floc quality must be

monitored throughout the treatmentprocess to ensure plant compliance.Poor process performance can befound early and corrective measurescan be taken.


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4.614.61 Process ActionsProcess Actions

Depending on the size of the facility, the Operator may have various tasks toperform each shift. In smaller plants, the Operator is required to control almost allof the process actions as well as most maintenance activities.

Typical Tasks performed by the Operator:4. Monitor process performance5. Evaluate water quality conditions6. Check and adjust process controls7. Visually inspect facilities

Observation are typically made via online instrumentation or via manuallaboratory methods (Bench-top Turbidimeter, Color Comparator). The frequencyon these tests depends on the quality of the source water.

Determination of Flocculator speed adjustment should include:12.Volume of the floc to be formed. Clear water may indicate a slower speed.13.Visual Observations. Short-Circuiting may indicate flocculator mixing intensity

is not sufficient, while floc breakup may indicate that the mixing is too fast.14.Water Temperature. Lower water temperature requires higher mixing

turbulence, so speed should increase.

Chemical feed equipment should be maintained and the flow verified to ensurecorrect chemical dosages.


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4.624.62 Process OperationProcess Operation


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4.624.62 Process OperationProcess Operation


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4.6244.624 Evaluation of Plant PerformanceEvaluation of Plant Performance

B. Walk Through Plant

The best way to evaluate the performance of the coagulation/flocculation process

is to observe the actual process. While walking through the plant, take some clearplastic beakers and take grab samples throughout the process.4. Observe the floc as it enters the flocculation basins. The floc should be small

and well dispersed. If not, the flash mixer may need adjustment.5. Tiny (pinpoint) floc may mean inadequate coagulant dosage. Popcorn flake is

the desired appearance. If the water is milky or blue the alum dose is too high.6. What does the floc look like as it moves through the basins? The size should be

increasing, but if it appears that it is sheering apart, the flocculator speed maybe too fast.

7. Does the floc settle out in the Sedimentation basins? If the floc is too light itmay not settle out. Try adding more coagulant and a polymer to increase thesize and weight of the floc. Fine floc particles may indicate that the alumdosage is high.

8. Bring some beakers with samples from various locations back to the

Laboratory, let them sit for awhile, and observe the whether or not the flocsettles.

9. How are the filters are performing?


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4.644.64 RecordkeepingRecordkeeping

Record keeping is one of the most important functions of an Operator. An Operatorshould maintain a record on daily operations in logs or diaries. These provide aaccurate day-to-day account of Facility Operations.

In the Coagulation-Flocculation process, you should keep records of the followingrecords:4. Source Water Quality (pH, turbidity, temperature, alkalinity, chlorine demand,

and color)5. Process Water Quality (pH, turbidity, and alkalinity)6. Process Production Inventories (Chemicals used, chemical feed rates, amount

of water processed, and the amount of chemicals in storage)7. Process Equipment Performance (Types of equipment in operation,

maintenance procedures performed, equipment calibration and adjustments)

Records entries should be neat, legible, and easily found. Records should beinitialed for future reference.

4.65 Safety ConsiderationsPotential safety hazards for the Operator include: Electrical equipment, Rotatingmechanical equipment, water treatment chemicals, Laboratory reagents, Slipperysurfaces caused by polymers, drowning, and confined spaces.


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4.74.7 Abnormal Process ConditionsAbnormal Process Conditions

4.70 Indicators of Abnormal ConditionsAny sudden change in the source or filtered water turbidity, pH, alkalinity,temperature, or chlorine demand are signals that require the Operator to evaluate

the coagulation/flocculation process.

4.71 Process ActionsPerform a jar test to confirm coagulation/flocculation effectiveness.Conduct a visual observation of the flash mixing intensity and condition of thefloc.Changes to the alkalinity, pH, and temperature in the source water may have an

impact of the clumping together of floc.Sudden increases in filtered water turbidity could be caused by poor filterperformance. The Operator may need to backwash the filter. Poorcoagulation/flocculation performance is usually the culprit.


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Table 4.4 Coagulation-FlocculationTable 4.4 Coagulation-FlocculationProcess TroubleshootingProcess Troubleshooting


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4.924.92 RecordkeepingRecordkeeping

Good records of actions taken during start-up/shutdown operations will assist theOperator, as well as other Plant personnel, in conducting future start-ups/shutdowns. The results of all inspections, equipment adjustments, and any

unusual events should be accurately recorded.

4.1034.103 Safety ConsiderationsSafety Considerations

Laboratory work could expose the Operators to numerous safety hazards. Careshould be taken when handling glassware and reagents. Operators should wear

protective clothing when performing a wet analyses. Lab work should beperformed in a well ventilate space with plenty of light. Be familiar with thelocation and use of eyewash stations.

4.1044.104 Record KeepingRecord Keeping

Operator should record all laboratory analyses on the appropriate forms and

document any unusual results.

2 S f C id i


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4.1124.112 Safety ConsiderationsSafety Considerations

Operators work around various equipment and each have their own safety considerations:Electrical Equipment3. Lock-out/Tag-out all electrical equipment you are working on.4. Wear protective gloves to prevent shock.

5. Use a multimeter to test for energized lines and equipment.6. Check grounds and avoid grounding yourself in water or on pipes.7. Ground all electrical tools8. Use the buddy system

Mechanical Equipment11. Use protective guards on equipment12. Do not wear loose clothing, worn gloves, or long hair near equipment13. Clean up all lubricant spills

Open Water Structures16. Do not avoid or defeat protective devices, such as handrails, by removing them17. Close all openings when finished18. Know the locations of all life preservers and wear one when necessary.

Valve and Pump Vaults21. Be sure the area is clear of hazardous atmospheres (lack of oxygen, hydrogen sulfide)22. Only work in well-ventilated structures23. Use the Buddy system

Failure to respect your chemicals will be hazardous to your


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Commonly Used FormulasCommonly Used Formulas

Determining Chemical FeederDetermining Chemical Feeder


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Phone (828) 584-1460Phone (828) 584-1460

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[LECTURE] Coagulation and Flocculation

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