BEIRUT ARAB UNIVERSITYFACULTY OF ENGINEERING

CIVIL & ENVIRONMENTAL ENGINEERING DEPT.

SANITARY COURSE(CVLE453)

COAGULATION & FLOCCULATION

FALL 2015-2016

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General Layout of Water Treatment PlantClic

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Pre-treatment

• Iron and Manganese removal• Hardness treatment

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Iron and Manganese Removal

• Iron and Manganese are frequently found in groundwater and insome surface waters.

• They do not cause health problems, but

• They cause aesthetic problems:-Discoloration of water (iron = reddish water, manganese = brown or black water)-Staining of plumbing fixtures-Impartation of a bitter taste of the water-Stimulation of the growth of microorganisms

• Economic problems include damage to textiles and food. Iron residue in pipesincreases pumping head and decreases carrying capacity. It may also clog pipesand corrode through them.

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Iron and Manganese Removal Techniques

• Precipitation (pH adjustment)This requires pH of the water to be in the range of 10 to 11.

• OxidationUse of oxidants such as air, chlorine, potassium permanganate.Oxidation is followed by settling and filtration.

• Ion exchangeThe water passes through a bed of resin that absorbs undesirableions.

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Hardness Treatment

• Hardness is the presence of positively charged metallic ions in thewater. Calcium, magnesium, iron, strontium, barium, etc…

• It is of concern in domestic water consumption because hard waterincreases soap consumption, leaves a soapy scum in the sink, cancause water heater electrodes to burn out quickly. In industrial uses,it can damage equipment.

• It is expressed in mg/l as CACO3.

• Hardness (mg/l) = Metal ion concentration * 50/ Eq. Wt. of M

• The ion exchange is the most frequently used process for softeningwater.

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• Raw water contains suspended particles of color, turbidity, andbacteria that are too small to settle in a reasonable time period andcannot be removed by simple filtration.

• The object of coagulation is to alter these particles in such a way asto allow them to adhere to each other. Thus they can grow to a sizethat will allow removal by sedimentation and filtration.

• Coagulation is considered to be a chemical treatment process asopposed to the physical treatment operations of flocculation,sedimentation, and filtration that follow.

CoagulationClic

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CoagulationClic

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• The coagulants must be added to raw water and perfectly distributedinto the liquid. This can be done through RAPID MIXING orAGITATION.

• Coagulation results from adding salts of ironor aluminum to the water. Common coagulants:

o Alum (Aluminum Sulfate)o Sodium sulfateo Ferric sulfateo Polymers

Detention time in the rapid mix tank istypically less than one-half minute.

Coagulation

Flash Mixing Tank

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• Coagulation is the reaction between one of these salts and water. Thesimplest coagulation process occurs between alum and water. Whenalum is added to water, a chemical reaction occurs that producespositively charged aluminum ions. The overall result is the reductionof electrical charges and formation of sticky substance called “Floc”.

• Liquid alum is preferred in water treatment because:o Ease of handlingo Lower costso Less labor requiredo Less storage spaceo Greater accuracy in measuremento Elimination of nuisanceo Easier maintenance

CoagulationClic

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Factors Influencing Coagulation• pH• Alkalinity• Temperature• Time• velocity• Zeta potential

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Jar Test• Optimum pH for coagulation and other factors must be considered

before deciding which chemical to be use and at what levels.

• Jar or Coagulation test is performed. A laboratory procedure todetermine the optimum pH and the optimum coagulant dose

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Determination of optimum pH

• Fill the jars with raw water sample (250 or1000 mL) – usually 6 jars

• Add the coagulant and start mixing for 1min at a speed of 100 rpm

• Reduce the stirring speed to gentle 30 rpm• and continue mixing for 20 min

• This slower mixing speed helps promotefloc formation by enhancing particlecollisions, which lead to larger flocs

• Turn off the mixers and allow flocs to settlefor 20 to 30 min

Jar TestClic

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Optimum pH: 6.3• Measure the final residual

turbidity in each jar

• Plot residual turbidity againstpH

Jar TestClic

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• Repeat all the previous steps

• This time adjust pH of all jars at optimum (6.3 found from first test)

• Add different doses of the selected coagulant (Coagulant dose: 5; 7; 10; 12; 15; 20 mg/L)

• Rapid mix each jar then,Reduce the stirring speed

• Turn off the mixers andallow flocs to settle, thenmeasure the final residual turbidity in each jar

• Plot residual turbidityagainst coagulant dose.

The coagulant dose with lowerresidual turbidity will be theoptimum coagulant dose.

Jar Test

Coagulant Dose mg/L

Optimum coagulant dose: 12.5 mg/L

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• Detention Time : 20 to 60 seconds• The tanks are usually, baffled horizontally into two or three compartments in-

order to provide sufficient residence time.• Tanks should also be vertically baffled to minimize vortexing.• Chemicals should be added below the impeller, point of most mixing.

Rapid Mixing Tank Design

Mixing impeller types used in water treatment

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• Flocculation is the physical process of slowly mixing the coagulated water.

• It’s a gentle mixing process

• The primary objective of flocculation is to bring particles in contact with oneanother so they will collide, stick together, and grow to a size that will easilysettle.

• Detention Time : 20 to 40 minutes

FlocculationClic

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FlocculationClic

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