Flocculation

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SHAHRUL ISMAIL, DESc.University College of Science and Technology MalaysiaCHAPTER 3:Environmental Microbiology

Water Treatment Process :Flocculation

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Flocculation theory3TYPESMECHANICAL DEVICESHYDRAULIC METHODSFLOCCULATION - TYPES4

Vertical-shaft, turbine-type impeller Horizontal-shaft paddle 5

In-line jet rapid mixing deviceRotating blade flocculators 6

Horizontal paddle flocculator Vertical paddle flocculator

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Baffled chamber flocculator8

CLARI-FLOCCULATOR

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10Temp (C)Density (kg/m3)100958.480971.860983.240992.230995.650225997.047922997.773520998.207115999.102610999.70264999.97200999.839510998.11720993.54730983.854The density of water in kilograms per cubic metre (SI unit)at various temperatures in degrees Celsius.The values below 0 C refer to supercooled water.Density of water (at 1 atm)11

12Drag force and Coefft. Of DragInfluid dynamics,drag(fluid resistance) refers to forces that oppose the relative motion of an object through afluid.

Thedrag equationcalculates the force experienced by an object moving through afluidat relatively large velocity (i.e. highReynolds number,Re>~1000)

Thepowerrequired to overcome the aerodynamic drag is given by: Power = Drag force X Velovity

Influid dynamics, thedrag coefficient s adimensionless quantitythat is used to quantify thedragor resistance of an object in a fluid environment such as air or water. It is used in thedrag equation, where a lower drag coefficient indicates the object will have lessaerodynamicorhydrodynamicdrag. The drag coefficient is always associated with a particular surface area.[1]13S.NoParameters Flash Mixer Flocculator1.Detention time 20- 60 s10 - 40 min2.Tank depth3 4.5 m3 4.5 m3. Peripheral velocity of paddles (paddle tip vel.)1.75 2 m/s0.2 0.6 m/s4.Velocity gradient, G

> 300 (1/s)

10 75 (1/s)DESIGN CRITERIA FOR FLASH MIXER & FLOCCULATOR14S.NoParameters Value1.Detention time 10 - 40 min2.Tank depth3 4.5 m3.Total area of paddles10 25% of CS area of tank4. Peripheral velocity of paddles 0.2 0.6 m/s5.Velocity gradient, G

In tapered flocculation, I stage II stage III stage 10 75 (1/s)

100 (1/s) 50 60 (1/s)< 20 (1/s)6. Dimensionless parameter, Gt

For Alum coagulantsFor Ferric coagulants 104 - 105

2 6 x 104 1 1.5 x 104 DESIGN CRITERIA FOR MECHANICAL FLOCCULATOR15S.No Parameters Value7.Power consumption10 36 KW/MLD8.Velocity of flow:

From rapid mixing unit to flocculator

Through flocculation basin

From flocculation basin to settling tank through pipe or channel (to prevent settling or breaking of flocs)0.45 0.9 m/s

0.2 0.8 m/s

0.15 0.25 m/s16Step 1 Design of Influent Pipe Assume velocity of flow through pipe and use continuity eqn. to find Diameter of pipeV = 1 m/s (assume)Q =A V255.1 (m3/h)= (D2/4) (1 x 3600)D =0.24 mProvide an influent pipe of 300 mm diameter.17Step 2 Design of flocculator 1) Assume G & t, calculate Gt.2) If Gt is ok, calculate volume of flocculator (V = QxDT)3) Assume water depth & from volume, obtain plan area of flocculator (V = A x Depth)4) From area of flocculator, obtain tank diameter. 18Step 2 Design of flocculator Provide a tank diameter of 6.6 m.G =40 (1/s) (assume)t =20 min (assume)G t = =40 (1/s) (20 x 60) (s)4.8 x 104 (Hence Ok)V = =255.1 (m3/h) (20/60) (h) 8.5 m3Water depth =2.5 m (assume)Plan area of floccualator =8.5/2.5 = 34 m2Let D =Diameter of flocculator Dp =Diameter of inlet pipe/4 (D2 Dp2) = 34 m2D = 6.58 m19Step 3 Dimensions of PaddlesApproach:

Calculate power input to flocculator (P = G2V)

Calculate area of paddles (P = CDApVr3 )

Check total area of paddles = 10 25% of CS area of tank.

Find out no. of paddles & paddle dimension (HeightxWidth)

Find out no. of shafts to support paddles.

Calculate the distance of shaft from the center line of flocculator

Calculate the distance of paddle edge from the center line of vertical shaft (V = 2 rn/60), assume no. of revolutions of paddles.

20Step 3 Dimensions of Paddles8) Assume velocity of water below partition wall between Flocculator & clarifier and use continuity eqn (Q = A V) for calculating Area of opening required below the partition wall. Finally calculate depth below partition wall.

9) Calculate depth to be provided for sludge storage (as 25% additional depth)

10)Calculate total depth of tank at partition wall, assuming free board. 21Total Area of Paddles (Ap) Provide a tank diameter of 6.6 m.P =

=

P =G2V

(0.89x10-3) (40)2 (x6.62x2.5/4)

122 W

P = CDApVr3 CD =

1.8 (assume) @ 25 C =

997 Kg/m3Vr =(0.75 x Vel. Of tip of blades)Vel. Of tip of blades =0.4 m/s (assume)Vr ==0.75 x o.4 0.3 m/s 122 W = x 1.8 x 997 x Ap x (0.3)3Ap = 5.04 m222Total Area of Paddles (Ap) CS Area of flocculator = (6.6 0.3) 2.5(Total area of paddles / CS Area of tank) = =5.04 / (6.6 0.3) 2.5

10.2% (Hence ok)Provide 8 no. of paddles of height 2 m & width 0.32 m23Total Area of Paddles (Ap) No. of shafts = 2 (each shafts will support 4 paddles) Dis. Of shaft from centre line of flocculator = = (6.6 0.3) / 4

1.58 mn (paddles) =4 rpmDis. Of paddle edge (r) from center line of vertical shaft, V = 0.4 =

r = 2rn / 60

2r4 / 60

1 m24Total depth of tank @ partition wallVel. Of water below partition wall bt. Flocculator & clarifier = 0.3 (m/min) (assume)Area of opening required below partition wall, Q = 250 (m3/h)=

A = A V

A x (0.3x60) (m/h)

13.9 m2Depth below partition wall = =13.9 / (x6.6) 0.67 mDepth provided for sludge storage = =

Total depth of tank assuming a free board of 0.3 m = = 0.25 x 2.5 (25% extra) 0.63 m

0.3+2.5+0.67+0.63 4.1 m25Design of ClarifierAssume SLR, calculate Dia. Of clariflocculator Calculate length of weir & check weir loading rate.SLR = 40 m3/m2/day (assume)SA of clarifier = =(255.1 (m3/day) x 24) / 40 (m3/m2/day) 153.06 m2/4 (Dcf2 6.62) =153.06 m2Dcf =15.44 mLength of weir = = x 15.44 48.53 mWeir loading =

= dimensionless drag coefficient

CD = (24/NR) + (3/NR) + 0.34

NR = Reynolds No. = Vsd/ (dimensionless)30Types of tanks Horizontal flow tanks or vertical flow tanks on the basis of direction of flow of water in the tanks.

Rectangular, Square, Circular in plan.

31Horizontal flow tanksRadial flow circular tank with central feed: -> Water enters at the centre of the tank -> Through openings in the circular well in the centre of the tank, it flows radially outwards in all directions equally. -> Horizontal vel. decreases as water flows towards periphery -> sludge is taken to central sump mechanically

Radial flow circular tanks with peripheral feed: -> Water enters the tank from periphery

3)Rectangular tanks with longitudinal flow

4) Rectangular tanks with longitudinal flow where sludge is mechanically scrapped to sludge pit located near influent end.32Vertical flow tanks They combine sedimentation with flocculation.

They are square or circular in plan

Influent enters bottom of the unit, where flocculation takes place

Upflow vel. decreases with increased CS area of the tank.

There is a formation of blanket of floc through which rising floc must pass.

It is also known as Sludge Blanket Clarifier.

The clarified water is withdrawn through circumferential weir. 33CLARI-FLOCCULATORS 2 Or 4 flocculating paddles placed.

The paddles rotate on their vertical axis.

Paddles may be rotor-stator type, rotating in opposite direction around this vertical axis.

Clarification unit is served by inwardly raking rotating blades.

Flocculated water passes out from bottom of flocculation tank to clarifying zone through a wide opening, the area of opening being large enough to maintain a very low velocity.

Clear effluent overflows into the peripheral launders. 34Tank Dimensions Rectangular tank upto 30 m length.

L to W ration = 3:1 to 5:1

Circular tanks upto 60 m in dia are in use, but upto 30 m better to reduce wind effects.

Square tanks upto 20 m

Depths = 2.5 5 m (preferably 3 m)

Bottom slopes vary from 1% in rectangular tanks to 8% in circular tanks.

The slopes of sludge hoppers range from 1.2:1 to 2:1 35SLR & DTTANK TYPESLR (m3/m2/d)

DT (h)Particles normally removed RangeTypical value for designRange

Typical value for design

Plain sedimentationUpto 600015 30 0.01 153 4 Sand, slit & clayHorizontal flow (circular)25 75 30 40 2 8 2 2.5Alum & Iron flocVertical flow (Upflow) clarifiers - 40 50-1 1.5 Flocculent 36Inlets & Outlets To obtain uniform vel. of flow, water is passed through dispersion perforated by holes/slots.

Vel. Of flow through slots = 0.2 0.3 m/s

Headloss = 1.7 x velocity head

Outlet structures:Weir, Notches, Orifices, Effluent trough, Effluent launder, Outlet pipe.

V-notches attach to sides of troughs and are placed 150 300 mm c/c

Weir length relative to surface area determines strength of outlet current.

Normal weir loadings are upto 300 m3/d/m37Sludge removal Sludge is normally removed under hydrostatic pr. through pipes.

Pipe dia = 200 mm or more (for non-mechanized units) = 100 200 mm (for mechanized units)

Floor slope = shall not be flatter than 1 in 12 (for circular tanks with scrapper) = 1 in 10 (for manual cleaning)

Scrapping mechanism is rotated slowly to complete 1 revolution in 30 40 min Tip vel. Of scraper should be 0.3 m/min or below Power requirements are 0.75 W/m2 of tank areaSettling tanks are capable of giving settled water having turbidity not exceeding 20 NTU, preferably < 10 NTU38Presedimentation & StorageDT = 0.5 3 h

High SLR = 20 80 m3/m2/d39Tube Settlers Settling of basin is dependent on SA, independent of depth.

Very small dia tubes inserted in the basin provide laminar flow conditions.

Such tube settling devices provide excellent clarification with DT equal to or less than 10 min

Tubes cab be horizontal or steeply inclined.

In tubes inclined @ 60, sludge will not slide down the floors.

Tubes may be : Square, Rectangular, Triangular, Circular, Hexagonal, Diamond shaped Thin plastic sheet (1.5 mm) black in color40Settling Tank Efficiency The of basins is reduced by currents induced by inertia of incoming water, wind, turbulent flow, density & temp gradients. Such currents short circuit the flow.

The of real basin,Y/Yo = 1 [1+(nVo/(Q/A))]-1/n Y/Yo -> of removal of suspended particles

-> Coeff. that identifies basin performance

Vo -> Surface overflow rate for ideal settling basin

Q/A -> Required surface overflow rate for real basin to achieve an of Y/Yo for given basin performance 41Settling Tank Efficiency Values of n:

n = 0 for best performance = 1/8 for very good performance = for good performance = for ave. performance = 1 for very poor performance

A well designed tank should be capable of having a volumetric of atleast 70%

To achieve better clarification, flow regime in settling basin should be as close as possible to ideal plug flow.

A narrow and long rectangular tank approximates plug flow conditions than peripheral feed circular tank & centre feed radial flow tank. 42Design of Radial Circular Settling Tank Design a secondary circular settling tank to remove alum floc with following data.

1) Ave. output from settling tank = 250 m3/h2) Amt. of water lost in desludging = 2%3) Ave. design flow = (250/98%)x100% = 255.1 m3/h4) Min. size of alum floc to be removed = 0.8 mm5) Sp.gr. Of alum floc = 1.0026) Expected removal of alum floc = 80%7) Assumed performance of settling tank = very good (n = 1/8)8) Kinematic viscosity of water @ 20 C = 1.01x10-6 m2/s 43Design of Radial Circular Settling Tank Approach:

Using Stokes law calculate settling vel. Of particles (Vs) & check NR