Clariflocculator

CLARIFLOCCULATOR

The constituents of a coagulation sedimentation plant or Clariflocculator are:

(1) Feeding device

(2) Mixing device or mixing basin

(3) Flocculation tank or flocculator

(4) Settling or Sedimentation tank

A Clariflocculator containing all these four units is shown in figure

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The chemical coagulant is fed (either dry or solution) into raw water through the feeding device.

The (water + coagulant) mixture is thoroughly mixed and agitated in the Mixing

The coagulant causes the very fine suspended and colloidal particles to agglomerate and form 'floc', which happens in the flocculation tank.

The flocculated water is finally passed into the sedimentation tank where these flocculated particles settle and are removed.

Each of these four units is discussed in detail below:

 

1. Methods of feeding coagulants / Feeding devices

a) Dry feeding

b) Wet feeding

 

a) Dry feeding

The chemical coagulant is fed into raw water in powdered form.

Simple in operation, requires less space and cheaper. However dosage control is difficult. Coagulant (powder) is kept in hopper bottom of tank. Agitating plates prevent arching of coagulant. The dosage is regulated by the speed of toothed wheel/helical screw which in turn is controlled by venturi device installed in the raw water pipes.

The quantity of coagulant released is in proportion to the quantity of the raw water entering the mixing tank.

(b) Wet feeding:

The chemical coagulant is fed into raw water in solution form. The wet feeding equipments are costlier, but they can be easily controlled and adjusted. The coagulant solution is prepared and stored in a tank, from where it is allowed to trickle down into mixing tank. A conical plug type arrangement is used to regulate the rate of coagulant discharge proportionate to the rate of raw water flow. The mixing basin and float chamber are interconnected so that water level is same in both. As the flow of water increases, the depth of water in mixing basin as well as float chamber increases and thereby lifting the float. As float rises, the pinion and pulley lifts the conical plug allowing more flow of coagulant solution into mixing basin. When water flow decreases, conical plug descends down and reduces the coagulant feeding rate. Thus coagulant dosage is automatically controlled. The chemical which are corrosive in nature create problems in wet feeding.

 

2. Mixing Devices

After addition of coagulant to raw water, the mixture should be thoroughly and vigorously mixed so that the coagulant gets fully dispersed into water. This violent agitation is achieved by mixing devices such as centrifugal pumps, compressed air and mixing basins. The mixing basins are normally adopted and they are of the following two types:

a) Mixing basins with baffle walls

b) Mixing basins equipped with mechanical devices

(a) Mixing basins with baffle walls: Rectangular tanks which are divided by baffle walls. The baffles are provided in such a way that the water flows in the following pattern.

(i) Horizontally around the ends of baffles.

(ii) Vertically over and under the baffles.

The disturbances/ hindrances created by the baffles to the flow creates sufficient  agitation and mixing The velocity of flow is controlled to a value of 0.15 to 0.45 m/s.

The detention period is 20 to 50 minutes. They are less efficient and are used only i small. WTPs. Awoll

b) Mixing basins with mechanical devices - Flash Mixer

• The mechanically agitated mixing basins provide the best type of mixing. T chemical added to raw water is vigorously mixed and agitated by a flash mix for rapid dispersion in water.

• A Flash mixer consists of a rectangular tank provided with an impeller fixed an impeller shaft. The impeller is driven by an electric motor at a high a variable speeds. The intensity of mixing depends on the temporal me velocity gradient (G).

• The coagulant is brought by the coagulant pipe which discharges it under impeller. The raw water from inlet is also deflected by baffles towards impeller. The thoroughly mixed water is taken to the flocculator.

• A drain valve is provided to remove sludge from the bottom of flash mixe

• The speed of impeller is kept between 100 to 200 rpm.

• The detention period is 1/2 to 2 minutes (30 to 60 seconds).

• The value of G is 300 s^-1

• Power required is 1 to 3 Watts per m3/hr of flow.

c) Centrifugal pump

Raw water is pumped into the settling tank. Chemical is added to the suction line of

When water with coagulant passes through the impeller of pump, mixing is created.

d) Compressed air agitation

In this, raw water and coagulant are agitated vigorously by diffusing compressed air from bottom of the mixing basin.

(e) Narrow mixing channel with flume

The Coagulant is fed from the feeding tank'

The turbulance caused by vertical baffles and flumes mixes the chemical thoroughly.

3. Flocculation Tank or Flocculator

The violent agitation in a Flash Mixer should be followed by a slow and gentle stirring to permit agglomeration of floc particles.

From the mixing basin, water is taken to the flocculator where it is given slow and gentle stirring motion.

They are rectangular tanks with paddles operated by electric motors. The water from the flocculator is taken to the sedimentation tank.

The paddles rotate at 2 to 3 rpm speed. The detention time is 20 to 60 min (30 m normally). The velocity gradient is 20 to 80 s1. The clear distance between paddles an wall or floor is 15 to 30cm.

The purpose of the flocculator is to facilitate 'floc' formation. The floc so forme should not be allowed to settle in the flocculator.

The velocity of flow is not important in the design of flocculators, because th rolling motion created by the paddles prevents settling of floc.

Design Criteria for Flocculator

(i) Depth of tank - 3 to 4.5 m

(ii) Detention time - 10 to 40 min (30 min normal)

(iii) Velocity of flow - 0.2 to 0.8 m/s (0.4 m/s) normal.

(iv) Area of paddles - 10 to 25% of cross-sectional area of tank.

(v) Outlet flow velocity - 0.15 to 0.25 m/s

(vi) Power consumption - 10 to 36 KW/mld.

(vii) Velocity gradient (G): 10 to 75 s-1

4. Sedimentation tank (Clarifier)

Similar to plain sedimentation tanks, except that the detention period is less (2 to 4 hrs) and has a higher surface loading (overflow) rate of 1000 to 1250 1/hr/m² or 24 to 30 m3/d/m² of plan area.

 

COMBINED COAGULATION-CUM-SEDIMENTATION TANKS

It is a combination of coagulation and sedimentation tank. It consists of a plain floc chamber (without mechanical devices) followed by a sedimentation tank. Detention period for floc chamber is 15 to 40 minutes and for settling tank is 2 to 4 hours. The depth of floc chamber is half the depth of settling tank. The water from mixing basin enters this tank and the clarified water comes out at the outlet end. The design is similar to a sedimentation tank except that it is deeper 3 to 6 m. They are cleaned at 6 months interval.

Problem 2.2:

Design a coagulation-cum-sedimentation tank with continuous flow for a population of 60,000 persons with a daily per capita water allowance of 120 litres. Make suitable assumptions where needed.

Solution:

1. Design of Settling Tank

Average daily consumption = Population × Per capita demand

= 60,000 x 120 = 7.2 × 10⁶ litres

Maximum daily demand = 1.8 × Average daily demand

02 = 1.8 × (7.2 × 106) = 12.96 × 10⁶ litres

Assuming detention time of 4 hours (between 2 to 4 hours)

Capacity or Volume of Tank = Discharge x Detention Time

Volume = 12.96×10⁶ / 24 x 4

= 2.16 x 10⁶ litres = 2.16 x 10³ m³

Assuming, surface overflow rate as 1000 litres /hr /m2 (between 1000 to 1250 l/hr/

Q = 12.96 × 10⁶ l/d = 540 x 10³ 1/hr

Assuming width of tank as 12 m,

12 x L = 540 m²

L= 45 m

Volume = L x B x H = 2.16 × 10³ m³

45 x 12 x H = 2.16 x 10³

H = 4 m

Extra depth for sludge storage (1 in 50 slope) = 45/50 = 0.9m

Assume Free Board = 0.5 m

Overall depth = water depth + sludge storage + free board

= 4 m +0.9 m + 0.5 m

= 5.4 m

Provide settling tank of dimensions 45 m x 12 m x 4m

2. Design of the Floc chamber

Depth of floc chamber = ½ x depth of settling tank

1/2 x 4.5 = 2.25 m

Assuming period of flocculation (detention period) as 20 minutes. (between 15 to

40 min)

Volume or Capacity of chamber =Q x Detention time

12.96×10³ /24×60 =180m³

Area required = Volume / depth = 180/2.25 = 80m³

Using same width = 12 m,

Length of flocculation chamber = 80 /12 = 6.67 m

The dimensions of Floc chamber are = 6.7 m x 12 m x 2.25 m