Design of a clariflocculator

CLARIFLOCCULATOR
A clariflocculator consists of the following units

• Feeding device
• Mixing device or mixing basin
• Flocculation tank or flocculator and
• Settling or sedimentation tank

Operations of each of the above listed components are listed below

• The feeding device is used to feed the chemical coagulant into the raw water
• The mixture is then thoroughly mixed and  agitated in the mixing basin
• The 'floc' which is formed because of the chemical reaction occurring in the mixing basin is allowed to consolidate in the flocculation tank.
• The flocculated water then passes into the sedimentation tank where flocculated particles settle down and are removed.
• The clarified water (water with low turbidity) is removed from the sedimentation tank.
• The entire process of coagulation and sedimentation helps in removal of significant amount of turbidity from water. In addition to this, it may also help in removing bacteria from water.
• The coagulant used must be fine and uniform in size
• The dosage of coagulant to be used for effective results is an important factor in choosing the type of feeding arrangement.
• The cost of coagulant and size of the plant are important factors to be considered in the design of a clariflocculator
• Coagulation and flocculation play a very important role in any water treatment plant.
• When coagulants are added, the stabilised colloidal system of water is disturbed and the coagulant forms primary flocculation particles.
• Rapid mixing or flash mixing quickly disperses coagulant in raw water and forms primary flocculation particles after destabilising the  double boundary layer.
• Flocculation induces primary particles to come together and form flocs
• Paddles are placed transversely across the tank.
• Paddle rows are arranged to impart barrel roll effect to water causing formation of floc particles that pass through every zone in water causing floc to become more dense floc into small globular particles that settle rapidly
• The clariflocculator is a device that combines the two processes of flocculation and sedimentation
• At the central position flocculation is performed while sedimentation takes place at the outer peripheral position

DESIGN CONSIDERATIONS

• The following data has to be assumed appropriately
• Capacity of the plant
• Capacity and steps of aeration fountain
• Capacity of flash mixer
• Number of clariflocculators, their capacity and method of construction

Based on previous work by Bhole (1975) in the journal of IWWA

• The area requirement for an aeration fountain aerator is 0.045 m2/m3/hr
• A measuring weir is adopted for first and second phase and for variable flow. A 5m long channel is used to give steady and uniform width of flow
• A flash mixer for both the flocculators is designed with capacity requirement for the fifth stage
• Design criteria are
• Detention time
• Velocity of flow
• RPM of blades
• Power requirement and
• Loss of head
• A circular flocculator is used in all water treatment plants
• Water entering the flocculator is first flocculated and then clarified
• Design criteria of flocculator
• Depth of tank = 3 to 4.5 m
• Detention period = 15 to 30 min
• Velocity of flow = 0.2 to 0.8 m/s (normally 0.4m/s)
• Total area of paddle = 10 - 25% of cross-sectional area of tank
• If the peripheral velocity of blade should be 0.2 to 0.6 m/s
• For best performance (in case of sedimentation of flocs) velocity of blades should be less than 0.1 m/s
•  A peripheral velocity greater than 0.6 m/s prevents growth of radially settling floc
• Paddle top should be sufficiently above the floor and below water surface to allow scrapers to pass through
• Number of rotor paddles = 4 (usually)
• Number of blades =3 (usually) 
• Range of velocity gradient = 10 - 75 s-1
• Dimensionless factor G.T = 10^4 to 10^5