This process removes fine suspensions, colloids and organic colors. Coagulation means the addition of chemicals to colloidal dispersions for the particle destabilization. This is achieved by adding proper dose of chemicals and rapid mixing. Flocculation is a second stage for floc formation from destabilized particles which is achieved by gentle and prolonged mixing.
Influencing Factors:- Coagulation and flocculation are greatly influenced by Physical and chemical forces such as electrical charges on particles, exchange capacity of particle size, concentration, pH, water temperature and electrolyte concentration. The optimum coagulant dose is determined from Jar Test.
Coagulant aids: - Finely divided clay, fuller’s earth, activated carbon, bentonites, polyelectrolyte (Polymers containing ionisable units). Addition of these aids will increase the solids concentration of water and will enhance collision efficiency. This in turn will improve flocculation and reduce the coagulant dose
Coagulant Choice: - Common coagulants used in water treatment are alum, ferrous sulphate, ferric chloride, sodium aluminate, chlorinated copper, magnesium carbonate and mixtures of them.
Coagulant, pH range, Normal dose, mg/l Density, kg/cum.
Sodium Aluminate, 5.0 – 8.0, 3.4 -34.0, --
Ferric Chloride, 5.5 – 11.0, 8.5 -51.0, 830.8
Aluminum Sulfate, 5.5 – 8.0, 5.15 – 8.5, 514.3
Ferric Sulfate, 5.5 – 11.0, 8.5 – 51.0, 923.1
Ferrous Sulfate,8.5 – 11.0, 5.1 – 51.0, 593.4
Note :- The above dosages are only representative and the actual dose is to be determined depending on quality of water.
Rapid Mixing :- It’s an operation by which coagulant is mixed rapidly and uniformly dispersed throughout the mass of water to create a more or less homogeneous system. It forms micro-flocs. A detention Time of one min. is recommended in the flash mixer. The velocity gradient G shall be in the range of 750-1000 per seconds.
Slow Mixing: - It is a hydrodynamic process which results in the formation of large settleable flocs by bringing the finely divided matter in contact with microflocs formed during rapid mixing and these can be removed in settling tanks and filters. Flocculation consist of revolving paddles at a speed of about 2-3 revolutions per minutes. A detention period of 30 minutes is recommended for flock formation. The velocity gradient G shall be in the range of 20-100 per seconds.
Mixing devices :- Hydraulic jump, flow through weirs and pressure conduits, mixing basin with baffle walls, mixing basin with mechanical means such as centrifugal pumps, compressed air, flash mixers and flocculators. This is useful if the quantity of water to be treated is small.