Clarification and flocculation

• Clarification involves removal of suspended matter like sediments and natural organic matter in addition to removal of colour resulting in clear water.
• Clarification is the first step in conventional treatment of water and consists of:
• Screening
• Coagulation-flocculation
• Sedimentation or flotation and
• Fine filtration
• Clarification refers to the sequence of operations used to remove suspended solids (mineral and organic) from raw water along with a proportion of dissolved organic matter
• Clarification primarily involves removal of settleable and floating solids
• If chemicals are added in order to enhance clarification process, an increased amount of solids can be removed due to:
• improved settling and 
• removal rates
• Chemical clarification results in lesser solids in effluent and more sludge to be disposed
• Particle size and settling time are inversely proportional implying that smaller particles require increased settling time; moreover, small particles are charged making them highly unstable and resistant to settling
• Chemical clarification is based on on two concepts:
• Increasing size of particle and
• Encouraging contact between particles
• Chemical clarification consists of :
• Coagulation
• Flocculation and
• Separation
• Coagulation is accomplished by:
• Addition of chemicals
• Rapid mixing and
• Pin-point floc formation
• Flocculation is accomplished by:
• Slow mixing and
• Floc growth resulting in increased diameter
• Fine suspended particles usually have a positive electrostatic charge
• Since like charges repel, natural stabilizing force keeps solids apart
• The purpose of chemical clarification is to:
• Destabilise charges and
• Encourage contact
• The three main mechanisms involved in chemical clarification are:
• Electrostatic charge reduction
• Interparticle bridging and
• Physical enmeshment
• Electrostatic charge reduction occurs due to:
• Solid particles usually have a negative electrostatic charge
• A cationic chemical is added to neutralize the charge
• Necessary precautions should be taken to avoid overdose causing excessive positive charge
• Interparticle bridging involves formation of fibers that attach to several colloids thereby capturing and binding them together. 
• Some synthetic polymers and organic polyelectrolytes are used to assist interparticle bridging
• For interparticle bridging to take place, polyelectrolytes with long chains of electrostatic charges are used
• Polyelectrolytes adsorb on the charged particles moreover, they must not be overdosed
• Physical enmeshment between non-repelling particles allows formation of large flocs
• Physical enmeshment involves combination of ions with hydroxyl ions
• Physical enmeshment results in the formation of a gelatinous precipitate consisting of flakes and void spaces
• These flakes and voids collect particles due to formation of co-precipitates thereby enlarging the floc
• Coagulation occurs due to due to:
• Rapid mixing and
• Adequate contact

• Flocculation occurs due to:
• Slow mixing and
• Increase in floc size
• Process control can be achieved by:
• Complete mixing and
• Controlled mixing for floc formation and
• Application of controlled chemical dose
• Commonly used metal coagulants are of two types:
• Aluminum Sulphate
• Aluminum Chloride
• Poly Aluminum Chloride
• Sodium Aluminate

and

• Ferric Sulphate
• Ferrous Sulphate
• Ferric Chloride

• Sometimes coagulation supporting agents are used. Examples are:
• Hydrochloric acid
• Sulphuric acid
• Hydrated Lime (CaOH)

Coagulation

• Coagulation is defined as the addition of a positively charged ion such as Al3+, Fe3+ or catalytic polyelectrolyte that results in particle destabilization and charge neutralization
•  The purpose of coagulation is removal of finely divided suspended solids and colloidal material from the waste liquid.
• These contaminants cannot be separated by sedimentation alone except by the use of reasonably long detention periods; truly colloidal particles cannot be removed by settling.
• If these suspended pollutants are organic, they can often be oxidized by biological means, as on trickling filter; biochemical oxidation, however, is slower for suspended matter than for dissolved organic contaminants. 
• If the quantity of insoluble organic matter is large, bio-oxidation equipment must be increased in size to care for this added duty; it is usually more economical to remove the greater part of such matter by chemical coagulation instead of in a trickling filter or activated sludge tank.