Advantages
- Reduction in treatment cost
- Finished water turbidity
- Sludge volume reductions
- Reduction in colloidal silica
- Increase in length of filter runs and reduction in wash water
CHEMICAL COAGULATION
In waste water treatment, coagulation and flocculation are employed to separate suspended
solids from water. Although the terms coagulation and flocculation are often used
interchangeably, or the single term “flocculation” is used to describe both; they are, in fact, two
distinct processes. Knowing their differences can lead to a better understanding of the
clarification and dewatering operations of wastewater treatment. Finely dispersed solids
(colloids) suspended in wastewaters are stabilized by negative electric charges on their
surfaces, causing them to repel each other. Since this prevents these charged particles from
colliding to form larger masses, called flocks, they do not settle. To assist in the removal of
colloidal particles from suspension, chemical coagulation and flocculation are required. These
processes, usually done in sequence, are a combination of physical and chemical procedures.
Chemicals are mixed with wastewater to promote the aggregation of the suspended solids in to
particles large enough to settle or be removed.
Coagulation is the destabilization of colloids by neutralizing the forces that keep them apart.
Cationic coagulants provide positive electric charges to reduce the negative charge (zeta
potential) of the colloids. As a result, the particles collide to form larger particles
(flocks).Coagulation, thus, implies formation of smaller compact aggregates. Rapid mixing is
required to disperse the coagulant throughout the liquid.
Care must be taken not to overdose the coagulants as this can cause a complete charge reversal
and restabilize the colloid complex.
Effluents are heterogeneous in nature. Chemical coagulation is an important unit process in
water treatment for the removal of turbidity. Its application in water treatment is followed by
sedimentation and filtration. Various types of coagulants are being used to condition water
before sedimentation and filtration. The most widely used coagulants are:
- Aluminium sulphate{Alum }
- Poly aluminium chloride {PAC}
- Ferrous sulphate
- Sodium Aluminate
- Silicon Derivatives
- Lime
- Synthetic Organic Polymers Currently Alum and PAC are most extensively used in water treatment .When brought in contact with water, they form positively charged aluminium hydroxide floc which agglomerates the negatively charged clays, slit, bacteria, algae organic matters etc causing them to settle down. The sludge formation in alum and PAC is very high. Also it suffers from high disposal cost making the treatment non user friendly. Suffer from a serious draw back Synthetic polymers are now increasingly being used as polymeric coagulants for water treatment. Commercial polymeric coagulants are highly charged cationic products. Cationic polymeric coagulants /flocculants contain positively charged groups such as amino {- NH3 +}, Imino {- CH2 NH2 + – CH2} or quaternary amino {- + NR4}. Although a number of poly sulphonium and poly phosponium compounds have been reported, quadrivalenrt nitrogen is the charged site in all commercial cationic polymeric coagulants at the present time. Cationicity derives from the quadrivalent nitrogen, either via protonation of primary, secondary or tertiary amines or via generation of quaternary nitrogen groupings. Coagulants of this latter types are referred to as Poly quats. Cationicity of the quaternary nitrogen is independent of Ph, although, other parts of the polymer molecule may exhibit sensitivity to Ph, such as hydrolysis of ester linkages. Cationicity dependent on protonation, however, is a function of Ph .Without getting enmeshed in details of polymer- surface interactions, the gross effects of Ph on effective cationicity can be obtained from colloid titration behavior of the polymer at fixed levels of Ph.