CHEMICAL COAGULATION
Industrial waters are clarified to remove turbidity and color from the effluent streams in the textile, paper and other polluting industries. The dictionary meaning of a coagulant is "an agent that induces curdling or congealing. In a water treatment, what it is a chemical that will remove color and turbidity present in raw water in the form of flocs. Coagulants neutralize the repulsive electrical charges (typically negative) surrounding particles allowing them to "stick together" creating clumps or flocks. Flocculants facilitate the agglomeration or aggregation of the coagulated particles to form larger floccules and thereby hasten gravitational settling. Some coagulants serve a dual purpose of both coagulation and flocculation in that they create large flocks that readily settle.
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 filteration. Various types of coagulants are being used to condition water before sedimentation and filteration.
The most widely used coagulants are:
Advantages
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 .With out 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.
Applicability
Coagulation-flocculation is a conventional pre-treatment method (typically in combination with sedimentation and rapid sand filtration) used to separate the suspended and dissolved compounds (turbidity) from the water in (semi-)centralised drinking water treatment plants. Many charged species such as suspended mineral, organics, pathogens, and dissolved species such as metal ions, phosphates, fluoride, and radionuclides can be separated by these processes. Typically, coagulation-flocculation is combined with sedimentation and rapid sand filtration) to be effective. Chemicals reactants (flocculants and coagulants) concentration must be adjusted properly to the exact composition of the water. The operation of coagulators, flocculators and clarifiers requires trained operators. Maintenance work should be undertaken regularly to guarantee an efficient treatment. Coagulation-flocculation is also often used to remove suspended solids in domestic and industrial wastewater treatment plants.