Industrial waters are clarified to remove turbidity andcolor from the effluent streams in the textile, paper and other polluting industries.The dictionary meaning of a coagulant is "an agent that induces curdlingor congealing. In a water treatment, what it is a chemical that will remove colorand turbidity present in raw water in the form of flocs. Coagulants neutralize the repulsive electrical charges(typically negative) surrounding particles allowing them to "sticktogether" creating clumps or flocks. Flocculants facilitate theagglomeration or aggregation of the coagulated particles to form largerfloccules and thereby hasten gravitational settling. Some coagulants serve adual purpose of both coagulation and flocculation in that they create large flocksthat readily settle.


In waste water treatment, coagulation and flocculation areemployed to separate suspended solids from water. Although the termscoagulation and flocculation are often used interchangeably, or the single term"flocculation" is used to describe both; they are, in fact, twodistinct processes. Knowing their differences can lead to a betterunderstanding of the clarification and dewatering operations of wastewater treatment.Finely dispersed solids (colloids) suspended in wastewaters are stabilized bynegative electric charges on their surfaces, causing them to repel each other.Since this prevents these charged particles from colliding to form largermasses, called flocks, they do not settle. To assist in the removal ofcolloidal particles from suspension, chemical coagulation and flocculation arerequired. These processes, usually done in sequence, are a combination ofphysical and chemical procedures. Chemicals are mixed with wastewater topromote the aggregation of the suspended solids in to particles large enough tosettle or be removed.


Coagulation is the destabilization of colloids by neutralizing theforces that keep them apart. Cationic coagulants provide positive electriccharges to reduce the negative charge (zeta potential) of the colloids. As aresult, the particles collide to form larger particles (flocks).Coagulation,thus, implies formation of smaller compact aggregates. Rapid mixing is requiredto disperse the coagulant throughout the liquid.


Care must be taken not to overdose the coagulants as thiscan cause a complete charge reversal and restabilize the colloid complex.


Effluents are heterogeneous in nature. Chemical coagulationis an important unit process in water treatment for the removal of turbidity.Its application in water treatment is followed by sedimentation andfilteration. Various types of coagulants are being used to condition waterbefore sedimentation and filteration. The most widely used coagulants are:

v     Aluminumsulphate{Alum }

v     Poly aluminumchloride {PAC}

v     Ferrous sulphate

v     Sodium Aluminate

v     SiliconDerivatives

v     Lime

v     SyntheticOrganic Polymers


Currently Alum and PAC are most extensively used in watertreatment .When brought in contact with water, they form positively chargedaluminium hydroxide floc which agglomerates the negatively charged clays, slit,bacteria, algae organic matters etc causing them to settle down. The sludgeformation in alum and PAC is very high. Also it suffers from high disposal costmaking the treatment non user friendly. suffer from a serious draw backSynthetic polymers are now increasingly being used as polymeric coagulants forwater treatment. Commercial polymeric coagulants are highly charged cationic products. Cationic polymeric coagulants /flocculants contain positively charged groups such asamino {- NH3 +}, Imino {- CH2 NH2 + - CH2} or quaternary amino {- + NR4}. Althougha number of poly sulphonium and poly phosponium compounds have been reported,quadrivalenrt nitrogen is the charged site in all commercial cationic polymericcoagulants at the present time. Cationicity derives from the quadrivalentnitrogen, either via protonation of primary, secondary or tertiary amines orvia generation of quaternary nitrogen groupings. Coagulants of this lattertypes are referred to as Poly quats. Cationicity of the quaternary nitrogen isindependent of Ph, although, other parts of the polymer molecule may exhibitsensitivity to Ph, such as hydrolysis of ester linkages. Cationicity dependenton protonation, however, is a function of Ph .With out getting enmeshed indetails of polymer- surface interactions, the gross effects of Ph on effectivecationicity can be obtained from colloid titration behavior of the polymer atfixed levels of Ph.


 

Theory of Coagulation:


Extensive studies have shown that understanding the nature of suspended solids in a water system and then understanding four major factors that affect removal can lead to more effective solids removal:

  1. Destabilization of particles
  2. Microfloc formation
  3. Agglomeration
  4. Physical entrapment


To coalesce colloids, interparticle repulsion forces must be neutralized. Certain cationic chemicals can be used to decrease the negative charge. This is destabilization. In hydrophilic colloidal system where colloids are strongly hydrated, it may be necessary to ad a chemical that not only neutralizes surface charge, but also forms an insoluble complex with the colloid. After destabilization of the colloid {decreased net electric charge}, collisions between particles can occur. Through chemical bridging { adsorption } hydrogen bonding and van der waals forces, the colloidal particles form small microflocs, which in turn continue to colloid in a dynamic system and they form macroflocs or larger agglomerates, some times physically entrapping other particles. Depending on which of the four factors is more critical to solids removal, different approaches may be needed.


Typical coagulant addresses the following areas:

  1. Reduction in treatment cost
  2. Finished water turbidity
  3. Sludge volume reductions
  4. Reduction in colloidal silica
  5. Increase in length of filter runs and reduction in wash water requirements
  6. Enhancement in plant capacity and treatment efficiency



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