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Flocculation by definition means a process in which individual particles of a suspension form aggregates. In the water treatment industry, the terms coagulation and flocculation imply different mechanisms. Flocculants consist of various molecular weight anionic, nonionic and cationic polymers. They are used to increase the efficiency of settling, clarification, filtration and centrifugation operations.
FLOCCULATION
FLOCCULATION is used to describe the action of polymeric materials which form bridges between individual particles. Bridging occurs when segments of a polymer chain adsorb on different particles and help particles aggregate. Flocculants carry active groups with a charge which will counterbalance the charge of the particles. Flocculants adsorb on particles and cause destabilization either by bridging or charge neutralization.
An anionic flocculant will usually react against a positively charged suspension (positive zeta potential). That is the case of salts and metallic hydroxides.
A cationic flocculant will react against a negatively charged suspension (negative zeta potential) like silica or organic substances.
However the rule is not general. For example, anionic flocculants agglomerate clays which are electronegative.
Three groups of flocculants are currently used
1.1 - MINERAL FLOCCULANTS
They are colloidal substances. Adsorption and charge neutralization play some part in the flocculation mechanism. They are:
• activated silica.
• certain colloidal clays (such as bentonite),
• certain metallic hydroxides with a polymeric structure (alum, ferric hydroxide)
1.2- NATURAL FLOCCULANTS
They are water soluble anionic, cationic or nonionic polymers. Nonionic polymers adsorb on the suspended particles. The most common natural flocculants are:
• the starch derivatives: mostly pregelatinized hence water-soluble. They are corn or potato-starches. They can be natural starches, anionic oxidized starches or amine treated cationic starches. The use of this class of products has decreased in water treatment but remains important in the paper industry.
• the polysaccharides: usually guar gums and mostly used in acid medium.
• the alginates: anionic and used in potable water treatment.
1.3- SYNTHETIC FLOCCULANTS
Polyacrylamides
The most common polymers are those based on polyacrylamide, which is a nonionic polymer. Their effect is due to bridging between particles by polymer chains.
Polymers can be given anionic character by copolymerizing acrylamide with acrylic acid. Cationic polymers are prepared by copolymerizing acrylamide with a cationic monomer. All available acrylamide based polymers have a specific amount of ionic monomer giving a certain degree of ionic character.
They have a specific average molecular weight (i.e. chain length) and a given molecular distribution.
For each suspension, a certain degree of anionic, cationic or nonionic character is beneficial. Usually, the intrinsic flocculating power increases with the molecular weight.
Polyacrylamides have the highest molecular weight among the synthesized industrial chemicals in the range of 10-20 millions. Other polymers display specific properties and are used under specific conditions.
They are mostly:
• Polyethylene-imines
• Polyamides-amines
• Polyamines
• Polyethylene-oxide
• Sulfonated compounds
• There are many types of flocculants and flocculents including flocculant agentsand flocculant chemicals. You can also use various flocculant aids such as pH adjustment.
• (Flocculants are the chemicals or substances that facilitate flocculation or floccing up of suspended solids in liquid, the resulting floc or wooly mass itself is called flocculentwith an "e" however sometimes the terms are used interchangeably.)
• alum flocculant
• anionic flocculant
• cationic flocculant
• chitosan flocculant
• polyacrylamide flocculant
• polymer flocculant
• nalco flocculant
Coagulants and flocculants are similar, however, a coagulant aids coagulation, notflocculation
. Coagulation and Flocculation are essentially the same type of process, thickening or clumping. Coagulation is more of a thickening process, whereas flocculation is a clumping together of substances or encapsulation of substances that form flocculent and in wastewater treatment hopefully precipitate out of solution.
You can find a flocculant definition here and our flocculant msds here.
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Flocculants and coagulants used as wastewater flocculants, and in flocculant water treatment are designed to do one thing, remove suspended solids from water and wastewater and or facilitate solids removal and subsequent dewatering.
Flocculants, or flocculating agents, are chemicals or compounds that causeflocculation by making suspended particles in liquids clump together forming a wooly lump or floc. Because they are woolly in appearance these clumps are called floc.
The coagulants
Trivalent cations : the neutralization of the negative surface of the colloid is accomplished by the addition of cations in the case of inorganic coagulants. The trivalent ions are ten times more effective than the divalent ion. Trivalent iron and aluminum salts keep on the be be widely used in all the water coagulation treatments.
The influence of the pH : Inorganic coagulant because of their hydrolysis change the physical-chemical characteristics of water to be treated (pH, conductivity, …) :
M 3+ + 3 H2O <=> M(OH)3 + 3 H+
The pH necessary for coagulation may be adjusted by addition of an acid or a base.
Cation Optimum pH for Coagulation-Flocculation
Al3+ 6.0 – 7.4
Fe3+ > 5
Sludge production : The formation of metallic hydroxide causes the production of a substantial amount of sludge. This sludge should be removed in the final solids-liquids separation process.
Organic coagulants may also be used. The advantage of those cationic polyelectrolytes is because they directly neutralize the negative colloids. Consequently to this direct action the amount of sludge is considerably reduced.
The flocculant
Inorganic polymers (activated silicia) and natural polymers (starches, alginate) were the first to be used. But the use of synthetic flocculants often results in a minimum amount of sludge. Combined with modern separation techniques can allow to produce very dense sludge that can be directly treated in a dewatering unit.