Flocculation
From Wikipedia, the free encyclopedia
Flocculation refers to a process where a solute comes out of solution in the form of floc or "flakes." The term is also used to refer to the process by which fine particulates are caused to clump together into floc. The floc may then float to the top of the liquid, settle to the bottom of the liquid, or can be readily filtered from the liquid.
In geology, flocculation is a condition in which clays, polymers or other small charged particles become attached and form a fragile structure, a floc. In dispersed clay slurries, flocculation occurs after mechanical agitation ceases and the dispersed clay platelets spontaneously form flocs because of attractions between negative face charges and positive edge charges.
In biology the process is used to refer to the asexual aggregation of microorganisms, most commonly brewing yeast at the end of a brew.
Flocculation is widely employed in the purification of drinking water as well as sewage treatment and treatment of other industrial wastewater streams.
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[edit] Flocculants
Flocculants, or flocculating agents, are chemicals that are used to promote flocculation by causing colloids and other suspended particles in liquids to aggregate, forming a floc. Flocculants are used in water treatment processes to improve the sedimentation or filterability of small particles. For example, a flocculant may be used in swimming pool or drinking water filtration to aid removal of microscopic particles which would otherwise cause the water to be cloudy and which would be difficult or impossible to remove by filtration alone.
Many flocculants are multivalent cations such as aluminium, iron, calcium or magnesium. These positively charged molecules interact with negatively charged particles and molecules to reduce the barriers to aggregation. In addition, many of these chemicals, under appropriate pH and other conditions, react with water to form insoluble hydroxides which, upon precipitating, link together to form long chains or meshes, physically trapping small particles into the larger floc.
Long-chain polymer flocculants, such as modified polyacrylamides, are manufactured and sold by the flocculant producing business.
Modified Polyacrylamides can be supplied in dry or liquid form for use in the flocculation process. The most common liquid polyacrylamide is supplied as an emulsion with 10-40% actives and the rest is a carrier fluid, surfactants and latex. Emulsion polymers require activation to invert the emulsion and allow the electrolyte groups to be exposed.
Other factors such as pH, temperature, and salinity can induce flocculation or influence flocculation rates.
The following chemicals are used as flocculants:
- alum
- aluminium chlorohydrate
- aluminum sulfate
- calcium oxide
- iron(III) chloride
- iron(II) sulfate
- sodium aluminate
- sodium silicate
The following natural products are used as flocculants:
- Moringa oleifera seeds
- Papain
- A species of Strychnos (seeds)
- Isinglass
[edit] Coagulants
The terms flocculant and coagulant are sometimes used interchangeably, but it is more accurate to use the term coagulant for a chemical that contributes to molecular aggregation, rather than particular aggregation.[citation needed] Usually dissolved substances are aggregated into microscopic particles by a coagulant and then these particles may be flocculated into a macroscopic floc with a flocculant. In general, coagulants will have higher net charge and a lower molecular weight than flocculants.
[edit] Deflocculation
A deflocculant is a chemical that is added to prevent a colloid from coming out of suspension.
