Colloids

Very fine solid particles, typically between 0.1 and 0.001 microns in diameter, which are suspended in a liquid or gas and will not settle out of a solution and cannot be removed by conventional filtration alone.

An good example of this is homoginized milk. The particles in the milk are small enough so that they stay suspend indefinitely. In the case of water treatment, this may be clay or iron. Sometimes iron will begin to precipitate and form particles that will remain in suspension and cannot be filtered without the use of coagulants.

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