These particles or colloids, typically of a diameter between 0.1 and 0.01 micron, pass through very fine filters and therefore have to be removed by other methods.
Additionally, the settling velocity of colloids is measured in years per metre! This is primarily due to their size and their stability caused by the typical negative charges carried at their surface which causes repulsion in the presence of other colloids.
The traditional first step to achieve the removal of these suspended solids is coagulation. Coagulation is the process of neutralising those repulsing charges by addition of a strongly positively charge reagent.
These reagents, coagulants, can be organic, inorganic (Fe, Al, Ca) or a combination of both.
Choice of the right coagulant and its dosage is made according to the effluent composition; also, pH for the reaction can be adjusted to reach the optimum efficiency. At design stage, different coagulants are tested in so called “Jar tests” to validate their efficiency to precipitate the suspended solids.
Once this result has been obtained, flocculation can take place to make this particles bigger.
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