What is flocculation?
Flocculation is the separation of a solution, commonly the removal of sediment from a fluid. The term is derived from floc, which means flakes of material; and when a solution has been flocculated, the sediment has formed into larger aggregated flakes, making them easier to see and remove. This process occurs naturally, or it can also be forced using flocculants and/or physical processes.
What are flocculants?
Flocculants are substances that promote the agglomeration of fine particles present in a solution, creating a floc, which then floats to the surface (flotation) or settles to the bottom (sedimentation). This can then be more easily removed from the liquid.
Flocculants can be organic or inorganic, and come in various charges, charge densities, molecular weights, and forms.
Organic polymeric flocculants are most widely used today, due to their ability to promote flocculation with a relatively low dosage. Although, their lack of biodegradability and the associated dispersion of potentially harmful monomers into water supplies is causing the focus to shift to biopolymers, which are more environmentally friendly. The problem with these is they have a shorter shelf-life, and require a higher dosage than organic polymeric flocculants. To combat this, combined solutions are being developed, where synthetic polymers are grafted onto natural polymers, to create tailored flocculants for water treatment that deliver the optimum benefits of both.
What are flocculants used for?
Flocculants are used across many different industries, from textile industries, civil engineering companies, earth sciences and biotechnology, to breweries and cheesemakers. However, flocculants are primarily used in the wastewater treatment industry for solids removal, water clarification, lime softening, sludge thickening, and solids dehydration.
Water can contain colloidal solids, like clay particles, bacteria, plankton, decaying plant material or other organic matter; and using coagulation and flocculation to purify water has been an active practice since as early as 2000BC, when the ancient Egyptians used almonds smeared around vessels to purify river water.
Flocculation leading to sedimentation is both used in the purification of drinking water, as well as for sewage, storm water, and industrial wastewater treatment. This is why flocculants are largely known for being water treatment plant chemicals.
How do flocculants work?
Flocculants can either be applied on their own, or in combination with coagulants, depending on the charge and chemical composition of the solution being separated.
Coagulants work by destabilising particles present in a stable solution, causing them to aggregate, and allowing them to be bonded together by flocculants. The flocculants join the particles together into flocs, which are then separated from the solution, once they have either fallen as sediment or floated to the surface.
The appropriate combination of organic or inorganic flocculants and coagulants will depend on the type of substances being removed from the water, as well as the method of separation being used by the water treatment facility (i.e. sedimentation or floatation etc).
What are the differences between flocculants and coagulants?
While both coagulation and flocculation are both common processes used in the treatment and purification of water, they are in fact very different. Coagulation is a chemical process, whereby the chemical properties of the solution are altered to promote coagulation. Coagulate means to curdle, and coagulants initiate the same process that naturally occurs in milk, when the pH of the liquid changes and the milk solids clump together. Coagulants are usually salts, which break down to release positive or negative charges. Flocculation, on the other hand, is a physical process that causes particles to floc together, first forming a cloud and then a precipitate. Flocculants are often polymers, which induce the settling of particles into larger and larger flakes or flocs. Physical agitation or other techniques are often required to promote flocculation, while coagulation will occur as soon as the coagulant has been added to the solution, without any physical processes being required.
