The definition of coagulation and flocculation is not uniform throughout the industry and to prevent any misunderstandings we define the terms as:
Coagulation: Particles that aggregate with themselves e.g. by the influence of a change in pH.
Flocculation: Particles that aggregate by the use of polymers that binds them together.
Coagulation and flocculation are well-known techniques within wastewater treatment. They are typically used for treatment of wastewater containing colloids (suspended particles) and metal ions. But the phenomena are also well-known from everyday life – especially coagulation.
A good example is sour milk. When milk gets sour the pH decreases, which causes the fat particles to destabilize and then coagulate.
To better understand coagulation and flocculation the theory of particles needs to be explained. We use particles as an overall term for all ions or molecules in solution or suspension.
Particles in water will to some extend be electrically charged. The area nearest to the particle can be divided into two layers. Closest to the electrically charged particle counter-ions will gather and create the first layer (called the Stern layer). The next layer is composed of both counter-ions and co-ions but with a surplus of counter-ions. Finally, is the bulk which is the surrounding water with an equally distribution of counter-ions and co-ions.
These two layers around the particle cause it to be stable in the water. When the conditions within the water are changed e.g. by a change in pH or conductivity the amount of ions in the water changes well. This change in ions in the water will affect the amounts of ions in the two layers and thereby affect the stability of the particles. This mechanism is used in coagulation.
Electrically charged particles can also precipitate by the use of flocculation polymers. A flocculation polymer is a polymer with charged sites. By using a polymer with the opposite charge as the particles that need to be flocculated the particles will (caused by the opposite charges) be bound to the polymer combining them in larger particles, which cannot stay suspended.
When particles or ions are precipitated from the solution further treatment with e.g. sedimentation, sand filter, bag filters or even membrane filtration (MF/UF) can be necessary to obtain the wanted water quality. Finally, the precipitated particles which now is a sludge can be further treated by e.g. a filter press to a much denser sludge to ensure a more economically attractive total solution for wastewater treatment.
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Biological water treatment