In wastewater treatment plants in Australia, all the waste streams consist of liquid and solid components. The first step of the process is to remove as much as is possible of the solid components from the liquid ones. The steps may involve filtration technologies, bar screens, sedimentation basins, clarifiers and other technologies such as chemical treatment, physical means and centrifugation. The second step of the treatment involves the treatment of liquid components using biological and chemical methods. Today, we are going to talk about flocculation and coagulation as essentials of treatment of wastewater.
Coagulation
Coagulation means to curdle. It refers to the process of clotting of blood and the formation of large lumps or clots or even to sour milk-making curds. It is mainly the process by which the colloidal particles and fine suspensions of solids present in a stream of wastewater are combined into bigger agglomerates that can be easily separated utilizing filtration, sedimentation, centrifugation, flocculation or other reparatory methods. This refers to chemical processes where destabilisation of particles that cannot settle is realised.
The particles that cannot settle can refer to most colloids and extremely small solid particles and all dissolved particles. The particles form clumps with the help of coagulants. It is normally achieved by adding different coagulants to the water to promote destabilisation of colloid dispersion present. Coagulation is typically the destabilisation of colloids by neutralisation of the electrostatic forces that separate them. In wastewater treatment systems, the cationic coagulants yield positive electrostatic charges. This helps in the reduction of the negative electrostatic charges better known as zeta potential of the colloids. This results in the particles colliding to form larger floc particles.
The coagulation process requires rapid mixing to disperse the coagulant throughout the water. Care should be taken to reduce an overdosing of the coagulants because this can result in a complete electrostatic charge reversal. It can also lead to the destabilisation of the colloid complex.
Flocculation
This is the coming together of particles. At times, this requires a microscope to be observed. It is a mechanical or physical process that results in the joining together of flocs or large aggregated clumps to form larger masses. These larger masses, later on, precipitate from the liquid phase and are converted into a solid phase for more separation. The forcesresponsible for keeping these particles dispersed and suspended after they contact each other are typically reduced in coagulation. This is referred to as precipitate formation or the collapsing of the colloid.
Flocculation joins the de-established dispersion of colloidal into large aggregates that move into the solid phase. Both wastewater treatment and water treatment methods use flocculants and coagulants to achieve the ends of removing the particles from the liquid phase. This renders them components of the solid phase. Wastewater and water commonly contain colloidal particles including clay particles, organic matter, bacteria, small particles of plant material that has decayed and other dispersed moieties in colloidal range. Addition of some common coagulants to a stream of wastewater produces coagulation of colloids and results in the precipitation of soluble compounds.
