Physical-chemical treatment

Whether producing drinking water or treating wastewater, a crucial step in the treatment process is removing particles. These particles have different sizes. They are divided into 4 categories:

• dissolved(< 0,08 μm),
• colloidal(0,08 – 1 µm),
• supracolloidal(> 1 – 100 µm)
• settleable(> 100 µm).

The type of treatment and its effectiveness depend on the size of the particles present in the water to be treated. Coarse particles are easy to remove using a lamella clarifier, for example. Finer particles settle poorly, but a drum filter can easily trap them. However, colloidal and dissolved particles cannot be properly treated by mechanical processes. In this case, we use physico-chemical treatments! By adding chemicals (known as coagulants and flocculants), we can create the right physical conditions for them to agglomerate and be removed. Physico-chemical treatment is a combination of two stages known as coagulation-flocculation. This is a classic method for separating suspended solids in treatment plants. Physico-chemical treatment is generally combined with settling processes or filtration. After a pH correction stage called “neutralization”, the water undergoes coagulation and then flocculation, where the particles agglomerate to form “flocs”. Finally, a physical separation stage separates the flocs from the treated water. We can describe the steps involved in this process as follows:

• Neutralization: pH correction to achieve optimum coagulant efficiency (value indicated on product data sheet).
• Coagulation: quick mixing of a coagulant with the property of neutralizing the negative charges of particles contained in water.
• Flocculation:By slowly mixing another chemical called flocculant for several tens of minutes, flocs form and aggregate.
• Separation: This is the physical separation of solids (by settling, flotation or filtration), which settle easily thanks to their increased volume and density!