Physical-chemical treatment

For water pre-treatment

What is physico-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 classified 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 capture them. However, colloidal and dissolved particles cannot be properly treated by mechanical processes. So, we resort to 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.

The treatment steps

The physico-chemical treatment Is the 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 detail the steps of this process as follows:

  • Neutralization: pH correction to reach the optimum efficiency of the coagulant (value indicated on the product’s technical data sheet).
  • Coagulation: Rapid mixing of a coagulant product with the property of neutralizing the negative charges of particles in the water.
  • Flocculation: By slowly mixing another chemical called a flocculant for several minutes, flocs form and come together.
  • Separation: It involves the physical separation of solids (by settling, flotation, or filtration) that easily settle due to their increased volume and density!
The word clarifloculation refers to all these steps combined

What is physico-chemical treatment used for?

This type of treatment is used in several cases for many applications :
  • Accelerate particle settling speed (Hazen speed) to reduce the size of structures and save floor space
  • Remove fine, colloidal and dissolved particles
A physico-chemical treatment is very effective in removing suspended solids (TSS). This process also removes some phosphorus, BOD5, COD, TOC (drinking water) and heavy metals. Treatment performance depends on a number of aspects and the physico-chemical properties of the effluent. Therefore, laboratory tests, called jar tests, must be performed to determine
  • the most suitable chemical products
  • optimal chemical doses
  • optimal contact time.

Some examples of packages involving physico-chemical treatment:

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