Physical-chemical treatment

For water pre-treatment

What is physico-chemical treatment?

Whether it’s producing drinking water or cleaning up wastewater, a crucial step in the treatment process is to remove 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 step called “neutralization”, the water is subjected to coagulation and flocculation. Thus, the particles agglomerate and we obtain “flocs”. Finally, a physical separation step separates the flocs from the treated water. We can detail the steps of this process as follows:

  • Neutralization: pH correction to achieve the optimum coagulant effectiveness (value indicated on the product data sheet)
  • Coagulation: rapid mixing of a coagulant product with the property of neutralizing the negative charges of particles contained in water.
  • Flocculation: By slowly mixing another chemical called a flocculant for several tens of minutes, the flocs form and clump 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 many 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 of the 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 carried out to determine
  • the most suitable chemical products
  • optimal chemical doses
  • optimal contact time.

FAQ

What are the advantages of physico-chemical treatment compared to biological treatment?

The physico-chemical treatment is particularly effective in quickly removing suspended solids, heavy metals and non-biodegradable pollutants. It is often used in pre-treatment or final treatment, especially in industries where effluents contain substances that are difficult to decompose biologically.

The choice between settling and flotation depends on the density of the flocs formed:

  • Decantation : used when the flocs are heavier than water and can settle at the bottom of the tank.

  • Flotation : preferable when the flocs are lighter than water; they rise to the surface thanks to the injection of dissolved air.

The nature of the particles to be disposed of and the characteristics of the effluent influence this decision.

Essential parameters include:

  • The pH: it must be adjusted to optimize the effectiveness of the coagulants.

  • Turbidity: measures the clarity of the water and the effectiveness of the treatment.

  • The suspended solids concentration (TSS): indicates the amount of solid particles present.

  • Chemical oxygen demand (COD): reflects the amount of organic matter.

Continuous monitoring of these parameters allows the process to be adjusted in real time to ensure optimal quality of the treated water.

The Jar Test is a laboratory method used to determine the optimal dose of coagulants and flocculants needed for water treatment. It involves mixing water samples with different concentrations of reagents and then observing the formation and sedimentation of flocs. This procedure allows the treatment process to be optimised according to the specific characteristics of the water to be treated.

Yes, physico-chemical treatment is often used as a key step in the reuse of industrial wastewater. It effectively removes specific pollutants, such as heavy metals, oils and non-biodegradable organic substances, making the treated water compliant with reuse standards for industrial or agricultural applications.

Some examples of packages involving physico-chemical treatment:

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