Chemicals used for Wastewater Treatment Plants:
Particles finer than 0,1 micron (10-7 m) remains continuously in motion due to electrostatic charge (most often negative) which causes them to repel each other. Once their electric charge is neutralized by the use of a coagulant chemical (FeCl3 Ferric Chloride or FeSO4 Ferric Sulphate), all particles start to collide and agglomerate (combine together) under the influence of Van der Waals’s forces. These heavier particles are called flocs. Thus, by coagulation we have a) neutralized the electrostatic charge of the particle and eliminated there continuous motion and b) agglomerated large numbers of fine particles into flocs that can be separated.
In Physical Chemistry, the van der Waals force (or van der Waals interaction), named after Dutch scientist Johannes Diederik van der Waal is the attractive or repulsive force between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules. The term includes:
force between permanent dipole and a corresponding induced dipole
instantaneous induced dipole-dipole forces.
pH Correction Chemicals
pH correction is necessary because:
· The coagulant is an acid and the wastewater stream to the DAF Unit would become corrosive if the pH is not corrected.
· See second paragraph under “Flocculents’ about appropriate pH and reaction of the flocculent with water to form insoluble hydroxides.
The Product Specification from the Chemical Supplier should specify the appropriate pH for optimal reaction of the flocculent with water.
Flocculents, or Flocculating agents, are chemicals that promote flocculation by causing colloids and other suspended particles (the Pin Flocs) in water to aggregate, forming a large strong floc. Flocculents in water treatment are used in water treatment processes to improve the sedimentation or flotation or filterability of small particles. For example, a flocculent may be used in a wastewater treatment system to remove fine particles in addition to the larger particles where the fine particles represent a high COD loading and which would be difficult or impossible to remove by Flotation or filtration only.
Many flocculents are multivalent cations such as aluminium, iron, calcium or magnesium. These positively charged molecules interact with negatively charged particle and molecules to reduce the barriers to aggregation In addition , Many of these chemicals, under appropriate pH and other conditions such as temperature and salinity, react with water to form insoluble hydroxides which, upon precipitating, link together to form long chains or meshes, physically trapping small particles into a larger floc.