Coagulation and flocculation process in wastewater treatment

a. Nature: In water there exist many different suspended substances. These substances can be treated differently depending on their size:

• d > 10-4 mm: use sedimentation and filtration method.
  • d < 10-4 mm: must combine mechanical methods with chemical methods. That is, adding substances that create adhesion and pulling suspended particles to settle => is called flocculation method in water treatment. To carry out this process, appropriate reactants are added to water: Al2(SO4)3; FeSO4; or FeCl3.

b. Consider the coagulant as aluminum alum:

• Alum: put in water, they dissociate into Al 3+ ——-> Al(OH)3

Al ³⁺ + 3H₂O  à Al(OH)₃ + 3H⁺

• The pH of water directly affects the hydrolysis process:
  
  • pH < 4.5 : no hydrolysis occurs.
  • pH = 5.5 – 7.5 : achieve the best.
  • pH > 7.5 : The flocculation effect is not good.
  • The appropriate water temperature is approx 20-40^oC, the best 35-40^oC.
• In addition, other influencing factors such as: Ion composition, organic matter, dosage…