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 20-40oC, the best 35-40oC.
- In addition, other influencing factors such as: Ion composition, organic matter, dosage…