Question

1) Particles can be classified into Colloids (size 1 nm- 1 μm) and Suspended solids (larger than 1 μm). Give 2 examples of colloids and suspended solids. Explain suitable treatment to remove colloids and suspended solids.

2) Coagulation refers to all the reactions and mechanisms that result in particle aggregation in the water being treated. Give example of 2 coagulants (metal salts). Describe the functions of coagulant.

Answer 1

Question 1:

Answer: Colloids is a mixture that consists of microscopically insoluble or soluble particles of size raging from 1nm -1000 nm dispersed throughout a second substance. Examples of colloids includes Milk and gelatin. Coagulation is one method to treat colloids. In coagulation, the colloid particles dispersed in a second solution forms large clumps or clots in presence of coagulant agent. Coagulant are chemicals that destabilize the electrostatic charges on the colloid particles and forms larger floc particles. These large folc particles or clumps combine to form agglomerates that are separated by various methods such as flocculation, sedimentation and centrifugation etc.,

Suspended solids: They are small particles of size larger than 1μm that remains as colloid in suspension in water. Examples: Mud, silt and organic colloids. Suspended solids are treated or removed by various methods such as sedimentation and filtration. As the size of suspended particles are more than 1μm, then can be removed by allowing them to sediment or by centrifugation. Filters of different pore sizes are used to filter suspended particles in suspension in water.

Question 2

Answer: Coagulation is a process in which liquids changes to solid or semi-solids in presence of coagulants. Coagulants are chemicals mostly metallic salts. Example includes aluminum Sulfate (Alum) and ferric sulfate. The main function of coagulant is to neutralize/destabilize the electrostatic charges on the colloid particles, leading to formation of larger floc particles or clumps. In generally, coagulants with opposite to the colloidal particles in solution are added to neutralize the repulsive forces and to destabilize the suspension. After destabilization, van der Waals force allows the particles to agglomerate.