Test: Purification of Water - 2 - Civil Engineering (CE) MCQ

In catch basin grit, sand etc. settle and these solids are prevented from entering into the sewer.

The turbidity denotes the concentration of colioids. Alkalinity is used for restoring pH after alum (coagulant) addition.
In the case of low turbidity small number of colloids make the coagulation difficult. If the alkalinity is high, moderate alum doses will result in the formation of AI(OH)₃ which is amorphous and gelatinous. So sweep coagulation takes place. High turbidity-low alkalinity: With relatively small doses of coagulant, water can be coagulated by adsorption and charge neutralization. Depression of pH makes this method more effective because aquometallic ions are more effective at low pH.
High turbidity-high alkalinity: The pH will be relatively unaffected by coagulant addition. Because of high alkalinity, adsorption and charge neutralization will be less effective than in waters of low alkalinity. Higher coagulant dosage should be used to ensure sweep coagulation.

The most important factor contributing to the stability of colloidal suspensions is the excessively large surface to volume ratio from their very small size. The surface phenomenon results in accumulation of electrical charge at the particle surface.

In a clariflocculator. The chemical coagulant is, first of all, fed (either dry or in solution form) into the raw water through the feeding device. This mixture is then thoroughly mixed and agitated in the mixing basin. The 'floc', which is formed as a result of the chemical reaction taking place in the mixing basin, is then allowed to consolidate in the flocculation tank. The flocculated water is finally passed into the sedimentation tank where these flocculated particles settle down and get removed.