Question

Short questions:
a. Name and discuss the mechanisms thought to occur during coagulation.
b. If there is no bicarbonate present in a well water that is to be softened to remove magnesium, which chemical must you add?
c. Water softening is a chemical process aimed at reducing the presence of certain ions naturally present in the water. What are those ions, and, for each, under which new chemical form are they extracted from the water at the end of the treatment?
d. In a few words, explain physically why the depth H of a settling tank does not enter the expression for the critical settling velocity, Vc=Q/WL.
e. A can of a volatile chemical (benzene) has spilled into a small pond. List the data you would need to gather to calculate concentration of benzene in the stream leaving the pond using the mass balance technique. Assume steady state condition and no sorption of benzene to the bottom of the pond.

Answer 1

Question (a) Mechanism of Coagulation

(b)

1. Na2CO3 can be added to remove Mg which could be present in the form of chloride

MgCl₂  + Na₂CO₃  -------> MgCO₃ + 2NaCl

2. Other way is by Sodium-aluminum silicate where Mg ions present in hard water are exchanged with sodium ions in the permutit (Na⁺Al-Silicate).

MgSO₄ + 2Na⁺ (Al-Silicate^- ) ---> Mg(Al-Silicate)₂  + Na₂SO₄

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(C) Mg⁺² and Ca⁺² are the ions responsible for hardness of water and they are usually removed as insoluble carbonates when treated with appropriate reagents like Na₂CO₃ or in other form like silicates based on the kind of salt form these ions are in.

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