The raw water supply for a community has a pH = 7.0 and an alkalinity = 65 mg/L

Question

The raw water supply for a community has a pH = 7.0 and an alkalinity = 65 mg/L as CaCO_3. Essentially all the alkalinity is attributable to the carbonate acid/base system. The water is treated by adding 25 mg/L ferric chloride FeCl_3, causing Fe(OH)_3(s) to precipitate and aiding the removal of suspended solids and dissolved organic matter from the raw water. The precipitation of ferric chloride proceeds according to the following reaction: FeCl_3 + 3 H_2O rightarrow Fe(OH)_3(s) + 3 CI^- + 3H^+ (a) If we assume that all the added ferric chloride precipitates as Fe(OH)_3(s) (i.e., the reaction goes to completion as written), what is the pH and alkalinity of the water in the effluent of the coagulation-flocculation-sedimentation unit? Assume that gas exchange during the various treatment steps is slow enough that the solution can be treated as a closed system. (b) If we were to take gas exchange into account, would Ct increase or decrease? Would gas exchange assist in taking the above reaction to completion?

Explanation / Answer

Answer a:

For the given reaction pH will be decreased and equal to 4.7 due to the addition of Fecl3. And alkalinity would be decrease and reached at 42mg/L, by the following reason;

Alkalinity consumed by ferric chloride is about 0.92 mg/L, therefore it consumed by adding of 25mg/L equal to 23mg/L. Hence, 42mg/L alkalinity left in the water.

Answer b:

As the gases would be evolved through reaction, therefore the concentration would be decrease. Yes the gas exchange would assist the reaction in completion by reacting with components.

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