Question

The raw water supply for a community contains 18 mg/L total particulate matter. It is to be treated by addition of 60 mg alum (Al2(SO4)3 14H2O) per liter of water treated. Essentially, all the added alum precipitates represented by the following reaction:

Required:
a. For a total flow of 8000 m^3/d, compute the daily alum requirement and the concentration of solids in the water following alum addition, assuming the alum all precipitates as Al(OH)3(s).
b. The water is initially at pH 7.5 and has ALK=40 mg/L as CaCO3. It is desired to maintain solution pH at 6.5 or higher. Will the pH be in the acceptable range after the chemical addition and Al(OH)3(s) precipitation?
c. What will the pH be if the treated solution is bubbled with air, so that it reached equilibrium with atmospheric CO2?

Answer 1

Solution :

Given :

The steady state flow = 8000 $m^3 /d$

                                    $= 80 \times 10^5 \ I/d$

The concentration of the particulate matter = 18 mg/L

Therefore, the total quantity of a particulate matter in fluid $= 80 \times 10^5 \ I/d \times 18 \ mg/L$

$= 144 \times 10^6 \ mg/g$

$= 144 \ kg/d$

If 60 mg of alum $[Al_2(SO_4)_3.14 H_2O]$ required for one litre of the water treatment.

So Alum required for  $80 \times 10^5 \ I/d$

$= 80 \times 15^5 \ I/d \times 60 \ mg \ alum /L$

$= 480 \times 10^6 \ mg/d$

or 480 kg/d

Therefore the alum required is 480 kg/d

1 mg of the alum gives 0.234 mg alum precipitation, so 60 mg of alum will give $= 60 \times 0.234 \text{ of alum ppt. per litre}$

      $= 14.04$ mg of alum ppt. per litre

480 kg of alum will give = 480 x 0.234 kg/d

                                        = 112.32 kg/d ppt of alum

Daily total solid load is  $= 144 \ kg/d + 112.32 \ kg/d$

                                       = 256.32 kg/d

So, the total concentration of the suspended solid after alum addition $= 18 \ mg/L + 60 \times 0.234$

= 32.04 mg/L

Therefore total alum requirement = 480 kg/d

b). Initial pH = 7.4

 The dissociation reaction of aluminium hydroxide as follows :

$Al(OH)_3 \rightleftharpoons Al^{3+} + 3OH^{-}$

After addition, the aluminium hydroxide pH of water will increase due to increase in $OH^-$ ions.

Therefore, the pH of water will be acceptable range after the addition of aluminium hydroxide.

c). The reaction of $CO_2$ and water as follows :

$CO_2 (g) + H_2O (l) \rightarrow H_2CO_3$

For the atmospheric pressure :

$p_{CO_2} = 3.5 \times 10^{-4} \ atm$

And the pH is reduced into the range of 5.9 to 6.4