Environmental Engineering Reference
In-Depth Information
the containers, it is not possible using it as cooling
water or cleaning water for equipments. In cases
it is used in wet-charging, white traces will be left
on the surface of containers. Hard water is not
preferred for cleaning the equipments since it
reduces the performance of detergents used.
Therefore, using another chemical to adjust pH
instead of CaO, for example NaOH, may decrease
the water consumption.
The hardness of water is mainly caused by
divalent or trivalent ions, Mg
2+
and Ca
2+
, and
water can be softened by chemical precipitation
technique (Patterson, 1985, 220-226). If the chemi-
cal (CaO) used for pH adjustment is replaced by
another chemical containing an ion, which does
not cause any additional hardness, then the treated
wastewater can be recycled back to the production
system to be used in wet-charging, cleaning, etc.
This change is expected to decrease the water
consumption of the company significantly. To be
able to compare the benefits of using NaOH instead
of CaO, there is a need to find out the amount
of NaOH required to neutralize the wastewater
containing H
2
SO
4
and to raise its pH to 8.5.
Calculation of CaO and NaOH Amounts:
The theoretical calculation of the amounts of CaO
and NaOH to be added to neutralize the wastewater
containing 496 mg SO
4
2-
/L (5.2 x 10
-3
mol SO
4
2-
)
and to adjust the pH to 8.5 requires the following
assumptions:
4. The dissociation of NaOH in H
2
O is
complete.
NaOH → Na+ + OH-……………... (Equation 1)
5. The CaO and NaOH used are 100% pure.
Dissociation of H2O:
Kw= [H+] [OH-]=10-14
=
10
14
−
−
OH
[
H
+
]
Mass balance on Ca
2+
and SO
4
2 -
:
number of moles
total volume
Ca
2
+
C
T,Ca
2+
=
[
]
=
number of moles
total volume
2
−
C
−
=
[
SO
]
=
4
T,SO
2
Charge Balance:
2
[
Ca
2
+
]
+
[
H
+
]
=
2
[
SO
2
−
]
+
[
OH
−
]
4
10
−
14
2
[
Ca
2
+
]
[
H
+
]
2
[
SO
2
−
]
+
=
+
4
[
H
+
]
1. The ionic strength of the wastewater is very
low, so that the influence of other ions in
concentration estimations can be ignored.
2. The temperature of the wastewater is 25°C.
3. Dissociation of Ca(OH)
2
, which is the prod-
uct of a reaction between CaO and H
2
O is
complete.
pH=8.5= - log[H
+
+
−
8 .
[H ]=10
M
Ca
2
+
−
3
[
]
=
5 20
.
×
10 M
Therefore, Ca
2+
to be added:
CaO (s) + H
2
O → Ca(OH)
2
(aq) ……(Reaction 1)
Ca(OH)
2
→ Ca
2+
+ 2OH
-
…….....…(Reaction 2)
