Environmental Engineering Reference
In-Depth Information
of hydroxyl ions in the compositions of the
chemicals. It should be noted here that there is a
significant difference in the solubility of NaOH
(1,080 gr/L) and CaO (1.39 gr/L) (Dean, 1992).
The NaOH can be dissolved in the water in almost
instantly requiring mixing enough to sustain the
homogeneity of the wastewater. On the other
hand, the solubility of CaO is lower than NaOH.
In the experiments made in the wet-charging
operation, it is seen that caustic does not leave
white traces in the surface of containers. Therefore
it is feasible to use caustic instead of lime. In ad-
dition, with the use of lime, it will be possible to
treat and reuse the same water for several days.
The wet-charging operation uses 150 tones of
water every day; so the reuse of the treated water
in wet-charging will remarkably reduce the water
consumption. The remaining 150 m
3
of refined
water may be used in the cleaning of equipments,
in the negative drying of plates and as lead sup-
pressant. Table 4 shows the comparison of lime
and caustic. Since 100 tones of treated wastewater
will be used in equipment cleaning once a day, this
amount of wastewater can not be recycled back
to production system. Otherwise, there would
be a need for tank with capacity of at least 100
tones. Since the water needed in the facility is
300 tones/day, the company will have to use tap
water of 100 m
3
to clean the equipments each day.
In the calculations, it is assumed that 300 m
3
of
tap water is captured in the first day of the week
and 100 m
3
of water in each of the remaining 5
10
mol
300
m
day
3
L
m
1
1
mol CaO
mol Ca
(
40
) gr
mol CaO
+
16
1
1000
kg
gr
=
5 20
.
×
−
3
×
×
1000
×
×
×
3
2
+
L
1
kg
=
87 36
.
day
The calculated amount of CaO is much lower
than the amount added into the pH adjustment
tank in the wastewater treatment plant, which is
413 kg/day. The difference is huge, but it could
be due to the assumptions made in theoretical
calculations especially the grade of the CaO used
in the plant, which may be very low. Moreover,
the dissociation of CaO and Ca(OH)
2
may not be
completed in the pH adjustment unit as a result
of poor mixing conditions.
Mass Balance on Na
+
and SO
4
2 -
:
number of moles
total volume
C
= Na
+
T,Na
+
number of moles
total volume
C
−
=
[
SO
2
−
]
=
T,SO
2
4
Charge Balance:
+
+
2-
-
[Na ]+[H ]=2[SO ]+[OH ]
4
The equation was similarly solved and the
amount of [Na
+
] was found to be 0.01037 M
which corresponds to 124.46 kg/day. The reason
for adding more NaOH than CaO is the number
Table 4. Comparison of cash flow analysis of the current system and lime alternative
Current System
Lime Alternative
Water Usage (m
3
/month)
7,200
3,200
Total Cost of Water ($/ month)
15,930
7,080
Chemical Usage (tone/month)
2.1 (0.08735 x 6 x 4)
2.9 (0.124460 x 6 x 4)
Unit Cost of Chemical ($/tone)
76.25
562.5
Total Cost of Chemical ($/month)
160
1,680
