W3T331759 / WT.050.585.003.UE.IM.1014
78
MICRO/2000® AND DEOX/2000® MODULE
NaHSO
3
is used, 1.38 mg/l of hardness is removed. The stoichiometric
relationship means that for every mg/l chlorine, 0.9 mg/l SO
2
is required
(often this is increased to 1.05 to eusure complete dechlorination) while for
sodium bisulphite, 1.46 mg/l must be used. The alkalinity of chlorinated
wastewater effiuent is usually in the region of 150 to 200 mg/l as CaCO
3
.
Therefore if S0
2
is used, dechlorination of 10 mg/l would result in a
28 mg/l reduction in alkalinity, and for NaHSO
3
, the result would be
14 mg/l reduction.
The process has to be controlled by a dechlorination monitor. This can be
done in a number of ways. A traditional chlorine monitor can be used to
control the process to zero chlorine residual. The disadvantage with this
approach is that the monitor only responds to the chlorine residual and not
SO
2
. Hence the zero of the instrument is zero chlorine, addition of SO
2
beyond this will not be detected and there is a possibility of excessively
dosing SO
2
, especially if there is a long process time.
An alternative approach is to use a centre zero analyser such as the SFC
DEOX/2000®. In this approach an iodine bias is applied to the cell.
Iodine (I
2
) is reduced in the cell analogous to chlorine Cl
2
and hence a
zero current is established. The presence of chlorine results in both the
I
2
and the Cl
2
being reduced which creates a current greater than the zero
current. The presence of SO
2
results in a reaction with I
2
which decrease
the I
2
bias in the cell. This yields a current lower than the zero current.
This is achieved by the following chemistry.
Iodine is not stable long term and hence has to be generated in the cell.
Using a peristaltic pump, Potassium Iodate (KIO
3
) and an excess of
Potassium Iodide (KI) are mixed in a acid solution.
KIO
3
+ 5KI+ 6HAc ------> 31
2
+ 6KAc + 3H
2
O
This is constantly fed to the cell to create the zero current bias, the feed
rate being 0.08 ml/min of reagents in total, i.e. negligible compared to
the volume of water being treated.
The reactions are then as follows:-
If excess chlorine is present, this reacts with excess KI in the analyser
as follows:-
HOCl +2I
-
------> I2 + Cl
-
+ OH
-
Every mg/l of chlorine produces an extra mg/l of I
2
which adds to the
zero current signal.
Summary of Contents for WALLACE & TIERNAN DEOX/2000
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