Impact of Mixing Chlorine and Chlorine Dioxide on Total Trihalomethane Formation - Part 2

Part 1 described the search for more potent disinfection technologies and began the discussion of the laboratory studies.

Comparison of the Effect of 2 mg/L of ClO2 and Chlorine Doses on TTHMs

Figure 5 shows the effect on TTHM formation of 2 mg/L of
chlorine dioxide dosages with chlorine doses ranging from 0 percent chlorine to
200 percent. The 0 percent chlorine sample shows a 15 ppb reduction of TTHMs
from the raw TTHM level of 42 ppb. The 2 mg/L chlorine dioxide dose apparently
is sufficient to exceed the immediate chlorine dioxide demand than the 1 mg/L
dose. At the 33 percent and 66 percent chlorine doses, the TTHMs are reduced
significantly more than the 1 mg/L chlorine dioxide dose with comparable
chlorine levels, indicating that more chlorine dioxide is being formed from the
reaction between the chlorite byproduct and free chlorine. As before, the 100
percent, 150 percent and 200 percent chlorine dosages are showing increasing
TTHM levels but lower than comparable chlorine levels with 1 mg/L chlorine
dioxide doses.

Chlorine Effect on Chlorite and Chlorate at 2 mg/L Chlorine Dioxide Dose

Figure 6 shows the chlorite and chlorate levels plotted for
chlorine levels from 0 percent to 200 percent for the same samples depicted in
Figure 5. As before, the chlorite levels are decreasing while the chlorate level
increases at a lower rate. The chlorite levels range from 0.90 to 0.69 or 0.21
mg/L difference as compared to the chlorate difference of 0.11 mg/L. The
difference in the chlorite range of values is twice the amount compared to the
1 mg/L chlorine dioxide dose indicating that more chlorine dioxide reformation
is possible with the greater amount available from the higher chlorine dioxide
dose. Therefore, the 2 mg/L chlorine dioxide doses with chlorine reactions seem
to have a similar consistent pattern as the 1 mg/L chlorine dioxide doses.

Comparison of the Effect of 3 mg/L of ClO2 and Excess Chlorine Doses on
TTHMs

Figure 7 shows the average effect on TTHM formation from two
sets of data depicting 3 mg/L of chlorine dioxide dosages with chlorine doses
ranging from 0 percent chlorine to 200 percent chlorine level. The 25 ppb TTHM
reduction from the raw to the 0 percent chlorine dose is greater than the 15
ppb reduction obtained at the previous 2 mg/L chlorine dioxide dose. Again, the
25 percent and 50 percent chlorine doses show decreasing TTHMs while the 75
percent, 100 percent, 150 percent and 200 percent chlorine doses have
increasingly higher TTHM values. It seems apparent that 66 percent chlorine is
the most beneficial for TTHM reduction for all three chlorine dioxide dosages.

Full Plant Studies

Since the previous laboratory studies indicated that mixing
chlorine with chlorine dioxide is beneficial in reducing TTHM formation, it was
important to verify the results on a plant scale. Because El Paso's Canal
Plant has two 20-mgd treatment trains, it was ideal for comparison testing.
Figure 8 depicts TTHM results for samples taken from Plant 1 and Plant 2
secondary effluents. The samples were spiked, like previous laboratory samples,
with 7-mg/L chlorine and held for a 1-hour contact time in order to form TTHMs.

In the first round of plant testing, the Plant 1 raw water
was dosed with about 2.5 mg/L chlorine dioxide only. At the same time, Plant 2
also was dosed with 2.5 mg/L of chlorine dioxide, but with about 50 percent
chlorine. The results showed that plant 2 with the chlorine had a greater
reduction by about 12 ppb TTHMs as compared to Plant 1 with chlorine dioxide
alone.

Next, it was decided to repeat the experiment by reversing
the treatments for each plant. If there were other factors affecting the
results in the plant processes, then the results would not repeat. In the
second round of plant testing, Plant 1 received the chlorine with chlorine
dioxide while Plant 2 received only chlorine dioxide. The TTHM results were
almost the same, showing a significantly greater reduction in Plant 1, which
was receiving the chlorine dioxide with chlorine.

Conclusions

Based on the laboratory and plant studies conducted in this
investigation, the following conclusions can be made with reasonable certainty.

A
1 mg/L chlorine dioxide dose alone did not reduce TTHMs significantly. However,
at 1 mg/L chlorine dioxide dose with 66 percent chlorine, TTHMs were reduced by
7 ppb or 15 percent.

The
2 mg/L and 3 mg/L chlorine dioxide doses with 0 percent chlorine were able to
reduce the TTHMs. However, the 66 percent chlorine dose in combination with
these higher chlorine dioxide doses was able to reduce TTHM formation
considerably more.

Increasing
chlorine dioxide doses with 66 percent chlorine showed a higher rate of TTHM
reduction, perhaps because higher chlorite byproduct levels are available for
reaction with higher chlorine levels to reform chlorine dioxide.

The
laboratory and plant data suggest that about 66 percent chlorine mixed with
chlorine dioxide reduces TTHMs better than chlorine dioxide alone at all three
chlorine dioxide dosages studied in this investigation.

Chlorite
reduction occurs at a greater rate than chlorate formation with increasing chlorine
doses added to chlorine dioxide.

The
chlorine dioxide generator was able to obtain additional TTHM reduction when
chlorine was added.

Recommendations for Future Studies

On the basis of the results presented in this paper,
additional investigations are warranted to

Determine if synergistic disinfection benefits are possible with 66 percent chlorine mixed with chlorine dioxide,

Determine
if CT disinfection credit can be determined for both the chlorine and chlorine
dioxide in the same disinfection zone, and

Determine
if the oxidation of iron and manganese occurs with the 66 percent chlorine
mixed with chlorine dioxide and if TTHM reduction is adversely impacted when
these elements are present.

Acknowledgments

Thanks are extended to the El Paso Water Utilities/Public
Service Board for funding and making their facilities available to complete
this study. Special thanks go to Richard Wilcox for his invaluable
contributions and outstanding laboratory work in analyzing our samples.

Douglas Rittmann, Ph.D., P.E., is a water/wastewater consultant and a lecturer at the University of Texas at El Paso CivilEngineering Department. He was previously the division manager for the El Paso Water Utilities.

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