IHE Ozone Bench Scale Schematic
Observed Analytical and Equipment Issues
Several analytical and equipment issues arose throughout the progression of this research in the experimental period. These issues can be broken down into problems with the equipment used in the bench scale experiments, and issues with the analytical
measurements and procedures undertaken.
Analytical Issues
The analytical issues experienced during the experimental period largely occurred during the measurement of the dissolved ozone concentration with the indigo blue method. This method was employed in lieu of a functioning dissolved ozone analyzer in the bench scale setup. Inaccuracies observed with the measurement of the dissolved ozone concentration in the non-spiked and spiked experiments originated from analytical, measurement and human error that occurred during use of the indigo blue method. This method required sampling of ~5ml of the ozonated water sample at specific
time measurements, often within 1 minute of each other. In the time that the sample was taken from the reactor chamber to the addition of the indigo reagent and capping of the sample jar, dissolved ozone gas will transfer from its dissolved state to a gaseous state and leave the sample. This loss of ozone gas potentially contributed to the lower than expected measurements of dissolved ozone concentrations.
In the early stages of the experimental phase, when the indigo blue method was first employed, varying volumes of the samples taken at required time intervals were added to the fixed volume of indigo blue reagent. This procedure was designed around the basis that sample would be added to the reagent until an acceptable conversion of the reagent from exposure to the dissolved ozone occurred. This was observed as the
chemical reaction of ozone with the reagent, resulting in a change of the solution from a dark blue color to a pale blue color. Samples taken at the beginning and end of the experiment, where the dissolved ozone concentration was minimal, resulted in large sample weights in comparison to later samples. To achieve a similar conversion of the indigo reagent as samples with a higher dissolved ozone concentration, a sample volume of roughly 20ml was often required. Samples with high dissolved ozone concentrations often required 5ml or less of sample water to achieve the required conversion of the indigo reagent. The weight of each sample jar and sample is used in the subsequent calculation of the ozone concentration. After review of initial results, the procedure was amended to allow only a maximum addition of 5ml of the sample taken at each sampling time, resulting in a standard sample volume addition and less variation.
The indigo blue method uses collected data from each sample including, weight of added sample and UV absorbance at 600nm to determine the dissolved ozone
concentration. This calculation is heavily influence by both of these data points, and the previously mentioned large sample weight coupled with a low UV absorbance resulted in the calculation of negative dissolved ozone concentrations. As a negative ozone
concentration is not possible, this error is contributed mainly to the failure of the method to accurately calculate dissolved ozone concentrations below a certain threshold in combination with the excessive volume of sample added by the researcher.
Equipment Issues
The equipment issues that arose during the 6-week experimental period with the IHE bench scale ozonation setup were limited to the KI 100 dissolved ozone analyzer (MI-3001), the Ritter gas flow meter (FI – 3000) and the ozone generator. In the early stages of the experiments conducted with the benchscale equipment, the KI 100 analyzer operated as designed, however after a number of experiments it’s reliability decreased and eventually ceased to function normally. The decision to stop using the analyzer was due to its constant status in ‘error’ and inability to produce consistent, accurate dissolved ozone measurements. In order to obtain the required dissolved ozone measurements, the indigo blue method was employed.
Additionally, functional issues with the Ritter gas flow meter were encountered. The flow meter would occasionally stop measuring the flow completely, or stop for a short period time and begin again. This caused issues with measuring the flow of ozone gas and calculating the ozone gas volume delivered to the reactor chamber. The Ritter meter was serviced twice during the experimental period and appeared to function
normally during the trials reported in this research, although several trials were discounted due to the issues with the meter.
Production of ozone gas from the ozone generator created a further issue with the operation of the bench scale equipment and experiments. The Trailigaz ozone generator operated in cycles, where at certain points during its operation the ozone gas flow would vary. This was observed with the change in ozone gas flow on the Ritter flow meter. During the end of a cycle, observed by an audible click and hum from the generator, the ozone gas flow would increase significantly for a short time before returning to the prior flow. This behavior was observed both during experiments and while generator was on standby and the valve to the reactor chamber was closed.
Adjustments Made in Demineralized Water Graph and Experimental Results
In contrast with the demineralized water trial, where the dissolved ozone
concentration reaches the 6mg/l equilibrium trial after the ozone uptake occurs, the tap water ozone concentration remains below equilibrium. A substantial difference of 2mg/l is present in the dissolved ozone concentration in the tap water sample after the initial uptake. This difference remains constant as the dissolved ozone concentration plateaus at 4mg/l (Figure 11), remaining below the equilibrium determined from the demineralized water sample at 6mg/l. Further analysis of the tap water experimental data showed both the blank and measurement taken at t=0 resulted in a measurement of 2mg/l dissolved ozone. This concentration is inconceivable due to both the lack of dissolved ozone in tap water and inability of ozone to instantaneously dissolve into water. Therefore it is concluded that these measurements were taken in error, and the graph was adjusted accordingly by shifting the measured dissolved ozone concentration down by 2mg/l to account for this error. Practically this discrepancy between the measured ozone
concentration and that of the equilibrium is impossible, as it indicates that there is a constant ozone demand in the tap water sample. Despite this difference between the equilibrium concentration and the measured dissolved ozone concentration in tap water, it is impossible that a constant ozone demand persisted in the sample for 45 minutes after the initial ozone uptake. It is more likely that instead error occurred in the measurement. The expected behavior of the tap water dissolved ozone concentration over time is a immediate increase in concentration as the ozone gas begins to dissolve into the water, then increase gradually as it is consumed and begins to trend towards equilibrium. As seen in other experiments throughout this research, dissolved ozone’s equilibrium within a water matrix is typically seen as a plateauing of the concentration over time. This indicates that the water matrix no longer has an ozone demand and any ozone reactive compounds or matter has been consumed. If a significant ozone demand exists in a water sample, such as that seen with the secondary wastewater matrix, the dissolved ozone concentration would gradually rise as the ozone demand is met and reactive particles and matter are consumed. In this situation the concentration would continue this gradual increasing trend until the ozone demand was satisfied and equilibrium is met, resulting in a plateauing of the concentration of dissolved ozone. The tap water sample exhibited this trend, with the major difference that the ozone demand seems to have been met and the dissolved ozone concentration did not rise to the 6mg/l equilibrium described by the demineralized trial.
Several possible factors could have lead to the presence of the discrepancy between the dissolved ozone concentration in tap water and the equilibrium derived from the demineralized water trials. The most likely factor is that error occurred in measurement of the dissolved ozone concentration with the indigo blue method. Possible off gassing of dissolved ozone can occur when a sample is taken for measurement, reducing the
dissolved ozone concentration. Additionally, the HWL measured results yielded a 2mg/l concentration in the blank sample. It is not expected that any ozone be present in the non- ozonated blank, and it is feasible that the 2mg/l discrepancy is due to human error in use of the indigo blue method. The possibility of error also exists in measuring the
absorbance of the ozonated indigo blue sample with the spectrophotometer, resulting in the 2mg/l concentration seen in both the blank and t=0 sample.
However, although error in the use of the indigo blue method likely occurred during this tap water trial, it is not indicative of a structural issue with the procedure, method or other data generated in this research. Experiments performed with the other three water matrices yielded sensible results that align with the expected behavior of ozone with varied water matrices, as seen in previous studies and research. Similar error in measurement did not occur with prior preliminary tap water trials conducted with the indigo blue method and the bench scale equipment, resulting in the conclusion this error was purely incidental.
Despite the issue with tap water reaching equilibrium, the results aligned with the expected outcome of the interaction of ozone with a water matrix such as tap water. Due to the level of treatment and presence of ozone reactive material in the tap water sample a minimal ozone demand and small ozone uptake was anticipated. This was confirmed with an ozone uptake greater than that of demineralized water, and ozone demand present in the tap water matrix.