Dissolved ozone concentration

Bubble columns and various modifications such as airlift reactors, impinging-jet-reactors, downflow bubble columns are frequently used in lab-scale ozonation experiments. Moderate /qa-values in the range of 0.005-0.01 s l can be achieved in simple bubble columns (Martin et al. 1994 Table 2-4 ). Due to the ease of operation they are mostly operated in a cocurrent mode. Countercurrent mode of operation, up-flow gas and down-flow liquid, has seldom been reported for lab-scale studies, but can easily be achieved by means of applying an internal recycle-flow of the liquid, pumping it from the bottom to the top of the reactor. The advantage is an increased level of the dissolved ozone concentration cL in the reactor (effluent), which is especially important in the case of low contaminant concentrations (c(M)) and/or low reaction rate constants, i. e. typical drinking water applications... 

Normally, it makes little sense to apply such systems in lab-scale ozonation experiments, since the high mass transfer rates are only achieved at high gas flow rates which because of the typical operation characteristics of EDOGs accordingly means low ozone gas concentrations. An appropriate field of application was, however, presented in the study of Sunder and Hem pel (1996) who operated a tube-reactor for the ozonation of small concentrations of perchloroethylene. An injector nozzle coupled with the highly efficient Aquatector ozone-absorption unit was installed in front of the tube-reactor. Both the gas and liquid were partially recycled in this system. According to the authors more than 90 % of the ozone produced was absorbed in demineralized water and dissolved ozone concentrations ranged up to 100 pmol L-1 (cL = 5 mg L-1, T= 20 °C). 

Ozone decay. Is it necessary to know the ozone decay rate (exactly) in waste water treatment studies No, not in every case. Often the reactions of ozone with organic compounds occur in the liquid film (fast reactions), so that cL is (approx.) zero and ozone decay cannot occur. Generally, measure the dissolved ozone concentration cj B 3.2, B 4... 

With the help of this correction term the atrazine concentration could be calculated with a precision of 15 %. The B-term showed no trend, no general considerations were possible. The prediction of the dissolved ozone concentration was only possible in the presence of scavengers. The effect of ozone consumption due to direct reactions with the intermediates was estimated. 

In a first approximation a pseudo-first order reaction rate is often assumed. This must be checked against what really happens in the reactor. In semi-batch or nonsteady state oxidation, the concentration of the pollutants as well as the oxidants can change over time. A common scenario initially a fast reaction of ozone with the pollutants occurs, the reaction is probably mass transfer limited, the direct reaction in the liquid film dominates, and no dissolved ozone is present in the bulk liquid. As the concentration of the pollutants decreases, the reaction rate decreases, less ozone is consumed, leading to an increase in the dissolved ozone concentration. Metabolites less reactive with ozone are usually produced. This combined with an increase in dissolved ozone, may also shift the removal mechanism from the direct to the indirect if radical chain processes are initiated and promoted (see Chapter A 2). These changes are often not observed in waste water studies, mostly because dissolved ozone is often not measured. 

In steady-state experiments, although all concentrations remain constant, the simplification to pseudo-first order should be avoided or used only conditionally. It is important to remember that the pseudo-first order rate coefficient k is also dependent on the concentrations of the oxidants. For example if the steady-state dissolved ozone concentration changes due to changes in the operating conditions, k also changes. The same is true for the OH° concentration. 

Ozone Measuring the dissolved ozone concentration may also be more complicated in three-phase systems. The analysis of dissolved ozone by the photometrical indigo method (Hoigne and Bader, 1981) is disturbed by compounds or materials that scatter or absorb... 

This method has a distinct advantage in that it produces ozone in pure water (electrolyte free), making this system more desirable for wastewater treatment. Higher dissolved ozone concentrations can be achieved by pressurizing the system [16] which will eliminate the need for low efficiency gas-liquid contact spargers. However, some of the disadvantages of this system are ... 

Figure 5.10 showed a relationship between dissolved ozone concentration of MF permeate and membrane filtration flux. Membrane filtration flux substantially increased with up to 0.3 mg/L dissolved ozone concentration, while rising gradually with 0.3-1 mg/L of the concentration. Based on this result, it became obvious that it is necessary to maintain dissolved ozone concentration more than 0.3 mg/L. 

Figure 5.10 Relationship between dissolved ozone concentration of MF permeate and filtration flux.

Effects of Membrane Filtration Method on Membrane Filtration Flux Figure 5.12 shows the changes in ozone dosage, dissolved ozone concentration of MF permeate, and membrane filtration fluxes when ozone was injected with an ejector and water was filtrated with cross-flow (Run 11) and dead-end (Run 12) methods. Membrane filtration flux for the dead-end method was higher than that of the cross-flow method, and high membrane filtration flux operation (about 6 m /m /day) was achieved with 3 mg/L ozone dosage. 

Figure 5.14 shows the changes in raw water turbidity, dissolved ozone concentration of MF permeate, and transmembrane pressure. In this pilot plant, we continuously monitored the dissolved ozone concentration of MF permeate and controlled the ozone dosage so that the dissolved ozone concentration was kept at the given value (ca.l mg/L). As a result, raw water turbidity rose up to about 150°, but stable filtration could be maintained during the period. 

Based on these results, by controlling ozone dosage to have dissolved ozone concentration of MF permeate at around 1 mg/L, continuous high membrane filtration flux operation could be maintained for about 3 months even during the low water temperature period and the high turbidity period. 

Figure 5.14 Changes in raw water turbidity, dissolved ozone concentration of MF permeate and transmembrane pressure, data corresponding to Run 14 of Table 5.5.

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