Ozone consumption

The first term in equation I refers to ozone consumption via the initiation step. This term is negligible compared to the second. Thus,... 

Due to high activity in reactions with free radicals, ozone undergoes the chain decomposition in solutions also. The chain reaction of ozone decomposition was evidenced in 1973 in the kinetic study of cyclohexane and butanone-2 oxidation by a mixture of 02 and 03 [146-151], It was observed that the rate of ozone consumption obeys the equation [112] ... 

Chain decomposition of ozone was also observed in the oxidation of cumene by an O3-O2 mixture [151]. The rate of ozone consumption was found to be... 

The more complicated kinetic equation was found for ozone consumption in oxidized cyclohexane [295 K, CC14 [146]) ... 

The total ozone consumption per unit time, 0ozone is given by ... 

The value of the conversion factor s is determined by the required efficiency of the conversion of component A. By substituting equations (5,6) into equation (4) we find for the ratio of the total ozone consumption to the amount of ozone needed for the oxidation of component A, defined as the Ozone consumption Factor (OF) ... 

With given flow Q and influent concentrations CAi and Cs and the required efficiency s the reactor volume and ozone consumption can be calculated from this equation. 

In the case of a plug flow reactor (PFR) the ozone consumption rate Oozone can be given by an equation comparable to that for the CFSTR ... 

Substitution of equation (12) and (15) into equation (8) yields the ozone consumption factor (OF) for the PFR ... 

The consumption of ozone by component B is lower when using a PFR instead of a CFSTR, as can be calculated from the second terms in the right hand sides of equations (4) and (8). In Figure 3 the ratio of the ozone consumption by the conversion of component B in a PFR and a CFSTR is given in dependence on the ratio kA/kB and s. When a high conversion of component A is desired (e 1) and the kA and kB differ by more than one order of magnitude, which is mostly the case when only oxidation by molecular ozone is considered the ozone losses due to the conversion of component B can be reduced by more than 90%. 

Figure 3. Ratio of the ozone consumption by the conversion of component B (second term in right hand side of equations (7) and (16)) in a PFR and a CFSTR as a function of kA/kB. Parameter is = CAe/CAi.

All three terms which contribute to ozone losses are minimal in a PFR. The relative importance and the absolute values of these terms and the ozone consumption factor can be calculated from equations (7) and (16), provided the reaction rate constants and the reaction conditions are known. 

The removal of color and UV-absorbance is one of the easier tasks due to quick reactions and comparatively low required specific ozone consumptions in the range below... 

The reduction in DBP-formation also depends on the specific ozone consumption. Typical reductions are in the range of 10 to 60 % (compared to non-ozonated water), at specific ozone dosages between 0.5 to 2 g 03 g 1 DOC initially present. If bromide is present, brominated organic DBPs and bromate formation may occur. 

Calculate parameters, e. g. ozone consumption rate r(03), ozone yield coefficient Y(03/ M) (see Table 1 -2). 

For the experimenter in the laboratory, not only do materials have to be chosen on the basis of their corrosion-resistance, but also for their effect on ozone decay. Some metals (e. g. silver) or metal seals enhance ozone decay considerably. This can be especially detrimental in drinking water and high purity water (semiconductor) ozone applications, causing contamination of the water as well as additional ozone consumption. Moreover, the latter will cause trouble with a precise balance on the ozone consumption, especially in experiments on micropollutant removal during drinking water ozonation. With view to system cleanliness in laboratory experiments, use of PVC is only advisable in waste water treatment, whereas quartz glass is very appropriate for most laboratory purposes. 

A very simple type of a bubble column, which was not mentioned above is a gas-wash bottle. This very small-scale system (VL = 0.2-1.0 L) may be used for basic studies, in which general effects (e. g. influence of pH and/or buffer solutions specific ozone dose) are to be assessed. Its use is not recommended for detailed studies, because the mass-transfer coefficient is often low and its dependency on the gas flow rate is unknown or difficult to measure. Often there is no possibility to insert sensors or establish a reliable measuring system for exact balancing of the ozone consumption. An optimal mode of operation would comprise treatment of the (waste-)water for a certain period of time, preferably without withdrawal of solution during the ozonation. In this way different ozonation conditions can be tested by varying the ozonation time or the ozone gas concentration. A variation of the gas flow rate is not recommended. 

In waste water ozonation experiments with high concentrations of (highly reactive) contaminants it is recommended that the ratio of QG to VL not be too low, i. e. the liquid volume not be too large (VL= 1-5 L) and an appropriate ozone generator be used. This is important because of the high reaction rates which may be achieved and which may cause a total depletion of ozone in the off-gas, causing problems in balancing the ozone consumption in the systems. 

Recalling the remarks on reaction engineering (see above, Section 2.3.4) batch ozonation experiments are the best method to minimize the specific ozone consumption (unless simple biodegradation with an adapted biomass is possible). 

Regimes 2 and 3 - moderate reactions in the bulk (2) or in thefdm (3) and fast reactions in the bulk (3) For higher reaction rates and/or lower mass transfer rates, the ozone concentration decreases considerably inside the film. Both chemical kinetics and mass transfer are rate controlling. The reaction takes place inside and outside the film at a comparatively low rate. The ozone consumption rate within the film is lower than the ozone transfer rate due to convection and diffusion, resulting in the presence of dissolved ozone in the bulk liquid. The enhancement factor E is approximately one. This situation is so intermediate that it may occur in almost any application, except those where the concentration of M is in the micropollutant range. No methods exist to determine kLa or kD in this regime. 

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. 

When using an amperometric electrode as the measuring technique, interference from the solutes and solvent can occur. The solutes and solvent can adsorb to the semipermeable membrane of the electrode, therefore giving an additional resistance to the diffusion of ozone through this membrane to the electrolyte chamber. The use of an amperometric electrode is not recommended for water containing particles. In these cases the ozone consumption can only be calculated from the ozone gas balance. 

Assess results Several methods can be applied to minimize the ozone consumption in combined processes of ozonation and biodegradation. For example, the different operational behavior of batch and continuous-flow systems, with the resulting differences in oxidation products, etc. has to be considered, as well as the fact that multi-stage systems for each treatment process may be of advantage over single stage systems. 

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