Modeling of Waste Water Oxidation

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. 

---