**4.1. Advantages of advanced oxidation processes**

AOPs have a number of advantages when compared to conventional oxidation processes (Gabardo Filho, 2005; Domènech et al., 2001):

Multivariate Analysis in Advanced Oxidation Process 67

**Figure 1.** Electronic scheme of a photochemical process for heterogeneous photocatalysis (Ciola, 1981)

TiO2 is a solid with a melting point of 1800 °C, and the ninth most abundant element corresponding to 0.63% of crust. The element described features 4 crystalline forms of anatase (tetragonal), brookita (orthorhombic), rutile (tetragonal) and TiO2 (B) monoclinic, as

Design of Experiments (DOE) has been widely used in the optimization of process parameters and improving of quality products by the application of engineering concepts

Design of Experiments is defined as a set of statistical techniques applied to planning, conduction, analysis and interpretation of controlled tests, in order to find define factors that influence the values of a parameter or group of parameters (Bruns; Neto; Scarmínio, 2010).

**Figure 2.** Crystal structures: anatase (a), rutile (b) and brookita (c)

shown in Figure 2 (Carp et al., 2004).

and Statistics (Wang, Huang, 2007).

**6. Design of experiments** 


### **4.2. Disadvantages of advanced oxidation processes**

AOPs can be applied to certain types of waste under some restrictions, as follows (Domenech et al., 2001; Moral, 2005):

