## 3 Results and Discussion
### Response Surface Modelling and Optimization  
#### 3.6.1 Turbidity  
Synthetic wastewater was characterized and used for the RSM experimental runs. Turbidity and COD were 317 NTU and 870 mg/L. Table 3 shows the operating parameters (factor 1 and factor 2) generated by RSM. Once these runs were conducted, the results for the output parameters were entered. Next, the two responses were then analyzed by the Design Expert. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Catalyst** & **Wastewater** & \begin{tabular}{c} **Initial** \\ **Conditions** \\ \end{tabular} & \
\begin{tabular}{c} **Removal** \\ **Efficiency** \\ \end{tabular} &
\begin{tabular}{c} **References** \\ \end{tabular} \\ \hline \multirow{4}{*}{TiO\({}_{2}\)} & \multirow{4}{*}{Municipal Sewage} & COD: 620 mg/L & \multirow{4}{*}{12\%} \\  & & Colour: 878 Pt.Co & 12\% \\  & & Turbidity: 365 NTU & 76\% \\  & & pH: 6.85 & 80\% \\  & & Cat. Loading: 0.1 g/L & pH: 7.1 & \\  & & Reaction time 150 min & & \\ \hline \multirow{4}{*}{TiO\({}_{2}\)} & \multirow{4}{*}{Real greywater} & COD: 620 mg/L & \multirow{4}{*}{92\%} \\  & & pH: 5 & \\  & & Cat. Loading: 0.1 g/L & pH: 4.45 & \\ \cline{1-1}  & & Reaction time 150 min & & \\ \hline \multirow{4}{*}{TiO\({}_{2}\)} & \multirow{4}{*}{Petroleum refinery} & COD: 1226 mg/L & \multirow{4}{*}{93} \\  & & PH: 8 & \\ \cline{1-1}  & & Cat. Loading: 1.5 g/L & Reaction time: 150 min & \\ \hline \multirow{4}{*}{TiO\({}_{2}\)} & \multirow{4}{*}{Petroleum refinery} & COD: 8200 mg/L & \multirow{4}{*}{60\%} \\  & & pH: 4.5 & \\ \cline{1-1}  & & Cat. Loading: 1 g/L & \\ \hline \multirow{4}{*}{TiO\({}_{2}\)} & \multirow{4}{*}{Paper mill} & COD: 2075 mg/L & \multirow{4}{*}{75\%} \\  & & pH: 6.5 & \\ \cline{1-1}  & & Cat. Loading: 0.75 g/L & \\ \cline{1-1}  & & Reaction time 180 min & & \\ \hline \multirow{4}{*}{TiO\({}_{2}\)/calcium aluminosilicate} & \multirow{4}{*}{Municipal Sewage} & COD: 2487 mg/L & \multirow{4}{*}{94\%} \\ \cline{1-1}  & & pH: 6.5 & \\ \cline{1-1}  & & Cat. Loading: 0.75 g/L & \\ \cline{1-1}  & & Reaction time: 180 min & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparative studies on TiO\({}_{2}\) photo-catalytic degradation of actual wastewater. The fit summary for response 1, shown in Table 4, was generated, and the quadratic model was suggested based on the \(p\)-value; see Equation (3). The results for the analysis of variance (ANOVA) for the quadratic model can be seen in Table 5, and the model F-value of 4.91 indicates the model is significant. It also suggests that an F-value this large only has a 3.01% chance of occurring due to noise. The \(p\)-value generated is 0.0301 and indicates that the model term is significant as it is less than 0.0500. Specific to this case, A\({}^{2}\) is a significant model term. Any \(p\)-value that is greater than 0.1000 suggests the model terms are not significant; however, hierarchy terms are not included in the terms counted. Therefore, this model is significant and acceptable as only model terms AB and B\({}^{2}\) are above 0.100. Lastly, the value of 0.9639 for the lack of fit indicates that it is not significant relative to pure error, and a non-significant lack of fit value indicates that the model will fit. The predicted R\({}^{2}\) value of 0.5647 is in reasonable agreement with the adjusted R\({}^{2}\) value of 0.6196, as the difference between the two values is less than 0.2. The adequate precision (Adeq. Pr) of 4.8711 shows an adequate signal as the value is greater than 4, which is desirable. The predicted values were plotted against the experimental values using an experimental predicted data interactive plot to approximate the extent of the correlation. Figure 5a depicts a reasonably strong linear correlation between the experimental and predicted data with a high regression coefficient (Table 5), with only a couple of data points as outliers. The standard error (SE) for the line of best fit showed an insignificant deviation, as the \(p\)-value was less than 0.05 at a 95% confidence level.