## 1 Introduction
In addition to light intensity, light wavelength also affects the efficiency of photocatalytic degradation, especially when using common photocatalysts such as ZnO (zinc oxide), SnO\({}_{2}\) (tin oxide), and TiO\({}_{2}\), as they are only active when exposed to UV light, which accounts for only 5% of the solar spectrum or 388 nm [21]. As a result of their large bandgap (Eg > 3.0 eV), most photocatalysts are unable to take advantage of visible and infrared light, which accounts for 43% and 52% of solar energy, respectively. Additionally, refs. [22; 23; 24] stated that using commercial Degussa P-25, which has a crystalline ratio of anatase to rutile of 70/80:20/30 and a light wavelength of less than 380 nm, is adequate for photon activation. UV light provides a wavelength that ranges from 315 nm to 400 nm and therefore provides sufficient light photons for the photonic activation of the catalyst. Additionally, refs. [25; 26; 27; 28; 29] studied the effect of reaction time on the degradation efficiency of organic pollutants and found that an increase in reaction time increased the degradation efficiency, as the highest degradation efficiency was correlated to the longest reaction time. This was attributed to an increased interaction time between the pollutant and the surface of the photocatalyst. [PERSON] et al. [30] studied the effect of the mixing speed on the photocatalytic degradation of aqueous ammonia and found that an increase in mixing speed led to a higher percentage of ammonia removal. In addition, [PERSON] et al. [31] also studied the effect of mixing speed with respect to decolourization and determined that when the mixing speed was increased, the bulk solution improved its transfer to the catalyst's surface, therefore increasing decolourization. The optimization of process parameters in the photocatalytic degradation process is complex and may require many experimental results. However, the Response Surface Methodology (RSM) employed in this study has the ability to produce a large amount of information from a limited number of experiments [32]. The numerical optimization technique will be utilized to ascertain the optimum conditions and interactions between the operating parameters. The application of photocatalysis concerning the treatment of actual industrial wastewater is very limited [33]. In addition, the study explored the different light sources (UV and UV-visible) for the photocatalytic degradation of wastewater. In this study, the efficiency of TiO\({}_{2}\) was studied and analyzed based on the removal of turbidity and chemical oxygen demand (COD) from actual municipal wastewater in a photo-catalytic system. Three operating parameters were varied: reaction time (20-100 min), mixing speed (20-100 rpm), and catalyst load (0.3-1.5 g/L), and their effect on pH, colour, turbidity, and COD removal from the wastewater was analyzed. RSM was used to optimize and study the interactions between the operating parameters using simulated wastewater by generating the values of the reaction time and catalyst load that produced the best (COD) removal and turbidity removal. These optimized parameters were then applied to UV/UV-Vis light sources, and a comparative study between the two light sources was performed to determine which light source achieved better results for COD and turbidity removal. The advantage of visible-light photocatalysis lies in its use of clean, renewable, and cheap visible light as a driving force. The sampled wastewater effluent used for the experiment was obtained from a local South African Municipal wastewater treatment plant.