The Application of a Surface Response Methodology in the Solar/UV-Induced Degradation of Dairy Wastewater Using Immobilized ZnO as a Semiconductor

An Advanced Oxidation Process (AOPs) was carried out in this study with the use of immobilized ZnO and solar/UV as an energy source to degrade dairy wastewater. The semibatch reactor system consisted of metal plate of 800 × 250？mm and a glass tank. The reaction time was of 3？h for 3？L of dairy wastewater. Experiments were performed based on a surface response methodology in order to optimize the photocatalytic process. Degradation was measured in percentage terms by total organic carbon (TOC). The entry variables were ZnO coating thickness and pH, using three levels of each variable. The optimized results showed a TOC degradation of 31.7%. Optimal parameters were metal-plate coating of 100？μm of ZnO and pH of 8.0. Since solar/UV is a constant and free energy source in most tropical countries, this process tends to suggest an interesting contribution in dairy wastewater treatment, especially as a pretreatment and the optimal conditions to guarantee a better efficiency of the process. 1. Introduction The use of ZnO as a semiconductor was studied for possible application in a photo-excitation-initiated degradation of the catalyst followed by the formation of a surface bandgap (see (1)). The oxidation potential ( ) permits the formation of active intermediates by the direct oxidation of an organic matter (see (2)). Many reactive hydroxyl radicals can be formed either by decomposition of water or by a bandgap reaction with (see (3) and (4)). The Hydroxyl radical is a powerful nonselective oxidation agent leading to organic pollutants degradation [1–3]. Consider that The methodologies used in the design of experiments allow a similar result as the one obtained from conventional experiments with the advantage of the use of fewer experiments. Thus, a good design of experiments can provide sufficient results for an effective statistical analysis [4]. In order to obtain the optimized variables for the study of dairy wastewater photocatalytic treatment, a surface response methodology was employed. Dairy wastewater does not generally contain inherently toxic chemical substances, but it is composed of dissolved organic compounds that are not easily degradable by biological treatment without a prior treatment. In fact, this limitation affects the efficiency of the treatment through pH (depending on the type of dairy), overload of the system, and sludge volume. Moreover, dairy wastewater also produces an unpleasant odor and consists of a liquid with a significant color if the organic load is high enough [5–7]. 2. Materials and Methodology 2.1. Sampling and