Trimethoprim (TMP) is a pharmaceutical compound, which is commonly found in the water environment. The UV/chlorine process forms several reactive species, including hydroxyl radicals (HO•) and reactive chlorine species, to degrade contaminants. The influencing factors and the optimal operational conditions for the degradation of TMP by the UV/chlorine process were investigated. The degradation of TMP was much faster by the UV/chlorine process as compared to the UV alone or free chlorine alone process. A kinetic model was developed to simulate the degradation of TMP and determine the unknown rate constants. This study also predicted the relative contributions of each of the reactive species and photolysis using the developed kinetic model. It was found that the ClO• radical was the major reactant responsible for the degradation of TMP. Furthermore, the most important finding was the identification of the best operational conditions. The best operational conditions resulted in the lowest use of energy and electrical energy per order (EE/O), namely, (1) for the ultrapure water, the optimum intensity of the UV light and the free chlorine dosage were 2.56 Einstein/L·s and 0.064 mM, respectively, with a minimum EE/O of 0.136 kWh/m³; and (2) for the water matrix containing 3 mg/L NOM, the optimum intensity of the UV light and the free chlorine dosage were 3.45 Einstein/L s and 0.172 mM, respectively, with a minimum EE/O of 0.311 kWh/m³.

Pharmaceuticals and personal care products (PPCPs) in drinking water have attracted increasing attention from domestic and international scholars, and Trimethoprim (TMP) is one of the common drugs in the aqueous environment. Since reactive substances such as HO• and Cl• and ClO• are generated in the UV/free chlorine oxidation system, it is considered as an emerging alternative to the UV/H2O2 process. However, since the background conditions of the water body and the oxidant concentration have an effect on the oxidation effect, a comparison of the UV/free chlorine and UV/H2O2 oxidation systems under different conditions was carried out in this paper. It was found that the removal efficiency of TMP by UV/free chlorine was 16% higher than that by UV/H2O2 at an oxidant concentration of 0.05mM, and the removal efficiency of TMP degradation by UV/free chlorine was 17% higher than that by UV/H2O2 at a HCO3 -/CO3 2- concentration of 1mM. The removal efficiency of TMP by UV/free chlorine was 17% higher than that by UV/H2O2. The UV/free chlorine oxidation system was superior to the UV/H2O2 oxidation system in degrading TMP. The main reason for this phenomenon is that the quantum yields of HClO and OCl- are higher than those of H2O2. Under the same UV intensity and concentration conditions, UV/free chlorine oxidation system can produce more free radicals in the same time, so UV/free chlorine oxidation effect is better and less costly.