UV/H₂O₂ treatment: an essential process in a multi barrier approach against trace chemical contaminants

The presence of pesticides, endocrine disruptors and pharmaceuticals caused PWN to implement multiple barriers for organic contaminant control in their surface water treatment plants. A combination of advanced oxidation by UV/H₂O₂ treatment and granular activated carbon (GAC) filtration is installed. Medium pressure UV experiments in a standard pilot reactor have been carried out into the degradation of a representative selection of pesticides found in PWN's main raw water source, the IJssel Lake. It was observed that atrazine and diuron are more sensitive to direct photodegradation while bentazone and bromacil are primarily degraded by hydroxyl radical oxidation. Addition of H₂O₂ increased the decay rate of all selected herbicides. Using computational fluid dynamics, irradiance distribution and kinetic models developed by Trojan Technologies Inc., an optimized UV-reactor was designed. In tests with a pilot reactor according to this new design, the predicted performance was confirmed, both for photodegradation and hydroxyl radical oxidation. During the research period, the scope broadened from pesticides to pharmaceuticals, endocrine disrupting compounds, solvents and algae toxins. At process conditions 0.56 kWh/m³ and 6 mg/L H₂O₂, 80–100% degradation was achieved for compounds such as mecoprop, clofibric acid and diclofenac. A somewhat lower degradation was found for dicamba, 2, 4-D, bentazone, ibuprofen, carbamazepine and sulphametoxalol. The developed modelling was used to design a full scale UV/H₂O₂ system with an electric energy of 0.56 kWh/m³ for treatment of 3,000 m³/h. In a site acceptance test, degradation of atrazine was measured at two UV-doses at a fixed H₂O₂ dose of 6 mg/L. The installation performed as predicted by the design models and design criteria were met. At wtp Andijk, UV/H₂O₂ is integrated in the existing process train, preceded by conventional surface water treatment (coagulation, sedimentation and filtration) and followed by GAC filtration providing a robust barrier against reaction products from both oxidation and photolytic degradation (assimilable organic carbon, nitrite). Replacing the conventional pretreatment by ion exchange followed by ceramic microfiltration will further improve the economics of UV/H₂O₂ treatment.