UV/H₂O₂Treatment: A Practical Solution for Organic Contaminant Control and Primary Disinfection

PWN's water treatment plant Andijk was commissioned almost 40 years ago. It services water from the IJssel Lake by conventional surface water treatment. In view of taste and odor problems the plant was retrofitted with GAC filtration 25 years ago. The finished water quality still complies with all E.C. and Dutch drinking water standards. Nevertheless an upgrade is desired to avoid the use of chlorine and to extend the barriers against pathogenic micro-organisms and a broad range of organic micropollutants such as pesticides, rocket fuel by-products (NDMA), fuel oxygenates (MTBE), solvents (dioxane), endocrine disruptors, algae toxins, pharmaceuticals, etc. UV/H2O2 treatment was selected for both primary disinfection and organic contaminant control. The disinfection requirements were based on a 10−4 health risk. The required 3 log inactivation for Giardia and Cryptosporidium was achieved by an UV dose lower than 20 mJ/cm2. The highest UV dose, 105 mJ/cm2, was needed for the inactivation of spores of Sulphite Reducing Clostridia. Reactivation of protozoa was established for UV doses up to 25 mJ/cm2, for doses higher than 45 mJ/cm2 no reactivation was observed. In view of the raw water concentrations the required organic contaminant degradation was set at 80%. Collimated beam and pilot-plant work showed that the required degradation can be achieved by the proper combination of electric energy and H2O2. In a UV reactor optimized for organic contaminant control, UV dose of 540 mJ/cm2 (about 0.5 kWh/m3) and 6 mg/L H2O2 were needed. Under those conditions pesticides (atrazine), NDMA, MTBE, dioxane, endocrine disruptors (bisphenol A), microcystine and pharmaceuticals (diclofenac, ibuprofen) could be removed up to the required 80%. Bromate formation was absent while formation of primary metabolites was insignificant. The UV dose for organic contaminant control is about five times higher than the dose needed for disinfection. The UV/H2O2 process was implemented into the existing treatment train between the sand and GAC filters. In the GAC filters excess H2O2 is degraded, nitrite is converted into nitrate and biodegradable reaction products are consumed by bacteria. The full-scale installation with 3 streets of 4 Trojan Swift 16L30 reactors has been in operation since October 2004. Disinfection and organic contaminant control are as expected.