Influence of process conditions and water quality on the formation of mutagenic byproducts in UV/H2O2 processes

• UV disinfection of drinking water at low UV dose does not seem to give mutagenic byproducts.
• LP UV lamps do not cause the formation of mutagenic byproducts.
• Formation of mutagenic byproducts by MP UV lamps largely depends on dose, nitrate and NOM concentration.
• Formation of mutagenic byproducts is caused by photolysis of nitrate and reaction with NOM.

UV/H2O2 processes in drinking water treatment may generate byproducts which cause an increased response in Ames fluctuation assays. As this probably involves a mixture of substances in very low concentrations, it is challenging to identify the individual byproducts. Therefore it was studied under which conditions mutagenic byproducts are formed and how this can be prevented. It was found that positive Ames fluctuation test responses only are obtained when Medium Pressure UV lamps are used, and not with Low Pressure lamps. This probably is explained by the photolysis of nitrate, which plays an important role in the formation of mutagenic byproducts. The most important parameters involved in the formation of such byproducts were demonstrated to be the nitrate concentration, the natural organic matter, the UV spectrum of the lamps, and the UV dose applied. These factors explain up to 74–87% of the Ames fluctuation test responses after UV/H2O2 drinking water treatment. By taking this into account, drinking water utilities can estimate whether UV processes applied in their case may cause the formation of mutagenic byproducts, and how to take measures to prevent it.