UV photooxidation facilitating biological treatment for the removal of NOM from drinking water

The effects of UV-C (254 nm) radiation on the biodegradability and chlorine demand of natural organic matter (NOM) were investigated. Biodegradability was assessed using the biological regrowth potential and biodegradable dissolved organic carbon (BDOC) methods and remained unchanged for doses of 40 to 1,000 mJ cm⁻², but increased when the water was exposed to higher doses. The DOC concentration and UV absorbance values were reduced in UV-treated waters, but the chlorine demand remained the same as for raw water. Exposure of the UV-treated water to microbial treatment (via the BDOC test) gave a reduced DOC concentration and chlorine demand, indicating that the chlorine-consuming compounds resulting from irradiation were biodegradable. High performance size exclusion chromatography showed that both biodegradability assessment methods preferentially removed smaller molecules. Formaldehyde, acetaldehyde, glyoxal and glyoxylic acid were detected in UV-treated waters and were removed by the BDOC test. Chlorine demand correlated well with the concentration of low molecular weight (LMW) carbonyl compounds, which can be considered to be a surrogate for a large number of unidentified organics contributing to the chlorine demand and biodegradability.