Effect of intracellular algal organic matter and nitrate on disinfection byproduct formation in chlorinated water after UV/H2O2 and UV/Cl2 advanced oxidation processes†

Abstract

Advanced oxidation processes (AOPs) are one of the highly effective alternatives for treatment of algal toxins in drinking water. Water that contains algal toxins commonly has organic matter of algal origin and elevated nitrate. Organic matter undergoes transformations during advanced oxidation processes and may change in a way that increases disinfection byproduct (DBP) formation when water is chlorinated post-AOP. Nitrate forms reactive nitrogen species under certain UV wavelengths that can also interact with organic matter and change its properties in a way that increases post-AOP DBP formation. Two types of advanced oxidation processes (UV/H₂O₂ and UV/Cl₂) were compared in their ability to change the formation potential of regulated DBPs [four trihalomethanes (THMs) and nine haloacetic acids (HAAs)] and an unregulated nitrogenous DBP (N-DBP) N-nitrosodimethylamine (NDMA) due to the interaction of the process with algal organic matter (AOM) and nitrate in the water. The two AOPs showed no significant differences in post-treatment DBP formation under any of the tested conditions. Higher levels of treatment with both processes led to slightly higher formation potential of some THMs. AOM made a poor precursor for additional THMs and three HAAs (six not consistently detected), but had a higher NDMA yield than background organic matter (0.59 ng mg⁻¹-C vs. 0.18 ng mg⁻¹-C, p = 0.038). Nitrate suppressed chlorinated THMs and favored increased concentrations of brominated THMs and HAAs, resulting in higher percent incorporation of background bromide into DBPs. Moreover, nitrate addition (20 mg-N L⁻¹ of added nitrate compared to the background level of 0.47 mg-N L⁻¹) led to 11 times higher NDMA formation. Formation of N-DBPs during post-AOP chlorination in the presence of AOM and nitrate warrants additional investigation.