EDTA Degradation by UV/Chlorine Treatment: Efficiency, Formation of Oxidation Byproducts and Toxicity Evaluation

UV and chlorination are worldwide employed tertiary treatment for disinfection/oxidation of secondary effluents in sewage treatment plants. However, However, the combined UV and chlorination process (UV/chlorine) for removing non-biodegradable EDTA in secondary effluents is still unknown. In this study, we evaluated UV/chlorine for EDTA degradation and compared its formation of typical oxidation byproducts and toxicity change with chlorination. UV/chlorine showed synergistic effect on EDTA degradation, with removal efficiency of higher than 90% within 60 s at chlorine dosage of 0.30 mM, compared to UV irradiation of less than 2% and chlorination of about 65%. Increasing chlorine dosage from 0.050 to 0.30 mM and solution pH from 5.0 to 7.0 accelerated EDTA degradation. The coexisting bicarbonate and nitrate showed slight effects on EDTA degradation, but natural organic matter exhibited remarkable inhibition. As well, about 50%-80% of EDTA in realistic secondary effluent was removed by UV/chlorine. Three chlorinated byproducts including trichloromethane (TCM), dichloroacetic acid and trichloroacetic acid were formed in both chlorination and UV/chlorine, and UV/ chlorine treatment slightly enhanced the production of TCM. The acute toxicities of treated wastewater by UV/chlorine to luminescent bacterium and rice seeds were greater than that by chlorination, which deserves extensive concerns in advanced treatment processes for secondary effluents.