Effect of the ultraviolet/chlorine process on microbial community structure,typical pathogens,and antibiotic resistance genes in reclaimed water

Urban wastewater contains a wide range of pathogens and antibiotic resistance genes(ARGs),which are a serious concern if reusing treated wastewater.However,few studies have explored the microbial communities in reclaimed water using ultraviolet(UV)/chlorine treatment and assessed the changes of the resistome.This study investigated the occurrence of typical pathogens,ARGs,and bacterial communities in UV/chlorine-treated reclaimed water samples.The numbers of culturable and viable but non-culturable pathogens were effectively reduced to 0 CFU/mL within 1–10 and 10–30 min after UV/chlorine treatment,respectively.Meanwhile,the physicochemical indices of water quality were not affected.UV/chlorine treatment could significantly change the bacterial community structure of reclaimed water,showing a decrease in bacterial abundance and diversity.Chlorine-resistant Acinetobacter and Mycobacterium were the dominant bacterial genera(>50%)after UV/chlorine treatment.Moreover,the number of ARGs and mobile genetic elements(MGEs)decreased with an increase in UV/chlorine exposure.However,eight ARGs and three MGEs were consistently detected in more than three seasons,making these major concerns because of their potential role in the persistence and dissemination of antibiotic resistance.Overall,the results of this study suggest that UV/chlorine treatment can potentially improve the microbiological safety of reclaimed water.And more attention should be paid to the pathogens that are both chlorine-resistant and carry MGEs because of their potential for resistance transmission.

DBP formation from degradation of DEET and ibuprofen by UV/chlorine process and subsequent post-chlorination

The formation of disinfection by-products（DBPs） from the degradation of N,N-diethyl-3-methyl benzoyl amide（DEET） and ibuprofen（IBP） by the ultraviolet irradiation（UV）/chlorine process and subsequent post-chlorination was investigated and compared with the UV/H_2O_2 process.The pseudo first-order rate constants of the degradation of DEET and IBP by the UV/chlorine process were 2 and 3.1 times higher than those by the UV/H_2O_2 process, respectively, under the tested conditions. This was due to the significant contributions of both reactive chlorine species U（RCS） and hydroxyl radicals（HO） in the UV/chlorine process. Trichloromethane, 1,1,1-trichloro-2-propanone and dichloroacetic acid were the major known DBPs formed after 90% of both DEET and IBP that were degraded by the UV/chlorine process. Their yields increased by over 50%after subsequent 1-day post-chlorination. The detected DBPs after the degradation of DEET and IBP comprised 13.5% and 19.8% of total organic chlorine（TOCl）, respectively, and the proportions increased to 19.8% and 33.9% after subsequent chlorination, respectively. In comparison to the UV/H_2O_2 process accompanied with post-chlorination, the formation of DBPs and TOCl in the UV/chlorine process together with post-chlorination was 5%–63% higher,Ulikely due to the generation of more DBP precursors from the attack of RCS, in addition to HO.

Removal of C.I.Reactive Red 2 by low pressure UV/chlorine advanced oxidation

Azo dyes are commonly found as pollutants in wastewater from the textile industry,and can cause environmental problems because of their color and toxicity.The removal of a typical azo dye named C.I.Reactive Red 2（RR2） during low pressure ultraviolet（UV）/chlorine oxidation was investigated in this study.UV irradiation at 254 nm and addition of free chlorine provided much higher removal rates of RR2 and color than UV irradiation or chlorination alone.Increasing the free chlorine dose enhanced the removal efficiency of RR2 and color by UV/chlorine oxidation.Experiments performed with nitrobenzene（NB）or benzoic acid（BA） as scavengers showed that radicals（especially OH） formed during UV/chlorine oxidation are important in the RR2 removal.Addition of HCO_3^- and Cl^- to the RR2 solution did not inhibit the removal of RR2 during UV/chlorine oxidation.