Ultraviolet-based synergistic processes for wastewater disinfection: A review

Ultraviolet (UV) irradiation is widely used for wastewater disinfection but suffers from low inactivation rates and can cause photoreactivation of microorganisms. Synergistic disinfection with UV and oxidants is promising for enhancing the inactivation performance. This review summarizes the inactivation effects on representative microorganisms by UV/hydrogen peroxide (H2O2), UV/ozone (O3), UV/persulfate (PS), UV/chlorine, and UV/ chlorine dioxide (ClO2). UV synergistic processes perform better than UV or an oxidant alone. UV mainly attacks the DNA or RNA in microorganisms; the oxidants H2O2 and O3 mainly attack the cell walls, cell membranes, and other external structures; and HOCl and ClO2 enter cells and oxidize proteins and enzymes. Free radicals can have strong oxidation effects on cell walls, cell membranes, proteins, enzymes, and even DNA. At similar UV doses, the inactivation rates of Escherichia coli with UV alone, UV/H2O2, UV/O3, UV/PS (peroxydisulfate or peroxymonosulfate), and UV/chlorinated oxidant (chlorine, ClO2, and NH2Cl) range from 2.03 to 3.84 log, 2.62-4.30 log, 4.02-6.08 log, 2.93-5.07 log, and 3.78-6.55 log, respectively. The E. coli inactivation rates are in the order of UV/O3 approximate to UV/Cl2 > UV/PS > UV/H2O2. This order is closely related to the redox potentials of the oxidants and quantum yields of the radicals. UV synergistic disinfection processes inhibit photoreactivation of E. coli in the order of UV/O3 > UV/PS > UV/H2O2. The activation mechanisms and formation pathways of free radicals with different UV-based synergistic processes are presented. In addition to generating HO center dot, O3 can reduce the turbidity and chroma of wastewater to increase UV penetration, which improves the disinfection performance of UV/O3. This knowledge will be useful for further development of the UV-based synergistic disinfection processes.