Chlorine and UV are the most common disinfectants at wastewater treatment plants, and combining them may provide advantages such as better disinfection efficacy, improved reliability, and fewer disinfection byproducts. This work investigated disinfection efficacy using free chlorine
and UV with poliovirus, MS2 coliphage (MS2), adenovirus, and total coliform in bench-scale experiments with fully nitrified secondary and filtered effluents. Combining UV and free chlorine achieved disinfection benchmarks for all microorganisms, and inactivated a wider range of microorganisms
than either disinfectant alone; UV alone provided relatively poor disinfection of adenovirus, and free chlorine alone did not always achieve total coliform benchmarks. Operationally, water quality and relative UV/chlorine dose affected disinfection efficacy, but disinfectant order (whether
UV is applied before, after, or simultaneously with chlorine) had little effect. Overall, the results demonstrate improved disinfection with combined UV and free chlorine, and provide insight into the effects of operating conditions on disinfection efficacy.
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