Reduction of Photoreactivation with the Combined UV/Peracetic Acid Process or by Delayed Exposure to Visible Light

Abstract

Photoreactivation of microorganisms following UV inactivation is a well‐known, but complex, phenomenon. It is affected by several factors, including UV fluence, wavelength, light intensity, and exposure time to photoreactivating light. The effect on photoreactivation of a combined peracetic acid (PAA)/UV process has not been investigated. Accordingly, this study compared the degree of photoreactivation, under both sunlight and artificial lights, following UV and combined PAA/UV inactivation of fecal coliforms. Effluent samples from the Montreal Wastewater Treatment Plant (MWTP) (Quebec, Canada) were exposed, for 3 hours, to both low‐ and high‐intensity artificial lights and sunlight. All resulted in similar photoreactivation levels. However, average photoreactivation for UV‐treated wastewater samples was 1.2 logs, compared with 0.1 log for the combined PAA/UV treatment. Hence, the use of PAA in combination with UV can significantly reduce the potential for photoreactivation. To simulate the photoreactivation conditions of the MWTP effluent (which passes through a 4‐km outfall tunnel with approximately 3 hours detention time), UV‐treated samples were kept in the dark for 3 hours before photoreactivating light exposure. After this period, photoreactivation levels were close to zero. Hence, the effects of photoreactivation may be diminished by use of a combined disinfection scheme and/or by delaying exposure of the disinfected wastewater to light.

Photoreactivation of Escherichia coli is impaired at high growth temperatures

“…If photolyase is oxidized in vivo by some reagents, photoreactivation will be more readily suppressed. This inference is in line with the report that combined peracetic acid and UV processes can reduce photoreactivation [34].…”