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Photoreactivation of ultraviolet (UV)-disinfected wastewater of different qualities was experimentally assessed. Photoreactivation ability of secondary effluent and microstrained inflow was analyzed in different samples of 50 mL (Petri dish) and 7,000 mL volume to describe open channel effluent situations of wastewater treatment plants in a more realistic approach. The small sample of secondary effluent revealed a total log10 inactivation of 1.8 units and the small sample of microstrained inflow a total log10 inactivation of 3.2, with an applied UV-254 fluence of 84 and 253 J/m2, respectively. Maximum net photoreactivation for secondary effluent and microstrained inflow was in the order of 1.2 log10 and 0.37 log10 units, respectively, for both sample sizes. However, significantly faster photoreactivation performance was generally determined for small sample volumes. The photoreactivation processes were completely compensated for by solar disinfection within a 120 min exposure time. Solar disinfection processes were negligible in the larger sample volumes of microstrained inflow. For municipal wastewater treatment systems with open channel effluents, it is essential to take into consideration the dependence of solar UV-365 fluence rate on water depth and wastewater characteristics. [ABSTRACT FROM AUTHOR]

Microcystis aeruginosa forms algal bloom in lakes. They produce toxic compounds such as microcystin. Against such algal problems, the effect of UV treatment was examined. In UV treatment, the effect of photoreactivation should be examined. Photoreactivation is a repair mechanism of genomic DNA damage by sunlight irradiation. UV treatment causes DNA damages on target cyanobacteria, however sunlight can repair some of these DNA damages. To examine the effect of photoreactivation, both white and yellow light incubations were employed. White light allows both photoreactivation and photosynthesis, while yellow light prohibits photoreactivation and only allows photosynthesis. Microcystis aeruginosa NIES 98 strain and PCC 7806 strain were used as the test cultures. Those cultures were exposed to low-pressure (LP) or medium-pressure (MP) ultraviolet (UV) lamp, then incubated under white or yellow light. Yellow light incubation method was effective to examine photoreactivation. It was revealed that almost six times UV fluence was required to inactivate 99% of Microcystis aeruginosa, under photoreactivation condition, compared with non-photoreactivation condition. Inhibition of photoreactivation could greatly enhance UV treatment efficiency against Microcystis aeruginosa. One of the practical suggestions is to conduct UV treatment at night, when photoreactivation by sunlight rarely takes place. Highly efficient inactivation was achieved by avoiding photoreactivation. [ABSTRACT FROM AUTHOR]

Ultraviolet (UV) light has become widely accepted as an alternative to chlorination or ozonation for wastewater disinfection. There are now over 2,000 wastewater treatment plants worldwide using either low-or medium-pressure UV technology. Recent studies investigating UV lamp technology, configuration, cleaning requirements and ageing, as well as long-term performance tests, have demonstrated beyond any doubt the effectiveness of UV in inactivating pathogens in wastewater. Research has also shown that, to ensure permanent inactivation and prevent the recovery of microorganisms following exposure to UV, a broad, "polychromatic" spectrum of UV wavelengths is necessary. These wavelengths inflict irreparable damage not only on cellular DNA, but on other molecules, such as enzymes, as well. Only medium-pressure UV lamps produce the necessary broad range of wavelengths; low-pressure lamps emit a single wavelength peak which only affects DNA. Polychromatic medium-pressure UV light is so effective because of the lamp's exceptionally high UV energy output at specific wavelengths across the UV spectrum. It has been shown, for example, that pathogenic E. coli O175:H7 was able to repair the damage caused by low-pressure UV, but no repair was detected following exposure to UV from medium-pressure lamps. [ABSTRACT FROM AUTHOR]

Photoreactivation of Escherichia coli K12 (IFO 3301), in the presence or absence of yeast extract (YE), was investigated after inactivation by low-pressure UV lamp. An endonuclease sensitive site (ESS) assay was used to determine the UV-induced pyrimidine dimers in the genome of E. coli, while a colony-forming ability (CFA) test was also used to examine the survival ratio of E. coli. The YE solution reduced the CFA recovery at a final concentration of 125 mg/L. A dialysis of the YE solution indicated that the YE fraction (with nominal molecular weight >1,000 and <3,500) was effective at repressing the CFA recovery. Interestingly, the repair of ESS was equivalent regardless of the presence of the YE dialysate, while the CFA recovery was significantly repressed in the presence of YE. It was, therefore, suggested that YE components, probably with molecular weights of 1,000-3,500, were effective at repressing the CFA recovery of E. coli without affecting the ESS repair during photoreactivation. [ABSTRACT FROM AUTHOR]

In the present study, the effect of UV radiation on the inactivation of a range of microorganisms was studied. Each organism was seeded into sterile tap water and exposed to UV in batch experiments with changing turbidities. In addition, the effect of UV on microbial communities in river Nile water was examined. It was found that Imin contact time (0.5L/min flow rate) was effective against vegetative cells levels almost reaching zero (except with Staphylococcus aureus). On the other hand, spore-forming bacteria, Candida albicans and coliphage were more resistant to UV. This contact time caused coenobia cells in single form with Scenedesrnus obliquus while for Microcystis aeruginosa colonies broke into smaller groups. Exposure of Nile water microbial communities to UV showed that yeasts and Aeromonas survived better than the other organisms while in the phytoplankton partial fragmentation occurred in some algal groups. The protective effect of turbidity differed between organisms, with increased contact time under conditions of stable turbidity having no effect on the organisms. At 20 NTU the UV radiation had no effect on the morphological characters of algal cells. In reactivation experiments, it is clear that photoreactivation, and not dark repair, takes place with bacterial cells. Only coliphage had no photoreactivation and dark repair responses although with coliphage and host, both reactivation processes worked well. Moreover, the irradiated algae regained their normal shape after 3 days in suitable media and enough light. [ABSTRACT FROM AUTHOR]